things related to time travel

Time Travel Words

Words related to time travel.

Below is a massive list of time travel words - that is, words related to time travel. The top 4 are: time , space , science fiction and spacetime . You can get the definition(s) of a word in the list below by tapping the question-mark icon next to it. The words at the top of the list are the ones most associated with time travel, and as you go down the relatedness becomes more slight. By default, the words are sorted by relevance/relatedness, but you can also get the most common time travel terms by using the menu below, and there's also the option to sort the words alphabetically so you can get time travel words starting with a particular letter. You can also filter the word list so it only shows words that are also related to another word of your choosing. So for example, you could enter "time" and click "filter", and it'd give you words that are related to time travel and time.

You can highlight the terms by the frequency with which they occur in the written English language using the menu below. The frequency data is extracted from the English Wikipedia corpus, and updated regularly. If you just care about the words' direct semantic similarity to time travel, then there's probably no need for this.

There are already a bunch of websites on the net that help you find synonyms for various words, but only a handful that help you find related , or even loosely associated words. So although you might see some synonyms of time travel in the list below, many of the words below will have other relationships with time travel - you could see a word with the exact opposite meaning in the word list, for example. So it's the sort of list that would be useful for helping you build a time travel vocabulary list, or just a general time travel word list for whatever purpose, but it's not necessarily going to be useful if you're looking for words that mean the same thing as time travel (though it still might be handy for that).

If you're looking for names related to time travel (e.g. business names, or pet names), this page might help you come up with ideas. The results below obviously aren't all going to be applicable for the actual name of your pet/blog/startup/etc., but hopefully they get your mind working and help you see the links between various concepts. If your pet/blog/etc. has something to do with time travel, then it's obviously a good idea to use concepts or words to do with time travel.

If you don't find what you're looking for in the list below, or if there's some sort of bug and it's not displaying time travel related words, please send me feedback using this page. Thanks for using the site - I hope it is useful to you! 🐪

• science fiction
• theory of relativity
• novikov self-consistency principle
• time machine
• h. g. wells
• special relativity
• inertial frame of reference
• general relativity
• arrow of time
• time dilation
• fourth dimension
• the time machine
• time interval
• buy souvenir
• frank tipler
• robert forward
• chronological
• chronograph
• railway time
• time standard
• speed of light
• specious present
• clock watcher
• chronology protection conjecture
• plesiosauria
• exotic matter
• the clock that went backward
• twin paradox
• faster-than-light neutrino anomaly
• half century
• cosmic string
• compossibility
• synchronous
• simultaneity
• time of arrival
• time of departure
• amsterdam time
• morningtide
• crunch time
• between time
• greenwich mean time
• time horizon
• compile time
• time of day
• absolute time
• geological time
• spring break
• new york minute
• time measure
• standard time
• round trip ticket
• time measurement
• supper hour
• time signal
• visit other country
• canonical hour
• time keeper
• quantum mechanics
• mahabharata
• relativity of simultaneity
• mahākāśyapa
• quantum field theory
• einstein field equations
• philosophy of space and time
• gödel metric
• time travel in fiction
• quantum mechanics of time travel
• theoretical physics
• gautama buddha
• urashima tarō
• nihon shoki
• honi ha-m'agel
• louis-sébastien mercier
• washington irving
• rip van winkle
• circumnavigation
• the sleeper awakes
• samuel madden
• memoirs of the twentieth century
• guardian angel
• hypertravel
• alexander veltman
• peregrination
• peregrinate
• alexander the great
• anachronism
• cybertravel
• august derleth
• anonymous author
• simultaneously
• newcastle upon tyne
• synchronization
• charles dickens
• simultaneous
• theretofore
• a christmas carol
• pierre boitard
• edward everett hale
• alternate history
• edward page mitchell
• enrique gaspar y rimbau
• andrew sawyer
• safe-conduct
• closed timelike curve
• timewasting
• chronostratigraphy
• proper time
• interference
• quantum entanglement
• grandfather paradox
• stephen hawking
• fermi paradox
• semiclassical gravity
• experience different culture
• lose something
• see new place
• quantum gravity
• faster than light
• choose destination
• book holiday
• spacetime interval
• go to airport
• reverse commuter
• postulates of special relativity
• hand luggage
• motion sickness
• minkowski diagram
• plane ticket
• tachyonic antitelephone
• father time
• closed time-like curve
• hibernation
• vehicle propulsion
• one-way light time
• round-trip light time
• time weight
• back to the future
• launch window
• time period
• casimir effect
• energy condition
• matt visser
• tipler cylinder
• willem jacob van stockum
• cauchy horizon
• thermoregulation
• delayed choice quantum eraser
• alcubierre drive
• four-dimensionalism
• marlan scully
• fourier analysis
• double-slit experiment
• günter nimtz
• new scientist
• quantum tunneling
• ronald mallett
• university of toronto
• shengwang du
• photonic crystal
• relativistic speed
• time traveler convention
• albert einstein
• frame of reference
• gravity well
• global positioning system
• extraterrestrial life
• miles per hour
• seasonableness
• timekeeping
• go back home
• united states
• kornel lanczos
• weak energy condition
• magnetic field
• university of koblenz
• daylight save time
• st patrick's day

That's about all the time travel related words we've got! I hope this list of time travel terms was useful to you in some way or another. The words down here at the bottom of the list will be in some way associated with time travel, but perhaps tenuously (if you've currenly got it sorted by relevance, that is). If you have any feedback for the site, please share it here , but please note this is only a hobby project, so I may not be able to make regular updates to the site. Have a nice day! 🐞

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

If you liked this, you may like:

Is time travel even possible? An astrophysicist explains the science behind the science fiction

Published: Nov 13, 2023

By: Magazine Editor

Written by Adi Foord , assistant professor of physics , UMBC

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago. https://www.youtube.com/embed/QeRtcJi3V38?wmode=transparent&start=0 Telescopes are a kind of time machine – they let you peer into the past.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

This article is republished from The Conversation under a Creative Commons license. Read the original article and see more than 250 UMBC articles available in The Conversation.

Tags: CNMS , Physics , The Conversation

Related Posts

Watch: A recap of URCAD 2024

• Science & Tech

UMBC partners with American Statistical Association to organize annual African International Conference on Statistics

From thousands to millions to billions to trillions to quadrillions and beyond: Do numbers ever end?

Search UMBC Search

• Accreditation
• Consumer Information
• Equal Opportunity
• Privacy PDF Download
• Web Accessibility

Search UMBC.edu

Is time travel even possible? An astrophysicist explains the science behind the science fiction

Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County

Disclosure statement

Adi Foord does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

University of Maryland, Baltimore County provides funding as a member of The Conversation US.

View all partners

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

• Time travel
• Special Relativity
• Thermodynamics
• Stephen Hawking
• Curious Kids
• Curious Kids US
• Time travel paradox

Program Manager, Teaching & Learning Initiatives

Lecturer/Senior Lecturer, Earth System Science (School of Science)

Sydney Horizon Educators (Identified)

Deputy Social Media Producer

Associate Professor, Occupational Therapy

• April 24, 2024 | Quantum Computing Meets Genomics: The Dawn of Hyper-Fast DNA Analysis
• April 24, 2024 | Scientists Turn to Venus in the Search for Alien Life
• April 24, 2024 | NASA Astronauts Enter Quarantine As Boeing Starliner Test Flight Approaches
• April 23, 2024 | Peeking Inside Protons: Supercomputers Reveal Quark Secrets
• April 23, 2024 | A Cheaper and More Sustainable Lithium Battery: How LiDFOB Could Change Everything

Exploring the Reality of Time Travel: Science Fact vs. Science Fiction

By Adi Foord, University of Maryland, Baltimore County November 16, 2023

Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein’s theory of relativity shows time’s relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and paradoxes, such as the “grandfather paradox,” complicate the feasibility of actual time travel.

Will it ever be possible for time travel to occur?

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The Science Behind Time Travel

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time Is Relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Theoretical Possibilities and Challenges

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes, or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and Failed Dinner Parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time, and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Artist’s rendering of the James Webb Space Telescope. Credit: Northrop Grumman

Telescopes Are Time Machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel . As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope , the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang , about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies, and dreams.

Written by Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County.

More on SciTechDaily

A Significant Surge: How Tropical Cyclones Increase the Cost of Carbon

Voyager’s New Horizon: NASA Engineers Tackle Thruster Buildup & Software Glitches

New ScienceCast Video Previews the ‘Opposition of Mars’

“Our Solar System May Be Unusual”: Rogue Planets Unveiled With NASA’s Roman Space Telescope

AI-Powered Lasers: A Modern Solution to Space Debris

IBS Relief: Common Drug Improves Symptoms of Irritable Bowel Syndrome

Scientists Discover Chinmo – “The Youth Gene”

Jupiter’s Moon Europa Could Be Pulling Oxygen Down Below the Ice To Feed Life

8 comments on "exploring the reality of time travel: science fact vs. science fiction".

Until the problem of the second law of thermodynamics(entropy) is solved, the concept of time travel will remain the subject of science fiction. Since this is a basic law of our universe, there is no conceivable way that we know of to do this. The great thing about our knowledge of the universe is that it continues to grow and with that our view of what is possible continues to change. After all, at one time it was believed we could never leave earth!

The 7 planets are soul pollen in the space @ life has been the world has been prepared @ The pollen of the universe is hidden @ Around the axis of the galaxy, the universe is hidden@@ The pollen of the galaxies is a hidden cluster Itis made that we thought about what wisdom is in God’s work and how it is arranged in the form of words.The verse that is made is as follows @@ John, you are in time @ worlds, planets around the axis of the branches of galaxies @@ 8 Prophets, divine prophets, God-aware witnesses @ God’s words of revelation, they are aware @@ 48 What the words of revelation that every prophet has about @ sometimes Sometimes the message of God has become a verbal cliché in the head @@ 62 The message of God was given to every prophet @ The message was made around the power of God @@ 46 The truth of the religions of the cradle of time is said in the world @ Prophets always came for justice in time @@ 62 A warrior became brave in time @ Delaver Ghahrmani Boud Taarani@@ 43 Omar Noah never died, who knows!@ Imransan Is there an unseen world, immortality!?@@ 49 Men’s rights in the sign @ Human rights, the observance of world justice @@ 40 We have God’s love @ A love that is not patched, separation!!!: @@ 39 Take one word from the end of the first and second stanzas to the bottom of these eight verbal verses, and this sentence is made @@ God-aware, the world is born, you know the sign of God @ Agah,the beginning of time, the world, eternity, the world of separation @@ The meaning of this sentence is That God, who before us humans lived on the earth, formed the earth’s crusts with full knowledge, and we humans know the sign of God, which is on the earth, on the continents and countries, and some names have been shown by God for our knowledge since the end of time.In a later video, if I have a lifetime left, I will explain exactly about these poems, God willing @@ The word of the Prophet 17 is the 17th and Muhammad (PBUH) said that the Bedouin Arabs should pray 17 rakats so that theyperform ablution and be clean and not kill and loot.From the caravans, these were all God’s will, and he is good everywhere, in every nation, God does not like evildoers, sinners, and oppressors.Muhammad (PBUH) was God’s last messenger to the Arab people, he was God’s best prophet, and the third verse is because they do not accept Muhammad (PBUH).Some Iranians who were in contact with me, that’s why in the third verse of this surah, he said that his message was given to every prophet, the message is based on divine power, and the word truth, the first two letters of which istruth, is truth, and truth is the 43rd and forty-third word, and the word is time.It is exactly 46, and this song of the Prophets was revealed at the age of 46 ببخشید اگر قبلآ مطالبی فرستادم که به مذاهب مذاهم ارتباط داد شاید به درد شما نمیخورد من در کامنت های بعدی سعی میکنم از نجوم و حیات زمین سخن بگویم این چند بیت را به خارجی انگلیسی که تبدیل کردم معنی آن حیرت انگیز تعغیر کرد گفتم برای شما بفرستم و بداند که این کلام من نبوده کلام خداوند بوده شما نظرتان در مورد خداوند چیست من میگویم خداوند که پدر ومادر انسان بودند قادر به ساختن ما بودند اما آیا آنها قادر به ساختن ستاره ها هم بودند

Your comment has validity to God. But it surely has no place here, it is only fare if the hole comment were in english and has less of a convincing push in a belief a person either believes or not.

It’s too bad physics can’t come to a complete consensus about time, I would like to add some thought about discoveries it has been proven that time travels in only one direction forward, the experiment dealt with light thru glass and how it reacts in the middle and what change happens after light exits the other side, a simple explanation of this experiment. Brings me to theorize and start that time existed before the big bang and is outside of our universes influence, when time is acted on by gravity the ( Form ) of time is changed until the influence no longer has effect, this could go hand in hand with light photons the photon has a influence in the Form that time has. This can not be a observed difference unless we were to see beyond the speed of light. We do agree that physics changes at a subatomic state and also does some strange changing once the speed of light has been exceeded.

The experiment I referred to was posted on IFL in October 2023 headlined ( Solution to complex light problem shows that time can only go Forward ).

One of the problem with travel time is the one people keep forever. And, that is that the earth is always move through space. Matter of fact, the earth is not in the same place that it was 50 years ago. So you will have to move through space as well as time.

Ironically, the only science fiction that seems to handle this is the original story “The Time Machine”.

Time travel is happening now. It has been done since the 1950s. The method satisfies all the requirements. Traveler can’t change the past, but only observe. You can’t go farther back than when the machine was first invented (1950s). There’s one more limitation, you can only observe what the machine was directed. The time machine, the common video camera, and video tape recorder. Now it’s the camera, and file capture computer. Yes, viewing a video tape is effectively going back in time. It’s more than the video, it’s the sound too. There are working versions of smell, and touch which can be recorded too. If you record, and replicate all the senses, you have effectively complete time travel. The most primitive form is the picture. This technology has been around for thousands of years, and is manually intensive. Later many pictures were strung together to make a film. Using a camera to record film was the first example of complete visual time travel (back to when the film was made). Later sound was added, and we have the movies. A way of going back in time that included sight, and sound. Now we have video systems (YouTub) that can play back past events selected by you. Yes, video systems are virtual time travel machines we have now.

How Stellar Cannibalism Illuminates Cosmic Evolution

جزایر فیلیپین دایناسوری که توسط انسان خلق شده در بیش ده ها میلیون سال و بخاطر ریخته شدن دوهزار متر خاک غرق شده بخاطر بالا آمدن آب اقیانوس اما جزایر فیلیپین شبیه دایناسوریست

The address of the above comment on the site about a thief who was trained by a dinosaur bird who was trained by humans tens of millions of years ago and who arrested murderers and robbers.The police were arrested by big birds.don’t the scientists of the world think about this?They were buried in the bed of important cities, they were buried with all the tools and machines, the traces of humans tens of millions of years ago, they had a civilization and a history of hundreds of thousands of years, they built a base underthe earth, from the meteorites that explode from the planets when they hit the sun, and they knew that several thousand meters of soil is poured on the surface of the seas and islands, and they knew that the shapes they made of the islands in thecountry of Papua may go under the ocean, of course, the Philippine islands.The picture is of a baby dinosaur that went under the ocean, but the northern island of Australia, which is Papua, is quite clear.It is a big dinosaur whose tail is towards the east and its mouth is open towards the west.There is the Philippines, but it was more difficult to take the soil to the Philippine islands to create a dinosaur than the island of Papua, that is why the height of the soil in the Philippines is lower, and when the meteorites fell a few kilometerson the surface of the ocean, the image created by humans under the ocean in the shape of a dinosaur is hidden in the American continent The picture is of a bird in the shape of a dinosaur that is flying, and this bird was made to flyby the Indians of the tribe, that bird was talking to people, but its spirit might have heard something from the police because a thief while in the bird’s mouth He was handcuffed by the police and the weapon, which is a machine gun, is fromthe east of the American continent on the coast

The country of Florida is a machine gun.When you continue to New York City, you will reach the mouth of the dinosaur, where a thief is trapped in the mouth of a bird, and the little finger of the police handcuffed the thief’s hand, and a small colt is in the hand ofthe thief, who the police caught in the mouth of the dinosaur.put in the mouth of the bird dinosaur, you can clearly see that the thief fell on the ground and was shot in the head, and it is clear that his forehead was pierced, the bird’s mouth is open in flight, the head of the birdis from the east of the American continent, and a fish is placed in the bird’s mouth in the water of the ocean.The stretched glove of the police, which is in the form of a fist, with a handcuff attached to the left hand of the thief who fell on the ground in the sea and the mouth of the bird, the head of the thief and the killer, is located towards the southwest and west coast of America.All these images were created from the American continent and islands by Humans were created, but they didn’t have enough fuel and time to create more accurate images and meteorites ruined the beauty of the images, but it is clear that all these changes were createdby humans, but you have to consider that two thousand meters of soil from meteorites are fish.And they buried the whales under the beaches, and after a very long time, the bodies of the whales turned into oil under the two thousand meters of soil on the beaches, and on the other hand, the presence of two thousand meters of soil onthe surface of the seas and droughts could not make the created images disappear.

Leave a comment Cancel reply

Email address is optional. If provided, your email will not be published or shared.

Save my name, email, and website in this browser for the next time I comment.

Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines. 

Satellites watch as 4th global coral bleaching event unfolds (image)

Happy Earth Day 2024! NASA picks 6 new airborne missions to study our changing planet

Boeing's Starliner spacecraft will not fly private missions yet, officials say

Most Popular

• 2 SpaceX launching Falcon 9 rocket on record-tying 20th mission today
• 3 Watch 2 gorgeous supernova remnants evolve over 20 years (timelapse video)
• 4 Boeing Starliner 1st astronaut flight: Live updates
• 5 US Space Force picks Rocket Lab for 2025 Victus Haze space domain awareness mission

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

Sign up for the Live Science daily newsletter now

Get the world’s most fascinating discoveries delivered straight to your inbox.

Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

• If There Were a Time Warp, How Would Physicists Find It?
• Can Animals Tell Time?
• Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

Why do people feel like they're being watched, even when no one is there?

Why do babies rub their eyes when they're tired?

Space photo of the week: Little Dumbbell Nebula throws a wild party for Hubble telescope's 34th anniversary

Most Popular

• 2 James Webb telescope confirms there is something seriously wrong with our understanding of the universe
• 3 Giant, 82-foot lizard fish discovered on UK beach could be largest marine reptile ever found
• 4 Global 'time signals' subtly shifted as the total solar eclipse reshaped Earth's upper atmosphere, new data shows
• 5 'I nearly fell out of my chair': 1,800-year-old mini portrait of Alexander the Great found in a field in Denmark
• 2 New UTI vaccine wards off infection for years, early studies suggest
• 3 Tweak to Schrödinger's cat equation could unite Einstein's relativity and quantum mechanics, study hints
• 4 Why does striking flint against steel start a fire?
• 5 32 times lasers revealed hidden forts and settlements from centuries ago

• Newsletters

Time travel: five ways that we could do it

Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later (No guests showed up). Time travel is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

But travel to the future? That’s a different story.

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.

But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

1. Time travel via speed

This is the easiest and most practical way to time travel into the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

This is not a just a conjecture or thought experiment – it’s been measured. Using twin atomic clocks (one flown in a jet aircraft, the other stationary on Earth) physicists have shown that a flying clock ticks slower, because of its speed.

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

And the closer you get to the speed of light, the more extreme the time-travel.

Computer solves a major time travel problem

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months , for us.

Amazingly, particle physicists have to take this time dilation into account when they are dealing with particles that decay. In the lab, muon particles typically decay in 2.2 microseconds. But fast moving muons, such as those created when cosmic rays strike the upper atmosphere, take 10 times longer to disintegrate.

2. Time travel via gravity

The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.

As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.

And anyway, the effect is not that strong so it’s probably not worth the trip.

Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

(The situation described in the movie Interstellar , where one hour on a planet near a black hole is the equivalent of seven years back on Earth, is so extreme as to be impossible in our Universe, according to Kip Thorne, the movie’s scientific advisor.)

The most mindblowing thing, perhaps, is that GPS systems have to account for time dilation effects (due to both the speed of the satellites and gravity they feel) in order to work. Without these corrections, your phones GPS capability wouldn’t be able to pinpoint your location on Earth to within even a few kilometres.

3. Time travel via suspended animation

Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

Could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards achieving inducing a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13 °C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects.

Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces a hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh.

4. Time travel via wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

5. Time travel using light

Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

According to Mallet, the right geometry could lead to time travel into either the past and the future.

Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

His ideas have not grabbed the rest of the physics community however, with others arguing that one of the assumptions of his basic model is plagued by a singularity, which is physics-speak for “it’s impossible”.

The Royal Institution of Australia has an Education resource based on this article. You can access it here .

Related Reading: Computer solves a major time travel problem

Originally published by Cosmos as Time travel: five ways that we could do it

Please login to favourite this article.

28 Fascinating Facts About Time

By kerry wolfe | may 2, 2022.

Did you know that a day on Earth used to be around six hours shorter than it is today? Or that Julius Caesar once implemented a 445-day-long year? Learn those fascinating facts about time and more in this list, adapted from an episode of The List Show on YouTube. 

1. Every person on Earth is living in the past.

This may sound like the plot to some sci-fi, time-travel thriller, but it’s actually a fact of human biology and the trickiness of time. Our brains don’t perceive events until about 80 milliseconds until after they’ve happened. This fine line between the present and the past is part of the reason why some physicists argue that there’s no such thing as “now” and that the present moment is no more than an illusion. 

2. Throughout history, different cultures around the world have experienced time in different ways.

In the Western world, we tend to think of time as linear and flowing from left to right . But this isn’t the case for everyone. Language affects how people conceptualize time, particularly the spatial metaphors they use to describe and map it. 

Those who read languages that flow from right to left, such as Arabic and Hebrew, generally view time as flowing in the same direction. The Aymara , who live in the Andes Mountains in South America, consider the future to be behind them, while the past is ahead. In their view, because the future is unknown, it’s behind you, where you can’t see it. Some Indigenous Australian cultures, which rely heavily on direction terms like north, south, east, and west in their languages, visualize the passage of time as moving from east to west. If they’re facing north, for example, the past would be to their right, or east, whereas the future would be to their left, which would be west.

3. Individual people can experience time differently, too.

You’ve probably noticed how time seems to speed up when you’re racing against a deadline or having fun, and how it tends to drag when you’re bored. This is because when you’re focused on something, like a big work project or a party, your brain pays less attention to how time passes . But when you’re bored, or your brain is less stimulated, you become more aware of the passing of time, making it feel slower. One study proposed that dopamine —the neurotransmitter and hormone that helps us feel happy—may be an additional culprit. It showed that increased dopamine production, which happens when you’re enjoying something , may slow down your body’s internal clock, making time feel like it’s flying by.

4. Science has a number of different ways of defining time.

To cover just a couple : There’s astronomical time, which is measured in relation to how long it takes Earth to rotate on its axis. In astronomical time, a second is 1/60th of a minute . And then there’s atomic time, which dictates the numbers that you’ll see on a clock. According to atomic time, one second equals 9,192,631,770 oscillations of a cesium-133 atom. Measuring the vibration of an atom—which, in simple terms, is the gist of what oscillation is —is the most accurate way to track time.

5. We can thank Albert Einstein for a lot of our current understanding of the physics of time.

Rather than viewing time as a set order, he proved that it’s actually relative. For example, according to Einstein’s theory of special relativity , there’s an inverse relationship between your speed and the speed of time. The faster you move, the slower time moves.

This is why someone blasting through space will age slower than the people still hanging out on Earth: Astronaut Scott Kelly was born several minutes after his twin brother, Mark, but after Scott spent 340 days living on the International Space Station, he returned to Earth around an extra 5 milliseconds younger than his “big” brother. Had Scott been traveling at a speed close to the speed of light, that age difference would have become much more pronounced.

6. Einstein’s theory also states that gravity can warp time.

If you’ve seen the 2014 movie Interstellar , this concept may seem familiar . The closer you are to a massive body—which, in the case of Interstellar , is a giant black hole—the slower time would pass for you. 

7. Gravity’s effect on time isn’t limited to intergalactic travel.

Here on Earth, gravity can vary for a number of reasons, including your altitude, since you’re changing your distance from the center of the Earth. That means if you put a bunch of synchronized atomic clocks at various altitudes , eventually those clocks would fall out of sync. A clock at the top of Mount Everest and one at sea level would, over the entire 4.5 billion year history of the planet, have diverged by about a day and a half.

8. Gravity is also the reason why our days are getting longer.

Over a billion years ago, a day on Earth lasted around 18 hours . Our days are longer now because the moon’s gravity is causing Earth’s spin to slow down. In Earth’s earlier days, the moon wasn’t as far away, which caused Earth to spin much faster than it currently does. 

Longer days also mean shorter years—kind of. The time it takes the Earth to orbit the sun hasn’t changed, but the amount of days within a year has. Back when the dinosaurs ruled 70 million years ago , days were only around 23.5 hours long, and a year was made up of 372 of those slightly shorter days. 

9. There are two ways to think of the length of a day on Earth.

Though you probably learned that one day on Earth is 24 hours, it actually takes the planet 23 hours, 56 minutes, and 4.0916 seconds to rotate on its axis. This is the difference between a solar day and a sidereal day—a solar day is 24 hours, whereas a sidereal day is roughly four minutes shorter. We measure solar time based on the sun’s position in the sky; a sidereal day is measured based on the location of the “fixed” stars. In other words, a sidereal day is the time it takes for a distant star or constellation to appear on the same meridian .

10. Because astronomical time and atomic time don’t always line up, every so often, we get a leap second.

Earth’s spin speed can be a bit unpredictable. Atmospheric winds, Northern Hemisphere winters with heavy snow, and other big weather systems can affect how fast the planet rotates. In order to keep the difference between astronomical time and atomic time to less than .9 seconds, the International Earth Rotation and Reference Systems Service will occasionally announce the need for a leap second . 

Most people won’t notice a leap second, but they can be a huge pain for tech companies . Because leap seconds are added irregularly, developers have no way of working them into their codes, which has caused websites like LinkedIn and Reddit to crash in the past. A bug caused by 2012’s leap second created so much chaos on Qantas’s servers, more than 400 flights wound up being delayed.

11. The length of a year on Earth can also get a bit complicated.

The original Roman calendar was a bit of a mess, so much so that in 46 BCE Julius Caesar mandated a 445-day-long year to help bring the calendar back in sync with the seasons.

12. At the same time, Caesar asked the astronomer Sosigenes to help reform the calendar.

Most years were set at 365 days, but to make up for the fact that the earth's revolution around the sun doesn't take exactly 365 days, leap years were implemented . Every four years the month of February was given an extra day to make up for what is a sort of rounding error in the calendar.

13. But Sosigenes made a bit of a miscalculation, so the calendar continued to be a little off.

He thought a year lasted 365.25 days. It’s actually around 365 days, five hours, 48 minutes, and 45 seconds, equivalent to about 365.242 days. This tiny error had some pretty big consequences: By 1577, the Julian calendar was off by 10 days, meaning key Christian holidays were being celebrated on incorrect dates. 

Pope Gregory XIII took issue with this and established a commission to get the calendar back on track. In 1582, the Gregorian calendar was created. Rather than having an extra day every four years without exception, years that are divisible by 100—like 1700 or 1900—skip leap year. Unless the year is also divisible by 400, like the year 2000, in which case the Leap Year is back on!  Even this system isn’t perfect, though: It has an error of one day in 3236 years.

14. We can thank the railroad industry for standardizing our time zones.

Until the 19th century, towns and villages synchronized their clocks to the local solar noon. This created thousands of local times that all varied and made scheduling transportation a major headache. Train schedules in different cities had to list dozens of arrival and departure times for each train to account for all the mini time zones. On November 18, 1883, railroad companies in the United States and Canada began using a system very similar to the standardized time zones we still use today. In the UK, the railroad companies began using a standard London-based time in 1840.

15. After an engineer named Sandford Fleming missed a train in 1876, he set out to change the way time worked.

Fleming originally proposed a concept he called “Cosmic Time,” in which the world would run off an imaginary clock located at the planet’s center, essentially a line from the center of the planet to the sun. He then suggested splitting the world into 24 time zones labeled with a letter of the alphabet, with each zone spanning 15 degrees of longitude. His original plan to create a standard “Cosmic Time” was rejected, but it did lay the groundwork for a similar standardization, so-called Universal Time . And nations present at the 1884 International Meridian Conference laid the groundwork for dividing the world into 24 time zones, with the Prime Meridian, also known as Longitude 0°, running through Greenwich, England. 

16. Even with the advent of standardized time, people still struggled to keep their clocks in sync.

One London family used this to their advantage, and made a living by selling people the time. An astronomer named John Belville would set his pocket watch to the time at the Royal Observatory Greenwich. He would then travel around the city and visit his network of subscribers, who paid to set their own clocks by Belville’s pocket watch. After Belville died in 1856, his wife, and then later their daughter Ruth, carried on the tradition. Ruth continued to sell the time until World War II. By then she was in her eighties, and a couple of factors led to her timely retirement: Improved technology had made her role less important, and the war was making treks around London too dangerous.

17. Time zones can still be a bit complicated.

Big countries like Canada and the United States have multiple time zones, whereas China, another large country, only has one . China adopted the Beijing Standard Time to foster unity, but the effect can feel a bit uncanny—two cities in the country can be at roughly the same latitude, but experience sunrise hours apart, according to their clocks. In some parts of China, for example, the sun doesn’t rise until nearly 10 a.m. 

18. Though a lot of people believe daylight saving time was adopted to keep farmers happy, that’s a myth.

The first person to seriously advocate for daylight saving time was an entomologist who wanted more sunlit hours to look for insects after work in the summer. He proposed his idea to a scientific society in New Zealand in 1895. 

19. Daylight saving time wasn’t officially implemented until 1916.

Germany became the first country to adopt it in an effort to conserve coal during World War I. The United States didn’t follow suit until 1918 .

20. Daylight saving time ended on a national level after the war, but individual states and municipalities kept it going until World War II.

At the end of World War I, the entire nation went on what was effectively a year-round daylight saving time. After World War II, the entire nation was again picking and choosing daylight saving time. It’s been reported that in Iowa, back in 1964, there were 23 different combinations of dates that communities turned on and off daylight saving time. In 1966, the government officially mandated a standardized daylight saving time for the entire United States, though individual states can opt out.

Until 2007, daylight saving time ended in October. It’s been reported that the candy industry lobbied to wait until after Halloween to change the clocks back an hour. 

21. Daylight saving time does more than make people lose an hour of sleep.

In fact, it can have some pretty concerning health effects. Studies have linked daylight saving time with an uptick in heart attacks, car crashes, and mining injuries. The extra hour of daylight is good for koalas, though: Researchers found that koala-car collisions went down by up to 11 percent during daylight saving time.

22. People have been tracking time for thousands of years.

In 2013, archaeologists found what’s thought to be the world's oldest lunar calendar while excavating a field in Scotland. The calendar, which is made of a series of 12 pits that mimic the moon’s phases, dates back to around 8000 BCE. 

23. Sundials read differently depending on the hemisphere you’re in.

In the Northern Hemisphere, the sun casts a shadow that moves from north, to east, to south, to west. In the Southern Hemisphere, the shadow moves in the opposite direction. Our concept of “ clockwise ” is based on the way sundials in the Northern Hemisphere told time.

24. An innovative clock was built in China in 1090.

A man named Su Song created a water-powered clock tower that measured time and tracked the movements of the planets and stars in the night sky. Su Song built a giant water wheel within the clock tower. Buckets attached to the wheel would fill with water and then tip once full, causing the wheel to rotate, demarcating time.

25. The Maya had multiple calendars to measure time.

The most familiar is the Long Count Calendar. These calendars measured around 5125 years, beginning around August 3114 BCE. The Long Count calendar’s cycle came to an end around December 21, 2012, sparking a craze of Armageddon conspiracy theories.

26. You’ll find the most accurate clock at the National Institute of Standards and Technology in Boulder, Colorado.

The clock keeps time by measuring the vibration of a single aluminum ion , and should remain accurate for 33 billion years. The clock sitting on your bedside table isn't quite as precise.

27. New clocks are set at 10:10 for a reason.

If you’ve bought a new clock or watch recently, you may have noticed that the default setting was 10:10, give or take a few minutes. There are various theories behind this particular choice of time, but really, it all comes down to aesthetics. Setting the time to around 10:10 allows the hands of an analog clock to be displayed in a neat, symmetrical way that doesn’t obscure any logos in the center of the clock’s face. Clocks were once set to 8:20, and occasionally still are, but the hands’ downward angles can make it look like the timepieces are frowning.

28. Traveling back in time is possible—theoretically, at least.

According to Einstein’s theory, you could travel back in time by moving faster than the speed of light, as long as you could somehow have infinite mass. Since that probably won’t work, you could create “ wormholes ” between two points in space-time. (This would also be tough, since humanity still hasn’t invented the technology to actually build a wormhole.) Or you could try bending space-time by plucking some “cosmic strings.” Two of these theoretical strings, which are thin streams of pure energy that are moving in opposite directions at very near the speed of light, could theoretically warp space-time enough to create a closed time-like curve—also known as a time machine. 

• Table of Contents
• Random Entry
• Chronological
• Editorial Information
• About the SEP
• Editorial Board
• How to Cite the SEP
• Special Characters
• Advanced Tools
• Support the SEP
• PDFs for SEP Friends
• Make a Donation
• SEPIA for Libraries
• Entry Contents

Bibliography

Academic tools.

• Friends PDF Preview
• Author and Citation Info
• Back to Top

Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

• Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

• Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
• Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
• Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage \(S_1\) the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage \(S_2\) of the life of the film, it has been developed and is about to enter the time machine. Stage \(S_3\) occurs just after it exits the time machine and just before it is photographed. Stage \(S_4\) occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage \(S_1\) in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages \(S_1\) and \(S_2\). During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage \(S_2\) depends on the shade of gray it has at stage \(S_3\). The influence of the shade of gray of the film at stage \(S_3\), on the shade of gray of the film at stage \(S_2\), can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function \(f\) from \([0,1]\) to \([0,1]\) must intersect the line \(x=y\) somewhere, and thus there must be at least one point \(x\) such that \(f(x)=x\). Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines \(x, y\) and \(z\), that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system \(P\) which, as in the above example, has the following life. It starts in some state \(S_1\), it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of \(P\) can be represented by a Cartesian product of \(n\) closed intervals of the reals, i.e., let us assume that the topology of the state-space of \(P\) is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of \(P\) in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state \(S_3\) of the older system (as it emerges from the time travel machine) that will influence the initial state \(S_1\) of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state \(S_3\). Thus without imposing any constraints on the initial state \(S_1\) of the system \(P\), we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle \(x\), then we will set the dial to \(x+90\), and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage \(S_2\) must be a continuous function of the state of the dial at stage \(S_3\). But, the state of the dial at stage \(S_2\) is arrived at by taking the state of the dial at stage \(S_1\), and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage \(S_2\), should be a continuous function of the cause, namely the state of the dial at stage \(S_3\). It is additionally the case that path taken to get there, the way the dial is rotated between stages \(S_1\) and \(S_2\) must be a continuous function of the state at stage \(S_3\). And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle \(x\) in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle \(y\) in the vertical plane. All possible \(\langle x,y\rangle\) pairs can thus be visualized as a torus with each \(x\) value picking out a vertical circular cross-section and each \(y\) picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle \(i\) which picks out vertical circle \(I\) on the torus. The initial angle \(i\) that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle \(I\) on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle \(i\) will develop to some other angle. One can picture this development by rotating each point on \(I\) in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle \(I\) during this development will deform into some loop \(L\) on the torus. Loop \(L\) thus represents all possible intermediate angles \(x\) that the dial is at when it is thrown into the time machine, given that it started at angle \(i\) and then encountered a dial (its older self) which was at angle \(y\) when it emerged from the time machine. We therefore have consistency if \(x=y\) for some \(x\) and \(y\) on loop \(L\). Now, let loop \(C\) be the loop which consists of all the points on the torus for which \(x=y\). Ring \(I\) intersects \(C\) at point \(\langle i,i\rangle\). Obviously any continuous deformation of \(I\) must still intersect \(C\) somewhere. So \(L\) must intersect \(C\) somewhere, say at \(\langle j,j\rangle\). But that means that no matter how the development of the dial starting at \(I\) depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value \(I\). As before we can represent the combination of this initial position and all possible final positions by the line \(I\). Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line \(I\) into line \(L\), which is indicated by the arrows in figure 2 . This development is fully continuous. Points \(\langle x,y\rangle\) on line \(I\) represent the initial position \(x=I\) of the (young) pointer, and the position \(y\) of the older pointer as it emerges from the time machine. Points \(\langle x,y\rangle\) on line \(L\) represent the position \(x\) that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position \(y\). Since the pointer is designed to develop to half the value of the pointer that it encounters, the line \(L\) corresponds to \(x=1/2 y\). We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point \(\langle x,y\rangle\) on line \(L\) such that \(x=y\). However, there is no such point: lines \(L\) and \(C\) do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, \(L\) and \(C\) would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state \(S_1\) there is a solution, but this solution may not be “close” to the solution for a nearby initial state, \(S'\). We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth \(A\)” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth \(B\)” and re-enter “mouth \(A\)” again. CTCs can even arise when the spacetime is topologically \(\mathbb{R}^4\), due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points \(p\) such that there exists a (non-zero length) timelike curve that starts at \(p\) and returns to \(p\). Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface \(S\) a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

• every point in the manifold can be connected by a timelike curve to \(S\), and
• any timelike curve which connects a point in one region to a point in the other region intersects \(S\) exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point \(p\) of \(S\) is in the time travel region, then any timelike curve which intersects \(p\) can be extended to a timelike curve which intersects \(S\) near \(p\) again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from \(S)\) and its future (ditto mutatis mutandis ). If the whole past of \(S\) is in the normal region of the manifold, then \(S\) is a partial Cauchy surface : every inextendable timelike curve which exists to the past of \(S\) intersects \(S\) exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of \(S\) intersects \(S\). Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface \(S\), such that all of the temporal funny business is to the future of \(S\). If all you could see were \(S\) and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on \(S\) and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on \(S\) if there is to be time travel to the future of \(S\). We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “\(X\)” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line \(L-\) with the line \(L+\). Particles “going in” to \(L+\) from below “emerge” from \(L-\) , and particles “going in” to \(L-\) from below “emerge” from \(L+\).

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice \(S\) and continue it to a unique solution. After the change in topology, \(S\) is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on \(S\) always be continued to a global solution? Second, is that solution unique? If the answer to the first question is \(no\), then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on \(S\) even though that region lies completely to the future of \(S\). If the answer to the second question is \(no\), then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on \(S\) does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in \(S\)’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on \(S\), but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from \(S\) as required by the initial data. If a line hits \(L-\) from the bottom, just continue it coming out of the top of \(L+\) in the appropriate place, and if a line hits \(L+\) from the bottom, continue it emerging from \(L-\) at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on \(S\) is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into \(L+\) from the bottom match the data coming out of \(L-\) from the top, and the data going into \(L-\) from the bottom match the data coming out of \(L+\) from the top. So we can add any number of vertical lines connecting \(L-\) and \(L+\) to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches \(L+\) for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on \(S\), then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach \(L+\), we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, \(S\) is no longer a Cauchy surface, so it is perhaps not surprising that data on \(S\) do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of \(L-\) must exactly match data “going in” to \(L+\), even though what comes out of \(L-\) helps to determine what goes into \(L+\). This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on \({L+}/{L-}\).

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit \(L+\). Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches \(L+\): it is deflected first by a particle which emerges from \(L-\). But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at \(S\) then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of \(L-\): the only requirement is that whatever comes out must match what goes in at \(L+\). So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on \(L-\) outweighs the constraint that the same data go into \(L+\), instead of paradox we get an embarrassment of riches: many solution consistent with the data on \(S\), or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set \(\{a, b, c\}\), and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node \(a\) is 3 and at node \(b\) is 7, then its value at node \(d\) will be \(3W_{ad}\) and its value at node \(e\) will be \(3W_{ae} + 7W_{be}\). By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from \(z\) back to itself, e.g., \(z\) to \(y\) to \(z\).

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on \(v\) and \(w\), and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at \(x, y\), and \(z\) are:

Solving these equations for \(z\) yields

which gives a unique value for \(z\) in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where \(1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0\). If we choose weighting factors in just this way, then arbitrary data at \(v\) and \(w\) cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at \(v\) and \(w\): the field there must be zero (assuming \(W_{vx}\) and \(W_{wy}\) to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at \(v\) and \(w\) must be so chosen that \(vW_{vx}W_{xz} = -wW_{wy}W_{yz}\). In either case, the field values at \(v\) and \(w\) are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on \(S\) does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on \(S\), and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of \(A\) there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region \(T\)? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point \(p\) in \(T\), and any space-like surface \(S\) that includes \(p\) is there a neighborhood \(E\) of \(p\) in \(S\) such that any solution on \(E\) can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface \(S\), such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields \(\boldsymbol{E}\) on an initial surface to be related to the (simultaneous) charge density distribution \(\varrho\) by the equation \(\varrho = \text{div}(\boldsymbol{E})\). (If we assume that the \(E\) field is generated solely by the charge distribution, this conditions amounts to requiring that the \(E\) field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that \(\varrho = \text{div}(\boldsymbol{E})\) could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by \(\varrho = \text{div}(\boldsymbol{E})\). The constraints imposed by \(\varrho = \text{div}(\boldsymbol{E})\) on the state on a space-like hypersurface are:

• local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
• quite independent of the global space-time structure,
• quite independent of how the space-like surface in question is embedded in a given space-time, and
• very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point \(p\) are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near \(p\) in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near \(p\) had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of \(p\) been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface \(S\) can always be embedded into a space-time such that that state on \(S\) consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold \(S\) with positive definite metric has a unique embedding into a maximal space-time in which \(S\) is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has \(S\) as a Cauchy surface and contains a consistent evolution of the initial value data on \(S\). Now since \(S\) is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which \(S\) is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes \(n\) bits as input, we can imagine taking some number \(i \lt n\) of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these \(i\) bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state \(S_1\), interacts with its older self, after the interaction is in state \(S_2\), time travels while developing into state \(S_3\), then interacts with its younger self, and ends in state \(S_4\) (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state \(S_1\), any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states \(S_2\) and \(S_3\) such that when \(S_1\) interacts with \(S_3\) it will change to state \(S_2\) and \(S_2\) will then develop into \(S_3\). The states \(S_2, S_3\) and \(S_4\) will typically be not be pure states, i.e., will be non-trivial mixed states, even if \(S_1\) is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states \(\vc{+}\) and \(\vc{-}\). Let us suppose that when state \(\vc{+}\) of the young system encounters state \(\vc{+}\) of the older system, they interact and the young system develops into state \(\vc{-}\) and the old system remains in state \(\vc{+}\). In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that \(\vc{+}_2\) develops into \(\vc{+}_3\), and that \(\vc{-}_2\) develops into \(\vc{-}_3\).

Now, if the only possible states of the system were \(\vc{+}\) and \(\vc{-}\) (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state \(\vc{+}_1\) is impossible. For if \(\vc{+}_1\) interacts with \(\vc{+}_3\) then it will develop into \(\vc{-}_2\), which, during time travel, will develop into \(\vc{-}_3\), which inconsistent with the assumed state \(\vc{+}_3\). Similarly if \(\vc{+}_1\) interacts with \(\vc{-}_3\) it will develop into \(\vc{+}_2\), which will then develop into \(\vc{+}_3\) which is also inconsistent. Thus the system can not start in state \(\vc{+}_1\).

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states \(\vc{+}\) and \(\vc{-}\). Suppose that the older system, prior to the interaction, is in a state \(S_3\) which is an equal mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\). Then the younger system during the interaction will develop into a mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), which will then develop into a mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\), which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state \(S_1\) there is always a state \(S_3\) (which typically is not a pure state) which causes \(S_1\) to develop into a state \(S_2\) which develops into that state \(S_3\). Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states \(\vc{+}\) and \(\vc{-}\) not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is \(\vc{+}_1\). One might suggest that that if state \(S_3\) is

one will obtain a consistent development. For one might think that when initial state \(\vc{+}_1\) encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state \(\vc{+}_1\) when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state \(S_3\) of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state \(S_1\) younger system. After this interaction there is an entangled state \(S'\) of the two systems. Deutsch computes the mixed state \(S_2\) of the younger system which is implied by this entangled state \(S'\). His demand for consistency then is just that this mixed state \(S_2\) develops into the mixed state \(S_3\). Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state \(\vc{+}_3\) or in state \(\vc{-}_3\) prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state \(\vc{+}_1\) in all worlds. In some of the many worlds the older system will be in the \(\vc{+}_3\) state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the \(\vc{-}_3\) state. Thus in A -worlds after interaction we will have state \(\vc{-}_2\) , and in B -worlds we will have state \(\vc{+}_2\). During time travel the \(\vc{-}_2\) state will remain the same, i.e., turn into state \(\vc{-}_3\), but the systems in question will travel from A -worlds to B -worlds. Similarly the \(\vc{+}\) \(_2\) states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

• Aaronson, Scott, 2013, Quantum Computing since Democritus , Cambridge: Cambridge University Press. doi:10.1017/CBO9780511979309
• Arntzenius, Frank, 2006, “Time Travel: Double Your Fun”, Philosophy Compass , 1(6): 599–616. doi:10.1111/j.1747-9991.2006.00045.x
• Clarke, C.J.S., 1977, “Time in General Relativity” in Foundations of Space-Time Theory , Minnesota Studies in the Philosophy of Science , Vol VIII, Earman, J., Glymour, C., and Stachel, J. (eds), pp. 94–108. Minneapolis: University of Minnesota Press.
• Deutsch, David, 1991, “Quantum Mechanics near Closed Timelike Lines”, Physical Review D , 44(10): 3197–3217. doi:10.1103/PhysRevD.44.3197
• Deutsch, David and Michael Lockwood, 1994, “The Quantum Physics of Time Travel”, Scientific American , 270(3): 68–74. doi:10.1038/scientificamerican0394-68
• Dowe, Phil, 2007, “Constraints on Data in Worlds with Closed Timelike Curves”, Philosophy of Science , 74(5): 724–735. doi:10.1086/525617
• Earman, John, 1972, “Implications of Causal Propagation Outside the Null Cone”, Australasian Journal of Philosophy , 50(3): 222–237. doi:10.1080/00048407212341281
• Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
• Earman, John, Christopher Smeenk, and Christian Wüthrich, 2009, “Do the Laws of Physics Forbid the Operation of Time Machines?”, Synthese , 169(1): 91–124. doi:10.1007/s11229-008-9338-2
• Echeverria, Fernando, Gunnar Klinkhammer, and Kip S. Thorne, 1991, “Billiard Balls in Wormhole Spacetimes with Closed Timelike Curves: Classical Theory”, Physical Review D , 44(4): 1077–1099. doi:10.1103/PhysRevD.44.1077
• Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press. doi:10.1093/oso/9780198842507.001.0001
• Fletcher, Samuel C., 2020, “The Principle of Stability”, Philosopher’s Imprint , 20: article 3. [ Fletcher 2020 available online ]
• Friedman, John and Michael Morris, 1991, “The Cauchy Problem for the Scalar Wave Equation Is Well Defined on a Class of Spacetimes with Closed Timelike Curves”, Physical Review Letters , 66(4): 401–404. doi:10.1103/PhysRevLett.66.401
• Friedman, John, Michael S. Morris, Igor D. Novikov, Fernando Echeverria, Gunnar Klinkhammer, Kip S. Thorne, and Ulvi Yurtsever, 1990, “Cauchy Problem in Spacetimes with Closed Timelike Curves”, Physical Review D , 42(6): 1915–1930. doi:10.1103/PhysRevD.42.1915
• Geroch, Robert and Gary Horowitz, 1979, “Global Structures of Spacetimes”, in General Relativity: An Einstein Centenary Survey , Stephen Hawking and W. Israel (eds.), Cambridge/New York: Cambridge University Press, Chapter 5, pp. 212–293.
• Gödel, Kurt, 1949, “A Remark About the Relationship Between Relativity Theory and Idealistic Philosophy”, in Albert Einstein, Philosopher-Scientist , Paul Arthur Schilpp (ed.), Evanston, IL: Library of Living Philosophers, 557–562.
• Hocking, John G. and Gail S. Young, 1961, Topology , (Addison-Wesley Series in Mathematics), Reading, MA: Addison-Wesley.
• Horwich, Paul, 1987, “Time Travel”, in his Asymmetries in Time: Problems in the Philosophy of Science , , Cambridge, MA: MIT Press, 111–128.
• Kutach, Douglas N., 2003, “Time Travel and Consistency Constraints”, Philosophy of Science , 70(5): 1098–1113. doi:10.1086/377392
• Lewis, David, 1976, “The Paradoxes of Time Travel”, American Philosophical Quarterly , 13(2): 145–152.
• Lloyd, Seth, Lorenzo Maccone, Raul Garcia-Patron, Vittorio Giovannetti, and Yutaka Shikano, 2011, “Quantum Mechanics of Time Travel through Post-Selected Teleportation”, Physical Review D , 84(2): 025007. doi:10.1103/PhysRevD.84.025007
• Malament, David B., 1977, “Observationally Indistinguishable Spacetimes: Comments on Glymour’s Paper”, in Foundations of Space-Time Theories , John Earman, Clark N. Glymour, and John J. Stachel (eds.), (Minnesota Studies in the Philosophy of Science 8), Minneapolis, MN: University of Minnesota Press, 61–80.
• –––, 1984, “‘Time Travel’ in the Gödel Universe”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1984(2): 91–100. doi:10.1086/psaprocbienmeetp.1984.2.192497
• –––, 1985, “Minimal Acceleration Requirements for ‘Time Travel’, in Gödel Space‐time”, Journal of Mathematical Physics , 26(4): 774–777. doi:10.1063/1.526566
• Manchak, John Byron, 2009, “Can We Know the Global Structure of Spacetime?”, Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics , 40(1): 53–56. doi:10.1016/j.shpsb.2008.07.004
• –––, 2011, “On Efficient ‘Time Travel’ in Gödel Spacetime”, General Relativity and Gravitation , 43(1): 51–60. doi:10.1007/s10714-010-1068-3
• Maudlin, Tim, 1990, “Time-Travel and Topology”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1990(1): 303–315. doi:10.1086/psaprocbienmeetp.1990.1.192712
• Novikov, I. D., 1992, “Time Machine and Self-Consistent Evolution in Problems with Self-Interaction”, Physical Review D , 45(6): 1989–1994. doi:10.1103/PhysRevD.45.1989
• Smeenk, Chris and Christian Wüthrich, 2011, “Time Travel and Time Machines”, in the Oxford Handbook on Time , Craig Callender (ed.), Oxford: Oxford University Press, 577–630..
• Stein, Howard, 1970, “On the Paradoxical Time-Structures of Gödel”, Philosophy of Science , 37(4): 589–601. doi:10.1086/288328
• Thorne, Kip S., 1994, Black Holes and Time Warps: Einstein’s Outrageous Legacy , (Commonwealth Fund Book Program), New York: W.W. Norton.
• Verch, Rainer, 2020, “The D-CTC Condition in Quantum Field Theory”, in Progress and Visions in Quantum Theory in View of Gravity , Felix Finster, Domenico Giulini, Johannes Kleiner, and Jürgen Tolksdorf (eds.), Cham: Springer International Publishing, 221–232. doi:10.1007/978-3-030-38941-3_9
• Wallace, David, 2012, The Emergent Multiverse: Quantum Theory According to the Everett Interpretation , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199546961.001.0001
• Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press. doi:10.1093/oso/9780198793335.001.0001
• Weyl, Hermann, 1918/1920 [1922/1952], Raum, Zeit, Materie , Berlin: Springer; fourth edition 1920. Translated as Space—Time—Matter , Henry Leopold Brose (trans.), New York: Dutton, 1922. Reprinted 1952, New York: Dover Publications.
• Wheeler, John Archibald and Richard Phillips Feynman, 1949, “Classical Electrodynamics in Terms of Direct Interparticle Action”, Reviews of Modern Physics , 21(3): 425–433. doi:10.1103/RevModPhys.21.425
• Yurtsever, Ulvi, 1990, “Test Fields on Compact Space‐times”, Journal of Mathematical Physics , 31(12): 3064–3078. doi:10.1063/1.528960

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
• Rovelli, Carlo, unpublished, “ Can We Travel to the Past? Irreversible Physics along Closed Timelike Curves ”, arXiv: 1912.04702. doi:10.48550/ARXIV.1912.04702

causation: backward | determinism: causal | quantum mechanics | quantum mechanics: retrocausality | space and time: being and becoming in modern physics | time machines | time travel

Copyright © 2023 by Christopher Smeenk < csmeenk2 @ uwo . ca > Frank Arntzenius Tim Maudlin

• Accessibility

Support SEP

Mirror sites.

View this site from another server:

• Info about mirror sites

The Stanford Encyclopedia of Philosophy is copyright © 2023 by The Metaphysics Research Lab , Department of Philosophy, Stanford University

Library of Congress Catalog Data: ISSN 1095-5054

National Geographic content straight to your inbox—sign up for our popular newsletters here

Time travel for travelers? It’s tricky.

Scientific theories suggest it’s possible to travel through time. But the reality isn’t so clear.

Time travel has fascinated scientists and writers for at least 125 years. The concept feels especially intriguing now, when physical travel is limited. Here, a photo illustration of Tokyo’s Robot Restaurant captures the idea of speeding through time.

I’m stuck at home, you’re stuck at home, we’re all stuck at home. Jetting off to some fun-filled destination like we used to might not be in the cards for a little while yet. But what about travelling through time? And not just the boring way, where we wait for the future to arrive one second at a time. What if you could zip through time at will, travelling forward to the future or backward to the past as easily as pushing buttons on the dashboard of a souped-up DeLorean, just like in the movie Back to the Future ?

Time travel has been a fantasy for at least 125 years. H.G. Wells penned his groundbreaking novel, The Time Machine , in 1895, and it’s something that physicists and philosophers have been writing serious papers about for almost a century.

What really kick-started scientific investigations into time travel was the notion, dating to the closing years of the 19th century, that time could be envisioned as a dimension, just like space. We can move easily enough through space, so why not time?

At the end of the 19th century, scientists thought of time as a dimension like space, where travelers can go anywhere they want. This photo illustration of Tokyu Plaza in Tokyo’s Omotesando Harajuku evokes the feeling of visiting endless destinations.

“In space, you can go wherever you want, so maybe in time you can similarly go anywhere you want,” says Nikk Effingham, a philosopher at the University of Birmingham in the United Kingdom . “From there, it’s a short step to time machines.”

( Why are people obsessed with time travel? Best-selling author James Gleick has some ideas .)

Dueling theories

Wells was a novelist, not a physicist, but physics would soon catch up. In 1905, Albert Einstein published the first part of his relativity theory, known as special relativity . In it, space and time are malleable; measurements of both space and time depend on the relative speed of the person doing the measuring.

A few years later, the German mathematician Hermann Minkowski showed that, in Einstein’s theory, space and time could be thought of as two aspects of a single four-dimensional entity known as space-time . Then, in 1915, Einstein came up with the second part of his theory, known as general relativity . General relativity renders gravity in a new light: Instead of thinking of it as a force, general relativity describes gravity as a bending or warping of space-time.

But special relativity is enough to get us started in terms of moving through time. The theory “establishes that time is much more similar to space than we had previously thought,” says Clifford Johnson, a physicist at the University of Southern California. “So maybe everything we can do with space, we can do with time.”

Well, almost everything. Special relativity doesn’t give us a way of going back in time, but it does give us a way of going forward— and at a rate that you can actually control. In fact, thanks to special relativity, you can end up with two twins having different ages, the famous “twin paradox.”

Suppose you head off to the Alpha Centauri star system in your spaceship at a really high speed (something close to the speed of light), while your twin remains on Earth. When you come back home, you’ll find you’re now much younger than your twin. It’s counterintuitive, to say the least, but the physics, after more than a century, is rock solid.

“It is absolutely provable in special relativity that the astronaut who makes the journey, if they travel at very nearly the speed of light, will be much younger than their twin when they come back,” says Janna Levin, a physicist at Barnard College in New York . Interestingly, time appears to pass just as it always does for both twins; it’s only when they’re reunited that the difference reveals itself.

Maybe you were both in your 20s when the voyage began. When you come back, you look just a few years older than when you left, while your twin is perhaps now a grandparent. “My experience of the passage of time is utterly normal for me. My clocks tick at the normal rate, I age normally, movies run at the right pace,” says Levin. “I’m no further into my future than normal. But I’ve travelled into my twin’s future.”

( To study aging, scientist are looking to outer space .)

With general relativity, things really start to get interesting. In this theory, a massive object warps or distorts space and time. Perhaps you’ve seen diagrams or videos comparing this to the way a ball distorts a rubber sheet . One result is that, just as travelling at a high speed affects the rate at which time passes, simply being near a really heavy object—like a black hole —will affect one’s experience of time. (This trick was central to the plot of the 2014 film, Interstellar , in which Matthew McConaughey’s character spends time in the vicinity of a massive black hole. When he returns home, he finds that his young daughter is now elderly.)

To get around the “grandfather paradox,” some scientists theorize there could be multiple timelines. In these images of Nakagin Capsule Tower in Tokyo, Japan, time seems to pass at different rates.

But black holes are just the beginning. Physicists have also speculated about the implications of a much more exotic structure known as a wormhole . Wormholes, if they exist, could connect one location in space-time with another. An astronaut who enters a wormhole in the Andromeda Galaxy in the year 3000 might find herself emerging from the other end in our own galaxy, in the year 2000. But there’s a catch: While we have overwhelming evidence that black holes exist in nature—astronomers even photographed one last year—wormholes are far more speculative.

“You can imagine building a bridge from one region of space-time to another region of space-time,” explains Levin, “but it would require kinds of mass and energy that we don’t really know exist in reality, things like negative energy.” She says it’s “mathematically conceivable” that structures such as wormholes could exist, but they may not be part of physical reality.

There’s also the troubling question of what happens to our notions of cause and effect if backward time travel were possible. The most famous of these conundrums is the so-called “ grandfather paradox .” Suppose you travel back in time to when your grandfather was a young man. You kill him (perhaps by accident), which means your parent won’t be born, which means you won’t be born. Therefore, you won’t be able to travel through time and kill your grandfather.

Multiple timelines?

Over the years, physicists and philosophers have pondered various resolutions to the grandfather paradox. One possibility is that the paradox simply proves that no such journeys are possible; the laws of physics, somehow, must prevent backward time travel. This was the view of the late physicist Stephen Hawking , who called this rule the “ chronology protection conjecture .” (Mind you, he never specified the actual physics behind such a rule.)

But there are also other, more intriguing, solutions. Maybe backward time travel is possible, and yet time travelers can’t change the past, no matter how hard they try. Effingham, whose book Time Travel: Probability and Impossibility was published earlier this year, puts it this way: “You might shoot the wrong person, or you might change your mind. Or, you might shoot the person you think is your grandfather, but it turns out your grandmother had an affair with the milkman, and that’s who your grandfather was all along; you just didn’t know it.”

Which also means the much-discussed fantasy of killing Hitler before the outbreak of World War II is a non-starter. “It’s impossible because it didn’t happen,” says Fabio Costa, a theoretical physicist at the University of Queensland in Australia . “It’s not even a question. We know how history developed. There is no re-do.”

In fact, suggests Effingham, if you can’t change the past, then a time traveler probably can’t do anything . Your mere existence at a time in which you never existed would be a contradiction. “The universe doesn’t care whether the thing you’ve changed is that you’ve killed Hitler, or that you moved an atom from position A to position B,” Effingham says.

But all is not lost. The scenarios Effingham and Costa are imagining involve a single universe with a single “timeline.” But some physicists speculate that our universe is just one among many . If that’s the case, then perhaps time travelers who visit the past can do as they please, which would shed new light on the grandfather paradox.

( The Big Bang could have led to the creation of multiple universes, scientists say .)

“Maybe, for whatever reason, you decide to go back and commit this crime [of killing your grandfather], and so the world ‘branches off’ into two different realities,” says Levin. As a result, “even though you seem to be altering your past, you’re not really altering it; you’re creating a new history.” (This idea of multiple timelines lies at the heart of the Back to the Future movie trilogy. In contrast, in the movie 12 Monkeys , Bruce Willis’s character makes multiple journeys through time, but everything plays out along a single timeline.)

More work to be done

What everyone seems to agree on is that no one is building a time-travelling DeLorean or engineering a custom-built wormhole anytime soon. Instead, physicists are focusing on completing the work that Einstein began a century ago.

After more than 100 years, no one has figured out how to reconcile general relativity with the other great pillar of 20th century physics: quantum mechanics . Some physicists believe that a long-sought unified theory known as quantum gravity will yield new insight into the nature of time. At the very least, says Levin, it seems likely “that we need to go beyond just general relativity to understand time.”

Meanwhile, it’s no surprise that, like H.G. Wells, we continue to daydream about having the freedom to move through time just as we move through space. “Time is embedded in everything we do,” says Johnson. “It looms large in how we perceive the world. So being able to mess with time—I’m not surprised we’re obsessed with that, and fantasize about it.”

Related Topics

You may also like.

Why it's high time for slow travel in Gstaad, Switzerland

Scared to scuba? Here are 5 reasons it’s finally time to learn.

Free bonus issue.

Step inside the factory where the NFL’s footballs are made

Volcanoes blow smoke rings. Here's how they do it.

The true history of Einstein's role in developing the atomic bomb

The monarch butterfly’s spots may be its superpower

What is a sonic boom—and is it dangerous?

• Environment
• Perpetual Planet

History & Culture

• History & Culture
• History Magazine
• Mind, Body, Wonder
• Paid Content
• Terms of Use
• Privacy Policy
• Your US State Privacy Rights
• Children's Online Privacy Policy
• Interest-Based Ads
• About Nielsen Measurement
• Do Not Sell or Share My Personal Information
• Nat Geo Home
• Attend a Live Event
• Book a Trip
• Inspire Your Kids
• Shop Nat Geo
• Visit the D.C. Museum
• Learn About Our Impact
• Support Our Mission
• Advertise With Us
• Customer Service
• Renew Subscription
• Manage Your Subscription
• Work at Nat Geo
• Sign Up for Our Newsletters
• Contribute to Protect the Planet

Copyright © 1996-2015 National Geographic Society Copyright © 2015-2024 National Geographic Partners, LLC. All rights reserved

Time Travel: From Ancient Mythology to Modern Science

• Read Later  

Time travel and time machines have been a topic of science fiction and countless movies for many decades. In fact, it appears that the possibility to travel in time, either into the future or into the past, has appealed to the imagination of mankind for centuries. While many may think it is absurd to believe that we could travel backwards or forwards in time, some of the world’s most brilliant scientists have investigated whether it could one day be made a reality.

Research into Time Travel

Albert Einstein for example, concluded in his later years that the past, present, and future all exist simultaneously, and most are familiar with his well-known concept of relativity. That is, that time is relative and not absolute as Newton claimed. With the proper technology, such as a very fast spaceship, one person is able to experience several days while another person simultaneously experiences only a few hours or minutes. Yet the wisdom of Einstein's convictions had very little impact on cosmology or science in general.

• Passing Through the Gates of Time: The Mind, Time Travel, and St Augustine
• Chronicles from the Future: A True Story Kept Hidden by the Masons now Revealed

The majority of physicists have been slow to give up the ordinary assumptions we make about time. However, if time travel really was possible, one can hardly contemplate what this may mean for humanity. Whoever had the power to move through time, would have the power to modify history. While this may sound attractive, it would be impossible to know the consequences of any alteration of past events, and how this would affect the future.

Illustration from the Menologion of Basil II of the Seven Sleepers, a medieval legend about a group of youths who hid in a cave to escape persecution and emerged over 300 years later. ( Public domain )

Time Travel in Ancient Mythology

If we look at ancient texts, we can find a number of references to time travel. In Hindu mythology, there is the story of King Raivata Kakudmi who travels to meet the creator Brahma . Even if this trip didn’t last long, when Kakudmi returned back to Earth, 108 yugas had passed on Earth, and it is thought that each yuga represents about 4 million years. The explanation Brahma gave to Kakudmi is that time runs differently in different planes of existence.

Similarly, we have references in the Quran about the cave of Al-Kahf. The story refers to a group of young Christian people, who in 250 AD tried to escape persecution and retreated, under God’s guidance, to a cave where God put them to sleep. They woke up 309 years later. This story coincides with the Christian story of the  seven sleepers , with a few differences.

Another story comes from the Japanese legend of Urashima Taro, an individual who was said to visit the underwater palace of the Dragon God Ryujin. He stayed there for three days, but when he returned to the surface, 300 years had passed. In the Buddhist text , Pali Canon, it is written that in the heaven of the thirty Devas (the place of the Gods), time passes at a different pace where one hundred Earth years count as a single day for them. There are many more references to time travel to be found within ancient mythology.

Urashima Taro returning from the Dragon King's Palace, only to find that 300 years had passed. ( Public domain )

Scientific Research into Time Travel

Probably the most well-known story of accidental time travel is the Philadelphia experiment which allegedly took place in 1943 with the purpose of cloaking a ship and making it invisible to enemy radar . However, it was said that the experiment went terribly wrong – the ship not only vanished completely from Philadelphia but it was teleported to Norfolk and went back in time for 10 seconds.

When the ship appeared again some crew members were physically fused to bulkheads, others developed mental disorders , a few disappeared completely, and some reported travelling into the future and back. Allegedly, Nikola Tesla, who was the director of Engineering and Research at Radio Company of America at the time, was involved in the experiment by making all the necessary calculations and drawings and also providing the generators.

Have you heard about the Chronovisor? Supposedly invented in the 60's, it was a machine that could catch electromagnetic information remaining in the æther and visualize it, so you could literally watch history. pic.twitter.com/D24QsvxYEU — N'Golo 69%… (@NgoloTesla) June 22, 2020

In 1960, we have another interesting case report of scientist Pellegrino Ernetti, who claimed that he developed the Chronivisor, a machine that would enable someone to see in the past. His theory was that anything that happens leaves an energy mark that can never be destroyed (something like the mystical Akashic Records). Ernetti allegedly developed this machine that could detect, magnify and convert this energy into an image – something like a TV showing what happened in the past.

Scientists have long been curious as to the possibility of time travel. ( drawlab19 / Adobe Stock)

Controversial Experiments Related to Time Travel

In the 1980s, there are reports of another controversial time travel experiment, the  Montauk project , which again allegedly experimented with time travel among other things. Whether the Philadelphia and Montauk experiments actually took place is still under debate. However, it is common sense to assume that the military would definitely be interested in the possibility of time travel and would engage in extensive research on the subject.

In 2004, Marlin Pohlman, a scientist, engineer, and member of Mensa with a Bachelor, MBA and PhD, applied for a patent for a method of gravity distortion and time displacement. In 2013 Wasfi Alshdaifat filed another patent for a space compression and time dilation machine that could be used for time travel.

• Iranian Scientist Builds Machine That Can Predict the Future
• Chronovisor: The Time Machine That Captured The Crucifixion of Jesus

According to PHYS.ORG , the physicist Professor Ronald Lawrence Mallett of the University of Connecticut was working in 2006 on the concept of time travel , based on Einstein’s theory of relativity. At the time, Mallett was absolutely convinced that time travelling was possible. He predicted that human time travel will be possible in our century. Particle physicist  Brian Cox , quoted in a 2013 article published in HuffPost , agreed that time travel is possible but only in one direction.

We also have the mysterious story of Ali Razeqi, managing director of the Iranian Centre for Strategic Inventions, who The Daily Mail reported had claimed to have developed a device that could see anywhere from 3 to 5 years in the future . His initial story disappeared from the internet a few hours after it was published.

In theory, time travel is possible, even if it is difficult to comprehend. Has the research cited above brought us closer towards making time travel a reality? If so, we can only hope that the technology does not get into the wrong hands.

Top image: Is time travel possible? Source: rolffimages / Adobe Stock

By John Black

Arnold, L. 23 July 2013. “The Time Machine Chronicles: Where Nuts and Pencil-necks Collide” in Mysterious Universe.

Ozaki, Y. T. No date. “The Story of Urashima Taro, The Fisher Lad” in Japanese Fairy Tales . Available at: https://etc.usf.edu/lit2go/72/japanese-fairy-tales/4881/the-story-of-urashima-taro-the-fisher-lad/

Zyga, L. 4 April 2006. “Professor predicts human time travel this century” in PHYS.ORG . Available at: https://phys.org/news/2006-04-professor-human-century.html

In fact the more I think of it you don't need clunky machines. That would speak to the paradox theory. I often go back to sorcery as means of technology. Think of time travel or time more like a spell that you have to break in order to travel.

Deonte N. Ferino

The concept of time is like a cipher or puzzle. All the terminology reads like some sort of anagram. For instance all of its integrals add up to 666 60 secs 60 mins 24hr. Also the terms allude to bondage. Ti me, C Lock & "Watch" wouldn't be surprised if the inventors of the terms were adepts in the concept of time travel. There's lots more...have fun.

RATHER THAN DEBATING SENSELESSLY FOR DECADES ON THE SAME ISSUE OF WHETHER ALIENS EXIST OR NOT? WHY NOT SIMPLY QUESTION YOURSELF WHO YOU ARE IN SPACE? THE SIMPLE ANSWER TO THE MOST COMPLEX AND COMPLICATED QUESTION IS---- YOU ARE AN ALIEN....WE ARE ALL ALIENS TO SOMEONE WHO CALL THEMSELVES AS HUMAN....JUS LIKE WE DO

'Aliens' are really time travellers from the very distant future. They can't return to the past physically, in flesh and blood, but they can do so as sort of holographs, travelling fasted than the speed of light. The people of the very distant future may have actually dispensed with their flesh and blood bodies by that time, or they may have simply evolved into skinny people with huge heads and eyes. Either way, they cannot intefere with the past. It's physically impossible for them. Two things are for certain - there are aliens, but no aliens have a human form - which as that of an ape, after all.

What about the theory that states a paradox cannot exist? If you could go back in time and kill yourself or your parents then you never existed to be able to go back in time.

Dr John (Ioannis) Syrigos initially began writing on Ancient Origins under the pen name John Black. He is both a co-owner and co-founder of Ancient Origins.

John is a computer & electrical engineer with a PhD in Artificial Intelligence, a... Read More

Related Articles on Ancient-Origins

20 Best Time-Travel Shows Ranked

If you could travel back and forth through time, where would you go? What would you do? Who would you talk to? Even better, if you were writing a book, making a movie, or working on a television show about time travel, what would you include? The best TV shows about time travel all feature characters who visit other eras for various compelling (or even life-threatening) reasons. Maybe it's to prevent a coming apocalypse, maybe it's just to save one person's life — but as many of these shows teach, small changes can have big effects, and many of these characters learn that their time-traveling can change the world.  

Now, there are some great time travel-adjacent shows that don't quite fit this list. A fun romp like "Early Edition," for example, utilizes a time-traveling newspaper and potentially a time-traveling cat, but doesn't in and of itself feature a lot of time travel. Likewise, something like "Terminator: The Sarah Connor Chronicles" is rooted in a time travel premise, but stays mostly in one time. With all that said, here's a look at our choices for the 20 best time travel shows on TV.

Save the cheerleader, save the world. That's what future Hiro Nakamura (Masi Oka) tells present-day Hiro when he appears to him from the future, and that's what establishes "Heroes" as way more than just a superhero show.

The NBC series follows a group of regular people who develop special powers, not unlike mutants in the "X-Men" series, after a mysterious worldwide eclipse. Each character gains their own individual abilities. Claire Bennet (Hayden Panettiere) develops the ability to heal from any injury. Senator Nathan Petrelli (Adrian Pasdar) gains the ability to fly, while his brother Peter (Milo Ventimiglia) can temporarily absorb others' powers. Still, few of these characters have cooler abilities than Hiro, who can influence the space-time continuum. This means he can teleport, slow down time — and, of course, time travel.   

Understandably, Hiro's power set becomes a serious asset throughout the series, and his path to perfect his abilities is one of "Heroes'" strongest story arcs. The first few times he travels through time don't go as planned, and throughout the series, things can get in the way of him ending up where he wants to go or when he wants to be. While Hiro's time-traveling is just one part of the larger story, it's definitely one of the show's highlights – especially since Oka is so darn charming as the character.

19. 11.22.63

One of the best Stephen King TV series out there, the eight-episode "11.22.63" follows a man named Jake Epping (James Franco). He's a relatively normal guy who receives a chance to change history when his friend Al (Chris Cooper) tells him he's found a way to travel back in time. Al tells Jake that the portal he's discovered goes back to the year 1960 and that he's been working on a plan to stop the assassination of President John F. Kennedy.

Al's age and advancing cancer diagnosis prevent him from following through on the plan, however, and he asks Jake to take over for him. Jake agrees, but soon his quest is met with pushback from a mysterious source. As it turns out, the past doesn't want to be changed, and every step Jake takes toward preventing JFK's assassination leads to more cracks in the timeline. 

A charming and exciting time travel drama, "11.22.63" is a well-executed, twisty tale that only ranks so low on this list because it's in such great company. If you're looking for a quick, self-contained time travel miniseries that revolves around one of modern America's most notable events, this show is well worth a watch. 

When Oceanic Airlines Flight 815 crash lands on a deserted island, wacky and scary things start happening to the survivors. ABC's "Lost" deals with flashbacks, flash-forwards, mysterious groups that already have a presence on the island, a black smoke monster — and, as it turns out, an ancient battle between good and evil. One of the great appointment television shows before streaming broke through, "Lost" had fans talking about it and theorizing about its mysteries on a weekly basis.

The sci-fi drama captivated viewers for six seasons, and though time travel is referenced throughout the entire series run, it plays the biggest role in Season 4. As the island itself leaps from place to place and from time to time, the main group of characters jumps with it, encountering previous versions of themselves and island events that occurred in the past, and suffering from the effects of temporal displacement. The most beloved episode dealing with time travel is undoubtedly "The Constant," in which fan-favorite Desmond Hume (Henry Ian Cusick) figures out a way to stop his consciousness from jumping through time by finding his constant — his true love, Penny (Sonya Walger).

Of course, "Lost" is not just a time travel show, and famously covers such a wide variety of mysteries and sci-fi concepts that viewers might find it hard to keep up. As such, it ends up with this relatively low ranking. 

Like "Lost", "Fringe" is considered one of the most binge-worthy sci-fi shows of all time  but the fact that it isn't exclusively about time travel means it lands near the tail end of this particular list. The ABC show revolves around a science-fiction conglomerate that dabbles with interdimensional travel, wormholes, and alternate realities. Anna Torv stars as FBI Agent Olivia Dunham, who heads up the bureau's Fringe Division. With the help of "mad scientist" Dr. Walter Bishop (John Noble), his estranged son Peter (Joshua Jackson), and their lab assistant Astrid Farnsworth (Jakisa Nicole), Dunham explores cases involving fringe science — be they about time travel, mind control, experiments gone wrong or any other strange and obscure criminal activity.

Time travel is more of a looming presence early in "Fringe," particularly present in the character of the Observer (Michael Cerveris), a bald, pale, genetically advanced human from the future. While Season 1 and Season 2 deal with the battle between two dimensions and realities, time travel really becomes an element in Season 3. Seasons 4 and 5 then deal with alternate timelines and the Observers that infiltrate the world from the future, intent on wiping out humanity. As you might expect, things can get a bit confusing, but the show sure is fun.

16. The Umbrella Academy

You have to respect a show that's so high-concept that time travel doesn't even get top billing. "The Umbrella Academy" boasts mysterious events, family drama, dance numbers, a talking chimpanzee, some of the cleverest superpowers in superhero shows, and a robot mom — and that's just scratching the surface. Based on "The Umbrella Academy" comics created by Gerard Way of My Chemical Romance fame, the Netflix show is a saga that exploits everything from the butterfly effect to the grandfather paradox for emotional and comedic impact.

The central Hargreeves family consists of a group of kids all born on the same day, adopted by the same eccentric billionaire (Colm Feore). He has trained them to protect the world with their various superpowers, but they aren't particularly great at it, and their strict upbringing has left them with a wide array of issues and deep rifts between them. The dysfunctional bunch starts out fairly estranged, but slowly bonds to save humanity from an apocalyptic event ... only to cause another potential apocalyptic event by sprinkling themselves across time.

In between the tears in the space-time continuum, "The Umbrella Academy" is ultimately an ensemble story about found (and re-found) family, as well as a truly unique superhero show where personal failure and the side-effects of costumed crimefighter life play a huge role. However, since Season 1 largely approaches time travel through Number Five (Aidan Gallagher) and the Temps Aeternalis agency, and much of Season 3 focuses on a present-day alternate reality, only the 1960s-themed Season 2 goes truly all in on the concept of sending all main characters to a different era. 

15. Sliders

"Sliders" is a 1990s sci-fi adventure series that features Jerry O'Connell and friends getting lost across the multiverse. O'Connell ("Stand By Me") plays boy genius Quinn Mallory, inventor of the Timer — a device that lets him and his friends "slide" through a wormhole vortex into different versions of Earth. The thing about wormhole vortexes, though, is that they like to misbehave, meaning Quinn and his buds never know where they're headed next on their adventures. This makes their quest to get back home to their own Earth a tricky one.

"Sliders" starts off fun and strong, and is at its best when having bonkers fun — like when Rembrandt (Cleavant Derricks) discovers a world where he could have been Elvis-level famous — and when it's exploring real-world issues in a high-concept dimension, like when the crew visits an Earth that treats men worse than women. Even if you've seen it before, it's definitely worth a re-watch, because "Sliders" is one  TV show that's better than you remember.

14. Continuum

On "Continuum," Kiera Cameron (Rachel Nichols) is a Protector – think futuristic government agent from even more futuristic equipment — from the year 2077. She gets transported to the year 2012 along with a group of murderous terrorists, forcing Kiera to remain in the past as she chases them down. Fortunately, her gadgets and knowledge of the past soon come in handy and she finds loyal allies. Unfortunately, her enemies also know their history and plan on altering it for their own gain. 

"Continuum" milks the premise for all it's worth, while avoiding the pitfall of becoming a run-in-the-mill procedural with an unchanging status quo. While Kiera does handle her share of case-of-the-week story arcs, they're often connected to the group she pursues, and she never lets go of her primary target of stopping the terrorists. In order to avoid disrupting the timeline, she also has to go to great lengths to avoid revealing that either she or her targets are time travelers — and when their actions inevitably end up changing the future, she has to deal with the consequences. 

13. Timeless

If ever there was a time travel show that was canceled too soon, it's Eric Kripke and Shawn Ryan's "Timeless." The NBC sci-fi series stars Abigail Spencer as the historian Lucy, Matt Lanter as the soldier Wyatt, and Malcolm Barrett as Rufus, a scientist who makes up a team trying to prevent a mysterious organization from altering the courses of history through time travel. They're up againsts Garcia Flynn (Goran Višnjić), who travels throughout history intending to influence major events like the Hindenburg disaster. However, the team soon realizes that the villain they thought they were fighting is much larger and infiltrates the historical timeline in ways they never imagined. 

Instead of focusing on the usual historical suspects, "Timeless" often highlights forgotten people of color, women, and lesser-known historical figures, giving them their due and celebrating their contributions to society. This element of the show can be seen in the way Rufus, for instance, is reluctant to join the team because he knows how Black people are treated in the eras they visit. 

Despite its intriguing concept, the show was canceled after Season 1, but fans caused such an uproar that NBC reversed the decision of canceling "Timeless"  and renewed it for another season. After Season 2, NBC pulled the plug once more, and again, the fans cried foul. In a kind of compromise, NBC greenlit a special two-hour series finale that ties up loose ends and gives much-needed closure to the story. 

12. 12 Monkeys

The "12 Monkeys" SyFy series is based on the 1995 film of the same name that stars Bruce Willis and Brad Pitt — though the series makes a fair few changes to stretch the plot into a four-season sci-fi drama. The series stars Aaron Sanford as James Cole, a scavenger from the year 2024 who's tasked with traveling to 2015 in order to stop the release of a biological weapon. In the movie, James is helped by a psychologist named Kathryn Railly played by Madeleine Stowe, but here, he befriends a virologist named Dr. Cassandra "Cassie" Railly (Amanda Schull). Pitt's character, Jeffrey Goines, is also gender-swapped here, with Emily Hampshire playing Jennifer Goines.

Like the movie, the series deals with the Cassandra Complex, the idea that we have a hard time believing concerns about the future, no matter how likely and provable they are. It also deals with circular time and the idea that past events can be affected by future ones. If those aspects of the film lift your time travel antennae, the four-season show dives even deeper.

11. Paper Girls

"Paper Girls" is a brilliant time travel show that was canceled way ahead of its time. Based on the comics by Brian K. Vaughan and Cliff Chiang, this Amazon series tells the story of a group of 1990s tween girls who get attacked by futuristic invaders. They manage to escape into the future, where one of the girls, Erin (Riley Lai Nelet ), meets her adult self (Ali Wong).

The show dispenses with grandfather paradox hand-wringing and instead uses the concept of the girls confronting their past and future selves, to brutally honest and hilarious effect. Young Erin is horrified to find out how much of herself she's abandoned by the time she turns into Old Erin, and refuses to let life work out that way. It motivates Erin to want to return to her home time even more — this kid has a clock to beat. However, there are two sides to the coin, and Old Erin is also able to care for her young self in ways she never felt able to when she was younger. It's a beautiful and potent visual metaphor that other characters also make good on. 

All in all, "Paper Girls" is a feast for the eyes as much as its ensemble cast is a feast for the soul. Plus, Jason Mantzoukas playfully chewing scenery as the ominous Grand Father? This show could have lasted until the end of time — or at least until Season 2.

10. Timewasters

"Timewasters" is a time travel comedy about a Black British jazz band that accidentally time-slips back to 1920s London, among other timelines. The quartet stumbles into an earlier time perod via a disgusting elevator that, yes, doubles as a time machine. Once the crew shows up in the past, they're treated like freaks, but they gain some measure of success as musicians. While the crew eventually tries to return to the present, they also have a "Back to the Future" moment when they seemingly get stuck in the 1950s.

"Timewasters" is full of funny jokes and great music, and it's a groundbreaking show in a number of ways. "People like us never get to time travel — it's what white people do, like skiing or brunch," creator Daniel Lawrence Taylor told the Royal Television Society . "For me, race is so important." Taylor also stars in "Timewasters," along with Kadiff Kirwan ("Slow Horses"), Adelayo Adedayo ("Some Girls"), and Samson Kayo ("Our Flag Means Death"). The show is also an excellent destination if you're into spotting a variety of British actors and comedians ... including Joseph Quinn, who went on to rise to fame as Eddie Munson on "Stranger Things."

9. Outlander

Based on the series of novels by Diana Gabaldon, Starz's "Outlander" follows the story of a World War II nurse named Claire (Caitriona Balfe) who finds herself thrown back in time after visiting a circle of mysterious Druid stones. She arrives in 18th Century Scotland and, after being taken in by a band of gruff Scots, she marries the dashing young Jamie Fraser (Sam Heughan) in order to avoid being taken prisoner by her real husband's (Tobias Menzies) apparent evil ancestor, Black Jack Randall (Menzies). Claire lives through a time of great upheaval in Scotland when tensions with British control are rising and history-making battles loom in the near future. Despite being initially reluctant to stay, she and Jamie fall deeply in love, and their romance remains the backbone of the series.

The entire "Outlander" timeline  takes some time to explain, what with several 20th-century characters taking the trip to the 18th century and the show covering versions of notable real-world historical events. Without further spoilers, all there is to say is that if you enjoy time travel shows that lean heavily toward historical drama, "Outlander" is where it's at. Also, if you view Tobias Menzies as an incorrigible dweeb due to his performance as Edmure Tully on "Game of Thrones," his monstrous "Outlander" villain is guaranteed to erase that image.

8. Quantum Leap

"Quantum Leap" stars Scott Bakula as Dr. Sam Beckett, a physicist who invents a way to travel through time. When the corporation funding his project threatens to shut it down, Sam uses himself as a guinea pig to test out the method. He finds himself thrown back in time, but in another person's body. The only other entity aware of his 'leap" is a hologram of his colleague and best friend, Admiral Al Calavicci (Dean Stockwell). Al tells Sam that he must correct things that went wrong in the past before being allowed to leap back to his own time and body, and can only use the resources of the project's supercomputer, Ziggy.

With Sam leaping back and forth into different bodies at different times, the show uses a variant of the traditional procedural set up. New characters turn up to guest star and Sam gets to save the day, have a fling, and learn something new before leaping to the next destination, which just might be home one of these days.  

The series ran on NBC from 1989 to 1993, but its combination of time travel and case-of-the-week antics has proved enduring enough that "Quantum Leap" even gets a shout-out in "Avengers: Endgame." Despite being over three decades old, it remains a cool time travel series worth checking out.

7. The 4400

In the opening scenes of "The 4400," an enormous ball of light drops 4,400 people at the foot of Mount Rainier in Washington. They soon realize that they were all taken from some other point in time and deposited into the year 2004, unaged and without any memories of where they'd been. At first, everyone assumes that these people have been abducted by aliens. However, it soon turns out that the truth is far more time travel-related.

The returned people soon start developing "Heroes"-style powers that range from telekinesis to telepathy and super-strength, which people from the future have entrusted with to prevent various catastrophic events that they want to avoid in their timeline. Unfortunately, the 2004 government considers the powered folks a threat, and inhibits their powers with a neurological drug. 

The stories that unfold from this setup are exactly as complex and entertaining as you'd imagine, with various members of the titular group treating their powers in different ways and society having a hard time dealing with them. Unfortunately, "The 4400" ended abruptly after four seasons on a somewhat ambiguous note, but even so, it's a fun show to revisit.

6. Travelers

In Netflix's "Travelers," time-traveling operatives from a post-apocalyptic future are tasked with preventing certain events that have led to the downfall of society in their own present day of 2018. The travelers' consciousness takes over a person in the desired time who's just about to die, and the operative then lives out the rest of that person's days though with the mission in mind ... and a strict set of rules they must follow. Apart from a list of ways they're not allowed to interact with the past, they're also strictly forbidden from communicating with other known travelers outside their team, save for special circumstances dictated by the Director, who communicates by temporarily taking over children. 

It's a unique and complex premise, and the way the travelers scope out potential targets for takeover and learn to live as them is as timely as it comes — they use social media, GPS locations, and other readily available online information for their time-travel tricks. This adds a layer of present-day dread to the show's fascinating take on time travel. 

Loki Laufeyson (Tom Hiddleston) meets his match when he comes up against the Time Variance Authority in one of the Marvel Cinematic Universe's most ambitious Disney+ shows, "Loki." The TVA is so dedicated to maintaining a particular sacred timeline that they purge all alternate realities where someone made a choice they deem wrong, which might not always make sense, but precision isn't the point here. It's the idea of playfulness versus control. 

The Loki we see here is an alternate-timeline variant of the one the audiences are familiar with, and thus starts the show in full "The Avengers" villain mode before life — and time — starts grinding him down. Working with TVA agent Mobius M. Mobius (Owen Wilson), he starts redeeming himself by tracking down an apparently evil version of himself, Sylvie (Sophia Di Martino) ... and ultimately tackling the biggest challenges time can offer.  

The God of Mischief's surprisingly human path of reckoning is the heart of a show that's deliciously stylish, silly, and sometimes scary. "Loki" takes a cops-and-robbers crime caper into time travel territory and explores hefty themes with a light touch, from mindless compliance to self-serving overseers to criminalizing anyone deemed different. "Loki" isn't just a time travel show — it's a show about everything time can offer and more, with characters dancing between eras as you might step from room to room. Also, it has Alligator Loki, who's objectively the best Loki of all. 

If "Loki" is too light-hearted for you, Netflix's "Dark" might be your jam ... provided you can make sense of its incredibly convoluted time travel storyline. Four families weave a tangled web of time travel in this German-language psychological thriller about missing kids, a rotten town, and how almost all of our secrets come out in time. In other words, it's a good time travel show, but it's definitely not a feel-good time travel show. 

"Dark" follows its many characters over the course of their lifetimes and, at one point, has three timelines going at once. Part of the intrigue and challenge of watching the show is trying to understand how (and when) each timeline threads into the other. If you decide to watch it, it's best to have an evidence board and plenty of red yarn ready to chart the relationships and betrayals the town of Winden sees over the years.

While "Dark" is as much a show about human connection and how frayed it can become as it is about time travel, it's also the MVP of using as many time travel paradoxes as possible during its three-season run. "Dark" is also an innovator in the field of wormhole placement. Wormholes are already not to be trusted, but a wormhole underneath a nuclear power plant? No, thank you.

3. Beforeigners

What happens when a bunch of Viking-era warriors, 19th-century figures, and Stone Age people pop up in modern-day Oslo? "Beforeigners" attempts to answer that question while navigating twisty murder mysteries with such efficiency that the Norwegian series may be best described as "crime travel." Adding to the intrigue is the way it focuses more on the present-day relationship between the time refugees and their modern counterparts than on how they showed up in the first place.

"Beforeigners" centers around the odd-couple partnership between hardened police detective Lars Haaland (Nicolai Cleve Broch) and eager new Viking police recruit Alfhildr Enginnsdóttir (Krista Kosonen), who investigate things like the murder of a Stone Age victim and even look into crimes with possible ties to Jack the Ripper.

The metaphor of time migration is an apt one for immigration, and this sci-fi show explores tricky real-life issues with plenty of scope. Creators Anne Bjørnstad and Eilif Skodvin got their start in comedy writing, and their commitment to the bit is evident in the show, including the language used. "Early on, I contacted researchers, professors who helped us. We also constructed the language that Stone Age people spoke, and even with the language from the 19th century: We worked on it to make it sound right," Bjørnstad told Variety . "Why not invest in language, which is such a big part of a person's identity?"

2. Russian Doll

"Russian Doll" could be pitched as "Natasha Lyonne's 'Groundhog Day,'" but that still wouldn't hint at half of the show's charm and emotion. This Netflix offering is a mind-bending time loop dramedy that's a stylish and surreal exploration of life, death, and all the trauma in between. Season 1 of "Russian Doll" features Nadia (Lyonne) stuck reliving her 36th birthday until she inevitably dies and resets back to her friend's bathroom. Later in the season, she discovers a fellow time traveler (Charlie Barnett). They quickly realize that the way out of their dead ends and into a new life is through helping each other.

Season 2 takes some departures from the recursive reality set up in the first season, bending viewers' minds even more thoroughly. "Russian Doll" goes deep, but keeps a sense of humor even as it twists the knife in its characters' hearts — and their timelines. The show keeps audiences just oriented enough by linking its time loops to recognizable spaces and sound cues. You will never look at the subway the same way again, and you will probably never get Harry Nilsson's "Gotta Get Up" out of your head.

1. Doctor Who

Really, could any other show top a list like this?  The untold history of "Doctor Who"  goes all the way back to 1963, when the show premiered on the BBC. The series follows the adventures of a Time Lord who calls themselves the Doctor — an alien being from the planet Gallifrey who travels through space and time on a craft called the TARDIS, which is charmingly disguised as an old-fashioned British police call box and is famously bigger on the inside.  Every Doctor has their own companions  – humans who follow the Doctor throughout space and time, helping people, battling new and recurring villains, and dealing with the assorted wibbly-wobbly stuff on the Doctor's timeline .

The original series ran from 1963 through 1989 and established the neat trick of recasting the Doctor every few years or so, thanks to the premise that the character has multiple lives and can reincarnate himself into different physical bodies. The modern series was revived in 2005 with Christopher Eccleston as the Doctor, and talented actors like David Tennant (twice), Matt Smith, Peter Capaldi, Jodie Whitaker, and Ncuti Gatwa have followed in his footsteps. Even without the fact that no other show has time travel quite as integrated into its very premise as "Doctor Who," the show's sheer longevity and cultural impact are more than enough to make it the king of the time travel hill. 

• [ November 30, 2022 ] The Night Sky This Month: December 2022 Night Sky
• [ November 22, 2022 ] James Webb Telescope Turns Its Attention To The Kuiper Belt News & Events
• [ November 1, 2022 ] The Night Sky This Month: November 2022 Night Sky
• [ October 4, 2022 ] Are Wormholes Fact or Fiction? General Astronomy
• [ October 1, 2022 ] The Night Sky This Month: October 2022 Night Sky

5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

• Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
• Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

• Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
• Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

• Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
• Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

• Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
• Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

• Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

• The Solution: time travel is impossible because of the very paradox it creates.
• Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
• The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
• Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

Related Posts

© Copyright 2023 Astronomy Trek

Holiday Savings

cui:common.components.upgradeModal.offerHeader_undefined

Past, present, paradox: writing about time travel, crafting a believable time travel story requires careful consideration of the logic at play. let's crack the temporal code on traveling through time in fiction.

Table of Contents

Time travel in fiction can open your story to infinite possibilities. Ever wondered what it would be like if somebody taught the Romans how to make a nuclear bomb? Do you need to retcon an event in your story? Time travel!

It may seem simple for your time-traveling characters to hop in Tony’s Terrific Temporal Transport and whiz through time, but there are many hurdles to overcome when writing about time travel.

Chief among these is dealing with time travel paradoxes, so let’s look at those, discuss how you can write convincing time travel stories, and explore how some popular stories handle it.

The Problem With Time Travel

Consider an ordinary day in your life. It follows a sequence of events where one thing leads to another. This is called causality , the concept that everything that happens results from events that happened before it. The problem with time travel in fiction, especially travel to the past, is that it often breaks the rules of causality.

This can lead to time travel paradoxes and unforeseen results , including:

• Continuity paradoxes: The act of time travel renders itself impossible.
• Closed causal loop paradoxes: Traveling to the past creates a condition where an idea, object, or person has no identifiable origin and exists in a closed loop in time that repeats infinitely.
• The butterfly effect: Even the smallest action can have massive consequences.

With all that in mind, let’s embark on a journey through time and explore these further!

Grandfather Paradox

This thought experiment posits the idea of somebody traveling back in time and killing their grandfather before their parents were born. Because the grandfather never has children, the time traveler—his grandchild—cannot exist.

However, if the time traveler never existed, they couldn’t kill their grandfather, so he would go on to have children and grandchildren. One of those grandchildren is the time traveler, though, who might go back in time and kill their grandfather. If that seems confusing, it’s okay—it’s supposed to be.

The bottom line is that if somebody travels to the past and changes something that prevents them from ever traveling to the past, they have broken the timeline's continuity.

Polchinski’s Paradox

American theoretical physicist Joseph Polchinski removed human intervention from the time travel equation.

Imagine a billiard ball travels into a wormhole, tunnels through time in a closed loop, and emerges from the same wormhole just in time to knock its past self away.

Doing so prevents it from ever entering the wormhole and traveling through time, to begin with. However, if it does not travel back in time, it cannot emerge to knock itself out of the way, giving it a clear path to travel back in time.

Bootstrap Paradox

The Bootstrap Paradox is the first closed causal loop paradox we will explore. This presents a situation where an object, idea, or person traveling to the past creates the conditions for their existence, leading to it having no identifiable origin in the timeline.

Imagine sending the schematics for your time machine to your past self, from which you create a time machine. Where did the knowledge of how to create the time machine begin?

Predestination Paradox

The most nihilistic of paradoxes explores the idea that nothing we do matters, no matter what. Events are predetermined to still occur regardless of when and where you travel in time.

Suppose you time travel to the past to talk Alexander the Great out of invading Persia, but he hadn’t even considered this until you mentioned it. By traveling to the past to prevent Alexander’s conquest, you caused it.

Butterfly Effec t

Less of a paradox and more an exploration of unintended consequences, the butterfly effect explores the idea that any action can have sweeping repercussions, no matter how small.

In the 1960s, meteorologist Edward Lorenz discovered that adding tiny changes to computer-based meteorological models resulted in unpredictable changes far from the origin point. In traveling back in time, we don’t know what effect even minor changes might have on the timeline.

How to Write Convincing Time Travel Stories

Time travel can be pretty complex at the best of times, but that doesn't mean writing about it has to be a challenge. Here are a few practical tips to craft narratives that crack the temporal code.

Ask Yourself, "Why Time Travel?"

If your story has time travel, to begin with, it likely plays a pretty significant role in the narrative. Define the purpose that time travel has in your story by asking yourself questions like:

• How and why is time travel possible in your setting?
• What does it mean for your story and your characters?
• What are your characters meant to use time travel for?
• Is the actual practice of time travel different from its intent?

If you can't be clear about time travel's purpose in your story, how can you convincingly write about it? To get crafty with time, you first need to master its relevant mechanics.

Keep a Record of Everything

You're asking your reader to potentially make several mental leaps when time travel is involved in a story, so it's imperative to have all of your details sorted. Do the work of planning out dates and events ahead of time by creating a time map for yourself—like a mindmap, but for a timeline.

You'll be able to keep a birds-eye view of the narrative at all times, be more strategic about moving the order of events around, and ensure that you never miss a detail. You may even want to have multiple versions—a strictly linear timeline and a more loosely structured time map where you draw connections between events and in the order they appear in the narrative.

In Campfire, you can do both with the Timeline Module —create as many Timelines as you want by using the Page feature in the element. You can also connect your Timeline(s) to a custom calendar from the Calendar Module for extra fun with time wonkiness in your world.

If a new idea pops up while writing, don't stress! You'll have your handy time map already laid out so you can easily see if a new scene or chapter makes sense, as well as where it will best fit into the narrative.

Never Forget Causality

I mentioned this concept earlier in the article, but it should be reiterated: The most important rule of time travel is that every action results in a consequence. Remember cause and effect : an action is taken (your character time travels to the past), and causes an effect, the consequence (the timeline is forever changed).

"Consequence" doesn't have to be a negative thing, either, even though the word has that connotation. The resulting consequence of a given action could be a positive effect, too.

Regardless, seek to maintain causality so you don't confuse your readers (or yourself, for that matter). Establishing clear rules for how time travel works in your setting and sticking to them will help you keep your time logic consistent and avoid running into narrative dead ends or plot holes.

Tips & Tricks For the Time-Traveling Author

Now that we’ve examined several obstacles you can encounter when writing about time travel, let’s see how you can either avoid them or exploit them. That’s right! Even time travel paradoxes present opportunities for superb storytelling.

Focus on the Future

Fortunately, all the named paradoxes here involve the past, so the easy way to avoid them is to not go there! Thanks to Einstein’s theory of special relativity, you don’t even have to invent a clever way to travel instead to the future.

An aspect of Einstein's theory is time dilation , in which the faster an object moves through space, the slower it moves through time. With this, you need only zip around at near the speed of light for a few weeks or months, and when you come back to Earth, years or centuries will have gone by.

Create a Multiverse

A popular trope in science fiction today, and a theory gaining popularity among theoretical quantum physicists, is the multiverse concept. According to multiverse theory, whenever an event occurs, every possible outcome of the event happens simultaneously, splitting the universe into parallels that each contain differing outcomes.

Since all these realities exist, perhaps changing the past is simply a way for time travelers to travel between realities, shifting their perspective to a timeline where things occurred differently than in their original reality.

Get Creative With Consequences

Instead of avoiding paradoxes, maybe you want them to occur. Leading to some fascinating stories, this can be approached in a variety of ways. Perhaps you want to examine the unintended consequences of the butterfly effect, create a time-traveling police force that enforces the laws of time travel, or simply break time itself and revel in the chaos that ensues.

Just be sure to remember the action-consequence rule and keep your timeline handy for easy reference—especially if you're toying around with multiple timelines!

Best Time Travel Stories

What follows are what I think are some of the best time travel stories. As you will see, the first two fall victim to time travel paradoxes, while the other two do a great job of exploring various elements we’ve discussed.

Terminator 2: Judgment Day

The corporation Cyberdyne Systems has remnants of the Terminator from the first movie, which they use to create an artificial intelligence system called Skynet. Skynet then actually creates the terminators and sends one back in time. Thus, it gives humanity the technology to create itself in a classic example of a bootstrap paradox.

Back to the Future

In this film, Marty McFly travels to the past and inadvertently interrupts the event where his parents first meet. This causes a chain of events where Marty’s parents never get married and have children, threatening to erase Marty and his siblings from the timeline.

Some argue that the McFly offspring ceasing to exist is a great exploration of the consequences of time travel. However, they would never have been at risk had Marty not been in the past to impede their parents’ romance. And if he ceases to exist, he’ll never go back and get in the way, thus creating a grandfather paradox.

War of the Twins

In this second volume of the Dragonlance Legends trilogy by Margaret Weis and Tracy Hickman, the mage Raistlin Majere travels into the past, kills a wizard named Fistandantilus in a battle for power, and assumes his identity. Throughout the book, Raistlin unwittingly follows the historical fate of Fistandantilus, in a wonderful exploration of the predestination paradox.

It’s hard to talk about time travel in fiction these days without mentioning Loki. The show explores two suggestions from my list above: the multiverse and policing the timeline. In this series, varying outcomes of events lead to branching timelines, creating a multiverse of possibilities. However, an agency called the Time Variance Authority exists to prevent this from happening, and they set out to eliminate any branches separate from what they consider the Sacred Timeline.

Bon Voyage!

I hope this exploration of time travel leaves you prepared to tackle these obstacles and opportunities that naturally present themselves when playing around with time.

Just knowing about the complexities of time travel and the paradoxes it can bring about is the best way to avoid trouble and create innovative storytelling moments. So, dust off your DeLorean, polish your paradox-proof plot, and get ready to write your adventure through the ages!

Learn more about making a timeline with Campfire in the dedicated Timeline Module tutorial . And be sure to check out the other plotting and planning articles and videos here on Learn, for advice on how to plan your very own time travel adventures!

• Toys & Games
• Computer Hardware
• Video Games
• Books, Comics, Mags
• More Lists…
• The Gallery
• Argos Catalogues
• Social Media
• Most Wanted

11 Different Ways to Time Travel

• Posted by Retromash
• in Time Travel
• Tagged with

After my gentle introduction to Time Travel Week yesterday I thought I’d get a bit more heavy today. Time travel is a subject that has always fascinated me. I’m not quite sure what it is about it that is so alluring to me. It’s probably multi-faceted. Obviously, being a retrohead, there’s the nostalgic side of things that would make it really cool to relive some of the warm and fuzzy memories from our youth but I’m probably equally intrigued about what the future holds. It would be great to get a glimpse of what’s to come. But there’s also the aspects of going back in time to learn about history first hand and to see what really happened in some of the big turning points in human history that I find fascinating. I do also love sci-fi in general and so obviously it is a genre that appeals to me purely from that point of view but I also enjoy proper science too and regularly read articles about astronomy, space exploration and astrophysics. It’s fitting that I’m due to watch Interstellar this week as I believe it touches on some of this (but I’m not sure, so I’m not spoiling anything!).

And no matter which of the above reasons it is which grabs me in a particular film about time travel, I also love the puzzle element of it all. If you go back and change something then what does it affect in the future, or is it all predetermined because the future has already happened etc etc. It can be a bit of a mind-f*** but maybe I just quite like that sometimes.

Before we go into some of the many different ways of time travelling in sci-fi, I’d like to quickly touch upon the science. Stephen Hawking himself has said that time travel into the future is certainly possible . Either through a wormhole or just by travelling near the speed of light and then coming back to the Earth. But to me that just seems almost like cryogenic freezing. That is a form of time travel too I guess, in the sense that you suddenly find yourself in the future (from your perspective), but these ways are a one way road. You can’t travel back like that. Once you’re there you’re there. You’ve kind of just fast forwarded. And I don’t really consider that true time travel. Many people say that travelling into the past is impossible. But many scientists have still tried to prove it can be possible . I think even Stephen Hawking does leave a very, very tiny element of possibility there but says it is highly improbable and would need lots of power and gravity and other sciencey stuff. Plus he has done an experiment where he invited time travellers from the future to a party that he didn’t advertise and therefore only people from the future would know about. Nobody showed up . But there are many theories as to what would happen if you did go back in time and I even have thoughts of my own on this which should come out in the posts this week.

So let’s look at some of the ways that people time travel in sci-fi and popular culture and some of the issues or interesting points that arise from them. There are many different ways time travel is represented in movies. Warning: I get quite nerdy here. It’s all good fun, to explore what I find a fascinating scientific, or pseudo-scientific, discussion. So, in no particular order…

So ‘time travel’ is by no means a fixed, standard way of thinking. There are many interpretations. Obviously these are very much science-fiction with the emphasis on the fiction but it’s still something that is a great deal of fun to daydream about. I’ll be discussing my ideal ways to time travel and where I would go in my final post about time travel on Sunday.

Which method above would you use if you could time travel?

#timetravelweek

Time Travel: Where would I go? | Retromash

Nov 23, 2014 - Reply

[…] method of time travelling I discussed in my article on 11 Different Ways to Time Travel is voyuer travelling. Either just to events in your own life or to events anywhere in history in […]

Time Travel Week | Retromash

Oct 19, 2015 - Reply

[…] 17th Nov – Back to the Future Secret Cinema Tuesday 18th Nov – 11 Different Ways to Time Travel Wednesday 19th Nov – Time Travel Cliches and Tropes Thursday 20th Nov – Top Ten Time […]

Nov 15, 2018 - Reply

How to practically implement these meythods? What is the major requirement to travel time now??

On which thing we need to reserch to find out a way to travel time??

Reply me for sure…..im an aerospace student and i ll do research for sure

Nov 16, 2018 - Reply

If you find a way to actually time travel, please get back in touch.

Jul 3, 2020 - Reply

Please find a way! I will give you 1 billion dollars. Please hurry!

A Quick Guide To Vintage Lego Sets | Retromash

Jul 19, 2022 - Reply

[…] sum, vintage Lego sets are a great way to travel back in time and relive your childhood. Finding one in your personal stash or getting one online can be very […]

Leave a comment Cancel reply

Save my name, email, and website in this browser for the next time I comment.

This site uses Akismet to reduce spam. Learn how your comment data is processed .

Recent Posts

• BAFTA Games Awards 2024: Complete List Of Winners Revealed With Baldur’s Gate 3 Emerging As A Major Victor
• Exploring the 1980s Edition of Monopoly and How it Differs Today
• The Nostalgia Effect: How Popular Are Retro-Style Online Slots?
• How mobile phones have widened the online gaming audience
• How You Can Reinvigorate Your Love of Solo Gaming

Popular Posts

• Argos catalogues from the 70s, 80s and 90s
• Building a Home Arcade Machine
• Audio Mashup – How I edited ‘Thunder In Your Heart’
• How I met Keith Harmer, the artist behind the amazing 80s Lamborghini Countach poster
• This Matchbox catalogue from 1984 cost 5p and is filled with absolute awesome
• Johnuary – A Summary of the Month
• The Retromash Book List
• Possibly my favourite magazine of all time
• 11 Different Ways To Time Travel
• G.I. Joe v Action Force
• Top Ten Arnie Films
• Top Ten Lamborghini Countachs
• Photography
• Retrogaming
• Time Travel
• Toys & Games
• February 2024
• January 2024
• December 2023
• November 2023
• October 2023
• September 2023
• August 2023
• February 2023
• January 2023
• December 2022
• November 2022
• October 2022
• September 2022
• August 2022
• February 2022
• January 2022
• December 2021
• November 2021
• October 2021
• September 2021
• August 2021
• February 2021
• January 2021
• December 2020
• November 2020
• October 2020
• September 2020
• August 2020
• February 2020
• January 2020
• December 2019
• November 2019
• October 2019
• September 2019
• August 2019
• February 2019
• January 2019
• December 2018
• November 2018
• October 2018
• September 2018
• August 2018
• February 2018
• January 2018
• December 2017
• November 2017
• October 2017
• September 2017
• August 2017
• February 2017
• January 2017
• December 2016
• November 2016
• October 2016
• September 2016
• August 2016
• February 2016
• January 2016
• December 2015
• November 2015
• October 2015
• September 2015
• August 2015
• February 2015
• January 2015
• December 2014
• November 2014
• October 2014

Social Media Channels

No breach of copyright intended to the owners of any property or product mentioned in this site.

• BAFTA Games Awards 2024: Complete List Of Winners Revealed With Baldur’s Gate 3 Emerging As A Major Victor April 26, 2024
• Exploring the 1980s Edition of Monopoly and How it Differs Today April 20, 2024
• The Nostalgia Effect: How Popular Are Retro-Style Online Slots? April 19, 2024
• How mobile phones have widened the online gaming audience April 17, 2024
• How You Can Reinvigorate Your Love of Solo Gaming April 14, 2024
• How To Turn Your Love Of Sporting Video Games Into Real-Life Adventures April 14, 2024

Argos catalogues from the 70s, 80s and 90s Building a Home Arcade Machine Part 1 Poker to Pokemon – The Evolution of Card Games How I met Keith Harmer, the artist behind the amazing 80s Lamborghini Countach poster This Matchbox catalogue from 1984 cost 5p and is filled with absolute awesome Spider-Man’s Early Video Games Johnuary – A Summary of the Month The Retromash Book List Possibly my favourite magazine of all time G.I. Joe v Action Force Top Ten Arnie Films Top Ten Lamborghini Countachs

© Copyright Retromash 2014-2021. All Rights Reserved

• Skip to primary navigation
• Skip to main content
• Skip to primary sidebar
• Skip to footer

TravelAwaits

Our mission is to serve the 50+ traveler who's ready to cross a few items off their bucket list.

Best Things To Do in Tulum

• Destinations

Move over Cancun, it’s Tulum’s time. Over the past few years, this once-sleepy Caribbean town has become one of Mexico’s premier Riviera Maya destinations. From its quirky boutique shops to its white-sand, jungle-bordered beaches, there’s plenty to obsess over.

But Tulum has a few surprises in store for the average tourist.

Most are overwhelmingly positive. When you visit Tulum, you’ll learn about things like ancient Mayan ruins, indigenous feather painting, and how to identify a perfectly balanced taco. But you’ll also run into things like sargassum algae and tourist traps.

Never heard of sargassum before? Haven’t really thought about waiting in long lines in the blazing Yucatan sun to see the shrine of a Mayan god? That’s okay. I’ve done the leg work to help you dodge these types of surprises.

Let’s get into the details with this list of the best things to do in Tulum, starting with its free public beaches.

Things To Do in Tulum

Hang Out on the Beach

Sargassum warning : We need to talk about sargassum. Due to various factors, legions of this brown algae have started to drift from the Atlantic into the Caribbean. Along with Playa del Carmen and Cancun, Tulum’s beaches have been affected by the stinky tides.

The good news? Sargassum algae provides food and habitat for tuna, sea turtles, and other aquatic life. Though unsightly, it’s good for the environment. The bad news? It smells like death and it washes onto Tulum’s shores on and off from April to August.

If you plan on visiting Tulum during this period, just know that you might run into the stinky brown algae on a beach or two. However, sargassum doesn’t usually affect all of Tulum’s beaches at once. (Or, if your plans are flexible, consider visiting during the winter .)

You’ve been warned— now onto the fun stuff!

As a tropical paradise, beach bumming is one of the best things to do in Tulum. The lush natural landscape hugs the Caribbean Sea, allowing visitors to enjoy jungle views while they enjoy the clear waters. From Tulum town, you can easily access popular beaches like Playa Ruinas (which includes Tulum’s Ruins), Mayan Beach, and the famous Playa Paraiso.

Beach clubs dot most of these areas. These clubs offer visitors beach chairs, towels, cabanas, snacks, cocktails, and more. Some are specifically designed for photoshoots, too. Depending on your tastes, you can target more luxurious beach clubs, including Ziggy’s Beach Club and the Papaya Playa Project, or more casual hot spots, including Paraiso Beach Club and Mia Beach Club.

Snorkel & Scuba Dive

Given its emphasis on tourism, it’s easy to find tour companies that offer aquatic adventures in Tulum. Snorkeling and scuba diving are two of the most popular options. But it’s worth mentioning here that, along with the ocean, there are snorkeling and scuba diving options for nearby cenotes. (We’ll dive into cenotes in the next section… pun intended.)

Most of the Yucatan peninsula is bordered by what’s called the Mesoamerican Barrier Reef. That means you can snorkel and scuba dive at some of the Caribbean’s best coral reefs, home to tropical fish and colorful sea life. For the truly daring, you can dive with bull sharks.

If you’re a bit slower in the water like me, sea turtles might be a better option. You can easily book a trip just north of Tulum to Akumal (from the Mayan word for turtle, ‘aak’). It’s a coastal town where sea turtles come to feed on the large deposits of seagrass. But I recommend sticking with trusted and environmentally friendly tour groups—sea turtles rely heavily on this ecologically fragile habitat.

Swim in the Cenotes

Cenote is another Mayan word (from ‘tso’ono’ot’), which refers to a body of groundwater. Cenotes are sheltered deposits of crystal clear water, usually located inside cave-like sinkholes that dot the Yucatan Peninsula . Though Tulum’s tropical beaches are definitely worth writing home about, I found my time swimming in cenotes to be a lot more memorable.

Picture descending into a cave that’s at least ten degrees cooler than the humid jungle outside. It’s dark, but there are beams of golden sunlight piercing through the jungle above. You’ve officially entered another world—one that may have involved Mayan sacrifices back in the day.

Cenotes, as mentioned above, are a common feature on Yucatan and were used by the Mayans for a variety of purposes. That makes them an awesome blend of cultural archaeology and natural splendor. Some of the most popular cenotes around Tulum are Dos Ojos, El Gran Cenote, Zacil Ha, and Yal-Ku.

Once again, my inner tree-hugger compels me to bring up the fragile ecological balance of cenotes. Depending on the location, tour companies might ask you to wash lotions and oils off before taking a dip. Just remember: the more you scrub, you more you love!

Explore Ruins & Archaeological Sites

In terms of the most unique sightseeing attractions near Tulum, nothing beats the Mayan ruins. If you want to explore a UNESCO World Heritage Site , then you have two options. One is Chichen Itza, the world-famous ancient Mayan ruins located just two hours from Tulum.

Chichen Itza was a booming Mayan city—but you probably recognize the name for its association with pyramid-like stone temples. Though it’s well worth a visit, keep in mind that it’s one of Mexico’s crown jewels of tourism, meaning you’ll be surrounded by guests from around the world. Many guests. 

I suggest exploring other ruins closer to the Tulum area. They’re smaller, but they’re slightly less crowded. The Coba Ruins, for example, receive one-third the number of tourists as Chichen Itza. This former Mayan village includes stelae, raised roads called ‘sacbes’, altars, and other archaeological structures.

If Indiana Jones-esque adventures aren’t on your radar and you don’t feel like day-tripping outside of Tulum, then you’re in luck. The Tulum Ruins line Tulum Beach and are open to the public every day of the week. A fairly quick tour will clue you into Mayan history without diving too deeply into the details.

Wander Around Tulum (and Find the Perfect Pool Club)

Though it was once an ancient Mayan settlement, boutique shops, big-name restaurants, and flashy clubs line Tulum’s spread-out streets today. Given the hotel zone is vast and the city isn’t very walkable, you might be wondering how should you get around and what there is to do in town.

Most travelers rent a car when they land at Cancun Airport. Though having a car is definitely convenient, it’s not absolutely necessary. You can also rent a bike or take Tulum taxis to get around… but then what?

You can shop and eat your way through Tulum, which is a solid choice in any city. But I think Tulum’s museums and pool-beach club hybrids are also worth exploring. One exciting project is Mystika, a specialty museum that focuses on Mayan cosmology using interactive exhibits.

In case you didn’t get a degree in Global Studies and metaphysical mumbo-jumbo like me, ‘cosmology’ is a fancy word for how we make sense of the universe. Where did we come from? What’s the purpose of life? Why can’t white people dance? Things like that.

But if you’re more interested in having fun in Tulum than learning about cosmology, don’t worry. One of the most famous things to do in Tulum is lounge around looking good at its pool clubs and beach bars. In fact, most include Instagram-worthy backdrops… while also functioning as pool clubs that let you escape the tides of sargassum.

Bagatelle is famous for its pool that overlooks the sea, along with its champagne list and chic daybeds. Ma’xanab, by contrast, focuses on creating serene vibes rather than hosting daytime DJ sets. Amansala offers a similarly tranquil setting, along with a health-centric menu that includes salads with names like Green Goddess .

Go on a Jungle Adventure

Quick geography lesson: Tulum is located on the tip of the heavily forested Yucatan Peninsula. To my great disappointment, these are taxonomically referred to as ‘moist forests’ instead of jungles. But they’re home to creatures like jaguars, spider monkeys, margays, and more.

In Tulum, you have literally dozens of choices in terms of adventurous jungle experiences. Ziplining and ATVing are increasingly popular. However, I suggest thinking twice before booking tours that use terms like ‘ATV’ and ‘sacred’ side by side.

The jungles around Tulum are in an interesting position. Tourist dollars help protect them (yay), but irresponsible development threatens the ecosystem (boo). For this reason, you’ll find that ecotourism in Tulum is increasingly subject to debate .

This doesn’t mean you can’t have fun out in nature—but please consider researching tour companies before booking. High-flying adventures aside, Tulum is also home to amazing reserves where you can hike, kayak, bird-watch, and more.

The Punta Laguna Nature Reserve, sometimes called the ‘Spider Monkey Reserve’, is only a short drive from Tulum. There, you have access to a fantastic community-led tour that involves plenty of howler and spider monkeys. The lagoon itself is also stunning to explore.

I also recommend Sian Ka’an Biosphere Reserve, Tulum’s second reachable UNESCO World Heritage Site . It takes its name ‘ka’an’ from the Yucatec Maya word for heaven. It’s a fitting description, as this reserve offers shelter to the Riviera Maya’s most precious inhabitants, from monkeys to herons to crocodiles. 

Lastly, Laguna Kaan Luum is another jungle hot spot. Keep in mind that you won’t be allowed to enter the cenote in the center of the lagoon. Despite this, I think it’s still worth a visit thanks to its crystal clear waters—and the fact that it’s only around twenty minutes from Tulum Centro.

Take Care of Yourself with Yoga Retreats & Wellness Programs

It’s undeniable: there’s now a tourist vibe in Tulum that’s similar to nearby Cancun —just without the massive resorts. But before Tulum became a global sensation, locals placed a strong emphasis on environmentalism, healthy living, and other New Age ideals. Despite the growing shift toward large-scale tourism, there are still plenty of yoga retreats and wellness-centric activities around Tulum Pueblo.

They truly run the gamut, meaning you need to do a bit of research.

For immersive and personalized retreats, look into programs from Tribal Tulum. If you want to stay on the beach, you’ll be happy to hear that the aforementioned Amansala (home of the Green Goddess salad) offers yoga and fitness courses. Other companies place a stronger focus on relaxation via luxurious full-service spas, like Azulik Resort.

The sky is the limit when it comes to holistic wellness retreats in Tulum. Some programs even combine yoga practices with diving experiences. Others are multi-day courses geared toward certain goals, such as silent retreats, self-love, women’s empowerment, social media detoxes, and even couples getaways.

Hit the Dance Floor

If you’re anything like me, you’ve probably seen Tulum’s electronic shows pop up on your social media feed. (If you’re anything like me, you’re probably wondering what all those tropical birds do when the decibel level rises above 70? But that’s another story.) Partiers in Tulum can rejoice, as the town is heavily geared toward all-nighters.

Here are the hottest nightclubs that you need to know: Papaya Playa Project, Bagatelle, Vagalume, Tantra, and Taboo.

If electronic music and bottle service aren’t your thing, then feel free to explore Tulum’s live music and dancing scene. It’s just as vibrant. A local favorite is Ki’bok, which is a coffee shop that transforms into a dance club at night.

There are also events like La Zebra’s Sunday Salsa Night where you can rub elbows with friendly tourists and even a few twirling locals. Batey is another classic options that offers live music and mouth-watering cocktails.

Feed Your Soul — and Your Belly

Tulum is home to fantastic restaurants that feature the very best in Mexican and international cuisine. If you’re a true foodie, then I recommend booking a food tour. These walking tours offer a backstage pass to local tastes and multicultural cuisine alike. There are also cooking programs that teach visitors how to prepare Mayan cuisine.

But what about finding the best restaurants? You’ll probably notice that prices are a bit higher in Tulum than expected. As the town grows, so does its roster of Mexico City-calibre restaurants and their pricey menus. If you want to eat like a king or queen, then head to Autor, Arbolea, Ultramar, or Cetli. Just be prepared to fork over a small fortune.

Want to keep things casual? Local eateries serve up delicious meals at a reasonable price. Some favorites from Tulum Pueblo include Antojitos La Chiapaneca, Taqueria Honorio, El Camello Jr., and Los Aguachiles. (A quick heads up about eating local—be careful about raw fruits and vegetables that are washed with tap water. Locals are used to it, but tourists should avoid it at all costs.)

Learn More About Local Culture

I’ve written a lot about ruins in this article, which probably paints a picture of the Mayans as an ancient and distant people. In reality, Mayans live across the Yucatan Peninsula—including in Tulum. Many speak the Yucatec dialect of Mayan, called ‘Maya’ by its 800,000 native speakers.

If you’re a fan of cultural exchanges, then you’re in luck. Many of the locations above are at least partly managed by Mayans. Sian Ka’an Biosphere Reserve, for example, is overseen by the Amigos de San Ka’an, which includes Mayan communities who foster biodiversity and promote ecotourism.

But what about more direct ways to interact with modern Mayan culture? One of the most straightforward options is to explore works by local artists. Street art, for example, is one of the most visible canvasses that Mexican and Mayan artists work with. As you explore Tulum Pueblo, you’ll see dozens of colorful and photo-worthy projects.

Tinasah House, for example, is an arts collective that was formed in 2012 with the goal of promoting local talent. Many of the murals and installations you’ll see in beach bars and street corners are from Tinasah. You’ll also notice an abundance of art galleries around town. Balam Art On Feathers, for example, offers up traditional Mayan feather-painted works that you won’t find elsewhere.

Jump Around to Different Cities

Tulum might be the latest hotspot on the Riviera Maya , but it’s far from being the only one. If you have a rental car at your disposal, then you can easily hit the road to explore neighboring cities. Because Tulum sits on the border of the states of Quintana Roo and Yucatan, a short drive can take you closer to local culture. Here are three destinations I think are worth the trip.

• Valladolid. This small town is easy to reach from Tulum by bus or car. The buildings are colorful, the market is lively, and there’s a cenote (Cenote Zací) in the center of town that you can swim in for free after eating at the adjoined restaurant. Best of all, it’s a hub for traditional Yucatecan food.
• Cancun/Isla Mujeres: If you’re not bothered by a quick trip into the resort town of Cancun, then you’ll have access to things like luxury shopping and spa treatments. If staying on the beach is your top priority, then consider parking your car in Cancun and taking the ferry to Isla Mujeres . It’s a decidedly more rustic and hippy vibe. (As a bonus, you’ll pass the hugely popular Playa del Carmen on the way.)
• Coba: Above, I mentioned visiting the Coba Ruins. The town is also worth exploring, as it is home to cenotes like Multum Ha and popular hiking trails. There are even lux spa services at places like Coqui Coqui. Those with a rental car can build their own adventure to hit all of the hotspots around Coba.

Find the Hidden Gems

Tulum is home to dozens of adventures that you simply can’t find elsewhere, from feasting on Yucatecan-style fish tacos to swan diving into cenotes. But if you’re really looking to get off the beaten path, then I’ve got two more Hidden Gem-caliber suggestions.

• Keep an eye on the concerts. Boiler Room is active in Tulum, hinting that this location’s emphasis on EDM isn’t going anywhere. If you follow any international DJs, take a look at their tour dates before booking your trip. Artists who perform in Mexico City might also stop in Tulum to play at one of its nightclubs. Who doesn’t love the chance to get to see one of their favorite artists play in another country?
• Become a swamp creature with a clay bath. Don’t worry—it’s cultural. The Mayans were one of many ancient peoples to perfect the art of the clay bath. You can learn more about mineral-infused clay and how it aids in detoxification while you enjoy a treatment in Tulum. There are a handful of spas that offer Mayan clay treatments, along with a few companies that take you into the wild to bathe using actual clay deposits.

Regardless of what brings you to Tulum, chances are you’ll be back again. Just remember to travel mindfully into nature, enter the unknown with an open mind, and try to avoid sargassum season. Oh, and don’t drink the water.

Tay’s obsession with travel began with the Travel Channel show Anthony Bourdain: No Reservations. Her interests led her to a tiny experiential college where she earned her degree in Global Studies. Higher education took her to Costa Rica, Panama, Nicaragua, Thailand, Taiwan, Australia, Indonesia, and India. Her academic focus was on indigeneity, ecology, and pop culture, leading to studies like the spirituality of surfing (Costa Rica), the cultural implications of Sak Yant tattooing (Thailand), and grassroots community organizations/motorcycle clubs (Brooklyn). Over the years, she’s presented her research to national councils, helped launch NYC’s first Indigenous Peoples’ Day public powwow, and had her fantasy work (yes—she does that, too) shortlisted for major indie awards. As of 2024, Tay is a freelance writer with the same passion for global thinking, mindfulness, and self-discovery. She lives in Barcelona with her partner and her chihuahua.

Where Lonely Planet staffers are traveling this summer

Apr 19, 2024 • 10 min read

Banff National Park in the Canadian Rockies is one of the places Lonely Planet staffers plan to visit this summer © Paul Zizka Photography / Banff Tourism Board

Summer is around the corner in the northern hemisphere, and the team at Lonely Planet is already making (or has made) their travel plans.

If you're wondering where to go and what to do this summer, why not follow one of our leads and discover a new destination or rediscover an old favorite? I, unlike my colleagues, have yet to make plans, so finding out where everyone else is going has lit a fire under me. Likewise, I hope these trip plans inspire you to make some of your own – and that you'll turn to Lonely Planet for help when plotting your next getaway.

Here are just some of the places the staff at Lonely Planet traveling this summer.

"I'm going to a backcountry lodge in Banff . It's only accessible by hiking in. I’ve wanted to do one of these for ages, so this is how I’m celebrating my 40th!" – Jessica Lockhart, Senior Editor, Oceania

Banff and Jasper National Parks have several rustic backcountry lodges surrounded by unparalleled scenery. Each lodge has its own unique setting, hosts and history, but all have simple amenities, minimal (or no) electricity and running water, and welcoming common spaces where travelers can gather to read, play cards or recount the day’s adventures. Advance bookings are key – prices may seem steep, but factor in the included home-cooked meals and freedom from setting up camp or worrying about weather and wildlife, and the cost suddenly becomes worth it. 

Keen to go hiking in Canada? Here's our guide to the best trekking routes

"I will be visiting Makarska Riviera , including Brela , Makarska, Tucepi and the island of Korčula . The trip is all about promoting Croatia ’s great outdoors , so the itinerary includes kayaking, rafting, buggying and lots of hiking !" – Aoife Breslin, Publicity and Marketing Coordinator

Croatia’s tourism peaks between June and August, when the Adriatic’s warm waters charm countless visitors. It’s great fun, though afternoons are roasting hot, the lines at attractions are at their longest, and accommodation costs rise. Inland, temperatures are higher, but crowds are less noticeable. June is the quietest month of high season, but with clear skies, music festivals and the promise of early summer, it's a strong contender for Croatia’s best month. 

Ready to plan your trip to Croatia? Choose the right time for your visit with our seasonal guide

"I'm going island hopping  in Greece for four weeks in June. I'll be spending most of my time on Serifos (to start) as it's where the local Greeks holiday  – doing a pottery class, vineyard tour, taking boats to secluded coves, going to cooking school and generally trying to be as fabulous as possible – with additional stays on Santorini and Paros . – Chris Zeiher, Senior Director of Trade Sales and Marketing

Greece is ancient sun-bleached ruins piercing blue skies, the balmy Aegean lapping an endless coastline and a culture alive with passionate music, wonderful cuisine and thrill-seeking activities. Summer is when most travelers choose to explore its countless islands, and June affords the longest days of sunshine, peaking in the second fortnight. It’s also an opportune time for your first, refreshing dip of the summer. 

Going to Greece for the first time? Here our our top tips on things to know before you go

"I'm heading to Tuscany and the island of Elba ." – Annie Greenberg, Creative Director  "We are doing a girls' trip to Tuscany." – Aly Yee, Senior Director

Tuscany escapes easy definition. The Apennines – Italy ’s mountainous spine – slope into vineyard-covered rolling hills, which in turn fade into the Mediterranean coast. Late spring to early autumn is when most people visit Tuscany. It’s easy to understand why – days get longer and warmer, the countryside comes to life, outdoor dining opportunities abound, and festivals happen all around.  The island of Elba comes to life during summer, and it’s worth booking accommodation well in advance if you plan to visit in the high season. Elba offers both great beaches and hiking opportunities along its Grande Traversata Elbana (GTE, Elba’s Great Crossing) trail.

Ready to plan a trip to Tuscany?  Check out our regional guide  

"I'm off to car-free Isla Holbox , Mexico for a relaxing beach getaway. I'm planning on horseback riding and wataflow therapy, which I've never done, but should be interesting."  – Serina Patel, Marketing Manager

Isla Holbox (hol-bosh), meaning "black hole" in Mayan, lives up to its name – it's like a portal to one of Mexico’s last unspoiled tropical islands. Golf carts and bicycles serve as the main forms of transportation, and visitors will discover sandy streets, colorful Caribbean buildings, lazing, sun-drunk dogs, and sand so fine its texture is nearly clay. The greenish waters are a unique color from the mixing of ocean currents, and on land there's a mixing too: of locals and tourists, the latter hoping to escape the hubbub of Cancún .

Using Cancún as a base? Here are the best day trips into the wider area

"I'm heading to Granada , Spain for a week or so, maybe getting in Málaga too. This will be my second visit, so I'm hoping to take a more relaxed approach this time and also catch up with friends." – Alison Killilea, Production Support Editor

With serene Islamic architecture, monumental churches, old-school tapas bars and counterculture graffiti art, Granada is Spain’s cultured, creative southern city; a place with a storied past centering on the Alhambra , one of the world’s great human-made wonders. While Granada can be scorching in summer, the city of Málaga is deemed to have the best climate in the country, with about 3000 hours of sun a year – the most in all of Spain. 

Seen the Alhambra? Here are Granada's other top experiences

Switzerland

"I'm also going to Zürich , Lucerne , Grindelwald  and Zermatt . I will mostly be hiking and doing lots of outdoorsy activities. My number one priority while I am in Switzerland is to do the highest hiking trail in Europe , the Barrhorn."  – Aoife Breslin, Publicity and Marketing Coordinator

Nowhere is perfect, but let’s face it, Switzerland gets pretty darned close. With its supermodel looks, fine weather, easy-peasy public transport , multilingual mindset and penchant for cheese and wine, this is a country where it’s easy to get comfortable – even if it is a bit on the pricey side.  Peak summer in Switzerland is  tip-top , to borrow the Swiss German phrase. Barring the odd storm, it’s nearly always hot and sunny – oppressively so, sometimes, meaning the best place to be is in the cooler air of the high mountains.

Ready to experience the best of Switzerland? Here's our guide to the top things to do

The Bahamas

"I'm heading to Eleuthera  in the  Bahamas for our annual family trip – there are 16 of us so it's always an exciting week! With age ranges of 7–70, I can report that we are all excited to do a beach bonfire and barbecue, and go sailing to swim with some piggies." – Amy Nichols, Senior Marketing Manager

The sapphire waters and  sun-soaked sands  of the Bahamas beckon travelers with warm weather that never fades. Just a short flight from the east coast of the US, this island nation is a magnet for repeat visitors and last-minute bookers alike. Eleuthera, however, is a bit tougher to get to, but is well worth the expense and effort if you're looking for vacation bliss. With its pink-sand beaches, Atlantic-battered reefs, weather-warped rock and dense subtropical scrub, this incredibly narrow 109-mile (175km-long) crescent also offers boutique hotels, revered surf breaks and some fabulous restaurants. 

Can't decide where to go in the Bahamas? We can help with this guide to the best places to visit 

"I'm heading to Naples , Florida  on a family vacation for 2 weeks. I'm planning to relax mostly, with lots of trips to the beach and eating good food! But I'm hoping to take a trip to the Everglades , too." – Aoife Breslin, Publicity and Marketing Coordinator

For upscale romance and the prettiest, most serene city beach in southwest Florida, come to Naples, the Gulf Coast's answer to Palm Beach. The soft white sand is backed only by narrow dunes and half-hidden mansions. More than that, though, Naples is a cultured, sophisticated town, unabashedly stylish and privileged but also welcoming and fun-loving. With spectacular year-round sunshine, there's certainly no bad time to visit Florida . Summers can be pretty hot, but you'll probably spend less on lodging than the winter or summer months.

Traveling to Florida on a budget? We've got some money-saving tips for you

Massachusetts

"My friends and I are going to Martha’s Vineyard , Massachusetts in July. We’re planning a Midsommar -esque garden dinner one night and will spend the rest of our time beach hopping and strolling through Edgartown." – Ann Douglas Lott, Associate Editor

Martha's Vineyard remains untouched by the kind of rampant commercialism found on the mainland – there's not a single chain restaurant or cookie-cutter motel in sight. Sunny skies and consistently hot weather make July and August the best time for a traditional beach holiday with sunning, swimming and sand-digging. The tradeoff, of course, is that July and August are the months everyone goes to the Vineyard and it's likely to be jam-packed, so plan ahead.

Want to see more of Massachusetts? Here are the best road-trip routes

"My family is headed to Northern Michigan this summer. We're excited to paddleboard to a shipwreck, relax on the beach and sail on Lake Michigan. We'll hike and run down all the sand dunes, explore cute artsy towns full of galleries and good eats." – Sarah Stocking, Digital Editor

Summertime buzzes with travelers when draws like the  Great Lakes , charming islands and unspoiled wilderness are at their most accessible. Michigan’s high season kicks off on Memorial Day (the last Monday in May). Ferries start to depart more regularly to popular spots and while summer-only establishments lift their shutters. The weather is mostly sunny and warm, with temperatures ranging from 76°F (24°C) to 85°F (29°C). This means lots of summer-only outdoor activities begin.

There are loads of great beaches in Michigan. Here's our guide to the very best

North Carolina

"My wife and I are going to Kure Beach, North Carolina for the 4th of July for five days — my favorite things to do there are take a yoga class on the beach with Kure Beach Yoga (no need to bring a mat, bring a towel!), watch the sunrise from our hotel (The Lighthouse Inn, a very laid-back and recently revamped spot right by the water so you don’t have to stress about parking, which is awful every summer), and see if we can find the boardwalk cat, Bibi." – Rachel Lewis, Senior Social Media Manager

The height of summer in North Carolina is beach time, and with 322 miles (518km) of ocean shoreline reaching from the  Outer Banks  in the north to the  South Carolina  border in the south (and 12,000 miles/19,000km of estuarine coastline along the way),  North Carolina has plenty of beaches to choose from . Kure Beach has 6 miles (10km) of protected shoreline as well as lagoons teeming with wildlife that you can explore by renting a kayak or a stand-up paddleboard.

Explore some of North Carolina's epic landscapes by foot with our guide to the best hiking routes

Rhode Island

"I'm off to Newport , Rhode Island for Memorial Day Weekend and planning on going to restaurants, wineries, mansion tours, walking and hiking." – Serina Patel, Marketing Manager

It may be the yachting capital of the world, but you don’t need nautical stripes – or a summer cottage – to enjoy the seaside retreat of Newport, Rhode Island. With its fresh briny air, expansive sea views and stunning bays, it's obvious why cityfolk continue to follow in the footsteps of the American industrialists here. Enjoy a taste of the good life by touring Newport's Gilded Age mansions built in the late 1800s, taking sailing lessons, or going wine tasting.

Explore related stories

Apr 14, 2024 • 6 min read

Florida is famous for sun and sand, but for many families it's all about the theme parks. Here's our pick of the best theme parks in the Sunshine State.

Apr 10, 2024 • 6 min read

Mar 25, 2024 • 6 min read

Mar 22, 2024 • 9 min read

Mar 8, 2024 • 6 min read

Mar 2, 2024 • 8 min read

Feb 28, 2024 • 3 min read

Feb 23, 2024 • 5 min read

Feb 15, 2024 • 4 min read

Feb 7, 2024 • 5 min read