example travel time

Do you believe in time travel? I’m a skeptic myself — but if these people’s stories about time travel are to be believed, then I am apparently wrong. Who knows? Maybe one day I’ll have to eat my words. In all honesty, that might not be so bad — because the tradeoff for being wrong in that case would be that time travel is real . That would be pretty rad if it were true.

Technically speaking time travel does exist right now — just not in the sci fi kind of way you’re probably thinking. According to a TED-Ed video by Colin Stuart, Russian cosmonaut Sergei Krikalev actually traveled 0.02 seconds into his own future due to time dilation during the time he spent on the International Space Station. For the curious, Krikalev has spent a total of 803 days, nine hours, and 39 minutes in space over the course of his career.

That said, though, many are convinced that time dilation isn’t the only kind of time travel that’s possible; some folks do also believe in time travel as depicted by everything from H. G. Wells’ The Time Machine to Back to the Future . It’s difficult to find stories online that are actual accounts from real people — many of them are either urban legends ( hi there, Philadelphia Experiment ) or stories that center around people that I’ve been unable to verify actually exist — but if you dig hard enough, sincere accounts can be found.

Are the stories true? Are they false? Are they examples of people who believe with all their heart that they’re true, even if they might not actually be? You be the judge. These seven tales are all excellent yarns, at any rate.

The Moberly–Jourdain Incident

In 1901, two Englishwomen, Anne Moberly and Eleanor Jourdain , took a vacation to France. While they were there, they visited the Palace of Versailles (because, y’know, that’s what one does when one visits France ). And while they were at Versailles, they visited what’s known as the Petit Trianon — a little chateau on the palace grounds that Louis XVI gave to Marie Antoinette as a private space for her to hang out and do whatever it was that a teenaged queen did when she was relaxing back then.

But while they were there, they claimed, they saw some… odd occurrences. They said they spotted people wearing anachronistic clothing, heard mysterious voices, and saw buildings and other structures that were no longer present — and, indeed, hadn’t existed since the late 1700s. Finally, they said, they caught sight of Marie Antoinette herself , drawing in a sketchbook.

They claimed to have fallen into a “time slip” and been briefly transported back more than 100 years before being jolted back to the present by a tour guide.

Did they really travel back in time? Probably not; various explanations include everything from a folie a deux (basically a joint delusion) to a simple misinterpretation of what they actually saw. But for what it’s worth, in 1911 — roughly 10 years after what they said they had experienced occurred — the two women published a book about the whole thing under the names Elizabeth Morison and Frances Lamont simply called An Adventure. These days, it’s available as The Ghosts of Trianon ; check it out, if you like.

The Mystery Of John Titor

John Titor is perhaps the most famous person who claims he’s time traveled; trouble is, no one has heard from him for almost 17 years. Also, he claimed he came from the future.

The story is long and involved, but the short version is this: In a thread begun in the fall of 2000 about time travel paradoxes on the online forum the Time Travel Institute — now known as Curious Cosmos — a user responded to a comment about how a time machine could theoretically be built with the following message:

“Wow! Paul is right on the money. I was just about to give up hope on anyone knowing who Tipler or Kerr was on this worldline.

“By the way, #2 is the correct answer and the basics for time travel start at CERN in about a year and end in 2034 with the first ‘time machine’ built by GE. Too bad we can’t post pictures or I’d show it to you.”

The implication, of course, was that the user, who was going by the name TimeTravel_0, came from a point in the future during which such a machine had already been invented.

Over the course of many messages spanning from that first thread all the way through the early spring of 2001, the user, who became known as John Titor, told his story. He said that he had been sent back to 1975 in order to bring an IBM 5100 computer to his own time; he was just stopping in 2000 for a brief rest on his way back home. The computer, he said, was needed to debug “various legacy computer programs in 2036” in order to combat a known problem similar to Y2K called the Year 2038 Problem . (John didn’t refer to it as such, but he said that UNIX was going to have an issue in 2038 — which is what we thought was going to happen back when the calendar ticked over from 1999 to 2000.)

Opinions are divided on whether John Titor was real ; some folks think he was the only real example of time travel we’ve ever seen, while others think it’s one of the most enduring hoaxes we’ve ever seen. I fall on the side of hoax, but that’s just me.

Project Pegasus And The Chrononauts

In 2011, Andrew D. Basiago and William Stillings stepped forward, claiming that they were former “chrononauts” who had worked with an alleged DARPA program called Project Pegasus. Project Pegasus, they said, had been developed in the 1970s; in 1980, they were taking a “Mars training class” at a community college in California (the college presumably functioning as a cover for the alleged program) when they were picked to go to Mars. The mode of transport? Teleportation.

It gets better, too. Basiago and Stillings also said that the then- 19-year-old Barack Obama , whom they claimed was going by the name “Barry Soetero” at the time, was also one of the students chosen to go to Mars. They said the teleportation occurred via something called a “jump room.”

The White House has denied that Obama has ever been to Mars . “Only if you count watching Marvin the Martian,” Tommy Vietor, then the spokesman for the National Security Council, told Wired’s Danger Room in 2012.

Victor Goddard’s Airfield Time Slip

Like Anne Moberly and Eleanor Jourdain, senior Royal Air Force commander Sir Robert Victor Goddard — widely known as Victor Goddard — claimed to have experienced a time slip.

In 1935, Goddard flew over what had been the RAF station Drem in Scotland on his way from Edinburgh to Andover, England. The Drem station was no longer in use; after demobilization efforts following WWI, it had mostly been left to its own devices. And, indeed, that’s what Goddard said he saw as he flew over it: A largely abandoned airfield.

On his return trip, though, things got… weird. He followed the same route he had on the way there, but during the flight, he got waylaid by a storm. As he struggled to regain control of his plane, however, he spotted the Drem airfield through a break in the clouds — and when he got closer to it, the bad weather suddenly dissipated. But the airfield… wasn’t abandoned this time. It was busy, with several planes on the runway and mechanics scurrying about.

Within seconds, though, the storm reappeared, and Goddard had to fight to keep his plane aloft again. He made it home just fine, and went on to live another 50 years — but the incident stuck with him; indeed, in 1975, he wrote a book called Flight Towards Reality which included discussion of the whole thing.

Here’s the really weird bit: In 1939, the Drem airfield was brought back to life. Did Goddard see a peek into the airfield's future via a time slip back in 1935? Who knows.

Space Barbie

I’ll be honest: I’m not totally sure what to do with thisone — but I’ll present it to you here, and then you can decide for yourself what you think about it. Here it is:

Valeria Lukyanova has made a name for herself as a “human Barbie doll” (who also has kind of scary opinions about some things ) — but a 2012 short documentary for Vice’s My Life Online series also posits that she believes she’s a time traveling space alien whose purpose on Earth is to aid us in moving “from the role of the ‘human consumer’ to the role of ‘human demi-god.’”

What I can’t quite figure out is whether this whole time traveling space alien thing is, like a piece of performance art created specifically for this Vice doc, or whether it’s what she actually thinks. I don’t believe she’s referenced it in many (or maybe even any) other interviews she’s given; the items I’ve found discussing Lukyanova and time travel specifically all point back to this video.

But, well… do with it all as you will. That’s the documentary up there; give it a watch and see what you think.

The Hipster Time Traveler

In the early 2010s, a photograph depicting the 1941 reopening of the South Fork Bridge in Gold Bridge, British Columbia in Canada went viral for seemingly depicting a man that looked… just a bit too modern to have been photographed in 1941. He looks, in fact, like a time traveling hipster : Graphic t-shirt, textured sweater, sunglasses, the works. The photo hadn’t been manipulated; the original can be seen here . So what the heck was going on?

Well, Snopes has plenty of reasonable explanations for the man’s appearance; each item he’s wearing, for example, could very easily have been acquired in 1941. Others have also backed up those facts. But the bottom line is that it’s never been definitively debunked, so the idea that this photograph could depict a man from our time who had traveled back to 1941 persists. What do you think?

Father Ernetti’s Chronovisor

According to two at least two books — Catholic priest Father Francois Brune’s 2002 book Le nouveau mystère du Vatican (in English, The Vatican’s New Mystery ) and Peter Krassa’s 2000 book Father Ernetti's Chronovisor : The Creation and Disappearance of the World's First Time Machine — Father Pellegrino Ernetti, who was a Catholic priest like Brune, invented a machine called a “chronovisor” that allowed him to view the past. Ernetti was real; however, the existence of the machine, or even whether he actually claimed to have invented it, has never been proven. Alas, he died in 1994, so we can’t ask him, either. I mean, if we were ever able to find his chronovisor, maybe we could… but at that point, wouldn’t we already have the information we need?

(I’m extremely skeptical of this story, by the way, but both Brune’s and Krassa’s books swear up, down, left, and right that it’s true, so…you be the judge.)

Although I'm fairly certain that these accounts and stories are either misinterpreted information or straight-up falsehoods, they're still entertaining to read about; after all, if you had access to a time machine, wouldn't you at least want to take it for a spin? Here's hoping that one day, science takes the idea from theory to reality. It's a big ol' universe out there.

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Travel Time

Time spent traveling during normal work hours is considered compensable work time. Time spent in home-to-work travel by an employee in an employer-provided vehicle, or in activities performed by an employee that are incidental to the use of the vehicle for commuting, generally is not "hours worked" and, therefore, does not have to be paid. This provision applies only if the travel is within the normal commuting area for the employer's business and the use of the vehicle is subject to an agreement between the employer and the employee or the employee's representative.

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Handy Reference Guide to the Fair Labor Standards Act - Answers many questions about the FLSA and gives information about certain occupations that are exempt from the Act.

Coverage Under the Fair Labor Standards Act (FLSA) Fact Sheet - General information about who is covered by the FLSA.

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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.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

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How I Became Obsessed With Accidental Time Travel

The web is awash with ordinary peoples’ stories of “time slips.” Their real magic is what they can tell us about our relationship to time.

By Lucie Elven

This year, I turned 30, a development that came with a breathless sense of dread at time’s passing. It wakes me up in the early mornings: Nocturnal terror breaks through the surface of sleep like a whale breaching for air. My ambition and fear kick in together until I get up, pour myself some water and look out the window at the squid-ink sky and the string of lights along my neighbors’ houses. I lie down again after finding firmer mental ground, dry land.

So when a guy that my friend was seeing evangelized about “time slips” — a genre of urban legend in which people claim that, while walking in particular places, they accidentally traveled back, and sometimes forward, in time — I was a ripe target. Curious and increasingly existential, I Googled these supposed time slips. I found a global community of believers building an archive of temporal dislocations from the present. These congregants gathered in corners of the internet to testify about how, in the right conditions, the dusting of alienation that settles over the world as we age can crystallize into collective fiction.

I was initially skeptical of the vague language that time-slip writers employed to convey experiences I already found dubious: too many uses of foggy words like “blunder” and “sporting”; detail lavished on varieties of hats encountered. But I was drawn in by their secretive tone — I sensed that sharing these anecdotes was compromising, even shameful (“People would laugh at you,” one poster wrote). Disapproval became attraction, and I returned to the message boards throughout the summer.

Here’s a classic that, like the best of these stories, was related secondhand on a paranormal blog: In a Liverpudlian street in 1996, an off-duty policeman named Frank was going to meet his wife, Carol, in a bookshop called Dillons when “suddenly, a small box van that looked like something out of the 1950s sped across his path, honking its horn as it narrowly missed him.” More disorienting still, Frank “saw that Dillons book store now had ‘Cripps’ over its entrance” and that there were stands of shoes and handbags in the window instead of new fiction. The only other person not wearing midcentury dress was a girl in a lime green sleeveless top. As Frank followed her into the old women’s wear boutique, “the interior of the building completely changed in a flash”; it was once again a bookshop.

I found a global community of believers building an archive of temporal dislocations from the present.

As with a spell of déjà vu, the experience was short-lived, and time was regained. According to the blogger’s detective-like report, Cripps “was later determined” to have been a business in the 1950s. In response to Frank’s slip, posters have told their own or related accounts they’ve heard from others: “This happened to my ex-boss, Glyn Jackson in London, England,” one begins. “Glyn’s story is Highly believable as Glyn is person who lacks imagination on such a scale that he could not put together a grade one story for English to save his life.” And on it goes.

I have never appreciated stories about the passage of time. I resent that I won’t ever get back the hours of my life that Richard Linklater stole with “Boyhood” — his two-and-three-quarter-hour film, shot over a 12-year period in which time is the force that overwhelms everything, not least the idea that our own actions drive our life stories. There’s a whole lot of unwelcome profundity there.

Time-slip anecdotes, though fashioned out of the ambient dread of living with the ticking clock, are childlike in their sense of wonder. They are light, playful and irrational, as frivolous and folky as a ghost story if it were narrated by the confused ghost instead of the people it haunts. One poster, as a girl, used to see a woman in a blue bathrobe in her room: “Her hair was long and messy, a reddish brown. I didn’t see her face because she was usually turned away. I used to mistake her for my mom.” Years later, grown up, the poster’s daughter slept in her former bedroom. “One day I realized ... I was wearing the same blue bathrobe,” the mother writes. Paranormal trappings aside, this story speaks to the feeling of whiplash brought on by time’s passing.

Slipping can be significant, as any Freudian will tell you, and these narratives are riddles whose answers might tell us about our relationship to time. I have begun considering the message boards on which they are exchanged to be narrow but important release valves, allowing posters to talk about the feelings that arise from being time-bound: depression, midlife crises, the dysmorphia of living in a human body. What ailed Miss Smith, whose car slid into a ditch after a cocktail party, and who witnessed “groups of Pictish warriors of the late seventh century, ca. 685 AD,” if not an understanding of her smallness in history’s vast expanse? Why did two academics, famous in the time-slip community for writing a book about spotting Marie Antoinette in the Versailles grounds, encounter trees that looked lifeless, “like wood worked in tapestry”? Perhaps in that instant, like the last queen of France’s Ancien Régime, they felt radically out of joint with their present moment.

If you suspend disbelief, you’ll find these threads constitute a philosophical inquiry about the place of the spirit in our physical beings. They debate the merits of subjectivity and objectivity and question the idea that time is a one-lane highway to death. These writers argue that our past and future can suffuse our present, unveiling an epic dimension of our quotidian existences in moments when we slip and, like Frank, feel eternity.

Lucie Elven is a writer whose first book of fiction, “The Weak Spot,” was published this year in the United States by Soft Skull Press and in Britain by Prototype.

Background photograph: George Marks/Getty Images

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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!

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.

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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. 

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Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

• If time travel were ever to occur, we should see extensive uncaused correlations.
• It is extremely unlikely that we should ever see extensive uncaused correlations.
• Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

• Are the past, present and future equally real?
• Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

• the block universe view: eternalism + B-theory
• the moving spotlight view: eternalism + A-theory
• the presentist view: nowism + A-theory
• the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

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• Markosian, Ned, 2004, “Two arguments from Sider’s Four-Dimensionalism ”, Philosophy and Phenomenological Research , 68: 665–73.
• Markosian, Ned, 2020, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio , 12: 137–65.
• Maudlin, Tim, 2012, Philosophy of Physics: Space and Time , Princeton: Princeton University Press.
• Meiland, Jack W., 1974, “A two-dimensional passage model of time for time travel”, Philosophical Studies , 26: 153–73.
• Mellor, D.H., 1998, Real Time II , London: Routledge.
• Meyer, Ulrich, 2012, “Explaining causal loops”, Analysis , 72: 259–64.
• Miller, Kristie, 2005, “Time travel and the open future”, Disputatio , 1: 223–32.
• –––, 2006, “Travelling in time: How to wholly exist in two places at the same time”, Canadian Journal of Philosophy , 36: 309–34.
• –––, 2008, “Backwards causation, time, and the open future”, Metaphysica , 9: 173–91.
• Monton, Bradley, 2003, “Presentists can believe in closed timelike curves”, Analysis , 63: 199–202.
• –––, 2009, “Time travel without causal loops”, Philosophical Quarterly , 59: 54–67.
• Nerlich, Graham, 1981, “Can time be finite?”, Pacific Philosophical Quarterly , 62: 227–39.
• Ney, S.E., 2000, “Are grandfathers an endangered species?”, Journal of Philosophical Research , 25: 311–21.
• Price, Huw, 1996, Time’s Arrow & Archimedes’ Point: New Directions for the Physics of Time , New York: Oxford University Press.
• Putnam, Hilary, 1975, “It ain’t necessarily so”, in Mathematics, Matter and Method , Cambridge: Cambridge University Press, vol. 1 of Philosophical Papers , 237–49.
• Reinganum, Marc R., 1986, “Is time travel impossible? A financial proof”, Journal of Portfolio Management , 13: 10–2.
• Riggs, Peter J., 1991, “A critique of Mellor’s argument against ‘backwards’ causation”, British Journal for the Philosophy of Science , 42: 75–86.
• –––, 1997, “The principal paradox of time travel”, Ratio , 10: 48–64.
• Savitt, Steven, 1994, “The replacement of time”, Australasian Journal of Philosophy , 74: 463–73.
• –––, 2005, “Time travel and becoming”, Monist (Special Issue on Time Travel), 88: 413–22.
• Sider, Theodore, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Clarendon Press.
• –––, 2002, “Time travel, coincidences and counterfactuals”, Philosophical Studies , 110: 115–38.
• –––, 2004, “Replies to Gallois, Hirsch and Markosian”, Philosophy and Phenomenological Research , 68: 674–87.
• –––, 2005, “Traveling in A- and B- time”, Monist (Special Issue on Time Travel), 88: 329–35.
• Simon, Jonathan, 2005, “Is time travel a problem for the three-dimensionalist?”, Monist (Special Issue on Time Travel), 88: 353–61.
• Slater, Matthew H., 2005, “The necessity of time travel (on pain of indeterminacy)”, Monist (Special Issue on Time Travel), 88: 362–9.
• Smart, J.J.C., 1963, “Is time travel possible?”, Journal of Philosophy , 60: 237–41.
• Smeenk, Chris and Wüthrich, Christian, 2011, “Time travel and time machines”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press, online ed. doi:10.1093/oxfordhb/9780199298204.003.0021
• Smith, Joseph Wayne, 1985, “Time travel and backward causation”, Cogito , 3: 57–67.
• Smith, Nicholas J.J., 1997, “Bananas enough for time travel?”, British Journal for the Philosophy of Science , 48: 363–89.
• –––, 1998, “The problems of backward time travel”, Endeavour , 22(4): 156–8.
• –––, 2004, “Review of Robin Le Poidevin Travels in Four Dimensions: The Enigmas of Space and Time ”, Australasian Journal of Philosophy , 82: 527–30.
• –––, 2005, “Why would time travellers try to kill their younger selves?”, Monist (Special Issue on Time Travel), 88: 388–95.
• –––, 2011, “Inconsistency in the A-theory”, Philosophical Studies , 156: 231–47.
• –––, 2015, “Why time travellers (still) cannot change the past”, Revista Portuguesa de Filosofia , 71: 677–94.
• –––, 2017, “I’d do anything to change the past (but I can’t do ‘that’)”, American Philosophical Quarterly , 54: 153–68.
• van Inwagen, Peter, 2010, “Changing the past”, in Oxford Studies in Metaphysics (Volume 5), Dean W. Zimmerman (ed.), Oxford: Oxford University Press, 3–28.
• Vihvelin, Kadri, 1996, “What time travelers cannot do”, Philosophical Studies , 81: 315–30.
• Vranas, Peter B.M., 2005, “Do cry over spilt milk: Possibly you can change the past”, Monist (Special Issue on Time Travel), 88: 370–87.
• –––, 2009, “Can I kill my younger self? Time travel and the retrosuicide paradox”, Pacific Philosophical Quarterly , 90: 520–34.
• –––, 2010, “What time travelers may be able to do”, Philosophical Studies , 150: 115–21.
• Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press.
• Williams, Donald C., 1951, “The myth of passage”, Journal of Philosophy , 48: 457–72.
• Wright, John, 2006, “Personal identity, fission and time travel”, Philosophia , 34: 129–42.
• Yourgrau, Palle, 1999, Gödel Meets Einstein: Time Travel in the Gödel Universe , Chicago: Open Court.

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.

• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

causation: backward | free will: divine foreknowledge and | identity: over time | location and mereology | temporal parts | time | time machines | time travel: and modern physics

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Travel Policy Nonexempt Employees

The purpose of this policy is to outline pay rules that apply to nonexempt employees when traveling on company business.

Employees in positions classified as nonexempt (overtime eligible) under the Fair Labor Standards Act may be eligible for compensation for the time they spend traveling. The compensation an employee receives depends on the kind of travel and whether the travel time takes place within normal work hours.

"Normal work hours," for the purposes of this policy, are defined as an employee's regularly scheduled work hours (e.g., 8:30 a.m. to 5:30 p.m.). This definition applies to normal workdays (Monday through Friday) and to weekends (Saturday and Sunday). Employees with variable work hours will have their normal work hours defined by human resources prior to travel, based on a review of time records over the previous month.

  "Travel time" is defined according to the type of travel involved:

Travel for a one-day assignment in another city : An employee who regularly works at a fixed location and is given a special one-day assignment in another city and returns home the same day will be paid for the time spent traveling to and from the other city, except for the time the employee would normally spend commuting to and from the regular worksite.

Travel during the workday : Time spent by an employee traveling as part of his or her regular job duties, such as travel from jobsite to jobsite during the workday, is work time and will be paid as such.

Travel away from home : Travel that keeps an employee away from home overnight is travel away from home. Travel time that takes place within normal work hours, regardless of the day of the week, is treated as work hours. When an employee travels between time zones, the time zone associated with the point of departure should be used to determine whether the travel falls within normal work hours. Time spent traveling from home to an airport terminal or train station is considered commute time and is not treated as hours worked. Time spent waiting at a terminal until arrival at the destination is compensable when it falls during normal work hours.

Travel time as the driver of an automobile : All authorized travel time spent driving an automobile (as the driver, not as a passenger) is treated as work hours, regardless of whether the travel takes place within normal work hours or outside normal work hours.

If an employee requests a specific travel itinerary or mode of transportation that is different from the one authorized by the company, only the estimated travel time associated with the itinerary and mode of transportation that has been authorized will be eligible for compensation. For example, if an employee drives a car as a matter of personal preference when an authorized flight or other travel mode is available, and the travel time by car would exceed that of the authorized mode, only the estimated travel time associated with the authorized mode will be compensated.

Travel time as a  passenger  in an automobile is not automatically treated as work hours. Travel as a passenger in an automobile is treated the same as all other forms of travel.

Calculating and Reporting Travel Time

Employees are responsible for accurately tracking, calculating and reporting travel time on their time sheets in accordance with this policy.

Meal periods should be deducted from all travel time.

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Einstein's equation and time travel

How is Einstein's equation (Gμν = 8πG Tμν) actually applied? And how does it support the theory of time travel.

Many physicists refer to this as Einstein’s Equations (plural) because it's actually a set of several equations. The symbol Gμν denotes the "Einstein tensor," which is a measure of how much space-time is curving. The symbol Tμν denotes the "energy momentum tensor," which measures the density and flux of the energy and momentum of matter. The energy and momentum of matter causes space-time to curve in a way that is described by Einstein’s Equations. The curvature of space-time is what causes all the effects that we associate with "gravity."

There are many textbooks that explain how to apply Einstein’s Equations equations to various situations, but they require a very advanced background in physics and mathematics. I will therefore have to summarize things in words.

Before talking about time travel, let me first explain what a "world-line" is. Suppose that an object at a particular time is sitting at some particular point in space. At another time, it will be sitting at another point. Its locations at different times trace out a path through space-time, which is called the object’s world line. This path extends forward to points farther and farther in the future (until the object ceases to exist for some reason, at which point its world-line ends). But imagine that an object’s world-line bends around in a loop, so that at it arrives at a point in space-time where it had already been? For example, there may be a point on my world line that was me at my fifth birthday party at my parents’ home in New York City. Another point (farther along my world-line) is me now, aged 61, sitting in my office at the University of Delaware. If I continue farther and farther along my world line, might I end up again at my fifth birthday party in New York City in 1958? Could my five-year old self meet my much older self at that party? That is what most people mean by "time travel." In the jargon of physics, such a world line that bent around and intersected itself is called a "closed time-like loop" or "closed time-like curve."

The question is whether world-lines that are closed time-like loops are possible in the real world. It is comparatively easy to show that if space-time were not curved world-lines would definitely not be able to bend around to form closed time-like loops. But one might hope that gravity, by curving space-time, could bend some world-lines around to make closed loops. The way people have studied this question is to assume that Einstein’s equations correctly describe how the energy and momentum of matter curves space-time. Then they look for solutions of those equations where space-time curves in such a way as to make closed time-like loops possible, and therefore time-travel possible. No one has ever found such solutions. People have on occasion found what seemed to be such solutions, but on closer inspection it was found that when realistic conditions are imposed, no time travel is possible. An example is the Gödel Universe (or Gödel metric).

One kind of solution that would allow time travel is called a "Minkowski wormhole." A Minkowski wormhole would be like a tunnel that took you from one point in space-time to what seemed like a far distant point--a point far away in space, or in the past, or in the future. But people have shown that for a Minkowski wormhole to exist, there would have to be a type of matter whose energy and momentum were unlike any kind of matter ever seen and extremely unlikely to exist in the real world.

The great majority of experts believe that time travel is not allowed by the laws of physics. But no one has proved that rigorously. If you want to learn more, you could try to find discussions of closed time-like loops and Minkowski wormholes in books or on the internet.

-Stephen Barr

• General relativity

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Compensatory Time Off for Travel - Questions & Answers to Fact Sheet

• Q1. What is compensatory time off for travel? View more A. Compensatory time off for travel is a separate form of compensatory time off that may be earned by an employee for time spent in a travel status away from the employee's official duty station when such time is not otherwise compensable.
• Q2. Are all employees covered by this provision? View more A. The compensatory time off provision applies to an "employee" as defined in 5 U.S.C. 5541(2) who is employed in an "Executive agency" as defined in 5 U.S.C. 105, without regard to whether the employee is exempt from or covered by the overtime pay provisions of the Fair Labor Standards Act of 1938, as amended. For example, this includes employees in senior-level (SL) and scientific or professional (ST) positions, but not members of the Senior Executive Service or Senior Foreign Service or Foreign Service officers. Effective April 27, 2008, prevailing rate (wage) employees are covered under the compensatory time off for travel provision. (See CPM 2008-04 .)
• Q3. Are intermittent employees eligible to earn compensatory time off for travel? View more A. No. Compensatory time off for travel may be used by an employee when the employee is granted time off from his or her scheduled tour of duty established for leave purposes. (See 5 CFR 550.1406(b).) Also see the definition of "scheduled tour of duty for leave purposes" in 5 CFR 550.1403. Employees who are on intermittent work schedules are not eligible to earn and use compensatory time off for travel because they do not have a scheduled tour of duty for leave purposes.
• Q4. What qualifies as travel for the purpose of this provision? View more A. To qualify for this purpose, travel must be officially authorized. In other words, travel must be for work purposes and must be approved by an authorized agency official or otherwise authorized under established agency policies. (Also see Q5.)
• Q5. May an employee earn compensatory time off when he or she travels in conjunction with the performance of union representational duties? View more A. No. The term "travel" is defined at 5 CFR 550.1403 to mean officially authorized travel—i.e., travel for work purposes approved by an authorized agency official or otherwise authorized under established agency policies. The definition specifically excludes time spent traveling in connection with union activities. The term "travel for work purposes" is intended to mean travel for agency-related work purposes. Thus, employees who travel in connection with union activities are not entitled to earn compensatory time off for travel because they are traveling for the benefit of the union, and not for agency-related work purposes.
• Q6. An employee receives compensatory time off for travel only for those hours spent in a travel status. What qualifies as time in a travel status? View more A. Travel status includes only the time actually spent traveling between the official duty station and a temporary duty station, or between two temporary duty stations, and the usual waiting time that precedes or interrupts such travel.
• Q7. Is travel in connection with a permanent change of station (PCS) creditable for compensatory time off for travel? View more A. Although PCS travel is officially authorized travel, it is not travel between an official duty station and a temporary duty station or between two temporary duty stations. Therefore, it is not considered time in a travel status for the purpose of earning compensatory time off for travel.
• Q8. What is meant by "usual waiting time"? View more A. Airline travelers generally are required to arrive at the airport at a designated pre-departure time (e.g., 1 or 2 hours before the scheduled departure, depending on whether the flight is domestic or international). Such waiting time at the airport is considered usual waiting time and is creditable time in a travel status. In addition, time spent at an intervening airport waiting for a connecting flight (e.g., 1 or 2 hours) also is creditable time in a travel status. In all cases, determinations regarding what is creditable as "usual waiting time" are within the sole and exclusive discretion of the employing agency.
• Q9. What if an employee experiences an "extended" waiting period? View more A. If an employee experiences an unusually long wait prior to his or her initial departure or between actual periods of travel during which the employee is free to rest, sleep, or otherwise use the time for his or her own purposes, the extended waiting time outside the employee's regular working hours is not creditable time in a travel status. An extended waiting period that occurs during an employee's regular working hours is compensable as part of the employee's regularly scheduled administrative workweek.
• Q10. Do meal periods count as time in a travel status? View more A. Meal periods during actual travel time or waiting time are not specifically excluded from creditable time in a travel status for the purpose of earning compensatory time off for travel. However, determinations regarding what is creditable as "usual waiting time" are within the sole and exclusive discretion of the employing agency.
• Q11. What happens once an employee reaches a temporary duty station? View more A. Time spent at a temporary duty station between arrival and departure is not creditable travel time for the purpose of earning compensatory time off for travel. Time in a travel status ends when the employee arrives at the temporary duty worksite or his or her lodging in the temporary duty station, wherever the employee arrives first. Time in a travel status resumes when an employee departs from the temporary duty worksite or his or her lodging in the temporary duty station, wherever the employee departs last.
• Q12. When is it appropriate for an agency to offset creditable time in a travel status by the amount of time the employee spends in normal commuting between home and work? View more A. If an employee travels directly between his or her home and a temporary duty station outside the limits of the employee's official duty station (e.g., driving to and from a 3-day conference), the agency must deduct the employee's normal home-to-work/work-to-home commuting time from the creditable travel time. The agency must also deduct an employee's normal commuting time from the creditable travel time if the employee is required—outside of regular working hours—to travel between home and a transportation terminal (e.g., an airport or train station) outside the limits of the employee's official duty station.
• Q13. What if an employee travels to a transportation terminal within the limits of his or her official duty station? View more A. An employee's time spent traveling outside of regular working hours to or from a transportation terminal within the limits of his or her official duty station is considered equivalent to commuting time and is not creditable time in a travel status for the purpose of earning compensatory time off for travel.
• Q14. What if an employee travels from a worksite to a transportation terminal? View more A. If an employee travels between a worksite and a transportation terminal, the travel time outside regular working hours is creditable as time in a travel status, and no commuting time offset applies. For example, after completing his or her workday, an employee may travel directly from the regular worksite to an airport to attend an out-of-town meeting the following morning. The travel time between the regular worksite and the airport is creditable as time in a travel status.
• Q15. What if an employee elects to travel at a time other than the time selected by the agency? View more A. When an employee travels at a time other than the time selected by the agency, the agency must determine the estimated amount of time in a travel status the employee would have had if the employee had traveled at the time selected by the agency. The agency must credit the employee with the lesser of (1) the estimated time in a travel status the employee would have had if the employee had traveled at the time selected by the agency, or (2) the employee's actual time in a travel status at a time other than that selected by the agency.
• Q16. How is an employee's travel time calculated for the purpose of earning compensatory time off for travel when the travel involves two or more time zones? View more A. When an employee's travel involves two or more time zones, the time zone from point of first departure must be used to determine how many hours the employee actually spent in a travel status for the purpose of accruing compensatory time off for travel. For example, if an employee travels from his official duty station in Washington, DC, to a temporary duty station in San Francisco, CA, the Washington, DC, time zone must be used to determine how many hours the employee spent in a travel status. However, on the return trip to Washington, DC, the time zone from San Francisco, CA, must be used to calculate how many hours the employee spent in a travel status.
• Q17. How is compensatory time off for travel earned and credited? View more A. Compensatory time off for travel is earned for qualifying time in a travel status. Agencies may authorize credit in increments of one-tenth of an hour (6 minutes) or one-quarter of an hour (15 minutes). Agencies must track and manage compensatory time off for travel separately from other forms of compensatory time off.
• Q18. Is there a limitation on the amount of compensatory time off for travel an employee may earn? View more A. No.
• Q19. How does an employee request credit for compensatory time off for travel? View more A. Agencies may establish procedures for requesting credit for compensatory time off for travel. An employee must comply with his or her agency's procedures for requesting credit of compensatory time off, and the employee must file a request for such credit within the time period established by the agency. An employee's request for credit of compensatory time off for travel may be denied if the request is not filed within the time period required by the agency.
• Q20. Is there a form employees must fill out for requests to earn or use compensatory time off for travel? View more A. There is not a Governmentwide form used for requests to earn or use compensatory time off for travel. However, an agency may choose to develop a form as part of its internal policies and procedures.
• Q21. How does an employee use accrued compensatory time off for travel? View more A. An employee must request permission from his or her supervisor to schedule the use of his or her accrued compensatory time off for travel in accordance with agency policies and procedures. Compensatory time off for travel may be used when the employee is granted time off from his or her scheduled tour of duty established for leave purposes. Employees must use accrued compensatory time off for travel in increments of one-tenth of an hour (6 minutes) or one-quarter of an hour (15 minutes).
• Q22. In what order should agencies charge compensatory time off for travel? View more A. Agencies must charge compensatory time off for travel in the chronological order in which it was earned, with compensatory time off for travel earned first being charged first.
• Q23. How long does an employee have to use accrued compensatory time off for travel? View more A. An employee must use his or her accrued compensatory time off for travel by the end of the 26th pay period after the pay period during which it was earned or the employee must forfeit such compensatory time off, except in certain circumstances. (See Q24 and Q25 for exceptions.)
• Q24. What if an employee is unable to use his or her accrued compensatory time off for travel because of uniformed service or an on-the-job injury with entitlement to injury compensation? View more A. Unused compensatory time off for travel will be held in abeyance for an employee who separates, or is placed in a leave without pay status, and later returns following (1) separation or leave without pay to perform service in the uniformed services (as defined in 38 U.S.C. 4303 and 5 CFR 353.102) and a return to service through the exercise of a reemployment right or (2) separation or leave without pay due to an on-the-job injury with entitlement to injury compensation under 5 U.S.C. chapter 81. The employee must use all of the compensatory time off for travel held in abeyance by the end of the 26th pay period following the pay period in which the employee returns to duty, or such compensatory time off for travel will be forfeited.
• Q25. What if an employee is unable to use his or her accrued compensatory time off for travel because of an exigency of the service beyond the employee's control? View more A. If an employee fails to use his or her accrued compensatory time off for travel before the end of the 26th pay period after the pay period during which it was earned due to an exigency of the service beyond the employee's control, the head of an agency, at his or her sole and exclusive discretion, may extend the time limit for up to an additional 26 pay periods.
• Q26. May unused compensatory time off for travel be restored if an employee does not use it by the end of the 26th pay period after the pay period during which it was earned? View more A. Except in certain circumstances (see Q24 and Q25), any compensatory time off for travel not used by the end of the 26th pay period after the pay period during which it was earned must be forfeited.
• Q27. What happens to an employee's unused compensatory time off for travel upon separation from Federal service? View more A. Except in certain circumstances (see Q24), an employee must forfeit all unused compensatory time off for travel upon separation from Federal service.
• Q28. May an employee receive a lump-sum payment for accrued compensatory time off for travel upon separation from an agency? View more A. No. The law prohibits payment for unused compensatory time off for travel under any circumstances.
• Q29. What happens to an employee's accrued compensatory time off for travel upon transfer to another agency? View more A. When an employee voluntarily transfers to another agency (including a promotion or change to lower grade action), the employee must forfeit all of his or her unused compensatory time off for travel.
• Q30. What happens to an employee's accrued compensatory time off for travel when the employee moves to a position that is not covered by the regulations in 5 CFR part 550, subpart N? View more A. When an employee moves to a position in an agency not covered by the compensatory time off for travel provisions (e.g., the United States Postal Service), the employee must forfeit all of his or her unused compensatory time off for travel. However, the gaining agency may use its own legal authority to give the employee credit for such compensatory time off.
• Q31. Is compensatory time off for travel considered in applying the premium pay and aggregate pay caps? View more A. No. Compensatory time off for travel may not be considered in applying the biweekly or annual premium pay limitations established under 5 U.S.C. 5547 or the aggregate limitation on pay established under 5 U.S.C. 5307.
• Q32. When are criminal investigators who receive availability pay precluded from earning compensatory time off for travel? View more A. Compensatory time off for travel is earned only for hours not otherwise compensable. The term "compensable" is defined at 5 CFR 550.1403 to include any hours of a type creditable under other compensation provisions, even if there are compensation caps limiting the payment of premium pay for those hours (e.g., the 25 percent cap on availability pay and the biweekly premium pay cap). For availability pay recipients, this means hours of travel are not creditable as time in a travel status for compensatory time off purposes if the hours are (1) compensated by basic pay, (2) regularly scheduled overtime hours creditable under 5 U.S.C. 5542, or (3) "unscheduled duty hours" as described in 5 CFR 550.182(a), (c), and (d).
• Q33. What constitutes "unscheduled duty hours" as described in 5 CFR 550.182(a), (c), and (d)? View more A. Under the availability pay regulations, unscheduled duty hours include (1) all irregular overtime hours—i.e., overtime work not scheduled in advance of the employee's administrative workweek, (2) the first 2 overtime hours on any day containing part of the employee's basic 40-hour workweek, without regard to whether the hours are unscheduled or regularly scheduled, and (3) any approved nonwork availability hours. However, special agents in the Diplomatic Security Service of the Department of State may count only hours actually worked as unscheduled duty hours.
• Q34. Why are criminal investigators who receive availability pay precluded from earning compensatory time off when they travel during unscheduled duty hours? View more A. The purpose of availability pay is to ensure the availability of criminal investigators (and certain similar law enforcement employees) for unscheduled duty in excess of a 40-hour workweek based on the needs of the employing agency. Availability pay compensates an employee for all unscheduled duty hours. Compensatory time off for travel is earned only for hours not otherwise compensable. Thus, availability pay recipients may not earn compensatory time off for travel during unscheduled duty hours because the employees are entitled to availability pay for those hours.

A. When an employee who receives availability pay is required to travel on a non-workday or on a regular workday (during hours that exceed the employee's basic 8-hour workday), and the travel does not meet one of the four criteria in 5 U.S.C. 5542(b)(2)(B) and 5 CFR 550.112(g)(2), the travel time is not compensable as overtime hours of work under regular overtime or availability pay. Thus, the employee may earn compensatory time off for such travel, subject to the exclusion specified in 5 CFR 550.1404(b)(2) and the requirements in 5 CFR 550.1404(c),(d), and (e).

Under the provisions in 5 U.S.C. 5542(b)(2)(B) and 5 CFR 550.112(g)(2), travel time is compensable as overtime hours of work if the travel is away from the employee's official duty station and—

(i) involves the performance of work while traveling, (ii) is incident to travel that involves the performance of work while traveling, (iii) is carried out under arduous conditions, or (iv) results from an event which could not be scheduled or controlled administratively.

The phrase "an event which could not be scheduled or controlled administratively" refers to the ability of an agency in the Executive Branch of the United States Government to control the scheduling of an event which necessitates an employee's travel. If the employing agency or another Executive Branch agency has any control over the scheduling of the event, including by means of approval of a contract for it, then the event is administratively controllable, and the travel to and from the event cannot be credited as overtime hours of work.

For example, an interagency conference sponsored by the Department of Justice would be considered a joint endeavor of the participating Executive Branch agencies and within their administrative control. Under these circumstances, the travel time outside an employee's regular working hours is not compensable as overtime hours of work under regular overtime or availability pay. Therefore, the employee may earn compensatory time off for such travel, subject to the exclusion specified in 5 CFR 550.1404(b)(2) and the requirements in 5 CFR 550.1404(c), (d), and (e).

• Q36. If an employee is required to travel on a Federal holiday (or an "in lieu of" holiday), is the employee entitled to receive compensatory time off for travel? View more A. Although most employees do not receive holiday premium pay for time spent traveling on a holiday (or an "in lieu of" holiday), an employee continues to be entitled to pay for the holiday in the same manner as if the travel were not required. Thus, an employee may not earn compensatory time off for travel during basic (non-overtime) holiday hours because the employee is entitled to his or her rate of basic pay for those hours. Compensatory time off for travel may be earned by an employee only for time spent in a travel status away from the employee's official duty station when such time is not otherwise compensable.
• Q37. If an employee's regularly scheduled tour of duty is Sunday through Thursday and the employee is required to travel on a Sunday during regular working hours, is the employee entitled to earn compensatory time off for travel? View more A. No. Compensatory time off for travel may be earned by an employee only for time spent in a travel status away from the employee's official duty station when such time is not otherwise compensable. Thus, an employee may not earn compensatory time off for travel for traveling on a workday during regular working hours because the employee is receiving his or her rate of basic pay for those hours.
• Q38. May an agency change an employee's work schedule for travel purposes? View more A. An agency may not adjust the regularly scheduled administrative workweek that normally applies to an employee (part-time or full-time) solely for the purpose of including planned travel time not otherwise considered compensable hours of work. However, an employee is entitled to earn compensatory time off for travel for time spent in a travel status when such time is not otherwise compensable.
• Q39. Is time spent traveling creditable as credit hours for an employee who is authorized to earn credit hours under an alternative work schedule? View more A. Credit hours are hours an employee elects to work, with supervisory approval, in excess of the employee's basic work requirement under a flexible work schedule. Under certain conditions, an agency may permit an employee to earn credit hours by performing productive and essential work while in a travel status. See OPM's fact sheet on credit hours  for the conditions that must be met. If those conditions are met and the employee does earn credit hours for travel, the time spent traveling would be compensable and the employee would not be eligible to earn compensatory time off for travel. If the conditions are not met, the employee would be eligible to earn compensatory time off for travel.
• Q40. May an agency restore an employee's forfeited "use-or-lose" annual leave because the employee elected to use earned compensatory time off for travel instead of using his or her excess annual leave? View more A. Section 6304(d) of title 5, United States Code, prescribes the conditions under which an employee's forfeited annual leave may be restored to an employee. (See fact sheet on restoration of annual leave .) There is no legal authority to restore an employee's forfeited annual leave because the employee elected to use earned compensatory time off for travel instead of using his or her excess annual leave.

A. No. Compensatory time off for travel may be earned by an employee only for time spent in a travel status away from the employee's official duty station when such time is not otherwise compensable. The term "compensable" is defined at 5 CFR 550.1403 to make clear what periods of time are "not otherwise compensable" and thus potentially creditable for the purpose of earning compensatory time off for travel. Time is considered compensable if the time is creditable as hours of work for the purpose of determining a specific pay entitlement (e.g., overtime pay for travel meeting one of the four criteria in 5 CFR 550.112(g)(2)) even when the time may not actually generate additional compensation because of applicable pay limitations (e.g., biweekly premium pay cap). The capped premium pay is considered complete compensation for all hours of work creditable under the premium pay provisions.

In other words, even though an employee may not receive overtime pay for all of his or her travel hours because of the biweekly premium pay cap, all of the travel time is still considered to be compensable under 5 CFR 550.112(g)(2). Under these circumstances, the employee has been compensated fully under the law for all of the travel hours and the employee may not earn compensatory time off for any portion of such travel not generating additional compensation because of the biweekly cap on premium pay.

• Q42. May an employee who receives administratively uncontrollable overtime (AUO) pay under 5 U.S.C. 5545(c)(2) earn compensatory time off for travel? View more A. If such employee's travel time is not compensable under 5 CFR 550.112(g) or 5 CFR 551.422, as applicable, and meets the requirements in 5 CFR part 550, subpart N, the employee is eligible to earn compensatory time off for travel for time spent in a travel status.
• Q43. If a part-time employee's regularly scheduled tour of duty is Monday through Friday, 8:00 a.m. to 2:30 p.m., and the employee is required to travel on a Friday from 2:30 p.m. to 4:30 p.m., is the employee entitled to earn compensatory time off for travel for those 2 hours? View more A. It depends. If the travel qualifies as compensable hours of work under 5 U.S.C. 5542(b)(2)(B) and 5 CFR 550.112(g)(2)—i.e., the travel involves or is incident to the performance of actual work, is carried out under arduous and unusual conditions, or results from an event which could not be scheduled or controlled administratively—the employee may not be credited with compensatory time off for travel hours. (Such travel time outside a part-time employee's scheduled tour of duty, but not in excess of 8 hours in a day or 40 hours in a week, would be non-overtime hours of work compensated at the employee's rate of basic pay.) If the travel time does not qualify as compensable hours of work and meets the other requirements in 5 CFR part 550, subpart N, the part-time employee would be entitled to earn compensatory time off for those 2 hours. We note travel time is always compensable hours of work if it falls within an employee's regularly scheduled administrative workweek. (See 5 U.S.C. 5542(b)(2)(A) and 5 CFR 550.112(g)(1).) For a part-time employee, the regularly scheduled administrative workweek is defined in 5 CFR 550.103 as the officially prescribed days and hours within an administrative workweek during which the employee was scheduled to work in advance of the workweek. An agency may not adjust the regularly scheduled administrative workweek normally applied to an employee (part-time or full-time) solely for the purpose of including planned travel time otherwise not considered compensable hours of work.
• Q44. Does an upgrade in travel accommodations impact an employee's entitlement to compensatory time off for travel? View more A. Allowing an employee to upgrade his or her travel accommodations (e.g., to business class) does not eliminate his or her eligibility to earn compensatory time off for travel.

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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.

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the eight types of time travel

Time travel is a stable in science fiction. Countless books, comics, movies, and TV shows have used it as their main plot device. Even more have incorporated it into a key moment of the story. Over the years, eight major types of time travel logic emerged. Recently, YouTubers Eric Voss and Héctor Navarro examined all eight types, and looked at which one gets it most correct in term of the real world science behind science fiction.

Type 1 Anything goes

Definition: Characters travel back and forth within their historical timeline.

This approach frees you to have fun and not get lost in the minutiae of how time travel works. Usually, there’s a magical Maguffin that to quote the great Dr. Ememett Brown, “makes time travel possible”. Writers have used a car, a phone booth, and a hot tub, among other options. This approach leads to inconsistent limits on the logic of the time travel, but this doesn’t mean the story is poorly plotted, won’t be enjoyable or won’t be an enormous hit. This approach is more science fantasy than science fiction with no basis in real-world science.

Examples: Back to the Future , Bill and Ted’s Excellent Adventure , Hot Tube Time Machine , Frequency , Austin Powers , Men In Black 3 , Deadpool 2 , The Simpsons , Galaxy Quest , Star Trek TOS , Doctor Who , 11/22/63 by Stephen King.

Type 2 Branch Reality

Definition: Changes to the past don’t rewrite history. They split the timeline into an alternate branch timeline. This action does not change or erase the original timeline.

As authors got more familiar with the science behind time travel in theoretical physics, this type, based upon the many worlds theory in quantum mechanics, emerged. When the character travels back into the past and changes events, they create a new reality. Their original reality is unchanged. Branches themselves can branch leading to a multiverse of possibilities.

Examples: The Disney Plus series, Loki , used this extensively. See also: Back to the Future Part II , Avenger’s Endgame , the DC Comics multiverse, the Marvel Comics multiverse, Rick and Morty , Star Trek (2009), A Wrinkle in Time by Madeleine L’Engle.

Type 3 Time Dilation

Definition: Characters traveling off-world experience time moving more slowly than elsewhere in the universe, allowing them to move forward in time (but not backward).

This type is the based upon our scientific understanding of how time slows down as you approach the speed of the light. This is a forward-only type of time travel. There’s no going backwards.

Examples: Planet of the Apes , Ender’s Game , Flight of the Navigator , Interstellar , Buck Rodgers .

Type 4 This Always Happened

Definition: All of time is fixed on a predestined loop in which the very act of time travel itself sets the events of the story into motion.

This one can confuse and delves closer to the realm of theology than science. It feels gimmicky, and has become something of a trope making it hard to pull this off in a satisfying way for your audience. This type also invites the audience to question if your protagonist ever had free will or agency in the story.

Examples: Terminator , Terminator 2 , Harry Potter and the Prisoner of Azkaban , Game of Thrones -Season 6, Twelve Monkeys , Interstellar , Kate and Leopold , The Butterfly Effect , Predestination , Ricky and Morty -Season 5, Looper .

Type 5 Seeing the Future

Definition: After seeing a vision of their fate, characters choose to change their destiny or embrace their lot.

We’re stretching to call this time travel, but it provides your story with built-in conflict and stakes. Will the hero choose to walk the path knowing how it will end, or will they choose a different path?

Examples: Oedipus Rex , A Christmas Carol , Minority Report , Arrival , Next (Nicolas Cage), Rick and Morty -Season Four. Star Trek:Discovery -Season 2, Avenger’s EndGame with Dr. Strange and the Mind Stone.

Type 6 Time Loop / Groundhog Day

Definition: Characters relive the same day over and over, resetting back to a respawn point once they die or become incapacitated.

This type gained popularity after the movie, Groundhog Day , became a tremendous hit. Most of the other examples take the Groundhog Day idea and put a slight twist on it. Like Type 4 “This Always Happened”, the popularity of this type can make it harder to pull off in a fresh and innovative way.

Examples: Obviously, Groundhog Day with Bill Murray. Edge of Tomorrow , Doctor Strange in the ending battle with Dormammu, Russian Dolls (Netflix), Palm Springs , Star Trek TNG .

Type 7 Unstuck Mind

Definition: Characters consciousness transport through time within his body to his life at different ages.

Nostalgia for the past and dreaming of the future are core parts of the human experience. This type runs more metaphorically than scientific.

Examples: Slaughterhouse 5 by Kurt Vonnegut, X-Men: Days of Future Past , Desmond in the series Lost .

Type 8 Unstuck Body

Definition: A character’s body or object becomes physically detached from the flow of time within the surrounding universe, becoming inverted or younger. Only certain objects or bodies are unstuck from time. Also called Inverted Entropy.

This one will blow your mind if you think about it for too long. Like Type 2 “Branch Reality”, this one comes from the realm of quantum mechanics and theoretical physics. Scientists and mathematicians have all the formulas worked out to make this de-aging a reality, but currently lack the technology to control all the variables in the ways needed. It would like scientists working out than an object could break the speed of the sound in 1890. It would look inconceivable, given the technology of the day, but I wouldn’t put limits on human ingenuity.

Examples: Dr. Strange (the Hong Kong battle). Tenet , briefly in Endgame with Scott Lang and Bruce, Primer .

If you’re writing a time travel story, you’ll need to decide which one of these types you want to deploy. They all have their advantages and disadvantages. In many ways, its similar to designing your magic system, especially if you go with a Type 1 time travel story. The most important thing remains to have relatable characters and to tell a great story while being internally consistent with the rules and logic of your story world.

Ted Atchley  is a freelance writer and professional computer programmer. Whether it’s words or code, he’s always writing. Ted’s love for speculative fiction started early on with Lewis’  Chronicles of Narnia,  and the Star Wars movies. This led to reading Marvel comics and eventually losing himself in Asimov’s Apprentice Adept and the world of Krynn ( Dragonlance Chronicles ). 

After blogging on his own for several years, Blizzard Watch ( blizzardwatch.com ) hired Ted to be a regular columnist in 2016. When the site dropped many of its columns two years later, they retained Ted as a staff writer. 

He lives in beautiful Charleston, SC with his wife and children. When not writing, you’ll find him spending time with his family, and cheering on his beloved Carolina Panthers. He’s currently revising his work-in-progress portal fantasy novel before preparing to query. 

Ted has a quarterly newsletter which  you can join here . You’ll get the latest on his writing and publishing as well as links about writing, Star Wars, and/or Marvel.

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