parallel universe travel definition

• Skip to main content
• Keyboard shortcuts for audio player

A Physicist Explains Why Parallel Universes May Exist

The supermassive black hole at the center of our galaxy. Smithsonian Institute/via Flickr hide caption

The supermassive black hole at the center of our galaxy.

Our universe might be really, really big — but finite. Or it might be infinitely big.

Both cases, says physicist Brian Greene, are possibilities, but if the latter is true, so is another posit: There are only so many ways matter can arrange itself within that infinite universe. Eventually, matter has to repeat itself and arrange itself in similar ways. So if the universe is infinitely large, it is also home to infinite parallel universes.

Does that sound confusing? Try this:

Think of the universe like a deck of cards.

"Now, if you shuffle that deck, there's just so many orderings that can happen," Greene says. "If you shuffle that deck enough times, the orders will have to repeat. Similarly, with an infinite universe and only a finite number of complexions of matter, the way in which matter arranges itself has to repeat."

Greene, the author of The Elegant Universe and The Fabric of the Cosmos , tackles the existence of multiple universes in his latest book, The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos .

Recent discoveries in physics and astronomy, he says, point to the idea that our universe may be one of many universes populating a grander multiverse.

"You almost can't avoid having some version of the multiverse in your studies if you push deeply enough in the mathematical descriptions of the physical universe," he says. "There are many of us thinking of one version of parallel universe theory or another. If it's all a lot of nonsense, then it's a lot of wasted effort going into this far-out idea. But if this idea is correct, it is a fantastic upheaval in our understanding."

How Quantum Mechanics And General Relativity Play A Part

Greene thinks the key to understanding these multiverses comes from string theory, the area of physics he has studied for the past 25 years.

In a nutshell, string theory attempts to reconcile a mathematical conflict between two already accepted ideas in physics: quantum mechanics and the theory of relativity.

"Einstein's theory of relativity does a fantastic job for explaining big things," Greene says. "Quantum mechanics is fantastic for the other end of the spectrum — for small things. The big problem is that each theory is great for each realm, but when they confront each other, they are ferocious antagonists, and the mathematics falls apart."

The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos By Brian Greene Hardcover, 384 pages Knopf List price: $29.95

Read An Excerpt

String theory smooths out the mathematical inconsistencies that currently exist between quantum mechanics and the theory of relativity. It posits that the entire universe can be explained in terms of really, really small strings that vibrate in 10 or 11 dimensions — meaning dimensions we can't see. If it exists, it could explain literally everything in the universe — from subatomic particles to the laws of speed and gravity.

So what does this have to do with the possibility that a multiverse exists?

"There are a couple of multiverses that come out of our study of string theory," Greene says. "Within string theory, the strings that we're talking about are not the only entities that this theory allows. It also allows objects that look like large flying carpets, or membranes, which are two dimensional surfaces. And what that means, within string theory, is that we may be living on one of those gigantic surfaces, and there can be other surfaces floating out there in space."

That theory, he says, might be testable in the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research.

"If we are living on one of these giant membranes, then the following can happen: When you slam particles together — which is what happens at the LHC — some debris from those collisions can be ejected off of our membrane and be ejected into the greater cosmos in which our membrane floats," he says. "If that happens, that debris will take away some energy. So if we measure the amount of energy just before the protons collide and compare it with the amount of energy just after they collide, if there's a little less after — and it's less in just the right way — it would indicate that some had flown off, indicating that this membrane picture is correct."

Greene explains that when he began studying string theory and parallel universes, it wasn't because he could one day measure energy at CERN or develop new mathematical equations. He simply liked the idea, he says, of studying something on such a large scale.

"We're trying to talk about not just the universe but perhaps other universes — but all within a logical framework that allows us to make some definitive statements," he says. "To me, that's enormously exciting, to step outside the everyday and really look at the universe, within these mathematical terms, on its grandest scales."

Related NPR Stories

Brain candy, want to clean up at poker study physics, krulwich wonders…, a light take on the gravity-time relationship, short of 'all,' string theorists accused of nothing, physicists debate the merits of string theory.

The Hidden Reality

Buy featured book.

Your purchase helps support NPR programming. How?

• Independent Bookstores

Excerpt: 'The Hidden Reality'

What is the multiverse—and is there any evidence it really exists?

Scientists can only see so far before they run into the edge of the universe. Will we ever know if anything lies beyond?

What lies beyond the edges of the observable universe? Is it possible that our universe is just one of many in a much larger multiverse? Movies can’t get enough of exploring these questions. From Oscar winners like Everything Everywhere All at Once   to superhero blockbusters like Dr. Strange in the Multiverse of Madness , science fiction stories are full of creative interactions between alternate realities. And depending on which cosmologist you ask, the concept of a multiverse is more than pure fantasy or a handy storytelling device.

Humanity’s ideas about alternate realities are ancient and varied—in 1848 Edgar Allan Poe even wrote a prose poem in which he fancied the existence of “a limitless succession of Universes.” But the multiverse concept really took off when modern scientific theories attempting to explain the properties of our universe predicted the existence of other universes   where events take place outside our reality.

“Our understanding of reality is not complete, by far,” says Stanford University physicist Andrei Linde . “Reality exists independently of us.”

If they exist, those universes are separated from ours, unreachable and undetectable by any direct measurement (at least so far). And that makes some experts question whether the search for a multiverse can ever be truly scientific.

Will scientists ever know whether our universe is the only one? We break down the different theories about a possible multiverse—including other universes with their own laws of physics—and whether many versions of you could exist out there.

What is a multiverse?

The multiverse is a term that scientists use to describe the idea that beyond the observable universe, other universes may exist as well. Multiverses are predicted by several scientific theories that describe different possible scenarios—from regions of space in different planes than our universe, to separate bubble universes that are constantly springing into existence.

FREE BONUS ISSUE

The one thing all these theories have in common is that they suggest the space and time we can observe is not the only reality.

Okay … but why do scientists think there could be more than one universe?

“We cannot explain all the features of our universe if there’s only one of them,” says science journalist Tom Siegfried , whose book The Number of the Heavens investigates how conceptions of the multiverse have evolved over millennia.

“Why are the fundamental constants of nature what they are?” Siegfried wonders. “Why is there enough time in our universe to make stars and planets? Why do stars shine the way they do, with just the right amount of energy? All of those things are questions we don’t have answers for in our physical theories.”

Siegfried says two possible explanations exist: First, that we need newer, better theories to explain the properties of our universe. Or, he says, it’s possible that “we’re just one of many universes that are different, and we live in the one that’s nice and comfortable.”

What are some of the more popular multiverse theories?  

Perhaps the most scientifically accepted idea comes from what’s known as inflationary cosmology , which is the idea that in the minuscule moments after the big bang, the universe rapidly and exponentially expanded. Cosmic inflation explains a lot of the observed properties of the universe, such as its structure and the distribution of galaxies.

“This theory at first looked like a piece of science fiction, although a very imaginative one,” says Linde, one of the architects of cosmic inflationary theory. “But it explained so many interesting features of our world that people started taking it seriously.”

One of the theory’s predictions is that inflation could happen over and over again , perhaps infinitely, creating a constellation of bubble universes. Not all of those bubbles will have the same properties as our own—they might be spaces where physics behaves differently. Some of them might be similar to our universe, but they all exist beyond the realm we can directly observe.

You May Also Like

How fast is the universe really expanding? The mystery deepens.

This supermassive black hole was formed when the universe was a toddler

The 11 most astonishing scientific discoveries of 2023

What are some other ideas.

Another compelling type of multiverse is called the many-worlds interpretation of quantum mechanics , which is the theory that mathematically describes how matter behaves. Proposed by physicist Hugh Everett in 1957, the many-worlds interpretation predicts the presence of branching timelines, or alternate realities in which our decisions play out differently, sometimes producing wildly different outcomes.

“Hugh Everett says, Look, there’s actually an infinite number of parallel Earths, and when you do an experiment and you get the probabilities, basically all that proves is that you live on the Earth where that was the outcome of that experiment,” says physicist James Kakalios of the University of Minnesota, who has written about the physics (or not) of superheroes. “But on other Earths, there’s a different outcome.”

According to this interpretation, versions of you could be off living the many different possible lives you could have led if you’d made different decisions. However, the only reality that’s perceptible to you is the one you inhabit.

So where do all of those other Earths exist?

They’re all overlapping in dimensions we can’t access. MIT’s Max Tegmark refers to this type of multiverse as a Level III multiverse , where multiple scenarios are playing out in branching realities.

“In the many-worlds interpretation, you still have an atomic bomb, you just don’t know exactly when it’s going to go off,” Linde says. And maybe in some of those realities, it won’t.

By contrast, the multiple universes predicted by some theories of cosmic inflation are what Tegmark calls a Level II multiverse, where fundamental physics can be different across the different universes. In an inflationary multiverse, Linde says, “you do not even know if, in some parts of the universe, atomic bombs are even possible in principle.”

So if I want to meet myself, how do I get there? Can we travel between multiverses?  

Unfortunately, no. Scientists don’t think it’s possible to travel between universes, at least not yet.

“Unless a whole lot of physics we know that’s pretty solidly established is wrong, you can’t travel to these multiverses,” Siegfried says. “But who knows? A thousand years from now, I’m not saying somebody can’t figure out something that you would never have imagined.”

Is there any direct evidence suggesting multiverses exist?  

Even though certain features of the universe seem to require the existence of a multiverse, nothing has been directly observed that suggests it actually exists. So far, the evidence supporting the idea of a multiverse is purely theoretical, and in some cases, philosophical.

Some experts argue that it may be a grand cosmic coincidence that the big bang forged a perfectly balanced universe that is just right for our existence. Other scientists think it is more likely that any number of physical universes exist, and that we simply inhabit the one that has the right characteristics for our survival .

An infinite number of alternate little pocket universes, or bubbles universes, some of which have different physics or different fundamental constants, is an attractive idea, Kakalios says. “That’s why some people take these ideas kind of seriously, because it helps address certain philosophical issues,” he says.

Scientists argue about whether the multiverse is even an empirically testable theory ; some would say no, given that by definition a multiverse is independent from our own universe and impossible to access. But perhaps we just haven’t figured out the right test.

Will we ever know if our universe is just one of many?

We might not. But multiverses are among the predictions of various theories that can be tested in other ways, and if those theories pass all of their tests, then maybe the multiverse holds up as well. Or perhaps some new discovery will help scientists figure out if there really is something beyond our observable universe.

“The universe is not constrained by what some blobs of protoplasm on a tiny little planet can figure out, or test,” Siegfried says. “We can say, This is not testable, therefore it can’t be real—but that just means we don’t know how to test it. And maybe someday we’ll figure out how to test it, and maybe we won’t. But the universe can do whatever it wants.”

Related Topics

• ASTROPHYSICS

This is what the first stars looked like as they were being born

The world’s most powerful telescope is rewriting the story of space and time

Colossal gravitational waves—trillions of miles long—found for the first time

The most ancient galaxies in the universe are coming into view

Here's how astronomers found one of the rarest phenomenons in space

• Environment
• Perpetual Planet

History & Culture

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

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

Do parallel universes exist? We might live in a multiverse.

Sci-fi loves parallel universes. But could we really be in one?

• Eternal inflation & the Big Bang theory
• Quantum mechanics
• Infinite space

The mirror-image universe

• Multiverse: For and against

Parallel universes in fiction

Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science. 

Our universe is unimaginably big. Hundreds of billions, if not trillions, of galaxies spin through space, each containing billions or trillions of stars . Some researchers studying models of the universe speculate that the universe's diameter could be 7 billion light-years across. Others think it could be infinite.

But is it all that's out there? Science fiction loves the idea of a parallel universe, and the thought that we might be living just one of an infinite number of possible lives. Multiverses aren't reserved for "Star Trek," "Spiderman" and "Doctor Who," though. Real scientific theory explores, and in some cases supports, the case for universes outside, parallel to, or distant from but mirroring our own. 

Multiverses and parallel worlds are often argued in the context of other major scientific concepts like the Big Bang , string theory and quantum mechanics . 

Related: How big is the universe?

Eternal inflation, the Big Bang theory and parallel universes

Around 13.7 billion years ago, everything we know of was an infinitesimal singularity. Then, according to the Big Bang theory, it burst into action, inflating faster than the speed of light in all directions for a tiny fraction of a second. Before 10^-32 seconds had passed, the universe had exploded outward to 10^26 times its original size in a process called cosmic inflation . And that's all before the actual expansion of matter that we usually think of as the Big Bang itself, which was a consequence of all this inflation: As the inflation slowed, a flood of matter and radiation appeared, creating the classic Big Bang fireball, and began to form the atoms, molecules, stars and galaxies that populate the vastness of space that surrounds us. 

Related: How an inflating universe could create a multiverse  

That mysterious process of inflation and the Big Bang have convinced some researchers that multiple universes are possible, or even very likely. According to theoretical physicist Alexander Vilenkin of Tufts University in Massachusetts, inflation didn't end everywhere at the same time. While it ended for everything that we can detect from Earth 13.8 billion years ago, cosmic inflation in fact continues in other places. This is called the theory of eternal inflation. And as inflation ends in a particular place, a new bubble universe forms, Vilenkin wrote for Scientific American in 2011. 

Those bubble universes can't contact each other because they continue to expand indefinitely. If we were to set off for the edge of our bubble, where it might butt up against the next bubble universe over, we'd never reach it because the edge is zipping away from us faster than the speed of light, and faster than we could ever travel.

Related: How many stars are in the universe?  

But even if we could reach the next bubble, according to eternal inflation (combined with string theory), our familiar universe with its physical constants and habitable conditions could be totally different from the hypothetical bubble universe next to our own. 

"This picture of the universe, or multiverse, as it is called, explains the long-standing mystery of why the constants of nature appear to be fine-tuned for the emergence of life," Vilenkin wrote. "The reason is that intelligent observers exist only in those rare bubbles in which, by pure chance, the constants happen to be just right for life to evolve. The rest of the multiverse remains barren, but no one is there to complain about that."

Vilenkin's explanation implies that in some of the infinite bubble universes outside our own, there could be other intelligent observers. But in every instant that passes, we get farther away from them, and we will never intersect.

Quantum mechanics and parallel universes

Some researchers base their ideas of parallel universes on quantum mechanics, the mathematical description of subatomic particles. In quantum mechanics, multiple states of existence for tiny particles are all possible at the same time — a "wave function" encapsulates all of those possibilities. However, when we actually look, we only ever observe one of the possibilities. According to the Copenhagen interpretation of quantum mechanics as described by the Stanford Encyclopedia of Philosophy , we observe an outcome when the wave function "collapses" into a single reality. 

But the many-worlds theory proposes instead that every time one state, or outcome, is observed, there is another "world" in which a different quantum outcome becomes reality. This is a branching arrangement, in which instant by instant, our perceived universe branches into near-infinite alternatives. Those alternate universes are completely separate and unable to intersect, so while there may be uncountable versions of you living a life that's slightly — or wildly — different from your life in this world, you'd never know it. 

Related: Black holes, aliens, multiverse & Mars: Space TED talks you need to watch  

The many-worlds theory is the most "courageous" take on the quandary of quantum mechanics, physicist Sean Carroll wrote in his book, " Something Deeply Hidden: Quantum Worlds and the Emergence of Spacetime " (Dutton, 2019). He also argued that it is the most straightforward theory, although not without wrinkles. 

One of those wrinkles is that the many-worlds idea is not really falsifiable. This is an important component of scientific thought and is the way the scientific community develops ideas that can be explored with observation and experimentation. If there's no opportunity to find evidence against a theory, that's bad for science as a whole, science journalist John Horgan argued in a blog post for Scientific American .

Infinite space, infinite universes?

Some physicists believe in a flatter version of multiple universes. That is, if the universe that we live in goes on forever, there are only so many ways that the building blocks of matter can arrange themselves as they assemble across infinite space. Eventually, any finite number of particle types must repeat a particular arrangement. Hypothetically, in a big enough space, those particles must repeat arrangements as large as entire solar systems and galaxies.

So, your entire life might be repeated elsewhere in the universe, down to what you ate for breakfast yesterday. At least, that's the theory.

But if the universe began at a finite point, as nearly every physicist agrees that it did, an alternate version of you likely doesn't exist, according to astrophysicist Ethan Siegel's 2015 Medium article. 

According to Siegel, "the number of possible outcomes from particles in any Universe interacting with one another tends towards infinity faster than the number of possible Universes increases due to inflation." 

"So what does this mean for you?" Siegel wrote. "It means it's up to you to make this Universe count."

In a relatively recent addition to the pantheon of multiverse theories, researchers from the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, have proposed that the universe began at the Big Bang — and on the opposite side of the Big Bang timeline, stretching backwards in time, a universe once existed that was the exact mirror image of our own. 

"Instead of saying there was a different universe before the bang, we're saying that the universe before the bang is actually, in some sense, an image of the universe after the bang," Neil Turok, a Perimeter Institute researcher, told Space.com sister site Live Science .

That means everything — protons, electrons, even actions like cracking an egg — would be reversed. Antiprotons and positively charged electrons would make up atoms, while eggs would un-crack and make their way back inside chickens. Eventually, that universe would shrink down, presumably to a singularity, before expanding out into our own universe.

Seen another way, both universes were created at the Big Bang and exploded simultaneously backward and forward in time.

Multiverse: Arguments for and against

Arguments for the multiverse theory.

Cosmic inflation 

Our universe grew exponentially in the first moments of its existence, but was this expansion uniform? If not, it suggests different regions of space grew at different rates — and may be isolated from one another.

Mathematical constants 

How are the laws of the universe so exact? Some propose that this happened only by chance — we are the one universe out of many that happened to get the numbers right. 

The observable universe

What is beyond the edge of the observable space around us? No one knows for sure, and until we do (which could be never), the thought that ou universe extends indefinitely is an interesting one. 

Arguments against the multiverse theory

Falsifiability 

There is no way for us to ever test theories of the multiverse. We will never see beyond the observable universe, so if there is no way to disprove the theories, should they even be given credence? 

Occam's razor 

Sometimes, the simplest ideas are the best. Some physicists argue that we don't need the multiverse theory at all. It doesn't solve any paradoxes, and only creates complications. 

No evidence 

Not only can we not disprove any multiverse theory, we can't prove them either. We currently have no evidence that multiverses exists, and everything we can see suggests there is just one universe — our own. 

Countless works of myth and fiction draw from ideas of parallel universes and the multiverse. Overlapping worlds make appearances in Norse mythology as well as Buddhist and Hindu cosmology. The idea of multiple universes coming into contact showed up in print as early as Edwin A. Abbott's novella "Flatland: A Romance of Many Dimensions" (Seeley & Co., 1884), and can still be seen in recent movies such as the 2016 Marvel film "Doctor Strange." An entire genre of Japanese graphic novels, called isekai, deals with characters transported to parallel worlds, as described by the New York Public Library . 

Nearly every "Star Trek" series incorporates some form of mirror universe, and the 2009 reboot film starring Chris Pine and Zachary Quinto took subsequent "Star Trek" movies into an entirely new timeline that explicitly branches off from the original series.

And comics, as well as their corresponding movies, delve deeply into the idea of parallel worlds. Recent Marvel Comics' storylines (both film and in print), DC's Flashpoint arc and 2018's "Into the Spider-Verse" all explore multiple universes and the intersections between them.

This is an incomplete list of some appearances of multiverses, split-timeline universes and parallel universes in fiction:

• Into the Spider-Verse (2018)
• Terminator Genisys (2015) 
• Phineas and Ferb the Movie: Across the Second Dimension (2011) 
• Star Trek (2009) 
• Donnie Darko (2001) 
• Run Lola Run (1998) 
• Sliding Doors (1998) 
• Back to the Future 1-3 (1985, 1989, 1990) 
• The Adventures of Buckaroo Banzai Across the 8th Dimension (1984)
• Star Trek: Discovery, multiple episodes 
• Star Trek: Enterprise, multiple episodes 
• Star Trek: Deep Space Nine, multiple episodes 
• Star Trek: The Next Generation, "Parallels" (Episode 11, Season 7) (1993) 
• Star Trek: The Original Series, "Mirror, Mirror", (Episode 4, Season 2) (1967) 
• Doctor Who, multiple episodes  
• Sliders, entire series 
• Community, "Remedial Chaos Theory" (Episode 4, Season 3) (2011) 
• Rick and Morty, multiple episodes 
• Futurama, multiple episodes 
• Eureka, multiple episodes 
• Agents of Shield, multiple episodes
• "The Chronicles of Narnia" series (Geoffrey Bles, 1950-56) by C. S. Lewis
• "His Dark Materials" series (Scholastic, 1995-2000) by Phillip Pullman
• The "Discworld" series (HarperCollins, 1983-2015) by Terry Pratchett
• "Men Like Gods" (Macmillan, 1923) by H. G. Wells
• "The Dark Tower" series (Donald M. Grant, 1982-2012) by Stephen King

Video games

• BioShock Infinite, 2013
• Kingdom Hearts, 2002-2020
• Chrono Cross, 1999
• Half-Life, 1998-2020
• Metroid Prime 2: Echoes, 2004
• Zero Escape, 2009-2016

This article was adapted in part from previous work by Space.com contributor Elizabeth Howell. 

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

Get the Space.com Newsletter

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

Daisy Dobrijevic joined Space.com in February 2022 having previously worked for our sister publication All About Space magazine as a staff writer. Before joining us, Daisy completed an editorial internship with the BBC Sky at Night Magazine and worked at the National Space Centre in Leicester, U.K., where she enjoyed communicating space science to the public. In 2021, Daisy completed a PhD in plant physiology and also holds a Master's in Environmental Science, she is currently based in Nottingham, U.K. Daisy is passionate about all things space, with a penchant for solar activity and space weather. She has a strong interest in astrotourism and loves nothing more than a good northern lights chase! 

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

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

Satellites spot clusters of 'spiders' sprawled across Mars' Inca City (photo)

Most Popular

• 2 5 sci-fi movies that weren't great but still deserve a sequel
• 3 NASA begins delivering 1st Artemis Moon Trees to be planted across United States
• 4 Superheroes fight acid-spewing xenomorphs in Marvel Comics' 'Aliens vs. Avengers'
• 5 James Webb Space Telescope discovers some early universe galaxies grew up surprisingly fast

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

Bibliography

Academic tools.

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

Time Travel

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 ]

• Adams, Robert Merrihew, 1997, “Thisness and time travel”, Philosophia , 25: 407–15.
• Arntzenius, Frank, 2006, “Time travel: Double your fun”, Philosophy Compass , 1: 599–616. doi:10.1111/j.1747-9991.2006.00045.x
• Asimov, Isaac, 1995 [2003], Gold: The Final Science Fiction Collection , New York: Harper Collins.
• Baron, Sam and Colyvan, Mark, 2016, “Time enough for explanation”, Journal of Philosophy , 113: 61–88.
• Belot, Gordon, 2005, “Dust, time and symmetry”, British Journal for the Philosophy of Science , 56: 255–91.
• Benovsky, Jiri, 2011, “Endurance and time travel”, Kriterion , 24: 65–72.
• Berkovitz, Joseph, 2001, “On chance in causal loops”, Mind , 110: 1–23.
• Black, Max, 1956, “Why cannot an effect precede its cause?”, Analysis , 16: 49–58.
• Brier, Bob, 1973, “Magicians, alarm clocks, and backward causation”, Southern Journal of Philosophy , 11: 359–64.
• Carlson, Erik, 2005, “A new time travel paradox resolved”, Philosophia , 33: 263–73.
• Carroll, John W., 2010, “Context, conditionals, fatalism, time travel, and freedom”, in Time and Identity , Joseph Keim Campbell, Michael O’Rourke, and Harry S. Silverstein, eds., Cambridge MA: MIT Press, 79–93.
• Craig, William L., 1997, “Adams on actualism and presentism”, Philosophia , 25: 401–5.
• Daniels, Paul R., 2012, “Back to the present: Defending presentist time travel”, Disputatio , 4: 469–84.
• Deutsch, David and Lockwood, Michael, 1994, “The quantum physics of time travel”, Scientific American , 270(3): 50–6.
• Dowe, Phil, 2000, “The case for time travel”, Philosophy , 75: 441–51.
• –––, 2001, “Causal loops and the independence of causal facts”, Philosophy of Science , 68: S89–S97.
• –––, 2003, “The coincidences of time travel”, Philosophy of Science , 70: 574–89.
• Dummett, Michael, 1964, “Bringing about the past”, Philosophical Review , 73: 338–59.
• Dwyer, Larry, 1977, “How to affect, but not change, the past”, Southern Journal of Philosophy , 15: 383–5.
• Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
• Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press.
• Effingham, Nikk and Robson, Jon, 2007, “A mereological challenge to endurantism”, Australasian Journal of Philosophy , 85: 633–40.
• Ehring, Douglas, 1997, “Personal identity and time travel”, Philosophical Studies , 52: 427–33.
• Elliott, Katrina, 2019, “How to Know That Time Travel Is Unlikely Without Knowing Why”, Pacific Philosophical Quarterly , 100: 90–113.
• Fulmer, Gilbert, 1980, “Understanding time travel”, Southwestern Journal of Philosophy , 11: 151–6.
• Gilmore, Cody, 2007, “Time travel, coinciding objects, and persistence”, in Oxford Studies in Metaphysics , Dean W. Zimmerman, ed., Oxford: Clarendon Press, vol. 3, 177–98.
• Goddu, G.C., 2003, “Time travel and changing the past (or how to kill yourself and live to tell the tale)”, Ratio , 16: 16–32.
• –––, 2007, “Banana peels and time travel”, Dialectica , 61: 559–72.
• Gödel, Kurt, 1949a [1990a], “An example of a new type of cosmological solutions of Einstein’s field equations of gravitation”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 190–8; originally published in Reviews of Modern Physics , 21 (1949): 447–450.
• –––, 1949b [1990b], “A remark about the relationship between relativity theory and idealistic philosophy”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 202–7; originally published in P. Schilpp (ed.), Albert Einstein: Philosopher-Scientist , La Salle: Open Court, 1949, 555–562.
• Godfrey-Smith, William, 1980, “Travelling in time”, Analysis , 40: 72–3.
• Gorovitz, Samuel, 1964, “Leaving the past alone”, Philosophical Review , 73: 360–71.
• Grey, William, 1999, “Troubles with time travel”, Philosophy , 74: 55–70.
• Hafele, J. C. and Keating, Richard E., 1972a, “Around-the-world atomic clocks: Observed relativistic time gains”, Science , 177: 168–70.
• –––, 1972b, “Around-the-world atomic clocks: Predicted relativistic time gains”, Science , 177: 166–8.
• Hales, Steven D., 2010, “No time travel for presentists”, Logos & Episteme , 1: 353–60.
• Hall, Thomas, 2014, “In Defense of the Compossibility of Presentism and Time Travel”, Logos & Episteme , 2: 141–59.
• Hanley, Richard, 2004, “No end in sight: Causal loops in philosophy, physics and fiction”, Synthese , 141: 123–52.
• Harrison, Jonathan, 1980, “Report on analysis ‘problem’ no. 18”, Analysis , 40: 65–9.
• Hawking, S.W., 1992, “Chronology protection conjecture”, Physical Review D , 46: 603–11.
• Holt, Dennis Charles, 1981, “Time travel: The time discrepancy paradox”, Philosophical Investigations , 4: 1–16.
• Horacek, David, 2005, “Time travel in indeterministic worlds”, Monist (Special Issue on Time Travel), 88: 423–36.
• Horwich, Paul, 1975, “On some alleged paradoxes of time travel”, Journal of Philosophy , 72: 432–44.
• –––, 1987, Asymmetries in Time: Problems in the Philosophy of Science , Cambridge MA: MIT Press.
• Ismael, J., 2003, “Closed causal loops and the bilking argument”, Synthese , 136: 305–20.
• Keller, Simon and Nelson, Michael, 2001, “Presentists should believe in time-travel”, Australasian Journal of Philosophy , 79: 333–45.
• Kiourti, Ira, 2008, “Killing baby Suzy”, Philosophical Studies , 139: 343–52.
• Le Poidevin, Robin, 2003, Travels in Four Dimensions: The Enigmas of Space and Time , Oxford: Oxford University Press.
• –––, 2005, “The Cheshire Cat problem and other spatial obstacles to backwards time travel”, Monist (Special Issue on Time Travel), 88: 336–52.
• Lewis, David, 1976, “The paradoxes of time travel”, American Philosophical Quarterly , 13: 145–52.
• Loss, Roberto, 2015, “How to Change the Past in One-Dimensional Time”, Pacific Philosophical Quarterly , 96: 1–11.
• Luminet, Jean-Pierre, 2011, “Time, topology, and the twin paradox”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press. doi:10.1093/oxfordhb/9780199298204.003.0018
• 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

Copyright © 2024 by Nicholas J.J. Smith < nicholas . smith @ sydney . edu . au >

• Accessibility

Support SEP

Mirror sites.

View this site from another server:

• Info about mirror sites

The Stanford Encyclopedia of Philosophy is copyright © 2024 by The Metaphysics Research Lab , Department of Philosophy, Stanford University

Library of Congress Catalog Data: ISSN 1095-5054

The Real Science of the Multiverse

Explaining some of the mind-bending science behind the popular science fiction trope.

The Marvel Cinematic Universe is immense—and with the addition of the “multiverse,” it’s growing even bigger. It’s a classic science fiction trope, allowing characters to jump between timelines and realities and even encounter alternative versions of themselves. Marvel’s latest Spider-man film showcases a rogues gallery from across parallel universes. And as fantastical as it sounds, Marvel’s web of timey-wimey weirdness may not be that far off from reality.

“ The notion of parallel universes leapt out of the pages of fiction into scientific journals in the 1990s ,” writes cosmologist George Ellis in Scientific American . “Many scientists claim that mega-millions of other universes, each with its own laws of physics, lie out there, beyond our visual horizon. They are collectively known as the multiverse.”

Real-life multiverse theories include everything from branching timelines to exact copies of our world . Physicist Max Tegmark has arranged four distinct “levels” of multiverse into a hierarchy , where each type of universe grows progressively different from our own.

The most straightforward multiverse scenario is the Level I Multiverse. First, we must assume that space is infinite, stretching out in all directions, forever. However, the observable universe (everything we can see) is not infinite. Parallel worlds lie beyond this cosmic horizon. Basically, space is so mind-blowingly big that, eventually, it has to repeat itself. This includes the existence of perfect doppelgängers (Tegmark uses probability to estimate that your nearest duplicate is 10^118 meters away). According to Ellis, “Nearly all cosmologists today (including me) accept this type of multiverse.” In a Level I multiverse, Tom Holland’s Spider-man could certainly exist alongside Andrew Garfield’s and Toby Maguire’s Spider-men.

The Level II multiverse is trippier. According to Tegmark, it is comprised of “an infinite set of distinct Level I multiverses, some perhaps with different spacetime dimensionality and different physical constants.” As the vacuum of space continues to expand and spawn other universes, “some regions of space stop stretching and form distinct bubbles, like gas pockets in a loaf of rising bread,” explains Tegmark. When Dr. Strange travels to an unfamiliar, psychedelic dimension, he may have popped into one of the Level II multiverse bubbles.

Weekly Newsletter

Get your fix of JSTOR Daily’s best stories in your inbox each Thursday.

Privacy Policy   Contact Us You may unsubscribe at any time by clicking on the provided link on any marketing message.

At Level III, we find the multiverse scenario has “drawn the most fire in the past decades,” writes Tegmark. Unlike the other models, this theory doesn’t add qualitatively new universes. Instead, the multiverse forms around you, as random events cause the timeline to split. Imagine rolling a die, and instead of landing on a single number, it lands on all values at once. We can “conclude that the die lands on different values in different universes,” writes Tegmark. Voila ! Six new branches of reality are formed. This mind-bending model is called the “many-worlds interpretation.” It may seem familiar if you’ve watched the Marvel show Loki , where time-traveling agents work to prune the branching timeline, and avert random events that could cause it to split out of control.

Once you reach the final level, Level IV, all bets are off. It is comprised of multiverse models that don’t obey even the most fundamental laws of nature. Tegmark calls it “the ultimate type of parallel universe.” This models “opens up the full realm of possibility,” he continues. “Universes can differ not just in location, cosmological properties or quantum state but also in the laws of physics. Existing outside of space and time, they are almost impossible to visualize; the best one can do is to think of them abstractly, as static sculptures that represent the mathematical structure of the physical laws that govern them.” Perhaps we’ll get to see this type of multiverse in Marvel’s next project?

Support JSTOR Daily! Join our new membership program on Patreon today.

JSTOR is a digital library for scholars, researchers, and students. JSTOR Daily readers can access the original research behind our articles for free on JSTOR.

Get Our Newsletter

More stories.

• Alfalfa: A Crop that Feeds Our Food

Ostrich Bubbles

Charles Darwin and His Correspondents: A Lifetime of Letters

Who Can Just Stop Oil?

Recent posts.

• What Convenience Stores Say About “Urban War Zones”
• But Why a Penguin?
• When All the English Had Tails
• Shakespeare and Fanfiction

Support JSTOR Daily

Sign up for our weekly newsletter.

December 1, 2015

13 min read

Does the Multiverse Really Exist?

Proof of parallel universes radically different from our own may still lie beyond the domain of science

By George F. R. Ellis

In the past decade an extraordinary claim has captivated cosmologists: that the expanding universe we see around us is not the only one; that billions of other universes are out there, too. There is not one universe—there is a multiverse. In Scientific American articles and in books such as Brian Greene's The Hidden Reality , leading scientists have spoken of a super-Copernican revolution. In this view, not only is our planet one among many, but even our entire universe is insignificant on the cosmic scale of things. It is just one of countless universes, each doing its own thing.

The word “multiverse” has different meanings. Astronomers are able to see out to a distance of about 42 billion light-years, our cosmic visual horizon. We have no reason to suspect the universe stops there. Beyond it could be many—even infinitely many—domains much like the one we see. Each has a different initial distribution of matter, but the same laws of physics operate in all. Nearly all cosmologists today (including me) accept this type of multiverse, which Max Tegmark calls “level 1.”

Yet some go further. They suggest completely different kinds of universes, with different physics, different histories, maybe different numbers of spatial dimensions. Most will be sterile, although some will be teeming with life. A chief proponent of this “level 2” multiverse is Alexander Vilenkin, who envisions an infinite set of universes containing an infinite number of galaxies—and infinitely many people with your name who are reading this article.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

Similar claims have been made since antiquity by many cultures. What is new is the assertion that the multiverse is a scientific theory, with all that implies about being mathematically rigorous and experimentally testable. I am skeptical about this claim. I do not believe the existence of those other universes has been proved—or ever could be. Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are implicitly redefining what is meant by “science.”

Over the horizon How would universes proliferate in a multiverse, and where would they all reside? Advocates have suggested several alternatives. The universes might be sitting in regions of space far beyond our own, as envisaged by the chaotic inflation model of Alan H. Guth, Andrei Linde and others [see “ The Self-Reproducing Inflationary Universe ,” by Andrei Linde; Scientific American , November 1994]. They might exist at different epochs of time, as proposed in the cyclic universe model of Paul J. Steinhardt and Neil Turok [see “ The Myth of the Beginning of Time ,” by Gabriele Veneziano; Scientific American : A Matter of Time, November 2014]. They might exist in the same space we do but in a different branch of the quantum wave function, as advocated by David Deutsch [see “ The Quantum Physics of Time Travel ,” by David Deutsch and Michael Lockwood; Scientific American , March 1994]. They might not have a location, being completely disconnected from our spacetime, as suggested by Tegmark and Dennis Sciama [see “ Parallel Universes ,” by Max Tegmark; Scientific American , May 2003].

Of these options, the most widely accepted is that of chaotic inflation, and I will concentrate on it; however, most of my remarks apply to all the other proposals as well. The idea behind chaotic inflation is that space at large is an eternally expanding void, within which quantum effects continually spawn new universes like a child blowing bubbles. The concept of inflation goes back to the 1980s, and physicists have elaborated on it based on their most comprehensive theory of nature: string theory.

String theory allows bubbles to differ from one another in fundamental ways. In effect, each begins life not only with a random distribution of matter but also with random kinds of matter. Our universe contains particles such as electrons and quarks interacting through forces such as electromagnetism; other universes may have distinct particles and forces—thus, different local laws of physics. The full set of allowed local laws is known as the landscape. In some interpretations of string theory, the landscape is immense, ensuring a tremendous diversity of universes.

Many physicists who talk about the multiverse, especially advocates of the string landscape, do not care much about parallel universes per se. For them, objections to the multiverse as a concept are unimportant. Their theories live or die based on internal consistency and, one hopes, eventual laboratory testing. They assume a multiverse context for their theories without worrying about how it comes to be—which is what concerns cosmologists.

For a cosmologist, the basic problem with all multiverse proposals is the presence of a cosmic visual horizon. The horizon is the limit to how far away we can see, because signals traveling toward us at the speed of light (which is finite) have not had time since the beginning of the universe to reach us from farther out. All the parallel universes lie outside our horizon and remain beyond our capacity to see, now or ever, no matter how technology evolves. In fact, they are too far away to have had any influence on our universe whatsoever. That is why none of the claims made by multiverse enthusiasts can be directly substantiated.

Proponents tell us that one can state in broad terms what happens 1,000 times as far as our cosmic horizon, 10

100 times, 10 1,000,000 times, an infinity—all from data we obtain within the horizon. It is an extraordinary extrapolation. Maybe the universe closes up on a very large scale, and there is no infinity out there. Maybe all the matter in the universe ends somewhere, with empty space forever after. Maybe space and time come to an end at a singularity that bounds the universe. We just do not know, because we have no information about these regions. And we never will.

Seven questionable arguments Most multiverse proponents are careful scientists who are quite aware of this problem but think we can still make educated guesses about what is going on out there. Their arguments hew to seven broad themes, each of which runs into trouble.

Space has no end. Few dispute that space extends beyond our cosmic horizon and that many other domains lie beyond what we see. If this limited type of multiverse exists, then we can extrapolate what we see to domains beyond the horizon, with our uncertainty increasing along with distance. More elaborate variations, including alternative physics out where we cannot see, are easy to imagine. The trouble with this type of extrapolation from the known to the unknown is that no one can prove you wrong.

How can scientists decide whether their picture of an unobservable region is a reasonable or an unreasonable extrapolation of what we see? If other universes have different initial distributions of matter, might they also have different values of fundamental physical constants, such as those that set the strength of nuclear forces? Different assumptions imply either or both.

Known physics predicts other domains. Proposed unified theories predict entities such as scalar fields, a hypothesized relative of other space-filling fields such as the magnetic field. Such fields should drive cosmic inflation and create universes ad infinitum. These theories are well grounded theoretically, but the nature of the hypothesized fields is unknown, and experimentalists have yet to demonstrate their existence, let alone measure their supposed properties. Crucially, physicists have not substantiated that the dynamics of these fields would cause different laws of physics to operate in different bubble universes.

The theory that predicts an infinity of universes passes a key observational test. The cosmic microwave background radiation reveals what the universe looked like at the end of its hot, early expansion era. Patterns in it suggest that our universe really did undergo a period of inflation. But not all varieties of inflation continue on to create an infinite number of bubble universes. Observations do not single out the required type of inflation from other types. Some cosmologists such as Steinhardt even argue that eternal inflation would have led to different patterns in the background radiation than we see “ The Inflation Debate ,” by Paul J. Steinhardt; Scientific American : Secrets of the Universe, August 2014]. Linde and others disagree. Who is right? It all depends on what you assume about the physics of the inflationary field.

Fundamental constants are finely tuned for life. A remarkable fact about our universe is that physical constants have just the right values needed to allow for complex structures, including living things. Steven Weinberg, Martin Rees, Leonard Susskind and others contend that an exotic multiverse provides a tidy explanation for this apparent coincidence: if all possible values occur in a large enough collection of universes, then ones that permit life are inevitable. This reasoning has also been used to explain the density of the dark energy that is speeding up the expansion of the universe today.

I agree that the multiverse is one possible explanation for the value of this density; it may even be the best option we have right now. But we have no hope of testing it observationally. And most analyses of the issue assume the basic equations of physics are the same everywhere—only the constants differ. But if one takes the multiverse seriously, this need not be so [see “ Looking for Life in the Multiverse ,” by Alejandro Jenkins and Gilad Perez; Scientific American , January 2010].

Fundamental constants match multiverse predictions. This argument refines the previous one by suggesting that the universe is no more finely tuned for life than it strictly needs to be. Proponents have assessed the probabilities of various values of the dark energy density. The higher the value is, the more probable it is, but the more hostile the universe would be to life. The value we observe should be just on the borderline of uninhabitability, and it does appear to be so.

We cannot apply a probability argument, however, if there is no multiverse; statistics simply are not applicable if only one universe physically exists. This argument thus assumes the desired outcome before it starts. Probability can probe the consistency of the multiverse proposal but cannot prove its existence.

String theory predicts a diversity of universes. In current string theory, almost anything is possible. It predicts that many essential properties of our universe are pure happenstance. If the universe is unique, those properties seem inexplicable.

How can we understand, for example, the fact that physics has precisely those highly constrained properties that allow life to exist? If the universe is one of many, those properties make perfect sense. Nothing singled them out; they are simply the ones that arose in our region of space. Had we lived elsewhere, we would have observed different properties, if we could indeed exist there. (Life would be impossible in most places.)

Unfortunately, string theory is not tried and tested; it is not even a complete theory. If we had proof that string theory is correct, its theoretical predictions could be a legitimate, experimentally based argument for a multiverse. We do not have such proof.

All that can happen, does happen. Some physicists and philosophers speculate that nature never chose to obey some laws and not others; all conceivable laws do apply somewhere.

The idea is inspired in part by quantum mechanics, which, as Murray Gell-Mann memorably put it, holds that everything not forbidden is compulsory. A particle takes all the paths it can; we see the weighted average of all those possibilities. Perhaps the same is true of the entire universe, implying a multiverse.

But astronomers have no chance of observing this multiplicity of possibilities. We cannot even know what the possibilities are. This proposal requires some unverifiable organizing principle that decides what is allowed and what is not—for example, that all possible mathematical structures must be realized in some physical domain (as Tegmark has proposed). Yet we have no idea what kinds of existence this principle entails, apart from the fact that it must include the world we see around us. And we have no way whatsoever to verify the existence or nature of any such organizing principle. It is in some ways an attractive proposition, but its proposed application to reality is pure speculation.

Absence of evidence Cosmologists have also suggested various empirical tests for parallel universes. The cosmic microwave background radiation might bear some traces of other bubble universes if, for example, our universe has ever collided with another bubble of the kind implied by the chaotic inflation scenario. The background radiation might also contain signs of universes that preceded the big bang in an endless cycle. These are indeed ways one might get real evidence of other universes. Some cosmologists have even claimed to see such signs. The observational claims are strongly disputed, however, and many of the hypothetically possible multiverses would not lead to such evidence. So observers can test only some specific classes of multiverse models in this way.

A second observational test is to look for evidence that one or more fundamental constants actually vary, which would corroborate the premise that the laws of physics are not so immutable after all. Some astronomers claim to have found such variations [see “ Inconstant Constants ,” by John D. Barrow and John K. Webb; Scientific American : A Matter of Time, November 2014]. But most consider the evidence dubious.

A third test is to measure the shape of the observable universe: Is it spherical (positively curved), hyperbolic (negatively curved) or “flat” (uncurved)? Multiverse scenarios generally predict that the universe is not spherical, because a sphere closes up on itself, allowing for only a finite volume. Unfortunately, this test is not a clean one. The universe beyond our horizon could have a different shape from that in the observed part. Moreover, not all multiverse theories rule out a spherical geometry.

The topology of the universe offers a better test: Does it wrap around, like a doughnut or pretzel? A wrapped universe is finite, so definitely inconsistent with the chaotic inflation scenario. A closed shape would produce recurring patterns in the sky, such as giant circles in the background radiation. Observers have failed to find any such patterns. But this null result just rules out specific types of a single universe—it does not require a multiverse.

Finally, physicists might hope to prove or disprove some of the theories that predict a multiverse. They might find observational evidence against chaotic versions of inflation. Recent observations by the Planck space observatory of directional unevenness in the cosmic microwave background radiation, for example, tend to call these models into question. Or they might discover a mathematical or empirical inconsistency that forces them to abandon the landscape of string theory. Either scenario would undermine much of the motivation for supporting the multiverse idea, although it would not rule the concept out altogether.

Too much wiggle room All in all, the case for the multiverse is inconclusive. The fundamental issue is the extreme flexibility of the proposal: it is more a concept than a well-defined theory. Most proposals involve a patchwork of different ideas rather than a coherent whole. The basic mechanism for eternal inflation does not itself cause physics to be different in each domain in a multiverse; for that, it needs to be coupled to another speculative theory. Although they can be fitted together, there is nothing inevitable about it.

The key step in justifying a multiverse is extrapolation from the known to the unknown, from the testable to the untestable. You get different answers depending on what you choose to extrapolate. Because theories involving a multiverse can explain almost anything, some multiverse variant can accommodate any observation. The various “proofs,” in effect, ask us to accept a theoretical explanation instead of insisting on observational testing. But such testing has, up until now, been the central requirement of the scientific endeavor, and we abandon it at our peril. If we weaken the requirement of solid data, we weaken the core reason for the success of science over the past centuries.

Now, a unifying explanation of some range of phenomena does carry greater weight than a hodgepodge of arguments for the same phenomena. If the unifying explanation necessarily assumes the existence of unobservable entities such as parallel universes, we might feel compelled to accept those entities.

But a key issue here is how many unverifiable entities are needed. Specifically, are we hypothesizing more or fewer entities than the number of phenomena to be explained? In the case of the multiverse, we are supposing the existence of a huge number—perhaps even an infinity—of unobservable entities to explain just one existing universe. It hardly fits 14th-century English philosopher William of Ockham's stricture that “entities must not be multiplied beyond necessity.”

Proponents of the multiverse make one final argument: that there are no good alternatives. As distasteful as scientists might find the proliferation of parallel worlds, if it is the best explanation, we would be driven to accept it. Conversely, if we are to give up the multiverse, we need a viable alternative. This exploration of alternatives depends on what kind of explanation we are prepared to accept. Physicists' hope has always been that the laws of nature are inevitable—that things are the way they are because there is no other way they might have been—but we have been unable to show this to be true. Other options exist, too. The universe might be pure happenstance—it just turned out that way. Or things might in some sense be meant to be the way they are—purpose or intent somehow underlies existence. Science cannot determine which is the case, because these are metaphysical issues.

Scientists proposed the multiverse as a way of resolving deep issues about the nature of existence, but the proposal leaves the ultimate issues unresolved. All the same issues that arise in relation to the universe arise again in relation to the multiverse. If the multiverse exists, did it come into existence through necessity, chance or purpose? That is a metaphysical question that no physical theory can answer for either the universe or the multiverse.

To make progress, we need to keep to the idea that empirical testing is the core of science. We need some kind of causal contact with whatever entities we propose; otherwise, there are no limits. The link can be a bit indirect. If an entity is unobservable but absolutely essential for properties of other entities that are indeed verified, it can be taken as verified. But then the onus of proof devolves to showing that it is absolutely essential to the web of explanation. The challenge I pose to multiverse proponents is: Can you prove that unseeable parallel universes are vital to explain the world we do see? And is the link essential and inescapable?

As skeptical as I am, I think that the contemplation of the multiverse is an excellent opportunity to reflect on the nature of science and on the ultimate nature of existence: why we are here. It leads to new and interesting insights and so is a productive research program. In looking at this concept, we need an open mind—though not too open. It is a delicate path to tread.

Parallel universes may or may not exist; the case is unproved. We are going to have to live with that uncertainty. Nothing is wrong with scientifically based philosophical speculation, which is what multiverse proposals are. But we should name it for what it is.

Physicists Weigh In: Could We Ever Travel to a Parallel Universe?

Intellectual face-off: which side are you on.

Are Your Carbon Copies Out There?

One very prominent mind twister in both science fiction and real-life science is the concept of parallel universes. This is hardly surprising, since the idea of multiple copies of yourself existing at the same time is both existentially disturbing and thrilling.

The idea of a multiverse is not considered a scientific theory but rather, as Ethan Siegel of Forbes puts it , “a theoretical consequence of the laws of physics as they’re best understood today.” The idea that space-time begins and stretches infinitely implies that existence is mathematically bound to repeat itself at some point—a notion sometimes called the "quilted multiverse."

Or, forgetting the idea of repetitious cosmic clones, there's the possibility that multiple big bangs begat multiple space-time bubbles, in a foamy multiversal sea of infinite potentialities. Here's how it works:

How to Get There?

But what we want to know is: could you ever get to another space-time?

That depends. The American theoretical physicist and string theorist extraordinaire Brian Greene, of Columbia University, argues that the plausibility of multiversal travel—conceding that parallel universes really do exist—hinges on which multiverse concept you subscribe to. If you are an advocate of a multiple big bang multiverse, then that would mean that leaving our universe to travel to another would be just as impossible as travelling back to the time before the big bang that resulted in our universe even happened.

Now, if you believe a quantum physics-dominated notion of parallel universes, then there’s no need to travel to other universes, because you are already inhabiting multiple alternate universes (though not necessarily all of them). Can't decide which dress to wear? No matter—you've worn them both, in two separate parallel universes.

Meanwhile, theoretical physicist Michio Kaku believes that our universe will end up in a “ big freeze ,” and that technology can one day allow us to travel between universes.

Neil deGrasse Tyson, on the other hand, says that if you come from a universe with higher dimensions , then it could be as easy to move between dimensions as stepping from one room to another. And in string theory —one of the leading contenders in bridging the seemingly insuperable gulf sundering quantum mechanics and general relativity—the assumption is that we actually have far more dimensions in this universe than we previously thought and that we just fail to detect them because they are actually very small, curled up in the infinitely minute, trans-subatomic realms beyond the reach of our instruments.

But how can we prove (or disprove) any of these arguments without gaining first-hand experience of it? Much as many aspects of our universe still remain elusive to us, it's currently impossible to acquire any proof to confirm which of these hypotheses is right. But while we don't have the means to definitively prove whether alternate universes do exist , and whether we could traverse borders to move from one to another, it’s highly unlikely that a topic as stimulating as this will disappear anytime soon, either in science fiction or in real-life science.

Meanwhile, physicists are at it. Watch this brief video of physicists going head to head with each other on string theory, Math, and potentially embarrassing alien encounters.

Share This Article

• Brain and Body

Search form

The science behind parallel universes explained.

August 16, 2016 | Kelly Tatera

MinutePhysics breaks down some of the most popular multiverse theories. 

If parallel universes exist, there could be another version of yourself living a completely different life on some otherworldly planet. Unfortunately, we’re not at the point where scientists can confidently say there’s any solid evidence that confirms these parallel universes truly exist, but there are some compelling multiverse theories.

MinutePhysics delves into some of the most popular multiverse theories, including Bubble “universes” and the Many Worlds hypothesis, explaining them in simplified terms — all in under five minutes.

Check out their video below and prepare for your mind to be blown.

Facebook comments

The Science Behind the Multiverse in ‘Everything Everywhere All At Once’

The movie that won Best Picture imagines a reality composed of an uncountable number of universes

Will Sullivan

Daily Correspondent

The science fiction film Everything Everywhere All At Once dominated Sunday night’s Academy Awards with seven victories, including the coveted Best Picture. The movie stars Michelle Yeoh as Evelyn Wang, a laundromat owner trying to pay her taxes.

It’s a grounded premise, but the film is actually one of the wildest of the year. In it, Evelyn learns early on that—to prevent a multiversal apocalypse—she must connect with different versions of herself from parallel dimensions.

The Best Picture winner isn’t alone in imagining the concept of a multiverse for the silver screen—parallel universes have also been a central plot point in recent Marvel movies and television shows. (Though, Everything Everywhere All At Once is the only one with a parallel universe in which people have hot dog-like fingers, or where people’s consciousnesses are trapped inside rocks with googly eyes.)

Evelyn’s escapades are fantastical and fictional, of course, but the multiverse might not be. Scientists today actually grapple with the idea that more universes than the one we live in may exist.

The concept of a multiverse arises from a couple of scientific theories, writes National Geographic ’s Nadia Drake. One stems from the idea of cosmic inflation, which states that the universe expanded exponentially shortly after the Big Bang. If this rapid expansion happened repeatedly, the theory says, it could have resulted in an infinite number of other universes . These hypothetical universes, like separate bubbles in space, might have their own laws of physics and occasionally bump into each other.

Scientists have tried searching for evidence of such collisions in the cosmic microwave background , the distant light released shortly after the Big Bang, writes the Washington Post ’s Carolyn Y. Johnson.

The idea pops up again in quantum mechanics , a field based in part on the notion that quantum particles exist in every possible state until they are measured. In the “many-worlds interpretation,” every time an experiment takes place, the universe branches into a number of new realities that contain all the possible outcomes of the experiment.

This theory means that all events could have parallel outcomes in other universes. In other words, for every action you take, a different version of you might be experiencing the option that you did not choose.

“I actually try to think, when I get a parking ticket, ‘Hey, there’s another version of a parallel universe where I didn’t get ticketed,’ so I can feel a bit better. And there’s another version where my car got towed,” Max Tegmark , a physicist at MIT, tells the Post .

So far, though, scientists have no concrete evidence that other universes exist, nor any ways to find them, noted the New York Times ’ Dennis Overbye last April.

The multiverse “doesn’t really have a mathematical basis—it is a collection of ideas,” Geraint Lewis , a cosmologist at the University of Sydney in Australia, says to  Forbes ’ Jamie Carter. “In the cycle of science it remains at the hypothesis stage and needs to become a robust proposition before we can truly understand the consequences.”

The characters in Everything Everywhere All At Once are able to do something we probably won’t ever do, even if the multiverse does exist: reach across the boundaries between universes. In the film, characters tap into the minds and experiences of other versions of themselves by performing some weird and random action, like eating chapstick or peeing in their pants. In doing so, they take on the abilities of their other selves. Evelyn “verse jumps” into the mind of another Evelyn who works as a sign spinner outside a pizza store, for example, then uses those spinning talents in her own universe to twirl a riot shield and ward off interdimensional attackers.

The movie’s not just about traveling between universes, though. Co-writers and co-directors Daniel Kwan and Daniel Scheinert told a sci-fi story, but it’s one that’s rooted in real-world themes surrounding an immigrant family, relationships between mothers and daughters and grappling with our own expectations for our lives.

Get the latest stories in your inbox every weekday.

Will Sullivan | | READ MORE

Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next .

Cracking the Door to Other Universes

Does the multiverse really exist.

Everything You Need to Know About the Multiverse

It could be the result of out-of-control rapid expansion that causes patches of universe to branch off and make their own worlds.

.css-2l0eat{font-family:UnitedSans,UnitedSans-roboto,UnitedSans-local,Helvetica,Arial,Sans-serif;font-size:1.625rem;line-height:1.2;margin:0rem;padding:0.9rem 1rem 1rem;}@media(max-width: 48rem){.css-2l0eat{font-size:1.75rem;line-height:1;}}@media(min-width: 48rem){.css-2l0eat{font-size:1.875rem;line-height:1;}}@media(min-width: 64rem){.css-2l0eat{font-size:2.25rem;line-height:1;}}.css-2l0eat b,.css-2l0eat strong{font-family:inherit;font-weight:bold;}.css-2l0eat em,.css-2l0eat i{font-style:italic;font-family:inherit;} A staple of Hollywood storytelling, the concept of the multiverse has its roots in serious scientific study. One catch: we don’t know for sure if it exists.

Parallel Worlds

But to get to the multiverse we have to start with just the universe. By definition, the universe is “all the things”—it is the sum total of complete physical existence. If it’s a thing, it’s in the universe. But even with that definition, we can start to crack the door to “other” universes. One way to do that is to recognize that the universe is only so old, and light can only travel at a finite speed. So there’s a limit to what we can observe in the universe (that limit is about 45 billion light years away).

However, that explanation gives you a very weak vision of the multiverse. Actually, there’s more to the universe than what we can observe, so there’s more “stuff” out there—more stars, more galaxies, maybe even more intelligent creatures—than we could ever make contact with. All of these things are in the universe, but are definitely not a part of our own world.

Those regions outside our observable bubble of the universe still look and act the same as inside it. It’s all the same physics at play, just with various other combinations. But that’s not where the story has to end.

✅ What is cosmic inflation? After the Big Bang, space expanded at an exponential rate. This explosive expansion, or inflation, lasted from 10 −36 seconds to between 10 −33 and 10 −32 seconds, then slowed down. However, space is still expanding at a rate of about 73 kilometers per second per megaparsec, as the Hubble Space Telescope estimated in 2022.

A Universe Is Born

We currently do not have a solid understanding of the earliest moments of the Big Bang . We know the general outline: our universe was once much smaller and hotter in the past; nowadays it’s not so small, and it’s a whole lot colder. We’ve tested this basic idea against a variety of experiments, too. But as we rewind the clock to the Big Bang, we reach a scale where our physics simply breaks down. When the universe was less than a second old, the conditions of the cosmos were so extreme that we have no theory of physics to guide us.

That said, we do suspect that in its earliest moments the universe underwent a radical transformational event, known as inflation . It appears from all available evidence that when our cosmos was only a fraction of a second old, it rapidly expanded to enormous proportions, growing by at least a factor of 10 60 .

This inflationary event set the stage of the remainder of the Big Bang, when our universe flooded with particles and radiation that would then grow up to become galaxies , stars, and planets.

Here’s where we get a multiverse: maybe inflation never ended. Maybe the entire universe is constantly undergoing this out-of-control rapid expansion, but pieces of it branch off and settle down into something more sedate. Thus, what we call “the universe” is just a tiny bubble of the true, ever-inflating, ever-expanding huge universe.

Making a Multiverse

In this view of inflation, the entire universe never stops inflating. It just keeps getting bigger at an accelerated pace (faster even than the speed of light ). What triggers a patch to slow down and pinch off is merely a random quantum fluctuation. Our patch of the universe just happened to randomly stop inflating (compared to the larger universe), but the rest of the universe outside our bubble continues to do what it was doing before, and what it will always do.

If you point your finger in any random direction, somewhere out there, past some unfathomably huge distance, is another universe, and beyond that, another, and beyond that, another.

Our patch is not alone. Different patches can also randomly settle down and become a normal, calmly expanding universe. To observers in any of those patches, they will see a Big Bang in their past (just like we do); they will have a cosmos filled with matter and radiation (just like we do); and they will have a limit to what they can observe (just like we do).

In this never-ending-inflation scenario, each of these patches (or bubbles, or pockets, or whatever metaphor makes the most sense to you) appears as its own universe, with each universe separated by a vast and ever-growing expanse of absolutely nothing . This is a physically motivated, and possibly very real, multiverse: a collection of independent, separate universes, filled with entities (stars, planets, people), each doing their own thing.

In the multiverse, our universe is not the first bubble to arise, but merely one of an infinite chain of universes. Imagine a giant foam, like the top of a bubble bath. The multiverse is the foam itself, always growing and always creating new bubbles, with each bubble acting as its own independent cosmos.

All of these bubble universes exist within the same framework of spacetime. If you point your finger in any random direction, somewhere out there, past some unfathomably huge distance, is another universe, and beyond that, another, and beyond that, another.

If this kind inflation truly never ends, then there are an infinite number of universes out there in the multiverse. Each one of those universes could have ended their local inflation in the same way, but it’s also possible that as each universe pinched off, it got a brand new set of physics to go along with it, with different collections of forces and particles.

Some of those universes would look incredibly similar to our own. Others may have failed, full of nothing but void. Some may be far stranger than we can possibly imagine.

Testing Reality

And some may be exact replicas of us. If — and this is a big if— there are only a finite number of ways to arrange all the particles in a given universe, then with an infinite number of universes you’re bound to get repeated copies. That means that not only is there another universe out in some random direction, but that if you follow that line far enough, you’ll encounter a duplicate of you doing the exact same thing, right now, in this present moment.

This is all pretty wild, but difficult to test. The problem is that all the bubbles of the multiverse are completely inaccessible from each other. They exist, but not in any connected way. So we can’t just get in a rocket and fly off to head to our nearest neighbor.

But there may have been some cosmic accidents in our ancient past. When our universe was younger, it had just broken off from the larger inflation-driven flow. If another bubble universe just happened to nucleate close to ours, then there’s a small chance that our universes may have briefly intersected before being permanently driven away from each other.

🚀 Could We Achieve Warp Speed?

There’s a Theoretical Limit to How Fast Warp Speed Really Could Be

The chances of that happening are incredibly small—but not zero—which provides a way to test the multiverse. Unfortunately, no observations of the larger cosmos have revealed any indications that we have suffered such a collision. While those experiments don’t rule out the multiverse idea, they don’t exactly help.

The only thing left we have to go on is our theoretical understanding of the early universe . . . which we don’t really understand. We have only a vague picture of what inflation is like; we do not know what powered it, why it had the energies that it did, or why it shut off in our cosmos. We don’t even know if inflation automatically leads to a multiverse, or if we’re misunderstanding our own math.

Still, while physicists continue to debate the idea, it does make for a good story.

Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page .

.css-cuqpxl:before{padding-right:0.3125rem;content:'//';display:inline;} Pop Mech Pro: Space .css-xtujxj:before{padding-left:0.3125rem;content:'//';display:inline;}

How a Lunar Supercollider Could Upend Physics

The History of Pi

The Strange Origin of the Hollow Moon Conspiracy

What Do Alien Space Probes Look Like?

Physicists Are Pretty Sure We Can Reach Warp Speed

How NASA’s Next Super Telescope Could Find Aliens

Will Mars Astronauts Need Sunscreen?

Sunquakes May Be the Key to Nuclear Fusion

7 Solid Reasons to Actually Believe in Aliens

The 7 Greatest Cosmic Threats to Life on Earth

Your Guide to Every Stargazing Event in 2024

• Share full article

Advertisement

Supported by

The Big Ideas: What Is Reality?

In a Parallel Universe, Another You

As they probe the secrets of the cosmos, scientists question whether our reality is but one in a multiverse.

By Michio Kaku

Dr. Kaku is a physicist.

This essay is part of a series called The Big Ideas, in which writers respond to a single question: What is reality? You can read more by visiting The Big Ideas series page .

When I was 8 years old, a revelation forever changed my life.

The year was 1955, and newspaper headlines announced the death of a renowned scientist. A photo accompanied one article, showing his office desk strewn with papers and books. As I recall, the photo caption noted that among the stacks of material was an unfinished manuscript.

I was captivated by this discovery. What could be so challenging that this man, often hailed as one of the greatest scientists of all time, could not complete this work? I had to find out, and over the years I visited libraries to learn more about him.

His name was Albert Einstein. His unfinished work explored what would be known as the theory of everything, an equation, perhaps no more than an inch long, that would allow us to unify all the laws of physics. It would, as Einstein had hoped, give us a glimpse into the mind of God. “I want to know his thoughts,” he famously said. I was hooked.

Today, many of the world’s top physicists are embarking on this cosmic quest, whose far-reaching reverberations span our understanding of reality and the meaning of existence. It would be the crowning achievement of thousands of years of scientific investigation, since ancient civilizations also wondered how the universe was created and what it is made of. The ultimate goal of the theory of everything is to combine Einstein’s theory of relativity with the bizarre world of quantum theory.

In essence, the theory of relativity delves into the cosmos’s most massive phenomena: things like black holes and the birth of the universe. The domain of relativity is nothing less than the entire cosmos. Quantum theory, on the other hand, explores the behavior of matter on the most minuscule level. Its domain encompasses the tiniest particles of nature, those hidden deep inside the atom.

Unifying these two spheres of thought into a single and coherent theory is an ambitious undertaking, one that builds on and adds to the work that Einstein began. But to do this, scientists must first determine a crucial truth: where the universe came from.

This is where our two spheres of thought pointedly diverge.

If we subscribe to Einstein’s relativity theory, the universe is a bubble of some sort that is expanding. We live on the skin of this bubble, and it exploded 13.8 billion years ago, giving us the Big Bang. This birthed the singular cosmos as we know it.

Quantum theory is based on a radically different picture — one of multiplicity. Subatomic particles, you see, can exist simultaneously in multiple states. Take the electron, a subatomic particle that carries a negative charge. Wondrous devices in our lives, such as transistors, computers and lasers, are all possible because the electron, in some sense, can be in several places at the same time. Its behavior defies our conventional understanding of reality.

Here is the key: In the same way that quantum theory forces us to introduce multiple electrons simultaneously, applying that theory to the entire universe makes us have to introduce multiple universes — a multiverse of universes. By that logic, the solitary bubble introduced by Einstein now becomes a bubble bath of parallel universes, constantly splitting in two or bumping into other bubbles. In this scenario, a Big Bang could perpetually happen in distant regions, representing the collision or merging of these bubble universes.

In physics, the concept of a multiverse is a key element of a leading area of study based on the theory of everything. It’s called string theory, which is the focus of my research. In this picture, subatomic particles are just different notes on a tiny, vibrating string, which explains why we have so many of them. Each string vibration, or resonance, corresponds to a distinct particle. The harmonies of the string correspond to the laws of physics. The melodies of the string explain chemistry.

By this thinking, the universe is a symphony of strings. String theory, in turn, posits an infinite number of parallel universes , of which our universe is just one.

A conversation I once had with the theoretical physicist and Nobel laureate Steven Weinberg illustrates this. Imagine sitting in your living room, he told me, listening to the radio. In the room are the waves from hundreds of different radio stations, but your radio is tuned to just one frequency. You can hear only the station that is coherent to your radio; in other words, it vibrates in unison with your transistors.

Now, imagine your living room is filled with the waves of all the electrons and atoms vibrating in that space. These waves might hint at alternate realities — ones with, say, dinosaurs or aliens — right there in your living room. But it’s difficult to interact with them, because you don’t vibrate coherently with them. We have unfastened ourselves from these alternate realities.

There’s an exercise my colleagues and I sometimes present to our Ph.D. students in theoretical physics. We ask them to solve a problem by calculating the probability that one will wake up on Mars tomorrow. Quantum theory is based on what is known as Heisenberg’s uncertainty principle, allowing for a small probability that we can exist even on distant places like Mars. So there’s a tiny but calculable likelihood that our quantum wave will tunnel its way through space-time and wind up there.

But when you do the calculation, you find that for this to happen you’d have to wait longer than the lifetime of the universe. That is, most likely you’ll wake up in your bed tomorrow, not on Mars. To paraphrase the great British geneticist J.B.S. Haldane, reality is not only queerer than we suppose, but queerer than we can suppose.

It’s been more than six decades since Einstein’s death, yet I keep going back to that photo of his desk that I saw as an 8-year-old, the work he left unfinished and its profound implications. In the quest to meld two opposing perspectives of the universe, we’re left with a host of deeply unsettling questions. Might we also exist in multiple states? What might we be doing if we had chosen a different career? Married someone else? What if we could somehow change important episodes in our past? As Einstein once wrote, “The distinction between past, present and future is only a stubbornly persistent illusion.”

Maybe there are copies of us living entirely different lives. If this theory of everything is correct, then perhaps there is a parallel universe where we are billionaires plotting our next escapade, or where we subsist as vagrants desperately searching for our next meal. Who knows? A simple quantum fork in the road might have made all the difference.

Michio Kaku is a professor of physics at the City University of New York and the author of “The God Equation.”

What’s Up in Space and Astronomy

Keep track of things going on in our solar system and all around the universe..

Never miss an eclipse, a meteor shower, a rocket launch or any other 2024 event  that’s out of this world with  our space and astronomy calendar .

Scientists may have discovered a major flaw in their understanding of dark energy, a mysterious cosmic force . That could be good news for the fate of the universe.

A new set of computer simulations, which take into account the effects of stars moving past our solar system, has effectively made it harder to predict Earth’s future and reconstruct its past.

Dante Lauretta, the planetary scientist who led the OSIRIS-REx mission to retrieve a handful of space dust , discusses his next final frontier.

A nova named T Coronae Borealis lit up the night about 80 years ago. Astronomers say it’s expected to put on another show  in the coming months.

Is Pluto a planet? And what is a planet, anyway? Test your knowledge here .

What Physicists Mean by Parallel Universes

• Cosmology & Astrophysics
• Physics Laws, Concepts, and Principles
• Quantum Physics
• Important Physicists
• Thermodynamics
• Weather & Climate

• M.S., Mathematics Education, Indiana University
• B.A., Physics, Wabash College

Physicists talk about parallel universes, but it's not always clear what they mean. Do they mean alternate histories of our own universe, like those often shown in science fiction, or whole other universes with no real connection to ours?

Physicists use the phrase "parallel universes" to discuss diverse concepts, and it can sometimes get a little confusing. For example, some physicists believe strongly in the idea of a multiverse for cosmological purposes, but don't actually believe in the Many Worlds Interpretation (MWI) of quantum physics.

It is important to realize that parallel universes are not actually a theory within physics, but rather a conclusion that comes out of various theories within physics. There are a variety of reasons for believing in multiple universes as a physical reality, mostly having to do with the fact that we have absolutely no reason to suppose that our observable universe is all that there is. 

There are two basic breakdowns of parallel universes that might be helpful to consider. The first was presented in 2003 by Max Tegmark and the second was presented by Brian Greene in his book "The Hidden Reality."

Tegmark's Classifications

In 2003, MIT physicist Max Tegmark explored the idea of parallel universes in a paper published in a collection titled "Science and Ultimate Reality " . In the paper, Tegmark breaks the different types of parallel universes allowed by physics into four different levels:

• Level 1: Regions Beyond Cosmic Horizon: The universe is essentially infinitely big and contains matter at roughly the same distribution as we see it throughout the universe. Matter can combine in only so many different configurations. Given an infinite amount of space, it stands to reason there exists another portion of the universe in which an exact duplicate of our world exists.
• Level 2: Other Post-Inflation Bubbles: Separate universes spring up like bubbles of spacetime undergoing its own form of expansion, under the rules dictated by inflation theory. The laws of physics in these universes could be very different from our own.
• Level 3: The Many Worlds of Quantum Physics: According to this approach to quantum physics, events unfold in every single possible way, just in different universes. Science fiction "alternate history" stories utilize this sort of a parallel universe model, so it's the most well-known outside of physics.
• Level 4: Other Mathematical Structures: This type of parallel universes is sort of a catch-all for other mathematical structures which we can conceive of, but which we don't observe as physical realities in our universe. The Level 4 parallel universes are ones which are governed by different equations from those that govern our universe. Unlike Level 2 universes, it's not just different manifestations of the same fundamental rules, but entirely different sets of rules.

Greene's Classifications

Brian Greene's system of classifications from his 2011 book, "The Hidden Reality," is a more granular approach than Tegmark's. Below are Greene's classes of parallel universes, but we've also added the Tegmark Level that they fall under: 

• Quilted Multiverse (Level 1): Space is infinite, therefore somewhere there are regions of space that will exactly mimic our own region of space. There is another world "out there" somewhere in which everything is unfolding exactly as it unfolds on Earth.
• Inflationary Multiverse (Level 1 & 2): Inflationary theory in cosmology predicts an expansive universe filled with "bubble universes," of which our universe is just one.
• Brane Multiverse (Level 2): String theory leaves open the possibility that our universe is on just one 3-dimensional brane , while other branes ​of any number of dimensions could have whole other universes on them.
• Cyclic Multiverse (Level 1): One possible result from string theory is that branes could collide with each other, resulting in universe-spawning big bangs that not only created our universe but possibly other ones.
• Landscape Multiverse (Level 1 & 4): String theory leaves open a lot of different fundamental properties of the universe which, combined with the inflationary multiverse, means there could be many bubble universes out there which have fundamentally different physical laws than the universe we inhabit.
• Quantum Multiverse (Level 3): This is essentially the Many Worlds Interpretation (MWI) of quantum mechanics; anything that can happen does... in some universe.
• Holographic Multiverse (Level 4): According to the holographic principle, there is a physically-equivalent parallel universe that would exist on a distant bounding surface (the edge of the universe), in which everything about our universe is precisely mirrored.
• Simulated Multiverse (Level 4): Technology will possibly advance to the point where computers could simulate each and every detail of the universe, thus creating a simulated multiverse whose reality is nearly as complex as our own.
• Ultimate Multiverse (Level 4): In the most extreme version of looking at parallel universes, every single theory which could possibly exist would have to exist in some form somewhere.
• Multiverse Definition and Theory
• The Many Worlds Interpretation of Quantum Physics
• Five Great Problems in Theoretical Physics
• Understanding the "Schrodinger's Cat" Thought Experiment
• What Is the Anthropic Principle?
• The Copenhagen Interpretation of Quantum Mechanics
• Leonard Susskind Bio
• The Basics of String Theory
• The Different Fields of Physics
• The Discovery of the Higgs Energy Field
• Understanding Cosmology and Its Impact
• Quantum Physics Overview
• Quantum Computers and Quantum Physics
• History and Properties of M-Theory
• The History of Gravity
• Description & Origins of Inflation Theory

What is Einstein’s theory of parallel universes?

By that logic, the solitary bubble introduced by Einstein now becomes a bubble bath of parallel universes, constantly splitting in two or bumping into other bubbles. In this scenario, a Big Bang could perpetually happen in distant regions, representing the collision or merging of these bubble universes.

What is the concept of parallel universe?

A parallel universe, also known as a parallel dimension, alternate universe, or alternate reality, is a hypothetical self-contained plane of existence, co-existing with one’s own. The sum of all potential parallel universes that constitute reality is often called a “multiverse”.

Is parallel universe a thing?

Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science. Our universe is unimaginably big.

How many parallel universes do scientists believe in?

In a new study, Stanford physicists Andrei Linde and Vitaly Vanchurin have calculated the number of all possible universes, coming up with an answer of 10^10^16.

How do you enter a parallel universe?

How are parallel universes created?

According to Everett, observing quantum matter causes an actual split in the universe rather than a choice of one state over another. The universe literally duplicates, splitting into multiple parallel universes for each possible state of the quantum matter.

Is the multiverse theory a paradox?

The theory resolves a cosmic paradox of the late physicist’s own making. It also points a way forward for astronomers to find evidence of the existence of parallel universes.

What is quantum multiverse?

Quantum. The quantum multiverse creates a new universe when a diversion in events occurs, as in the real-worlds variant of the many-worlds interpretation of quantum mechanics.

What did Stephen Hawking think of the multiverse?

So, in his very last paper in 2018, Hawking sought, in his own words, to “try to tame the multiverse.” He proposed a novel mathematical framework that, while not dispensing with the multiverse altogether, rendered it finite rather than infinite.

Who gave the theory of parallel universe?

One evening in 1954, in a student hall at Princeton University, grad student Everett was drinking sherry with his friends when he came up with the idea that quantum effects cause the universe to constantly split. Read more: “Multiverse me: Should I care about my other selves?

What’s the difference between multiverse and parallel universe?

The difference therefore is that a Multiverse is the name for all the parallel universes within the multiverse and parallel universe is just one instance of an universe.

What are the 10 dimensions?

• Probability (Possible Universes)
• All Possible Universes Branching from the Same Start Conditions.
• All Possible Spectrums of Universes with Different Start Conditions.

What is beyond the multiverse?

The Beyond is an unobservable space outside the Multiverse. It is the remnants of the Second Cosmos and is inhabited by the Beyonders.

How many dimensions do we live in?

In everyday life, we inhabit a space of three dimensions – a vast ‘cupboard’ with height, width and depth, well known for centuries. Less obviously, we can consider time as an additional, fourth dimension, as Einstein famously revealed.

How many dimensions are there?

The world as we know it has three dimensions of space—length, width and depth—and one dimension of time. But there’s the mind-bending possibility that many more dimensions exist out there. According to string theory, one of the leading physics model of the last half century, the universe operates with 10 dimensions.

Is it possible to travel between universes?

It’s not transporters or spore drives: According to Futurism, theoretical physicist Michio Kaku suggests our universe will eventually wind up in a “big freeze” as expansion slows and stops; combined with technology, this may allow inter-universe transport.

How does the multiverse work?

What is the Multiverse? To put it simply, the multiverse is the theory that there are infinite variations of the same universe that each have unique distinctions from the last; some drastic and some small. While in one alternate universe you may like different food, in another you may not even exist.

How many dimensions are there in quantum physics?

Superstring theory posits that the universe exists in 10 dimensions at once.

What dimension is omniverse?

The Omniverse is defined as (1) the totality of all physical universes in the Multiverse, plus (2) the Spiritual Dimensions including the Afterlife, Intelligent Civilization of Souls, Spiritual Beings, and Source.

What is bigger than the multiverse?

megaverse = more then one multiverse, possibly infinite number of multiverses. Omniverse = every single universe, so every piece of fiction, infinite number of megaverses.

What is beyond the universe?

The trite answer is that both space and time were created at the big bang about 14 billion years ago, so there is nothing beyond the universe. However, much of the universe exists beyond the observable universe, which is maybe about 90 billion light years across.

What is a omniverse?

: a universe that is spatiotemporally four-dimensional.

What is parallel paradox?

Parallel Paradox is a graphic novel based on Ben 10: Omniverse. It takes place sometime between So Long, and Thanks for All the Smoothies and The Frogs of War: Part 1.

Did Stephen Hawking believe in M theory?

Hawking contends that M-theory shows great promise in explaining the circumstances of the universe’s dense, hot beginning, known as the Big Bang, and the unique characteristics of the cosmos that resulted.

What is decoherence physics?

The term decoherence is used in many fields of (quantum) physics to describe the disappearance or absence of certain superpositions of quantum states. Decoherence is a consequence of the unavoidable interaction of virtually all physical systems with their environment.

Privacy Overview