time travel theories

Is Time Travel Possible?

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

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

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

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

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

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

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

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

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

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

In Summary:

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

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Time travel: Is it possible?

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

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

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

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

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

Related: The best sci-fi time machines ever

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

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

Related: Controversially, physicist argues that time is real

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

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

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

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

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

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

General relativity and GPS time travel

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

Read more: Can we stop time?

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

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

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

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

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

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

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

Can wormholes take us back in time?

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

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

Related: Best time travel movies

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

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

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

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

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

Alternate time travel theories

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

Infinite cylinder theory

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

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

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

Time donuts

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

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

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

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

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

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

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

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

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

Related: Marvel movies in order: chronological & release order

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

Books about time travel:

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

Movies about time travel:

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

Television about time travel:

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

Games about time travel:

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

Additional resources

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

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

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April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

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“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

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

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

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

Turns out, King might have been on to something.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• time travel
• grandfather paradox

Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

Disclosure statement

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

Brock University provides funding as a member of The Conversation CA-FR.

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time? 

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative. 

Learn more: Best travel insurance

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said. 

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said. 

She also mentioned the anecdote of  Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

What did Albert Einstein invent?: Discoveries that changed the world

Just Curious for more? We've got you covered

USA TODAY is exploring the questions you and others ask every day. From "How to watch the Marvel movies in order" to "Why is Pluto not a planet?" to "What to do if your dog eats weed?" – we're striving to find answers to the most common questions you ask every day. Head to our Just Curious section to see what else we can answer for you. 

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

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

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

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

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

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

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

Time is fleeting

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

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

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

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

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

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

Time after time

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

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

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

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

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

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

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

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

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

Originally published on Live Science .

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

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Time Travel: Theories, Possibilities, and Paradoxes Explained

By Neha Rastogi

Time Travel has been a matter of great interest for Science fiction since ages. Whether it’s the movies like Planet of the Apes (1968) or modern franchises like “Doctor Who” and “Star Trek” ; the concept is grabbing a lot of eyeballs. Not only movies and shows but even some mythological tales like Mahabharata and the Japanese story of Urashima Taro support the evidence that time travel exists. We often see stories where characters use time machines to jaunt through the years but the reality is far more complex and inexplicable.

Understanding the Concept of Time Travel

Time Travel is defined as the phenomenon of moving between different points in time through a hypothetical device called “Time Machine”. Despite being predominantly related to the field of philosophy and fiction, it’s somehow supported to a small extent by physics in conjunction with quantum mechanics. However, before getting into the argument of how real it is, let’s comprehend the fundamental meaning of time.

Basically, the whole idea of Time Travel is administered by the concept of time. Usually, people believe that time is constant but the famous Physicist Albert Einstein introduced the “Theory of Relativity” as per which, time is relative. In other words, time slows down or speeds up depending on how fast the observer moves relative to something else. According to him, a person traveling inside a spaceship at the speed of light would age much slower than his/her twin back at home.

Time is Relative

After Einstein’s Theory of Relativity, his teacher Herman Minkowski emphasized on space-time, a mathematical model that joins both space and time in a continuum. This implies that time and space cannot exist without each other. Space is a 3-dimensional arena consisting of length, width, and height. This is joined by Time with the fourth dimension called direction. So anything that happens in the universe takes place in this space-time continuum. Although this validates that space travelers are slightly younger than their twins when they return to earth, yet a huge leap in the past or future is not possible with the current technology.

Time Machines

It is believed that in order to travel back or forward in time, one would require a device called Time Machine . The research on such a device would involve bending space-time to such an extent that time lines turn back on themselves to form a loop, which is termed as “closed time-like curve.” Such an action demands the use of an exotic form of matter with “negative energy density” that has a unique property of moving in the opposite direction of the normal matter when pushed. Even if it exists, the quantity would be too small to construct a machine.

Pictorial Representation of Time Travel through closed time-like curve

However, some another research suggests that time machines can also be constructed by building a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped hole filled with vacuum, gravitational force can be used to bend the space-time so as to form a closed time-like curve. After racing around inside this doughnut a traveler would be able to go back in time with each lap. But in reality, it’s quite complex because the gravitational fields have to be very strong and would demand precise manipulation.

Time Travel Approaches in Physics

After studying and researching about Time Travel, various physicists have come up with approaches that may support its possibility, at least theoretically. Let’s take a look at these concepts so as to understand how Time Travel could actually work someday.

Time Dilation

Time Dilation Explanation

An important aspect of Einstein’s relativity theory is the term “time dilation” , which is defined as the difference of elapsed time between two events as measured by observers who are either moving relative to each other or are situated at different locations from the gravitational mass. As per the theory, time dilation can be summarized as a phenomenon which occurs due to the difference in either gravity or relative velocity.

In special relativity the time dilation effect is reciprocal i.e. when two clocks are in motion with respect to each other, for both the observers, the other one will be time dilated or the other clock will move slower. However, in general relativity, an observer at the top of the tower will find the clock closer to the ground to be slower and the other observer would agree about the direction and magnitude of this difference.

Due to the concept of time dilation, the current human time travel record is held by Russian cosmonaut Sergei Krikalev . Owing to the high-speed (7.66 km/s) of ISS and the length of time spent in space, it is believed that the cosmonaut actually arrived 0.02 seconds in the future while returning to the earth.

Cosmic String

Diagram Depicting Cosmic Strings

In 1991 J Richard Gott gave the idea of Cosmic Strings , which are believed to be left over from the early cosmos. These are defined as string-like objects or narrow tubes of energy that are stretched across the entire length of the universe. Owing to the huge amount of mass and massive gravitational pull, it would allow objects attached to the Cosmic Strings to travel at the speed of light.

So if two strings are pulled close to each other or one of them is stretched near the black hole, it might warp space-time to such an extent that would lead to creating a closed time-like curve and hence leading to the possibility of time travel. Theoretically, the gravity generated by these two Cosmic strings would help in propelling a spaceship into the past.

However, coming to the reality, the loop of strings is required to contain half the mass-energy of an entire galaxy so as to travel one year back in time. This implies that powering a time machine would require splitting half the atoms present in the whole galaxy.

Black holes

Illustration of Kerr Hole

When stars (having a mass of more than four times our sun) reach their end of life and all their fuel is burned up, they collapse under the pressure of their own weight creating “Black Holes” . The boundary of a Black Hole, called Event Horizon , has such a strong gravitational pull that it doesn’t even allow light to pass through it. Since light travels at the fastest speed, everything else traveling through a black hole is also dragged back. Such a non-rotating black hole is named as Schwarzschild black hole .

However, traveling to a parallel universe is possible through a rotating black hole named Kerr Hole . It was proposed in 1963 by a mathematician named Roy Kerr . As per his theory, if dying stars collapse into a rotating ring of neutron stars, that would produce enough centrifugal force to prevent the formation of singularity.

Note: Singularity can be perceived as the point into which the black hole tapers much like an ice-cream cone. At this point, the laws of Physics cease to exist and all the matter is crushed beyond recognition.

Since there will be no singularity, it would be safe to pass through a black hole without being crushed and exit out of a “White Hole” . A white hole is believed to be the exhaust end of a black hole which pushes everything away from it. Hence we may travel into another time or even another universe.

Although Kerr Holes are just theoretical, if they exist then we may find our way to a one-way trip to the past or future. However, physicist Kip Thorne believes that such a black hole doesn’t exist and it would suck everything before someone even reaches the Singularity.

Diagrammatic Representation of Wormhole 

Wormholes, also known as Einstein-Rosen Bridges , are believed to be the most potential means for time travel. It could allow us to travel several light years from earth and in much less time as compared to the conventional space travel methods. The possibility of wormholes is based on Einstein’s theory of relativity which says that any mass curves space-time. The following example is used to explain this curvature.

If two persons are holding a bed sheet stretching it tight and a baseball is placed on the sheet, its weight will make it roll to the middle of the sheet creating a curve at that point. Now if a marble is placed on the sheet, it would travel towards the baseball because of the curve. Here space is depicted as a two-dimensional plane than the four dimensions that actually makes up space-time.

Now if this sheet is folded over leaving a space at the top and bottom, placing the baseball on the top would form a curvature. If an equal mass is placed at the bottom part at a point corresponding to the location of the baseball, the second mass would eventually meet with the baseball. Similarly, wormholes might develop.

In space, masses that place pressure on different parts of the universe combine together to form a tunnel. Theoretically, this tunnel joins two separate times and allows passage between them. However, it’s possible that certain unforeseen physical properties may prevent the occurrence of wormholes and even if they exist, these might be really unstable.

Possibly someday human may learn to capture, stabilize and enlarge these tunnels but according to Dr. Hawking, prolonging the life of a tunnel through folded space-time may lead to a radiation feedback loop destroying the time tunnel.

Time Travel Paradoxes

If we ever work out a theory for time travel, we would give way to certain complexities known as paradoxes. A paradox is something that contradicts itself. In other words, time travel is not believed to be a practical concept because of certain situations that are likely to arise as the after-effects. These are broadly classified as -:

1. Closed Casual Loops: The cause and effect run in a circle causing a loop and is also internally consistent with the timeline’s history.

Diagram depicting time loop

• Predestination Paradox

It is defined as a situation when a traveler going back in time causes the event which he is trying to prevent from happening. It implies that any attempt to stop any event from occurring in the past would simply lead to the cause itself. The paradox suggests that things are destined to turn out the way they have happened and anyone attempting to change the past would find himself trapped in the repeating loop of time. For example, if you travel in the past to prevent your lover from dying in a road accident, you will find out that you were the one who accidentally ran over her.

• Bootstrap Paradox

A bootstrap paradox, also known as an Ontological Paradox where an object, person, or piece of information sent back in time leads to an infinite loop where the object has no discernible origin and is believed to exist without ever being created. It implies that the past, present and future and not defined, thus making it complicated to pinpoint the origin of anything. It raises questions like how were the objects created and by whom.

2. Consistency Paradox: It generates a number of timeline inconsistencies related to the possibility of altering the past. It can be further divided into the following categories.

• The Grandfather Paradox

Grandfather Paradox

This paradox talks about a hypothetical situation where a person travels back in time and kills his paternal grandfather at the time when his grandfather didn’t even meet his grandmother. In such a situation, his father would never have been born and neither would the traveler himself. So if he was never born, how would he travel to the past to kill his grandfather?

The paradox also talks about auto-infanticide where a time traveler goes into the past to kill himself when he was an infant. Now if he killed himself when he was a kid, how would he exist in the future to come back in time? Some physicists say that you would be able to go back in time but you won’t be able to change it, while others suggest that you would be born in one universe but unborn in another universe.

• The Hitler Paradox

Similar to the grandfather paradox, the killing Hitler paradox erases the reason for which you would want to go into the past and kill Hitler. Moreover, killing grandfather might have a “butterfly effect” but killing Hitler would have a far-reaching impact on the History as it would change the whole course of events. If you were successful in killing Hitler, there’d be no reason that would make you want to go back in time and kill him.

This paradox has been explained very well in a Twilight Zone episode called “Cradle of Darkness” as well as an episode “Let’s Kill Her” from Dr. Who.

• Polchinski’s Paradox

American physicist Joseph Polchinski proposed a paradox where a billiard ball enters a wormhole and emerges out of the other end in the past just in time to collide with its younger version and prevents it from entering the wormhole in the first place. While proposing this scenario, Joseph had Novikov’s Self Consistency Principle in his mind which states that time travel is possible but time paradoxes are forbidden.

A number of solutions have been suggested to avoid these inconsistencies like the billiard ball will deliver a blow which changes the course of the younger version of the ball but it would not stop it from entering the wormhole. This also explains that if you go back in time to kill your grandfather then something or the other will happen to prevent you from making it happen thus preserving the consistency of the History.

Solutions for the Paradoxes

In order to come up with a solution for these above-mentioned paradoxes, scientists have proposed some explanations which are enlisted below

The Solution: Time Travel is impossible because of the paradoxes that it creates.

Self-Healing Hypothesis: If we succeed to change the events in the past, it will set off another set of events that will keep the present unchanged.

The Multiverse: Every time an event in the past is altered, an alternate parallel universe or timeline is created.

Erased Timeline Hypothesis: A person traveling to the past would exist in the new timeline but their own timeline would be erased.

Is Time Travel Possible?

Nobody seems to have a definite answer in support or against the existence of Time Travel. On one hand, Einstein suggested to traveling at the speed of light in order to jaunt through the future but this would mean an unimaginable amount of energy would be required. Moreover, the centrifugal force on the body would prove to be fatal. Although it has been observed that space travelers age a little slower as compared to their identical twin on earth but some believe that there is no definite answer to travel back in space.

Theoretical physicist Brian Greene of Columbia University says that “No one has given a definite proof that you can’t travel to the past. But every time we look at the proposals and detail it seems kind of clear that they’re right at the edge of the known laws of physics.” Besides, Prof. Hawking feels that “Today’s science fiction is tomorrow’s science fact.”

However, the paradoxes, especially the grandfather paradox, have imposed a big question mark on the possibility of Time Travel. Basically, with the present laws and knowledge of Physics, the human won’t be able to survive in the process of Time Travel. So, we need certain developments in the quantum theories till we are sure as to how the paradoxes can be solved.

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Discussions of the nature of time, and of various issues related to time, have always featured prominently in philosophy, but they have been especially important since the beginning of the twentieth century. This article contains a brief overview of some of the main topics in the philosophy of time—(1) fatalism; (2) reductionism and Platonism with respect to time; (3) the topology of time; (4) McTaggart’s argument; (5) the A-theory and the B-theory; (6) presentism, eternalism, and the growing block theory; (7) the 3D/4D debate about persistence; (8) the dynamic and the static theory; (9) the moving spotlight theory; (10) time travel; (11) time and physics and (12) time and rationality. We include some suggestions for further reading on each topic and a bibliography.

Note: This entry does not discuss the consciousness, perception, experience, or phenomenology of time. A historical overview and general presentation of the various views is available in the entry on temporal consciousness . Further coverage can be found in the SEP entry on the experience and perception of time . For those interested specifically in phenomenological views, see the entries on Husserl (Section 6), and Heidegger (Section 2: Being and Time).

1. Fatalism

2. reductionism and platonism with respect to time, 3. the topology of time, 4. mctaggart’s argument, 5. the a-theory and the b-theory, 6. presentism, eternalism, and the growing block theory, 7. three-dimensionalism and four-dimensionalism, 8. the dynamic and the static theory, 9. the moving spotlight theory, 10. time travel, 11. time and physics, 12. time and rationality, other internet resources, related entries.

Many logical questions about time historically arose from questions about freedom and determinism—in particular worries about fatalism. Fatalism can be understood as the doctrine that whatever will happen in the future is already unavoidable (where to say that an event is unavoidable is to say that no agent is able to prevent it from occurring). Here is a typical argument for fatalism:

The conclusion appears shocking. Future moral catastrophes are unavoidable. Every weighty decision that now feels up to you is already determined.

The argument for fatalism makes some significant metaphysical assumptions that raise more general questions about logic, time, and agency.

For example, Premise (1) assumes that propositions describing the future do not come into or go out of existence. It assumes that there are propositions now that can accurately represent every future way things might go. This is a non-trivial logical assumption. You might, for instance, think that different times becoming present and actual (like perhaps possible worlds) have different associated sets of propositions that become present and actual.

Premise (2) appears to be a fundamental principle of semantics, sometimes referred to as the Principle of Bivalence.

The rationale for premise (4) is that it appears no one is able to make a true prediction turn out false. (4) assumes that one and the same proposition does not change its truth value over time. The shockingness of the conclusion also depends on identifying meaningful agency with the capacity to make propositions come out true or false.

A proper discussion of fatalism would include a lengthy consideration of premises (1) and (4), which make important assumptions about the nature of propositional content and the nature of agency. That would take us beyond the scope of this article. For our purposes, it is important to note that many writers have been motivated by this kind of fatalist argument to deny (2), the Principle of Bivalence. According to this line, there are many propositions—namely, propositions about events that are both in the future and contingent—that are neither true nor false right now. Consider the proposition that you will have lunch tomorrow. Perhaps that proposition either has no truth value right now, or else has a third truth value: indeterminate. When the relevant time comes, and you either have lunch or don’t, then the proposition will come to be either true or false, and from then on that proposition will forever retain that determinate truth value.

This strategy for rejecting fatalism is sometimes referred to as the “Open Future” response. The Open Future response presupposes that a proposition can have a truth value, but only temporarily—truth values for complete propositions can change as time passes and the world itself changes. This raises further questions about the correct way to link up propositions, temporal passage and truth values. For example, which of the following formulas expresses a genuine proposition about the present?

Tensed Proposition: “Sullivan is eating a burrito”.

Tenseless Proposition: “Sullivan eats a burrito at <insert present time stamp>”.

The tensed proposition will no longer be true when Sullivan finishes her lunch. So it has, at best, a temporary truth value. The tenseless proposition expresses something like “Sullivan eats a burrito at 3pm on July 20th 2019”. That proposition is always true.

Some philosophers argue that only the latter, eternally true kind of proposition could make sense of how we use propositions to reason over time. We need propositions to have stable truth values if we are to use them as the contents of thoughts and communication. Other philosophers—particularly those who believe that reality itself changes over time—think that tensed propositions are needed to accurately reason about the world. We’ll return to these issues in Section 4 and Section 5 .

Suggestions for Further Reading: Aristotle, De Interpretatione , Ch. 9; Barnes and Cameron 2009; Boethius, The Consolation of Philosophy , Book V; Crisp 2007; Evans 1985; Lewis 1986; Markosian 1995; McCall 1994; Miller 2005; Richard 1981; Sullivan 2014; Taylor 1992; Torre 2011; Van Inwagen 1983.

What if one day things everywhere ground to a halt? What if birds froze in mid-flight, people froze in mid-sentence, and planets and subatomic particles alike froze in mid-orbit? What if all change, throughout the entire universe, completely ceased for a period of, say, one year? Is such a thing possible?

If the answer to this last question is “yes”—if it is possible for there to be time without change—then time is in some important sense independent of the events within time. Other ways of investigating whether time is independent of the events within time include asking whether all of the physical processes that happen in time could happen at a faster or slower rate, and asking whether all events could have happened slightly earlier or later in time. After all, if every physical process could suddenly happen twice as fast, or if every event could take place slightly earlier or later in time, then it follows that in some important sense time can remain the same even if the way that events are distributed in time changes wholesale.

Aristotle and Leibniz, among others, have argued that time is not independent of the events that occur in time. This view is typically called either “reductionism with respect to time” or “relationism with respect to time”, since according to this view, all talk that appears to be about time can somehow be reduced to talk about temporal relations among things and events. The opposing view, normally referred to either as “Platonism with respect to time” or “substantivalism with respect to time” or “absolutism with respect to time”, has been defended by Plato, Newton, and others. On this view, time is like an empty container into which things and events may be placed; but it is a container that is independent of what (if anything) is placed in it.

Another way to present this distinction is to say that those like Plato and Newton who think that time is independent of the events that occur in time believe in “absolute time”. Those like Aristotle and Leibniz, who think that time is not independent of the events that occur in time, deny the existence of absolute time, though they still endorse “relative time”, where relative time is nothing over and above the temporal relations between events.

These views about time are closely connected to views about space and about motion. Most obviously, these views about time have straightforward spatial analogues—one may be a substantivalist about space (and thus endorse the existence of absolute space in addition to spatial relations between things), or one may be a relationist about space (and thus deny the existence of absolute space). Substantivalism and relationism about time have traditionally been taken to stand or fall with their spatial counterparts. In addition, the choice between substantivalism and relationism about space and time has consequences for your theory of motion. If you are a relationist about space and time then you must also be a relationist about motion: all motion is motion relative to something. If you are a substantivalist about space and time, you will endorse, in addition to relative motion, the notion of absolute motion, where absolute motion is motion relative to absolute space and time. If you are a substantivalist, in addition to facts about whether and how fast a train car is moving relative to the track, whether and how fast it is moving relative to the cars, and so on, there will also be a fact about whether and how fast the train car is really moving—whether and how fast it is moving relative to absolute space and time.

Why would someone endorse the existence of absolute time? One reason is that the empty container metaphor has a lot of intuitive appeal. Another reason is that some philosophers have thought that there must be such a thing as absolute motion—as opposed to merely relative motion—in order to explain certain physical phenomena, like the forces felt during acceleration. Newton had an especially famous argument along these lines involving a spinning bucket of water—the entry on Newton’s views on space, time , and motion has a careful discussion of this argument.

Why would someone deny the existence of absolute time? Some relationists have put forward arguments that are supposed to show that absolute space and time are philosophically problematic in some important way. Perhaps most famously, Leibniz argued that the existence of absolute space or time would lead to violations of the principle of sufficient reason and violations of the identity of indiscernibles.

In order to see why, consider two ways of describing the way things could be. On the one hand, everything is as it actually is. On the other, every event happens one second later than it actually does, but is otherwise exactly the same. If there is such a thing as absolute time then these two descriptions would pick out distinct possible worlds. But this, Leibniz claimed, would violate the principle of sufficient reason. For given that the actual world and the one-second-late world are exactly the same except for where things are located in absolute time, there could not (at least according to Leibniz) be any reason why one exists rather than the other. Moreover, Leibniz claimed, the actual world and the one-second-late world are indistinguishable; so if they were in fact distinct possible worlds, that would violate the principle that if two things are indistinguishable, then they are identical.

Leibniz’s arguments are examples of arguments that attempt to identify something philosophically problematic with absolute time and space. Perhaps more generally, many philosophers have been moved by the idea that even if absolute time and space are not problematic in a way that makes them unacceptable, they are still the kinds of things that we should do without if we can. This kind of attitude can be motivated by a straightforward kind of parsimony—we should always make do with the fewest types of entities possible. Or it can be motivated by a more specific worry about the nature of absolute space and time. You might, for instance, be especially loath to admit unobservable entities into your ontology—you are willing to admit them if you must, but you would rather eliminate them wherever possible. As absolute space and time are unobservable, someone who endorses this attitude will be inclined to think there are no such things.

Suggestions for Further Reading : Alexander 1956; Ariew 2000; Arntzenius 2012; Coope 2001; Mitchell 1993; Newton, Philosophical Writings ; Newton-Smith 1980; Shoemaker 1969.

It’s natural to think that time can be represented by a line. But a line has a shape. What shape should we give to the line that represents time? This is a question about the topology, or structure, of time.

One natural way to answer our question is to say that time should be represented by a single, straight, non-branching, continuous line that extends without end in each of its two directions. This is the “standard topology” for time. But for each of the features attributed to time in the standard topology, two interesting questions arise: (a) does time in fact have that feature? and (b) if time does have the feature in question, is this a necessary or a contingent fact about time?

Questions about the topology of time appear to be closely connected to the issue of Platonism versus relationism with respect to time. For if relationism is true, then it seems likely that time’s topological features will depend on contingent facts about the relations among things and events in the world, whereas if Platonism is true, so that time exists independently of whatever is in time, then time will presumably have its topological properties as a matter of necessity. But even if we assume that Platonism is true, it’s not clear exactly what topological properties should be attributed to time.

Consider the question of whether time should be represented by a line without a beginning (so a line, rather than a line segment). Aristotle has argued (roughly) that time cannot have a beginning on the grounds that in order for time to have a beginning, there must be a first moment of time, but that in order to count as a moment of time, that allegedly first moment would have to come between an earlier period of time and a later period of time, which is inconsistent with its being the first moment of time. (Aristotle argues in the same way that time cannot have an end.)

Aristotle’s argument may or may not be a good one, but even if it is unsound, many people will feel, purely on intuitive grounds, that the idea of time having a beginning (or an end) just does not make sense. And here we have an excellent illustration of what is at stake in the controversy over whether time has its topological properties as a contingent matter or as a matter of necessity. For suppose we come to have excellent evidence that the universe itself had a beginning in time. (This seems like the kind of thing that could be supported by empirical evidence in cosmology.) This would still leave open the question of whether the beginning of the universe occurred after an infinitely long period of “empty” time, or, instead, coincided with the beginning of time itself. There are interesting and plausible arguments for each of these positions.

It is also worth asking whether time must be represented by a single line. Perhaps we should take seriously the possibility of time’s consisting of multiple time streams, each one of which is isolated from each other, so that every moment of time stands in temporal relations to other moments in its own time stream, but does not bear any temporal relations to any moment from another time stream. Likewise we can ask whether time could correspond to a branching line (perhaps to allow for the possibility of time travel or to model an open future), or to a closed loop, or to a discontinuous line. And we can also wonder whether one of the two directions of time is in some way privileged, in a way that makes time itself asymmetrical. (We say more about this last option in particular in the section on time and physics.)

Suggestions for Further Reading: (1) On the beginning and end of time: Aristotle, Physics , Bk. VIII; Kant, The Critique of Pure Reason (especially pp. 75ff); Newton-Smith 1980, Ch. V. (2) On the linearity of time: Newton-Smith 1980, Ch. III; Swinburne 1966, 1968. (3) On the direction of time: Price 1994, 1996; Savitt 1995; and Sklar 1974. (4) On all of these topics: Newton-Smith 1980.

In a famous paper published in 1908, J.M.E. McTaggart argued that there is in fact no such thing as time, and that the appearance of a temporal order to the world is a mere appearance. Other philosophers before and since (including, especially, F.H. Bradley) have argued for the same conclusion. We will focus here only on McTaggart’s argument against the reality of time, which has been by far the most influential.

McTaggart begins his argument by distinguishing two ways in which positions in time can be ordered. First, he says, positions in time can be ordered according to their possession of properties like being two days future, being one day future, being present, being one day past, etc. These properties are often referred to now as “ A properties” because McTaggart calls the series of times ordered by these properties “the A series”. But he says that positions in time can also be ordered by two-place relations like two days earlier than, one day earlier than, simultaneous with, etc. These relations are now often called “ B relations” because McTaggart calls the series of times ordered by these relations “the B series”.

McTaggart argues that the B series alone does not constitute a proper time series; the A series is essential to time. His reason for this is that he assumes change is essential to time, and the B series without the A series does not involve genuine change (since B series positions are forever “fixed”, whereas A series positions are constantly changing).

McTaggart also argues that the A series is inherently contradictory. For, he says, the different A properties are incompatible with one another. No time can be both future and past, for example. Nevertheless, he insists, each time in the A series must possess all of the different A properties, since a time that is future will be present and then will be past. McTaggart concludes that, since neither the A-series nor the B-series can order the time series, time is unreal.

One response to this argument that McTaggart anticipates involves claiming that it’s not true of any time, t , that t is both future and past. Rather, the objection goes, we must say that it was future at some moment of past time and will be past at some moment of future time. But this objection fails, according to McTaggart, because the additional times that are invoked in order to explain t ’s possession of the incompatible A properties must themselves possess all of the same A properties (as must any further times invoked on account of these additional times, and so on ad infinitum ). Thus, according to McTaggart, we never resolve the original contradiction inherent in the A series, but, instead, merely generate an infinite regress of more and more contradictions.

McTaggart’s argument has had staying power because it organizes crucial debates about the metaphysics of temporal passage, because it hints at how those debates connect to further debates about where evidence for the time series and the nature of change come from, and because the difference between A-theoretic and B-theoretic approaches to the debate has continued in the intervening century.

Suggestions for Further Reading: Bradley 1893; Dyke 2002; McTaggart 1908; Mellor 1998; Prior 1967, 1968.

In Section 1 , we introduced the distinction between a tensed proposition and a tenseless proposition. Tensed propositions can fully and accurately describe the world, but nevertheless change truth value over time. Tenseless propositions, on the other hand, are always true or always false—they reference a particular time in the proposition and never change. Propositions represent ways reality could be. So, which view of propositions we adopt depends on what we think it means for reality itself to undergo change.

In section 4 , we discussed McTaggart’s distinction between time conceived of as a B-series (events ordered by which come before and which come after) and time conceived of as an A-series (events ordered by which are present, which are past, and which are future). Though not particularly creative as names, the A/B distinction has stuck around as a way of classifying theories of change.

B-theorists think all change can be described in before-after terms. They typically portray spacetime as a spread-out manifold with events occurring at different locations in the manifold (often assuming a substantivalist picture). Living in a world of change means living in a world with variation in this manifold. To say that a certain autumn leaf changed color is just to say that the leaf is green in an earlier location of the manifold and red in a later location. The locations, in these cases, are specific times in the manifold. And all of the metaphysically important facts about change can be captured by tenseless propositions like “The leaf is red at October 7, 2019”. “The leaf is not red at September 7, 2019”.

A-theorists, on the other hand, believe that at least some important forms of change require classifying events as past, present or future. And accurately describing this kind of change requires some tensed propositions—there is a way reality is (now, presently) which is complete but was different in the past and also will be different in the future. These tensed propositions also explain why we tend to attribute significance to the past-present-future distinction. For example, you might think the A-theorist is in a better position to explain why we care whether a horrible event is already in the past versus still in the future. Some A-theorists will argue that we aren’t concerned with location—we care that the event is over with in reality.

Note, also, there is a significant range of views within the A-theory camp about whether there is a spacetime manifold (Moving Spotlighters think there is), or whether only present events are real (the presentist view), or whether only present and past events are real (the Growing Block view). We say more about all of these views below. A-theorists also debate whether objects themselves undergo A-theoretic change or whether it is only entire regions of spacetime that change this way.

A-theorists and B-theorists appeal to different sources of evidence for their different views of passage. A-theorists typically emphasize how psychologically we seem to perceive a world of robust passage or “flow” of time. In physics, the laws of thermodynamics seem to imply a strong past-to-future direction to time. And quantum mechanics seems to identify an important sense of simultaneity, which could be identified with presentness (see section 11 below). Finally many commonsense ways of thinking of change seem to rely on A-theory descriptions of passage. For instance, they will use the fact that we care so much about whether bad events are past as evidence that there are ineliminable tensed propositions and those propositions represent ineliminable A-properties.

B-theorists typically emphasize how special relativity eliminates the past/present/future distinction from physical models of space and time. Thus what seems like an awkward way to express facts about time in ordinary English is actually much closer to the way we express facts about time in physics. Moreover, thinking of change in tenseless terms makes it easier to describe in a logically consistent way how objects survive change—objects have properties only relative to particular times, so there is no worry about attributing absolutely inconsistent properties to anything. We’ll consider some of these arguments in more detail in the remaining sections of this entry, as we consider more specific variations on A-theories and B-theories of time.

Suggestions for Further Reading: For general discussion of The A theory and The B theory: Emery 2017; Le Poidevin 1998; Le Poidevin and McBeath 1993; Markosian 1993; Maudlin 2007 (especially Chapter 4); Mellor 1998; Paul 2010; Prior 1959 [1976], 1962 [1968], 1967, 1968, 1970, 1996; Sider 2001; Skow 2009; Smart 1963, 1949; Smith 1993; Sullivan 2012a; Williams 1951; Zimmerman 2005; Zwart 1976.

A further question that you might ask about time is an ontological question. Does whether something is past, present, or future make a difference to whether it exists? And how do these ontological theses connect to debates about the A-theory and the B-theory?

According to presentism, only present objects exist. More precisely, presentism is the view that, necessarily, it is always true that only present objects exist. Even more precisely, no objects exist in time without being present (abstract objects might exist outside of time). (Note that some writers have used the name differently, and unless otherwise indicated, what is meant here by “present” is temporally present, as opposed to spatially present.) According to presentism, if we were to make an accurate list of all the things that exist—i.e., a list of all the things that our most unrestricted quantifiers range over—there would be not a single merely past or merely future object on the list. Thus, you and the Taj Mahal would be on the list, but neither Socrates nor any future Martian outposts would be included. (Assuming, that is, both (i) that each person is identical to his or her body, and (ii) that Socrates’s body ceased to be present—thereby going out of existence, according to presentism—shortly after he died. Those who reject the first of these assumptions should simply replace the examples in this article involving allegedly non-present people with appropriate examples involving the non-present bodies of those people.) And it is not just Socrates and future Martian outposts, either—the same goes for any other putative object that lacks the property of being present. No such objects exist, according to presentism.

There are different ways to oppose presentism—that is, to defend the view that at least some non-present objects exist. One version of non-presentism is eternalism, which says that objects from both the past and the future exist. According to eternalism, non-present objects like Socrates and future Martian outposts exist now, even though they are not currently present. We may not be able to see them at the moment, on this view, and they may not be in the same space-time vicinity that we find ourselves in right now, but they should nevertheless be on the list of all existing things.

It might be objected that there is something odd about attributing to a non-presentist the claim that Socrates exists now, since there is a sense in which that claim is clearly false. In order to forestall this objection, let us distinguish between two senses of “ x exists now”. In one sense, which we can call the temporal location sense, this expression is synonymous with “ x is present”. The non-presentist will admit that, in the temporal location sense of “ x exists now”, it is true that no non-present objects exist now. But in the other sense of “ x exists now”, which we can call the ontological sense, to say that “ x exists now” is just to say that x is now in the domain of our most unrestricted quantifiers. Using the ontological sense of “exists”, we can talk about something existing in a perfectly general sense, without presupposing anything about its temporal location. When we attribute to non-presentists the claim that non-present objects like Socrates exist right now, we commit non-presentists only to the claim that these non-present objects exist now in the ontological sense (the one involving the most unrestricted quantifiers).

According to the eternalist, temporal location does not affect ontology. But according to a somewhat less popular version of non-presentism, temporal location does matter when it comes to ontology, because only objects that are either past or present exist. On this view, which is often called the growing block theory, the correct ontology is always increasing in size, as more and more things are added on to the leading “present” edge (temporally speaking). (Note, however, that the growing block theory does not involve any commitment to four-dimensionalism as discussed in section 7 . In this way, the name “growing block” is somewhat misleading and the view is probably better described as the growing universe theory.) Both presentism and the growing block theory are versions of the A-theory.

Despite the claim by some presentists that theirs is the commonsense view, it is pretty clear that there are some major problems facing presentism (and, to a lesser extent, the growing block theory; but in what follows we will focus on the problems facing presentism). One problem has to do with what appears to be perfectly meaningful talk about non-present objects, such as Socrates and the year 3000. If there really are no non-present objects, then it is hard to see what we are referring to when we use expressions such as “Socrates” and “the year 3000”.

Another problem for the presentist has to do with relations involving non-present objects. It is natural to say, for example, that Abraham Lincoln was taller than Napoleon Bonaparte, and that World War II was a cause of the end of The Depression. But how can we make sense of such talk, if there are no non-present objects to be the relata of those relations?

A third problem for the presentist has to do with the very plausible principle that for every truth, there is a truth-maker—something whose existence suffices for the truth of the proposition or statement. If you are a presentist, it is hard to see what the truth-makers could be for truths such as that there were dinosaurs and that there will be Martian outposts.

Finally, the presentist, in virtue of being an A-theorist, must deal with the arguments against the A-theory that were mentioned above, including especially the worry that the A-theory is incompatible with special relativity. We will discuss these physics-based objections below.

Suggestions for Further Reading: Adams 1986; Bourne 2006; Bigelow 1996; Emery 2020; Hinchliff 1996; Ingram 2016; Keller and Nelson 2001; Markosian 2004, 2013; McCall 1994; Rini and Cresswell 2012; Sider 1999, 2001; Sullivan 2012b; Tooley 1997; Zimmerman 1996, 1998.

In Section 4 and Section 5 we saw that there have been two main theories developed in response to McTaggart’s Argument: The A-theory and The B-theory. Then, in Section 6 we saw that there are two main ways of thinking about the relation between ontology and time: presentism and eternalism. (There was also a third way, The Growing Block Theory, which we will mainly set aside for the sake of simplicity in this section.) Two main ways of thinking about time emerge from these discussions. On the one hand, A-theorists and presentists think that our pre-theoretical idea of time as flowing or passing, and thus being very different from the dimensions of space, corresponds to something objective and real. B-theorists and eternalists, on the other hand, reject the idea of time’s passage and instead embrace the idea of time as being a dimension like space. There is another important way in which philosophers in the second camp (the B-theory/eternalist camp) think time is like space, and it has to do with how objects and events persist over time. The debate typically centers around the doctrine of “temporal parts”, which those in the B-theory/eternalist camp tend to accept while those in the A-theory/presentist camp tend to reject.

To get an intuitive idea of what temporal parts are supposed to be, think of a film strip depicting you as you walk across a room. It is made up of many frames, and each frame shows you at a moment of time. Now picture cutting the frames, and stacking them, one on top of another. Finally, imagine turning the stack sideways, so that the two-dimensional images of you are all right-side-up. Each image of you in one of these frames represents a temporal part of you, in a specific position, at a particular location in space, at a single moment of time. And what you are, on this way of thinking, is the fusion of all these temporal parts. You are a “spacetime worm” that curves through the four-dimensional manifold known as spacetime . Moreover, on this view, what it is to have a momentary property at a time is to have a temporal part at the time that has the property in question. So you are sitting right now in virtue of the fact that your current temporal part is sitting.

The doctrine of temporal parts that B-theorists and eternalists tend to like can be stated like this:

Four-Dimensionalism: Any physical object that is located at different times has a different temporal part for each moment at which it is located.

On this view you have a temporal part right now, which is a three-dimensional “time slice” of you. And you have a different temporal part at noon yesterday, but no temporal parts in the year 1900 (since you are not located at any time in 1900). Also on this view, the physical object that is you is a fusion of all of your many temporal parts. (Note: there is a variation on the standard four-dimensional view, which is sometimes called “the worm view”. The variation, known as “the stage view”, holds that names and personal pronouns normally refer, not to entire fusions of temporal parts but, rather, to the individual person-stages, each of which is located at just an instant of time, and each of which counts as a person, rather than a mere part of a person).

The opposing view is three-dimensionalism, which is just the denial of the claim that temporally extended physical objects must have temporal parts. Here is a formulation of the view:

Three-Dimensionalism: Any physical object that is located at different times is wholly present at each moment at which it is located.

According to three-dimensionalism, the thing that was doing whatever you were doing at noon yesterday was you. It was you who was doing that, and now you are doing something different (namely, reading this sentence). So the relation between “you then” and “you now” is identity . According to four-dimensionalism, on the other hand, the thing that was doing whatever you were doing at noon yesterday was an earlier temporal part of the thing that is you, and the thing that is doing what you are doing now is the present temporal part of you. The relation between “you then” and “you now” is the temporal counterpart relation. (This is similar to the relation between your left hand and your right hand, which is the spatial counterpart relation. Your two hands are distinct parts of a bigger thing that contains them both.)

David Lewis, one of the main proponents of four-dimensionalism, suggests that the principal reason to accept the view is to solve what he calls “the problem of temporary intrinsics”. How can a single thing—Lewis, for example—have different intrinsic properties—like being straight, while he is standing, and then being bent, while seated—at different times? Not by standing in different relations—the being straight at and being bent at relations—to different times, he argues. (Since, he says, being straight and being bent are genuine properties rather than disguised relations.) And not in virtue of there being only one reality—such as the time when Lewis is bent—so that reality consists of Lewis, and every other thing, being the way it is now and not any other way. (For Lewis points out that we all believe we have a past and a future, in addition to a present.) So Lewis suggests that the best answer to the question about how a single thing can have different intrinsic properties at different times is that such an object has different temporal parts which themselves have the different intrinsic properties.

There is, however, a natural three-dimensionalist response to this argument. It involves appealing to a certain way of thinking about time, truth, and propositions that we touched on briefly in Section 1 , namely, the idea that propositions are in some way “tensed” as opposed to “tenseless”. Here is a way to formulate the relevant semantic thesis:

The Tensed Conception of Semantics

• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time.
• We cannot eliminate verbal tenses like is , was , and will be from an ideal language.

On this view, a sentence like “Sullivan is eating a burrito” expresses a proposition that used to be true, but is false now.

The alternative to the tensed conception of semantics is the tenseless conception of semantics . On the latter view, an utterance of a sentence like “Sullivan is eating a burrito” expresses a proposition about a B-relation between events—it says that Sullivan’s eating a burrito is simultaneous with the utterance itself (or perhaps with the time of the utterance). Here is a way of stating this view:

The Tenseless Conception of Semantics

• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time.
• We can in principle eliminate verbal tenses like is , was , and will be from an ideal language.

Consideration of Lewis’s argument from temporary intrinsics has shown that a three-dimensionalist should probably endorse the tensed conception of semantics, in order to account for changing truths about the world and its objects. And once we have seen this, it also becomes clear that A-theorists, presentists, and proponents of the growing block theory all have similar reasons for adopting the tensed conception of semantics. For the A-theorist is committed to there being changing truths about which times and events are future, which are present, and which are past; and presentists and growing block theorists are both committed to there being changing truths about what exists.

Suggestions for Further Reading: Hawley 2004 [2020]; Lewis 1986; Sider 2001; Thomson 1983; van Inwagen 1990

Many of the above considerations—especially those about McTaggart’s Argument; the A-theory and the B-theory; presentism, eternalism, and the growing block theory; and the dispute between three-dimensionalism and four-dimensionalism—suggest that there are, generally speaking, two very distinct ways of thinking about the nature of time. The first is the Static Theory of Time, according to which time is like space, and there is no such thing as the passage of time; and the second is the Dynamic Theory of Time, according to which time is very different from space, and the passage of time is a real phenomenon. These two ways of thinking about time are not the only such ways, but they correspond to the two most popular combinations of views about time to be found in the literature, which are arguably the most natural combinations of views on these issues. In this section we will spell out these two popular combinations, mainly as a way to synthesize much of the preceding material, and also to allow the reader to appreciate in a big-picture way how the different disputes about the nature of time are normally taken to be interrelated.

The guiding thought behind the Static Theory of Time is that time is like space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But it is not inevitable. It is also possible to mix and match from among the tenets of the Static Theory and its rival, the Dynamic Theory.)

The Static Theory of Time

• The universe is spread out in four similar dimensions, which together make up a unified, four-dimensional manifold, appropriately called spacetime .
• Any physical object that is located at different times has a different temporal part for each moment at which it is located.
• There are no genuine and irreducible A-properties; all talk that appears to be about A-properties can be correctly analyzed in terms of B-relations. Likewise, the temporal facts about the world include facts about B-relations, but they do not include any facts about A-properties.
• The correct ontology does not change over time, and it always includes objects from every region of spacetime.
• Propositions have truth values simpliciter rather than at times, and so cannot change their truth values over time. Also, we can in principle eliminate verbal tenses like is , was , and will be from an ideal language.
• There is no dynamic aspect to time; time does not pass.

Static Theorists of course admit that time seems special to us, and that it seems to pass. But they insist that this is just a feature of consciousness—of how we perceive the world—and not a feature of reality that is independent of us.

The second of the main ways of thinking about time is the Dynamic Theory of Time. The guiding thought behind this way of thinking is that time is very different from space. Here are six ways in which this thought is typically spelled out. (Note: The particular combination of these six theses is a natural and popular combination of related claims. But, like the Static Theory, it is not inevitable. It is also possible to mix and match from among the tenets of the Dynamic Theory and the Static Theory.)

The Dynamic Theory of Time

• The universe is spread out in the three dimensions of physical space, and time, like modality, is a completely different kind of dimension from the spatial dimensions.
• Any physical object that is located at different times is wholly present at each moment at which it is located.
• There are genuine and irreducible A-properties, which cannot be correctly analyzed in terms of B-relations. The temporal facts about the world include ever-changing facts involving A-properties, including facts about which times are past, which time is present, and which times are future.
• The correct ontology changes over time, and it is always true that only present objects exist.
• Propositions have truth values at times rather than simpliciter and can, in principle, change their truth values over time. Also, we cannot eliminate verbal tenses like is , was , and will be from an ideal language.
• The passage of time is a real and mind-independent phenomenon.

Opponents of the Dynamic Theory (and sometimes proponents as well) like to characterize the theory using the metaphor of a moving spotlight that slides along the temporal dimension, brightly illuminating just one moment of time, the present, while the future is a foggy region of potential and the past is a shadowy realm of what has been. The moving spotlight is an intuitively appealing way to capture the central idea behind the Dynamic Theory, but in the end, it is just a metaphor. What the metaphor represents is the idea that A-properties like being future , being present , and being past are objective and metaphysically significant properties of times, events, and things. Also, the metaphor of the moving spotlight represents the fact that, according to the Dynamic Theory, each time undergoes a somewhat peculiar but inexorable process, sometimes called temporal becoming . It goes from being in the distant future to the near future, has a brief moment of glory in the present, and then recedes forever further and further into the past.

Despite its being intuitively appealing (especially for Static Theorists, who see it as a caricature of the Dynamic Theory), the moving spotlight metaphor has a major drawback, according to some proponents of the Dynamic Theory: it encourages us to think of time as a fourth dimension, akin to the dimensions of space. For many proponents of the Dynamic Theory, this way of thinking—“spatializing time”—is a mistake. Instead, we should take seriously the ways that time seems completely different from the dimensions of space—for instance, time’s apparent directionality, and the distinctive ways that time governs experience.

Suggestions for Further Reading: Hawley 2001; Lewis 1986; Markosian 1993; Markosian 2004; Markosian (forthcoming); Moss 2012; Price 1977; Prior 1967; Prior 1968; Sider 2001; Smart 1949; Sullivan 2012a; Thomson 1983; and Williams 1951.

Above we mentioned that a metaphor sometimes used to characterize the Dynamic Theory is that of a moving spotlight that slides along the temporal dimension and that is such that only objects within the spotlight exist. A similar sort of metaphor can also be used to characterize the Moving Spotlight Theory, which is an interesting hybrid of the Static Theory and the Dynamic Theory. Like the Static Theory, the Moving Spotlight Theory incorporates the idea of spacetime as a unified manifold, with objects spread out along the temporal dimension in virtue of having different temporal parts at different times, and with past, present, and future parts of the manifold all equally real. But like the Dynamic Theory, it incorporates the thesis that A-properties are objective and irreducible properties, as well as the idea that time genuinely passes. The metaphor that characterizes the Moving Spotlight Theory is one on which there is a moving spotlight that slides along the temporal dimension and that is such that only things that are within the spotlight are present (but things that are outside the spotlight still exist).

Thus the Moving Spotlight Theory is an example of an eternalist A-theory that subscribes to the dynamic thesis. Unlike presentist or growing block theories, spotlighters deny that any objects come into or out of existence. Unlike the B-theories, however, spotlighters think that there is an important kind of change that cannot be described just as mere variation in a spacetime manifold. Spotlighters think instead that there is a spacetime manifold, but one particular region of the manifold is objectively distinguished—the present. And this distinction is only temporary—facts about which region of spacetime count as the present change over time. For example, right now a region of 2019 is distinguished as present. But in a year, a region of 2020 will enjoy this honor. The term “moving spotlight theory” was coined by C.D. Broad—himself a growing blocker—because he thought this view of time treated passage on the metaphor of a policeman’s “bull’s eye” scanning regions in sequence and focusing attention on their contents.

Just as there are different understandings of presentism and eternalism, there are different versions of the moving spotlight theory. Some versions think that even though the present is distinguished, there is still an important sense in which the past and future are concrete. Other versions (like Cameron 2015) treat the spotlight theory more like a variant of presentism—past and future objects still exist, but their intrinsic properties are radically unlike those of present objects. Fragmentalists (see Fine 2005) think that there is a spacetime manifold but that every point in the manifold has its own type of objective presentness, which defines a past and future relative to the point.

Why be a spotlighter? Advocates think it combines some of the best features of eternalism while still making sense of how we seem to perceive a world of substantive passage. It also inherits some of the counterintuitive consequences of eternalism (i.e., believing dinosaurs still exist) and the more complicated logic of the A-theories (i.e., it requires rules for reasoning about tensed propositions involving the spotlight).

Suggestions for Further Reading: Broad 1923; Cameron 2015; Fine 2005; Hawley 2004 [2020]; Lewis 1986 (especially Chapter 4.2); Sider 2001; Skow 2015; Thomson 1983; Van Inwagen 1990; Zimmerman 1998.

We are all familiar with time travel stories, and there are few among us who have not imagined traveling back in time to experience some particular period or meet some notable person from the past. But is time travel even possible?

One question that is relevant here is whether time travel is permitted by the prevailing laws of nature. This is presumably a matter of empirical science (or perhaps the correct philosophical interpretation of our best theories from the empirical sciences). But a further question, and one that falls squarely under the heading of philosophy, is whether time travel is permitted by the laws of logic and metaphysics. For it has been argued that various absurdities follow from the supposition that time travel is (logically and metaphysically) possible. Here is an example of such an argument:

Another argument that might be raised against the possibility of time travel depends on the claim that presentism is true. For if presentism is true, then neither past nor future objects exist. And in that case, it is hard to see how anyone could travel to the past or the future.

A third argument, against the possibility of time travel to the past, has to do with the claim that backward causation is impossible. For if there can be no backward causation, then it is not possible that, for example, your pushing the button in your time machine in 2020 can cause your appearance, seemingly out of nowhere, in, say, 1900. And yet it seems that any story about time travel to the past would have to include such backward causation, or else it would not really be a story about time travel.

Despite the existence of these and other arguments against the possibility of time travel, there may also be problems associated with the claim that time travel is not possible. For one thing, many scientists and philosophers believe that the actual laws of physics are in fact compatible with time travel. And for another thing, as we mentioned at the beginning of this section, we often think about time travel stories; but when we do so, those thoughts do not have the characteristic, glitchy feeling that is normally associated with considering an impossible story. To get a sense of the relevant glitchy feeling, consider this story: Once upon a time there was a young girl, and two plus two was equal to five . When you try to consider that literary gem, you mainly have a feeling that something has gone wrong (you immediately want to respond, “No, it wasn’t”), and the source of that feeling seems to be the metaphysical impossibility of the story being told. But nothing like this happens when you consider a story about time travel (especially if it is one of the logically consistent stories about time travel, such as the one depicted in the movie Los Cronocrímenes (Timecrimes) ). One task facing the philosopher who claims that time travel is impossible, then, is to explain the existence of a large number of well-known stories that appear to be specifically about time travel, and that do not cause any particular cognitive dissonance.

Suggestions for Further Reading: Bernstein 2015, 2017; Dyke 2005; Earman 1995; Markosian (forthcoming); Meiland 1974; Miller 2017; Sider 2001; Thorne 1994; Vihvelin 1996; Yourgrau 1999.

Our best physical theories have often had implications for the nature of time, and by and large, it is assumed that philosophers working on time need to be sensitive to the claims of contemporary physics. One example of the interaction between physics and philosophy of time that was mentioned in Section 2 was Newton’s bucket argument, which used the observed effects of acceleration to argue for absolute motion (and thus absolute space and time). Another example mentioned above was the worry that the A-theory conflicted with special relativity. The latter has proved especially influential in contemporary metaphysics of time and so deserves some further discussion.

According to standard presentations of special relativity, there is no fact of the matter as to whether two spatially separated events happen at the same time. This principle, which is known as the relativity of simultaneity , creates serious difficulty for the A-theory in general and for presentism in particular. After all, it follows from the relativity of simultaneity that there is no fact of the matter as to what is present, and according to any A-theory there is an important distinction between what is present and what is merely past or future. According to presentism, that distinction is one of existence—only what is present exists.

A different way of describing the relativity of simultaneity involves the combination of two claims:

• the claim that whether two spatially separated events happen at the same time depends on the reference frame you use to describe them, and
• the claim that no reference frame is privileged.

This way of putting the relativity of simultaneity requires a new bit of technical jargon: the notion of a reference frame. For our purposes, a reference frame is nothing more than a coordinate system that is used to identify the same point in space at different times. Someone on a steadily moving train, for instance, will naturally use a reference frame that is different from someone who is standing on the station platform, since it is natural for the person on the train to think of themselves as stationary, while for the person on the platform it seems obvious that they are moving.

The reason why it is worth introducing this bit of jargon is that once you present the relativity of simultaneity as the combination of claims (i) and (ii), you can also note that the motivation for claim (i) is importantly different from the motivation for claim (ii). The motivation for (i) is a series of empirical results at the end of the nineteenth and beginning of the twentieth century, including, most famously, the Michelson-Morley experiment. No one should deny this part of the relativity of simultaneity. The motivation for (ii), by contrast, is less often explicitly discussed, and seems to involve the commitment to some sort of general extra-empirical principle like “eliminate unobservable entities whenever possible”, or “eliminate excess spacetime structure whenever possible”. This means that presentists and other A-theorists have a way of avoiding the worry from relativity without any conflict with empirical results—they can reject whatever extra-empirical principle motivates (ii). Whether you think the costs associated with this move are worth paying will depend on your degree of commitment to the A-theory, what exactly you think of the relevant extra-empirical principle supporting (ii), and whether that principle plays an important role elsewhere in physics.

It is often said that philosophers should defer to physics with respect to what the latter says about time. But the interaction between the A-theory and special relativity illustrates one way in which that claim is more complicated than it first appears. Must philosophers respect both the empirical and the extra-empirical aspects of our best physical theories? Or is it sufficient that they respect the former?

Another way in which this assumption is complicated is that different physical theories often seem to imply different things about the nature of time. Consider, for instance, the fact that in general relativity there is sometimes (though not always!) a preferred way of “foliating” spacetime into instants of time and thus reintroducing a notion of absolute simultaneity, or the fact that on some interpretations of quantum mechanics, the dynamical laws seem to require a notion of absolute simultaneity.

Two additional questions about the nature of time that have been especially influenced by contemporary physics have to do with the arrow of time and the extent to which time itself might be emergent.

To motivate the first question, start from the observation that the order in which events happen in time seems to matter a great deal. There seems to be an important difference, for instance between a train traveling from Boston to Providence and a train traveling from Providence to Boston. This is because, even though both of these sequences may be constituted by the very same events, those events are in a different order in each sequence. In the former sequence the train being in Boston happens earlier than the train being in Providence. In the latter, the train being in Boston happens later.

These straightforward observations show that we experience time as having a direction. This is what philosophers call “the arrow of time”. But is the arrow of time a fundamental feature of the world? Or can it be reduced to some other feature, thus simplifying our metaphysics as a whole?

One way to try to eliminate the arrow of time at the fundamental level is to make use of certain interpretations of statistical mechanics inspired by Ludwig Boltzmann’s work. Imagine the history of the universe as a long timeline, but with no indication of which end of the time line represents the first moment of time and which end represents the last moment. It follows from certain interpretations of statistical mechanics that there is a physical quantity, the entropy of the universe, that will be relatively low at one end of the timeline and relatively high at the other end and will always increase as you move from the former end of the timeline to the latter. (More carefully, the entropy will almost always increase or at least stay constant.) The thought, then, is that we might be able to reduce the arrow of time to this entropy gradient. Earlier moments of time are just moments of time when the entropy of the universe is lower.

This way of eliminating the arrow of time from the fundamental level is promising, but has at least some unintuitive consequences. For instance, it seems natural to think that entropy could have decreased over time, instead of increasing over time as it actually does. But given the reduction described above, it is not in fact possible for entropy to decrease over time.

The second question mentioned above is a question about whether time itself—as opposed to just some particular feature of time, like time’s arrow—might merely be an emergent feature of the world. This question has become especially pressing as philosophers of physics have turned their attention to theories of quantum gravity in which there does not seem to be anything like temporal structure at the fundamental level. Work in this area is nascent, but some of the questions of interest include: Does quantum gravity eliminate time entirely or does it merely make time a non-fundamental feature of the world? What would it mean for something temporal to be grounded in something atemporal and what sort of grounding relation would be involved? What is the distinction between causal structure (especially the causal structure in causal set theory—one approach to quantum gravity) and temporal structure? And how can a theory that eliminates time entirely be empirically confirmed or disconfirmed?

Suggestions for Further Reading: Albert 2000; Emery 2019 & forthcoming; Godfrey-Smith 1979; Healey 2002; Huggett and Wüthrich 2013; Knox 2013; Markosian 2004; Maxwell 1985; Monton 2006; Price 1996; Putnam 1967; Rovelli 2017; Savitt 2000; Stein 1968, 1970; Weingard 1972; Wüthrich and Callender 2017.

A final important question concerns how considerations about the nature of time ought to impact the ways that we reason about time. For example, if it turns out that a B-theory is true, and there is no metaphysically important difference between the past and future, then should we adopt a more neutral attitude about events in our personal past and future? Epicurean philosopher Lucretius famously suggested that if there is no substantive difference between the times in the past before we came to exist and the times in the future after we die, we should care much less about the deprivation that death will bring. But we may think that even if the B-theory can describe everything that is metaphysically important without positing an important difference between the past and future, there is still an indispensable psychological importance to the past/future distinction that rational agents honor. Still other A-theorists argue that while there is an important metaphysical distinction between the past and future, the distinction has no normative importance.

If we deny three-dimensionalism and instead view ourselves as objects that persist through time by having temporal parts, then does that justify caring less about temporal parts in the distant future that are less strongly linked with our present part? Derek Parfit famously argued that a proper understanding of what we care about when we care about our own future persistence should motivate us to be less self-interested and more interested in redistributing resources to others. Endurantists have argued that facts about how we persist through time underwrite a strong distinction between moral principles (which concern what we owe to others now) and prudential rationality (which concerns what we owe to our future selves).

Another interesting line of research uses empirical work in psychology to better understand what is happening cognitively when we judge time as passing. This is especially pressing for B-theorists, who must explain why time seems to pass in psychologically or rationally significant ways, even though all passage is really just variation in an eternal manifold. Some B-theorists explain the apparent passage of time as an illusion of flow caused by perceptual processes that attribute apparent motion to events that happen in sequence. Another, compatible approach considers the way that evolutionary pressures might have shaped emotions and cognitive heuristics to give us a strong past/future distinction in our reasoning.

Suggestions for Further Reading: Brink 2003; Suhler and Callender 2012; Parfit 1971; Paul 2010; Prosser 2016; Sullivan 2018.

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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.

• A.N. Prior: The Founding Father of Temporal Logic , a web site devoted to the study of Arthur Norman Prior (the founder of tense logic), at the Danish Centre for Philosophy and Science Studies
• The Centre for Time at the Department of Philosophy, University of Sydney.
• Eternalism , entry in Wikipedia
• Philosophy of space and time , entry in Wikipedia
• Philosophical presentism , entry in Wikipedia
• Time , entry in the Internet Encyclopedia of Philosophy
• Time , entry in Wikipedia
• Time Travel , entry in the Internet Encyclopedia of Philosophy

causation: backward | consciousness: temporal | fatalism | future contingents: medieval theories of | Heidegger, Martin | Husserl, Edmund | indexicals | logic: temporal | McTaggart, John M. E. | Newton, Isaac: views on space, time, and motion | presentism | Prior, Arthur | propositions: singular | quantum theory: quantum gravity | space and time: absolute and relational space and motion, post-Newtonian theories | space and time: being and becoming in modern physics | space and time: conventionality of simultaneity | temporal parts | time: the experience and perception of | time: thermodynamic asymmetry in | time travel | time travel: and modern physics | Zeno of Elea: Zeno’s paradoxes

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3 Popular Time Travel Theory Concepts Explained

Time travel theory. It’s one of the most popular themes in fiction. But every plotline falls into one of these three Time Travel Theories.

Time travel is one of the most popular themes in cinema . Although most time travel movies are in the sci-fi genre, every genre, even comedy, horror, and drama, have tackled complicated storylines involving time travel theory. Chances are, you’ve seen at least a few of the movies listed below:

• But what about...

The Possibility Of Time Travel

Time travel theory.

• Bill & Ted’s Excellent Adventure (1989)
• The Time Machine  (2002)
• Timeline (2003)
• Time Cop (2004)
• Back to the Future  (1985)
• 12 Monkeys  (1995)
• Terminator Series (1984)
• Star Trek (2009)
• Harry Potter and the Prisoner of Azkaban (2004)
• Freejack (1992)
• Looper (2012)

But one thing you might not have realized, even if you’ve seen hundreds of time travel-related films, is that there are only  3 different theories of time travel. That’s it. Every time travel movie or book that you’ve ever enjoyed falls into one of these time travel theories.

Fixed Timeline: Time Travel Theory

Want to change the future on Earth by modifying the past or present? Don’t even bother according to this time travel theory. In a fixed timeline, there’s a single history that is unchangeable. Whatever you are attempting to change by time-traveling is what created the problems in the present that you’re trying to fix ( 12 Monkeys ). Or you’re just wasting your time because the events you are trying to prevent will happen anyway ( Donnie Darko ).

Dynamic Timeline: Time Travel Theory

History is fragile and even the smallest changes can have a huge impact. After traveling back in time, your actions may impact your own timeline. The result is a paradox. Your changes to the past might result in you never being born, like in Back to the Future (1985), or never traveling in time in the first place. In The Time Machine (2002), Hartdegen goes back in time to save his sweetheart Emma but can’t. Doing so would have resulted in his never developing the time machine that he used to try and save her.

One common way to explore this paradox theory is by killing your own grandfather. The grandfather paradox is when a time traveler attempts to kill their grandfather before the grandfather meets their grandmother. This prevents the time travel’s parents from being born and thus the time traveler himself from being born. But if the time traveler was never born, then the traveler would never have traveled back in time, therefore erasing his or her actions involving the death of their grandfather.

Multiverse: Time Travel Theory

Travel all over time and do whatever you want. It doesn’t matter because there are multiple universes and your actions only create new timelines. This is a common theory used by the science fiction TV series, Doctor Who . Using the multiverse theory of time travel, it’s assumed that there are multiple coexisting alternate timelines.

Therefore, when the traveler goes back in time, they end up in a new timeline where historical events can differ from the timeline they came from, but their original timeline does not cease to exist. This means the grandfather paradox can be avoided. Even if the time traveler’s grandparent is killed at a young age in the new timeline, he/she still survived to have children in the original timeline, so there is still a causal explanation for the traveler’s existence.

Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe. There’s just one problem… you can’t go back ( The One , 2002).

But what about…

Some may argue that people who are “trapped” in time are time travelers as well. This happens in countless time travel movies including Robin Williams ‘ character in the 1995 film Jumanji who gets trapped inside a board game. The list of “people who are cryogenically frozen and then successfully thawed out in the future” is even longer and includes Austin Powers: The Spy Who Shagged Me  (1999), Planet of the Apes (1968) and so on.

Although these characters are “moving” through time, they are doing so by pausing and then rejoining the current timeline. The lack of a time machine device disqualifies them from technically being “time travelers” and included in this list of theories on time travel.

So will time travel ever be possible? All we know for sure is that the experts don’t agree. According to the Albert Einstein theory of relativity, time is relative, not constant and the bending of spacetime could be possible. But according to  Stephen Hawking , time travel is not possible. The Stephen Hawking time travel theory suggests that the absence of present-day time travelers from the future is an argument against the existence of time travel — a variant of the Fermi paradox (aka where the hell is everybody?). But it’s fun to think about.

NERD NOTE:  What happens to time in a black hole? We don’t know for sure, but according to both Stephen Hawking and Albert Einstein’s theory, time near a black hole slows down. This is because a black hole’s gravitational pull is so strong that even light can’t escape. Since gravity also affects light, time would also slow down.

If you could successfully travel into the future, or back in time, what would you do? Warn people about natural disasters? Buy a winning lottery ticket ? Try to prevent your own death? What do you think about these time travel theory ideas or the time travel movies that we included in this article? Please tell us in the comments below.

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Mar 24, 2015 at 11:24 PM

are there really only 3 theories? i feel like there are more but i cant think of any besides the movies listed here. hummmmmmmmm

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5 Bizarre Paradoxes Of Time Travel Explained

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

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

Types of Temporal Paradoxes

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

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

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

1: Predestination Paradox

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

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

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

– Cinema Treatment

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

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

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

2: Bootstrap Paradox

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

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

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

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

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

3: Grandfather Paradox

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

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

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

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

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

4: Let’s Kill Hitler Paradox

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

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

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

5: Polchinski’s Paradox

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

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

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

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

Are Self-Fulfilling Prophecies Paradoxes?

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

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

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

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

Are Time Paradoxes Inevitable?

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

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

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

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

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

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Self-Esteem

Mental time travel boosts sense of control and self-esteem, imagining oneself in the future or recalling a nostalgic past can be beneficial..

Posted May 12, 2024 | Reviewed by Jessica Schrader

• What Is Self-Esteem?
• Find a therapist near me
• “Mental time travel” may increase self-esteem and a sense of coherence and control.
• One aspect of mental time travel is reliving nostalgic events from the past (called retrospection).
• Another aspect involves imagining oneself in the future (called prospection).

Published in Personality and Social Psychology Review , a recent study by Stephan and Sedikides suggests mental time travel can increase self-esteem , coherence, and sense of control.

But what is mental time travel? Let me use an example.

A friend of mine who is in a committed romantic relationship and very much in love tells me that he often relives the day he and his partner first met. He also imagines what their future would be like (e.g., their wedding day, their life as parents).

In both cases, my friend is engaging in mental time travel .

Simply put, mental time travel involves projecting oneself either forward or backward in time. As illustrated, reliving a past event requires reconstructing it from memory , whereas pre-living a potential future event requires imagination .

Benefits of Imagining the Future

Imagining yourself in the distant future (called prospection) allows you to step back and see the bigger picture—to focus on living according to your core values, achieving long-term goals , and living a satisfying and meaningful life.

It also enhances the feeling that future outcomes are under your control. It promotes the belief that your intentions, determination, and commitment will make goal pursuit possible no matter what challenges come your way. Finally, prospection may even increase self-esteem.

Benefits of Nostalgia and Recalling the Past

The same is true of moving backward in time and nostalgizing.

Nostalgia refers to longing and affection for valued past events and a desire to re-experience them. Previous research shows nostalgia can be an important psychological and relationship resource.

For instance, it can help people cope with boredom and meaninglessness . Romantic nostalgia may enhance passion, intimacy, commitment, and relationship satisfaction. Even nostalgic memories of an ex-romantic partner may have benefits and influence perceptions of self-growth .

Self-Affirmation

Mental time travel emphasizes aspects of the self that are positive, abstract, and central to one’s identity , as opposed to aspects that are negative or situation-specific. In other words, it stresses long-term goals, personal values, and positive personality traits, instead of current behaviors and more immediate plans and goals.

Therefore, mental time travel serves as self-affirmation . The core facets of self-affirmation include:

• Control: Feeling empowered to pursue desired goals; believing one can influence valued outcomes.
• Self-esteem: Having a positive subjective evaluation of one’s worth (i.e., liking yourself).
• Coherence: Being able to make sense of one’s experiences and to see life as meaningful.

How Does Mental Time Travel Foster Self-Validation?

To explain how mental time travel can promote self-validation and self-affirmation, let's look at another example, this one courtesy of an old classmate (let’s call her Emma).

Emma told me that before applying to nursing school, she failed a genetics course, which really affected her self-confidence . She felt stupid, incompetent, and worthless.

Eventually, Emma decided to engage in mental time travel—both recalling nostalgic events and imagining herself in the future. Doing so facilitated perspective-taking and seeing the bigger picture: She became more deeply aware of her core values (e.g., authenticity , social justice, making a difference) and positive personality traits (e.g., creativity , compassion , sensitivity, and gratitude ).

And failing genetics no longer defined her.

Additionally, Emma found that by engaging in prospection and retrospection regularly, subsequent challenges became increasingly manageable, including those she faces now in her teaching career as a nursing professor. Consistent with this, Stephan and Sedikides propose that “self-affirmation might serve to strengthen the psychological immune system in non-threatening situations, thus protecting against potential future threats.”

Threats to the adequacy and integrity of one’s sense of self come in many shapes and forms: getting fired, being rejected romantically by a desired mate, receiving negative health news, failing a course or getting a bad grade, etc.

The research discussed suggests that by imagining one’s future self or recalling nostalgic memories, one could get in touch with who they truly are—as defined by their positive personality traits, core values, and long-term goals.

Several positive psychology interventions could be helpful for this purpose, such as nostalgia interventions .

Or, consider the Best Possible Self exercise. Take some time to picture a future where everything in your life has gone as well as possible. Imagine living life in full accordance with your values, having accomplished all your most important goals and realized all your dreams .

What does such a happy, successful life look like? Write about it. This exercise has been shown to improve mood, optimism , and well-being.

Arash Emamzadeh attended the University of British Columbia in Canada, where he studied genetics and psychology. He has also done graduate work in clinical psychology and neuropsychology in U.S.

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All 16 Marvel Easter Eggs & References In X-Men '97 Episode 9

How gambit can still return before the end of x-men '97, what time x-men '97 episode 10 releases on disney plus.

Warning! This article contains spoilers for X-men '97!

• X-Men '97 hints at time loop after character deaths, sparking speculation about resurrection possibilities.
• Cryptic Marvel writer teases time travel implications in series, leaving fans intrigued and hopeful for surprises in upcoming episode.
• Despite denial of time travel retcon, Beau DeMayo's reference to a time loop in Star Trek episode raises questions about future plot twists.

A cryptic post on social media just made Marvel theories surrounding time travel in X-Men '97 much more tenable. X-Men '97 has been a welcome reprieve for MCU fans seeking out top-quality Marvel programming despite the series ostensibly occurring outside the wider MCU narrative. Part of its distance from typical MCU lore means that all bets are off when it comes to figuring out the rules of time travel that have become central to the MCU's Multiverse Saga . That was, at least, until one of its main characters referenced a multiversal concept recently introduced in the MCU via What If...? .

Recent developments in X-Men '97 have seen the beloved Gambit and multiple other prominent mutants meet their end thanks to a rampaging monster Sentinel in Episode 5. With Gambit being such a popular figure within the X-Men, fans have been scrambling for possible routes in which his death can be undone with the most convincing surrounding Cable's time-traveling abilities. Cable's recent mention of " absolute points in time " in Episode 8 seemed to put those theories to bed. That is until the series' writer, Beau DeMayo, just stirred the time-travel pot once more.

Marvel Studios' X-Men '97 episode 9 is full of iconic Easter eggs and references to both the original comics, the original animated series, and more.

X-Men '97's Creator Teases A Time Loop With Cryptic Social Media Post

In a post via Twitter/X , Beau DeMayo cryptically assigned viewers " homework " before X-Men '97 's grand finale, which comprised the Star Trek: The Next Generation episode " Cause and Effect ." The episode in question features the crew of the USS Enterprise becoming embroiled in a time loop for 17 days. It even features the destruction of the Enterprise before the crew's shrewd thinking allows them to avert being perpetually trapped in the tragic sequence and move away from the spatial anomaly causing it.

The implications here are stark. Unless Beau DeMayo is using the episode to hint at a Kelsey Grammer cameo (as he debuts as Captain Morgan Bateson in " Cause and Effect "), it looks as if a time loop or something similar will feature in X-Men '97 Episode 10 . The clue is confusing, given that DeMayo himself recently seemed to thwart all suggestions that time-travel shenanigans would help retcon the current state of affairs, unambiguously stating in another social media post, " No time travel to get us outta this one, folks ."

Could There Be Hope For Gambit After All?

Gamibit died after sacrificing himself on genosha in "remember it".

The apparent permanence of Gambit's death was especially shocking given how beloved the character is and how little attention he has received in live-action, furthering the neglect of the character. Yet DeMayo's cryptic comment seems to suggest that there is hope yet, as it is difficult to reconcile the idea that a time loop will play a role in the final episode of X-Men '97 Season 1 without bringing back Gambit . For that matter, mutants like Sebastian Shaw, Callisto, and the Morlocks also feel like significant losses that require a classic comic book retcon.

Yet DeMayo's insistence that time travel would not retcon this situation can't be ignored as much as Cable's insistence that the assault on Genosha was an absolute point in time. Doing so now might cheapen the moment and make future deaths seems less impactful. Instead, the time loop DeMayo seems to refer to may bring the X-Men back to a point past Genosha, leaving Gambit's resurrection to become a larger plot point in X-Men '97 's future.

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Kingdom Of The Planet Of The Apes' Biggest Theory Was Wrong & I'm Shocked

Warning: Contains SPOILERS for Kingdom of the Planet of the Apes!

• The 2024 film Kingdom of the Planet of the Apes takes a different direction from previous theories.
• The film debunks the theory that Mae is a time-traveling astronaut, revealing she comes from a human colony on Earth in scattered bunkers.
• The ending sets up sequels, revealing intelligent human colonies and the looming threat they pose against ape societies.

A common theory about Kingdom of the Planet of the Apes was proven instantly wrong by the 2024 film, with the franchise taking an alternate direction that no one saw coming. Picking up 300 years after War for the Planet of the Apes , the latest installment shows the drastic shift in the world after the Simian Flu devastated the human population and improved the intelligence of apes. While the previous movie saw human forces dwindling, they're far worse off in Kingdom and are no longer the dominant species on Earth.

While Owen Teague leads the Kingdom of the Planet of the Apes cast as Noa, The Witcher actress Freya Allan plays the lead human character, Mae . Their dichotomy as representatives of their species is one of the film's central narratives, examining the potential for a relationship between humans and apes. In the previous 2010s trilogy, participants from both species instigated conflict, leading to the state of the world as seen in the new movie.

Mae Isn't A Time-Traveling Astronaut In Kingdom Of The Planet Of The Apes

Mae comes from an unexpected colony of intelligent humans.

Given context from the broader franchise and what audiences knew from the trailers, the going theory about Kingdom of the Planet of the Apes' Mae was that she was an astronaut from the past who'd landed hundreds of years later . In the recent film's timeline, the primary population of humans is feral due to the Simian Flu virus, adding to the shock when Mae starts speaking mid-way through the film. However, this isn't because she was in space; it's because a colony of humans has taken shelter in bunkers, protecting them from the virus and rampant ape civilizations.

The modern Planet of the Apes movies aren't connected to the originals, though they could follow a similar narrative.

While Mae isn't an astronaut, Kingdom of the Planet of the Apes did note the potential of space travel for the franchise. Shortly after beginning his journey, Noa finds himself in an old human university observatory, where he looks through a telescope and sees the stars. At the end of the film, he returns there with Soona and shows her. While it's meant as a romantic moment between the pair, it also seems like foreshadowing for the franchise, especially given that the original Planet of the Apes movie was centered around a human astronaut returning to Earth.

Every Faction & Colony In Kingdom Of The Planet Of The Apes Explained

Kingdom of the Planet of the Apes features several distinct factions and clans, establishing the harsh environment for the new trilogy.

Where Mae Comes From In Kingdom Of The Planet Of The Apes

Mae's human colony are protected from the virus by sheltering in a bunker.

The Kingdom of the Planet of the Apes ending has major implications for the franchise going forward, setting up a premise for sequels to Wes Ball's film. The bulk of the film sees the ape ruler Proximus Caesar attempting to breach a mysterious vault, looking for weaponry to strengthen his reign. Mae also seeks the vault's contents, though for a different reason. She's on a mission from a human colony to collect a SATCOM key from the vault, which is later used to reactivate satellites , establishing communication for the remaining intelligent humans.

The fact that there are still intelligent humans on Earth means Wes Ball's Planet of the Apes trilogy won't simply be a conflict between apes. There's still a long way to go before the events of the original 1968 film, and humanity still has some fight left in it. Mae's human colony may be small in number, but its societal structure and advanced technology make it a threat to the apes , whose societies resemble ancient human civilization. Once the satellites turn on, it's also revealed that there are more human bunkers like Mae's around the country.

Mae's Planet Of The Apes Future Could Make The Astronaut Theory Correct

Space travel could be in humanity's future in planet of the apes.

While Mae certainly hasn't come from a spaceship, it's not impossible that she, or another character, could end up in space. Space travel has always been a part of the Planet of the Apes franchise , and it would make sense for humanity to make a desperate play for their survival by sending a ship into space, similar to what's seen in the original movie. The other option, which could be more far-fetched, would be for humankind to attempt some sort of time travel, hoping to reverse the effects of the Simian virus to begin with.

Kingdom Of The Planet Of The Apes' Eagles & Deeper Meaning Explained

The main apes in Kingdom of the Planet of the Apes are part of the Eagle Clan. Both literally and metaphorically, here's the purpose of the eagles.

The modern Planet of the Apes movies haven't embraced the outlandish science fiction elements of the original, but it will be fascinating to see how the series develops. Wes Ball intends to direct a trilogy for Kingdom of the Planet of the Apes , and talks of another subsequent trilogy have been discussed. For the sake of the narrative not becoming redundant, incorporating more science fiction elements could be an excellent move, embracing older eras of the franchise.

Kingdom of the Planet of the Apes

Director Wes Ball

Release Date May 10, 2024

Cast Sara Wiseman, Neil Sandilands, Eka Darville, Kevin Durand, Owen Teague, Freya Allan, Peter Macon

Rating PG-13

Genres Sci-Fi, Action

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The Morning

When travel plans go awry.

There are ways of keeping ourselves anchored, even when we enter a parallel universe disconnected from time.

By Melissa Kirsch

The weekend trip is, in theory, the perfect break. Two nights someplace else, just a small duffel bag and limited logistics standing between you and a reset. Leave on Friday, come back Sunday, fill the hours in between with enough that’s novel and return refreshed, or at least with a slightly altered perspective. You might take a weekend trip for vacation or work or to see family, but the effect is the same. You’re a little changed on return. You see your regular life a little bit differently.

I took what was meant to be a quick trip last weekend to attend a college graduation, and it was, strictly speaking, quick: I was scarcely away for 48 hours, but extreme weather marooned me for most of those hours in the liminal spaces of transit — airports, grounded planes, traffic jams — where time loses legibility. An old friend used to call these neither-here-nor-there realms the “zero world” for the way they feel unfastened from reality, parallel to daily life but separate. The flight cabin after an announcement of a fourth lightning delay is a world detached from the one you know, a temporary society populated by temporary citizens with perhaps not much in common save one deeply held belief: We need to get out of here.

I was as cranky and impatient as the rest of my fellow travelers at each complication in our journeys, but also fascinated by the communities and customs and Cibo Express markets of the zero world. Each of us was, at any given time, one captain’s announcement away from a temper tantrum, but we were also competitively careful to be polite to one another and to the airline staff, as if determined to demonstrate that those wild videos of short-tempered passengers being duct-taped to their seats did not represent us, the makeshift civilization of this departure lounge.

Graduation, when I finally arrived, was a joyous affair despite the glitches. The speaker, an astronaut, showed a photo of the farm where she grew up, the place she thought of as home for much of her life. Then she showed a photo of the limb of the Earth, the glowing edge of the atmosphere, and described how, when she went to space, home was no longer a town on a map but this planet, a shift in perspective so massive I felt a little queasy contemplating it.

On Hour 3 in the airport bar on Sunday morning, beside two German travelers practicing Spanish, I ordered an omelet and imagined my own home, which felt very far away and lit by its own otherworldly halo. What would I be doing if I were there? Reading, texting, catching up on emails — the same things I was doing here. What was so bad about this? Was it the lack of choice? The lack of fresh air?

It was all those things, and also the feeling of being trapped in a warp between origin and destination. My emotions felt out of proportion to the situation: I hadn’t traveled very far for very long, was in no peril and would still arrive in New York with enough day left to do whatever needed to be done, but I felt on the verge of tears, loosed from my moorings, floating between fixed points, dislocated. I put on my headphones, put on a favorite band whose songs are so familiar they provide a home base no matter where I am. I listened to the same album on repeat for the duration of the flight, in the car on the way home, even at home once I finally made it there.

There’s a story in The Times today about how A.S.M.R., the pleasant, brain-tingling feeling we get when hearing certain sounds or watching certain comforting scenes, has become a feature of all viral internet content, not just specialized videos devoted to inducing the sensation. You can still put on a very specific video of someone whispering into a microphone or crinkling paper, but you’re just as likely to find the stimuli in videos of people cooking or cleaning their pools. This seems like a logical extension. We’re restless beasts in need of soothing. Sometimes we’re dramatically homesick, sometimes it’s just a bad day. Why not imbue the mundane with the choreography of comfort? Why not add pleasure whenever and wherever we can?

For weekend travel inspiration: The Times’s 36 Hours series.

How to deal with the increasing unpredictability of travel .

Stunning views of Earth from space .

How A.S.M.R. became a sensation.

THE WEEK IN CULTURE

The final round of the Eurovision Song Contest takes place in Sweden today. This year’s favorites include a Croatian techno act called Baby Lasagna. Read , or listen to , a guide to the competition.

“I won’t let anything break me”: Eden Golan, Israel’s 20-year-old entrant, spoke to The Times about the campaign to exclude her country from the event because of the war in Gaza.

The stage crew has 50 seconds to disassemble and reassemble sets. Watch a video from The Wall Street Journal .

Film and TV

“It’s easy to get caught up in the bigness of it all”: Owen Teague, the star of the latest “Planet of the Apes” film, and Andy Serkis, the lead in the earlier movies, sat down for a conversation .

“Kingdom of the Planet of the Apes” is not as transporting as the previous trilogy of films, the Times critic Alissa Wilkinson writes , but “there’s still a tremendous amount to mull over.”

The latest season of “Doctor Who,” starring Ncuti Gatwa as the 15th actor to play the doctor, opened with a double episode. Read a recap.

Disney and Warner Bros. Discovery announced a plan to bundle their Disney+, Hulu and Max streaming services this summer

The recording engineer Steve Albini, who died this week at 61, was “arguably the most influential figure ever to emerge from indie rock,” Pitchfork wrote . Listen to 10 of his essential tracks , which shaped the sound of alternative rock music.

Kendrick Lamar and Drake’s rap beef crashed the website Genius , where users can annotate lyrics to songs. Times critics discussed where the rappers’ sonic conflict goes next .

Other Big Stories

A stage version of the beloved animated film “Spirited Away” is running in London, after premiering in Japan. The adaptation is opulent and impressive, but it could use more heart , our critic writes.

A federal judge granted a preliminary injunction that would bar the Des Moines Art Center from dismantling “Greenwood Pond: Double Site,” an environmental work by Mary Miss that includes wooden walkways and sitting areas in need of repair.

The owners of the Los Angeles house where Marilyn Monroe last lived, and died, sued the city, accusing officials of “backroom machinations” to save it from a planned demolition .

David Shapiro, a lyrical poet who appeared in a famous photograph from the 1968 uprising at Columbia University, died at 77 .

THE LATEST NEWS

Israel-Hamas War

A Biden administration report said that Israel may have broken international law in Gaza, but that Israel’s “credible and reliable” assurances mean the U.S. can keep sending weapons.

The Biden administration is still waiting for Israel to show how it plans to evacuate and protect civilians in Rafah ahead of a possible invasion.

The U.N. General Assembly voted to support Palestinian statehood , a symbolic move. The U.S. voted no, and Israel accused delegates of “shredding the U.N. charter.”

Michael Cohen, who paid Stormy Daniels hush money and whom prosecutors say Donald Trump reimbursed, is expected to testify Monday in Trump’s Manhattan criminal trial. The judge asked prosecutors to stop Cohen from criticizing Trump .

Russia tried to break through Ukrainian lines in the country’s north using shelling and armored columns. Ukraine said it had repelled the attacks.

Russia is upgrading a munitions depot in Belarus, possibly to house nuclear weapons , a Times analysis of satellite imagery found.

The Biden administration plans to raise tariffs on Chinese electric vehicles to protect U.S. auto manufacturers.

Apple is revamping Siri to offer more advanced A.I. responses , akin to ChatGPT.

An appeals court upheld Steve Bannon’s conviction for defying a subpoena from the House Jan. 6 committee. He could soon have to serve prison time.

A Virginia school board voted to restore the names of Confederate leaders — including Robert E. Lee and Stonewall Jackson — to two schools, reversing its 2020 decision to rename them .

CULTURE CALENDAR

By Desiree Ibekwe

🎥 Back to Black (Friday): You may well have seen the online discussion about this movie, an Amy Winehouse biopic directed by Sam Taylor-Johnson. The movie — which focuses on Winehouse’s relationship with Blake Fielder-Civil — was No. 1 at the British box office but divided viewers and critics, some of whom found fault with the appearance of its star, Marisa Abela. “I don’t need to convince people that they’re actually watching Amy,” Abela told The Times . “I need to remind people of her soul.”

RECIPE OF THE WEEK

By Melissa Clark

Strawberry Shortcake

It’s Mother’s Day tomorrow, and if your mom has a sweet tooth (and if so, I can relate), Jane Grigson’s strawberry shortcake as adapted by Nancy Harmon Jenkins might be just the thing for a celebratory brunch. Make the biscuit dough and cut out the rounds the day before (just keep them in the fridge until baking time). Then, while they’re in the oven, you can macerate the berries (any kind you like) with sugar and prep the whipped cream. Be sure to save any leftover biscuits. They’re excellent toasted for breakfast the next day.

REAL ESTATE

The hunt: An American took a chance on the Lake Geneva area of Eastern France, with a $300,000 budget. Which home did she buy? Play our game .

What you get for $900,000: A Frank Lloyd Wright house in Wilmette, Ill.; an 1879 three-bedroom house in Wilmington, N.C.; or a renovated ranch house in Scottsdale, Ariz.

Free help: A filmmaker, feeling unhelpful in her daily life, decided to offer small favors to passers-by in Union Square.

Made for walking: Brides are increasingly pairing cowboy boots with relaxed silhouetted dresses.

Scarlett Johansson: The actress shared her beauty regimen with T Magazine.

How to: Restoring a chair is easier than one might think . Here’s how a couple known as the Brownstone Boys did it.

ADVICE FROM WIRECUTTER

Food processors, blenders and choppers.

Countertop appliances can help you get a meal on the table faster, often with less work and a quicker cleanup. But deciding which gizmo is best for you can be a challenge. It depends on what kinds of foods you most frequently prepare, Wirecutter’s kitchen experts say. For example, if your main goal is to reduce the time you spend prepping ingredients, a food processor is likely your best bet. If you demand the smoothest, silkiest textures from your soups, sauces and smoothies (and have ample storage space), consider a full-size blender. Oh, and those TikTok-famous manual vegetable choppers ? No one needs those. — Rose Lorre

GAMES OF THE WEEK

W.N.B.A. season openers: A once-in-a-generation group enters the W.N.B.A. next week. You may already know their names: Caitlin Clark, Angel Reese, Cameron Brink, Kamilla Cardoso. Their college matchups shattered viewership records, and their pro draft last month did the same. The W.N.B.A. is trying to seize the moment: Nearly all of Clark’s games with the Indiana Fever will be national broadcasts , and some of her games are moving to bigger arenas to meet fan demand.

The season begins Tuesday, as Clark and the Fever face the Connecticut Sun and M.V.P. contender Alyssa Thomas. After that, the two-time defending champion Las Vegas Aces play Brittney Griner and the Phoenix Mercury. 7:30 p.m. and 10 p.m. Eastern on ESPN2

More coverage

Clark and Cardoso are featured in a documentary series , “Full Court Press,” airing on ABC this weekend, which follows them through their final season of college.

The W.N.B.A. is expanding : The league plans to add a 13th team, in the San Francisco area, next season, and a 14th, in Toronto, the year after.

NOW TIME TO PLAY

Here is today’s Spelling Bee . Yesterday’s pangram was uncloak .

Take the news quiz to see how well you followed this week’s headlines.

And here are today’s Mini Crossword , Wordle , Sudoku , Connections and Strands .

Thanks for spending part of your weekend with The Times. — Melissa

Sign up here to get this newsletter in your inbox . Reach our team at [email protected] .

Melissa Kirsch is the deputy editor of Culture and Lifestyle at The Times and writes The Morning newsletter on Saturdays. More about Melissa Kirsch