A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Actor Rod Taylor tests his time machine in a still from the film 'The Time Machine', directed by George Pal, 1960.

Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time. 

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future. 

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly. 

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

Navstar-2F GPS satellite

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

An artist's impression of a pair of neutron stars - a Tipler cylinder requires at least ten.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them? 

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers 

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible. 

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time 

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox 

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on … 

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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Image that reads Space Place and links to spaceplace.nasa.gov.

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!

Image of galaxies, taken by the Hubble Space Telescope.

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).

Illustration of GPS satellites orbiting around Earth

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.

Illustration of a hand holding a phone with a maps application active.

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

In Summary:

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

If you liked this, you may like:

Illustration of a game controller that links to the Space Place Games menu.

Time travel: Is it possible?

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

time travel graphic illustration of a tunnel with a clock face swirling through the tunnel.

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. 

Scott and Mark Kelly sit side by side wearing a blue NASA jacket and jeans

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

Graphic showing the path of GPS satellites around Earth at the center of the image.

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.

Graphic illustration of a wormhole

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.

Graphic illustration of the TARDIS (Time and Relative Dimensions in Space) traveling through space, surrounded by stars.

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:

A sketch from the Christmas Carol shows a cloaked figure on the left and a person kneeling and clutching their head with their hands.

  • 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:

Image of the Star Trek spaceship USS Enterprise

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

Games about time travel:

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

Additional resources

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

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

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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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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz 2018 square

Matthew S. Schwartz

time trip

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

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

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

Turns out, King might have been on to something.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Time is relative

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

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

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

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

Paradoxes and failed dinner parties

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

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

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

Telescopes are time machines

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

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

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

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

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

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Time Travel Probably Isn't Possible—Why Do We Wish It Were?

Time travel exerts an irresistible pull on our scientific and storytelling imagination.

Since H.G. Wells imagined that time was a fourth dimension —and Einstein confirmed it—the idea of time travel has captivated us. More than 50 scientific papers are published on time travel each year, and storytellers continually explore it—from Stephen King’s JFK assassination novel 11/22/63 to the steamy Outlander television series to Woody Allen’s comedy Midnight in Paris . What if we could travel back in time, we wonder, and change history? Assassinate Hitler or marry that high school sweetheart who dumped us? What if we could see what the future has in store?

These are some of the ideas that bestselling author James Gleick explores in his thought-provoking new book, Time Travel: A History. Speaking from his home in New York City, he recalls how Stephen Hawking once sent out invitations to a party that had already taken place ; why the Chinese government has branded time travel as “incorrect” and “frivolous” ; and how the idea of time travel is, ultimately, about our desire to defeat death.

Let’s cut right to the chase: What is time?

Oh, no, you didn’t! [ Laughs. ] In A.D. 400, St. Augustine said—and many people have said the same thing since, either quoting him consciously or unconsciously—“What, then, is time? If no one asks me, I know. If I wish to explain it to one that asks, I know not.” I think that is actually not a quip, but quite profound.

The best way to understand time is to recognize that we actually are very sophisticated about it. Over the past century-plus, we’ve learned a great deal. The physicist John Archibald Wheeler said, “Time is nature’s way to keep everything from happening all at once.” If you look it up in a dictionary, you get stuff like, “The general term for the experience of duration.” But that’s just completely punting because what is duration ?

I try to steer away from aphorisms and dictionary definitions, just to say two things. First, that we have a lot of contradictory ways of talking about time. We think of time as something we waste, spend, or save, as if it’s a quantity. We also think of time as a medium we are passing through every day, a river carrying us along. All of these notions are aspects of a complicated subject that has no bumper sticker answer.

When does the idea of time travel first appear in the West? And how did it impact popular culture?

I assumed, as a person who always read sci-fi a lot when I was a kid, that time travel is an obvious idea we’re born knowing and fantasizing about. And that it must always have been part of human culture, that there must be time travel Greek myths and Chinese legends. But there aren’t! Time travel turns out to be a very new idea that essentially starts with H.G. Wells’s 1895 novel, The Time Machine . Before that nobody thought of putting the words time and travel together. The closest you can come before that is people falling asleep, like Rip Van Winkle, or fantasies like Charles Dickens’s A Christmas Carol .

For Hungry Minds

The beginning of my book is an attempt to answer the question, “Why? Why not before? Why suddenly at the end of the 19 th century was it possible— necessary— for people to dream up this crazy fantasy?” Even though it’s H.G. Wells who does it, people pick up his ball very quickly and run with it. You find it in American science fiction that started appearing in pulp magazines in the 1920s and 1930s, or in the great new modernist literature of Marcel Proust’s In Search of Lost Time , James Joyce, and Virginia Woolf.

All these writers were suddenly making time their explicit subject, twisting time in new ways, inventing new narrative techniques to deal with time, to explore the vagaries of memory or the way our consciousness changes over time.

In 1991, Stephen Hawking wrote a paper called “Chronology Protection Conjecture , ” in which he asked: If time travel is possible, why are we not inundated with tourists from the future? He has a point, doesn’t he?

Yes! He even scheduled a party and sent out an invitation inviting time travelers to come to a party that had taken place in the past. Then he observed that none of them had shown up. [Laughs.] Hawking is one of these physicists who love playing with the idea of time travel. It’s irresistible because it’s so much fun! When he talks about the paradoxes of time travel it’s because he’s reading the same science fiction stories as the rest of us.

The paradoxes started appearing in magazines aimed mostly at young people in the 1920s. Somebody wrote in and said, “Time travel is a weird idea, because what if you go back in time and you kill your grandfather? Then your grandfather never meets your grandmother and you’re never born.” It’s an impossible loop.

Hawking, like other physicists, decided, “Time is my business. What if we take this seriously? Can we express this in physical terms?” I don’t think he succeeded but what he proposed was that the reason these paradoxes can’t happen is because the universe takes care of itself. It can’t happen because it didn’t happen. That’s the simple way of saying what the chronology protection conjecture is.

How have the Internet and other new technologies changed our perception and experience of time?

We are just beginning to see what the Internet is doing to our perception of time. We are living more and more in this networked world in which everything travels at light speed. We are multitasking and experiencing new forms of simultaneity, so the Internet appears to us as a kind of hall of mirrors. It feels as though we’re embedded in an ever expanding present.

Our sense of the past changes because in some ways the past becomes more vivid than ever. We’re looking at the past on our video screens and it’s just as vivid if the movie is about something that happened 20 years ago, as if it is a live stream. We can’t always tell the difference. On the other hand, the past that’s more distant—and isn’t available in video form—starts to seem more remote and fuzzier. Maybe we are forgetting how to visualize the past from reading histories. We’re entering a new period of time confusion, in which we suddenly find ourselves in what looks like an unending present.

In 2011, the Chinese government issued an extraordinary denunciation of the idea of time travel. What was their beef?

They thought it was corrupting and decadent. It’s a reminder that time travel is neither a simple nor innocent idea. It’s very powerful. It enables us to imagine alternative universes, and this is another line that science fiction writers have explored. What if someone was able to go back in time and kill Hitler?

Time travel is also a powerful way of allowing us to imagine what the future might bring. A lot of futurists nowadays tend to be dystopian. Time travel gives us ways of exploring how the worst tendencies of our current societies could grow even worse. That’s what George Orwell did in 1984 . I imagine the Chinese government doesn’t particularly want the equivalent of 1984 to be published in Beijing. [ Laughs. ]

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More than 50 scientific papers a year are now published on the idea of time travel. why are scientists drawn to the subject.

Scientists live in the same science fictional universe as all the rest of us. Time travel is a sexy and romantic idea that appeals to the physicist as much as it appeals to every teenager. I don’t think scientists are ever going to solve the problem of time travel for us but they still love to talk about wormholes and dark matter.

There’s a fascinating coincidence in the early history that when H.G. Wells needed to set the stage for his time machine hurtling into the future, he decided not to just jump right into his story but set the scene with a framing device—his time traveler lecturing a group of friends on the science of time—in order to justify the possibility of a time machine. His lecture introduces the idea that time is nothing more than a fourth dimension, that traveling through time is analogous to traveling through space. Since we have machines that can take us into any of the three special dimensions, including balloons and elevators, why shouldn’t we have a machine able to travel through the fourth dimension?

A decade later, Einstein burst onto the scene with his theory of relativity in which time is a fourth dimension , just like space. Soon after that, Hermann Minkowski pronounced that, henceforth, we were not going to talk about space and time as separate quantities but as a union of the two, spacetime , a four-dimensional continuum in which the future already exists and the past still exists.

I’m not claiming that Einstein read H.G. Wells 10 years before. But there was something in the air that both scientists and imaginative writers were empowered to visualize time in a new way. Today, that’s the way we visualize it. We’re comfortable talking about time as a fourth dimension.

You quote Ursula K. Le Guin , who writes, “Story is our only boat for sailing on the river of time.” Talk about storytelling and its relationship to time.

One of the things that has happened, along with our heightened awareness of time and its possibilities, is that people who invent narratives have learned very clever new techniques. Literal time travel is only one of them. You don’t actually need to send your hero into the future or into the past to write a story that plays with time in clever new ways. Narrative is also how everybody, not just writers, constructs a vision of our own relationship with time. We imagine the future. We remember the past. When we do that, we’re making up stories.

Psychologists are learning something that great storytellers have known for some time, which is that memory is not like computer retrieval. It’s an active process. Every time we remember something we are remembering it a little bit differently. We’re retelling the story to ourselves.

If time travel is impossible, why do we continue to be so fascinated with the idea?

One of the reasons is we want to go back and undo our mistakes. When you ask yourself, “If I had a time machine, what would I do?” sometimes the answer is, “I would go back to this particular day and do that thing over.” I think one of the great time travel movies is Groundhog Day , the Bill Murray movie where he wakes up every morning and has to live the same day over and over again. He gradually realizes that perhaps fate is telling him he needs to do it over, right. Regret is the time traveler’s energy bar. But that’s not the only motivation for time travel. We also have curiosity about the future and interest in our parents and our children. A lot of time travel fiction is a way of asking questions about what our parents were like, or what our children will be like.

At some point during the four years I worked on this book, I also realized that, in one way or another, every time travel story is about death. Death is either explicitly there in the foreground or lurking in the background because time is a bastard, right? Time is brutal. What does time do to us? It kills us. Time travel is our way of flirting with immortality. It’s the closest we’re going to come to it.

This interview was edited for length and clarity.

Simon Worrall curates Book Talk . Follow him on Twitter or at simonworrallauthor.com .

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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

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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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Time Travel

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

1.1 Time Discrepancy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2. The Grandfather Paradox

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

We can set out Horwich’s argument this way:

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

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

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

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

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

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

3. Causation

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

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

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

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

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

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

4. Time and Change

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

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

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

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

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

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

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

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

Earlier we posed two questions:

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

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

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

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

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

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

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

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

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Time travel: five ways that we could do it

time travel_travel through time

Cathal O’Connell

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

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

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

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

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

050416 timetravel 1

1. Time travel via speed

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

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

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

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

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

Computer solves a major time travel problem

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

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

2. Time travel via gravity

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

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

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

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

050416 timetravel 2

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

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

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

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

3. Time travel via suspended animation

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

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

Could humans ever do the same?

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

050416 timetravel 3

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

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

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

4. Time travel via wormholes

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

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

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

5. Time travel using light

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

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

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

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

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

Related Reading: Computer solves a major time travel problem

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“Time-trip.” Merriam-Webster.com Dictionary , Merriam-Webster, https://www.merriam-webster.com/dictionary/time-trip. Accessed 26 May. 2024.

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Guardians ace Shane Bieber joins Cleveland during West road trip for first time since surgery

Cleveland Guardians' Shane Bieber stands in the dugout during the third inning of a baseball game against the Los Angeles Angels in Anaheim, Calif., Saturday, May 25, 2024. (AP Photo/Ashley Landis)

Cleveland Guardians’ Shane Bieber stands in the dugout during the third inning of a baseball game against the Los Angeles Angels in Anaheim, Calif., Saturday, May 25, 2024. (AP Photo/Ashley Landis)

FILE - Cleveland Guardians starting pitcher Shane Bieber throws against the Seattle Mariners during a baseball game April 2, 2024, in Seattle. Bieber marked being six weeks removed from Tommy John surgery with a pair of milestones. The Guardians ace not only had the bulky brace from his right arm removed, he is back with his teammates for the first time since early April. (AP Photo/Lindsey Wasson, File)

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ANAHEIM, Calif. (AP) — Shane Bieber marked being six weeks removed from Tommy John surgery with a pair of milestones.

The Cleveland Guardians ace not only had the bulky brace from his right arm removed, he is back with his teammates for the first time since early April.

Bieber is on the Guardians’ six-game road trip out West that began Friday with a weekend set against the Los Angeles Angels. Cleveland goes to Colorado for a three-game series Monday before Bieber heads back to the team’s training facility in Goodyear, Arizona, to continue rehab.

Bieber has certainly rejoined his teammates at the right time. The AL Central-leading Guardians have won eight straight after Saturday night’s 4-3 win over the Angels . It is Cleveland’s longest winning streak since it ran off an AL-record 22 straight in 2017.

“It’s awesome to to be able to meet up with the guys, when they’re on such a heater. It’s been fun to watch, both in person and from afar,” Bieber said before Saturday’s game.

The 2020 AL Cy Young Award winner had reconstructive surgery on the ulnar collateral ligament in his right elbow on April 12 in Texas. Besides having the brace removed, Bieber has been going through the early rehab process of building back range of motion.

Colorado Rockies' Jordan Beck runs to first base after hitting an RBI single during the eighth inning of a baseball game against the San Diego Padres, Wednesday, May 15, 2024, in San Diego. (AP Photo/Gregory Bull)

“It’s had its difficulties over the first six weeks, but hopefully the process continues to go smoothly,” he said. “I’m feeling good and staying positive.”

When Bieber took the mound in Seattle on April 2, he had a feeling during the first inning that it was going to be his last start for some time as the pain in his elbow became too overwhelming. Bieber was limited to 21 starts last season also due to elbow issues.

Despite that, Bieber threw 83 pitches (55 strikes) in six scoreless innings and struck out nine. That followed not allowing a run and striking out 11 in six innings in the March 28 opener at Oakland.

“I saw some people ask why I was going through that? That’s a natural question to ask, but with the context of what I dealt with last year and rehabbing it,” Bieber said. “I needed more feedback on what I was dealing with. I have no regrets and I’m happy to be moving forward.

“I knew what was going on and inevitably going to happen, that I was going to have to get surgery. Looking back, I’m thankful to have had that outing. Those first two outings going so well made it a little more difficult in a sense. It was an interesting experience in Seattle pitching and competing knowing it was going to be awhile.”

Besides discussing the next steps in rehab, the bigger question hanging over Bieber is his future in Cleveland. He is slated to be a free agent in the offseason since this is his final year under club control.

Bieber is 62-32 with a 3.22 ERA in 134 starts over seven seasons with Cleveland. The 28-year old said he would consider a short-term deal if it made sense.

“Honestly it’s too far down the road for me. It’s a little bit different than I expected, but my mindset is the same. I’m confident in myself and there’s always something to be done, but at the same time I think everybody’s kind of focused on what’s going on right now.”

Even with Bieber and Trevor Stephan out for the year due to Tommy John surgeries, the Guardians’ pitching staff has the fourth-lowest ERA in the majors.

Manager Stephen Vogt said having Bieber and Stephan back this weekend has provided another jolt of energy for a ballclub that has won seven straight.

“It’s a huge pick-me-up for them and us. Any time you have Shane Bieber sitting around talking baseball, everybody’s going to get better,” Vogt said.

Tanner Bibee, who allowed three runs in six innings en route to his first win in over a month, said it was reassuring seeing Bieber in the dugout between innings.

“Other than Cookie (Carlos Carrasco), Shane is our vet,” Bibee said. “He helped me out a lot last year during my rookie year so it’s been great having him here these past two days.

AP MLB: https://apnews.com/hub/MLB

JOE REEDY

Loaded with in-state talent, UT and Texas A&M softball teams tussle for WCWS trip

Adam Winkler Image

AUSTIN, Texas (KTRK) -- For the first time in the programs' history, the University of Texas and Texas A &M will play for a trip to the Women's College World Series.

The Longhorns, seeded No. 1 in the NCAA softball tournament, host the 16th-seeded Aggies in a best-of-three Super Regional series beginning Friday in Austin.

According to the team's rosters, 25% of the players involved in the Super Regional are from the Greater Houston area. Among that group are Texas outfielder Kayden Henry (Dickinson High School), Texas A &M third baseman Kennedy Powell (Conroe High School), and Texas A &M pitcher Emiley Kennedy (Lake Creek High School).

"I look back to my little 10-year-old self when I started playing softball, and I would not have imagined me being here today," Kennedy, a first-team All-SEC pitcher, admitted during a Zoom interview with ABC13. "And it's my first super! I'm excited to be out there."

"I planned on being here when I transferred," Powell, who transferred to A &M from UCLA, noted. "I always knew the school I went to was going to have potential to get this far. We're just excited to get on the field and get going."

"It's an amazing thing to know I'm two wins away from where I dreamed of being when I was seven or eight years old," Henry, who helped lead Texas to the program's first-ever No. 1 overall seed, said.

The winner of the best-of-three series will advance to the Women's College World Series.

For more sports news, follow Adam Winkler on Facebook , X and Instagram .

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AAA forecasts nearly 44M travelers for Memorial Day weekend: See best times to travel

time trip

Memorial Day weekend marks the unofficial start of summer, which means people will be flooding highways and airports this weekend looking to get away for some rest and relaxation.

The American Automobile Association is projecting nearly 44 million travelers will head 50 miles or more from home over the Memorial Day holiday travel period, which is defined by the organization as the five-day period from Thursday, May 23 to Monday, May 27.

"We haven't seen Memorial Day weekend travel numbers like these in almost 20 years," said Paula Twidale, senior vice president of AAA Travel, in a news release. "We're projecting an additional one million travelers this holiday weekend compared to 2019, which not only means we're exceeding pre-pandemic levels but also signals a very busy summer travel season ahead."

The organization projects over 38 million people will travel by car over Memorial Day weekend, over 3.5 million will travel by air and nearly 2 million will travel by other modes of transportation, including buses, cruises and trains.

The 38 million drivers is the highest number for Memorial Day since AAA began tracking data in 2000 and is up 4% compared to last year. The 3.5 million air travelers is an increase of nearly 5% over last year and, according to AAA, this will be the most crowded Memorial Day weekend at airports since 2005.

Learn more: Best travel insurance

When is Memorial Day?: Here's when the holiday falls and what to know about its history.

Best, worst times to travel for Memorial Day weekend

Drivers leaving Thursday or Friday should hit the road early to avoid mixing with commuters, AAA said, citing data from transportation data and insights provider INRIX. Drivers going back home Sunday or Monday should avoid the afternoon hours when return trips will peak.

“Travel times are expected to be up to 90% longer than normal. Travelers should stay up to date on traffic apps, 511 services, and local news stations to avoid sitting in traffic longer than necessary,” said Bob Pishue, transportation analyst at INRIX, in the news release.

Here's a breakdown of the best and worst times to travel by car depending on which day you are traveling:

Thursday, May 23

  • Best travel time: Before 11 a.m. and after 7 p.m.
  • Worst travel time: Between noon and 6 p.m.

Friday, May 24

  • Best travel time: Before 11 a.m. and after 8 p.m.
  • Worst travel time: Between noon and 7 p.m.

Saturday, May 25

  • Best travel time: Before 1 p.m. and after 6 p.m.
  • Worst travel time: Between 2 p.m. and 5 p.m.

Sunday, May 26

  • Best travel time: Before 1 p.m.
  • Worst travel time: Between 3 p.m. and 7 p.m.

Monday, May 27

  • Best travel time: After 7 p.m.

Gas prices continue to decrease heading into Memorial Day weekend

According to AAA, the national average for a gallon of gasoline has fallen four cents since last week, to $3.60. The primary reasons are weak domestic demand and oil costs below $80 a barrel, according to the organization.

“Barring some unforeseen event, this pokey drop in pump prices is not likely to change anytime soon,” said Andrew Gross, AAA spokesperson, in a news release. “There are seven states with county gas averages at less than $3 a gallon. This trend will likely accelerate as more gas outlets east of the Rockies drop their prices.”

This Memorial Day weekend, drivers can expect similar gas prices as last year when the national average was about $3.57.

"Pump prices rose this spring but have held somewhat steady in recent weeks," AAA said in the news release, while also noting that prices may creep higher as the summer driving season gets underway.

The wildcard remains the cost of oil, according to the organization, and unlike last year, there are now two wars – in the Middle East and Ukraine – that could roil the oil market.

Gabe Hauari is a national trending news reporter at USA TODAY. You can follow him on X  @GabeHauari  or email him at [email protected].

I went on one date with a man, and then we decided to travel together indefinitely. It's not as romantic as it seems.

  • After traveling full-time, I joined a dating app and went on a date; we fell for each other.
  • Since we have similar values, we decided to travel together indefinitely. 
  • It's been difficult at times, and our future is uncertain, but it's the perfect life for us.

Insider Today

Last year, I finally fulfilled my dream of traveling full-time when I got a job aboard a cruise ship. I was loving every moment of my nomadic life. I was too busy enjoying myself to worry about a relationship.

But in my second year of the job and third year of being single, I joined the Nomad Soulmates dating app . I wasn't necessarily looking for my soulmate because travel was my priority for most of my life. But at 32, I knew I had to make a proactive choice to prioritize relationships and dating.

In April of 2023, I disembarked the ship in Lisbon and tried out the app. However, I was skeptical it would lead anywhere because I only had two months off the ship before I had to head to Norway to join my next contract. Plus, I had already scheduled my travel plans for the next two months.

But then I matched with someone that made me rethink everything.

We had an instant connection

I matched with a man who was in Lisbon for the month. He asked me out for Greek food, and I figured it was worth a try. Once again, I didn't imagine it would go anywhere because I only planned to stay for a week before heading to Porto for the weekend and then off to Spain for the next leg of my trip .

Related stories

But our date ended up being so much fun that I was interested in seeing where this could go. He was silly, and that was refreshing. We shared many of the same values and loved traveling, so the conversation never lagged.

I surprised myself when I invited him to stay with me for a few nights in Porto. During our visit to Porto, I canceled my trip to Spain and accepted his invitation to return to Lisbon with him, beginning our partnership.

We decided to keep traveling together

I had to choose between my travel plans and my partner . Having been a solo traveler, giving up my travel plans for something that may or may not work out was not a decision I took lightly.

But I wanted to take a chance on this as we found ourselves on the same page about everything — especially what we were looking for in a long-term relationship and what we wanted in the future. I never saw myself returning to the US; I wanted to return to Italy or continue traveling full-time with my future partner and kids. He also had no plans on returning to his home country of Germany and, having been traveling full-time since 2018, was uninterested in stopping. He also wants children and plans to travel with them full-time.

I saw that we were on the same page regarding how we wanted to incorporate parenting, traveling, and our careers, so we decided to continue traveling together — which was a decision we made exceptionally quickly.

I was fearful at the beginning of our relationship because I heard horror stories from friends about traveling with partners they just met. Luckily, being seasoned travelers , we face inconveniences quite similarly and calmly. Within the last year, we have dealt with being separated for months at a time and spending every second together for months at a time. We have also experienced missed flights, scooter accidents, cockroach infestations, and a lack of necessities such as hot water. It's not all romance and dreamy vacations.

Like so many other relationships I've had, I thought each new inconvenience would fracture our relationship, causing us to decide that maybe this wasn't the right partnership for us. But it all had the opposite effect, building the foundation of our relationship as we moved into the second year of traveling full-time together.

Bound by wanderlust , we aren't sure what our futures hold, but we will sculpt it with our shared dreams.

Watch: Marriott International's Tina Edmundson tells Insider that the travel mindset has changed since the pandemic

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All you need to know for Memorial Day weekend travel, traffic and weather

The price of a getaway this Memorial Day weekend may be a headache at the airport or on the highway as the hectic summer travel season kicks off.

As travelers set out for a brief vacation over Memorial Day weekend before heading home, it’s likely that jams will hold up highway traffic and clog airport security lines over the coming days.

AAA predicts that this Memorial Day weekend may be the busiest in terms of holiday travel in nearly 20 years. Amid this summer’s potentially record-breaking travel season, here’s how to speed up your commute this Memorial Day weekend.

How many Americans will travel for Memorial Day?

AAA anticipates that from Thursday to Monday (Memorial Day), 43.8 million Americans will be traveling at least 50 miles away from home. Road trips seem to be especially popular, with AAA projecting that nearly 9 in 10 of these Americans will travel by car, the highest number of Memorial Day weekend road trippers since AAA started tracking these numbers in 2000.

While the association projects that more than 38 million Americans will be traveling by car, airports won’t be immune to jams either.

In fact, the Transportation Security Administration is preparing for more than 18 million people to pass through airport security checkpoints from Thursday to Wednesday, May 29.

“We haven’t seen Memorial Day weekend travel numbers like these in almost 20 years,” said Paula Twidale, senior vice president of AAA Travel, in a press release. “We’re projecting an additional 1 million travelers this holiday weekend compared to 2019, which not only means we’re exceeding pre-pandemic levels but also signals a very busy summer travel season ahead.”

The travel headache over Memorial Day weekend may be only the start of a potentially record-breaking summer travel season, as U.S. airlines are projecting that 271 million passengers will fly from June 1 to Aug. 31, up from last summer’s record of 255 million travelers. In fact, the TSA has forecasted that it will see the highest volumes of travelers passing through airport security checkpoints ever this summer travel season.

What is the busiest travel day for Memorial Day weekend?

As Americans have widely reported in polls that they are concerned about the country’s economy, road tripping has emerged as a popular method of travel this Memorial Day weekend due to its flexibility and convenience, according to AAA.

Bob Pishue, a transportation analyst at the traffic management company INRIX, predicted that travel times for drivers would be up to 90% longer than they typically are. Sunday, May 27 and Memorial Day itself were expected to be especially busy, as trips back home would peak then, according to INRIX.

For those picking up rental cars, rental car company Hertz expected that Thursday and Friday, May 24 would be the busiest days.

In terms of air travel, the TSA anticipated that Friday, May 24 would rank as the busiest travel day for the entire Memorial Day weekend, with nearly 3 million people passing through airport security checkpoints on that day alone.

“In close coordination with airport, airline and travel partners, we are more than ready to handle this summer’s increased travel volumes,” David Pekoske, a TSA administrator, said in a press release. “We are also continuing to deploy state-of-the-art checkpoint technology that increases security effectiveness, efficiency and enhances the passenger experience, and our retention and recruitment numbers are the highest they’ve ever been.”

When is the best time to travel on Memorial Day weekend?

For the millions predicted to hit the road for a weekend getaway in the coming days, INRIX advised that drivers setting out on Thursday or Friday get on the road early to avoid running into traffic from other travelers. In addition, the company said that drivers heading home on Sunday, May 26 or on Memorial Day should avoid traveling during the afternoon, as trips home would peak then.

AAA predicted that these would be the best times for drivers to travel over Memorial Day weekend:

  • Thursday: Before 11 a.m. and after 7 p.m.
  • Friday, May 24: Before 11 a.m. and after 8 p.m.
  • Saturday, May 25: Before 1 p.m. and after 6 p.m.
  • Sunday, May 26: Before 1 p.m.
  • Memorial Day: After 7 p.m.

What time is heaviest traffic on Memorial Day weekend?

AAA projected that these times would be the busiest for drivers:

  • Thursday: Noon to 6 p.m.
  • Friday, May 24: Noon to 7 p.m.
  • Saturday, May 25: 2-5 p.m.
  • Sunday, May 26: 3-7 p.m.
  • Memorial Day: 3-7 p.m.

Tips for speeding up travel time on Memorial Day weekend

With weekend highway closures for maintenance, like the unpopular closures of portions of Interstate 17 last weekend and Mother’s Day weekend that led to hours-long traffic jams, drivers should carefully plan their trips in accordance with highway conditions. Pishue advised that drivers avoid being stuck in traffic by doing the following:

  • Checking services like az511.com , which shows Arizona highway conditions in real-time
  • Staying up to date on traffic through local news outlets. The Republic publishes a list of Valley freeway closures for the weekend using data from the Arizona Department of Transportation that drivers can consult to plan their commutes.
  • Looking at traffic apps, like Waze , which displays a live traffic map. Chandler became the first Arizona city to share traffic information through Waze, meaning that users can view the locations of street closures, construction sites and maintenance projects in Chandler based on official information from the city itself.

While the TSA said that it’s prepared to deal with the influx of passengers that will be flooding airports nationwide this Memorial Day weekend, the administration and Phoenix Sky Harbor International Airport advised air travelers to follow these tips to keep from holding up lines through airport security checkpoints:

  • Allow extra time to travel to the airport and get through security checkpoints amid the busy holiday travel season. The TSA advised that passengers factor in extra time for all the activities they may need to do before getting to their gates — from parking to returning rental cars to taking shuttles to the airport to checking in with their airlines to going through security. Check current wait times at Sky Harbor’s security checkpoints at skyharbor.com or at the flight information screens throughout the airport.
  • Reserve parking at Sky Harbor beforehand at the airport’s parking website .
  • Review the status of flights in advance through airline websites or skyharbor.com before arriving at Sky Harbor for a flight, picking up travelers or dropping off passengers.
  • Make a free reservation in Sky Harbor’s PHX RESERVE program to save a spot in the TSA security checkpoint lines in Terminal Three and Terminal Four. Travelers are able to make reservations and walk-in appointments through PHX RESERVE from 4-11 a.m. at Terminal Three if they’re flying out any time from 5 a.m. to 2 p.m. In Terminal Four, passengers are eligible for PHX RESERVE from 5:30-11 a.m. as long as their flight is scheduled from 7 a.m. to 3 p.m. Because availability for walk-in appointments is limited, reservations, which travelers can make up to six days before their flight, are advised.
  • Begin packing for the airport with an empty bag. The TSA said that travelers who empty their bags before packing them are less likely to be stopped by airport security due to having prohibited items. The administration advised that passengers consult TSA guidelines to learn what they can — and can’t — bring on flights. Travelers heading to the beach for Memorial Day weekend must pack sunscreen in containers that are larger than 3.4 ounces in check-in bags.

How much will gas prices be on Memorial Day weekend?

AAA projected that this Memorial Day weekend, pump prices will be similar to last year’s, when gas nationwide cost about $3.57 on average. While gas prices increased over the spring, the association said they’ve been relatively steady in the last few weeks.

AAA reported on Thursday that the national average price for a gallon of gas wavered only slightly as compared with the price a week before until it increased by a penny to $3.61, a nickel less than a month ago but 7 cents more than a year before. In Arizona, the average gas price on Thursday was over a quarter higher at about $3.88, which was still nearly a quarter less than it was a month ago and over 70 cents less than it was a year ago.

The lack of movement in pump prices in the days before Memorial Day was typical of the time, according to the association.

However, that may change as the summer driving season begins, which may cause gas prices to creep up. The business of Memorial Day weekend may also drive pump prices up — at least temporarily — AAA wrote.

“Since the pandemic, the summer driving season has not seen a surge in demand, which can push pump prices higher,” AAA spokesperson Andrew Gross said. “So it will be interesting to see if this year bucks that trend.”

Geopolitical factors, like conflict in the Middle East and Ukraine, may also lead to changes in pump prices, as they could shock the oil market. In addition, Gross said that while the Biden administration’s decision to sell almost 1 million barrels of gas in northeastern states, which was announced Tuesday, may keep regional gas prices from surging, the national average pump price probably will not change much.

Do flight prices go up on Memorial Day weekend?

Even though this Memorial Day weekend is slated to be the busiest for airports in nearly 20 years, AAA found that plane ticket prices for this weekend were comparable to last year’s. Domestic flight ticket prices increased by up to 2%, according. Popular American destinations included Miami beaches, Seattle cruise ports, New York and Las Vegas while European cities like Rome, London and Paris dominated the list of frequently booked international spots, according to AAA booking data.

The association wrote that numerous factors determine plane ticket prices for passengers, including where they’re flying, how many stops they’re making and the fare class they choose. As a result, travelers who book a nonstop flight, seat selection and an included carry-on will probably pay more than passengers who opt for basic economy and a layover flight.

Why will Memorial Day weekend be so busy?

Lower travel prices may be behind the busyness predicted to hit airports and highways this Memorial Day weekend. Airfare decreased by 6% and hotel rates fell by 0.4% as compared with prices from a year before, according to data released this week by the Bureau of Labor Statistics. In addition, the cost of renting a car or truck has decreased by 10%.

For road trippers, however, the national average price of gas is still over a nickel higher than a year ago.

What is the weather like on Memorial Day weekend?

It’s expected that triple-digit weather will make its return in the Valley over Memorial Day weekend, meaning that Phoenicians are slated to see some of the highest temperatures all year so far over the holiday weekend.

The triple-digit weather is predicted to continue over the weekend into much of next week and perhaps longer, signaling a hot and dry June. While temperatures will fall from a 98-degree high on Friday, May 24 to a high of 91 degrees on Saturday, May 25, they’ll climb to a 102-degree high on Memorial Day. By Wednesday, May 30, temperatures are slated to creep up to a high of 105 degrees.

What about fire risk?

Restrictions are in place for most of the Tonto National Forest as well as state trust lands in three counties.

That means no fireworks, no fires outside of grills, no target shooting and limitations on where people enjoying the great outdoors can go to grab a smoke.

The restrictions are the lowest level of limitations on public lands.

The Tonto Forest restrictions kicked in Thursday and will remain in place through Sept. 30. The areas immediately in and around Globe and Payson are exempt.

State trust land requires users to get a permit before accessing the land for hiking or other purposes. Restrictions are in place for these lands in Gila, Maricopa and Pinal counties.

Madeline Nguyen is a breaking news reporter for The Republic. Reach her at [email protected] or 480-619-0285. Follow her on X @madelineynguyen .

Watch CBS News

Memorial Day weekend 2024 could break travel records. Here's what to know.

By Emily Mae Czachor

May 20, 2024 / 5:04 PM EDT / CBS News

Despite another month of spring technically still ahead, travelers often view Memorial Day weekend  as an unofficial kickoff to the summer season — and traffic notoriously reflects that . Based on current forecasts, travel around the upcoming 2024 holiday next Monday, May 27, is not expected to relieve Memorial Day of its bad reputation for drivers and airline passengers. They may actually find themselves on some of the busiest highways and flights they've seen in decades.

The American Automobile Association, or AAA, warned of potentially unprecedented congestion on roads this weekend, along with airports that could be even more crowded than in years past. The organization, which looks at various economic factors and partners with other groups to project travel conditions, announced earlier this month that an estimated 43.8 million people across the United States would likely travel at least 50 miles from Thursday to Monday. That would mark a 4% increase in overall travel compared with 2023, according to AAA. It would also come close to the busiest Memorial Day weekend on record, which happened in 2005 when 44 million people left home for the holiday.

"We haven't seen Memorial Day weekend travel numbers like these in almost 20 years," said Paula Twidale, the senior vice president of the travel division at AAA, in a statement. "We're projecting an additional one million travelers this holiday weekend compared to 2019, which not only means we're exceeding pre-pandemic levels but also signals a very busy summer travel season ahead."  

For those planning to hit the road or board a plane this weekend, here's what to know.

Prepare to hit traffic

Memorial Day weekend in 2024 is expected to set a new record for road trips, according to AAA. The organization has estimated that 38.4 million people will travel in cars over the course of the weekend, which would be the biggest number recorded around this particular holiday since the group first started keeping track of Memorial Day travel patterns in 2000. 

Car rental company Hertz told AAA that demand for rentals this year will be highest in Atlanta, Boston, Las Vegas and Orlando, with most renters scheduled to pick up their cars on Thursday and Friday.

In general, drivers hoping to beat the traffic, or at least face less of it, should avoid the roads during afternoon hours on any day of the long weekend. Citing transit data from INRIX , AAA said the worst times to travel by car , in any U.S. time zone, are between 12 p.m. and 6 p.m. on Thursday, between 12 p.m. and 7 p.m. on Friday, between 2 p.m. and 5 p.m. on Saturday, between 3 p.m. and 7 p.m. on Sunday, and between 3 p.m. and 7 p.m. on Monday. 

The best times to drive will be before 11 a.m. or after 7 p.m. on Thursday, before 11 a.m. or after 8 p.m. on Friday, before 1 p.m. or after 6 p.m. on Saturday, before 1 p.m. on Sunday, and after 7 p.m. on Monday.

Airports will be crowded

Airports across the country are bracing for another spike in travelers, following an upward trend in flights booked around Memorial Day since last year's air travel numbers exceeded pre-pandemic levels. An estimated 3.51 million people are projected to fly this weekend, according to AAA, which is up from 3.35 million who traveled on planes to their destinations last year. If as many people fly as expected, this will be the most crowded Memorial Day weekend at airports since 2005, when AAA said 3.64 million people caught flights for the holiday.

United Airlines said more than 500,000 people are expected to fly each day from Thursday to Tuesday, which would be the airline's busiest Memorial Day weekend on record. Delta said 3 million people are expected to fly on its planes over that six-day period and American Airlines said it expects 3.9 million people to fly over the weekend.

Public transportation tips

Any of the projected 1.9 million people who use public transit systems to get to where they're going this weekend can plan ahead, too. INRIX projections show metro riders will face a degree of congestion in major cities, including Atlanta, Boston, Chicago, Denver, Detroit, Houston, Los Angeles, Minneapolis, New York, Philadelphia, Portland, Oregon, San Diego, San Francisco, Seattle, Tampa and Washington, D.C. Congestion on metros is expected to peak in those places in the late afternoon, early evening and mid-morning each day between Thursday and Monday. 

Forecasts suggest D.C., Los Angeles, Houston and Tampa will see the largest jumps in metro crowding compared with their respective norms. The worst is projected for one route from Gainesville to Tampa, where INRIX said metro congestion on Sunday at 9 a.m. local time could be 88% higher than usual.

Kris Van Cleave contributed reporting.

  • Memorial Day

Emily Mae Czachor is a reporter and news editor at CBSNews.com. She covers breaking news, often focusing on crime and extreme weather. Emily Mae has previously written for outlets including the Los Angeles Times, BuzzFeed and Newsweek.

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International Criminal Court Prosecutor Requests Warrants for Netanyahu and Hamas Leaders

While the request must be approved by the court’s judges, the announcement is a harsh rebuke of Prime Minister Benjamin Netanyahu of Israel and his war strategy in Gaza.

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Headshots of two men. Prime Minister Benjamin Netanyahu of Israel is on the left and Yahya Sinwar is on the right.

By Patrick Kingsley and Matthew Mpoke Bigg

Patrick Kingsley reported from Jerusalem, and Matthew Mpoke Bigg from London.

The chief prosecutor at the world’s top criminal court on Monday announced that he was seeking arrest warrants for the leaders of both Israel and Hamas on charges of crimes against humanity, a strong rebuke that equated Prime Minister Benjamin Netanyahu of Israel with his Hamas counterpart, Yahya Sinwar, and compounded the growing international alarm at Israel’s conduct in Gaza.

In a statement , Karim Khan, the chief prosecutor, said that after investigating Hamas’s Oct. 7 attack on Israel and Israel’s counterattack on Gaza he had decided to apply for arrest warrants for Mr. Sinwar, Hamas’s leader within Gaza; Muhammad Deif, Hamas’s military leader; and Ismail Haniyeh, the movement’s Qatar-based political leader. Mr. Khan also said he was requesting warrants for Mr. Netanyahu and for Israel’s defense minister, Yoav Gallant.

While Mr. Khan’s request must still be approved by judges from the court, the announcement forms one of the harshest rebukes of Israel’s strategy in its seven-month campaign against Hamas that has killed tens of thousands of Gazan civilians. It also heightens scrutiny of Hamas’s actions at the start of the war in October, when Hamas fighters led a raid that killed more than 1,000 people and abducted hundreds more.

“Today we once again underline that international law and the laws of armed conflict apply to all,” Mr. Khan said in his statement. “No foot soldier, no commander, no civilian leader — no one — can act with impunity.”

For now, the announcement is largely symbolic. Israel is not a member of the court and does not recognize its jurisdiction in Israel or Gaza, meaning that Israeli leaders would face no risk of arrest at home.

Judges can also take months to uphold requests for arrest warrants. But if they do issue warrants, those named could be arrested if they travel to one of the court’s 124 member nations , which include most European countries but not the United States.

Mr. Khan’s decision to simultaneously pursue Israeli and Palestinian leaders was criticized by Israeli government ministers and Hamas alike. Both sides questioned why their allies had been targeted instead of their enemies alone.

“How dare you compare the monsters of Hamas to the soldiers of the Israeli Army, the world’s most moral military?” Mr. Netanyahu asked in a statement on Monday evening.

Similarly, Hamas said in a statement that it “strongly denounces” the attempt to “equate the victim with the executioner by issuing arrest warrants against a number of Palestinian resistance leaders.”

The decision raised difficult questions for Israel’s allies who are members of the court and could be required to arrest Mr. Netanyahu and Mr. Gallant if the warrants are issued and the men subsequently travel to their territories. Qatar, which hosts several Hamas leaders including Mr. Haniyeh, is not a member of the court.

President Biden condemned the move, saying in a statement that, “Whatever this prosecutor might imply, there is no equivalence — none — between Israel and Hamas.”

Mr. Khan’s statement said that he had “reasonable grounds to believe” that Mr. Sinwar, Mr. Deif and Mr. Haniyeh were responsible for “war crimes and crimes against humanity” — including “the killing of hundreds of Israeli civilians in attacks perpetrated by Hamas.”

Mr. Khan said he sought their arrest both for the killing of civilians and the capture of hostages during the Oct. 7 attack, as well as on charges of maltreatment of and sexual violence against hostages during their captivity in Gaza.

The requests for warrants were based on interviews with survivors, review of documentary evidence including video and photographs, and field visits by Mr. Khan and his team. Mr. Khan visited the Israeli-occupied West Bank and a border crossing between Egypt and Gaza, but did not enter Gaza itself. He also went to some of the sites attacked in Israel during the Hamas-led raid, interviewing victims and witnesses.

Regarding Mr. Netanyahu and Mr. Gallant, the prosecutor said he believed the Israeli leaders bore criminal responsibility for war crimes and crimes against humanity, including using starvation as a weapon of war and “intentionally directing attacks against a civilian population.”

While Mr. Khan said that Israel was allowed to protect its citizens, he said that its forces had failed to uphold international law during its devastating response.

“Notwithstanding any military goals they may have, the means Israel chose to achieve them in Gaza — namely, intentionally causing death, starvation, great suffering, and serious injury to body or health of the civilian population — are criminal,” Mr. Khan wrote.

Mr. Khan also implicitly criticized Israel’s judicial system, saying that the I.C.C. is forced to act only when a country’s prosecutors fail to hold its own citizens to account.

The court defers to “national authorities only when they engage in independent and impartial judicial processes that do not shield suspects and are not a sham,” Mr. Khan said.

But Antony J. Blinken, the U.S. secretary of state, said that Mr. Khan had not given Israel enough time to show that its own prosecutors were investigating the case. Mr. Blinken said that Mr. Khan’s aides had called off a visit to Israel on Monday to address that very question, suggesting that they were not serious about finding out the answer.

Mr. Khan’s office said that it “has not received any information that has demonstrated genuine action at the domestic level to address the crimes alleged or the individuals under investigation.”

Within the Israeli government, which had been split over disagreements about war strategy in recent days, the announcement prompted ministers to set aside their differences and adopt a united front.

Benny Gantz, a minister in Israel’s war cabinet and a critic of Mr. Netanyahu, accused the prosecutor of “moral blindness” in drawing an equivalence between the leaders of Israel and Hamas. Mr. Gantz’s response came less than two days after he threatened to quit Mr. Netanyahu’s cabinet for failing to set in motion a plan for the governance of postwar Gaza.

Relatives of Israeli hostages praised the push to hold Hamas’s leaders to account, but criticized the decision to target both Israeli politicians and Hamas at the same time.

The Hostage Families Forum, an alliance representing hostages’ relatives and supporters, said it “applauds the issuance of warrants against senior Hamas officials” but was “not comfortable with the equivalence drawn between Israel’s leadership and the terrorists of Hamas.”

Palestinians in Gaza had the inverse reaction, questioning why Palestinian leaders had been targeted instead of only Israelis.

Jaber Yahia, 50, a teacher in central Gaza, said by telephone that he was “relieved” to hear of the requests for warrants for Mr. Netanyahu and Mr. Gallant. “Then I found out there were other warrants against Haniyeh, Sinwar and Deif. Why do they insist on putting the killers and victims in the same category?”

Appearing to anticipate such criticism from both sides, Mr. Khan wrote in his statement of the need to apply the law equally to all sides in a conflict.

“If we do not demonstrate our willingness to apply the law equally, if it is seen as being applied selectively, we will be creating the conditions for its collapse,” Mr. Khan said.

In recent weeks, Israeli and Western officials had predicted privately and publicly that leaders from Israel and Hamas could soon face prosecution.

In late April, Mr. Netanyahu said on social media that the country “will never accept any attempt by the I.C.C. to undermine its inherent right of self-defense.”

The I.C.C. is the world’s only permanent international court with the power to prosecute individuals accused of war crimes, genocide and crimes against humanity. It is separate from the International Court of Justice, another international tribunal in The Hague, which deals with disputes between states. The I.C.J. is currently assessing a claim, brought by South Africa, that Israel is conducting a genocide in Gaza , an accusation that Israel strongly denies.

Mr. Biden, too, rejects that idea, stating flatly on Monday during a celebration of Jewish Heritage Month in the Rose Garden at the White House that Israel’s military assault in Gaza in the wake of the October attacks led by Hamas “is not genocide.”

“We reject that,” he said, telling an audience of Jewish leaders and activists that Americans “stand with Israel.”

The I.C.C. cannot try defendants in absentia , but its warrants can make international travel difficult. The court has no police force, relying instead on its members to make arrests. An arrested suspect is typically transferred to The Hague to appear before the court.

Aid experts have said the hunger crisis in Gaza is a direct result of the war as well as Israel’s near-complete siege of the territory and its strikes on aid workers.

The Israeli military said it had safely coordinated thousands of aid operations and that it was investigating any “exceptional events that take place during war.”

More generally, Israel has denied placing limits on humanitarian aid entering the territory. It says Israeli officials have done all they can to bring food to the territory and that it is the fault of aid organizations for failing to adequately distribute food after the aid has crossed the border.

Analysts have also cited Israel’s failure to allow an alternative Palestinian leadership to take over in the territory, leading to a power vacuum and the breakdown of law and order, making it even harder to distribute food.

The food situation in Gaza was considered stable before the war began, despite a 16-year blockade on the territory instigated by Israel and Egypt. But food supplies fell sharply in October, when Israel cut off all aid deliveries for the two weeks that followed the Hamas attack. At that time. Mr. Gallant promised a “total siege” on the territory, describing Israel’s attackers as “human animals” and promising “no electricity, no food, no fuel” for Gaza.

Since reopening some aid routes in late October, Israeli officials have still imposed rigorous checks on aid going into Gaza, which is home to around 2.2 million people, and the prospect of famine has been looming for months.

Mr. Khan cited several of these restrictions as justification for issuing arrest warrants for Mr. Netanyahu and Mr. Gallant, saying that they were part of “a widespread and systematic attack against the Palestinian civilian population.”

By focusing on decisions by Israeli politicians, Mr. Khan avoided making detailed accusations about Israel’s military leadership and did not discuss the actions of its Air Force or ground forces.

Reporting was contributed by Gabby Sobelman in Rehovot, Israel; Johnatan Reiss in Tel Aviv; Abu Bakr Bashir in London; Marlise Simons in Paris; and Michael D. Shear in Washington.

Patrick Kingsley is The Times’s Jerusalem bureau chief, leading coverage of Israel, Gaza and the West Bank. More about Patrick Kingsley

Matthew Mpoke Bigg is a London-based reporter on the Live team at The Times, which covers breaking and developing news. More about Matthew Mpoke Bigg

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