to travel on time

Is Time Travel Possible?

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

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

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

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

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

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

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

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

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

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

In Summary:

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

If you liked this, you may like:

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.

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.

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.

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

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

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

Is time travel possible?

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

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

Learn more: Best travel insurance

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

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

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

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

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

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

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

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

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

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April 26, 2023

Is Time Travel Possible?

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

By Sarah Scoles

yuanyuan yan/Getty Images

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Time travel: five ways that we could do it

Cathal O’Connell

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

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

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

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

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

1. Time travel via speed

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

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

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

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

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

Computer solves a major time travel problem

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

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

2. Time travel via gravity

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

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

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

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

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

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

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

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

3. Time travel via suspended animation

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

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

Could humans ever do the same?

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

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

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

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

4. Time travel via wormholes

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

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

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

5. Time travel using light

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

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

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

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

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

Related Reading: Computer solves a major time travel problem

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

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Time travel: Is it possible?

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

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

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

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

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

Related: The best sci-fi time machines ever

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

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

Related: Controversially, physicist argues that time is real

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

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

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

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

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

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

General relativity and GPS time travel

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

Read more: Can we stop time?

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

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

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

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

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

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

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

Can wormholes take us back in time?

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

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

Related: Best time travel movies

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

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

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

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

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

Alternate time travel theories

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

Infinite cylinder theory

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

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

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

Time donuts

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

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

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

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

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

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

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

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

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

Related: Marvel movies in order: chronological & release order

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

Books about time travel:

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

Movies about time travel:

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

Television about time travel:

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

Games about time travel:

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

Additional resources

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

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

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Articles on Time travel

Displaying 1 - 20 of 25 articles.

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

Adi Foord , University of Maryland, Baltimore County

Are black holes time machines? Yes, but there’s a catch

Sam Baron , Australian Catholic University

What are wormholes? An astrophysicist explains these shortcuts through  space-time

Dejan Stojkovic , University at Buffalo

Curious Kids: is it possible to see what is happening in distant solar systems now?

Jacco van Loon , Keele University

Can we time travel? A theoretical physicist provides some answers

Peter Watson , Carleton University

Curious Kids: what would happen if someone moved at twice the speed of light?

Time travel could be possible, but only with parallel timelines

Barak Shoshany , Brock University

Why does gravity pull us down and not up?

Mario Borunda , Oklahoma State University

New warp drive research dashes faster than light travel dreams – but reveals stranger possibilities

Curious Kids: is time travel possible for humans?

Lucy Strang , The University of Melbourne and Jacqueline Bondell , Swinburne University of Technology

Rotating black holes may serve as gentle portals for hyperspace travel

Gaurav Khanna , UMass Dartmouth

The great movie scenes: Back to the Future

Bruce Isaacs , University of Sydney

Time travel is possible – but only if you have an object with infinite mass

Stephen Hawking’s final book suggests time travel may one day be possible – here’s what to make of it

Peter Millington , University of Nottingham

Like a TARDIS in your head, memory helps you travel through time

Alice Mason , The University of Western Australia

Time travel: a conversation between a scientist and a literature professor

Richard Bower , Durham University and Simon John James , Durham University

Star Trek’s version of time travel is more realistic than most sci fi

Lloyd Strickland , Manchester Metropolitan University

Anthill 1: About time

Annabel Bligh , The Conversation and Gemma Ware , The Conversation

How to build a time machine

Steve Humble , Newcastle University

It’s Back to the Future Day today – so what are the next future predictions?

Michael Cowling , CQUniversity Australia ; Hamza Bendemra , Australian National University ; Justin Zobel , The University of Melbourne ; Philip Branch , Swinburne University of Technology ; Robert Merkel , Monash University ; Thas Ampalavanapillai Nirmalathas , The University of Melbourne , and Toby Walsh , Data61

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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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• –––, 2007, “Banana peels and time travel”, Dialectica , 61: 559–72.
• Gödel, Kurt, 1949a [1990a], “An example of a new type of cosmological solutions of Einstein’s field equations of gravitation”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 190–8; originally published in Reviews of Modern Physics , 21 (1949): 447–450.
• –––, 1949b [1990b], “A remark about the relationship between relativity theory and idealistic philosophy”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 202–7; originally published in P. Schilpp (ed.), Albert Einstein: Philosopher-Scientist , La Salle: Open Court, 1949, 555–562.
• Godfrey-Smith, William, 1980, “Travelling in time”, Analysis , 40: 72–3.
• Gorovitz, Samuel, 1964, “Leaving the past alone”, Philosophical Review , 73: 360–71.
• Grey, William, 1999, “Troubles with time travel”, Philosophy , 74: 55–70.
• Hafele, J. C. and Keating, Richard E., 1972a, “Around-the-world atomic clocks: Observed relativistic time gains”, Science , 177: 168–70.
• –––, 1972b, “Around-the-world atomic clocks: Predicted relativistic time gains”, Science , 177: 166–8.
• Hales, Steven D., 2010, “No time travel for presentists”, Logos & Episteme , 1: 353–60.
• Hall, Thomas, 2014, “In Defense of the Compossibility of Presentism and Time Travel”, Logos & Episteme , 2: 141–59.
• Hanley, Richard, 2004, “No end in sight: Causal loops in philosophy, physics and fiction”, Synthese , 141: 123–52.
• Harrison, Jonathan, 1980, “Report on analysis ‘problem’ no. 18”, Analysis , 40: 65–9.
• Hawking, S.W., 1992, “Chronology protection conjecture”, Physical Review D , 46: 603–11.
• Holt, Dennis Charles, 1981, “Time travel: The time discrepancy paradox”, Philosophical Investigations , 4: 1–16.
• Horacek, David, 2005, “Time travel in indeterministic worlds”, Monist (Special Issue on Time Travel), 88: 423–36.
• Horwich, Paul, 1975, “On some alleged paradoxes of time travel”, Journal of Philosophy , 72: 432–44.
• –––, 1987, Asymmetries in Time: Problems in the Philosophy of Science , Cambridge MA: MIT Press.
• Ismael, J., 2003, “Closed causal loops and the bilking argument”, Synthese , 136: 305–20.
• Keller, Simon and Nelson, Michael, 2001, “Presentists should believe in time-travel”, Australasian Journal of Philosophy , 79: 333–45.
• Kiourti, Ira, 2008, “Killing baby Suzy”, Philosophical Studies , 139: 343–52.
• Le Poidevin, Robin, 2003, Travels in Four Dimensions: The Enigmas of Space and Time , Oxford: Oxford University Press.
• –––, 2005, “The Cheshire Cat problem and other spatial obstacles to backwards time travel”, Monist (Special Issue on Time Travel), 88: 336–52.
• Lewis, David, 1976, “The paradoxes of time travel”, American Philosophical Quarterly , 13: 145–52.
• Loss, Roberto, 2015, “How to Change the Past in One-Dimensional Time”, Pacific Philosophical Quarterly , 96: 1–11.
• Luminet, Jean-Pierre, 2011, “Time, topology, and the twin paradox”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press. doi:10.1093/oxfordhb/9780199298204.003.0018
• Markosian, Ned, 2004, “Two arguments from Sider’s Four-Dimensionalism ”, Philosophy and Phenomenological Research , 68: 665–73.
• Markosian, Ned, 2020, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio , 12: 137–65.
• Maudlin, Tim, 2012, Philosophy of Physics: Space and Time , Princeton: Princeton University Press.
• Meiland, Jack W., 1974, “A two-dimensional passage model of time for time travel”, Philosophical Studies , 26: 153–73.
• Mellor, D.H., 1998, Real Time II , London: Routledge.
• Meyer, Ulrich, 2012, “Explaining causal loops”, Analysis , 72: 259–64.
• Miller, Kristie, 2005, “Time travel and the open future”, Disputatio , 1: 223–32.
• –––, 2006, “Travelling in time: How to wholly exist in two places at the same time”, Canadian Journal of Philosophy , 36: 309–34.
• –––, 2008, “Backwards causation, time, and the open future”, Metaphysica , 9: 173–91.
• Monton, Bradley, 2003, “Presentists can believe in closed timelike curves”, Analysis , 63: 199–202.
• –––, 2009, “Time travel without causal loops”, Philosophical Quarterly , 59: 54–67.
• Nerlich, Graham, 1981, “Can time be finite?”, Pacific Philosophical Quarterly , 62: 227–39.
• Ney, S.E., 2000, “Are grandfathers an endangered species?”, Journal of Philosophical Research , 25: 311–21.
• Price, Huw, 1996, Time’s Arrow & Archimedes’ Point: New Directions for the Physics of Time , New York: Oxford University Press.
• Putnam, Hilary, 1975, “It ain’t necessarily so”, in Mathematics, Matter and Method , Cambridge: Cambridge University Press, vol. 1 of Philosophical Papers , 237–49.
• Reinganum, Marc R., 1986, “Is time travel impossible? A financial proof”, Journal of Portfolio Management , 13: 10–2.
• Riggs, Peter J., 1991, “A critique of Mellor’s argument against ‘backwards’ causation”, British Journal for the Philosophy of Science , 42: 75–86.
• –––, 1997, “The principal paradox of time travel”, Ratio , 10: 48–64.
• Savitt, Steven, 1994, “The replacement of time”, Australasian Journal of Philosophy , 74: 463–73.
• –––, 2005, “Time travel and becoming”, Monist (Special Issue on Time Travel), 88: 413–22.
• Sider, Theodore, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Clarendon Press.
• –––, 2002, “Time travel, coincidences and counterfactuals”, Philosophical Studies , 110: 115–38.
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• Simon, Jonathan, 2005, “Is time travel a problem for the three-dimensionalist?”, Monist (Special Issue on Time Travel), 88: 353–61.
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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

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

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

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What is TravelTime?

Create a Travel Time Map

Travel time map generator & isochrones, i know i can get from a to b by public transport within my selected time, but it's not showing up.

• Walking to the station platform
• Waiting for the next available departure
• Time spent boarding the train
• Giving enough time to take the A to B journey
• Depart on the station on the other side.

You can't drive that far / you can drive much further than that"

• Open another mapping app of your choice and enter an A to B route
• Select a departure time for tomorrow.

Still not convinced?

About this tool, what is a travel time map, how to create a drive time radius map or other modes.

• Select a start location
• Select a maximum travel time limit
• Select a mode of transport, for example driving
• Voila! There's your driving radius map

Use cases for consumers

• Create a commute time map so you can see where to live based on commute time.
• How far can i travel in a given time: compare transport coverage for different areas.
• Create a drive time radius map: explore how far you can travel on a road trip.

Use cases for businesses

• Travel time mapping up to 4 hours & cross reference other data sets in GIS such as population data
• Site selection analysis: analyse the best location to locate a business by adding thousands of analysis points
• Create a distance matrix or travel time matrix & calculate travel times from thousands of origins to thousands of destinations
• Network analysis / travelling salesman problem: use spatial analytics to solve routing problems
• Commute time map - plot thousands of employee commute times for an office relocation
• Create up to 3 time polygons visualising where's reachable within 2 hours or less. Our API can create large travel time areas, talk to sales.
• Calculate travel times from an origin to various points of interest - in this demo we use points from Foursquare Give A to B routing details

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TravelTime Features

• See 'How far can I get' in X minutes
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• Overlap many shapes & highlight overlap area
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Driving Time Calculator

Driving time between two cities.

Travelmath helps you find the driving time based on actual directions for your road trip. You can find out how long it will take to drive between any two cities, airports, states, countries, or zip codes. This can also help you plan the best route to travel to your destination. Compare the results with the flight time calculator to see how much longer it might take to drive the distance instead of flying. You can also print out pages with a travel map.

You may want to search for the driving distance instead. Or if you're thinking about flying, make sure you compare flight times between airports. For a long road trip, check the cost calculator to see if it's within your budget.

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Travel Time

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

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

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

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

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

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

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

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

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

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

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

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

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Fjords, Pharaohs or Koalas? Time to Plan for Your Next Eclipse.

If you can’t get enough of totality, or missed out this time, you’ll have three more chances in the next four years in destinations like Iceland, Spain, Egypt and Australia.

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By Danielle Dowling

Are you still a little giddy from the magical moments of totality during Monday’s solar eclipse? Or did clouds swoop in to block your view? Maybe you just couldn’t make it to the path of totality this time. No matter what, the question now is “ Where and when will it happen again?”

“People who have never seen it before, the first words out of their mouth after the totality ends is ‘I’ve got to see another one, this is incredible, this is unbelievable.’ That is when you become addicted to these things and end up traveling no matter where the next one is,” said Joseph Rao, an eclipse chaser and guest lecturer at the Hayden Planetarium.

So, if like Mr. Rao, you’ve developed a raging case of umbraphilia — the love of eclipses — you’ll have three chances over the next four years to see the moon blot out the sun. The first, on Aug. 12, 2026, will start above Greenland, then strafe the west coast of Iceland and move along the Atlantic Ocean and over Spain. Almost a year later, on Aug. 2, 2027, another will skirt the Mediterranean coast of North Africa then cross Egypt and part of the Arabian Peninsula. The third, on July 22, 2028, will cut across Australia and the southern tip of New Zealand.

Future Eclipses

Eclipse chasers will have several more chances this decade to view a total solar eclipse .

Last week, as Victoria Sahami , the owner of Sirius Travel , was preparing to guide a group of tourists in Mazatlán, Mexico, for Monday’s big event, she was also planning for these other upcoming eclipses. Ms. Sahami joined the ranks of the eclipse-obsessed when she witnessed one in Venezuela in the 1990s. “Like many people, I was hooked. There was no going back,” she said.

Total solar eclipses happen fairly regularly — about every one to two years — in locations scattered around the world. “That’s the great thing about them: You wind up in places that you don’t normally go,” Ms. Sahami said.

A major spoiler is weather, which will be a big variable in the 2026 eclipse — one Greenland, Iceland and Spain will see.

“Iceland normally has a lot of cloud during that time of year,” said Paul Maley , who runs Ring of Fire Expeditions . “The data shows Spain to have the higher good-weather prospects of all three. However, the sun is low in the sky and the eclipse ends as the sun hits the horizon at sunset.”

Because of Iceland’s mercurial meteorology, Ring of Fire Expeditions is going all in on Spain, with a 10-day excursion on the mainland. Sirius Travel is offering not only a five-day trip to Majorca but also an eight-day tour around Iceland. It will be based in Reykjavik, and the itinerary will remain flexible on the day of the eclipse so the tour can easily pivot toward the location with the least cloud cover. Ms. Sahami recommends the trip for those who already have a few eclipses under their belt and would be happy just to take in the sights of Iceland if the weather doesn’t cooperate.

The 2027 eclipse, on the other hand, promises to be truly stellar: Luxor, Egypt — the site of numerous ancient temples as well as the Valleys of the Kings and Queens — sits right in the middle of the path of totality and will be bathed in darkness for a full 6 minutes 23 seconds. Weather-wise, it is what Ms. Sahami called “a slam dunk.” “You know you’re going to see it. You know that you’re not going to get any clouds,” she said.

But for all its potential, those considering Egypt should be aware that the State Department has a Level 3 “Reconsider Travel” warning for the country because of the risk of terrorism.

The 2028 eclipse will darken the skies over Sydney, Australia, for 3 minutes 49 seconds. It will be the first time the city has experienced a total solar eclipse since 1857. Ms. Sahami has her eyes on a trip based out of there, while Mr. Maley has chartered a cruise ship off the northwest coast of Australia. It will be winter there, he said, but that isn’t likely to mean bad eclipse-viewing weather.

If you want to see any (or all) of these eclipses, you should get started on planning and booking now, particularly if you want to sign up for a trip organized by a tour company. One of Sirius Travel’s excursions to Luxor is already full.

Scrutinize refund policies and look into insuring your trip. Several companies will fully refund your deposit if you cancel a year in advance. A lot can happen, Ms. Sahami said, “but if you think you’re going to go, why not?”

Follow New York Times Travel on Instagram and sign up for our weekly Travel Dispatch newsletter to get expert tips on traveling smarter and inspiration for your next vacation. Dreaming up a future getaway or just armchair traveling? Check out our 52 Places to Go in 2024 .

Traveling Amtrak For The First Time? Here Are 14 Things To Know About Amtrak Trains

F eel the need to go outside New York City? Confused about which ride to select? Forget going to out-of-the-way airports or using crowded taxi services. The Amtrak train offers the ultimate solution to meet everyone's travel needs. Meet the luxurious train that makes passengers feel like they're at home. The endless amenities and friendly, respectful staff also make one's trip memorable.

Many Amtrak trains successfully run across hundreds of tracks in the United States (most Amtrak trains offer incredibly beautiful routes , too), and there's also a rich history behind the company. Multiple packages are available for every type of passenger, depending on budget, accommodation choice, and other factors. For a pleasant change, the Amtrak company charges no extra fees, i.e., passengers get what they see, unlike some airliners who keep adding extra surcharges to a customer's expense list till the last moment.

Related: Empire Builder: What Makes This Amtrak Route One Of The Most Scenic In The U.S.

Furthermore, passengers get extra legroom for a long journey compared to a car or air travel. The Amtrak also saves on fuel and protects one's personal vehicle from wear and tear if one uses it to travel. So, with that in mind, let's skim through some important things travelers should know about the trains, such as Amtrak train routes and prices, etc.

UPDATE: 2023/06/06 21:47 EST BY REENA JAIN

More Things To Know About Amtrak Trains For First Time Travelers

Everyone should be aware of a few things before embarking on their first Amtrak journey, regardless of the destination or length of the journey. This list has been updated with additional important information, including train routes, prices, schedules, destinations, and more to help travelers have a stress-free, relaxing, and enjoyable Amtrak journey.

The Amtrak App

The Amtrak app makes it simple for travelers to plan and reserve their trips, check Amtrak schedules and destinations, and receive real-time alerts for delays or changes. Users can access their e-tickets through the app, which provides information on Amtrak train routes and prices. Additionally, the app includes interactive maps that help passengers track the whereabouts of the train and get knowledge of the schedule. The Amtrak app has a user-friendly interface and convenient features, making train travel more simple, reliable, and enjoyable.

Expect Train Delays

Amtrak trains need to stop at some places to let large freight trains pass because they share the same tracks. It can result in unforeseen delays in travel times, especially on longer, cross-country routes. For instance, the California Zephyr from Chicago to Denver may need to stop before the Moffat Tunnel to allow a freight train to pass, delaying Amtrak schedules. Therefore, researching Amtrak schedules and destinations is beneficial before making travel plans.

Making A Pre-Departure Checklist Is Helpful

Passengers will require a number of items, including those necessary and desirable for a stress-free and memorable Amtrak trip. Some of these—like an ID—are necessary, while others are worthwhile for an enjoyable Amtrak journey.

Identification

Passengers must have a photo ID or passport with them.

Train Tickets

Passengers should have hard copies of their tickets or electronic copies of their tickets on hand.

Travel insurance

It's essential to protect oneself from unanticipated travel mishaps.

Passengers who will need to take any medication while traveling must bring it with them.

Other Essentials

Personal care items, snacks, beverages, books, and some forms of entertainment are desirable (but not necessary) for a comfortable trip, particularly on long Amtrak routes.

Stations Have Different Stopover Times

The train makes scheduled stops at various stations during a trip, and the stopover time varies depending on the type of station. If a station is a rest stop, the stopover time is comparatively longer than that of a passenger drop-off and pick-up location. Knowing if a specific stop is a designated rest stop where passengers are permitted to disembark is preferable for those who want to smoke because smoking is not permitted on the train or simply want to move around the station.

Reasonable Ticket Prices

Amtrak tickets are reasonably priced! The average price of a ticket from New York to Atlantic City is about $94 (although Amtrak's famous roomettes typically cost much more ). It is pretty budget-friendly compared to any airline fare on the same route, and it is not even the lowest price out there. The cheapest tickets on this Amtrak route can even be found for only $82.

The best thing is to avoid rush hour and book your Amtrak tickets well in advance. Plus, a cool tip: Amtrak offers saver fares on each route, where passengers can save up to 20% compared to the standard Coach fare when booked at least 7 days in advance. Furthermore, Amtrak fares for seniors (aged 65 and up) are 10% less than the standard fare on most trains in the United States, and seniors (aged 60 and up) pay 10% less for cross-border services operated by Amtrak and VIA Rail Canada.

Plenty Dining Choices

Amtrak offers many dining choices to its customers. There is something tasty and appetizing for every passenger, from adults to kids. Some of these main options include:

Passengers can access the café, which is open to passengers from all classes of the train from the early morning until late at night. Whether business or economy class, there are snacks, drinks, and food items for everyone.

• Traditional Dining

Traditional dining service for passengers in private rooms is provided in the dining car as a complimentary exclusive offering.

• Flexible Dining

Exclusive offer to First Class passengers to eat at flexible timings. Special diet menus are also available at Amtrak for customers with specific dietary requirements. The food menu comes with a detailed calorie breakdown as well.

A Variety Of Accommodations

The Amtrak from New York to Atlantic City offers various services and accommodations. Among the expert tips for first-time Amtrak passengers is to check the various room options available in order to best fit the needs and budget of a passenger.

• Seating Options

Even the Coach class has the best comfy seats. Imagine reclining sofa-like seats with spacious areas. The addition of multiple amenities combines to make the trip a relaxing one! For passengers with mobility impairments, Amtrak provides accessible seating arrangements.

• Private Rooms

Amtrak provides a unique luxurious experience to its passengers by offering private rooms to First Class. If one wants privacy and their own space on a short trip with added comfort, then a private room booking is the perfect step.

Related: California Zephyr Vs. Southwest Chief: Which Amtrak Train Route Is More Scenic?

Wi-Fi On Board

The Amtrak train comes with complimentary Wi-Fi during traveling. The speed of Amtrak Wi-Fi is less than a passenger’s home or work Wi-Fi network. Furthermore, the same Wi-Fi gets shared by all the passengers on the Amtrak from New York to Atlantic City. So, each passenger should try to surf the internet in such a limited way as not to overburden the network for the other onboard users.

The key is not downloading large files or streaming heavy videos online and keeping fellow passengers in mind while using the shared Wi-Fi. That is why limited access is provided by Amtrak so that more onboard bandwidth is available to all passengers. Access to websites with objectionable content is also restricted in this shared onboard Wi-Fi.

Onboard Upgrades

The trains are divided into classes: Coach, Business, and First Class. All these classes differ from each other, with a different set of rates and amenities for each of them. So, if space is available as unoccupied in a class, one can upgrade from one's original class to that higher class seating. It can be from Coach to Business or Business to First Class.

This smart feature can change the outlook of one's whole journey. Passengers only need to speak to the conductor onboard. They might inform about the availability of such an upgrade and let passengers purchase a new class seat.

Passengers Can Bring A Bicycle Along

Amtrak offers its customers the opportunity to bring their bicycles along. One can explore the stops along the way during a train journey - and what better way to do that than by bike? Ride the rails on board the train with a bike - sounds awesome! Amtrak offers several different services to transport a customer’s bike onboard.

Remember, though, that each train has different equipment and loading procedures that decide what service will be offered. The starting and ending destination stations also determine how or what is allowed by Amtrak regarding one's bicycle. For carry-on/train side, bicycles up to 50 lbs are allowed. Here, standard bicycle sizes apply with a maximum tire width of 2 inches.

Enjoy Guest Rewards

Amtrak offers a guest rewards system to its customers . The points earned can be used to travel on trains, stay at hotels, and shop.

• Earning Points

Customers can earn guest rewards by earning points through various means, both on and off the train. Passengers can earn 2 points for every $1 spent.

• Redeeming the Points

Customer of Amtrak can easily redeem their earned points. The reward travel begins at just 800 points, with train travel allowed to over 500 destinations.

• Member Benefits

Becoming an Amtrak member rewards customers for every ride on and off the train.

• Bonus Points

One can earn 20,000 bonus points with the new Amtrak Guest Rewards® Preferred Mastercard®. Customers can even choose from over 350,000 hotels worldwide to stay at using their collected points.

Passengers Can Bring Pets

Pets are allowed to travel on Amtrak. However, like with air travel, there are restrictions. Dogs and cats can come on trips that can take up to seven hours on most Amtrak routes in a pet carrier - of course, for a small fee. The pet carrier must also be spacious and highly ventilated, allowing for free movement of the animal, and the weight of the pet and carrier combined must be under 20 lbs.

Things to Note About Pets on Amtrak Trains

• Service animals are readily welcomed on all Amtrak services as they are not included in the pet category.
• Pets are not allowed to travel as baggage. A pet must travel with its human counterpart on a train.
• Only dogs and cats are allowed to travel as pets on Amtrak, and pets must be at least eight weeks old and up to date on all vaccinations.
• Amtrak highly recommends making pet reservations in advance of one's trip.

Related: Roomette Vs. Sleeper Bedroom: Knowing The Difference On An Amtrak

Baggage Limits

With Amtrak, customers can shed their baggage worries as the company handles everything meticulously. As baggage, a customer may bring 2 carry-on items. Here, limitations apply regarding weight and size. So, browse them beforehand. Checking this guide on what to pack for an Amtrak train ride may also be handy in terms of prioritizing what to bring (and what not to bring to save baggage weight and space).

Similarly, for checked luggage, each traveler can bring 4 bags, 2 are registered as free, and the other 2 are at $20 per bag. Again, size/weight limitations apply. For special items of baggage that require special handling or are outside the normal baggage category, Amtrak may ask for additional packing requirements and service fees. Amtrak also prohibits certain items onboard trains for security purposes.

Going with unreserved seating on a packed train? No worries. Travelers can find a Red Cap (baggage porter) to escort them to the train. They can access the track before the masses and carry their bags. Red Cap services are free, but a generous tip is recommended.

No Security Line No Waiting

Amtrak lets passengers simply go to the track and get on the train. There is no security line blockage as there is at the airport. With a reduction in airport TSA staff and more passengers flying, travelers need to get to the airport about two hours before their flight just to have enough time to check in or check a bag and clear security.

With Amtrak travel, one's ticket might be scanned before entering the platform or scanned onboard, but there is no waiting line. On the other hand, Amtrak recommends arriving 30 minutes before one's train departs.

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Being back in the beach house that witnessed much of my 20s feels strange and wondrous – like a sort of time travel

I run from room to room, touching things as if they’ll somehow transport me to the past. Not much has changed in the old weatherboard

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M any years ago, a friend from university invited some of us to his mum’s beach house at Walkerville South. His mum had bought the house super cheap before the world had discovered that there was another impressive coastline in Victoria, far away from the more established houses of the Mornington Peninsula or the Great Ocean Road.

The house was a weatherboard shack hidden in thick native bush. There were two bedrooms, and a large corner couch in the lounge that doubled as two extra beds when needed. Fronted by large windows, you could spy the ocean through the tall trees while standing in the kitchen and waiting for the kettle to boil. It was a house that didn’t need too much attention. From the straw matting on the floor to the green bathroom straight out of the 1970s, it was immediately welcoming, and once you arrived, you didn’t want to leave.

On that first visit, I slept on one of the couch-beds, preferring to keep the curtains open so I could see the darkness of the sky. And in the morning, I woke to a row of noisy rosellas hanging out on the edge of the deck, waiting for birdseed. We swam even in the height of winter, running into the cold, foamy water and lasting only minutes before tiptoeing with bare feet back up the hill to the waiting fire. We drank too much cheap red wine, ate simple meals of beans and rice, and laughed late into the night. It was one of those bonding weekends that was so joyful, it was repeated more than once.

After we left university and scattered in different directions, I still borrowed the house from time to time, introducing it to other friends, including the man who would one day become my partner. After he and I started going out, we visited just the two of us, and I remember mocking him for pulling on a wetsuit before wading into the sea. The house witnessed much of my 20s, those lost years when none of us knew what we were going to do with the rest of our lives, and coming together somehow made us feel safe.

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And then my university friend married his partner and moved to another state, and we lost contact. I stopped visiting the house because he wasn’t around to lend me the keys. But years later, each time I read Alison Lester’s wonderful Magic Beach to my children, I would be transported back to the wilds of Walkerville South, a place that had become almost mythical in my memory.

Three years ago, as the pandemic restrictions lifted, friends invited the kids and I to visit them on their summer holiday. I’d been to the Gippsland coast often as an adult and knew the roads well, but I hadn’t stayed at Walkerville South since that time. I was surprised to see how little had changed. A gravel road still led the way in, and the hill behind the beach was still dotted with only a handful of houses.

We pulled up outside the place my friends had rented and started unloading the car. As we walked in, I felt a prickle of familiarity. There was a corner couch that doubled as a bed in the lounge, large sliding glass doors out to the deck, and a green kitchen straight out of the 1970s. But it was the coloured Marimekko curtains that hung to the floor, more faded than when I’d last seen them, that did it. I knew immediately that it was the same house.

Excited, I asked who owned the house and my friend told me. I grinned when I heard that my old university friend’s mum still owned it. They didn’t know her well, but she was a friend of a friend and she still rented out the place to people sometimes. Now in her 90s, she didn’t visit it herself very often any more.

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I ran from room to room checking the fittings to see if they were the same, touching things as if they’d somehow transport me back to the past. I gripped the same green ball handles on the bathroom door. Ran my fingers along the same boxed-up board games stacked in the shelves in the lounge. And bent down to rub the fur back of the large grey, stuffed wombat that sat waiting near the fireplace, a little more loved looking than it had been all those years before.

Not much had changed in the old weatherboard. It held such stories in its walls. And now I was back, and it felt strange and wondrous like a sort of time travel. I stood on the deck, knowing the rosellas would soon land, and remembered a time when I was younger, freer, less worried about what was coming. When sleeping on a couch in the corner of a room was fought over, and when swimming in the winter sea was a given.

Nova Weetman is an award-winning children’s author. Her adult memoir, Love, Death & Other Scenes , is out in April 2024 from UQP

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Breaking news, work starts on america’s first high-speed train between la, las vegas that could cut travel time in half.

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The much-anticipated construction of the first high-speed passenger train between Los Angeles and Las Vegas got underway Monday — promising to ferry passengers between the two cities in half the the time it now takes to drive through the desert.

The $12 billion project is being spearheaded by Brightline West, which will lay 218 miles of track between a new terminal in Rancho Cucamonga, Calif., and another station just south of the Las Vegas Strip by 2028, according to Fortune .

In a statement, Brightline Holdings founder and chair Wes Edens said that breaking ground on the rail on Monday — thanks to $6.5 billion in backing from Joe Biden’s administration, plus $2.5 billion in tax-exempt bonds — was laying “the foundation for a new industry.”

Brightline also won federal authorization in 2020 to sell $1 billion in similar bonds, Fortune reported.

US Transportation Secretary Pete Buttigieg will reportedly participate in the rail’s Monday groundbreaking.

Upon its completion, Brightline West is expecting to welcome millions of passengers aboard the high-speed train, which will travel up to 200 miles per hour between the two major US cities in just over two hours — about half the time it takes to drive between LA and Vegas.

The high speeds are comparable to Japan’s Shinkansen bullet trains, whose network includes nearly 2,000 miles of lines on the country’s four major islands.

Brightline’s estimates are that 11 million passengers will take the electric-powered train one way per year, meaning there will be 30,000 travelers per day. The trains will also have restrooms, Wi-Fi, and food and beverage.

There will also be the option to check luggage — with ticket fares with or without large bags anticipated to be well below the cost of flying across the Mojave Desert.

It wasn’t immedately clear what a ticket aboard Brightline West will cost, though airline costs range between $80 and $230 depending on the time of the year.

Passenger traffic at Sin City’s Harry Reid International Airport set a record of 57.6 million people in 2023, Fortune reported.

Meanwhile, an average of more than 44,000 automobiles per day crossed the California-Nevada state line on Interstate 15 in 2023, according to Las Vegas Convention and Visitors Authority data — perhaps because of the Super Bowl and Formula 1 Grand Prix race that both took place in Las Vegas last year.

Las Vegas — which has nearly 3 million residents — draws more than 40 million visitors per year.

Brightline CEO Mike Reininger has said the goal is to have high-speed trains operating in time for the Summer Olympics in Los Angeles in 2028.

Almost the full distance of the forthcoming train will be along the median of I-15, according to Fortune, which begins in San Diego and runs through Los Angeles before passing through Nevada and into parts of Arizona, Utah, Idaho and Montana.

At this point, only one station stop along the route will take place in San Bernardino County’s Victorville area, per Fortune.

At a later date, Brightline West has said it may add more stops in Palmdale, Calif., and other US metros that are too close to fly between but too far to drive to, though details have yet to be finalized.

The goal is to relieve congestion on I-15, where motorists often get caught in slow-moving traffic.

Representatives for Brightline West did not immediately respond to The Post’s request for comment.

Florida-based Brightline already operates a Miami-to-Orlando high-speed line, where trains reach speeds of up to 125 miles per hour.

The service launched in 2018 and expanded to include service to the Orlando International Airport in September 2022, Fortune reported. It now offers 16 round trips daily, with one-way tickets to travel the 235-mile-long track going for about $80.

Another fast train in the US include Amtrack’s Acels, which can reach 150 miles per hour while transporting passengers between Boston and Washington, DC — but rails aren’t considered “high speed” unless they top at least 160 miles per hour.

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Dear Abby: Stop vacationing with a sibling who spends her time bad mouthing you!

• Published: Apr. 23, 2024, 4:00 a.m.

Dear Abby is written by Abigail Van Buren, also known as Jeanne Phillips.

• Abigail Van Buren

DEAR ABBY: A few years ago, my sister and I took our kids on vacation together. She and I get along but have never been close. Sometimes I walk on eggshells around her because I never know what might make her upset.

I thought we were having a good time on that vacation and getting along well, even though I was anxious. We were both taking pictures with our phones, and she handed me hers to forward myself some of the pictures. While I was looking at them, a text message pinged, and I checked it without thinking about it not being my phone.

It turns out my sister had spent the entire vacation texting about my “B.S.,” my inability to do anything competently and even referenced something that happened years before. I had noticed her constant texting but said nothing because I didn’t want to risk an argument.

I am still angry about this and don’t know how to let it go. I really don’t want to travel with her again, since I know the invisible third party will be with us the whole time. How do I deal with this? -- STAYING PUT IN NEW JERSEY

DEAR STAYING PUT: When you saw that message from your sister’s text partner you should have handed her the phone and started packing. You are not obligated to travel with someone who causes you to walk on eggshells, ignores you, puts you down and says nasty things behind your back. Don’t be mean about taking steps to protect yourself. Simply stop vacationing with her. (I know I sure would!)

Dear Abby is written by Abigail Van Buren, also known as Jeanne Phillips, and was founded by her mother, Pauline Phillips. Contact Dear Abby at www.DearAbby.com or P.O. Box 69440, Los Angeles, CA 90069.

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