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Justin Smith-Ruiu

A New Time-Travel App, Reviewed

We all know by now that the time-reversal invariance governing statistical mechanics at the microlevel maps by a simple equation onto the macroworld, making “time travel” a wholly unsurprising possibility … but damn! The first time you go back there’s just nothing like it.

I know all these first-person accounts of ChronoSwooping have become a cliché here on Substack, where, let’s face it, anyone can write pretty much whatever they want no matter how self-indulgent and derivative. Nonetheless I think I have some unusual insights to share, which derive from my own experience but which may offer some general lessons as to the nature and significance of time travel, both the original and long-prohibited “body-transit” method as well as the newer and more streamlined ChronoSwoop.

This is not only because I spent some years in the archives of the Stadzbybliotiēka of the Margravate of East K****, poring over the notebooks in which Quast first landed on the Quast equation, while in parallel jotting down sundry philosophical reflexions about the nature of Divine Tempus—as he called it—that have largely been neglected by other researchers. It is also because I have used the ChronoSwoop app in ways that are expressly prohibited by its makers, and indeed by the federal government. In light of this, while I am writing this product review for Substack and in the emerging “Substack style,” until the law changes or I depart permanently from the chronological present, I will be posting this piece only on the Hinternet-based Substack oglinda (Romanian for “looking-glass,” a hacking neologism supposedly coined by Guccifer 3.0), which I’m told is undetectable, remaining entirely unknown even to the original company’s founders. Fingers crossed.

Perhaps some readers on this oglinda will appreciate a brief summary of what’s been happening in the world of time travel since Quast first came up with his equation in 1962. I don’t know what sort of information has been circulating down here, and I don’t want anyone to feel left behind.

The early 1960s witnessed great leaps forward not just in time-travel technology, but in the technology of teletransportation as well—which is to say dematerialization of the body, and its rematerialization elsewhere, but without any measurable “metachrony.” By late 1966 poorly regulated teletransporters had begun to pop up on the state fair circuit, tempting daredevils into ever more foolish stunts. But this practice was curtailed already the following year, when, expecting to reappear kneeling before his sweetheart Deb at the stables with a ring in his hand, Roy Bouwsma, aka “the Omaha Kid,” got rematerialized instead with the stable door cutting directly through the center of his body from groin to skull—one half of him flopping down at Deb’s feet, the other half falling, like some neat bodily cross section carefully made for students of anatomy, into the stable with Deb’s confused horse Clem.

But while this atrocious moment, broadcast live on KMTV, nipped the new craze in the bud, the technology underlying it had already been adapted for use in what was then called “Tempus-Gliding,” which had the merely apparent advantage of concealing from those in the present any potential accident in the rematerialization of the voyager to the past. Of course, accidents continued to happen, and news of them eventually made its way back from past to present, bringing about all sorts of familiar paradoxes in the spacetime continuum. Tempus-Gliding, like any metachronic technology relying on body-transit, was a door thrown wide open to all the crazy scenarios we know from the time-travel tropes in science fiction going back at least to H. G. Wells: adults returning to the past and meeting themselves as children, meeting their parents before they were even born, causing themselves never to have been born and so suddenly to vanish, and so on. By the end of the 1960s people, and sometimes entire families, entire lineages, were vanishing as a daily occurrence (just recall the 1969 Harris family reunion in Provo!). You could almost never say exactly why, since the traveler to the past who would unwittingly wipe out all his descendants often had yet, in the present, ever to even try Tempus-Gliding.

Matt Burgess

Andrew Couts

Emily Mullin

C. Brandon Ogbunu

A campaign to end the practice quickly gained speed. By 1973 the “Don’t Mess With Spacetime” bumper stickers were everywhere, and by the following year Tempus-Gliding was outlawed—which is to say, as is always the case in such matters, that only outlaws continued to Tempus-Glide. Scattered disappearances continued, public outcry against illicit Tempus-Gliding became more widespread. In 1983 Nancy Reagan made an unforgettable guest appearance on Diff’rent Strokes to help get out the message about the dangers of illegal body-transit. (“More than 40,000 young lives are lost each year to illegal Metachron gangs.” “What you talkin’ ’bout Mrs. Reagan?”) By the late 1980s a combination of tough-on-crime measures and transformations in youth culture largely ended the practice, and time travel would likely have remained as dormant as moon-travel if it had not in the last decade been so smoothly integrated into our new mobile technologies, and in a way that overcomes the paradoxes and inconveniences of Tempus-Gliding. It does so, namely, by taking the body out of the trip altogether.

This is the mode of time travel, of course, that has shaped a significant subcurrent of science fiction scenarios, notably Chris Marker’s La Jetée (1962), later adapted into the better known Bruce Willis vehicle 12 Monkeys (1995). While these films might seem exceptional, they also share something important with the great majority of what may be called time-travel tales avant la lettre, in which, typically, a man such as Rip Van Winkle goes to sleep for a very long time and wakes up in “the future.” The “zero form” of time travel, we are reminded, is simply to live, which is to say to travel forward in time at a slow and steady rate that only appears to be sped up or “warped” through deep sleep.

Be that as it may, when the new app-based time-travel technologies began to emerge in the late 2010s—relying as they did on a loophole in the 1974 law against time travel that defined it strictly as “metachronic body-transit”—they were all confronted by the hard limit on innovation already predicted by Quast, who remained committed until the end to the impossibility in principle of future-directed time-travel. “If you want to get to the future, you’re just going to have to wait,” Quast wrote in an entry in his Hefte dated 6 October, 1959 (SB-1omk 21.237). “To live in time is already to travel in time. So be patient” [ In der Zeit zu leben, das ist schon in der Zeit zu reisen. Hab also Geduld ]. Rumors of future-transit apps downloadable from ultra-sketchy oglindas have been circulating for years, but I’ve never seen any, and having studied Quast’s work I have come to believe that they are a theoretical impossibility.

The earliest apps, popping up mostly from anonymous sources, were mostly perceived as too dangerous and illicit to gain widespread appeal. “We’ve got that legal cannabis here in California now,” Whoopi Goldberg said on an episode of The View in September 2019. “If I want to take a little trip, I’m sorry but there’s edibles for that. I’m not messing with spacetime [ audience laughter ].” In an echo of the panic leading to the prohibition of Tempus-Gliding in the early 1970s, the government began to issue PSAs sensitizing the public to the serious psychological trauma that a return to our own pasts can trigger. “This is not lighthearted fun,” the messaging went. “Metachronism can ruin your life.”

The campaign against these new technologies would probably have killed them, or at least pushed them so far down into the oglindas as to occlude them from the public’s consciousness, if in 2021, at the worst moment of the pandemic, the ChronoSwoop company had not appeared as if out of nowhere and dropped its addictive new app with its signature “Swoop left/Swoop right” functions. Key to ChronoSwoop’s success was the discovery that users will draw significantly more pleasure from being cast into random moments in the past (Swoop left) than from being permitted to choose particular moments they have deemed significant in the post-hoc construction of their autobiographical self-narrative. And if you find yourself thrown back into an unpleasant or dull moment, then a single swift Swoop right will bring you immediately back into the present. You can of course go into your settings and laboriously reconfigure the app to permit you to choose your precise dates, but the great miracle of ChronoSwoop’s success is that almost no one bothers to do this. The people want their time travel to come with streamlined, easy interfaces. They want to move through the past like they move through their feeds: going nowhere in particular, with no clear purpose.

Quast had remained agnostic as to the possibility of body-less time travel, though he always insisted that, if it turns out to be possible, this will amount to an empirical proof of body-soul dualism. If the “self” can easily be inserted into the body it possessed at an earlier stage of life, while retaining all the memories of experiences from after that stage, this means, he believed, that the memories, as well as consciousness itself, cannot be dependent on the physical substrate of the brain that supposedly hosts them. When people first started ChronoSwooping, there were rumors of “headaches,” which were supposed to have resulted from the transit back in time of the more fully developed neurological structure of the time traveler—essentially cramming, say, a 38-year-old’s brain into the cranium of his 10-year-old past self. But of course no such thing occurs, for what travels back, as Quast predicted, is the immaterial self alone, and the fact that this is possible does indeed demonstrate, whether the scientific establishment is ready to admit it or not, that we do not need to remain anchored to any parcel of matter at all in order to exist as conscious beings.

ChronoSwoop beat out its early competitors (remember TimeDig? 😂) not only by getting rid of the date-choosing option, but also by adding sensorimotor control to the package. The earliest apps only planted your consciousness into the body of your past self and permitted you to “ride along,” to see and feel everything your former self saw and felt, but not to exercise any control over any of this. Quast predicted that only such passive riding would ever be possible, in part because any will-driven intervention in the sequence of past events, such as ChronoSwooping now makes possible, seems to generate at least as many paradoxes for the spacetime continuum as old-fashioned body-transit.

It’s not clear how ChronoSwoop managed to pull it off, but we can at least affirm what the emerging scientific consensus says about this new option, namely that it demonstrates the truth of the so-called “Many Worlds” interpretation of quantum mechanics, where each new timeline created by a different course of action initiated by a time traveler through the vehicle of that traveler’s own former self simply places that self on a different timeline of a different world, of which there are in any case infinitely many. These worlds are all self-contained and non-interacting, unless you can call ChronoSwooping itself a form of interaction, so that, however strange it all is, we at least avoid the more awkward conundra of body-transit, as when, for the millionth time, some idiot gets it into his head to “kill baby Hitler,” which of course means that more or less everyone in the world from roughly 1933 on, being affected by different events of the world, also ends up having sex at different times, different spermatozoa end up fecundating different eggs, and virtually all of us children of the 20th century disappear, until someone else arranges to kill the idiot who killed baby Hitler and set us back on our course again.

As an early adopter, I first ChronoSwooped in November 2021. The particular experience might seem unremarkable when I describe it, but for me, beyond being an occasion to see my deceased father again, it was my initiation into a world from which I have not really returned. I ended up, at random, back in December 2003. It’s Christmastime, and I’m visiting with my dad in Little Rock, where, I quickly recall, he has recently relocated after some career difficulties in the wake of the dotcom crash. “Have you seen this guy called Crazy Frog?” he’s asking me, as we stand in front of his desktop. “He’s kind of dumb but he makes me laugh.” I look at the animated amphibian with the aviator glasses, singing his ringtone melody over a techno remix of Herbie Hancock’s “Rockit.” I had forgotten all about this. How many other fragments of lost culture, I wonder, lie dormant in me at every moment? Crazy Frog jumps on an invisible motorcycle and revs it along a Möbius-strip highway. “I like it,” my dad says, smiling childlike. I am filled suddenly with infinite love for him. I can’t bear it, and I Swoop right.

I go back again and ChronoSwoop tells me it’s June 21, 1998. I’m sitting on a barstool in a place I seem to remember, but only vaguely. I can tell immediately that it’s very late at night, and that the version of me I have just Swooped into was feeling considerable stress just seconds before. I don’t share his precise memories, or, rather, what happened for him just a moment ago is at a 24-year remove for me, but his cortisol levels are mine now too, and I can tell something’s wrong. After a minute or so my ex-girlfriend S**** bursts out of the men’s room, followed by some miserable low-life wearing a T-shirt with a dumb neon alien’s head on it. He wanders off and she comes sheepishly to me. “We were only doing lines, I swear.” She sniffs and rubs her nose. I am suddenly filled with rage. What a miserable time of my life this was, I think, and again I Swoop right.

I took a break for the next three days, believing I had already had enough. I found myself not quite traumatized, but far more melancholic than I usually am, and largely convinced that what the PSAs were saying was true. This is not lighthearted fun. And yet, for some reason, I went back. I landed this time on February 11, 1979. It’s morning, and I’m on the playground of my Montessori school with Jeremy. He’s wearing an Oakland Raiders windbreaker and has mushroom hair like Nicholas from Eight Is Enough. He’s holding his thumb up to his mouth like it’s a microphone and his hand over his ear as if he has a headset. “This is Howard Cosell,” he says in a funny voice—a “Howard Cosell” voice. I am staring at him confusedly. He sees that I’m not laughing at his imitation. Something in my face frightens him, and he begins to cry. I Swoop right.

What was that all about? Jeremy was always a crybaby, but not like this. What did he see in my face that frightened him so? I drink a Nespresso and I think about what to do next. Maybe I’ve had enough already? No, I Swoop left, and it’s August 18, 1975. I’ve just had a shower and I’m in my long red nightshirt. It’s a summer evening in Rio Linda, the windows are open, and the frogs and bugs are croaking and chirping. I’m lying on the couch, and Mom’s cutting my toenails. I have the strong sense that this entire composition and every being involved in it—the frogs, the bugs, Mom, the sun—is in fact only one being; or more precisely, that it is only one being, and that being is me . This is what life used to be like! Before what? Before things came apart. That’s what it is to grow up: to see the world come apart. It’s too much for me. I Swoop right.

I resolve to end my explorations here, and a good two weeks go by before I find myself quite unconsciously, lying on my back on the couch, moving through the well-hidden settings in my app. I click on “Set Target Date” and immediately I am taken to a screen requiring me to upload a scan of a state-issued ID, which will then confirm my date of birth and prevent me from choosing any target date preceding that all-important threshold. Once this formality has been handled, I aim it back to November 19, 1972, and I set the visit duration for just 30 seconds. (I presume that if I am not yet four months old, even if I have some sensorimotor control over my body as well as my usual 49-year old consciousness about me, I still might simply lack the coordination to Swoop right.) It’s hard to say what I experience when I arrive. It’s warm, it’s light, and all is one. I’m lying there next to a funny man who’s watching something on TV, but I don’t know it’s a TV, and the sound of laughter is coming out of it. “PB&J with pickles,” the man says, repeating what he has heard, laughing. Somehow I don’t understand what this means, but I’m thrilled that he finds it so funny. “Did you hear that one? PB&J with pickles !” he shouts to someone who is not in the room with us, but whose presence I can feel. Such joy. Such love. I disappear.

The block on pre-birth travel is ostensibly to prevent the risk of “ditching,” where someone gets permanently stuck in the past. But as long as we are able to preset the duration of the visit, this concern seems ill-placed, and we can only imagine that the real reason is the one that Quast foresaw: “If it ever becomes possible experimentally to prove the immateriality of the soul,” he wrote, “they will do everything in their power to prevent us from finding out about it” (SB-1omk 24.785).

I’m not the sort of person to break the law casually, but what I experienced in the autumn of 1972 was simply too powerful, and I wanted more. I went to the Pakistani mobile-phone shop down at the corner, and sure enough, what they always say about these places is true. Just as the agile shopkeeper will happily oblige any request to repair your touchscreen or to unblock some old battered phone, no questions asked, neither will he look surprised when you ask him, as the parlance has it, to “take away your birthday.”

When I got back home I drank a Diet Dr. Pepper and I pondered different dates and durations until one came to me as if in a message: 1 minute, July 30, 1971—exactly a year before my birth. I Swooped left. I cannot tell you how or why this is so, but I can tell you that exactly a year before I was born, I was floating in warm liquid, and although I had no eyes to see it, I can tell you that there was light. This scene too was charged up with love.

It was also, somehow, charged up with knowledge. Though I did not “know” anything—about PB&J sandwiches, for example, or about parents, or Howard Cosell, or Crazy Frog—it seemed to me after my return that this is only because I knew everything, and I knew it from a vantage where the sharp differentiation between these sundry things seemed a far greater error than their combination. Seeing them all as one, it seemed to me now, felt unmistakably like what is imagined under the idea of heaven. St. Augustine writes that in death the soul returns to regionem suae originis —to the region of its origin, and here he is adapting within a Christian context the broadly Platonic vision of a pre-life life spent in direct communion with the eternal and unchanging Forms. Is that what I was seeing in 1971? If so, then why was everything so wet? No Platonic philosopher, Christian or heathen, ever conceived “baby heaven” in precisely this way.

You probably have some idea of what I did next. I scrolled back to the earliest transit date possible—January 1, 1900. I would have gone back far earlier, to 500 BCE, to 50 million ybp, to God knows when, but the drop-down calendar made its cutoff the beginning of the 20th century. So that’s where I went; nor did I set a duration for the visit.

I can’t tell you what happened after that, or whether I’m still there, or what is even happening anymore. If you think I’ve been spending my days watching mustachioed men on velocipedes going to the beach and changing there into comical striped one-piece bathing suits to play beach-croquet with ladies in bloomers, you really haven’t understood what pre-birth ChronoSwooping is like. I set the thing for 1900, but the human calendar doesn’t mean very much when you’ve shed your body, and your senses, and any trace of your connection to the world of particulars.

I would not recommend doing what I have done. It is not a question of being able “to handle it”; we “handle” whatever comes our way, even or perhaps especially the most impossible things. Unlike the world I saw in 1971, here it’s not even wet or light, but neither is it dry or dark. I know everything, if by “everything” we mean the timeless and universal truths, but as for individuals, facts, things that come and go, contingent beings and the ever-vanishing traces of events, I just can’t make anything out anymore.

“God made time to prevent everything from happening at once,” the diminutive Billy ponders, while looking up at the bright North Star like some junior magus in a Family Circus cartoon circa 1988 that somehow remains vivid to me in its particularity, like the answer to a riddle I never meant to pose, even as almost all other particulars recede from my consciousness. This too is a cliché, of course. Albert Einstein said something similar; so did many other people in fact, and they were all drawing broadly on a theory of temporal idealism that runs through many philosophical systems, including, on at least one understanding, that of Augustine. But no matter, it’s Bil Keane’s cartoon version that sticks with me. I love the Sunday funnies: so stupid; so comforting; so warm. I love TV. I love memes. They’re kind of dumb but I love them.

On these and other such small things was I trained up, like some innocent AI that knows no temporal flow at all, so that the dim outlines of them still move across memory’s stage even after I have used my app against the rules and withdrawn from Time altogether—before Time was yet able to withdraw from me.

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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?

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

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

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

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

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

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

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

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

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

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

In Summary:

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

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Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

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

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

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

Turns out, King might have been on to something.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

1.1 Time Discrepancy

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

2. The Grandfather Paradox

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

We can set out Horwich’s argument this way:

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

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

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

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

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

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

3. Causation

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

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

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

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

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

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

4. Time and Change

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

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

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

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

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

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

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

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

Earlier we posed two questions:

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

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

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

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

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

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

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

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

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• Time Travel , 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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Best time travel games

Here's our list of the best time travel games with more timeloops and grandfather paradoxes than you can shake a stick at.

Looking for the best time travel games? These 10 entries mark the highlights of a genre that has been going strong for decades – promising the awe of hurtling forward in time, manipulating the present, or changing what’s happened in the past. Haven’t we all wanted to do that at some point?

Is time travel possible? Well, time travel is a pretty recognizable concept by now and has been around for as long as there have been people, whether in Hindu mythology or Japanese fairy tales. The idea of time travel in pop science fiction was widely popularized by H.G. Wells, who wrote the 1895 novel The Time Machine, as well as other stories that saw protagonists propelled into different eras.

While many modern games utilize some kind of repeating loop, like Hades or Loop Hero, the best time travel games take this a step further – allowing the player to distort time for their own ends, or forcing them to adapt to imprisonment within it.

These games don’t just use time travel as a story hook, but an integral part of how the game works – which is what helps them stand out among the bog-standard shooters, puzzle games, and platformers out there. 

So, if you loved watching the likes of Groundhog Day or Russian Doll , then these are the games for you. Who knows, maybe one of these titles will end up on our (or even your) space video games that should be movies or TV shows list. While you’re at it, make sure to check out our guide to the best time travel movies too, or if you’re after more games then these upcoming space games will interest you. 

10. Deathloop

• Release date: September 14, 2021
• Platform: PlayStation 5, PC, Xbox Series X/S

Possibly the best PS5 exclusive to have released on the console at the time (though it has since released on Xbox too), Deathloop is a thrilling time-travel first-person shooter (FPS) romp from the developers behind Dishonored.

You play as Colt, an assassin, who is on a mission to eliminate eight ultra-powerful individuals. These gifted beings are exploiting a time loop machine on a subarctic island to live the same day over and over, essentially becoming immortal and being able to do whatever they desire every night without consequence.

You must take out all eight to stop the loop from restarting – using what you learn across multiple loops to navigate the island and complete your mission in one perfect run. With slick gunplay, a gripping story, and a fascinating sci-fi premise, we don’t recommend you sleep on this one.

9. Twelve Minutes

• Release date: August 19, 2021
• Platform: Xbox One, PlayStation 4, Nintendo Switch, PC

It’s a fun, curious mystery game, with plenty of satisfaction to be had from toying with the objects and locations in your compact apartment to see how it’ll affect the outcome.

A curious time loop game, mostly in the star talent brought in to voice its trio of main characters: Daisy Ridley (Star Wars: Rise of Skywalker), James McAvoy (X-Men: First Class, Trance), and Willem Dafoe (Spider-Man, The Lighthouse).

The action takes place in a small, one-bedroom apartment, where McAvoy’s protagonist comes home to find a number of surprises waiting for him – one of which is an armed assailant claiming to be a police officer pounding on their door. When you’re knocked unconscious, you begin the time loop again and have to figure out what’s really going on in order to escape the loop for good.

8. Titanfall 2

• Release date: October 28, 2016
• Platform: Xbox One, PlayStation 4, PC

The sequel to Respawn’s futuristic mecha-combat FPS is a thrilling ride. It has a single-player campaign that pivots effortlessly between fascinating game mechanics, never feeling a need to dwell too long on each one.

The time travel mission, ‘Effect and Cause,’ is a real standout. It allows you to shift between the past and present mid-air as you run, shoot, and platform your way across an expansive military facility. Paired with Respawn’s excellent gunplay, this acclaimed shooter offers far more imagination than the average Call of Duty game.

The brisk campaign has plenty of other thrills to recommend it, but its seamless utilization of time travel as an integral game mechanic – even while limited to a single level – makes it well worth inclusion in this list.

7. Outer Wilds

• Release date: May 28, 2019
• Platform: Xbox One, PlayStation 4/5, Nintendo Switch, PC

Another time loop game, you say? Outer Wilds is a standout indie game, so much so that it’s made our best space games list. It sees you scour a solar system to unpack its mysteries, over a series of 22-minute time loops that reset your progress just as the nearest sun explodes into a supernova. Phew.

As with Deathloop, you’ll have to do more than simply map out the game – needing to learn when certain events occur, or which actions are available in the orbits of the solar system’s planetary bodies. Starting out as a student project, Outer Wilds has gone on to be one of the best indie games in recent years, with a heady mix of environmental exploration, galactic mystery, and survival sim all in one.

6. The Legend of Zelda: Ocarina of Time

• Release date: November 21, 1998
• Platform: Nintendo 64, GameCube, Wii, Wii U, Nintendo Switch

The best game of all time? It’s hard to argue with this assessment for The Legend of Zelda: Ocarina of Time (OoT), a landmark 3D action-adventure game that launched on the Nintendo 64 in 1998. In it, the young hero Link finds a magical ocarina which is able to change the weather, transport him across distant lands, and even propel him through time to a post-apocalyptic world.

It’s not a one-way trip, thankfully, and much of the puzzle work in OoT is in traveling back and forth between time periods, using what you learn and obtain to progress in each setting. As ever, you’re fighting a great evil, battling monsters, gathering weapons and tools for your journey, and figuring out troublesome dungeons – but time travel is what elevates Ocarina of Time into a game of truly mythical proportions.

It's an oldie, but a goodie. You can find the game through Nintendo Switch Online, or on Nintendo DS systems.

5. Quantum Break

• Release date: April 5, 2016
• Platform: Xbox One, PC

Back in 2016 when Quantum Break released, it was the best-selling game published by Microsoft on the Xbox One. While it soon ceded that title to Sea of Thieves (who needs time travel when you have sea shanties?) it’s still a landmark game, and a fascinating experiment in interactive storytelling.

The main conceit of Quantum Break is its mix of gameplay with an interactive TV show that breaks up the story’s five acts, allowing you to make key decisions that affect the narrative for the rest of the game.

Originally conceived as a sequel to Alan Wake, before pivoting into a new IP, Quantum Break is set in a world where a failed time travel experiment allows you to halt, freeze, accelerate, and generally manipulate time around you – inevitably used to ramp up combat as you take on hordes of soldiers and try to fix a fracture in time itself. Time ‘stutters’ can also freeze objects in your environment, creating obstacles or platforms that help and hinder your progress along the way.

4. Life is Strange

• Release date: January 20, 2015
• Platform: Xbox One, PlayStation 4, Nintendo Switch, PC, Android

While more recent Life is Strange games play around with the powers of empathy, or telekinesis, the original game is what started the franchise off with such aplomb – following the fortunes of a young girl who is able to rewind time.

Maxine ‘Max’ Caulfield discovers her time-manipulation ability when a classmate faces an unexpected danger – managing to undo the day’s events and keep them alive. As ever, time travel has unintended consequences, and while Max can aid or even save various people in her life, she finds the outcome isn’t always for the best in the long term. 

With a strikingly impressionistic art style, branching narratives, surprisingly high stakes for a high school story, and a good helping of teen angst, this episodic adventure is well worth a try.

3. Wanderer

• Release date: January 27, 2022
• Platform: PlayStation VR, SteamVR, Oculus

If you’re looking for a time-travel VR game, you’ve come to the right place. Wanderer is a first-person puzzle adventure that has you traveling back through time to prevent an apocalyptic present.

With a talking watch on your wrist, and the ability to jump to different places and periods, Wanderer lives up to its name, seeing you jettison across centuries of history and even to the moon. In each case, action is found in localized puzzles (escape rooms, essentially), some of which take imaginative back-and-forth travel to get the right objects for the right situation. 

You’ll get around 10 hours of play out of this one, along with some brain-bending puzzles that make the most of the VR medium (along with one of the best VR headsets )

• Release date: August 6, 2008
• Platform: Xbox 360, PlayStation 3, Switch, PC, Mac, Linux

Few games play so successfully with time as Braid, a 2008 indie platformer initially launched on Xbox 360 to great acclaim. The developer went on to create The Witness with the profits from this game.

You play Tim, who is trying to find and rescue a princess from some unspecified monster. So far, so Mario. However, things aren’t quite so simple as running and jumping on Goombas: Braid utilizes a number of different time-based mechanics, from a simple rewind (helpful after jumping and missing a ledge) to whole levels that move forward in time as you move right and backward as you move left.

It’s a mind-melting game in many ways, and you may find the story elusive at first, it being gradually pieced together by jigsaw pieces you collect throughout your journey – and blown open by its subversive conclusion. But a recurring theme is the idea of wanting to redo or undo the past, and you won’t find a game that needles this idea as effectively or imaginatively as Braid.

1. The Legend of Zelda: Majora's Mask

• Release date: April 27, 2000
• Platform: Nintendo 64, GameCube, Wii, Wii U, Nintendo Switch, Nintendo DS

How could we list the best time travel games without Majora’s Mask? This follow-up to Ocarina of Time takes place in the same land of Hyrule as its predecessor just two months later, even re-using many in-game assets. The key difference is that an unhinged-looking moon going to crash into the Earth, obliterating everything – and you find yourself reliving the same three days over and over in an attempt to prevent the cataclysmic event from happening.

Those three days come to around an hour of actual gameplay time, meaning you play through a loop quite swiftly, but the real magic of Majora’s Mask is in how it expands on the time-warping magic of OoT. Here you’ll use the ocarina to jump back to the start of the loop, slow down the passage of time, or jump forward to a later day, navigating time like a puzzle dungeon in need of solving.

Earning widespread acclaim, Majora’s Mask has been rereleased several times throughout the years. For modern gamers, you can find the game through Nintendo Switch Online, or on Nintendo DS systems.

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Oh, God. The enfant terrible of animated comedies is delivering unto us a Season 22 finale of biblical proportions. Because after Family Guy ’s near-quarter of a centu ry on the air, nothing is sacred!

“I never thought I’d hear myself say this as a writer,” confesses Richard Appel , a showrunner alongside Alec Sulkin , “but the Standards and Practices department at Fox is easy to work with and pretty good with us.”

Due in part to the sheer success of the cartoon, as well as its consistent ability to push the envelope without inspiring boycotts (anymore), Guy has gotten away with decades of gags other shows wouldn’t dare to attempt—like tonight’s trip back to the days of Jesus. A travesty, you say? Even the respected drama The Chosen has a sense of humor—witness its Jesus ( Jonathan Roumie ) cracking jokes!

The episode kicks off with Brian the dog (voiced by series creator Seth MacFarlane ) falling for a devout vet tech ( Mae Whitman ). When he pretends to be pious himself, things suddenly go from holy-roller rom-com to something far more irreverent. “We call that an Act 1 mislead,” reveals Sulkin. “You go a little bit down a road with something that seems like it could be a Family Guy story, and then it turns into something else.”

25 Hilarious Moments From 'Family Guy's First 25 Years (VIDEO)

Brian’s new girlfriend refuses to engage in premarital hanky-panky for fear of eternal damnation, though, so the incensed horndog has a novel solution. He hops into the time machine that Stewie (also voiced by MacFarlane) built back in Season 1. The lothario’s goal: to alter the past so he can score in the present.

Before you can say “Thou shalt not time travel,” the pooch has landed in Israel circa A.D. 30 (above), and he’s soon joined by Stewie in a misbegotten crusade to stop Jesus (voiced by exec producer Sulkin) from creating Christianity. Lordy.

Family Guy , Season 22 Finale, Wednesday, April 17, 9:30/8:30c, Fox

Family Guy where to stream

Alec Sulkin

Mae whitman, richard appel, seth macfarlane.

Explore the World: 5 Must-Visit Travel Websites for Wanderlust Seekers

I n today's fast-paced world, taking a break from the daily grind and exploring new horizons is essential for both physical and mental well-being. Travel opens doors to new cultures, cuisines, and experiences, broadening our perspectives and enriching our lives. With abundant online information, planning and executing a memorable trip has become easier than ever. Here are five outstanding travel websites that will inspire and empower you to embark on your next adventure:

1. Work Hard Travel Well

For busy professionals who crave travel experiences but struggle to fit them into their hectic schedules, Work Hard Travel Well is a beacon of inspiration. Founded by Kim, a Black travel blogger and content creator, this website provides practical tips and strategies for maximizing PTO, planning incredible journeys, and extracting every ounce of value from travel experiences. Kim's expertise is helping individuals make the most of their limited vacation time, ensuring every trip is a rejuvenating and unforgettable experience.

Follow her on YouTube here

2. The Points Guy

Travel enthusiasts and frequent travelers alike will find The Points Guy to be an invaluable resource. Founded by Brian Kelly, a travel expert and authority in frequent flyer programs and credit card rewards, this website provides a comprehensive guide to maximizing travel rewards and earning free flights and hotel stays. Their website features in-depth reviews of airlines, hotels, and credit cards and strategies for accumulating and redeeming points for valuable travel experiences . Whether you're seeking luxurious getaways or budget-friendly adventures, The Points Guy has the expertise to help you maximize your travel rewards and stretch your travel dollars further.

3. Lonely Planet

For travelers seeking in-depth insights into various destinations worldwide, Lonely Planet is a trusted and respected resource. Renowned for its comprehensive travel guides and destination-specific information, this website offers a wealth of travel knowledge, including destination guides, city guides, regional guides, and travel tips , all tailored to diverse travel styles and interests. Whether you're planning a backpacking adventure through Southeast Asia or a cultural immersion in Europe , Lonely Planet provides the tools and resources to make your trip informative, engaging, and unforgettable.

4. Condé Nast Traveler

Discerning travelers seeking high-end travel options will find inspiration and guidance on Condé Nast Traveler. A leading publication in the travel industry, this website features a curated selection of travel destinations, hotels, restaurants, and experiences, catering to those who appreciate luxury and refinement. Condé Nast Traveler's expert editors provide in-depth reviews and recommendations, ensuring that travelers make informed decisions and enjoy the best the world offers. From opulent accommodations to exclusive dining experiences, Condé Nast Traveler showcases the pinnacle of travel, inspiring dreams of unforgettable getaways.

5. National Geographic Travel

With a rich history of storytelling and exploration, National Geographic Travel offers a unique perspective on travel, combining stunning photography, insightful articles, and immersive experiences. Their website features a vast collection of travel stories, destination guides, and interactive features that inspire wanderlust and encourage exploration. National Geographic Travel takes readers beyond the typical tourist hotspots , venturing into remote corners of the world, uncovering hidden gems, and shedding light on diverse cultures and traditions. Their website is a treasure trove of travel inspiration, igniting a passion for discovery and encouraging travelers to embrace the unknown.

These five travel websites cater to diverse travel interests and preferences, providing valuable resources for planning and executing memorable travel experiences. Whether you're seeking budget-friendly backpacking adventures or luxurious getaways, these websites offer inspiration, guidance, and practical tips to make your travel dreams a reality. So, pack your bags, grab your passport, and let these travel websites fuel your wanderlust and guide you on your next adventure.

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5 Airport Lines You Can Ditch (and How to Skip Them for Free)

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At the airport, long lines for check-in, security screenings and even getting food can feel like a giant waste of time — and potentially disrupt even the most meticulously planned itinerary.

But with planning, you can skip some of the most annoying lines, and in some cases, get reimbursed if the line-skipping privilege requires an application fee. Here are five common airport bottlenecks and how to avoid them for free or cheap.

1. Check in

It is the year 2024, which means there’s a mobile version of all sorts of travel services . That includes the ability to check in to your flight without standing in line at the airline counter.

Most airlines allow you to check in for your flight directly through their mobile app or website within 24 hours of departure, allowing you to bypass the desk and proceed directly to security if you are flying with only carry-on luggage.

Even if you plan to check bags, checking in online via the airline app or website can help speed up the process so all you will have to do at the airport is print the bag tags at a kiosk and then hand the luggage off at the bag drop.

One way to avoid the checked-bags line altogether is to check your bag at the gate . Many airlines offer complimentary gate-checked baggage services on full flights, but it doesn’t hurt to ask the gate agent even if it’s not offered outright.

This trick doesn’t work if you’re packing common items that can’t go through the security screening , like pocket knives or liquids greater than 3.4 ounces. But assuming your stuff will pass through the security screening and you don’t need it during the flight, it might be worth waiting to check it at the gate.

2. Airport security ID check

An expedited Clear lane at San Francisco International Airport. (Photo by Sally French)

U.S. airport security technically consists of two lines: the line to get your identity checked, and the line to get you (and your stuff) screened.

You can get through airport security fast and skip to the front of the identity verification line with Clear, which is a private biometric screen company operating at more than 55 airports nationwide. Clear says it has more than 20 million members.

Once you pay for a Clear membership (which typically costs $189 per year), you can scan your fingerprints or eyes at Clear’s kiosks instead of having a Transportation Security Administration (TSA) agent look at your ID. From there, a Clear employee escorts you straight to the physical security screening, allowing you to cut in front of everyone else waiting to have their boarding pass or identification checked.

Though Clear membership fees are high, you might not actually have to pay them. Certain American Express credit cards offer annual statement credits to cover the cost.

on American Express' website

• Credit for the application fee for TSA PreCheck or Global Entry.

• $189 statement credit to cover the cost of an annual Clear membership.

Terms apply.

3. Baggage screening

The standard TSA screening process can be slow, as most people have to remove their jackets and shoes, plus large electronics, from their bags. But with TSA PreCheck , you can leave your shoes and jackets on — and keep your laptops tucked away.

Most airports have two separate screening lanes for your carry-on luggage — one for TSA PreCheck and one for standard screening. According to the TSA, 99% of TSA PreCheck passengers wait less than 10 minutes (while it’s not uncommon for the standard line to take about 30 minutes).

To access those TSA PreCheck lines, you’ll need to submit an application and pay the fee, which starts at $78 and covers five years of membership. More than 4 million people enrolled in the program in 2023, bringing the total to more than 18 million active members.

There are ways to get TSA PreCheck for free , including holding a credit card that offers TSA PreCheck statement credit or redeeming rewards from certain hotel and airline loyalty programs.

4. Ordering food

At certain airports, Starbucks allows ordering through its mobile app. (Photo by Sally French)

Many airport eateries now offer mobile food ordering, where you can place an order before you arrive, and pick it up before you catch your flight.

Some restaurants, such as Starbucks, offer mobile ordering through an app. Starbucks began rolling out mobile order functionality in 2022, making it possible to order ahead and pay on the Starbucks app at participating airport outposts.

Other airports offer websites or apps that allow you to order food and beverages from participating airport restaurants. For example, the SFO2Go website allows you to order food from one of about a dozen restaurants at San Francisco International Airport. Food is typically ready within about 10-20 minutes.

International travelers returning to the U.S. must pass through a Customs and Border Inspection site before leaving the airport, which typically entails yet another long line.

If you hold Global Entry, which is an expedited clearance program for preapproved, low-risk travelers, you can skip the customs line. Program members get access to specific Global Entry lanes where their photo is taken to verify their membership. The process is generally much faster than the standard line — so fast that you pause only briefly on your way out of the terminal.

To get Global Entry, you’ll need to fill out an application and pay a $100 fee, which is nonrefundable (even if your application is denied). Avoid that fee by paying on one of the myriad credit cards that will cover your Global Entry application fee . Global Entry also includes TSA PreCheck benefits, so if you travel internationally, it’s better to pay the slightly higher fee for Global Entry to get access to both special lanes.

on Bank of America's website

on Chase's website

$0 intro for the first year, then $95 .

Statement credit of up to $100 as reimbursement when you charge the application fee for TSA PreCheck or Global Entry to the card. Available once every 4 years.

Statement credit of up to $100 as reimbursement when you charge the application fee for TSA PreCheck, Global Entry or NEXUS to the card. Available once every 4 years.

How to maximize your rewards

You want a travel credit card that prioritizes what’s important to you. Here are our picks for the best travel credit cards of 2024 , including those best for:

Flexibility, point transfers and a large bonus: Chase Sapphire Preferred® Card

No annual fee: Bank of America® Travel Rewards credit card

Flat-rate travel rewards: Capital One Venture Rewards Credit Card

Bonus travel rewards and high-end perks: Chase Sapphire Reserve®

Luxury perks: The Platinum Card® from American Express

Business travelers: Ink Business Preferred® Credit Card

On a similar note...

• International

April 11, 2024 - Israel-Hamas war

By Heather Chen , Antoinette Radford, Tori B. Powell , Aditi Sangal and Elise Hammond , CNN

Our live coverage of Israel's war on Hamas in Gaza has moved  here .

UN committee fails to reach consensus on full Palestinian membership, Security Council president says

From CNN’s Richard Roth and Michael Rios

A specialized UN committee failed to reach a consensus Thursday on Palestinian membership in the United Nations, according to the president of the UN Security Council.

Two-thirds of the committee members were in favor of moving on with membership, with many countries arguing that “Palestine fulfills all the criteria that are required” to be granted full state member status, Malta's Ambassador and Security Council President Vanessa Frazier said. 

She added that no one explicitly objected to the membership qualifications.

Frazier also said she would circulate a draft report on the deliberations as soon as Friday. If the committee doesn’t agree on the report, it could hold another meeting to iron out any differences.

Asked whether the process of deliberating Palestinian membership in the committee is now over, Frazier said, “Unless the next step of agreeing (to) the report of the committee warrants another meeting to iron out the differences, it’s not foreseen that there would be any further committee action.”

But she noted that any UN Security Council member can still table a resolution to vote on Palestinian membership at any time, regardless of the committee's report.

The US and Middle East brace for a possible Iran attack that could escalate the conflict. Here's the latest

From CNN staff

Concerns about a possible Iranian attack against Israel has prompted many diplomatic conversations around the globe.

Here are some developments on diplomacy around the threat of an attack:

• Iran's statement: The imperative for Tehran to "punish" Israel for the deadly strike on the Iranian consulate in Damascus last week might have been avoided if the attack had been condemned at the United Nations, Iran’s Mission to the UN said Thursday.
• US and UK diplomacy: US Secretary of State Antony Blinken spoke with the foreign ministers of Turkey, China and Saudi Arabia on Wednesday night and Thursday morning to tell them that countries should be urging Iran not to escalate the conflict in the Middle East after  threats made by Tehran against Israel,  according to a State Department spokesperson. The top US general for the Middle East is also in Israel . Additionally, Britain's foreign secretary warned his Iranian counterpart on Thursday that Tehran “must not draw” the Middle East into a wider conflict .
• Israel receives US support: Israeli Defense Minister Yoav Gallant spoke with Blinken and US Defense Secretary Lloyd Austin. The US officials expressed the country's support for Israel against Iranian threats. Gallant warned that such an attack could lead to a regional escalation.
• Travel restrictions: The US State Department restricted the travel of US government personnel in Israel in the wake of public threats against Israel by Iran. “The security environment remains complex and can change quickly depending on the political situation and recent events,” the alert noted. US officials  are on high alert  for a potential retaliatory strike by Iran or its proxies against Israel.

Meanwhile, here's some other updates:

• Hamas on hostages: A member of Hamas' political bureau said   a prisoner-hostage exchange is being discussed as part of larger ceasefire negotiations. "Part of negotiations is to reach a ceasefire agreement to have enough time and safety to collect final and more precise data" on the hostages held in Gaza, Basem Naim said in a statement on Thursday. "Because they (hostages) are in different palaces, (being held) by different groups, some of them are under the rubble killed with our own people, and we negotiate to get heavy equipment for this purpose."
• Updates on aid to Gaza: It is clear that Israel is working to ramp up humanitarian aid to Gaza, but it has not yet implemented all of the measures it has announced, a top United Nations humanitarian official said. Aid coming in trucks from Israel has to be "segregated from water, from food, from medical items" before it goes into Gaza, said Jamie McGoldrick, the UN’s humanitarian coordinator in Jerusalem. “Getting 400 trucks from Kerem Shalom doesn't mean 400 trucks go into Gaza,” he said, adding that the logistical complications are numerous, and take time to resolve. He also said Israel’s restrictions on movement inside the strip complicate matters.

Anera charity resumes work in Gaza after pausing when Israeli strike killed 7 World Central Kitchen staff

From CNN's Tala Alrajjal and Mohammed Tawfeeq

American Near East Refugee Aid (Anera) has resumed Gaza operations "after a temporary pause" following an airstrike that killed seven World Central Kitchen (WCK) aid workers on April 1.

"As you know, the decision to temporarily pause our operations was not an easy one. We followed the direction of our staff in Gaza, who've faced death, loss, and destruction since the start of the war," Sean Carroll, the president and CEO of Anera, said in a statement on Thursday. "After the killing of Anera staff member Mousa Shawwa , followed by the attack that killed seven aid workers from World Central Kitchen, we made the difficult but necessary decision to pause aid operations on April 2," Carroll added in the statement.

Carroll said Israeli authorities informed him during a meeting Thursday that "certain measures would be taken to protect humanitarian aid workers in Gaza – including Anera's staff."

"With the full support of our Gaza team, we have determined that the circumstances have changed sufficiently to resume our vital humanitarian work in Gaza," Carroll said. 

Anera on Thursday resumed "full operations in Gaza to deliver meals, food parcels, hygiene kits, tents, medical treatments, and more to families in dire need," according to the statement.

Iran says its imperative to punish Israel could have been avoided had UN Security Council condemned attack

From CNN’s Natalie Barr and Adam Pourahmadi

The imperative for Tehran to "punish" Israel for the deadly strike on the Iranian consulate in Damascus last week might have been avoided if the attack had been condemned at the United Nations, Iran’s Mission to the UN said Thursday.

“Had the UN Security Council condemned the Zionist regime’s reprehensible act of aggression on our diplomatic premises in Damascus and subsequently brought to justice its perpetrators, the imperative for Iran to punish this rogue regime might have been obviated," the  mission said on X.

The UN Secretary-General António Guterres condemned the attack on April 1, according to a statement from the UN spokesperson Stephan Dujarric. A Security Council discussion was held on April 2 to discuss the attack, but differences among members prevented any formal action or condemnation from taking place.

The United States is on high alert and actively preparing for a “significant” attack by Iran targeting Israeli or American assets in the region  in response to the strike in Damascus  that killed top Iranian commanders.

UK foreign secretary warns Iran not to draw Middle East into wider conflict

From CNN's Natalie Barr

Britain's foreign secretary warned his Iranian counterpart on Thursday that Tehran “must not draw” the Middle East into a wider conflict following a series of escalating threats made by Iran toward Israel.

British Foreign Secretary David Cameron told Iran's Foreign Minister Hossein Amir-Abdollahian that the United Kingdom was "deeply concerned about the potential for miscalculation leading to further violence. Iran should instead work to de-escalate and prevent further attacks," according to a post on X .

Iran's state-aligned Tasnim news agency on Thursday reported that Amir-Abdollahian had told Cameron that the silence from the UK and the United States following Israel’s attack on the Iranian consulate in Damascus last week only served to encourage Israel to continue waging war in Gaza and expand its conflict in the region.

US and Israeli defense leaders discuss fears of Iranian attack

From CNN’s Michael Conte in Washington, DC, Tamar Michaelis in Jerusalem and Larry Register in Atlanta

US Defense Secretary Lloyd Austin spoke with Israeli Defense Minister Yoav Gallant today to “reaffirm the US ironclad commitment to Israel’s security against threats from Iran and its proxies,” according to a Pentagon spokesperson.

The call comes a day after Iran's Supreme Leader Ayatollah Ali Khamenei said Israel " must be punished and it will be" following a strike on an Iranian embassy compound in Syria that killed seven Iranian officials.

Israel “will not tolerate an Iranian attack on its territory,” the statement read, adding that the two defense leaders also discussed detailed preparations “for an Iranian attack against the State of Israel.”

Gallant said an Iranian attack on Israel could lead to a regional escalation.

It is the second discussion held between Gallant and Austin over the past week, according to the statement. Gallant expressed his appreciation for Austin’s personal commitment to the security of the State of Israel and for the deepening cooperation between the defense establishments of both countries, as well as between the Israel Defense Forces and US CENTCOM.

State Department restricts personnel travel in Israel amid concerns over Iranian threats

From CNN's Jennifer Hansler

The US State Department has restricted the travel of US government personnel in Israel in the wake of public threats against Israel by Iran.

“Out of an abundance of caution, U.S. government employees and their family members are restricted from personal travel outside the greater Tel Aviv (including Herzliya, Netanya, and Even Yehuda), Jerusalem, and Be’er Sheva areas until further notice,” a  security alert  posted by the US Embassy Thursday said. “U.S. government personnel are authorized to transit between these three areas for personal travel.” “The security environment remains complex and can change quickly depending on the political situation and recent events,” the alert noted.

State Department spokesperson Matthew Miller said he would not “speak to the specific assessments that led to us to restrict our employees and family members’ personal travel, but clearly we are monitoring the threat environment in the Middle East and specifically in Israel.”

“We have seen Iran making public threats against Israel in the past few days,” Miller said. “Israel is in a very tough neighborhood and we have been monitoring the security situation. You saw us slightly adjust for travel warnings at the beginning of this conflict and we conduct ongoing assessments all the time about the situation on the ground.”

US officials are on high alert for a potential retaliatory strike by Iran or its proxies against Israel.

Hamas says ceasefire with Israel is essential to collect data on hostages held in Gaza by different groups

From CNN's Abeer Salman and Mohammed Tawfeeq

A member of Hamas' political bureau said   a prisoner-hostage exchange is being discussed as part of larger ceasefire negotiations.

"Part of negotiations is to reach a ceasefire agreement to have enough time and safety to collect final and more precise data" on the hostages held in Gaza Basem Naim said in a statement on Thursday. "Because they (hostages) are in different palaces, (being held) by different groups, some of them are under the rubble killed with our own people, and we negotiate to get heavy equipment for this purpose," he added.

Naim's statement was in response to questions from media outlets about whether Hamas has been rejecting the latest proposal, which was made in Cairo over the weekend, because it can not release 40 hostages in the first phase of a three-stage ceasefire deal.

According to an Israeli official and a source familiar with the discussions, Hamas indicated it is currently unable to identify and track down those 40 Israeli hostages, raising fears that more hostages may be dead than are publicly known. 

CNN's record of the conditions of the hostages also suggests there are fewer than 40 living hostages who meet the proposed criteria.

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Vice President Kamala Harris returning to Wisconsin, visiting La Crosse on Monday

Vice President Kamala Harris will travel to La Crosse on Monday, marking her third visit to Wisconsin this year.

Her visit will focus on "fighting for workers, supporting families, and improving health care for people across America," according to the White House.

Harris was last in Wisconsin in March, when she announced an executive order aimed at promoting apprenticeships and visited her childhood home in Madison . She also rallied voters in typically red Waukesha County around abortion in January.

More: Women campaigning for Biden take abortion issue to Republican stronghold Waukesha County

Harris' visit will come days after the release of the latest Marquette University Law School poll , which showed President Joe Biden and former President Donald Trump locked in a tight race. Biden was two percentage points behind Trump among both registered voters and likely voters in Wisconsin, with Trump at 51% and Biden at 49%.

That poll also found voters considered Biden better at handling abortion policy and health care, while Trump led on the economy. A majority of Wisconsin voters — 54% — said they supported a national ban on abortion after 15 weeks, with exceptions for rape, incest and the life and health of the mother. It's the first time the Marquette poll asked that question.

Biden was last in Wisconsin in March , when he highlighted a multimillion dollar investment toward Milwaukee's "complete streets" initiative and  stopped at the campaign's new Wisconsin headquarters .

Trump has made one visit to Wisconsin this election cycle, rallying voters in Green Bay on April 2, the day of the state's presidential primary. He focused heavily on immigration and repeated false claims about the 2020 election in his remarks.

.css-xtinp9{display:inline-block;margin-right:0.5rem;color:#0c0c0c;font-family:Roboto Condensed,system-ui,-apple-system,BlinkMacSystemFont,"Segoe UI","Helvetica Neue",sans-serif;font-size:20px;font-weight:700;text-transform:uppercase;line-height:1.2;}@media screen and (min-width: 992px){.css-xtinp9{font-size:36px;margin-right:1rem;}} Match Atalanta v Liverpool: TV channels, live commentary and how to watch Europa League highlights

Liverpool travel to Atalanta on Thursday for the second leg of their Europa League quarter-final – here's how to follow the fixture.

The clash at Stadio di Bergamo kicks off at 8pm BST and will be broadcast live on TNT Sports in the UK. Worldwide broadcast listings are available here .

• Get fixture and broadcast information directly to your phone by downloading our eCalendar here

Matchday Live gets under way at 7pm BST on LFCTV , with build-up followed by live audio commentary, then post-match reaction from Italy.

Audio commentary is also available for every matchday at video.liverpoolfc.com .

Follow Match Centre on Liverpoolfc.com and the official LFC app [ iOS | Google Play ] for live build-up, confirmed team news, minute-by-minute updates, photos, videos, post-match reaction and more.

Highlights, a full replay and more will be available for subscribers on LFCTV GO from midnight BST on Thursday.

• Subscribe to LFCTV GO now and get a month free using the code GOFREE23. You can also sign up and watch via the official LFCTV GO app ( iOS | Google Play ). Terms and conditions apply.

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Mount Ruang Erupts in Indonesia, Spewing Lava Thousands of Feet Into the Sky

Hundreds of earthquakes were detected in the weeks preceding the eruption of the volcano in North Sulawesi province. Hundreds of people were evacuated.

By Christine Hauser

Mount Ruang, a volcano in Indonesia, erupted on Tuesday, spewing fiery lava and ash thousands of feet into the night sky and forcing the evacuation of hundreds of people in the North Sulawesi province, according to the authorities and local news reports.

The volcano erupted at about 7:19 p.m. local time, Antara, the national news agency, reported. The country’s National Disaster Mitigation Agency said on Wednesday that more than 800 people in nearby villages were displaced by the eruption, many using ferries and taking shelter in churches and community centers.

The authorities said supplies such as mats, blankets, cleaning materials, and tents were needed, and that more shelters might be opening for people fleeing the volcano.

Indonesia is the world’s largest archipelago nation. It is spread across what is known as the Ring of Fire, where tectonic plates clash under the surface of the Pacific Ocean and spawn earthquakes and eruptions from volcanoes.

Mount Ruang is a stratovolcano , or a steep, conical volcano that has built up over years in layers from explosive eruptions of lava, rock fragments, ash and other properties.

“It is in a part of the world where there are a lot of active volcanoes,” said Dr. Tracy K.P. Gregg , who chairs the geology department at the University at Buffalo.

Its last major eruption was in 2002, when the column of lava and ash that it spewed reached up to 17 miles, Dr. Gregg said.

She said the volcano in 2002 measured 4, a “large” volcano on the Volcanic Explosivity Index, a scale used to measure the strength of an eruption by looking at several factors, such as duration, ash volume and plume height. Mount Pinatubo in the Philippines in 1991 measured 6 on the index. Mount St. Helens in the United States in 1980 measured 5.

“So it is a little bit smaller than that,” she said of Mount Ruang. Right now, it is not as violent as the previous eruption, she added, but the volcano cannot be fully assessed while it is in progress.

More than 300 volcanic earthquakes were detected over a period of at least two weeks preceding the eruption of Mount Ruang.

It is not immediately clear why the volcano erupted when it did. “Every volcano has its own personality,” she said.

In the past few years, several volcanoes in Indonesia have erupted. In December, 2023, the bodies of 11 hikers were found on the slopes of Mount Marapi on the island of Sumatra, after an eruption that spewed an ash column of nearly 3,000 meters — about 10,000 feet high.

In December 2022, more than 1,900 people were evacuated from the area surrounding Mount Semeru as it erupted. In an eruption there the previous December , more than 50 people were killed and hundreds more were injured.

Christine Hauser is a reporter, covering national and foreign news. Her previous jobs in the newsroom include stints in Business covering financial markets and on the Metro desk in the police bureau. More about Christine Hauser