Scientists, or at least their wild-haired fictional counterparts, promised us time travel and still have not delivered. Forget walking with dinosaurs or killing baby Hitler; I’d be happy just to warn my month-ago self not to make all the mistakes he’s about to. It’d also be nice to zoom past the next few months (year? years??) to the nationwide orgy of the post-virus era. Of course, none of these things are possible, because time travel doesn’t exist. But could it? That is the subject of this week’s Giz Asks, for which we reached out to a number of physicists.
Senior Lecturer at the International Centre for Radio Astronomy Research at Curtin University
One obvious point to start with is that time travel works already. We’re all caught in the river of time, traveling inexorably into the future. A person or object can move freely in any direction through the spatial dimensions, yet time is directional. Despite these differences, space and time are inextricably linked into the four-dimensional concept of space-time, as formulated by Einstein over a century ago in his theory of “special relativity.” As you move faster through space, your measurement of time begins to disagree with that of observers at rest. This is the origin of the “twins paradox”; if one twin remains on Earth while the other takes a return interstellar journey at close to the speed of light, on their return, the clock of the well-travelled twin will have measured less elapsed time than that of the twin who stayed home. A time-traveller interested in Earth’s far future need only build an exceedingly fast spaceship, leave, and come back.
Einstein’s further insight was to realize that gravity is not a force, but a deformation of space-time caused by the presence of mass, which he mathematically formulated in his theory of “general relativity.” This is taken to extremes in the movie Interstellar, which essentially gets the physics correct, and characters who spend a long time in deep gravitational wells experience much less time passing than those outside. (My only issue with Interstellar is the portrayal of scientists more willing to act on hunches and feelings than back-of-the-envelope calculations! Well, that, and it is about an hour too long.) So, another method of traveling into the far future would be to spend a bit of time close to an enormous mass, such as a black hole. Of course, you’d need to a) find one that isn’t accreting (and so emitting lethal gamma rays); b) get there and back within a human lifetime (see above); c) find an orbit that doesn’t involve being shredded by tidal forces. Easier said than done.
Thus far we’re only considering a plucky adventurer journeying into the far future, leaving friends and family, never to return. Of course, what we usually want from time travel is the ability to move freely backward and forward. Here, relativity cannot help us: to travel backward in time would involve moving faster than the speed of light, at which point your mass becomes infinite and you explode (thanks, Einstein). Not to mention, moving backwards in time would essentially violate the second law of thermodynamics, and cause all sorts of headache-inducing temporal paradoxes. Still, physics seems to enjoy giving us headaches, and some of my colleagues have shown that, theoretically, light could be sent backwards in time through wormholes. Personally, I have a kind of anthropic philosophy: if time travel were possible, wouldn’t the time travelers already be here? But if we want a final answer to the question of time travel, a bit more funding toward theoretical physics wouldn’t go amiss.
Professor, Mechanical Engineering and Physics, MIT
The true answer is that we don’t really know. It’s consistent with the laws of physics; it’s consistent with Einstein’s theory of general relativity.
In 2008, I realized, from looking at the work we were doing on quantum computing, that we could develop a more robust and understandable quantum theory of time travel using the quantum theory of teleportation and the quantum theory of how to escape from a black hole.
Quantum mechanics people tend to believe, now, that you can get out of a black hole in a kind of scrambled form. I wouldn’t recommend it, and I’m definitely not going to be the first person to try it out—but if you could escape from a black hole, by definition you’d be going faster than the speed of light, and even in Einstein’s theory of special relativity, if you can go faster than the speed of light you can send signals back in time.
Taking these things together—the fact that general relativity, a very good theory that people believe in, allows for time travel, and that quantum mechanics combined with general relativity also seems to allow it—suggests that it’s possible. And in physics, the idea is that if something’s possible, it’s mandatory—you can do it.
Because quantum teleportation is something you can do in a laboratory, we were able to actually conduct an experiment. The experiment was what I call the moral equivalent of sending a photon, a particle of light, a few billionths of a second backwards in time. We wanted to test out certain paradoxes of time travel—for instance, the grandfather paradox, in which a time traveler goes back in time and accidentally (or on purpose) kills their own grandfather, resulting in her never being born and never going back in time. Naturally, then, we had to get our photon to kill its former self.
The way it works is, the photon steps into the time machine and closes the door; if the red light on the outside of the time machine goes off, that means the photon has traveled backwards, to the past, and you can look to see what’s happened. What happened in our experiment was that it failed—no matter how hard it tried, it couldn’t actually kill itself.
Not surprisingly, our papers generated a lot of media attention. And ever since then, on a monthly basis, I get an email from someone saying, ‘Dear Professor Lloyd: I am a time traveler trapped in time. I understand that you have a time machine. Can you help me get back to my own time?’ Some of these letters are very poignant: an Italian woman has written me a number of times, checking in, because she wants to go back in time and warn her sister about getting into the car in which she’s going to have a fatal accident. It’s very sad. I tell her we can’t do that—and that even if we could, you still couldn’t change the past.
Research Professor, Astrophysics, Stony Brook University
We’re interested in two different kinds of time travel. In one, you skip forward into the future; in the other, you go back to the past.
Skipping forward into the future is simply a matter of going super-super-super-fast. As long as you have a big-enough rocket, and can approach the speed of light, you can skip forward into the future. But good luck building a rocket big enough. It won’t happen in our lifetimes, or in our descendants’ lifetimes, but hey—at least it’s possible.
To travel back into the past—that’s the nasty one. Which is a shame, because that’s the one we’re really excited about. The frustrating thing is that every single time we come up with a clever way to time travel to the past, some law of physics comes in and ruins the party, and it’s a different law of physics every single time. There’s no unified equation that we can point to, that we can print on a T-shirt, that says: This is the reason why we can’t travel into the past. It just doesn’t work. But we don’t really know why it doesn’t work.
For example, you can build a time travel device if you have an infinitely long cylinder that’s rotating at nearly the speed of light. But you can’t have an infinitely long cylinder in the universe, so that’s out. Or you can build a wormhole, which would let you travel back in time, but it would need to be made of matter that has negative mass, which doesn’t exist. It’s all these random little headaches that prevent us, despite a lack of a unified explanation. It does seem unlikely, but answering ‘yes’ or ‘no’ conclusively is beyond the realm of known physics.
Assistant Professor, Physics, Creighton University
The passage of time is an essential notion here, because it highlights the fact that time is actually very different from space. In space, it’s possible to stay put—to not go forwards or backwards. With time, it seems it’s not even possible to sit still—we’re always having to move forwards.
Einstein’s theory of relativity says that if you experience a different amount of gravity, or if you are moving relative to another person, then the passage of time you experience is different from that other person’s. This isn’t just some wacky idea—this is one of the most well-established theories of how our universe works.
GPS, to function, actually has to take that into account. We have clocks on these GPS satellites, and because they’re farther out from the Earth, they’re experiencing weaker gravity than we are on Earth’s surface, which means those clocks are actually going to tick at a different rate. If we didn’t account for that difference, our GPS wouldn’t work. The coordinates would be way off.
Another example: There’s a famous experiment, conducted in the 1970s, in which scientists put clocks on two airplanes and flew them in opposite directions around the Earth. Einstein’s theory of relativity predicted some precise amount of time by which those clocks would be off from each other by the time they returned from their trips; and in fact, when the planes met up again, they were off by the exact amount of time predicted by Einstein’s theory.
You can take this to an extreme: If you went near a very strong force of gravity, a black hole for example, by the time we met up again you’d have aged just a few hours, whereas I might’ve aged 40 years.
Even living on Earth, if you work in one of the upper stories of a skyscraper, your clock is actually ticking differently from that of someone who is living or working on one of the bottom floors. It’s a small difference—over the course of a lifetime, if you live or work on an upper floor, you might live a slightly shorter life, but just by a fraction of a fraction of a fraction of a second. But it is effectively a kind of time travel, in the sense that you’re moving through time at a different rate than someone else.
Professor, Physics, UC Santa Barbara
It’s surprisingly difficult to rule out time travel using our currently understood laws of physics. Classically, our best-understood description of time and space—absenting any complications from quantum mechanics—is Einstein’s theory of general relativity, and we have examples of solutions of Einstein’s equation in which you can actually travel back to your original point in time.
If you’re going to rule out time machines, you’d probably need to include the effects of quantum mechanics, which makes things more difficult. I would say that most physicists today don’t believe time machines are possible. There have been lots of attempts to construct them, and they always have some sort of instability, and they tend not to survive—they’re not valid constructions. But we don’t have a concrete proof yet. So it’s possible.
This of course wouldn’t be a time machine that would allow you to travel back to the 18th century. When I talk about building a time machine, I mean a machine that allows you to return to some point earlier than the present but after you turned your time machine on. In physics, if you’re going to build something it can only affect things in the future.
Senior Researcher, Physics, University of Queensland
Time travel does work! This piece you are reading, for example, works as a time machine: by reading it, you will be transported to the future by some five minutes or so.
Okay, what you really want is to travel to the future faster, or perhaps even to the past. For the first case, the answer is a resounding yes! First thing, though, you should forget about the typical movie time travel, where someone or something suddenly disappears at some place and time, and reappears at a different place and time. This involves some type of teleportation, which is a whole different question. To travel to the future, all you need is to go on a very fast trip, or get very close to an object with large mass. According to the theory of relativity, time appears to slow down for objects that are fast moving or that are deep into a gravitational potential. If you were on such a trip, time would appear to run as usual for you and anybody around you. However, once you get back, you will find that more time has passed for everybody else. Perhaps the trip took one year from your perspective, but friends at home aged by 20 years. Effectively, you have travelled 19 years to the future.
Not only is this effect well- established, but it has been measured by sending atomic clocks on planes around the earth. The effect is significant enough that clocks on satellites have to take it into account, and things like GPS location on your phone would not work otherwise. So far, we are talking about microseconds; to get to anything interesting for us, we’d need to travel close to the speed of light. This means that the possibility of practical travel to the future is closely related to the possibility of interstellar travel.
But now for the real deal: how about traveling to the past? We don’t have a definite answer to this, but the theory of general relativity, Einstein’s theory of gravity, seems to leave some open possibility. According to the theory, space and time can bend so much that time can wrap back on itself. We don’t know if producing such a warped piece of spacetime is actually possible, but if you could get there, you could in principle make a round trip and end up not only at the starting place, but also at the starting time! You might worry that this could cause all sort of paradoxes: what if you go back and destroy your time machine before you start the trip? Then you cannot travel, so you cannot destroy the time machine. The question of whether paradoxes can be fully avoided is still open, but it turns out that, at least from the point of view of pure logic, paradox-free time travel is not only possible, but it imposes no restriction on the ability of the traveller to perform free actions. Simply, whatever you do, you will not be able to create a paradox: if you managed to travel back in time, it means you didn’t succeed in destroying the time machine after all.
You might also ask: if traveling back in time was possible, where are all the visitors from the future? In fact, what the theory suggests is that one could travel back at most to the time when the first time machine was built. So it seems we will never be able to travel to our past, before 2021. However, time travel could become reality one day, and perhaps we won’t have to worry about getting late to a meeting!
Assistant Professor, Applied Mathematics, Western University
In a sense, the possibility of time travel is uncontroversial and well-understood, by way of Einstein’s theory of spatial and general relativity.
To demonstrate: Take two identical clocks, keep one here on Earth, and send the other up to space with an astronaut. Put that astronaut in a spaceship traveling at the speed of light, or send the astronaut close to something with a very strong gravitational field, like Jupiter, or a black hole. To the astronaut, the clock won’t seem to be doing anything unusual—it will seem like it’s ticking at its normal rate. But when that astronaut gets back to Earth, they’ll find that their friends have aged much more than they have.
In this instance, it’s a matter of technology: If we were able to construct an appropriately powerful spacecraft, someone could theoretically have that experience. As it is, we still see this play out as part of our daily lives with GPS, which has to factor in the different rate at which clocks tick on Earth and in space. Without taking that difference into account, GPS wouldn’t work.
Of course, when people talk about time travel, they’re usually talking about traveling back into the past in order to change something, and I’m afraid my answer here is that this will never be possible. The examples above would not change time directionality; instead, it would break it. By time directionality, I mean a thermodynamic property: things go from being hot to cold, from being ordered to disordered, from some unstable position to a stable one. Of course, I decide to clean the dishes, going from disorder to order, but in doing so I have to burn some of my energy: the dishes could be cleaner but I would be sweating, and in creating in every case more disorder in the universe at some other level. So, yes, maybe there could be a way to bring things back as they were—but at what cost?
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