Is Time Travel Impossible?


If time travel turns out to be possible, I
hereby invite future time travelers to join me on set I’ll post the address one year from today. Okay. Right now. Well that’s a bummer. Time travel stories are cool because both
the past and future are somehow more interesting than the present and because everyone secretly wants a do-over. But so far it appears we are doomed to live
consumed by regret in the eternal, boring present. Time marches on, inexorably and only forward. Or so we thought until Einstein came along. His special and general theories of relativity
changed the way we think about time forever, and believe it or not, their raw equations
permit time travel. They even tell us how to do it. So let’s review the possibilities, and decide
how possible time travel really is. The first approach to time travel uses only
special relativity, which describes how intervals of time and space are stretched or contracted
depending on relative speeds. A fast-moving spaceship appears to experience
a slower rate of time compared to someone waiting back on Earth. Do a trip around the galaxy at close to the
speed of light and very little time might pass from the perspective of the traveler. But they’ll find a minimum of hundreds of
thousands of years have passed when they get back to Earth. However, that’s a one-way trip in time, and is really
just traveling in the same temporal direction at different rates. So the original Planet of the Apes style time
travel is possible. But it’d be nicer to be able to go back
in time. Actually, the math does sort of allow that. The spaceship’s clock slows down as it speeds
up, and it stops completely at the speed of light. And at faster speeds, time should actually
tick backwards. So if you could travel faster than light you
could navigate a path to a point in spacetime before you departed. We saw how to Iconstruct such a path in a
previous challenge episode. Of course we know that the laws of physics
forbid faster than light travel. Or do they? In order for any object with regular mass
to even reach light speed it would need infinite energy – which can never be obtained. But notice I said “regular mass.” We can hack the equations of special relativity
by allowing mass to take values in the weird realm of imaginary or complex numbers. An object with imaginary mass is now restricted
to only traveling FASTER than light, never slower. That means it can only travel backwards in
time, not forwards. We call a particle with imaginary mass a tachyon. If we could control tachyons then perhaps
at least we could send information back in time. But do they exist? Does imaginary mass exist? This is an example where the equations
of a theory technically allow something to be true, but there’s still no good reason
to believe that it is. We’ve seen no evidence of tachyons, and common sense tells us we probably never will. So special relativity isn’t much help. Fortunately we still have the general theory
of relativity, which incorporates special relativity, but also explains the force of
gravity as a result of curvature in the fabric of spacetime due to the presence of mass and
energy. But GR describes a warping of space AND time. So maybe we can warp them enough to take us
back to our own past. The best-known approach is through something
called a wormhole. A wormhole is a particularly bizarre hypothetical
consequence of general relativity. Now, if space can be warped, then perhaps it can
be stretched in such a way as to create a tunnel between two points – and one whose
internal distances could be very short, even if the mouths of the tunnel are far apart. This has the obvious benefit of allowing you
to teleport between distant points in space, but also between distant points in time. This is how you do it: take one stable wormhole
large enough to be traversed. Accelerate one end to close to the speed of
light or drop it into a deep gravity well – its rate of time flow will slow relative
to the other end of the hole. Now bring the two ends back together. They will be offset in time: one portal permanently
stuck in the past of the other by some set interval. Travel through the “future” end
and you’ll exit in the “past.” So this all sounds straightforward enough. But can wormholes even exist? There are a number of ways they might – from
connections between universes in the interiors of black holes to miniscule wormholes appearing
and vanishing on the tiniest scales of space and time. Now, these deserve their own episode. But for now, to build a useful time machine a wormhole has to be large enough to fit through and it has to be stable. The equations of GR do permit large wormholes,
but they are definitely not stable. They collapses on themselves instantly, leaving
inescapable black holes. In order to keep our wormhole time machine
operational, its throat needs to be kept open. We need to counter gravity, and to do that
we need another probably-non-existent form of mass – negative mass – also referred
to as exotic matter. As far as we know, mass can only take on positive,
real values, so a requirement of negative mass seems a non-starter. However there may be hope. Really what we need to open the wormhole
is a negative energy density. Some have argued that we already produce this
in the Casimir effect, in which the energy of the vacuum is lowered between two nearby
conducting plates. However there’s no clear path to translating
this to a large-scale negative energy distribution that could keep a wormhole open. And in fact we’d need entire planets – perhaps entire stars converted to negative energy to do this. Some other time travel options also involve
using negative energy densities – for example the Alcubierre warp drive, which we already
covered. In short – if you have exotic matter you
can probably time travel. But is negative mass-energy as much of a non-starter
as imaginary mass? While the actual equations of general relativity
themselves don’t prohibit it, there are a set of secondary rules in GR that do. These are the so-called energy conditions. They’re a set of requirements that do things
like prevent negative energies and enforce energy conservation. But the energy conditions don’t have a really
fundamental basis, and they’re seen to be violated in some cases – like with the Casimir effect. We can’t completely rule out wormhole or
warp drive time machines based just on the energy conditions. And as it turns out, there may be other ways to build
time machines without either negative or imaginary masses. One example is the Tipler cylinder, conceived
by Frank Tipler based on a solution to the Einstein equations by Willem van Stockum. It’s simple: just build an infinitely long
cylinder of extreme density and set it rotating insanely quickly about its main axis. It will drag spacetime in its vicinity into
a sort of vortex. This generates sub-lightspeed paths through
spacetime that form closed loops, ending up back where they started in both space and
time. We call such paths closed time-like curves. A spaceship traveling along one of these curves
could return to a point in its own past. If you don’t have the budget for an infinitely
long cylinder, you could try building just a very, very long cylinder… and be horribly
disappointed. Stephen Hawking showed that unless the cylinder
is infinitely long this doesn’t work – unless you also modify the spacetime with negative energy. In which case you might as well just build
a wormhole. So it turns out that it’s not so hard to
find solutions in general relativity with closed timelike curves. Kurt Goedel, famous for his incompleteness
theorem, discovered one and he wasn’t even a physicist. His involved an entire universe, rotating
about a central axis and with matter and dark energy perfectly balancing it against collapse
or expansion. So to build this time machine we just need
construct an entire universe – which allows us to travel back in time only within that
universe. Thanks Goedel. Dragging the fabric of space in a circle
can give us our time-loops in very special, and frankly useless cases. Another one is the interior of a rotating
black hole – a so-called Kerr black hole. The maelstrom of spinning spacetime may generate
closed timelike curves deep down below the event horizon. So that’s fun: you can travel back to your
own past, but never to the time before you fell into the black hole, which is probably
the only thing you really want to do at that point. Unless it’s an Interstellar-style black
hole . . . general relativity doesn’t directly refute black hole time machine libraries. Yet. So it seems we have lots of ways to send things
back in time, but it all seem useless for actually making time machines for one reason or another. But the weird thing is that we don’t know
of one consistent, fundamental law in physics that prohibits true time travel. And yet most physicists still think it’s
impossible because time travel threatens the common-sense chain of cause and effect. It threatens causality. Break causality and you can create paradoxes–
time-travel to kill your grandfather and you would ever be born to time travel in the first
place. But there are no true paradoxes – only seeming
paradoxes that point to a gap in our understanding. Stephen Hawking put it nicely with his Chronology
Protection Conjecture. It states that the laws of physics will always
prevent time travel or allow it only when doesn’t cause paradoxes. In other words, the universe has to make sense,
time-travel or no. One way for a closed timelike curve to exist
without causing a paradox is expressed in the Novikov Self-Consistency Principle. Igor Novikov suggested that closed timelike
curves are fine as long as they’re self-consistent. As long as the backwards time-traveling configuration
of matter always leads to exactly to the same forward-traveling configuration. In other words, the loop creates itself. So I don’t know – you try to kill your
grandfather, only to become your own grandfather? Like Fry, let’s not think too hard about
that. An alternative lies in Hugh Everett’s many-worlds
interpretation of quantum mechanics, in which every possible universe exists, splitting
off in an infinite tree. So if you travel back in time and prevent
yourself being born – no problem – your photo doesn’t slowly fade away because you
were still born in that other timeline. Or time-travel could be genuinely impossible. Kip Thorn suggests there should probably be
one fundamental law of physics that prohibits it – for example, the quantum vacuum may
be unstable in the infinitely iterating loops of a closed timelike curves. On the other hand, Kip was the consultant
in Interstellar, so who knows what to believe? In actual fact we can’t know until we have
a full theory of quantum gravity – until then we’re working with the approximate
theories general relativity and quantum theory. Approximate theories can make bad predictions
– like the possibility of time travel. One final argument that time travel is impossible
is that we don’t see time travelers. Stephen Hawking put this to rigorous test
when he organized a cocktail party for time travelers, only advertising the event after
it ended. Tellingly, no one showed up. Though I don’t know – maybe there was a slightly
better party somewhere else in, like, all of history. For now we seem doomed to time travel only
forwards, and very slowly at that. We remain firmly in the grip of that one dimension
that we can never halt nor reverse it’s pace: time.

100 Replies to “Is Time Travel Impossible?”

  1. Marvel's time heist must be the best example yet for following the logic. Making up what could pass for negative energy sources or even mass reduction with ant man.

  2. Stupid question – but wouldn't delayed choice quantum eraser experiments be the best place to catch us some tachyons? 😉

  3. Time travel is impossible and so is space travel, unless you have space ship the size of a planet., accompanied by neighboring star as energy source.
    Lets test that: If there are any visitors from Cosmos, reveal yourselves now! 3…2…1…0. …
    No, nobody showed. Hence no time nor space travel.

  4. What if you travel back in time. Yet time is reversed for all your matter too. Its not a continuation of eventa from the travel start but a time loop resets all your memories and places you back exactly as you were. Oh. Would you even notice? And since the cells and atoms in your body have been replaced wouldnt that produce other difficulties?

  5. To me one of the biggest questions we'd need to answer to work out of it's actually practically possible and not just mathematically, is would we even recognize it if we observed it? What would a time traveling object look like from our perspective, and how obvious would that behavior actually be?

    The first thing I think of is the CP/T transformation discussion from back in the one-electron universe video and how you could hypothetically model anti-matter as time reversed matter. That sounds like tachyonic behavior, running its tape backwards in comparison to our normal matter. But we see that as a charge-parity flip instead, presumably because it seems to be self-consistent. Our future can observe its past as our worldlines travel in parallel, and any interaction between the two can be played out in either direction.

    Since none of its behavior changes continuity or automatically suggests that it propagates backwards, we think we see a particle that follows our arrow of time and just has fundamentally different properties.

    Hypothetical non-self-consistent interaction would probably be even harder to pin down for a lot of reasons though I'm less inclined to believe that for… also a lot of reasons.

    Presumably something that follows a complex path to arrive at an earlier point in spacetime would be much more obvious, but the point still stands. Stuck on our path, just how obvious would any of it be?

    Bonus challenge question, since we're all so fond of those: could you use closed timelike geometry inside the event horizon to keep a singularity from ever forming? Essentially shape the interior to keep all in-falling matter below the critical radius but leave it endlessly circling the drain?

  6. Well, does the 'Present' actually exist? As soon as you even start to think about it, the moment has already gone. The absolute shortest period of time is incredibly small – a Planck time interval of approximately 5.39 × 10 −44 s. Even shorter periods are probably possible, but meaningless for our existence. https://en.wikipedia.org/wiki/Planck_time

  7. Another paradox is time travellers are going either be: smart enough they know its wrong to change causal events, or that it cannot be proven easily by walking into some professors office saying 'hey look, i has future clothes' , or dumb enough that they actually made it clear so now reside in a psych ward on a chemical lobotomy that prevents the intellect required to remember how to travel or prove it. Along with that time jesus came back and was deemed highly psychotic.

  8. If we could go back in time than somebody would go back in time to invent to go back in time to be the inventor to go back in time so we would allready be able to go back in time.

  9. I bet a few time travelers showed up, but you guys are hiding it from us. You are part of a big time traveling conspiracy! 🙂

  10. Ok… this episode got a truly bizarre timing… At Sunday we played with friends the board game "TIME stories" (You are a "time agent" who protects timeline from falling apart by preventing other people's from meddling with the right order of events etc. BTW. if such agency ever existed, PBS or Hawking's time-guests would be in jail before(?) reaching either of the events) Anyway after we finished the game we discussed briefly if time traveling is possible at all, and boom several hours later PBS realeses episode: "Is Time Travel Impossible?"

    Very neat coincidence…

    or some time traveler read that comment, travelled back in time and rescheduled PBS's publication queue just to mess with people…

    but if so, that would mean we are stuck in endless timeloop…?

  11. I have a question about travelling backwards:
    I am a big 30 years old man and want to go back in time to visit my 5 years old self.
    At time t=1s the total mass and energy in the universe is x + mass of 5 y. o. kid.
    At time t=2s I join in the past and the total mass and energy is now x + mass of 5 yo and mass of 30 yo. Doesn't this mess with conservation of energy?
    If my mass was to be accounted for by just taking a chunk of moon out of existence, then wouldn't that be a different place, a different universe: not effectively my past.
    If the mass was to be given to the past by borrowing it from the future… This is weird and I have heard energy is coupled with something else in quantum mechanics but still, isn't the energy uncertainty too large when compared to the plack's constant? Correct me if I'm wrong I don't really know much about quantum mechanics.
    This was to ask if conservation of energy was the fundamental physical theorem that doesn't allow backwards time travel.

  12. Hey Matt so I was wondering if (theoretically of course) particles with real mass travel at less than the speed of light and particles with imaginary mass travel at more thanthe speed of light, then what would particles with complex mass do? Like say a ball of mass (3+2i) Kg which is neither purely real nor purely imaginary.

  13. I never quite got the bit in relativity that says if you go super fast in a spaceship time goes slower than the ppl back on the ground. I get that it happens, but not how the universe knows which one is which, because isnt it all supposed to be relative, like whats the difference between that and getting in a spaceship that doesnt even have any engines but then somehow the earth zooms away at 99% of c. somehow the universe knows which one is supposed to have slower time, even when which one is moving and how fast its going is a totally relative question.

  14. time is relative to graven image its is in our heads. it is a man made idea based on obeservation of change around us.

  15. I tend to think we are all time travelers. Take a long flight, well you are now slightly younger than someone theoretically born at the exact same time, but he was on a couch at home. That seems like time travel to me, even if the actual time difference is trivial.

  16. If you are interested in time travel, meet me at the train station of my hometown Amberg.
    Exactly at 2 PM , August 4th, 1999…..

  17. If you travelled back in time 2000 years ago you wouldnt be traveling in space simultaniously so you would wind up in space somewhere dead,as the Earth wasnt in that same place as you are when first leaving!

  18. Space-time is woven together in front of my face as much as it is in outer space right? It’s got to be… Just thinking

  19. In Interstellar, the singularity was created in the future, and it appears in the past, in order for the continuous process of reality to proceed. It was a loop, but just a one time process.

  20. Nah, here's how you do it. QM & Heisenberg tell us there are certain quantities which cannot be measured simultaneously. The more accurately you measure how fast something is going, the less accurately you can determine its position at any given time, for example.
    So, all you do is construct an apparatus that will quantum-teleport your payload to some extremely specific location. Since you know exactly where it's going, your uncertainty as to when it'll arrive is near-infinite! Lock down the coordinates hard enough and you can literally teleport something into last week. Or 40 million years ago… Or 40 million years from now. No way of knowing for sure of course but hey! At least you got your time travel.
    The biggest bug left to work out is that for larger objects like, say, a sodium atom or a lab monkey, the particles tend not to all wind up at the same "when" as each other, so things can get a little messy… Well, nobody ever said science was gonna be pretty.

  21. What if the future time travelers don't feel any need to prove something to us? Hence, they don't appear in those moments, we ask them to.
    Or maybe our history is so ancient and so lost to them, they don't even know about us.

  22. What happens when a black hole eats so much that it begins to rotate at near light speed. Can it continue to accelerate or will it be a mini bang (white hole)

  23. Time travel, like the god in the bible, is a science fiction fantasy that the human imagination clings to persistently because it's fascinating and cheats death. It's also impossible, so get over it and quit wasting your time.

  24. Unless time travel is only possible to a point where the time machine exists, or it could be illegal to change anything before the point at which time travel becomes possible.

  25. Will the passengers in the spaceship that moves at 99.99999999999…% of the Speed of Light will experience time dilation effects if they will move around the ship? Or not?

  26. I had a wild thought when i was stoned, hear me out… ghosts may be real or not, but if we are to travel back in time there would laws set in place to stop us from killing our grandfather, maybe we become ethereal if we time travel and thats what ghosts are. Batshit crazy i know but fun to think about nonetheless

  27. It's true, if I were choosing from all the parties in the history of time I probably wouldn't be sipping cocktails with Stephen Hawking either.

  28. Some human inventions, especially clocks can easily be fooled by nature. It's true that clocks can be dilated. Unfortunately you will need to dilate more than just the clock on your arm. You will need to place your own consciousness ahead of others. So what will happen if you travel close to the speed of light? The clock on your arm will slow down, but your consciousness won't. I'm willing to bet that while you're up there, you will realise just how slow your clock is ticking. When you finally decide to return, you will find that you are far behind on the clock, not because you travelled forward in time, but simply because the little thing on your arm got dilated. Just readjust it and you will be good to go. Another example could be the battery of your watch dying overnight. Your clock will come to a stand-still while the rest of the world moves on. Now when you wake up, you will find that you are suddenly far behind on the clock while the world has moved on. Again, this is not the result of time travel, but rather the result of something messing with your clock. Of course, your own consciousness will save the day by realising this mistake and all you will need to do is once again re-adjust.

  29. If time is infinite than the present equals both the past and the future in terms of volume.

    Also, if space is infinite than every point is at the center.

  30. When I move in space, I'm in my new location and no longer at my old location. When I travel through time, am I not at my new time and no longer at my old time? If everything and everyone around me is traveling through time at the same rate, would there be anything waiting for us if we traveled back through time. This is making my head hurt.

  31. Maybe too simple question for you, but I'l be really happy if someone can answer this. You keep saying in PBS series that if something moves with the speed of light, time stops for that object according to relativity. so, what about light itself? what's the clock for the light itself? I mean, when we light the bulb, if time stops for light, how come we can see it moving? analogy is that, imagine the event horizon of a supermassive blackhole, at the edge, objects reach speed of light and time stops for them, we always see them at the edge, right?

  32. How would you even know if you've detected a tachyon? How does one distinguish between a normal subluminal particle traveling forward in time and a superluminal particle traveling backward in time?

  33. Another possible explanation for not having seen any backwards-time travelers can simply be that you might need an anchor in spacetime to travel backwards to.

  34. They can't even come up with a cure for male pattern baldness…how are they going to get us traveling into the future.

  35. But is Causality defined? Stuff happens and causes other stuff to happen is Common Sense just like the World is flat and at the center of the Universe is Common Sense and wrong

  36. I don't believe that you could create a time machine by accelerating one end of a wormhole to relativistic speed or dropping it into a deep gravity well. Sure, any observer passing through the "slow" end of the wormhole would experience some time dilation on their way. But the time dilation would not create any growing time difference between the ends of the wormhole itself. The wormhole is not an object passing through space-time. It doesn't experience time at all. It's a piece of space-time itself.

  37. I have a question. In the first example, a spaceship is traveling away from a planet at a great speed relative to a planet and time passes more slowly for the ship. But why is the planet the point of reference? Can't we also say that the planet is traveling at a great speed away from the spaceship and therefore time passes more slowly on the planet relative to the ship? Or that the planet and the spaceship are traveling away from each other at the same speed? I don't understand why a particular point in space, for example the planet, is "special" compared to any other point in space.

  38. I think the fact that we don't see time travelers today is not good evidence that time travel is impossible. It seems more likely to be that either time travel is only possible back to the point of creation of the time traveling device, or that we as a species either kill ourselves off, or don't discover time travel until long after we've left and forgotten Earth.

  39. Is the Novikov Self-Consistency Principle the reason why Donnie Darko had to get rid of a jet engine from the tangent universe?

  40. civilization is annihilated, a sole survivor has a time machine and returns to avoid the event or events that caused the catastrophe, everyone's a winner? except, now all those souls have to reincarnate… bummer

  41. Yes, when u travel, it costs time. Time-travel. Only forward tho. Time does not exist in the universe, we have made it up to calculate things.

  42. Big problem: the planet spins at 900 mph on its surface. The planet orbits the sun at 67,000 mph. The sun orbits the galaxy around 500,000 mph. And the galaxy moves with universal expansion.

    If you time travel an hour into the past or future, you will appear in the vacuum of space and promptly freeze into an ice cube before decompression or asphyxiation even have a chance of killing you.

  43. https://www.projectpegasus.net/
    Is this real?
    If yes then why we don't even have a prototype of that time machine.
    I know it's not related to the physics of time travel but I found it interesting so I just wanted to see how people think about this viral conspiracy projects.

  44. I would love a description of an experiment that could be designed to actually detect Tachyons… it sounds rather silly to say definitely that it's impossible. Damn string theory henchmen are so boring

  45. I feel like we don't actually need a stable worm hole. It just needs to be meta-stable. In fact, if we want ones that can be turned off and on again, full stability would be a problem, right?
    Not sure that actually changes this argument though. Are there meta-stable wormhole equations that are more reasonable than stable ones?

  46. It's not true that you need infinite energy to speed an object to speed of light. You just need the same amount of antimatter. And there you go, you're travelling with the speed of light.Also, it's what GRBs do, right. Speed things up to the speed of light. The truth is we cannot speak of time travel, because we don't know what time is, neither what define us at that point. Because we don't know what "us' is at any point – is it our body, is it our brain, if you copy your brain is the other brain still "you"? That's well described in "Dark matter" by Blake Crouch. I recommend it. P.S. Honestly, we don't even have a well defined time-arrow, apart from the expansion of the universe of the direction in which entropy grows. But since the universe has pass trough an equilibrium state where entropy was maximal and THEN it continued to grow, good luck with that definition.

  47. If time travellers showed up in a video explaining how we don't know how to time travel, it would have been a paradox, therefore they couldn't show up, duh

  48. The laws of physics in this universe allow time travel using infinitely long cosmic strings, wormholes, or black holes…but our bodies and thus our minds will not stand the test of time (pun intended). 💕 ☮ 🌎 🌌

  49. What about the "too perfect" fallacy? What if the reason we can't prove that time travellers exist, is because they do? Let's say that time travel does exist and humanoid beings are able to hop to and fro in time. But are only able to do so without ever interacting with subjective timeframe that they are in or leaving any evidence that they were there? Think of the movie "The Sound of Thunder" where they can go back in time to hunt a big bad dinosaur, just as long as they follow a few simple rules. and those rules prevent them from affecting the time that they're in or the future that they came from.

    My opinion is that time travellers do exist. But they can't stay in the past or leave any evidence that proves it right. Maybe because we destroy ourselves with it or somehow create a true utopia where everyone is content and the human race lives on.

  50. I always wondered, this will be a tad offtopic, sorry in advance. So space-time could stretch at higher speed than speed of light, right? Now, if we could only create such event that would inflate outwards at higher speed than light, and throw this 'seed' into a black hole. I would wonder what could happen, do inflation wins and bursts from black hole, or do black hole is just a construct that forbids anything complex to stay in? I know that this kind of 'seed' we would use would just expand space-time and nothing more, but if there were 2 points, one in some place then the seed between two of them and then the next point. Could space stretch enough to pull away these points faster than speed of light? And if yes, could it pull something from black hole this way?

    I understand that it sounds silly, as we have space-time already and somehow creating a machine that would inflate the already existing field doesn't exist, but I treat it as a thought experiment. What would happen if space-time would inflate faster than speed of light inside a black hole?

  51. It's quite a stretch imo, to say that "it's technically possible if imaginary mass exists". I mean negative mass itself doesn't make any sense and now we're talking about imaginary mass? I'm not against conjecture, but at this point it's just math. It's just too theoretical to be called "technically possible"

  52. time is always relative right? so there are methods to hack the speed of light speed limit. what if 2 ships left earth at 75% of the speed of light in opposite directions. their relative movement would be 150% of the speed of light. then after traveling for the same distance each they both turned around and came back at the same speed. according to each ship the other ship would appear to travel back in time right? and according to earth each ship just appears to slow down time at an equal rate.

    logic problems like this are why i can't make sense of relativity. according to my understanding of relativity, in this scenario there are 3 different answers and they are all true at the same time. is this where the many worlds theory takes over and each case is split off in it's own reality where it's specific observations are true?

  53. What if matter is slowly shrinking, and galactic expansion is really matter slowly losing mass and a time traveller could be a different size!. Universal expansion could be the rate of loss of mass.

  54. The Answer is Yes, We can travel in time. Relativity allows it. The real Question is, Can we build a Time Travel Machine?

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