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. Negative Mass is asentaly the flow of mass. He seems like form of negative mass Electrons on Electrons produce negative mass

  2. With backward time travel you could create a machine of infinite power by repeatedly sending energy back in time to the machine, in an exponential perpetual loop. So that violates the laws of thermodynamics and I guess potentially destroy the Universe?

  3. Wouldn't time travel be a kind of pocket or alternate universe though? What would seem like time travel to the traveler, would really be more like a new universe that forked off of the one the traveler left at a specific time.

    Consider if a time traveler did appear at PBS Studios and give an interview. Say they are someone from 300 years into the future. To us, in our own universe, that person shouldn't exist. They would come from a time and a place that does not exist in our universe and the very act of them coming to our universe, would prohibit our universe from ever getting to that point because it would change it. If somebody or some object were to suddenly appear out of nowhere in our own universe, we would say it came from a parallel universe or multiverse. We wouldn't think of it as an object of our universe but at the wrong time.

  4. If you have a flux capacitor you can and if you use a fuel that is scooped up from dark matter you dont have to carry your fuel….

  5. Time-Life paradox (a.k.a. Hawkins-Fermi): 1. No time-travelers implies no time travel; 2. No aliens implies we're alone. Discuss.

  6. The Vulcan science academy had proven that time travel is not possible.🤣🤣🤣

    Also btw how arrogant are humans to believe that it would be wise or even abusable to visit people in the past. I would assume that in the future people will figure this technology out, but it will have to be closely monitored to make sure time travellers do not alter the time line so that tech will also be developed.

  7. I've had dreams about time travel. I've had very lucid dreams where I could control time at will. I wish those dreams were real. So we can travel back in time, in our dreams apparently. Too bad traveling backwards in time (in our dreams) doesn't actually change the past. I hate this fact. Good video.

  8. I was writing a comment about how this is the first video where you dint end with "spacetime", but I went back in time and changed it so you did. 😉

  9. I was thinking about this, since your speed affect your own time and not the time around you, shouldn't going faster then light just reverse your own time? like you'll start regressing and actually become younger and "forget/lose" anything that happened to you in the time that you lost. What i mean is, if you go faster than light so that you go back one years in time, you actually lost a year and everything that happened to you in that year because for you, it never happened. But, since it's only your own time that went back, the universe would have still be going forward in time like normal. If it toke you one week of travel at that speed to regress one year, it mean that for you, the universe is one year and one week older than you know when you return to normal speed.
    And there's an other thought that i have. If time is the 4th dimension, it means that there is spatial coordinates associated to it, and moving in time is having a speed in the time axis. So then, every point in space have 4 coordinates (x,y,z,t). Also, we can see the universe as a volume moving at a fixed rate in time, changing its coordinate "t" at a fixed rate. And to help visualize that, you can subtract one dimension and imagine a plane (2D) moving at a fixed speed in a direction perpendicular to its surface (3rd D). If the universe is the plane here then time is the 3rd D. Now imagine leaping trough time, in the past or the future. It would mean changing your "t" coordinate compared to the rest of the universe. In that case, wouldn't you end up in a space outside of the universe? A space were the universe isn't anymore or is not yet there? But, i know there's a lot of "yes but…" you can say to what i just said and it's for theses reasons that i don't believe that time is a dimension. And i also believe that there's no past or future, only the present exist in it's infinitely evolving way (I’m too lazy to wright why tho).

  10. What does "large enough to fit through" for a wormhole mean, though? The important thing to get through your time machine is information; pulses of light are enough, or even some method of getting the wormhole itself to pulse or ring on the other end. I don't need to fit my spaceship through; I can just have somebody build me a body on the other end and download my consciousness one bit at a time.

  11. If you told me that this was actually a channel that tries to make the Star Trek technobabble sound real, I'd believe it.

  12. Would you really refer to that as imaginary mass? I‘ve heard mathematicians say that “imaginary” numbers are poorly named and part of the Fundamental Theory of Algebra.

  13. Instead of a tipler cylinder why not a cylindrical ring? Seems like it would solve the infinite length problem.

  14. Well mass can't go back in time but information sure can, in fact it's an irrefutable consequence of Maxwells equation that radio waves have a component that travels backwards in time. It should therefore be possible to decode these. Say for example you knew the lottery numbers would be transmitted on a certain radio station next Saturday and you knew the frequency of the carrier wave coming back at you in time – it should be possible to construct a device to anticipate the wave and decode the extra information in it (as what will be an incredibly faint echo) This will be easiler still if you're only looking out for simple repeating patterns of limited information like lottery numbers. If you get rich on this idea, please sub me as I'm a bit short this month!! My Bitcoin wallet is 1Kwq9wKWpGib8oAaKGLRjNUo1QJKbwChht

  15. Is time travel possible?


    Time is a construct of man. You would have to expand and contract space at whim while the traveller would stay still.

  16. Time travel will never become possible for us. We know this because no one ever came back to kill Hitler at birth. No one stopped Freddie Mercury from getting AIDS. And no one came back to sterilize Donald Trump’s mother.

    So even if it’s possible in some tortured theoretical equation, it doesn’t have any bearing on humanity.

  17. So if someone slows down time by moving very fast and then returns to earth, isn't the rest of the world technically going back in time when they meet him again?

  18. The many worlds interpretation of quantum mechanics is most very likely correct. There are ways of showing evidence of the theory except the meathod of evidence gathering is also questioned. That meathod being CRV. Controlled remote viewing. Dr. Courtney Brown wrote a paper in the Journal for scientific exploration and its overview is on youtube. First you must understand remote viewing and how it is consistant and reliable under very specific circumstances. Then you can understand how Dr. Brown was able to show very compelling evidence of the many worlds interpretation of quantum mechanics.

  19. Aliens haven't shown up, ergo no aliens. Time travellers don't show up when summoned, ergo no time travel. 🤔 Maybe ignorance is bliss, but it certainly isn't probative.

  20. I dont see why anyone even wastes their time trying to solve problems like time travel. Just ask the AI in like 10 years man, save yourself the trouble.

  21. If multiverse is "real", and works by, each reality, will at all times branch in to all possible realities; and also time travel backwards in time is "real", then we would be overrun by time travelers from all sorts of branches, constantly, endlessly.

    That's a lot of if's though.

  22. What if there has been time travel, but the universe played its cards in a way it would not mess with history or the outcome event that was meant to happen. That probably doesnt make sense im like half asleep right now.

  23. No it's not and never will be… You'd be stuck in an infinite loop of the time you did "time travel" Or if you DID time travel, now you were never THERE to time travel… So no…

  24. I know it's a fictional novel, but Michael Crichton wrote a book years ago called, Timeline, in it, it explained how time travel was possible through the use of Quantum Physics and Mechanics by traveling to another multiverse. Which would be like ours but a different timeline to ours.

  25. time travelling is real. we're travelling 60 seconds a minute into the future, and whem we think about the past, we are travelling back, mentally

  26. It appears any solution that would allow for backwards time-travel will also solve the ability to travel large distances at near-light speed or faster than light speed. However, it should be noted that there is no reason to believe that backwards time-travel is gravitationally locked to any one planet. Either build a spaceship time machine, or your might find your self reappearing millions of miles away from the earth in the vacuum of space.

  27. I always figured that if it was possible the moment the person appeared in the past it would instantly become an alternate reality. So even if you killed your childhood self it would have nothing to do with your reality. Also going back home would be impossible since the future would be the alternate one. There’s no way to prove it since from the original realities perspective you just disappeared.

  28. If an object goes through a worm hole, what would observers see on either side? Does conservation of energy and momentum (linear and angular) still hold?

  29. As depicted in your graphics negative mass is just a a mass on the other side of the 2D space-time mesh! And ofcourse a 2D mesh actually is 3D as it has two side and two axies.

  30. Hey I went to the address you posted a year from now but you didn't post when you filmed today's ep so the studio was closed when I got there.

  31. If putting metal plates very close together, as in the Casimir effect, can create negative energy densities, then, with nanotechnolgy we could build a ton of thin metal layers and keep them nanometers apart, creating a sort of "negative energy battery" and then… I don't know, coold science stuff I guess

  32. If there were better parties in history, you could still go to them all. Hawking's party would simply be the last one you attend!

  33. dont forget you have to actually send the invite one year from the day, not just hypothetically send it, so dont forget to send it

  34. You forgot one simple method: rearranging all the particles in the universe to a specific arrangement of position, energy, and momentum in space. This would only require a second universe with lower entropy to work from. Haha!

  35. What if the solution to the time travel paradoxes is that the paradoxes aren't actually real and we only think they are cause otherwise nothing would make sense?

  36. But wouldnt timetravel to the past be self contradictory? In the sense that at some point you would have to be "not going" to the past and "going" to the past at the same instant? Doesnt seem like there is some weird limbo between -1 and 0. Because in math you either reach 0 or your dont, if you dont reach it then you dont reach it. Idk , if this makes any sense Ive just thought it from the top of my head.

    Didnt understand anything from the episode, lol

  37. Wouldn't it violate Hindenburg's Uncertainty Principle? You measure an object's position, then time travel and find its speed, then time travel again and do "things" to take advantage knowing both with more precision than Hindenburg's principle would allow. How would you deal with conservation of energy as well? Couldn't you measure the absolute speed of the universe with time travel? How would tachyons and exotic matter behave when it comes into contact with regular matter? There's only a couple of ways I could see a way to do it, but I don't know if I would classify them as true time traveling. The easiest would be if we lived in a virtual world. If you walked through a time travel portal, all other observers would see is you getting annihilated, but you get to go to the requested point in time and I guess it would make two different "timelines." But, that's no fun. So, a better one would be a device that turns regular matter into tachyons. But it would still be weird as either the tachyon particles could interact with what they were made with and you would need another device to turn the tachyons back into regular matter. But, I don't think that tachyons are very plausible as what would observers see when a tachyon is made or before it's made. Since the particle is traveling back in time, the moment you "made" a tachyon, you would stop observing it, so then you would realize what you thought of as regular matter was actually a tachyon. Which begs the question is the destruction of matter, really just the birth of tachyon matter? I think the most plausible solution is localized time travel where time isn't moving backwards but you're just forcing objects in a certain kind space to rewind. As long as you're not in it, you could effectively time travel. But I have no idea on how to get all particles to reverse themselves nor what you observe.

  38. Or what if we build a space ship and decelerate it to 0 velocity over 0 inertia in space time and just allow the universe to expand away from us at FTL " faster then light" speeds while we use no energy. ?

  39. I am sure time travel exists but humans won't be around to discover it hence inviting humans to a dinner party goes unheard.

  40. One interesting idea to consider — I think you've yourself thrown it around at one point — is that unless there are natural wormholes, wormhole time travel back in time can only be possible up to the point in time when the first stable wormhole is created, not earlier. So one might argue that even if time travel is possible in principle, it's not possible yet — in the sense that nothing could possibly time travel to current point in time, even if time travel is achieved at some point in the future.

  41. Question: GR say future and the past already exists. When traveling forward in time we say our travel depends on quatum psysics (and that's why we can't 100 % predict the future). Why aren't we talking about the same thing traveling backwards in time? Can we be sure that events that I remember took me to this moment is the one I would see 100 % of the times I traveled backwards in time? Isn't there a small chance that (due to quatum psysics) that another chain of events could have taking me to this place in spacetime and therefore we can not be sure that we visits our "remembered" past if we could travel backwards? It feels to me like it would work the same way forward as backward.

  42. A Delorean and the year 1985. Only space time would throw that reference out there knowing their veiwers would get the reference. Big time

  43. I'll be honest, I only understood planet of the apes, once I watched a YT video explaining it. IDK if simply missed the time travel part, forgot it, or it wasn't even in the movie, but it was mindblowing, all that virus and time stuff.

  44. I bet it is possible but you can't change anything because chronically you already failed at stopping or starting whatever. For example I try to prevent Abraham Lincoln from being killed, something happens that makes it impossible for me to save him.

  45. There's a flaw in Steven Hawking's experiment (the cocktail party for time travellers that he advertised after it happened)? Is there not??.. because he advertised it after it happened, then even if time travellers did return to the party, Hawking himself would already be in a later part of time and so wouldn't see it, it would be like he "drove past" the party on the road – it may have happened but he missed it. Likewise in the world where the time traveller's did visit, THAT Steven Hawking would meet the party-goers BEFORE he sent the invites out, meaning that from his point of view, he must have accidentally let-slow his plans to host the party in advance, if the universe corrects itself that is. That means that in any possible universe, either he sends invites out after the party which was already unattended, or he meets party-goers in a world where the invites are indeed sent out by that time. To put it another way, in order for his experiment to make sense, it must be possible to (A) host a party without inviting anyone or letting the party information ripple out into the world until after it happened, but (B) the observer must be able to know that people at the party were in fact people that only got information about the party after it happened. But if (A) and (B) are true, then consider the situation where Hawking hosts the party, meets time-travelling party-goers, but then later does NOT invite people, maybe he changes his mind or dies or something without telling anyone his plans. That would create not just a time-paradox but a LOGICAL paradox whereby people both were and were not invited to a party. It's usually considered bad if not impossible to create time-paradoxes (ie cause-effects patterns in the wrong order) but it's always considered unacceptable to create logical paradoxes. It's like how no scientist will entertain the possibility of a world where 2+2=6 or worlds with married bachelors. Since all theories use maths and logic either directly or at least implicity (otherwise, for example, the scientific theory might be true and false, which is something we aren't willing to accept) we can't even design an experiment that can only refute the null-hypothesis by breaking the laws of logic. Experiments can and should be designed to break the (current best-guess) laws of PHYSICS, but never the laws of logic.

  46. The conservator of the data can invent a parallel world that allows time travel. If you do not have the time to do things right, you just have time to do them over. So, a sentient mind allows "Flashbacks". If you awaken in your own dreams , vividly, what then is the difference ? You continue to exist in your dream, until or unless something else wakes you up or strikes you dead.

  47. The direction of time seems to be a matter of entropy anyways. "Time travel" might not even be recognizable as "travel", were we to invent it.

  48. One horrible reason for why time travel could be possible even though we don’t see time travelers:
    Humanity ends itself before we invent time machines.
    I hope everyone has a chipper Monday! 😅

  49. I wish so bad I could jump onto a spaceship at speed and fast forward the world a few thousand years to see how it all turns out. So far it isn't looking so good…

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