Is Time Travel, Especially Into the Past Possible? | Matt Szydagis | TEDxSchenectady

so time travel has really captures our imagination I have some examples here on my title slide we've got the poster from Back to the Future this interesting painting by Salvador Dali and Marcel Duchamp and of course there's a Star Trek in countless science fiction media now I always in all my talks on time travel point out this quote or something like it this is from Simon Newcomb aerial flight is one of the classic problems with which men will never have to cope I didn't include it in my talk but the New York Times in July of 1969 you'll probably figure out why that's a significant date had to retract a statement they wrote in 1920 saying that a rocket will never leave the Earth's atmosphere because it has nothing to push against oops so my point here is is that even though as we'll see today that even if time travel is most likely impossible due to our best understanding of physics our best understanding has has been wrong so it's not just that a non-scientist writing article in The New York Times for example has been wrong but also experts have been wrong on the feasibility of air travel and the feasibility of space travel and I could have an entire talk on quotes of of famous astronomers saying how space travel was nonsense and will never and will never leave the Earth's gravity so no a talk on time travel really needs to start with Einstein's theory of special relativity so Einstein solved of the many epiphanies he had and many problems he was able to solve he saw the the an issue between classical physics of mechanics and electromagnetism and he was able to we don't have time of course brief time together today to go through all the details but he was able to solve and reconcile some contradictory models and parts of physics by postulating that the speed of light is constant in all frames of reference and we especially in science fiction like to think of that as a limitation all speed of light is not that fast when you think of intergalactic skills but is it results in some incredibly interesting and important mathematical consequences and one of them is the twin paradox which isn't really a paradox oh it's a terrible name for it because it's a real effect and in fact even though I'm illustrating with a cartoon with interstellar travel that we're not capable of yet as a species on the on this on small scales especially at the subatomic level we know that the the twin effect is I prefer to call it is real so the so Harold Lyons who's the father of sherry lines who's one of our organizers today actually invented the first atomic clock you can see post an atomic clock there and a photo of Harold Lyons over there was a the atomic clock which is our most precise timekeeping device was able to demonstrate that what I'm about to tell you is an actual real fact so the classic example of the twin paradox is that you have you have two twins and one takes an interstellar journey to some distant star planet closest speed of light turns around and comes back and the the the twin who took the journey it is one that when he returns earth is significantly younger than his brother or sister and this is this is a real effect that we cannot do on these large scales yet but the thanks to atomic clocks and very fast planes we can show that indeed this is a real effect and that time is plastic we talked a lot about we talked about neuroplasticity today we all have this intuition that time is absolute but that is shattered by Einstein in relativity time is not absolute and your time and my time do not floo at the exact same rate it's not like the universe has a universal timepiece and every star in Planet has a clock ticking at some at the same rate so if we take this now and so if this is a question now of Engineering and Technology not of physics so this is if we had the fuel probably something better than fossil fuels to do interstellar travel this is guaranteed to work but now this is maybe not as satisfying this is time travel to the future one way because your your your your contemporary friends or your this you may outlive your descendants by taking an interstellar trip coming back to earth but that's a one-way trip to the future now let's try to if we try to do some mathematical extrapolations in relativity to see well what can we do to get to the past so if we mathematically extrapolate that as you approach the speed of light we know that time slows down in some sort of it's an ill-defined sense but at the speed of light time stops then if we extrapolate that ah faster than the speed of light should mean time goes in Reverse unfortunately though you would meet an infinite amount of energy to push anything with nonzero mass to the speed of light itself let alone to exceed it so that doesn't seem to be something practical or realistic in any way so now let's expand though from special relativity to Einstein's other great epiphany which was general relativity where he essentially rewrote Newton's theory of gravity to have a just Co come up with a completely different theory of gravity that explains gravity in a different way through gravity being the geometry of space-time and not just some mysterious force and recently this year's you know Nobel Prize in Physics was awarded for the discovery of gravitational waves which is an astounding additional upon other existing mountains of evidence of the of the predictive power of general relativity and how it accurately explains the world around us if we look at general relativity you can come up with something known as a tippler cylinder so this is another attempt at trying to get some something that gets us time travel to the past and the idea here is is you have a very dense material and that if it's rotating fast enough and the materials dense enough this twist spacetime in a way that traveling around the cylinder in space should get get you back in time you can think of this a good analogy is a spiral staircase so we're all taught that if you if you walk around 360 degrees you return to the same point try that on a spiral staircase you do not return to the same point the idea here is is that with a spinning cylinder with you can set up the right space-time geometry that gets space-time twisting such that if you execute 360 degrees spatially you return to a different point in time including the past the problem with this is is well how would you get a material that's dense enough and one of the main issues pointed out by Stephen Hawking is that for any of these types of ideas you would need either an infinitely long cylinder or you'd need something called negative energy basically matter that would have a would have negative mass instead of positive which I'll come back to later so it doesn't look like warping space-time is gonna work we have these infeasible obstacles infinite length how you gonna make something infinitely long although I think the size of the universe would would work too so if the universe isn't infinite you can get around this what if instead of trying to twist space-time you try to punch a hole through it so this is the very classic here's a picture of the the idea of a wormhole where you can punch a hole through space-time and you get from one distant location to another or even another universe or hopefully even better to a point in the past or future and the this is just a car tune the class example of this is you take a sheet of paper or you poke a hole through it to point out how what if you can have a shortcut through space-time I should point out this is only a cartoons and so we have three spatial dimensions they got this right in the movie interstellar a wormhole would actually be a sphere not a 2d hole like we're used to in in the book so this is just an analogy because I can't give you a four dimensional projection and PowerPoint that that'll be in a future version so and there's a the problem with wormholes this is a great idea and you can write down the equations for this in general relativity and as I said before we believe in relativity we discovered gravitational waves we saw war this week and your your GPS needs relativistic Corrections we're both special and general relativity as we communicate with satellites they're traveling fast and are far from the US gravitational field but these the equations of general relativity tell us that unfortunately a wormhole would be unstable even a laser pointer even light attempting to traverse the wormhole would collapse it you need something with negative mass or at least negative mass squared to go through what just knows the the tachyon particle the hypothetical particle that's always traveling faster than the speed of light and so this again runs into this issue of negative mass seems to be a running theme of something we need for time travel and I'm going to now explain why that appears to be the case so this is a classic a cartoon that's used to illustrate it with a 2d analogy of our of our three dimensions of space what the how general relativity describes gravity in the universe so bodies with mass the earth the Sun will make dents in space-time the more massive an object is though the more significant the dent so our the Earth orbits the Sun not because there's this as there's this mysterious action at a distance that Isaac Newton would call it but because there's this invisible manifold of space-time that's being warped and if you have enough a velocity you all around something instead of into it but if we keep extrapolating and extrapolating we get from the earth to the Sun to entire clusters of galaxies which are have enough gravitational field to actually bend light the Sun does is well of course and even the earth emits a much smaller effect but if we keep going to black holes this is a picture from the movie interstellar then if we keep extrapolating in the classical theory of general relativity there comes a point where at me at the event horizon of a black hole the point of no-return beyond which even light can't escape if you if you throw something into a black hole to an external observer it would be as if time had stopped so the extrapolation here similarly to the extrapolation with speed as you approach the speed of light time slows down and stops if we do this with gravity in general and says special relativity as gravity goes to infinity time slows down to the point of stopping now again though time is relative if you're falling into the black hole you don't freeze at the border you fall in and die okay so this is the external observer so some day I think it will be a lot of fun to just park next to a black hole and throw stuff at it and create this ring of frozen objects that appear frozen in time again though from the objects perspective it falls in and an is crushed in the singularity most likely according to our current best understanding now let's let's look at a more recent so we've we talked about the Tipler cylinder and the and the twin paradox these are all go back decades I want to talk about a more recent thinking on this topic going in a different direction although still following one of the earlier themes of rotation one was negative energy and the other was in each force somehow twisting space-time and so a colleague of mine professor Ron mallet at University of Connecticut stores whom I met when I was giving a colloquy at at University of Connecticut on my research into on my research into Dark Matter he's actually the only real-life experimental physicist who's not a crackpot real tenured professor although my um my colleagues always say being a professor of patient he does not guarantee you're not a crackpot but he's he's a real deal and he's got a new idea that he came up with only I believe it was a decade ago now to to create a a time to create an effect by which you could travel to the past using he has a completely new idea of how to attack this problem this is a photo of him with one of his prototypes is not photoshopped it's a real device it doesn't take you to the past yet but but this is a real device that I might explain to you how this works the idea of dr. mallets idea is that you produce a cylinder of laser light and the you use mirrors or something like that you know that create a circulating laser so that's confined to to a spiral essentially along a cylinder and the idea is is that if the laser is strong enough you should be able to send something in on one side send a particle in on one side and emerges on the other side the cylinder in the past because if the laser light is strong enough it's like it's like this animation here of swirling a cup of coffee space and time itself is rotated and on a small scale this also just like the twin effect and the gravitational slowing of time this is a real effect we've that we've measured we've measured it's called frame dragging from the earth this is just extrapolating this on steroids now it remains to be seen does this really they're a document of course about arguing with is discussing with his colleagues would it need to be infinitely long to work does this still need does it need negative energy so the last topic I'm going to hit so these are all the examples of ideas of what we can do and a lot of these are from relativity but let's think also about quantum mechanics which is so relativity tells us about gravity and about speed with the speed of light but there's also a whole other area of physics explains the world of the very small that explains particle physics and that's quantum mechanics in quantum mechanics we can try to attack the question of whether time travel is possible looking at particles proton and electron or a photon traveling what if it interacts with itself in the past what does that even mean if it prevents its own trajectory through a wormhole for example and that's a variation on things like the grandfather a paradox about breaking it down into into subatomic particles so time travel if it is allowed we would need to under we need a full fury of physics combining our understanding of quantum mechanics and relativity to get a full answer as to whether it's possible or not right now we don't have we we don't have one simple set of equations we can write down that explains the entire universe including gravity and particle physics all at once there are if time travel is possible I can't give a talk on on that topic without mentioning paradoxes my favorite though is I think even more complicated than you know killing one of your ancestors or a particle bumping into itself in the past but there's the ontological paradox so here we see Calvin trying to steal his homework from himself so he doesn't have to do it so if time travel is possible it leads to this serious issue of information without a source which in quantum mechanics is a very serious problem we how can we if if time travel is real forget about the paradoxes of you know killing you're of killing one of your ancestors either by accident or on purpose but because what happens if you have objects or ideas without an origin that are essentially closed loops so this is very very very very bizarre so one solution by a physicist David Deutsch is that you've probably heard of the solution of parallel universes oh it's no time there are no paradoxes you just make another timeline another explanation is predestination if you try to assassinate Hitler your gun will always Jam personally I think that's a lousy explanation it's not intellectually satisfying then there's a third explanation that's not explored very often not even in fiction because it's so bizarre and that is the idea that if you did if you did try to create a paradox the universe becomes a mobius strip and you have two timelines existing simultaneously and so if you go back in time and kill your grandfather or your mother you become you're both dead and alive at like Schrodinger's cat you're 50% dead 50% alive what does that even mean though and that's that's a difficult question since quantum mechanics is about particles we are collections of vast numbers of particles tend to a very large number and so it's not clear what the what these what the eerie quantum effects mean for large collections of atoms like ourselves so all of these ideas parallel universes and and this predestination paradox are explored in fiction all the time we've got the flash Twilight Zone Star Trek things like that every so often we have supposedly real-life evidence of time travel that's always really easy to debunk though such as the infamous cell phone in the Charlie Chaplin movie the circus which was very easily explained away as a as an earpiece for for the hearing-impaired so these are usually pretty easy to debunk we don't have any really robust evidence that there are time travelers among us except of course there's the composer Gustav Mahler who looks exactly like me so I actually in fact I filled my wife with that one on Facebook she said how did you get a daguerreotype photo of yourself that's amazing that was it That's not me all right so tourists from the future though are always brought up by Stephen Hawking and Neil deGrasse Tyson who I hope see this on youtube I hope I can't to stare attention because I the humble professor from UAlbany I'm gonna tell them that they're wrong about something very important so I'm gonna boldly declare that time travel is not disproven by a lack of tourists in the future and the reason is easy and in fact since I'm not expert generativity they are they should know because in general relativity it's very easy to show that for example wormholes this is a classic example this you can't even if time travel is real you can't go back before you made the machine so if I'd use a wormhole for example time travel the day that I punch the wormhole that's the first day I can go back to from the future to that day I can't go back further than the past then the moment I created the machine or phenomenon there are exceptions of course what if there's naturally occurring wormholes that have been around since the big since the beginning of time things like that but in general I'm not worried about the lack of tourists from the future because it just means time travel hasn't been invented yet so I don't see a problem with that so in conclusion the problems we have with time travel based on our current understanding of physics are unfortunately numerous you need to go faster than light build a device of infinite length use negative energy or infinite energy you would need something gravitationally gravitationally repulsive if we extrapolate as I was talking about how increasing gravity slows time down and gets you to the future to get to the past you'd need something that we we don't know exists and we don't even have a model or hypothesis that predicts any sort of particle or material with the requisite type of negative energy so I'm gonna conclude that it's unlikely that a human will ever travel through time at least into the past with a fine tunable machine with an on/off switch like we want an in fiction however I will make the bold optimistic prediction that once we've figured out how to marry relativity and quantum mechanics and we have a full fury of everything that I think that time travel will be possible for subatomic particles so that it'll be possible to communicate for example have some sort of time telephone or telegraph and I'm gonna boldly predict with no nothing to back this claim up but that within a hundred years we'll get to the point where we have the technology to send a subatomic particle a tiny fraction of a second into the past not carrying any useful information initially now in order for that to be feasible though time travel would have to be taken seriously which it isn't and no one would actually be working on it other than other than Ron mallet I don't know of anyone else who's really taking it seriously in terms of experimental physicists there are many theoretical physicists who do write on papers on it who work on general relativity but I don't know other the other than run mail and I don't know anyone else who's taking it seriously enough to try to build something and I think in order for my optimistic prediction to come true it would have to be taken seriously and pursued by large numbers of the brightest minds in order for even in a hundred years even for my my this prediction is way too optimistic for my colleagues but people but in general nine nine scientists say no that's not good enough what do you mean subatomic particles I want to get in the time machine and so I'm gonna conclude with this ever a lot of what I talked about today wouldn't be possible obviously with Einsteins a seminal work and and I always throw out this photo of Einstein because we got to remember that he didn't always have gray hair you know he was born he was a baby and he there was a point in which he did not have gray hair yet so I always like throwing out that image so that's all I've got thank you

Channel: TEDx Talks

Views: 202,982

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Keywords: TEDxTalks, English, Design, Physics, Time, Transportation, Travel

Id: m2-I5HUwfRg

Length: 24min 2sec (1442 seconds)

Published: Tue Jan 16 2018