Sean Carroll: The Paradoxes of Time Travel

good evening welcome to the Linda hall library I am Lisa Brauer I'm the president of the library and I am just delighted to see everybody here tonight it's wonderful to see such a large and enthusiastic crowd we are nearing the end of one of our most successful series at the library and if you have missed any part of it let me remind you that you can catch up with the first two lectures in this series at our website WWE hall or --g where the first two lectures are available on videotape or whatever the lip digital version whatever the medium is I'm dating myself anyway and also you can follow all of the library's activities at our website or you can also follow us on facebook series like this do not happen by accident they take a lot of hard work by a lot of people and the list is quite long and I won't single people out by name but the members of the Linda hall library staff really worked very hard to put a series like this together and I thank them personally for this along with them I would like to thank our friends at rainy-day books for helping out with the book signing that will take place after this lecture this evening but most of all I want to thank Jim and Francie Flynn for their support for this series without their yes please thank you thank them without their support it just would not have happened so we are very grateful to them the idea for this lecture series was born out of our current exhibition thinking outside the sphere if you have not had a chance to see it don't worry you have until September 18th to see it it was curated by Cindy Rogers and we thank her for her hard work as well as a thanks to Jim and Marilyn heben Street and Dodie gates for their help in underwriting the exhibition so I think I have all of the thank-you sout of the way if I haven't thanked anybody who needs to be thanked let me thank you now and I apologize for that and before introducing our speaker let me please ask you to silence your cellphone's if they are still turned on Sean Carroll studies the structure and evolution of the universe in his quest to learn about its fundamental physics currently his interests are focused on inflation the arrow of time and the events occurring at or before the Big Bang his research involves dark matter and dark energy modified gravity topological defects extra dimensions and violations of fundamental symmetries dr. Carroll is a senior research associate in physics at the California Institute of Technology where he investigates the theoretical aspects of cosmology field theory and gravitation his most recent book entitled from eternity to here the quest for the ultimate theory of time is a popular book on cosmology and the arrow of time Sean also finds time to contribute to the blog cosmic variance we are so very fortunate that he found the time to visit us to discuss time travel and the subject of time in general please welcome Sean Carroll thank you and can everyone hear me in the back there certainly sounds loud up here I don't know it's been a tremendous experience already for me just today being here at the Linda hall library as far as I can tell this is a unique resource in the world a private research library devoted to science and I got to go downstairs and look at the stacks and the older journals and go through the rare books exhibit and everything and so you you it's a it's a wonderful privilege to be in the vicinity of this place and I'm very happy to see that they're having public talks especially on such an interesting topic as extraterrestrial life and aliens and other things that I'm going to have very little to say about so I want to start my own talk with a confession I hate it when speakers start by apologizing but nevertheless I'm going to do it because the thing I want to be talking about is time travel the idea especially of traveling backwards in time and clearly this is related to traveling faster than the speed of light and clearly both ideas are related to extraterrestrials with arbitrarily advanced technology what would they be able to do could they be actually warp space and time so much as to travel backwards in time travel size from the speed of light visit us have visited us in the past but what I'm gonna do is set the stage I'm really gonna talk about the physics of time travel what it would mean is it possible what do we know and we don't know and in fact there's a lot that we don't know it's kind of embarrassing that we don't know more than we do but I'm going to leave it up to you to decide what the aliens could do with that knowledge and what they're gonna learn that we don't know the first thing to emphasize is of course that the idea of time is a fascinating one in and of itself it takes us from our everyday lives we move through time you all knew that to be here you had to be here at 7 o'clock you knew what that meant you knew what to do do with it but it's a mystery we don't know how time works at a fundamental level and to quantify this researchers at the Oxford English Dictionary found that time was the most used noun in the English language now these days we don't care what dictionaries say we care what the internet says so I checked on the internet I looked at various words that you might think are popular space much less popular than time money much less popular than space which is that that's encouraging fun love sex piece versus war really not very optimistic there in terms of the number of pages found Harry Potter small but catching up and time with almost a trillion web pages that mention the word time in there so I don't even know how that is possible but they're literally a trillion right this would be sorry a billion literally a billion web pages you know one web page for every six human beings here on earth that talked about the notion of time so clearly we're interested in it we're especially interested in the notion of going backwards in time time travel and we get a lot of our feelings for what time travel is from fiction from movies and TV shows books and so forth it's a fascinating subject we approach it from all different ways traveling through space hopping into boxes of one form or another aging backwards in time here's Bill and Ted who had excellent adventures with historical figures clearly this is something that fascinates us for some reason so let's just start by asking why is it so interesting to contemplate going backwards in time what is it that makes it that's something we like to think about into dramatized in movies and books and so forth and the simple answer is that time travel seems paradoxical to us it seems without thinking about it too hard but a little bit that if you can travel backwards in time bad things can happen so the picture has the time-traveling cat about to sneak up on himself that would be bad we often dramatize it even a little bit more by saying well what if you travel backwards in time and you killed your parents before they met then how did you get born because they couldn't give birth to you and that is a legitimate puzzle of course it starts by asking if we could travel backwards in time what would happen and so the simple answer is well you can't travel backwards in time any questions I have more to say than that so it turns out that you can do better it's true that the simple ideas that you might have about time travel would lead to some paradoxes but you have to decide is that truly mean the time travel is simply logically impossible or is there some way around it could we possibly do better well you can do a little bit better and and this seems like a cop-out but the basic rule that has to be obeyed is that there are no paradoxes what this means is that even if you could travel into the past what you could not do is change it and I really want to emphasize this that that as far as we know even if it were possible to go back into the past nothing that you could do once you got there could affect the series of events that we know happened so as characters from TV show lost say whatever happened happened if you can travel back into the past you can't mess with it because we know it already happened it's different from the future we think that for the future we can make choices you can decide today what to have for dinner tomorrow night we think that that's something that you're allowed to do we do not think that you can decide today what to have had for dinner last night okay we believe there's a fundamental asymmetry a difference between the past and the future the past is settled the past happened it's set in stone but the future is up for grabs time travel gets in the way of this way of thinking about the difference between the past in the future and that's really what's at the heart of why it bugs us why it offers fertile ground for fiction and for movies and so forth if time travel were possible it would necessarily imply a form of predestination what you were able to do in your own personal future is limited if that personal future happens in the past if there's something that you know happened like you were born and you say okay let me a man and then I can travel into the past let me imagine that I can hop in a time machine and go back to a moment before my parents left what would stop me from killing them or somehow stopping them from meeting and the answer is something you might not know what but it's just like having a prophecy it's just like Harry Potter knowing that either he or Voldemort is going to have to die we don't know how it's gonna happen but it's going to happen even if you can go backwards to visit your parents before they left maybe that is possible what's not possible is that you can prevent them from having you because we know that already happened now the basic reason why this annoys us is because we think we have free will we think that if we did hop into a time machine and travel to the past and there we were with our parents we could then be free to make the choice to prevent them from getting together killing them or something less dramatic than that but somehow changing what happened the point is that what we think of as free will I hate to tell you this is really just kind of an approximation at the level of the deep down laws of physics for the most part the laws of physics don't have free will in them if you know what something is doing if you know what a box of gas is doing or a pendulum or a planet moving around the Sun and you know the laws of physics you can predict what it will do in the future the reason we think we have free will is because we don't have that information about ourselves we don't know that much about the universe we know some crude things but we certainly don't know what we're thinking much less what our friends are thinking much less what other people in the world are thinking or what other things are going on in the world so there's a tremendous amount of freedom open to us when we contemplate what could happen in the future just because even if it is determined there's nothing that we can do about it that doesn't help us any a lot of people are disturbed by the idea of determinism the idea that if we have the system of the universe right now perfectly we could predict the future I'm here to tell you don't be disturbed the idea of determinism is not like an ancient wise man who says this is what will happen in the future and there's nothing you can do it's not like that the idea of determinism is an annoying little kid who says I know what you're gonna do next and you say well what am I gonna do and the kid says I can't tell you that and then you do it and the kid says I knew you were going to do that that's what determinism is in the real world the universe knows what you're gonna do but you don't know and it's not going to tell you you do know what happened in the past there's an asymmetry between the past and the future because the early universe was a very very special state I can recommend a good book to read about this if you're more interested but the difference between the past and the future comes down to this idea of entropy the early universe was low entropy which means it was orderly it was neatly precisely arranged there was a boundary condition we know what the early universe was like the future is completely unconstrained as far as we know we don't know what the future is like so given what the universe is today and the past boundary condition we can more or less figure out what the past was like whereas we can't figure out what the future is like that's why we think we have freewill toward the past but not the future you see what the problem is is that if you have time travel if you can go from here into the future and then loop around back into the past then there's no clean distinction between the past in the future this idea that we know the past but the future is up for grabs gets destroyed if you can travel into the past if you could do that then your personal future would be part of the universes past and there's no way to say which is the end that is determined which is the end that we're able to affect in any way so the point is that you could imagine consistent histories in which you travel to the past but they're very very highly constrained knowing that your parents gave birth to you if you can go visit them in the past is like knowing what is going to happen to you next year it's not knowledge you're normally thinking of as something you have access to but that's what time travel does so again the simple answer would be time travel is impossible but we're going to think about it a little bit more carefully so I had to get there's one big aside one big caveat to all of that which is that I'm speaking the language right now of classical mechanics of mechanics as Isaac Newton would have understood it but we know about it classical mechanics is not how the world works there's something called quantum mechanics and quantum mechanics plays with our ideas of determinism we can't predict the future exactly probabilities become involved and there's a very good reason for thinking that when you observe something in quantum mechanics you could have a particle that is spread out there's no location for it there's different probabilities of observing it in different places but when you look at it the universe splits into multiple copies a copy where you saw it here a copy where you saw it there a copy where you saw it there in other words quantum mechanics seems to imply that there are multiple copies of our universe and that at least logically opens the possibility for going backwards in time and messing with the past namely you can mess with a different version of the past than the version you actually grew up in so here's my one slide about quantum mechanics quantum mechanics implies that there really could be multiple worlds like season six have lost there's multiple timelines okay where different things happen they branch out they're equal to some point and then some different event happens in the two worlds and they go their different ways if this is true we can imagine this is a one version of the universe with time going bottom-to-top this is another version of the universe time going from bottom to top and these universes are identical up to this point what happens in Universal number one is that your parents meet they give birth to you you grow up you're a very disturbed child but you're a eccentric genius and you build a time machine and you use your time machine to travel backwards in time and then you take out your anxieties by killing your parents but you do it in a different version of the universe and then you continue to exist in that version of the universe you're older you're orphaned now you're a little bit lesson more embittered depending on how that worked out for you but your parents are dead and they don't give birth to you in that universe now this is logically possible it has been suggested by this guy David Deutsch was a very respected Oxford physicist he's trying to point out has theoretical physicists like to do that these things might be conceivable to us but nobody has an actual plan for making this happen no one has a scheme for starting in one universe and going to the past of a different universe and if you did let me point out this universe still exists you're not in it anymore you moved you changed addresses but this whole history still happens your parents still met you are still born your parents are still alive there I think it's a cold comfort to think that you can change a version of the past if the version of the past with which you are familiar is still there you can't change that again because we know it happened there are never any paradoxes if you invent a scenario in time travel which involves paradoxes you're not playing by the rules so what are these rules about which we're talking and now we can get into some serious thinking about physics so this man Isaac Newton so today for the first time in my life I got to leave through a first edition of Newton's masterwork because it's here in the rare books section of the Linda hall library the principia mathematica Newton was the one who set out all the major well not all of but the major advances in both science and math that enabled our understanding of the clockwork universe the the analogy was suggested to me that Newton coming up with the his theory of classical mechanics out of the blue almost like that would be as if the Wright brothers had gone to Kittyhawk and built a 747 he you would have thought that what Newton did would have taken hundreds of years to get right but he did it over a couple years when there was a plague in England and he had to leave Cambridge and sit and he got hit by the Apple and you've heard that story so here is the setup that Newton proposed and even though there's a lot of diagrams that are intimidating to us when we're taking physics classes and so forth the basic idea is actually pretty intuitive and it makes sense to us there is space there's the space around us there's the arena in which things happen space is full of locations where there different objects some locations are empty somewhere occupied and there is time so what happens in Newton's universe is that space exists it's full of stuff and space happens over and over again and the stuff in the universe changes a little bit from place to place from moment to moment and time is the label on different moments of the universe time is what prevents everything from happening at once as someone once said so time is the ordering the universe is like a filmstrip in this way of thinking it's like a sheet of film and there's different frames except there's an infinite number of frames and every frame is a different moment in the history of the universe and this division of the universe into space and time is absolute it's there everyone agrees on it you can measure it nothing mysterious about it so given that set up you can ask can you travel through time in a Newtonian universe and the simple jokey answer is yes if you sit around here for 24 hours you will travel into tomorrow you always travel through time you always move through the Newtonian universe at 1 second per second or 1 minute per minute everyone does the same thing that's the secret to Newton's universe is that everyone moves forward in time one second per second and everyone agrees on what that means everyone knows what a second is you can build clocks you can measure time and you can agree on what how much time has elapsed between one event and another that's what it means for space and time to be absolute in our Newtonian universe you can always change your perception of how time feels you can go into suspended animation for example you can just go to sleep and time seems to go a little bit faster you could have a cup of coffee or you can be tired and your perception of time changes and that's because you your body is full of clocks a clock is just something that does the same thing over and over again in a predictable way your heartbeat is a clock your pulse your breathing the cycles of your central nervous system and the problem is that they're not very good clocks a clock should be accurate and predictable our bodies are affected by all sorts of things by going to lectures that sort of send you to sleep in all things like that so your perception of the passage of time might be sloppy and imprecise but time itself according to Newton is absolutely rigid it's absolutely doing the same thing for every reliable clock so there's a way of thinking about this Newtonian universe that Newton himself didn't think but we're allowed to think of it which is to say take the whole shebang all of space and all of time and call it space-time you're allowed to do that no one is stopping you from doing that if you want to locate something in the universe an event in the universe you need to tell me where it is in space and when it is in time to come to this lecture you had to say it's at the Linda hall library at 7:00 p.m. you had to give both of those pieces of information so we can think of them all together as one four-dimensional space-time space itself is three-dimensional there's up down left right forward backward there's no other perpendicular directions in space but there's also the direction of time so we can think about this as a collection of events in this four-dimensional space-time there's no reason to think of it that way because space and time act differently according to Newton but you can think of it that way the question then is how do you divide up space-time into space versus time and whenever you're asked a question like that you should think of it operationally in other words you should think about what is the experiment I would do what is the apparatus that I would build to actually make this happen don't think of the whole universe with things happening very far away different moments of time think about things you could build and what they would do so Newton says if you're here and you know that what time it is right now you want to ask the question what is happening far away but at the same moment of time what does that mean what that means is you build two clocks you synchronize them so they're agreeing with each other and then you send one clock far away and then it just sits there and you have a bunch of clocks you can do this throughout the universe even though it's an engineering problem you have infinite resources so you set up a clock at every point in space and you make them agree they're synchronized and that tells you what it means to have two different moments two different points in space but at the same time simultaneously in the universe so you need good clocks ones that remain synchronized once it flow at the same rate then along comes this guy Albert Einstein in 1905 to say actually you can't do that and you're always shown pictures of Einstein when he was in his dotage he let himself go a little bit the hair was going back and he was wearing sweaters but when he was a young guy inventing these theories of space and time someone was combing his hair and someone was dressing him very very nicely so Einstein says the following thing he invents a new theory of space-time called special relativity and he says actually you can't do that process that you need to do in the Newtonian universe in order to tell you what it means by the phrase equal times at different points in space and the reason why is because when you start a clock moving through the universe suddenly it goes out of synchronization and this is not a problem with your clock this is not some failure on your part to build an accurate clock this is a deep fact about the world but that if you have a clock and you keep you have two clocks to start the same and are synchronized and they're both very well built one stays stationary but the other one goes on a journey and comes back when the one that comes back is compared to the one that stayed behind they will not agree anymore that is just a true statement about the universe we can do experiments we've put clocks and airplanes and sent them around the world and compared them to ones that remain behind and they're different this is a true fact that you have to learn to deal with what this means is that this absolute time of Isaac Newton isn't right time is individual time depends on the trajectory that you take through the universe it's not a feature of the universe once and for all it's a feature of your path through the universe and different paths can experience different amounts of time now this seems very different than our everyday experience but let me try to convince you it's not quite as crazy as it sounds the motto of relativity is that time is like space so what does that mean well here is space okay that's just a tabletop some flat surface and you all know that if I draw two points on a tabletop I can certainly create different pads that connect those points such that the paths have different lengths different distances there can be a straight-line path that's gonna be the shortest distance between those two points but there can also be curvy paths every curvy path will have a longer distance than the short path so what Einstein says is that time is like that time is just like space the elapsed time that is read on your wristwatch as you go through the universe is analogous to the elapsed distance that you travel as measured by an a domitor if you're walking down the street it will be different to different people who take different paths there is one huge and important distinction which is that in space the shortest distance between two points is always a straight line in time the straight line corresponds to the longest possible time so what that means is that if you are sitting here and you have your watch and you're saying how long it is someone else starts next to you synchronized and flies away and then comes back they go on some curvy path through space-time when they come back they will be younger than you they will have experienced less time they will not have felt anything funny about it to them time was passing at 1 second per second that's what time does but your second is not the same as their second because you're travelling on different paths through space-time so if you start with a rocket ship the point is is that to make any of this real you need to go close to the speed of light and you don't go close to the speed of light in your everyday lives you don't experience this so we talked about it we don't normally talk about fast cars or running as fast as you can we talked about spaceships so if you have a spaceship here with two different little probes and it sends out these probes on different paths so the probe accelerates outward then it comes back close to the speed of light they can all have clocks that are in a initially synchronized but compared to the spaceship that just moves an uninterrupted path the one that accelerates a little bit will experience a little bit less time the one that accelerates a lot will experience a lot less time so if you want in stein is giving you a different way to travel into the future it's still true that you will travel into the future one second per second but because your second isn't the same as everyone else's you can choose to reach the future faster if that makes sense if everyone else stays behind you can zip out close to the speed of light come back you've experienced a week passing the rest of the world has experienced a hundred years what you can't do is go back okay according to Einstein there are still limits on what can happen and that limit is expressed by the ultimate speed limit on the universe this is the speed of light and I have to introduced one technical term that is extremely helpful for the rest of the talk and that's the idea of a light cone in space-time so I'm gonna try to explain to you what this means here is I'm as you know not only a scientist but a highly accomplished artist so here is my sketch of you and you have a light bulb and what you do is you flick the light bulb on and off very quickly so what happens is the light bulb emits light in every direction and those little rays of light travel outward from the light bulb in every direction at the speed of light so here is the this circle represents at some moment in time very soon after you flicked the light bulb on and off how far has the light gotten where has it traveled to so the light is traveling out in every direction in space at an equal speed and that's gonna keep happening and Einsteins speed limit says that you can never catch up to the light once it's past you you can never move fast enough to reach the speed of light or even go any faster so the light keeps moving away from you you can run as fast as you want you can hop in the Space Shuttle or in the DeLorean or whatever you want you will never be able to catch up to that beam that sphere of light spreading away from you so what that means is there are places in the universe in the four-dimensional space-time universe there are events that you can never reach so if someone says there's a great party next Friday night but it's on Alpha Centauri okay it's four light-years away it takes at least four years to travel at the speed of light to get there if you travel the speed of light but the party is next week that party is inaccessible to you by the laws of physics you cannot get there because you cannot travel fast enough and if we were to portray this state of affairs on a space-time picture so now we have again space and time here we are we let the light bulb go and what happens is that life in the light bulb travels outward and gets a bigger and bigger circle at every moment of time so the whole thing describes this cone and that is the light cone an Einstein's rule is that starting from any point I can draw a light cone into the future and everything that I can possibly do must be inside my light cone there are events out here parties I would love to go to but to travel to them I would have to move faster than the speed of light those points are outside my light cone I can never get there I can get to Alpha Centauri four light-years away but I cannot get to Alpha Centauri next week that is inaccessible to me so this idea this idea that there's a light cone and you have to stay inside the light cone no matter what you do is what replaces Isaac Newton's idea of space and time as absolute things what Einstein says is that these divisions these slices here time as opposed to space erase them they're not there they're not really part of the universe what Einstein would say is I wish I'd drawn this but what Einstein would say is that this light cone is real that's part of the fabric of reality these slices of the universe into time versus space are just fictions they would not be agreed upon by observers who were moving at different speeds so here is Einsteins version of space-time there's no slices there's just light cones so you could start with different clocks and they have to travel inside their light cones and the point is that you wouldn't notice any difference between Einsteins view of the world and Newton's view of the world if everything was moving much much slower than the speed of light that is our everyday experience that's why we think the time is absolute that's why we think it makes sense to talk about an event happening very very far away at the same time but this division of slices is absolutely arbitrary according to Einstein what really exists are light cones okay that was a little bit of a digression to understand what Einstein said about how you can move through space and time but it has nothing in no way has changed our conclusion which is that you still need to move forward in time you need to move toward the future in Newton's version you have to move toward the future because space and time are absolute you just have no choice in Einstein's view of the world you still have to move toward the future in fact you're a little bit more constrained not only do you have to move to the future but you have to stay inside these light cones at every moment so if you want to travel backwards in time this story of Einsteins is of no help to you you can travel to the future faster by moving close to the speed of light then coming back but if you want to go to the pass you're still stuck however this is Einstein's special theory of relativity he invented in 1905 he then spent the next 10 years thinking about how to reconcile this idea with gravity and he eventually in 1915 published the general theory of relativity which says something remarkable it says that space-time can be curved space-time is not a rigid structure that is there once and for all space-time has its own life it responds to stuff in the universe responds to matter and energy and we can bend it and warp it then when a beam of light gets deflected by some massive object it's not because that massive object is really pulling out with some force it's because that massive object is bending the fabric of space and time around it so when the earth moves around the Sun it's not because the Sun is pulling on us with the gravitational fields as Einstein it's because the Sun is warping the space-time around it the earth is just trying its best to move in a straight line but there are no straight lines because space-time itself is curved so we end up orbiting the Sun that's how gravity works gravity is the bending of space and time has its own geometry and this has dramatic effects not only for theoretical physicists but for the real world because this is really time as well as space that is getting bent what this means is that the amount of time that will elapse that you'll feel on your wristwatch depends on the gravitational field that you are in I already told you it depends on how fast you move through the universe but it's also true that how much gravity you feel affects the rate of time roughly speaking time moves more slowly in a strong gravitational field than in a weak gravitational field and this is relevant to technology this is the global positioning system the GPS system that you use do not get lost when you're traveling through a strange town you ask your car you know where is this party that I want to go to on Alpha Centauri and it gives you directions so the way that GPS works is that there's a set of satellites orbiting the Earth and you in your car have a GPS receiver that beam signals back and forth to the satellites and what they do is they compare clocks your receiver has a clock the satellites have clocks and they're comparing the amount of time that passes that it takes light the radio signal to get from your signal your receiver to the satellite so guess what it's crucially important to understand the rate at which time is moving to you here on earth and to these satellites in orbit but those satellites are in a different gravitational field their clocks are moving a bit faster than they would if it weren't for this general relativistic effect and that turns out to be measurable if you didn't know about general relativity if you didn't know that the rate of time at which these clocks was ticking was affected by gravity then within minutes you would be miles off in your GPS estimate you need to know what this effect is to get right where you are in the city so it works it's experimentally verified now it's also I have to admit fun for theoretical physicists we can imagine bending space and time quite a bit more you've heard of black holes black holes you've been told are places where something that is really heavy and massive has collapsed to such a tiny region of space that the gravitational field is so strong that light itself cannot escape and that's all correct by what a really emphasize how fundamental it is what's really going on in a black hole there's so much gravity but the light cones have tilted that you've warped space and time so much that this movement of light around you is being sucked into the black hole but remember Einstein's rule is that you need to stay inside your light cone as you move to the future so a black hole works like this this is as the singularity this is the point of infinite density where everything is collapsed together this cylinder is the black hole itself and what happens is viewer out here you have your light cone you're not bothered you can orbit around the black hole you could take observations of it whatever you want to do but as you move closer and closer to the black hole your light cone gets tilted by the force of gravity and the definition of the black hole is that the black hole is the place where the light cones have tilted so much that in order to go from inside the black hole to outside the black hole you would have to go faster than light you have to move outside your light cone and that's something you can't do so the point is that this singularity is not a place in space the singularity is your future the singularity is a moment in time when you're here you're just going to the future you're not going left or right you could just not do anything at all but you will inevitably age into the singularity the reason why black holes are so dramatic is that hitting that singularity that point of infinite density and space-time curvature is as inevitable as heading tomorrow you can't take evasive maneuvers and prevent yourself from hitting tomorrow it's going to happen inside the black hole everything crashes into the singularity that's an effect of general relativity however we can now with all that background material at our disposal get back to our topic so what about time travel I said that if you just believed in Newton's view of the universe with absolute space and time there's no way to travel backwards in time it's an inevitable march forward in special relativity there's no absolute notion of simultaneity there are light cones but it's still true you inevitably march forward in general relativity the light cones start tilting they're affected by gravity so a brand new possibility opens up we can at least imagine a gravitational field that is so dramatically strong and so twisted literally that the light cones form a circle in space-time remember there's no slicing of the universe into moments of time there's only at every point a light cone that tells you where you can go into future so if the geometry of space-time were like this here you are you say I need to go into the future so I'll go into the future I'm still going into the future I'm just inside my light cone I'm going into the future going into the future going into the future going in the future and there I meet my past self about ready to begin on that journey so I all I did was to move into the future I aged and yet when I left here when I entered that loop that time loop as I got on there I saw an older version of myself remember whatever happened happened so if at this point I go around and I meet myself then when I entered my older self was there meeting me okay I'm sure you all understand that perfectly but think it through at this point in space-time either there was one copy of me or two copies of me or three copies of me they're not different versions of this event over and over again there's one event the beginning of this journey so if I'm going to meet myself in the past then my past self met my future self so general relativity the idea that space and time are curved opens the possibility to an honest-to-goodness time machine if we can warp space and time so much then we can actually travel on perfectly respectable paths through the universe just aging into the future that nevertheless bring us into contact with our past selves so here is the set up just so you can get it right if your Newton you have to go forward in time if your special relativity you have to go forward in time because you have to stay inside your light cones and all the light cones are stuck if your general relativity the light cones can tilt and we can at least contemplate a warping of space-time that is so severe that you can visit a past version of yourself so this has very vivid lessons for JJ Abrams or Steven Spielberg or someone whoever is making the next great science fiction movies time travel just like space travel time travel is not you hop in your sled or your phone booth and you D materialize in a puff of smoke and you appear some other moment of time that's crazy talk what would really be like as you hop in a rocketship you hop in some vehicle you travel on the right path through the universe that takes you to a sufficiently strong gravitational field that you can be flung backward in time at no time do you disappear you do not teleport it's not that your consciousness is suddenly imported into a past version of yourself it's that you get in the seat and you push the rocket in the right direction so the Starship Enterprise is at least plausible as a time machine it's a rocket it can go someplace and if the gravitational field is arranged in the right way it could visit the past a DeLorean that aims in the right direction and D materializes in a puff of smoke no that is not the way it would actually work indeed Back to the Future is probably like the worst movie in the world in terms of getting time travel right okay different movies have different standards Back to the Future is the worst because Michael J Fox actually does things in the past that instantaneously change things in the future people disappear from photographs and things like that what is that even supposed to mean instantaneously changing things when you're in the past and things I don't know what that means but that's okay made a lot of money DeLorean nevertheless went out of business so what we want to know is can we warp space-time that much I'm Stein by allowing space and time to be flexible and dynamical has opened the door he said here's a way we can at least imagine building a time machine so very few people took up the challenge but in the late 80s Early 90s attempts were actually made to do this how could you actually do it you need to bend space-time so much that you could travel on what physicists call a closed timelike curve a path through space-time that at every moment moves into the future but globally the future gets mixed up with the past and so here are two ideas for doing this one is from richard gott and one is from Kip Thorne my colleague at Caltech this by the way every time you see a photograph of richard gott talking about time machines he'll be wearing this jacket and that's because early on after he was working on time machines he gave a talk and robert Kirchner and other astronomers in the audience and said richard where do you get that jacket it must be from the future that color doesn't exist in the present day so he always wears it now as a tribute so God's idea was based on something called cosmic strings is a very hypothetical idea remember we're doing here is we're not engineers okay we know better than that what we're asking is do the laws of physics allow us to imagine a scenario in which a time machine could be constructed in which we could bend space and time so much that we could visit the past so we are allowed to invent things that don't actually exist as long as they're not ruled out by the laws of physics so cosmic strings are hypothetical ideas they're basically leftover relics from the Big Bang that like little fractures in space-time little tubes of energy that could be formed as the universe cools down from its earliest moments and in the 1980s and 1990s it was very popular to imagine the cosmic strings were relevant or the explanation for the formation of galaxies that we see in the universe that idea is less popular now but the idea of a cosmic string is still perfectly respectable and these little tubes of energy can really pack a wallop they can really have an incredibly strong gravitational field that's one piece of good news the other piece of good news is that the equations that you need to solve to understand what is the gravitational field of a cosmic string turn out to be really really simple compared to the gravitational field of a planet for example so richard gott sat down and did it he said what happens if I have two cosmic strings infinitely long perfectly straight and absolutely parallel to each other that are moving past each other very very fast slower than the speed of light but still very very fast and this was a sufficiently simple situation that he could solve all the equations exactly it's not realistic you can't do it you can't build it in your lab but it doesn't violate the laws of physics and you can understand exactly what goes on in this situation and what goes on indeed is that there are closed timelike curves there are paths you can take around by looping around those cosmic strings that you actually get thrown into the past if you do it in the right way that's the good news and there's a lot of excitement about this little picture of richard gott in newsweek holding up two strings and a little space shuttle going around them there's also bad news the bad news is well what if you don't get lucky enough to find two cosmic strings doing this could you imagine finding slowly moving cosmic strings and accelerating them putting little rocket engines on them and pushing them fast enough to get to the right speed the point is that if these strings are moving slowly with respect to each other you don't get a time machine you need to have two cosmic strings moving really fast compared to each other so what if you had to slowly moving ones and you put little rocket engines on them and you accelerated them what would happen would you build a time machine no actually what would happen is that putting the rocket engines on them and accelerating them these strings this tremendous velocity puts so much energy in such a small region of space that the whole thing collapses it makes a black hole it's almost as if nature is saying no you should not have tried to do that this is a no-no you're trying to go backward in time I know what you're doing I'm gonna stop you by making a black hole so it's not definitive it's one attempt that didn't quite work here's another attempt this is by Kip Thorne who's a colleague of mine at Caltech and and actually I'm not talking you here is collaborating with Steven Spielberg on a movie on a feature film based on his ideas about time Travel he was also an inspiration behind Carl Sagan's novel contact which has made new a very successful movie and what kip imagines that there are wormholes a wormhole is just a particularly dramatic bending of space and time that creates a shortcut through the universe so here's an this is an artist's impression it's not a photograph but here's space and here's a little tube that cuts through it it's very hard to really represent accurately in a picture what a wormhole would be like because in these curved non-euclidean geometries you can't draw them faithfully the point is that this external region what looks like this piece of paper that has been is actually just supposed to be the universe this is about the space as we normally observe it there's no weird bendings or anything like that in it but there's one little hole in space that is connected to another little hole that from the point of view of someone outside is really far away but from the point of view of someone who goes through the wormhole it's really short so it's as if you want to travel from Kansas City to st. Louis and you built a tunnel underground that could get you there in five miles okay you can't actually do that because space-time is not warped that much but general relativity gives you the freedom to imagine doing that to building a tunnel through space that can connect to very distant regions with a shortcut that is actually very easy to get through now wormholes are even more unlikely to exist in the real world than cosmic strings are compared to cosmic strings compared to wormholes I should say cosmic strings are sort of everyday in mundane okay wormholes are truly exotic we honestly don't know if it's even allowed by the laws of physics to have a wormhole in the real world it's not just we don't know how to build them is that when we try to build them we find that we need exotic things like negative energies to keep the wormhole mouths from just collapsing and guess what making a black hole that's what these things tend to do but we're being open-minded we're saying let's for the moment that we can number one find a wormhole number two keep it open okay so the wormhole is a tunnel that we're supporting it's not gonna collapse but the way to think about it is really just some sphere some region of space that I would go into and I would pop out somewhere else very very quickly so I have two very distant objects and I can move them around so what Kip Thorne was able to show is that imagine these are the two ends of the wormhole okay so these are the two mouths so I haven't drawn the wormhole itself that's invisible but you imagine you hop into this sphere and you pop out the other one instantaneously okay and I put two clocks next to them they both are synchronized and you can sort of guess what's going to happen I keep this wormhole throat this mouth of this wormhole here stationary it just ages directly upward in time whereas this wormhole I put on a rocket and I move it out very fast close to the speed of light and I move it back by what we already talked about in special relativity this wormhole mouth it moves close to the speed of light and then backed experiences less time so this clock goes from reading 12 o'clock to 12:10 while this clock goes from reading 12 o'clock to 1 o'clock but here's the secret when you pass through the wormhole you do it instantaneously so you do it at the same time according to these clocks in other words if I go in here I come out there 12 o'clock if I come in here at 12:10 I go out there so I'm not only traveling through space quicker than I should be allowed to I'm traveling through time quicker than I should be allowed to and I can go backwards I can travel into here and I would pop out there and then I could move around and visit myself in the past in other words if you give me wormholes which I know is a lot to grant but I'm allowed to build these in my secret scientist laboratory but if I could do that and you let me move them around I could build a time machine where one was not there to begin with that's the good news the bad news is that outside the wormhole there's energy that flows and you can't help that that's quantum mechanics there's always a little bit of energy that you can't really pin things down absolutely perfectly there are quantum fluctuations and what happens is as you're trying to turn this into a time machine the energy that is flowing in between the two wormholes grows and grows and grows until it becomes so large that everything collapses into a black hole this is not a theorem this is not absolutely rigorously established but this seems to be the lesson of a lot of smart people have thought about this problem very hard so unfortunately it's kind of embarrassing I can't tell you the answer to the question in general relativity can we build a time machine I can tell you number one that we know how to try and number two that when we do try it never seems to work everything collapses into a black hole that's the best we can do in the current state of the art so it seems to be a hint that some people take some people choose to ignore I mean it's a lot of money to be made in ignoring the hint that everyone else pays attention to if you can really be clever enough to find a way around it but the bottom line as I see it is that it's easy to imagine building a time machine in general relativity but when you try you run into trouble we can't prove that you can do it we can't give you an example that we know works we also can't prove that you can't do it I would bet given even money odds that the ultimate laws of nature say no you cannot build a time machine you cannot travel back into the past everything is well behaved that's really why you can't kill your parents but I can't prove it right now I'm not absolutely sure we need to work harder to understand what actually goes on in the real world so the take-home message three of them one is that even if time travel is possible paradoxes are not paradoxes are not problems with their problems with logic if something happened then it happened you can try to wriggle out of it by invoking quantum mechanics or something like that but if there's someplace in the multiverse where something really happened then in that place in the multiverse that thing really did happen the second thing is that traveling through time is like traveling through space both in the relatively down-to-earth sense that by taking different paths through the universe I can experience different amounts of time and in the more advanced sense that I can curve time just like I can curve space and imagine looping backwards on myself therefore in general relativity according to Einstein's theory of space and time I can conceive of building a time machine it would not be puffs of puffs of smoke there would not be flashing lights I would get into a rocket ship I would go off in the right trajectory and I would come back however it doesn't seem to work according to current theoretical technology every time we try to do it we run to some problem or another now physicists are very bad at predicting what they'll be able to come up with in the future so I'm hoping that if this is wrong a physicist from the future comes in and tells me about it right now thank you now I think we're gonna take questions we have time for a few questions we have two microphones I have one there'll be one in the back - just please raise your hand it's my understanding at the time of the Big Bang just for you know in nanoseconds thing non mass entities went faster than the speed of light and I was thinking somewhere down the line probably past my lifetime would be possible to destroy ourselves into something without mass and then transport ourselves somewhere else without affecting anything and then reorganizing right I know what you mean but I don't think that that's the right way to think about it I think that I know what what you've heard and the problem is that physicists often get this a little bit mixed up when they try to explain it to people so I think the right way to think about it is that nothing travels faster than the speed of light not material things non material things your feelings your thoughts quantum mechanical wave functions nothing ever travels faster than the speed of light there are ways of describing what happens in the universe where you use language as if things are traveling faster than the speed of light for example as the universe expands two points far away from each other can be spoken of as if they have a velocity with respect to each other we talk about distant galaxies receding at a certain speed and Hubble's law the law of the expansion of the universe says that the velocity of a distant galaxy is proportional to its distance so the further away a galaxy is the faster it's moving away from us so you don't even need the early universe the first nano second after the Big Bang right now if you go far enough away there's some galaxy that is moving away from us faster than the speed of light but nothing is moving faster than the speed of light so how do i reconcile those two statements the point is that that's not really a velocity what Einstein really says that you have to stay inside your light cone and what that means is that no two objects can pass by each other faster than the speed of light this idea that a distant object is moving faster than the speed of light is just a fiction it's a convenient way of talking about things and what's really happening is this the amount of space in between us and that galaxy is growing as the universe expands but therefore we will never be able to use this phenomenon to actually cheat get around the light cones and actually get ourselves somewhere far away faster than the speed of light the microphone disappeared hello right here right here does not help me okay yes okay early on you said that a lot of the if you were getting closer to the speed of light then certain thing to behave to behave differently but isn't speed relative to a particular point so if I measure myself my speed of myself relative to an atom going through the hydrent Collider mm-hmm am I not approaching the speed of light and why don't those laws apply yeah no that's absolutely true so I hope I didn't say what you said that I said if if you travel close to the speed of light as far as you're concerned everything is absolutely normal you never see your watch doing anything funny you don't feel time passing slow or fast or anything like that you're just in a rocketship you're having a good time time is moving forward at one second per second the differences in the amount of elapsed time are only discernible when you then compare yourself to the person who didn't go in the rocket ship to the person who stayed behind so indeed it's all relative that is the point one journey was accelerated the other journey was not accelerated therefore they feel different amounts of time having passed also know hi so what I understood from these is that you can't really travel to the past but you can travel with that theory of the cone you can travel to your past present so like when you go around you can go around too far that you go before you started the journey but only through the point where you started the journey you're trying to say well that's certainly true so even if all this is possible there is a moment when you start to travel on this loop through time so if you're young here and then you get a little bit older a little bit older you can when you travel back you meet yourself at the point where you began that loop through time and if that loop through time didn't exist back here then you can't ever visit there another way of say excuse me other way of saying it is Stephen Hawking made the joke that we know the time travel is impossible because we have not been visited from tourists by tourists from the future okay but that's a silly joke because the truth is that even if time travel were possible you would never be able to travel to a point before you built the time machine okay there either is or is not a journey through the universe that goes around a loop in time and gets back to this room okay if there is not if there are no closed timelike curves that pass through this room the no future adventurer is ever going to be able to come back here time is just not warped enough to do that so yes there are definitely limitations on what you could possibly do what moments in time you could possibly visit even if this setup were were real professor I believe it's Ronald Mont has a theory where he's using lasers oh yeah you a I guess a time shift Analia against mallet where you're talking about the event horizon there and using the lasers where you would get information from the future to the past not actual physical time trial but I guess material where you can see what's happened in the future yeah unfortunately he is actually not respectable when we're talking about the hypothetical things here that would allow us to travel information or material around a loop in time we're really imagining that you're pushing around amounts of mass and energy the size of the Sun or the size of the galaxy in very small condensed regions of space you're not going to do it with lasers if you if the lasers were strong enough to have any kind of effect like this guess what they would collapse into a black hole and you wouldn't know okay so this it's just you know when you start trying to do very very precise measurements in very very controlled environments and look at very very tiny effects then you can fool yourself you can get noise that looks to you like a signal so I am absolutely convinced that the laws of nature do not let you build lasers in your lab that let you travel send signals backwards in time and and that's not not just me I'm not alone in being convinced thank you doctor Carol thank you you

Channel: Enlightainment

Views: 379,326

Rating: 4.4838314 out of 5

Keywords: sean carroll, the paradoxes of time travel, lecture, physics, science, philosophy, 2010, linda hall library

Id: qB_V1l8iLlc

Length: 66min 7sec (3967 seconds)

Published: Tue Apr 03 2018