How to Build A Time Machine - Paul Davies

okay so welcome along ladies and gentlemen to your weekly City seminar series today we're very fortunate to be joined by Paul Davies who's come to us from Arizona State University where he's the director of the Beyond Center Paul did a bachelor's in at in physics at the University College London and also his PhD in the quantum theory of wheeler environment and electrodynamics and then he did a postdoc at Cambridge with Fred Hoyle and Paul has since then carved a impressive career in science and science communication he has several awards that I'll just take a couple of for as examples here that he's been presented in his career including the Faraday price in the Royal Society for science communication in 2002 the Kelvin medal of the Institute of Physics in the UK and the Templeton Prize in 1995 and in 2007 he was made in order a member of the Order of Australia by the Queen the queen of the Great Britain of course so Paul Paul as I mentioned has recently moved from Australia where is a longtime resident at in South Australia and then Macquarie University he's moved across to the Arizona State University where he continues his research and has now opened an interest in oncology research as well but today he's going to talk to us about a long-standing interest of years which is time travel and how to build the time machine so if you'll join me in welcoming Paul and well Thank You Adrian for those kind words of introduction and the remarks about the Queen ladies and gentlemen Star Trek fans long have claims that space is the final frontier but they're wrong time is the final frontier 100 years ago space travel and time travel were just so much science fiction today space travel is commonplace so could it be that in another hundred years time travel will also be commonplace now the great thing about time travel is it's very easy to imagine you step into some machine or some sort of box you press a few buttons and you step out again not just somewhere else but somewhere else very easy to imagine but can it really be done well I hope by the end of the lecture this evening I shall have convinced you that the answer to that question is a resounding maybe the the subject of time travel really took off with the trailblazing novel written in 1895 by the science fiction writer HG Wells the time machine and that sort of set the gold standard for what time travel will be like and what time machines will be like we could almost think of this as the archetypal Time Machine as this cartoon makes very clear but but it does have a sort of Victorian contraption err to it and so these days the basic story of time travel has been copied again and again and again but these days in rather more technological guys and I suppose any particular year there's another time travel story another time travel movie lots and lots of them coming out but that's that's fiction if time travel could be done what what a time machine really be like would it be like HG Wells this is the remake incidentally by the great-grandson of HG Simon Wells the remake in of the Hollywood movie in 2002 still with that sort of Victorian feeling to it so would it be something made of brass and lenses and and light and levers of maybe steam coming out or would it be more like the DeLorean car of Back to the Future or better still would it be like the famous police box in the Doctor Who series now when I'm talking about this in the United States I usually have to explain what is a police box because I don't think they ever existed here and in fact very few of them existed in Britain but when I was a lad growing up in London you'd see these things now the traditional British telephone box is looks like that it's a red box you see but the police wear blue uniform and they wanted a blue box for themselves and so they used to go in to make telephone calls that was before the days when the British police forces could afford to way radios or cell phones I guess they use now so that was their means of communicating and then of course technology overtook it and these police boxes rather disappeared so I wrote a book on time travel in I think it was 2002 and the one of the London newspapers say well we must you know do some publicity for this book launch let's go and find a doctor who style police box we scoured the whole of London and for those who are interested just outside Earls Court underground station there remains a nice box oh there it is in the bottom-left it's still there so we did a photo shoot and that you can imagine that the vehicles passing by was sort of slowing down seeing me standing there being photographed obviously thought I was being auditioned for the next part of Doctor Who but so sadly not it was just just the book launch but all of these are ways of imagining how we might use some sort of technological gadget to transport us through time so the idea is easy to visualize but the question is what where is the science fact underlying this well it was only ten years after HG published his trailblazing novel that Albert Einstein came along with his special theory of relativity published in 1905 and that theory is fundamentally a theory about the nature of time or space and time but the centerpiece of the theory from far as our story is concerned is Einstein's claim that time is in some sense elastic what does that mean it means that time can be stretched or shrunk depending on your frame of reference it means briefly that your time and my time can get out of step very simply we merely need to move differently and I'll just demonstrate this year I just need to walk up and down my time that is that in my frame of reference the duration between two events like me turning around at each end is different from the duration that you judge it to be the only thing is this difference is so minuscule that nobody's going to notice now if I were to get on a plane and fly to London and come back again but I take an accurate clock with me and I'm talking about clocks which are really off-the-shelf technology not anything super duper fancy then when we compare our clocks at the end of the journey they will be discrepant by a few billionths of a second so hardly a doctor whose style adventure a few billionths of a second but nevertheless easily measurable with modern technology so there's no doubt about this prediction that I'm they made in 1905 that time is affected by motion there's no doubt about it it's it's simply a fact it's a done deal we've done it but to see what that actually means in terms of time travel let me just illustrate this by a parable so first of all here is me jetting off where you can't do it by Concorde anymore jetting off to London and in effect I'm the time runs slower in the airplane but a better way of envisaging this a very well-known way is the so called turns effect imagine a pair of twins it's called Sally and Sam and Sally the adventure of strength twin decides your golf in a rocket chipper closest to the speed of light because in this in relativity the speed of light is what determines the scale of effects so if you can get close to the speed of light the time warping or time stretching effects become dramatic so Sally goes off a close to the speed of light to some nearby star turns around again and zooms back to earth and Sally looks on her watch how long did the journey take the answer is two years say but when she gets back she notices that in the two years in the rocket 20 years have elapsed on earth so Sam her twin brother lazyboy stays at home is now no longer the same age as Sally so Sally and Sam were born on the same day but they're now no longer the same age because the duration of time in their separate frames of reference has got out-of-step by 18 years in effect sally has left 18 years into Sam's future and again this is a real effect so this isn't just some something hypothetical or something from science fiction it really happens now to make it happen by 18 years is not a matter of principle it's not a matter of fundamental physics we know the physics is correct it's just a matter of money and engineering that you need to move sufficiently fast for sufficiently long if you can get the case you can get to the speed of light the more you can leap ahead in time as I've indicated the moment it can be done by a billionth of a second with no problem at all but to do it by years that's beyond our current technology but it's but the principle has been established so in principle if you said well I really like to visit the year 3000 not a problem just find a very rich philanthropist and you might think well I'd like to go to the year 3000 maybe they'll finally have invented a proper can opener or maybe that maybe the world is inhabited by beautiful young women who live off the side of cliffs in strange shells so that like in the remake of HG Wells you know maybe all of these things well you could find out you just need to spend enough money and move fast enough and you can get to the year 3000 in a shorter time as you like one year one month whatever so it's an engineering challenge not a physics well let's see the important point I want to get across the only problem is that you can't come back again that is if we imagine in the Sally and Sam scenario Sally decides well it's all very well visiting Earth 18 years in the future so to speak but you'd like to go back again and be the same age as her brother she turns around and reverses her trajectory just takes another 18 years into the future so it's one way in time only so getting to the future quicker which in effect is time travel into the future is something that is straightforward now I mentioned that all you need to do is move very fast there's another way you can do it and Einstein already figured this out in 1908 and that is to use gravity gravity has the effect of slowing time time runs a little bit faster up on the roof than it does down in the basement and again this is a measurable effect you might think well nobody could ever notice such the image can be mission was measured in the 1950s of all things was measured the difference between the bottom and the top of a tower the rate at which clocks tick there's now no doubt whatever the most direct test is to put a clock in a rocket and shoot it into space so time so to speak runs a little bit faster in space and it does again we're talking billions of a second but it's a easily measurable effect now when these experiments were done back in 50s 60s and 70s it was really to test the fundamentals of Einstein's theory but today this is a matter not only a practical engineering but of money-making commercial activity because the most important example of time warping at work making dollars for somebody is the global positioning system because that depends on a network of satellites that are both moving in and out of the Earth's gravitational field and moving rather fast nowhere near the speed of light across and the relativistic effects the time warping effects of both motion and gravitation have to be factored in otherwise taxi drivers would get lost within half an hour or an hour or depende sir exactly where they are so in other words these time warping effects are so well known they are now matter of everyday Engineering so there's not the slightest doubt that these effects real the question is can they ever be big enough to give us a Doctor Who type of venture well obviously I've mentioned if you can get close to the speed of light you can go into the future a very long way if you could get a very large gravitational field you could go a long way into the future one place where there is a large gravitational field is here this is the Crab Nebula very famous this was the star seen to explode in 1054 by oriental astronomers you look at the sky now in that area and what you see is this ragged cloud of gas this was a star that blew itself up and the cloud of gas is the debris from the outer regions of the star but the core of the star imploded to form a ball of neutrons a so-called neutron star neutron star is an awesome object it's like a mass of one-and-a-half Suns squash down to about the size of San Francisco or the size of maybe the bay or something like that and probably spinning well up to hundreds of times a second so this is a fantastic concept so here the material is so dense that a teaspoonful if you went there and scooped up a teaspoonful of this neutron star matter will weigh more than all the continents way here on earth so gravity there is intense and if you put a clock on the surface of the neutron star it would ticket about 70% of the speed of the clock on the wall in other words time would be slowed by about 30% percent relative to earth time so that's a really big effect but is there any limit to the amount by which gravity can slow time well let me invite you to think about this experiments basically we took the earth and we keep the total amount of matter but we squash it to a smaller size now you probably know gravity weakens with distance so if you squash the earth to a smaller size the gravity of its surface goes up so you feel heavier and then you squash it more you feel heavier and heavier still and if you go on squashing it now what the other way you can measure the strength of the gravity is how fast you have to shoot something up in the sky for you to escape the escape velocity how fast does it have to go up to escape and never come back and the more you squash it that the more intense that gravity becomes the bigger the escape velocity and so when you squash the earth down to about this size here about the size of a pea the escape velocity from the surface of the earth would be equal to the speed of light and then you would conclude that light could not escape now astonishingly this was known already in the year 1783 by this man here john mitchell the rector of Thornhill clergyman and he applied Newton's theory of gravitation as before Einstein eunseo gravitation to work out how big a star should be or how compact a star should be in order that light should not flow out of it he said quite explicitly their light could not arrive at us so he predicted the possible existence out there in the universe of black stars stars that were black because their gravity was so intense it would not let life light escape now one way think about this is that if we think about gravity as slowing time and here we are on earth time is soda soda slowed so everything's moving slower and slower and slower relative to us so it could get to the point where light could not escape because light is in effect trapped it cannot move or make any progress out towards that's because time stands still their time is frozen and for John Mitchell this was not not his way of thinking of it in terms of time but after Einstein that's the way we think about it and we no longer call these black stars we call them black holes black holes are the ultimate time warp a black hole the surface of a black hole is in a careful sense you have to be careful a relative to us a place where time stands still so it's an infinite time ball so if you were to go very close to the surface of a black hole and reside there for a year or two years or ten years and then come back out again you would find that an enormous duration have passed in the outside universe and if you were unfortunate enough to fall into the black hole across the horizon then all of infinite future would have passed by in the universe outside so one of the reasons that a black hole is a one-way journey to nowhere you can fall into it but you can't get out of it is because if by the time you've fallen in everything that can happen in the universe outside has already happened then you couldn't come out again because to do so you'd have to come out before you fell in but you see that's the key to going back in time that's that brings me on to the next part of the talk because going forward in time as I've said is a done deal we've done it not in very exciting amounts but nevertheless we know in principle that we could travel forward in time for as far as we want it but going back in time that's a tougher proposition so that's what I'm going to come unto now traveling back in time and so the wormhole gives us the black hole rather gives us a bit of a clue but there's a history to this which I'll just take you through because when Einstein first formulated his general theory of relativity he was really worried out the possibility of travel back in time because he knew that would unleash all sorts of paradoxes that I'm going to talk about in a moment he didn't want it basically he was a Paris to the idea but he couldn't prove there was nothing in his equations to show that it was impossible now you probably know that in the 1930s he moved to the United States and he had a job at Princeton University was very old by then I'm not many people used to talk to him he was rather isolated figure physics have moved on and people working on particle physics quantum physics things like that and so Einstein used to say that the only reason that he went to work in Princeton was to walk home with Kurt girdle now Kurt gödel is an interesting character in his own right gurgled was an Austrian logician a mathematician did some of the most profound work in the foundations of mathematics but evidently when he was chatting to Einstein going home then they must have talked a bit about space and time and relativity and so on because Gogol in 1949 produced a solution to Einstein's equations that indicated that you could indeed travel back in time using a gravitational effect and the effect he had in mind was somewhat unrealistic it was that if the whole universe is rotating it will be possible for an astronaut to leave any one point in space and travel not only to any other point in space but any other point in time as well including into into past time if the universe is rotating well at that time it didn't seem unreasonable because after all the Earth rotates the Sun rotates the galaxy rotates why can't the whole universe rotate today not many cosmologists believe that because the cosmic background heat radiation left over the afterglow of the Big Bang gives us a wonderful frame of reference that would dramatically very sensitively show any rotation of the universe and we can't detect it incidentally young Freeman Dyson who was also Princeton at this time with them with girdle and an Einstein told me recently when he when he first went to Princeton girdles one of the few people who people who would talk to him and and this idea that you could have a time machine by a rotating universe was not just a an idle speculation he was really serious interested as to whether this was a possibility and he would phone Dyson from time to time to say have they found it yet had they found it meaning have their astronomers found the rotation of the universe anyway today we're not many astronomers really believe there but there are other possibilities for time travel and the most popular one is the wormhole in space now wormhole is like a black hole only different though say the black hole is a one-way journey to nowhere you can fall in but you can't get out again a wormhole will be like a black hole in as much as it would be a compact object with an intense gravitational field but this time you could fall through it and come out somewhere else and you could look through it and see somewhere else so there's all sorts of stuff out there on the Internet of what it would be like of a wormhole cross their line of sight here is one with a star or what happens if you have a spacecraft and plunge into a wormhole now a nice analogy to a wormhole is actually you there's a lot of Hollywood in this another Hollywood movie which is Stargate so wormhole is a bit like a Stargate because if we had a wormhole say where it says exit over there supposing that's not a door but that's actually a wormhole and I left through it I wouldn't come out in Mountain View I might come out on the other side of of the galaxy so that's the concept of the wormhole it's a shortcut between two distant points in space now how can that happen so let me just teach you a little bit of general relativity here on on the on the run I've mentioned that gravity warps time that was one of nine Stein's central predictions but it also warped space and people have difficulty understanding how can space be warped because space is sort of everywhere how would you notice because we can really only think in two dimensions but what space being warped would amount to is the following imagine drawing a triangle around the Sun a flat I'm not talking about a warped triangle a flat triangle now back in the girl days when students used to actually learn geometry at school to remember that some people write okay we used to land that the three angles of a triangle add up to 180 degrees to right angles so that's part of Euclid's geometry well if you draw a triangle the sound and measure of those angles carefully and people have done this it's not literally with a drawing a triangle but they've done it with radar and fancy techniques then the angles add up just a little bit more than 180 degrees and so one way of expressing this is that the Sun warps the space in its vicinity or that the Sun's gravitational field manifests itself by warping both time and space so space can be warped sometimes you hear the term that it can be curved now the difficulty about presenting talks about curved space is always that I can't show you a picture of curve curvature in three dimensions so I can only ever show you two dimensions so just go with that for a moment and I will show you what a wormhole looks like in if we had two dimensions to represent three and so what you see here is a curve sheet if I had a shimmy on let me demonstrate this supposing this is space three-dimensional space represented just by that two-dimensional sheet and what you're seeing here is it being curved around like this such that two points come together and touch and with the throat of that wormhole and so the the idea here is that the wormhole connects two points which would otherwise be a long way apart to form a shortcut and so if such a thing could exist then it would be a means of getting from A to B much faster than the conventional way of going through normal space and the term wormhole was coined by John Wheeler the physicists astrophysicist John Archibald wheeler in the 1950s for quite a different purpose he was also the person who coined the term black hole so they sound the same black hole wormhole but they're very different we now like holes exists they've been that they're up there astronomers have discovered them but we don't know at this stage that wormholes exists but we at least we know what they should look like it care are some of the people who work in this very building because you're all of st. jodie foster falling through one in the movie contact which was not a movie about time travel it was a movie about space travel and aliens and a Seth Shostak tells me that the movie people came here to the SETI Institute to ask you know what would it be like going through a wormhole so what so when you see the special effects of that movie you can blame them for for the sparks that you know looks like a bit like going through the London Underground but but anyway the point is not what would it really look like but but can these things actually exists now I've said that that that if a wormhole exists it's like a shortcut between two points in space but why is it also a time machine well his Jodi falling into the wormhole and as she's she's got to fall through from A to B and in that configuration it's it's just a rapid transit method it's not a time machine but the turning to a time machine what you have to do is this you have to take this this wormhole and bring this mama doing it bring a neutron star close to one end of the wormhole with one mouth of the wormhole now I already told you the neutron stars make time tick more slowly so to speak and so if you position it at one end of the wormhole then there'll be a discrepancy between the clock rates at one end of the wormhole a and the other end B and then if you take after a while take the neutron star away again you'll have a permanent time difference between a and B so if you jump through the wormhole in one direction you're go say ten years into the future and if you jump through in the other direction you'll go ten years into the past and now that raises the exciting possibility that you could jump through from A to B and go say ten years into the past and then zoom back through ordinary space where time isn't especially walked and get back to a before you start it out in other words you could not only visit the past but you could visit your own past you could go back to a time before you went through the wormhole so physicists call this closed timelike world line so I won't get into the terminology so just little point of history the movie contact was based on Carl Sagan's famous novel of the same name and it was primarily as I mentioned about SETI and aliens and space travel but he did want to check out and he invented the wormholes a fictional device but he did want to check out whether it would actually work and Kip Thorne and his colleagues at Caltech as I saw a bit of recreational physics decided they look at this and they soon discovered that the conditions for a wormhole to exists were rather stringent but not obviously completely unphysical but that they would soon recognize if wormholes could exists they could be turned into time machines so a more famous now I think for time machines then for space travel anyway they're not without their problems because to traverse a wormhole would be a risky undertaking and one of the problems about it is what we call spaghettification anytime there's an intense gravitational field there is this risk of spaghettification and let me just try and demonstrate this you identify only skydivers here I haven't done it myself but you know if you jump out of a plane then of course eventually reach terminal velocity but when you're in that period of freefall you're weightless and course astronauts would be better example but let's imagine you jump out of a plane vertically like this and you're weightless but your feet are a little bit closer to the center of the earth than your head so gravity is a bit stronger at your feet than it is at your head and so you're pulled your feet are pulled a bit more in your head is pulled so that stretches you length ways but each shoulder is trying to fall towards the exact center of the earth which is a point so they're on converging trajectories so you'll squeeze this way and stretch that way that's why it's called spaghettification you wouldn't notice the effect jumping out of a plane but you try jumping into a black hole or a wormhole and this effect could be totally huge there's other horrible things as well there could be radiation there could be unknown fields which are responsible for keeping the wormhole stable all sorts of other things that could be very risky so the fact that wormholes might in principle exists doesn't mean that this is an immediately practical proposition but while interests people like me that is physicists interested in fundamental questions about the nature of reality in the nature of the universe is can such a thing in principle exist now that this is the wormhole is not the only model on the market I mentioned the rotating universe doesn't seem to be rotating the wormhole maybe they're out there but there are other models as well this is ronald mallets design he he says if you have lights going around and around in a spiral like that that is one one way of creating a time loop then there's Richard Gott he's got cosmic strings which are threads that may or may not exist in the universe with enormous mass per unit length moving at close to the speed of light past each other and the time traveller would do sort of a wiggle in and out these strings as they moved and that could take you back in time but my all-time favorite alternative model time machine is John's model when I decided to write this book on time travel I thought well I'll just check you know on the internet see who's out there see what what people are working on and I came across John's model so John is from the year I think 20 34 and he works for General Electric so they're into time machines in 2034 and he made the trip back and was kind enough to provide some photographs of the time machine and even to provide the manual for operating it and I thought was really good if I was going to be publishing this book and undoing publicity wouldn't it be great to have John on hands and say well you know it's quite correct it's really be done I've come from 20:34 so I tried to track him down but unfortunately he's returned to 2034 so just have to wait a couple of decades before I can interview him about it but there we are so now I still favor the the wormhole as the best way of doing time trouble and so the obvious question is where do you get your wormhole now one possibility is that nature has been kind and when the Big Bang went bang it coughed out wormholes and that they're just out there and what we got to do is find one harvested harness it turn it into a time machine are they are they out there somewhere where our astronomers have half-heartedly search for wormholes in space haven't really seen anything and so we just don't know where the wormholes exist threaded into the nature of space out there on a large scale but one place where physicists are pretty sure wormholes do exist is not out of space but inner space that is not on a very large scale of size but on a very very small scale of size and I mean small I'm talking twenty powers of ten smaller than an atomic nucleus or so called Planck scale and here's a a picture to illustrate this and I'm not going to go too much into this I just want to say that when quantum physics which is all about uncertainty and fluctuations is combined with gravitation then the prediction is that on this ultra small scale of size the apparently continuous nature of space and time become disrupted and break up into a sort of foamy or frothy structure so that if there was some way of reaching down into this space-time foam and grabbing hold one of these little tubes of space and then inflating it up to every day Jodie Foster size and then stabilizing it and doing all the other stuff to it to turn it into a time machine well then you'd be in business now how do you go about that pressure of time I'm afraid I can't tell you but you can read about it in my book there four steps that you need to take to harvest one of these tiny wormholes right at the end I'll tell you about what is possibly an easier way of doing it but let me just move on to the inevitable paradoxes so let me take an optimistic view that one way or the other one day we will gain control over space time we will have wormholes or that there's some advanced alien community that did all this a long time ago and the aliens can come and lend us their time machine then we're faced with the problem that we can go back in time and all of the paradoxes that make time travel story is so fascinating would then be unleashed what are we going to do about that well you remember in Back to the Future Marty McFly goes back and become sort of embroiled with his mother's love life thus threatening his own existence well let me give you a more brutal version of the same thing so it's often called the grandfather paradox but I'm going to call it the mother paradox so supposing the time traveler goes back in time and shoots mother dead what what then you see so it would be a fascinating thing to try wouldn't it yeah so you know if mother is dead if the Time Traveler's mother is dead then the time traveler is never born so the time traveler can't go back in time and so we get sort of contradictory nonsense so that you know really brings it home to you that traveling back in into the past has paradoxical effects now it's not only going back say 50 years and carrying carrying out this murder that gets us into trouble we could go back just one day and then we would meet our younger self what would that be like that would be pretty weird you there would be two of you then and by extension you can just go on doing it again and again and there could be any number of you so physicists have a great belief in things for conservation laws and this looks like it would violate lots of conservation laws but it does have one bright side this time-travel research I've said what you needed lots of dollars if you want to do time-travel where the dollars going to come from you don't have to worry because you could do this with gold bars and I do it enough time and you'd have enough money to pay for the time machine now that the weirdest paradox of all is perhaps less dramatic but it's one that that sends shivers down the spines of most theoretical physicists because it let me tell you the the paradox first imagine that the time traveller goes ahead say five years it imagine it's me I go ahead five years in time and I get out say still enough in Phoenix what am I going to do as is amazing I've gone five years in the future probably soon get quite bored but the one thing I would do is go to the University Librarian check out the journals look at some mathematics journals what's new especially I then note down a wonderful new theorem that has been just been published and I think well that's pretty neat and then I decide to go back to in five years so I've gone forward in time that I can go back in time because we're talking about backward in time now go back and I go to one of my students and I say not cur this really neat theorem come up with a proof for this and the student danced on the student then publishes it in a mathematics journal that's the very journal that I have read and so that raises this profound question I'll leave the proof as an exercise for afterwards but it raises that profound issue about where has the information of this theorem come from it didn't come from the professor or the time program because he just ready in the university library but didn't come from the student because student learnt it from the professor so here is knowledge not just information but knowledge that's come into existence in the universe from nowhere and so that seems deeply and irrational so scientists who believe we live in a universe as rationally ordered then gets very upset by the prospect that we could have these sorts of causal loops that would introduce into nature this type of informational chaos and for that reason a lot of famous scientists say time travel is impossible his his one though Stephen keeps changing his mind he likes to do u-turns and I think he's changed his mind on this one a couple of times I'm not sure where he's at at the moment but by and large these paradoxes I think make a lot of physicists agree that we nothing in known physics to forbid travel back in time but if we permit it then we run into various serious problems now my take on this is that the problems are overdone because all that we're requiring there's nothing to say obviously we can't go back in time and change the past I think that's clear we can't change the past but there's nothing to prevent us being part of the past there's nothing to prevent me going back in time and becoming part of my own history or somebody else's history so long as the narrative is self consistent so let's take a happier version of the mother paradox supposing I go back in time and I save a young girl from being shot dead and then that young girl grows up to become my mother then that's self-consistent so this is the other version so then everything's just fine it's a self-consistent narrative and some people say well but then what about free will you can't go back in time and do what you want you're constrained by the laws of physics through to have these self consistent narratives but that's always true I'd love to walk on the ceiling I can't the laws of physics won't let me so we're used to the idea that we can't do everything we might want to do because of the laws of physics it's another example of that nevertheless some people are still not happy and they want both unrestricted time travel and pre will and there's one other possibility of doing that and that's that there isn't a single reality but a vast multiplicity of realities and that's a very favorite idea of the moment it's often called the multiverse sometimes the quantum multiverse and what it means is that there's a lot of us about but but we're all in separate universes so we don't see our other selves in these parallel worlds or parallel universes so very popular idea and the point about time travel is if there's a way of slipping between these parallel universes you could go back in time and you could shoot mother dead in a parallel universe and return to your own universe a mother will be alive and well and with tea ready and all that's something so maybe if you could have these parallel realities you could then have this unrestricted time travel with free will so just to summarize them before I I take some questions a time travel fact or fiction well travel into the future definitely facts we've done it not a problem only a problem dollars to do it by more dramatic amounts traveling back into the past very problematic and the dollars here are much bigger because if you want a wormhole you want to pluck out space-time firm or something you're in for a big budget but there's just one possibility and that is this machine here that you may recognize called the Large Hadron Collider it's in Switzerland here it is from the air it's not really that red tube it's under the ground it's a ring shaped tube and you probably know that it's designed to make a particle called the Higgs boson but the point is that it accelerates protons are very close to the speed of light and smashes them together at enormous energies now and to give you some idea of the scale of this machine this is you see the the track at the bottom right and this is just one of the detectors it sits on this ring shaped tube it's 27 kilometers in circumference and when the protons make collide they make all sorts of subatomic debris and the physicists are busy scouring that looking for the Higgs boson but there's another possibility and this possibility is that right up to now I've been talking about Einstein's theory of relativity that's our best understanding of gravitation but there are alternative theories as well some of them quite popular some of them involve for example additional dimensions of space and I'm sure you've heard about that now in those theories in some of those theories with additional space dimensions the scale at which you get that family stuff I talked about instead of being 20 powers of 10 smaller than an atomic nucleus with the corresponding energy 20 powers of 10 bigger than the Large Hadron Collider instead the the number that comes out where you start getting the wormholes and the black holes and all these funny little things is round about the scale that the Large Hadron Collider is reaching and that's why some people were scared that when they switch on it might make a black hole that would swallow first of all the Geneva and then Switzerland and then de then the earth and maybe the whole universe is so all nonsense trust me I'm a physicist not a point that the point being that these say if tiny black holes and tiny wormholes were to form from these collisions the process that makes them would destroy them very quickly it's it's reversible so if these collisions make tiny black holes or tiny wormholes they would live only for a very short period of time but maybe just long enough to see some peculiar time looping effect of the manner that I've been talking about so this subject of time travel and wormholes which were a long time just really seem to be the outer end of the realm of science fiction has really come back more into vogue when people think goodness we might actually be able to see some of these effects in the Large Hadron Collider needless to say nothing as has yet been seen I should say that there is a more serious side to doing this sort of stuff it is across great fun to think about time travel but the serious side is that many physicists working on fundamental problems would like to come up with a grand unified theory of the whole universe a unified theory of all of nature and for that what you need is a consistent reality there's no point in writing down equations to describe everything if the equations have come internal contradictions and looping in time looks like it threatens some of those logical foundations if you can have causal loops then there are issues I don't think that those issues are insurmountable but see there are clearly issues we need to know not is it a practical proposition but in principle is it possible for loop back in time because if it is we have to build that into our attempts to create a grand unified theory of nature so that is really the motivation for scientists to get involved but I want to finish things this is the SETI Institute by saying that there the time travel carries an implication for SETI as well and that's because something called the Fermi paradox I'm sure you've heard about this this is Enrico Fermi the famous Italian physicists and he must be famous because he's shown here on a postage stamp which is cunningly truncated to conceal the mistake on the blackboard which any physicist any physicists here will know that he got two sums wrong but anyway he was certainly a genius and and he in the late 1940s referred to the fact that that the world wasn't teeming with alien beings as he felt evidence that there were none out there because of had plenty of time to come here and the point is that if you can have time travel as well as space travel then history and geography really just become part of the same thing and so the family paradox now gets worse because it's not a question of why having aliens come here from other parts of space but why haven't they come here from other parts of time as well and so it opens up that whole issue it's a more forcefully concentrates our attention so there is a SETI dimension I think and a Fermi paradox dimension it's a time travel so I think on that note I'll say that this is the end or perhaps it is in fact only just the beginning thank you well I guess the Fermi paradox also gets extended in the in the multiverse as well if it's possible to do time travel backwards why have we not seen that in our version of the multiverse and what why would our part of the multiverse be different from other right right that once you're into the multiverse you're into all sorts of problems because in a multiverse if it's big enough anything that can happen will happen and so then there's a whole set of paradoxes as to why we do or don't see certain things and the most we won't get too technical most famous of these is called the Boltzmann brain problem that if at least in some of these multiverses they will have infinite futures and then there's always exponentially small but nonzero probability that in the vacuum quantum vacuum state of the power of our future particles will spontaneously appear and assemble themselves into a brain but we'll have experiences and because there's an infinite future there's infinitely more Boltzmann brain experiences than humans on earth experiences so why aren't we Boltzmann brains why why do we find ourselves such a typical observers all sorts of things like that but but leaving aside the multiverse which I think opens a whole can of worms one possible resolution as to why we don't see time travelers from even our own future is then in all of the designs that I've talked about of time machines you can't use them to go back before the time that you make the machine so with the wormhole you know you put the neutron star at one end and then you can imagine in the far future that you could go to the supermarket by a time machine and a 100 year time difference machine would cost you more than a ten year time difference between so you move the neutron star in you leave it there for a hundred years and you put a 30 year time difference here you take it away again so you can now go back thirty years but you can't go back more than 100 years so our descendants could not come back now and say Davis got it right but of course if there are alien communities that have been around for a long period of time they could have made a time machine a long time ago and they can lend it to us or they could come back from their own future so long as their ancestors made the time machine before now so why am i around it is that you can say well there that if there are alien communities that they're not that far along for farther ahead than us that they may time machines a long time ago all the time machines are impossible or you know there's number of other ways of getting around it um it occurs to me with wormholes that wouldn't more moles imply a very large mass I mean a mass large enough I mean to have the curvature of space be gentle enough that you could go through it and have the throat be big enough that you could fit through it without being torn apart I mean isn't that like thousands of solar masses right it's a large something but it turns out paradoxically that what you actually need to hold the throat of the wormhole open it's something like negative mass or negative energy or negative pressure because the if you think about the whole black hole thing what you've got is is an object that implodes because it's gravity is so intense so you don't want everything to shrink down you want up you want to hold the throat of the wormhole open so you actually need something like anti-gravity and the throat of the thing gravity around there but anti-gravity in the middle to stabilize it oh yes yes yeah it's going to be a very large mass so the large positive and negative masses assemble so yeah absolutely yes so if we were looking for a wormhole in space we'd be looking for a large mass with certain sorts of light bending and light chromatic effects yeah yes as for oma if you remember the famous opera incident where the physicists thought that the neutrinos had gone faster than they did no it was time dilation so my question is how would you cancel the facts of time dilation so it appeared so even if it appeared that the time machines going faster the light but if there was still time dilation you would realize that after a while know that it wasn't going faster than right right so the Opera experiment that this young gentleman is referring to is a spare and perform that certain the same place with the Large Hadron Collider very different experiment with neutrinos which were fired through the earth to Italy where they were apparently detected a tiny fraction of a second before they should have been assuming that they're traveling no faster than light and so this was much in the media of the past few months and I even wrote a paper on it myself which was published just before the retraction which was very timely because it now appears that this was a was where it's not completely certain but it looks like it was a problem in calibrating the clocks but so the I'm not sure what the question was now but the question was how would you cancel the facts of time dilation so it wouldn't just appear as I was going faster than light but it actually was right well you see if an object there's nothing in the laws of physics to forbid an object going past on the light that's a curious thing the the fear of relativity says you can't break the light barrier so you can't start off slower than light and then zoom faster and faster and go through the light barrier and then travel faster on the light but it doesn't say there's no reason why you can't have objects that always travel fast in the light and they called tachyons we don't know that they exist but they could exist and I mentioned the paper I've written so the paper is based on the fact that if tachyons can exist they'll have been made in the Big Bang and so then you have to think what happens as the universe expands what happens to these tachyons and so that's a different story but if tachyons can exist lian travel faster than light you could use them in principle so signals not to travel back in time but to send signals back in time so I could send you a tachyon message faster than the light and you could send me back a tachyon message fast on the light and if we're moving in the right way we can arrange that your answer comes back before I send my question and so then that gets you into the same type of loop so yes it doesn't have to be a wormhole doesn't have to be Time Machine just has to be attackee on them already in trouble what about the paradox that if you just go back in time you've just changed time by being there and during a different version of time with a different version of you and future you so then you would instantly just vapor be dissipated disappear so you need some way to combat that if it's true well I'm not sure about the disappear bit but you know the end I said that if there are there's not just one world or one reality there are many of them and this is a popular idea that could be many parallel realities or slightly different then you could go back and be part of the past of a reality that looks very much like your own world but you're not actually changing your world you would change that world and then if you return to your own world then then that's everything is normal but I think that this you don't even need to go that far I think the point is if you go back into the past you can become part of the past but only in a way that is consistent with the future you can't change something you can't can't go back and kill your mother for example before you're born that can't happen but you could go back and save your mother like in the other story and then everything's fine just means that there are certain things you can do and certain things you can't do yeah years ago I asked John Wheeler what would happen in a collapsing universe to the second law of thermodynamics would would entropy then be decreasing and what he said was we haven't been there yet and we don't know right I'll write a whole book on this I like that ah so my question is about things which seemed to me to be purely speculative and in my view are not yet science because there's not even a conceptual test for them yet right which would include multiverses and string theory maybe wormholes and I don't know what the situation is with quantum foam and and party springing out of a vacuum right well that yet well each one is different than each one would take me our lecture to get into but of those things that you've mentioned I mean I was careful to say travel into the future is is real traveling to pass very speculative and black holes are real wormholes very speculative particles coming out of vacuum certainly that's no problem and that's something that is yeah oh absolutely I've spent most of my career working in that field better be real and then string theory well yes of course it's a lot of people complain that's very speculative and so far there's nothing that it's not making any tests that distinguish it from any other attempts of unifying nature so that's that is a very speculative thing the multiverse I think I agree that's also very speculative there are certain we I wrote a book called the Goldilocks enigma wise universe just right for life and there I point out there are some weak statistical tests for a multiverse but they're not very not very good direct tests seem to be almost impossible and you're right so a lot of fun physics and a lot of fun science is off in this sort of never-never-land of crazy stuff which is fun to think about but it's not very solidly grounded in in experimental physics so your your complaint is verbally correct but the trouble is so that of course how is the general public to know because all these things get trotted out together we got Janna tachyons one minute and neutrinos and excellent you know they and they can't distinguish one in my books I always try to make it very clear when something is well-established and something is speculation because often they get completed Oh entangled photons are fine yeah that's again matter practical engineering the space-time foam stuff no I mean that's just it's Jeremy because we don't actually have a proper theory of quite a consistent agreed theory of quantum gravity that could tell us about the structure of it and the best that we can do which prank originally predicted is the scale at which you would expect something to happen and we there was a great of what we were now called space-time fit and also about the Recon tracting universe and I don't did my PhD on that and new wheeler very well and and and in the end I think he basically changed his mind but Stephen Hawking also flirted with the idea of time running backward in a reconnecting universe and then he retracted calling it its greatest mistake and so it's an idea that goes around the hill of or goes around comes around including the cyclic reversing time universe a few years back I was taking a physics course and they were talking about running an experiment they there was experiments supposed to be run with quantum entanglement oh that might show time travel and in Reverse you know using quantum entanglement as the vehicle to prove that did you ever hear anything about that I'm not sure what experiment that is exactly why I don't remember the exact experiment it's just another we're going to run it and I know you're very much so wheelers delay choices yes I think so but that was done it was in fact performed by Carol alley at the University of Mary landed about three years ago now it's actually many more is that but it's been done a few times and the the important part is very subtle point about quantum mechanics the important point here is is not that you are able to do an experiment that changes the past the point is that in quantum physics because of the uncertainty we used to think of the future is uncertain so we prepare a quantum state sets an electron moving in a certain way and we're we're used to the fact that we can't say definitely where it's going to be at a future time but the same thing is true in the past we can't say where it was in the past either so we we like to think there's a definite history for example a unique pathway connecting say the Big Bang to the present moment that in other words there is a single unique history but in quantum mechanics that isn't true so if you apply quantum mechanics to the universe as a whole there's there is no unique history so when we look back in time when we look when we do measurements that inform us about the past what we're doing is resolving some of that ambiguity so there's a difference between resolving an ambiguity and a affecting the past you can't use it to send messages back in time or to change what's in the past all you're doing is saying the past is fuzzy and what we do now affects the nature of reality that was in the past there's a sucker point but I've written a great length about it in my in in the book called the Goldilocks anemia already mentioned towards the end all about we distill a choices program what it really means and how this changes our notion of the nature of the past but it doesn't provide a way of changing the past so it's in that book very subtle I'm prayed yeah well at the risk of showing my lack of knowledge of language I'll throw out this question a lot of things I don't know does it make any difference in having this time effect slowing down time whether you are accelerating towards the speed of light or close to it or whether you're going you say you've got to the speed of light and then you just keep going at a constant velocity right here the speed of light right it's it's actually elementary mathematics to to work it out no no no no I'm talking about soon our junior high school level mathematics it really is very very simple yeah and the formulas very simple and it doesn't require anything except elementary algebra well for that one you actually have to be able to perform a simple integral but that's all you need to do and then you can work out precisely what the time warping effect is along that trajectory and basically the story is that if you continue to accelerate of course you're getting closer and closer to the speed of light and that makes the time warping effects even greater so if you accelerate at 1 G for example for a year I prayed higher once told me the answer I can't remember D was but but you know the effect is enormous you're traveling so close to the speed of light the time warping effect is is really enormous but another statistic to remember which I found very arresting these cosmic rays of particles moving very close to the speed of light and if you take the highest energy cosmic rays and imagine you were traveling with one of those particles across the galaxy it would take only 15 minutes in your frame of reference across the galaxy which is quite something in there that moving so fast I was looking at some Fineman diagrams and the implication of quantum tunneling electron implies that the electron is going backward in time is that true for quantum tunneling uh yeah I think that you mix up a couple of things there so Fineman had the idea that a positron might be an electron going backwards in time it's not quite the same thing as quantum tunneling about things you raise quantum tunneling because there's another research interest of mine I've written some papers on that and there's the old question about how long does it take a particle to tunnel so people don't know about quantum tunneling so I presume behind there there's a window okay so now if I got a rock and I threw to the window it might very well smash the window and go through or it might serve it's a tough window might bounce off what we wouldn't expect is to find the rock on the other side of the window without the window being broken but that's exactly what happens with quantum tunneling if an electron and a barrier and it appears on the other side without having shattered the barrier or apparently surmounted the barrier so this real effective cause very well known and then the question is when it disappears from one side and appears on the other side how long does that take and that's a very difficult problem not difficult in the sense of the mathematics is hard it's just difficult to decide what are your criteria for measuring the time interval and you know I've got my own take on that it would get too technical to talk about it but but there's nothing going backward in time but there was an experiment that was done a few years ago which gave that impression perhaps which was sending in this case it wasn't an electron that was tunneling it was a photon a photon can also tunnel through barriers and so it was a an experiment where a quantum interference experiment where one path went like this and another path went through a barrier like that and then you could take the barrier away and put it there and you would expect putting the barrier there would slow down the photon taking that path did the opposite sped it up and you for well if it's traveling at the speed of light it's now getting there doesn't that mean it's going faster than light the answer is no but for all sorts of subtle reasons concerning the shape of the wave pulsing and other things but there soon there are these effects in quantum mechanics and look like they're very coming very close to in some way breaching the light barrier but when you look at the fine print you find they never actually do it okay probably I'd encourage you all to come and chat to Paul laughs briefly anyway he's gotta catch a flight back to Arizona and to make sure you can take this time off in along and make sure you don't miss your flight or something like that I'm sure Oh

Channel: SETI Institute

Views: 164,727

Rating: 4.6683416 out of 5

Keywords: SETI Institute, Carl Sagan, Paul Davies, Time Machine, Space Science, time travel

Id: AECmzuzJoKo

Length: 67min 5sec (4025 seconds)

Published: Sun Apr 08 2012