Jim Al-Khalili: Is Time Travel Possible? Determinism, Relativity and the Arrow of Time (2011)

so please welcome Jim al-khalili excellent thanks very much for that well good afternoon everyone so a bit of a bit of hard science to entertain you for the next 3/4 of an hour or so this is a talk which I guess I've given over the years to a range of age groups and and coming up this morning I sort of have a whole array of PowerPoint files where it which I look over and swap things around try and keep things nice and fresh so the nature of time is something that as has been debated by scientists theologians philosophers great minds over the past few millennia what is time how fast does it flow one wag I think once said time goes by at the rate of one second every second she's a bloody stupid thing to say of course and you can't measure something against itself but our our sort of common-sense view of time is is one in which is one that we inherited from Isaac Newton basically it's one which we have we imagine there's this cosmic clock that ticks by the the seconds minutes the hours the days the years at the same rate for everyone and we know our subjective view of how fast time goes by is not something we can always trust if you enjoy this lecture to go by quickly if you're bored it'll drag I mean you know we know that but but we sort of assume there is this time some cosmic time that goes by at the same rate for everyone we also now know that of course that view of time is flawed that your time as separate from space and our universe is not the way we see things now in modern physics we I also want to touch upon the issue since I'm going to be talking about time the past in the future or to touch on the issue of determinism the idea that if in principle we could know the position and state of motion of all the it's the building blocks of the universe we knew all the forces between all the particles the atoms and the particles make up the atoms for knew what everything was doing and where everything was in the universe at a given moment in time then this view of time that we inherited from mutant tells us that we should in principle be able to compute how the future will evolve that's what I mean by deterministic universe it's what we call the Newtonian clockwork universe and of course since we our bodies are only you know our brains are made of atoms particles physical entities subject to the same laws of nature as anything else then in principle knowing the future means knowing what our actions will be in the future so determinism tends to suggest and this is a deep philosophical issue though I don't get into too deeply but determinism suggests that the future is preordained that we have no no free will well this was compounded I guess in that by the turn of the 20th century when Einstein developed his his German his theories of relativity in Newtonian mechanics we have the idea that fixing the initial conditions say in a game of Paul will will will tell us how how that we will know we were able to compute how the future will evolve now of course we know now there's something called chaos theory which I won't talk about you'll be relieved to know in this lecture which suggests that we can never know to infinite accuracy exactly where every ball is so that we can crank the handle the equations to work out what's happening in the future because it's just impossible you know the slightest change in temperature or humidity or grain of dust somewhere would eventually magnify and alter the future this is what's called the butterfly effect but it doesn't rule out this notion of determinism there in principle where we able to know then if you is fixed in 1905 Einstein published a number of papers which revolutionized physics he proved that theoretically the atoms exist he he described the nature of light as being made of particles but most famously he came up with his special theory of relativity now for most people non scientists when you say what do you know about relativity theory all equals mc-squared you know a lot of people know what the E and the M and the C stand for but actually equals M c-squared is boring it's it was even an afterthought as far as einstein was concerned it didn't even appear in his first paper on the electrodynamics of moving bodies in 1905 what was really profound about Einstein's special theory of relativity is that it gives us this new vision picture of space and time Einstein said that there isn't this cosmic clock that goes by at the same rate for every one space isn't just the stage on which things happen space and time are part of the fabric the structure of the universe itself which means we have to some extent some control over space and time or indeed we have different perspectives different views on distances in space and intervals durations of time well what does this mean it let it leads to this notion of what's called the block universe now we know we live in three dimensions of space right I can move forwards backwards left right up and down all solid objects in our universe we know we say there are three dimensional we know what that means and then there's time we can you can think of it even before I know some it can think of time as a fourth dimension if you want to define an event something has happened you have to say where it happened and when it happened for to know everything about it so there are sort of four numbers the x y&z coordinates these are the 3d coordinates that fix its position in space then you have time well Einstein said you can't separate space and time as set of three numbers and one number time is almost like another axis so you need to talk about four-dimensional space-time now we can't imagine what four dimensions looks like our brains are only three dimensional and so what we do is throw away one of the dimensions of space imagine our space is flat is rips of cardboard cut-out you know just in a flat picture on the screen and so what I've got here is a block universe it's a 3d block meant to signify the whole universe the whole of space but space is just this flat surface because I'm so thrown away the third dimension because then I can use it as a time axis okay so that's the whole of space and time creeps along for us along this axis so you can imagine a cross-section through this universe which is our now out this this moment and that now slice is moving along from left to right the high now our now is the past in front of us is the future and a point on the now is our here and now okay this is a very useful way of solving problems in physics certainly where Einstein theory of relativity becomes important as they when when where we might want to use that but what it suggests is is something even more dramatic even more bleak in a way the Newton's clockwork universe because in Newton's universe you say okay the futures fixed is preordained you know what's you know fate as it were but it hasn't happened there it's yet to unfold even if we could in principle predict what would unfold in Einsteins block universe the future is just as real as the past everything is frozen static what sort of viewing or we could never have this view of the universe this is like a picture out spot outside of space and time this is the view that God has her oh I say that sometimes just in cases you know people say oh well that would be the view that God has so does this really mean that you know the future is there it's happening you know that our present moment is no more special than any moment in the past any moment in the future well Einstein believed that but it's a very extreme view to have it's not really necessary to view all the future as fixed in this deterministic way it does suggest that the future is predetermined but it's not the whole story we tend to use relativity theory we do things called we draw space-time diagrams and we we solve problems about different observers moving at different speeds relative to each other and and measuring time according to each of them so those sort of tricks this is like this is like the block universe picture but I've only got one distance access of one time axis for physicists it's very useful but for the rest of the world we would like to know what does it tell us about the nature of reality you know is there such a thing as fate do I am I really responsible for my actions I mean does it matter what I if I think I'm doing something according to my own freewill am i really following just a preordained path into the future that I'd have been able to predict if I had a computer powerful enough well the thing about Einstein's dear relativity is that it gives us this new picture of time it tells us something about how time flows when I stand discovered in 1905 is that as objects move close to the speed of light strange things happen so if you strap a clock to a rocket and have the rocket fly past at saying 99% the speed of light if you could track it as it goes by you would see the clock ticking by the seconds more slowly than an identical clock you have on earth time is running slower on board the rocket if you were onboard the rocket you could say well all motion is relative relativity and it's actually the earth that's hurtling at 99% of speed of light in the opposite direction and indeed what you would see is the earth clocks running slower but this is more than just an optical illusion time really runs at different rates according to different observers how fast they're moving you can slow time down when you go very fast this leads to something and famous example in relativity theory called the twins paradox the idea is that you know you have two twins and they're always in physics by they're always could Alice and Bob it's a and B because that's a very imaginative and met way if ever you haven't you need a third observe it's always Carol so imagine Alice has the rocket and she can fly off close to the speed of light and zip around the galaxy where as Bob's a bit of a coward and he stays on earth if she travels close to the speed of light and I don't want to get into the technical details here or the fact that you know it should be a symmetric view one way or the other this is an assignment I set to my undergraduate students to resolve but you know bear with me if Alice travels around for what she would regard as one year so she will have aged one year before she returns to earth the computer on board the rocket will tell her that one year has gone by she feels perfectly normal she doesn't feel that timer's been going faster or slower when she returns to earth she could find that 10 years have gone by on earth and Bob a twin is now nine years older than her she's aged just one year and there's nothing special about the one year and 10 years difference it depends on how fast she's going how close you can nudge towards closer to the speed of light she will come back after just a week of traveling around and a thousand years a million years have gone by on earth by slowing time down you see this gives us a means of getting to the future in the sense this is time travel so is time travel possible I can answer the first part of that question because it turns out time travel into the future into the past or two different things time travel into the future is possible but it doesn't necessarily mean that the future is out there waiting for Alice that the futures already sort of unfolded and she's getting to that future all that's happening here is that she's moving out of Bob an Earth's time frame and fast forward into the future she's getting to the future before everyone else less time has gone by for her now you might argue well that's not so different to suspended animation you know or what you fall asleep and you think you've only been asleep for ten minutes and two hours have gone by but her time really has slowed down she really has time travelled into the future and it is not just some theory that some buffin Einstein and others have come up with we know this happens it's been proven time and time again experimenters back in the early 70s got to atomic clocks and synchronized them and put one on board an aircraft and flew it around the world and then compared them again and they saw that there was a tiny difference now the aircraft doesn't go anywhere near the speed of light but the time difference was enough that they could set such that they could measure a tiny fractions of a second difference between them and it agreed entirely with Einstein's theory of relativity now a missing part of the story here because there's another way that we can slow time down in 1915 Einstein completed an even greater theory the general theory of relativity so the first one 1905 on was called the special theory it was special because it was seen as sort of a only part of the story only part of this larger more exotic theory in general relativity Einstein explains about how not only we have 4d space and time space time but how space-time curves and it curves due to gravity so basically he's he's done another one on Newton whereas Newton had this idea of space and time being separate and Einstein said no they have to be combined now he's attacked Newton's law of gravity Newton was the guy sitting under the apple tree of the Apple falls on his head new things are things fall okay maybe people know that things fall Newton what he said was there's this invisible force and like a rubber band that that pulls all masses together the Apple falls to the earth for the same reason that the earth is kept in orbit around the Sun Einstein went further Einstein said that gravity is not you don't have to think of it as an invisible force between objects there's a deeper geometrical explanation it's about how space space-time in fact is curved and you know there are lots of examples in popular science books about space-time being like a sheet of rubber and that has a dent in it where there's a mass and other objects fall roll into the dense that's exactly the way objects move under the influence of a gravitational field but what's interesting here is that what gravity also does is slow time down in the same way that traveling close to the speed of light slows time down so does gravity and whereas when you travel close to the speed of light there's always that sort of confusion well hang on a minute what's really slowing down if Alice and Bob Alice could say Bob went off the earth went off in the opposite direction came back why is it Alice is the one who's whose clocks will run slow and she's gone to the future and not Bob the technical detail is is in the fact that Alice has had to speed up and slow down she's simulating the effects of gravity you know when you speed up on it's a honor on a plane about to take off on the runway and you feel the g-force g40 acceleration does the same thing as gravity it slows time down literally so here's a nice example if my watch is running slow as I'm holding it down by my side I could lift it up into the air so that it's slightly further away from Earth's gravitational pull and that will speed it up it would run a bit faster now of course in practice I'd have to hold my arm up in the effort billions and billions of years you know just to make up a second it's probably more sensible to buy a new watch but that really does happen time really albeit for tiny fraction seconds time really will run slower the closer it is for the earth time on the surface of the Sun runs slower than it does on the surface of the earth because Suns more massive time out in space where there's no gravity can run faster than it does on earth GPS satellites the fact anyone who's used GPS in their car or in their smartphone is making use of the fact that you know we understand how time slows down the satellites in orbit around the Earth have their clocks running slightly slower than ours because they're moving relative to us but at the same time their clocks should be running faster than ours because they don't feel gravity they're in freefall around the earth and the computers that work out you know the time very accurately in order to locate positions on on the surface of the earth very accurately have to take into account the fact that you've got the satellite clocks running slower because of special relativity the idea of you know get close to the speed of light in clocks run slower but at the same time they would run faster because of general relativity because they're not feeling any gravity and those two effects are in competition but they don't cancel each other out which one wins depends on how high an orbit the satellite is intense how fast it's moving you have to take this into account otherwise we would never be able to locate our positions to within the few metres that we start to we've become accustomed to these days okay so we can toast slow time down we can travel into the future we can slow time down through powerful gravitational fields much more than we can on earth this is a famous picture of the Crab Nebula it's a cloud of dust and gas thousands of light years away but still in our galaxy and this is the central heart of the nebula and what you see there's these two spots here ignore the one on the right but the one on the left is the center of this nebula it's actually the remnants of a supernova a star that's run out of its nuclear fuel and exploded and a lot of its material that's left over collapses under its own weight and forms this very dense object called a neutron star now stars even bigger than the one that created the Crab Nebula when they collapse their gravity is so strong that they will collapse into a black hole black holes are simply dead stars and they are predicted by Einstein's theory of relativity even though at the time he didn't really believe they existed well they do exist we're almost certain of it we see their signature out in space when we look through our telescopes and they provide us with the possible means of building a time machine which I'll come to at the end of my lecture ok determinism the idea of the future being predetermined and preordained because of relativity got a bit of a wake-up call in the 1920s quantum mechanics is the other great theory of 20th century physics and whereas general relativity Einstein's second theory which describes essentially the whole universe how gravity affects the nature of space and time itself from general relativity grew out the field of cosmology and prediction of the Big Bang and so on quantum mechanically other hand is the theory of the very tiny the theory of the subatomic world and it's much weirder than Braille theory believe me it's is to do with cats in boxes being dead alive at the same time my favorite term picture of what quantum mechanics is about is the quantum skier ridiculous as far as we're concerned there he is you know he's fine but somehow her now he's gone through past that dead tree on both sides that's what things down at the quantum level really do and no one has a has a common sense sensible way of explaining how that happens well I'm not going to talk about I will say is that quantum mechanics rescues us from this bleak determinism because down at the level of atoms even something like an when an atomic nucleus will radioactively decay when it's going to spit out an alpha particle is something subject to the laws of chance and probability statistically if we have trillions and trillions of atoms we can work out something called their half-life how long does it take before half the atoms are going to spit out alpha particles and decay but you look at one individual atom and you can't tell not because our theories aren't clever enough but because Nature herself hasn't decided yet when this is going to happen quantum mechanics gives us back in determinism so our future is not decided don't question mark where it shouldn't be okay even nature hasn't decided what's going to happen next it means they even with the most powerful computers once we have to take into account that down at the very basics the building blocks things haven't decided what's going to happen next that means the future is open it means we've rescued in a sense we've rescued freewill different philosophers argue about whether this is actually true or not the American philosopher Daniel Dennett I think would argue quantum mechanics doesn't rescue freewill so it says there is still actually a topic of of debate I mean I'm not to make the sound as though 1920s quantum mechanics and suddenly okay we have free will we are responsible for our actions there is still a big discussion going on okay but so that's opened up future and I've said well we can travel to the future but we're not really going to the future that's already there we're just sort of getting there faster than everyone else that's not real time travel real time travel is if we could go back to the past and the same way we could say real time travel will be if someone from the future that pre-exists came back to visit us they are traveling to their past so can we travel back in time well as it happens Einstein's general theory of relativity the theory about gravity curving space-time and slowing time down and so on but the theory that predicts black holes the theory that gave us the Big Bang also tells us that according to the mass time travel into the past should also be possible it allows it things called closed timelike curves despite that most physicists I think if were if you asked certain have put money on it they would say time travel into the past it can't be possible not ruled out because their theories or the mathematics or the equations tell them because it doesn't make sense because it leads to paradoxes so what I'd like to do is give you two examples of time travel paradoxes just to show you how weird stupid things would be if we were really able to travel back in time now that doesn't stop us from you know enjoying time travel stories and films and so on but let's try and force the paradox okay I'm assuming most people in the audience are okay with the Terminator films but just in case for those Sadow's who haven't seen any I should point out you should and I'll just quickly run you through the basic premise of the first of the Terminator films Arni here is the Android from the future sent to our time 1980 when the film was made by the machines that rule the future now the machines are in this battle against the rebel humans the humans are led by John Connor he's the good guy machines can't catch him in their time in the future so they send the terminator back to the Past to kill John Connors mother before she's given birth to him because if she doesn't give birth to him he would never have been born in the first place they've changed the future it's another take on what's famously called the grandfather paradox well this being a Hollywood film of course Arnie fails to kill John Connors mother Linda Hamilton this being a Hollywood film John Connor sends his best mate back into the past to protect his mother and if you remember the film he does much better than protect her he gets her pregnant and she gives birth to John Connor so his best mate is also his dad and it's all very silly but it's it's good fun okay where's the paradox well I'm going to force the paradox what if bear with me here the machines do capture John Connor but for some strange reason they decide not to kill him they send him back in the past to kill his own mother okay this is the take on the grandfather paradox if you kill your own mother in the original once you have to kill your grandfather before he's met your grandmother before she's given birth to your mother before and I've always wondered why you have to skip a generation why I have to kill the male kill your own mother and be done with it it's a cleaner scientific experiment so John Connor is given some kill your mother serum to brainwash him and he goes back to the past and encounters mother what happens as he shoots her dead you know does he sort of fade out of existence does he suddenly disappear that he was never there in the first place you know he was she's dead he was never born so he wasn't there this is this is the paradox you see of course you know the police come to investigate who is the murderer where wasn't John Connor perfect alibi never born in the first place you can't argue with that but of course he was never born in the first place you never would have grown up and gone back in time and killed her so she survived gave birth to him he grew up and went back and killed her so he wasn't born so he didn't kill her so he was born and so on standard time travel paradox well despite that there are physicists serious physicists who work in relativity theory who would argue that that you know they trust the maths and they say relativity theory says time travel to the past as possible there must be a way out of the paradox the quickest solution is to say that John can't kill his mother because there he is clearly he must fail in his attempt to kill her because she did give birth to him there he is standing trying to kill her as proof that she gave birth to him killing his mother would lead to what they would call an inconsistent solution and that's anaesthetise imaginative turn of phrase but it's not very satisfying is it again it's how it starts to play with this issue of free will why would John ferry on Connor failed to kill his mother what if the machines pull him back to their time you know say well why did he fail well they say well okay maybe you know he comes to his senses at the last minute maybe the trigger on the gun gets stuck maybe he's just a lousy shot you know and every time he shoots his mother does a back somersault out of the way and need to Hamilton was pretty fit in that film I should add in both senses but so it doesn't matter he must fail because there he is what are the machines pulling back dose him up and kill your mother serum extensive shooting lessons you know lots of lots of weapons that you can call upon is he going to fail every time he tries to kill her somehow it's back to it's now in your face determinism you know whereas we talked about is the future preordained and and you know do we have free will we're now the future is fixed in the same way that the past is fixed because they're linked together you know he knows is he trying to shoot her well I'm aiming it but something's going to go wrong I know it is because clearly I must fail because I'm here well there is another way of getting round the paradox and in my mind it's the only way that rescues the possibility of time travel into the past and that is that our universe is not the only one there are parallel universes now there are plenty of reasons within modern physics to argue that parallel universes might exist we have not a shred of evidence that they really do but they're very handy parallel universes would would remove what's called the anthropic problem the anthropic principle the fact that you know the our everything has been so specially designed for us to be here what how could you know all the laws of nature tweak perfectly for us to be here well if there are parallel universes then we just happen to be in the universe that was right for us to evolve in and ask the question how come we're here there are many other universes where we didn't evolve parallel universes are very useful in explaining some of the weirdness of quantum mechanics here it helps us because it would suggest that when you travel back in time you hook up your here and now to the Past inevitably that past will be not the past of your own universe there are some very serious scientific papers looking into this and so John Connor when he travels back into the past he slides into a parallel universe one very similar to his own in that universe he can kill his mother all that happens is that he would then never be born in that universe while his own original mother carries on living healthily in the original universe so parallel universes remove the paradox as long as you buy in two parallel universes let me give you another quick time travel paradox the Monalisa paradox unless let's imagine Leonardo da Vinci has just finished his great work of art and he's decided to take a break and pursue his other great love which is science and inventing things let's say along with all the pulleys and levers and screws and mechanisms invented he's figured out how to build Time Machine a hammers awareness shed building a time machine when it's finally complete he goes to bed that night rather concerned about how he's going to test it out you know clever guy he knows there are time travel paradoxes well imagine he goes down to the shed the next morning and to his surprise he sees the time machine has been activated in the pod is an item actually it's a painting and he quickly realizes this is a painting that's been sent to him from the future it's a painting of a woman with the same facial bone structure as his Mona Lisa the same long dark hair but without her famous enigmatic smile and he realizes quickly it's her ugly twin sister moan a lot whom he's been avoiding having to paint for some time now but he's got the sets you see so and there's a note there from the future Leonardo saying here the painting of the moan a lot as proof that the time machine works I sent this to you on such-and-such a date such as such a time in the future there are 2 power M paradoxes here one of them is a bit like the terminator paradox you see it's about this the future being preordained Leonardo da Vinci knows that one day he will become that future Leonardo da Vinci who put the the painting in the in the time capsule what if he thinks no I want to keep it I've got I've got the set now I'd like to keep both what's what's the worst that can happen is a Time Lord going to come and say you're gonna rip the fabrics of space and time you have to put in it's a paradox he has to put us in because he does the future is fixed he does become that future Leonardo ok we've sort of covered that in the last parallel so let's say nerado to Vinci puts the nimona lock painting in the time machine sends it back to the past at the allotted time goes on into the future quite relieved that he only has in his possession now the Mona Lisa painting and that rather strange episode is in his past there's still another paradox you see at a certain time Indiana DaVinci's life he had in his possession the Mon alot painting but at no point did he ever paint it he found it in a time machine kept it for a while put it back in time machine here's this wonderful work of art that's caught in a time loop that was never actually created at all there's something called a bilking paradox again the only way out of these paradoxes is to subscribe to this notion that's hyped that some parallel universes exists because then you could have two Leonardo's that Leonardo in the future who sends back the painting that he painted slides back to our Leonardo in our universe and there he can keep it time travel into the past seems to require parallel realities otherwise it leads to paradoxes that we can't explain and yet Einstein's general theory of relativity says nothing about parallel universes but it does say that time traveling to the past should be possible well how would we go about building a time machine the best current ideas go back to Carl Sagan's novel contact which is now opposition some years ago now made into a film starring Jodie Foster Carl Sagan being a being a practicing scientists himself when he was writing the story of contact about how it make contact with an alien civilization he was looking for a means within physics to have a sort of a shortcut through space-time that didn't invoke some nonsense to do with dilithium crystals and so he sent the manuscript to and to another physicist in California Kip Thorne who developed this idea of what we call a wormhole so you Kip Thorne was an expert on black holes and from what we know about black holes our theory of black holes is that you know they have an event horizon see I think yes okay a black hole has an event horizon which is the sort of the point of no return if you get too close and you go beyond the event horizon you're sucked in and you go all the way down to this the singularity a lot of work has gone into black holes research we've seen signatures of black holes out in space because of the way they affect other stuff around them like nearby stars or nearby debris and how its sucked in you don't see the black hole itself of course by definition but we're not sure exactly what the inside of a black hole would look like and it's not clear whether the inside of a black hole is a point a singularity to Folly and get squashed down to zero size or is it some sort of a gateway that would lead to a different space-time so Kip Thorne played with this idea for Carl Sagan's story he said if you could link to black holes and monkey around with the maths so that you remove the event horizon so you allow it so during a two-way travel and it's stable you create what's called a traversable wormhole Carl Sagan was very happy about this he could use this idea for his story and he did as a means of short-circuiting to distant points in space how does he do this well we know imagine our space as a sheet of paper I'm throwing away one of the dimensions now shortest distance between two points is a straight line a and B but if you were able to bend the paper you could imagine an even shorter distance between a and B that would involve jumping out of the paper this is what we could imagine the curvature of our universe or our space-time could be like well just think of it as our space being curved but we can't imagine it with we've got two dimensions here and then when you curve the two dimensions we can imagine it because that's that's turned it into curvature in a third dimension we can hold that picture in our heads but take a volume and curve it what does that mean we can't okay but so it's like this only one dimension more and it gives us a shortcut we'll Kip Thorne and his collaborators soon after he had provided this advice to Carl Sagan for his novel they realized of course shortcuts in space should also mean shortcuts in time so wormhole wouldn't only join two different points in space it should also be able to join two different points in time and hence act as a time machine led to all sorts of wonderful stories and movies and programs about entering a wormhole or Stargate or whatever and going sort of from one universe to the other or for one part of one universes to another part of the same universe at different space and time or just going back and forwards into the past if time travel were possible therefore this is how we would make a time machine build a wormhole easy enough Michiyo Kaku American physicist has written about this about how how you would create a wormhole and how you would turn a wormhole into a time machine I want to give you just in case I haven't induced a headache completely I see I have the countdown is that eight minutes okay III do want to give enough time for questions so I'll go through this very quickly so guaranteed to give you a headache remember Alice and Bob alice is the one who flies off in the rocket coaster speed of light she comes back to earth less time has gone by for her excuses you know she's come back in the future what if she and Bob both have one end of a wormhole so this is what you'd see the wormhole mouth the other mouth of the wormhole is out of our space just like in the folded paper the tube throat of the wormhole is not something in our space so actually the other end of it is over in the rocket that's where alice is in the rocket he can see her through the wormhole shortcut and of course she can see him from her end if she were to travel around the galaxy close to the speed of light when she comes back to earth let's say one year has gone by for her ten years has gone by for Bob the wormhole is now provided a fixed tunnel between those two time frames so it becomes a time machine that can take you back and forwards he can go nine years into the future she can go nine years into the past if it's that way around so you have got confused now so wormholes would allow the possibility of time travel into the past current scientific thinking is still not clear they on whether wormholes even exist in principle mathematically they you know we think may exist but then mathematically we think time travel to the past is possible no one knows whether there's a loophole in Einstein's theory of relativity that needs to be closed to rule out wormholes and time travel into the past it would seem as though we are looking for a new theory and a lot of physicists would say it's it's a theory of what's called quantum gravity so it's linking together these two important ideas in twentieth-century physics quantum mechanics the theory of they're very small and relativity the theory of the very large it seems if you we were able to mesh those two together we might be able to have a better understanding of the nature of time itself well how close are we I would argue not very close at all Stephen Hawking thinks otherwise he thinks we're very close to the final answer string theory or m-theory there's lots of other candidates as brain theory there's loop quantum gravity as far as I'm concerned they're just clever mathematical tricks at the moment a scientific theory is one that is testable so I theory is one where we can do experiments to verify or not we don't even know what sort of experiments even though some physicists at the Large Hadron Collider think they may have a way of testing I don't think it's clear yet whether we would know which of these is right if indeed any of them all right it will be nice to know in my lifetime whether time travel into the back into the past is possible where the parallel universes really exist or not and aren't answering some of those questions in the meantime we can enjoy time-travel stories but whether we will ever be able to build a time machine I think there's another matter entirely thank you very much thank you and I see from the the clock that's counting down I have five minutes so I'm happy to take questions for the next five minutes yes I think you have to come to the microphone yes sorry I think my head just exploded but Wow no it's fantastic and given all comes razor that the the simplest explanation is probably the best if paradoxes lead us to assume that there must be therefore parallel universes and how about you can't travel back in time at all is that possibly the better theory than parallel universe absolutely I think I think that's probably yeah I'm trying to entertain I'm trying to open up possibility that it might be possible but actually a push comes to shove I would rather rule out time travel to the past along with parallel universes any day yeah hey that answer might make my question sort of invalid I was just going to ask if time travels fast as possible what would you how would you answer like the Fermi paradox of time travel you know where are they okay yeah so this is the the idea that if tonto into the past as possible then surely some future generation scientist would have figured out how to build a time machine and they should have visit us and be among us today maybe that's what UFOs are and they're lots of ways of explaining that first of all it may well be that you know they're there among us but they just keep a low profile well either nutters see them but actually know the act that the the proper answer is that if san traveled to the past were possible and you know a new you to hook up different times you can only travel back the earliest time you can travel back to is the moment the time machine is switched on that's the earliest point in time that you can hook to so you can't build a time machine that will take you back to the age of the dinosaurs now if there was some naturally occurring time machine because of the weirdness of space-time from the early universe that's waiting out there for us to go through it maybe but if you build a time machine tomorrow the earliest we can ever travel back in time from the future is back to tomorrow so all that's if we don't see time travelers from the future all that tells us is no one's built a time machine yet yeah can ask how you define a year if Alice traveling around if we thought we define the year by the amount of time it takes the earth to go around the Sun Sophia that's gone wrong with some ten times it has gone wrong some ten times so surely Alice has just been away for a year and saying what so for ten years how are we defining else's you what you do is you say there are different definitions of time according to your frame of reference so if you're on earth and not moving you that's that's a year what Alice will see is you know the time going by and she'll shoot what she will see is the earth going around the Sun much much more you know more slowly well that the whole the whole solar system zipping away from her but if you could focus on the solar system as it moves away or towards her she goes and comes back she'll see the earth going around there's so much more so she said well in my frame of reference now the earth has slowed down completely and and you know so for her one year has gone by but on earth they will see her time slowing down it's it's all down to your frame of reference no one is right or wrong we would argue we know we're used to the earth spending a year going around the Sun that's what defines a year and anything else be damned but that's that's just from our perspective you're not nodding very Kuja now well and let's notice - I mean the idea of different perspectives about space and time is something it takes a while in a pop-up course on special relativity to understand it's a bit like saying you know how to look at a box and I see a side-on view and I see my myface rectangular you see my face we'll squashed up because of your perspective we see shapes in space differently because of our different perspectives and points of view while relativity is like that I only have to think about space and time altogether if you're moving fast relative to each other you see distances and times differently so you'll see lengths shorter and you see times differently and it's only when you combine space and time together in this beautiful block universe that we can all agree it's a matter of perspective but but the math starts to get a bit more complicated that's why so many people have published papers proof I mean I get I get the papers quite regularly you know from you know from nutters it's very sweet I have proved Einstein's theory of relativity wrong with nothing more than GCSE maths and this is my you know thirty years of research and [ __ ] I mean it's it's yeah really was that the nicest phrase of one he said I have proved that reality is orthogonal to the speed of light I keep them in a box they're nice so so yeah I mean there is the explanation but it's not one which which is you could be readily convinced by just with a few sentences I'm sorry yeah I think unfortunately we're out of time that's gonna be the last question I'm so sorry possibly later I'm around oh you'd last questions okay was that was it I think we're out of time oh we are just sorry but there is a chance to ask you later on so absolute how the race round of applause then gentleman please Ernest you know

Channel: Question Explore Discover

Views: 228,097

Rating: 4.763082 out of 5

Keywords: qedcon, 2011, Jim Al-Khalili, skeptic, science, education, physics, university of surrey, theoretical physics, manchester, qed, General Relativity, Special Relativity, Quantum Physics

Id: TQhNS3u6f5Q

Length: 50min 16sec (3016 seconds)

Published: Tue Feb 26 2013