NOVA Time Travel Episode

[Music] if it ever becomes possible to travel back in time it may happen because one man asked the right question [Music] in the early 1980s the astronomer carl sagan was writing a science fiction novel about earth's first contact with aliens unexpectedly it sparked a scientific investigation of time travel in contact the heroin eleanor araway was a radio astronomer engaged in the search for radio signals from extraterrestrial intelligence well she receives a signal and the single after much decoding turns out to be a machine and the machine is a means of traveling great distances inside the machine a space traveler would sit in what looked like an ordinary armchair [Music] in the movie of contact the chair went high-tech but the idea was the same a mysterious machine designed by aliens that would transport a human across the universe but in writing this story sagan ran into trouble our galaxy is so large that it would take his heroine thousands of years to reach her destination that was my problem to get her to a great distance away from the earth in the milky way galaxy to meet the extraterrestrials and come back and do all that within the lifetime of the people she has left behind what sagan needed was a shortcut through the vast reaches of outer space and this search would unravel the secrets of time travel in the early 1980s there was a common misunderstanding that you might be able to uh trap from one place to the other in the galaxy without covering the intervening distance by plunging into a black hole but there was something about the whole idea that made me nervous and it was for that reason that i contacted kipthorn kip thorne was an old friend and an expert on black holes i was a little upset because he had the heroine in his novel traveling through a black hole and i knew that you can't go into a black hole and come out somewhere else the fundamental laws of physics forbid it a black hole forms when a large star dies and collapses to a small infinitely dense point with immense gravitational pull this warps space so severely that everything nearby is sucked in and destroyed making travel through a black hole impossible if you go down through the horizon of a black hole at the center you don't find a tunnel that leads you to some other place in the universe what you find instead is a region where the material of which your body is made the atoms get stretched and squeezed beyond recognition and then space and time themselves get stretched and squeezed beyond recognition and destroyed but if you've gone inside a black hole trying to travel through it uh that doesn't matter uh you're dead there was no way you can get through i got back a long letter from kip with about 50 lines of closely reasoned equations which was a level of detail in response to my phone call that i had not anticipated rather quickly i recognize that what he probably should do is replace the black hole as a means for rapid interstellar travel with a wormhole at that time wormholes were not something that were part of science fiction they became part of science fiction as a result of this interaction between carl and me wormholes are like tunnels in the fabric of space and time predicted by einstein's equations thorne realized they might hold the answer to sagan's dilemma our universe it's three-dimensional but we can pretend it's two-dimensional so it's like this sheet of paper and we live in pasadena uh over here and london is over there and it's thousands of miles from uh pasadena to london this universe is curved up so that through hyperspace the distance from pasadena to london is only a few feet and there is this pipe this little wormhole that will lead this from pasadena to london across that very short distance then it's like looking through a crystal ball you see a distorted picture of what is going on at the other mouth of the wormhole which may be in the uh in another galaxy or maybe near the star of vega or it may be in london this shortcut was just what sagan's heroine needed i must have gone through a wormhole contact had started something new it introduced to the world to science fiction and also reintroduced to serious scientists the notion of a wormhole is something that is was really worthy of thinking about with wormholes on center stage thorne soon found they had other mysterious properties they could be used for more than traveling great distances in space they might also be used for time travel if you have a wormhole then you can turn them into time machines for going backward in time we thought how could we have been so stupid we should have realized that that's obvious what became obvious to thorn was that time could behave in strange ways inside a wormhole let's suppose that i have a wormhole with one mouth here and the other mouth over there now there are three different possibilities for how time could be hooked up through the interior of that wormhole the first is that when i stick my arm into this mouth it came out over there simultaneously the second possibility is that when i stick my arm into this mouth it comes out over there only after some delay the third possibility is that if i go into this wormhole mouth then i come out over there before i ever even go in let's just see that it was quite a surprise when i realized that with a single wormhole you could have time hook up toward the future or toward the past and that you could actually manipulate the wormhole and change how time hooked up that was a surprise but a very satisfying surprise when i really understood how it worked suddenly time travel seemed feasible at least in theory as a youngster who was fascinated by the possibility of time travel in science fiction to be in any way involved in in the possible actualization of time travel is it just brings goosebumps a physicist working on the possibility of travel into the past has to be careful not to be labeled a crank or accused of wasting public money on science fiction don't say a fantasy stand back in less than three minutes i shall have escaped this age of madness [Music] nevertheless it is an important question right now we are in one of those classic wonderfully evocative moments in science when we don't know when there are those on both sides of the debate and when what is at stake is is very mystifying very profound time is something which at a fundamental level we don't understand newton of course thought time was like a river that flowed time is the thing out there that flows and i go with the flow time is nature's way to keep everything from happening all at once it is one of those concepts that is profoundly resistant to simple definition [Music] we continue with the sale of albert einstein's manuscript for the theory of relativity einstein's theories of relativity are invaluable because they lay the foundations of modern physics including the possibility of time travel one of this catalog and i have a bit of two million dollars to start the bidding on this at two million dollars now in his theory of special relativity einstein declared that time can be altered by speed the faster you move the slower time passes three million dollars at two million three hundred thousand dollars in 1971 joe hafele and richard keating put the theory to the test they flew four atomic clocks around the world to compare the passage of time on an airplane with time on the ground [Applause] [Music] i don't trust these professors who get up and scribble in front of blackboards claiming they understand it all because i've made too many measurements where they they don't come up with the numbers they say richard had told me that in all previous work he had done he had never seen the effect of time debilitation i knew what was being predicted and not but it always seemed to me that the best proof is to measure it to see how time slows down with speed they had to fly all the way around the world in the original experiment today's atomic clocks are more accurate so the warping of time should be apparent even on an ordinary transatlantic flight [Music] champion yes one for me and one for my friend suddenly about five minutes past four local time which is five minutes past nine english time einstein said time is that which is indicated by clock i think i know what a clock is therefore i think i know what time it is seem to be following an unbelievably constant trend on route they collected data from the pilot then put it into einstein's equations to predict the time change we have accumulated so far a change of 34 nanoseconds and broadly speaking you know it seems to confirm the original the clock would have gained somewhere between 37 and 40 nanoseconds when we landed about half an hour's time suppose you would live for 100 years and you would spend your entire life on one of these aircraft flying around the world you could expect to be younger than people who did not do that by about 1 10 000 of a second [Music] to measure exactly how much time changed relative to time on earth the clock on the airplane had been synchronized with the international standard at the start of the trip then it was compared to the same standard when it landed in the united states if we put other cables in here that changes the clock on the plane disagreed with the ones on the ground by 40 billionths of a second just as einstein predicted at two million seven hundred thousand dollars now at two million eight hundred thousand at two million nine hundred thousand here at two million nine hundred thousand at three million dollars three million one hundred thousand three million two hundred thousand dollars three million three hundred thousand dollars this year don't wave back there fair warning now at three million three hundred thousand dollars but taken to an extreme einstein's theory that time slows down at high speeds has even stranger consequences this is sometimes described as the twin paradox two identical twins one of whom goes off on a voyage close to the speed of light the other one stays home when the space traveling twin returns home time has dilated for him or her that is he or she has aged only a little while the twin who has remained at home has aged at the regular pace and here have two identical twins who may be decades apart in age applied to a wormhole this curious phenomenon might make time travel possible [Music] there are several different ways to turn a wormhole into a time machine if you are a clever and infinitely advanced civilization by an infinitely advanced civilization i mean somebody who can do anything their heart desires except they can't violate the fundamental laws [Music] what they could do is take advantage of the twins paradox and send one mouth of a wormhole on a voyage into outer space [Music] as the wormhole mouth approaches the speed of light time slows down relative to the wormhole mouth that remains on earth at the end of its high-speed voyage the traveling wormhole mouth returns to earth where it can be picked up by its owner [Music] just like the twins paradox less time has passed for this mouth of the wormhole than for the other end that stayed behind on earth the wormhole is now a tunnel with each mouth located in a different time if i now go into this wormhole mouth today i will come out of that mouth yesterday so in theory an advanced civilization might turn a wormhole into a time machine but the practical details are something else again where would they find a wormhole in the first place the answer grew out of the work of kip thorne's former teacher i like to think of space and time as an elegance to the ocean and changes in it is analogous to waves on the surface of the ocean but those waves of course don't show up when one's miles above the ocean it looks flat this one gets down closer to the surface one sees the waves breaking and the foam i see no way to escape the conclusion that similar boom-like structure is developing in space and time wheeler thinks the space between atoms is filled with bubbles [Music] and this quantum foam as he calls it gives rise to incredibly tiny wormholes let's suppose we begin at the size of britain and we squeeze ourselves on down smaller and smaller past the size of a human being down to the size of an atom and on down to the size of an atomic nucleus we're then halfway to the scale of quantum form we have to then begin at that scale and go on down as much as we went from britain to an atomic nucleus we have to keep on going down it's another 10 20 powers of 10 on down we get to a minuscule scale far smaller than anybody has ever been able to reach with experimental apparatus and there the fundamental laws seem to tell us there must be quantum foam space as we know it must cease to exist in this smooth sort of a way and must become something more like a froth of soap suds [Music] this quantum foam we can think of as having bubbles but we can also think of it as having little handles like the handle on a coffee cup that is a wormhole it's something that reaches from over here in the foam to over there thorn speculates that if quantum foam exists an advanced civilization might be able to grab one of these tiny wormholes and enlarge it but there's another problem we then had to face the fact that the one kind of wormhole that we knew as a solution of einstein's equations was a wormhole that lives for only a very short time it has a throat that expands opens and shuts in a flash so fast that uh anybody who tries to travel through it in that flash while it's open uh can't manage to get through they get crushed in the pinch-off to travel through a wormhole a human being would have to find a way to hold it open so we then ask ourselves what is it that you need to thread through the wormhole to hold it open long enough for somebody to travel through it and einstein's equations told us the answer we just had to do a page or two of calculations what you needed was something very exotic some material that has negative energy the bad news is that if you want a wormhole about one meter across which is a really minimal requirement for something to put a human through you need about minus one jupiter's worth of this exotic matter so the repellent force of a planet-sized amount of exotic matter would be required to hold a wormhole open but it's not as far-fetched as it might sound scientists have found that tiny amounts of such negative energy can be made in the laboratory it comes down here and supports this triangle all ordinary matter has positive energy but exotic matter can be produced by squeezing energy out of a vacuum created in a tiny gap between two metal plates now when you bring two plates close together photons of long wavelengths can't exist between the plates so we exclude some of this energy from the system when you do so the energy between the plates is lower than the energy outside and so there's a force between these two plates the force actually pulls the plates together and that's critical because that tells you that the energy density between the plates has to be negative and that is the key to it actually being exotic matter we just did it for fun and uh it's uh built with this junk we found around the lab here but in fact we can measure an extremely tiny force with it [Music] that experiment from our point of view is a proof in principle that at least small amounts of exotic matter effectively negative energy do exist in the real world if enough negative energy could be collected time tourists of the future might be able to hold a wormhole open long enough to make a safe journey into the past hoping that large amounts of negative energy might be possible and speculating that wormholes could be extracted and enlarged from the quantum foam thorne took the plunge [Music] he published his ideas about time travel in one of the most prestigious physics journals my concern was the word time machine in the title and my worry was that the popular press would see this paper and would start to ballyhoo it in a manner that caused our serious scientific colleagues to pay no attention to it as being crackpot stuff very quickly the rest of the press grabbed hold of it here we are we've invented time travel there are other stories that basically had us building time machines in our own basements and i i rather quickly pulled back and told the cal tech public relations office i do not give interviews on this subject i told uh my research group's administrative assistant i do not return telephone calls from the press on the subject i will talk about anything else but not time travel but thorn's work brought other scientists out into the open when i realized that my friend and extremely great scientist keep thorn publish it i immediately called him and told thank you so much for that now i will publish i will work on on this uh subject also he was overjoyed on the other end of the phone he said oh kip it's absolutely wonderful your paper's wonderful you've broken the barrier if you can do research about time travel and so can i igor novikov had been working on time travel in secret for years he was intrigued by the trouble that time travelers might cause if they went back and tried to change history once it appeared the time machines were a real possibility we then had to face the question of paradoxes of going back in time and changing history and thereby uh causing the foundations of physics to crumble beneath us you're in focus now paul press the button harvey press the button my friend send me back into time time travel would seem to lead to contradictions if one was able to go back and change the past the grandfather paradox is a uh very simple science fiction based apparent inconsistency at the very heart of the idea of backwards time travel if i have a time machine then there is the danger that i may go into the time machine go back in time kill my grandfather before my mother is conceived so that she can conceive me where does that then leave you how do i get here do you instantly pop out of existence because you were never made but then you couldn't have gone back and so on hurtling back through time creates paradoxes that have fueled dozens of hollywood productions dudes you guys are gonna go back in time anything you do could have serious repercussions on future events do you understand this is gonna cost me a serious consideration of backwards time travel must confront these paradoxes vineyard balls provided us a way to study paradoxes with time travel without getting into the nasty business of free will of human beings billiard balls behave in predictable ways according to the laws of physics cause always precedes effect but what happens if time travel is possible if i have a time machine the story is quite different imagine the pockets of this billiard table are the mouths of a wormhole time machine in this case i have only one billiard ball and i send that billiard ball into this mouth of the uh wormhole and it will then come out of that mouth before it entered this mouth hit itself and prevent itself from going into the first mouth voila a paradox it's the billiard ball version of going back in time and changing history of course this problem was discussed a lot uh in literature in movies in science fiction but i'm talking not about fantasies but real science having developed simple versions of the paradox we then set about trying to figure out how to solve these how will the laws of physics behave if time machines are permitted in searching for a resolution of the paradox we were led by a principal introduced by igor novokov which said that nature will only allow those behaviors that are absolutely self-consistent in order to be self-consistent a ball emerging in the past must knock itself into the time machine so it can come out again along a trajectory that will knock itself back into the pocket novokov wrote mathematical equations for the possible outcomes of a ball hitting itself and found that a self-consistent solution always exists this implied that nature would not allow a paradox to arise this is the main principle all these events must be in self-consistency with each other it's so simple so obvious but more of that we gave the strict mathematical proof that this principle is the consequence of the basic ideas of the physics by the same token novukov's findings implied that the laws of physics would prevent a time traveler from rewriting history yes i'm here to prepare your woman what has happened has happened it cannot be changed it cannot be repeated twice in different ways human beings aren't billiard balls and we might like to believe in free will so a sufficiently stubborn human would seem to be able to get around any sort of consistency condition by just demanding that he does something different i can have a free will to walk along this wall without special equipment it's my free will can i do that no i can't why because of a law of physics because of the law of gravity it's forbidden laws of physics like gravity already restrain our free will [Music] novikov thinks these same laws would restrain the free will of the time traveler bent on changing the past and fresco open up please immediately immediately if time travelers can't change the past themselves could they alter history by sending a message into the past instead einstein's theories of relativity show that if something could travel faster than the speed of light it could be viewed as going backwards in time but relativity also says that's impossible yet this man may have taken a step in that direction because he claims to have sent information faster than light [Music] this signal is splitted into by an electronic mirror here into two parts so we can compare the signal one is moving through the air and the other one is moving through the barrier in this experiment gunter nymphs splits a microwave signal in two half goes through the air traveling at the speed of light and half is fired into a barrier to block the signal but that's not what happens this is a scope where you see the signal and then we can see which one is faster the two humps on the screen are not in the same place because the microwaves that went through the barrier got to the detector first apparently exceeding the speed of light only this very small part comes to the other side but it comes and this part comes at the velocity which is much faster than the velocity of light so how could the microwaves go faster than light and what was the role of the barrier nims chalks it up to a strange phenomenon called quantum tunneling at the subatomic or quantum level the world is ruled by probability and chance and the seemingly impossible occurs all the time for example when a stream of particles like photons meets a barrier most bounce off but a few of them materialize on the far side of the barrier and continue on their way detected the particles that appeared and measured how fast they got there i mean use about this for fun and when we figured out that it's faster than the velocity of light we did not think about its importance another expert in quantum tunneling is raymond chow he agrees with at least part of what nymphs has found in our experiments we have measured that a single photon can tunnel across a tunnel barrier at 1.7 times the speed of light what bothers chao is not that random photons seem to go beyond the speed of light but that nims claims he can use tunneling to send information faster than light to have a genuine signal you really have to control the signal but in quantum mechanical tunneling it's a completely random process fundamentally we cannot uh we cannot send information with this tunneling particle yeah some colleagues were claiming that you cannot send information and then we started to transmit mozart 40 and this is for instance the original tape that's what we sent at the speed of 4.7 times the velocity of light and a distance of about 14 centimeter whether you can recognize mozart 40 or [Music] not despite the randomness and uncertainty of the tunneling process mozart seems to have gone through the barrier the essential question is what is a signal or what constitutes information has he really sent a signal in the sense of information faster than the speed of light this is where professor nims and i part company because we don't really have a rigorous definition of what his information at the quantum level [Music] maybe that this is not information for american colleague but but british i think mozart 40 has some information in it transmitting mozart is one thing convincing others that you've sent it faster than light is another and so the debate continues with neither side budging i consist no no i'm not consistent i insist on it that we have and we can transmit signals faster than the velocity of light nymphs has found little support for this claim and few think this will ever allow information to go backwards in time still the idea that time travel for people might become a reality raises a provocative question time travel might be possible but if that is the case why haven't we been overrun by tourists from the future this argument i find very dubious it might be that time travel into the past is possible but they haven't gotten to our time yet they're very far in the future and it's the further back in time you go the more expensive it is then there's a possibility that they're here all right but we don't see them they have perfect invisibility cloaks or something if if they're so smart if they have such highly developed technology then why not then there's the possibility that they're here we do see them but we call them something else ufos or uh ghosts or i think that if people from the future were going to show themselves they would do so in a more obvious way what would be the point of revealing themselves only to cranks and weirdos who wouldn't be believed but physics does put a limit on how far into the past a time traveler could go relativity theory says in general that uh once you've made a time machine you can never use it to go backward in time before the uh period when it was made there's no way i can use it to travel back to the age of the dinosaurs or even back to the time of my own birth because i didn't make the time machine until recently so relativity forbids one of the most beloved scenarios of science fiction from ever happening because you cannot go back before the point at which the time machine was made i don't have to worry about the possibility of my going back and killing my father before i was conceived what i have to worry about is my grandson coming going back in time and killing me before he is conceived that is there's no possibility of changing our past but uh in the future one can change the future's past but there is a loophole that might still make even the most far-fetched scenarios possible according to quantum theory uncertainty and chance rule the subatomic world you can never know the exact position and speed of particles at the same time instead you can only give probabilities for their behavior in a huge extrapolation some people think that all these probabilities are occurring simultaneously anything that might happen actually is happening quantum mechanics is a theory of many parallel universes some of them are alike and some of them are very unlike there are nearby universes that differ from this one only in the position of one photon or one electron there are other more distant universes where we're not filming here at all and there are others where i was never even born this is a radical interpretation of quantum mechanics but david deutsch finds the evidence he needs for parallel universes in a well-known physics experiment i first saw this experiment uh demonstrated when i was an undergraduate uh in fact it's a very old experiment it was first done in 1909. this is our light source for the experiment so the light is being steered by these mirrors onto the slits here and there's two slits in this slide and that produces the young slits interference pattern which we see on the camera the interference pattern is the set of faint vertical stripes at the center of the red spot these only appear when two slits are open one slit two slits one slit two slits with two slits open the single beam of light is split into two beams which overlap like ripples on a pond in some places the ripples reinforce each other while in others they cancel each other out creating the pattern of stripes on the screen but what happens if the intensity of the beam is reduced by a filter so that only one photon at a time can reach the slits now you can't see the beam at the moment but if i introduce some liquid nitrogen we should be able to pick that up so now you can see the beam scattered by the nitrogen but you see nothing after the filter the filter essentially stops everything we can see the camera can pick up the few photons that are arriving the photons arrive at the slits one at a time so those that get through and reach the screen on the other side should form just two bright lines they shouldn't interfere and produce the full pattern of stripes but they do and this is how the stripes appear on a computer screen when one does the experiment with individual photons the pattern that builds up um after one has passed many photons through the apparatus is exactly the same as it was in strong light and that's something we just don't understand as an undergraduate we were told oh this is because the photon behaves partly as a particle and partly as a wave now that just doesn't make sense it's gibberish it's saying that the photon is both in one place and spread out at the same time deutsche believes that the single photons are producing the full pattern of stripes by interacting with other photons that we can't see the photon that we can't see is a photon in a parallel universe which a nearby parallel universe which is interacting with the photon in our universe and causing it to change its direction the result of the single photon interference experiment is the strangest thing i know it is conclusive evidence that reality does not consist of just a single universe because that result could not come about unless there were another nearby universe interacting with ours if true this idea has profound implications for time travel when one travels back in time one does not in general reach the same universe that one starts from one reaches the past of a different universe could this explain why we haven't met any time travelers i don't believe it personally mainly because this idea is not very constructive there are not many consequences of this idea which we can test in a real experiment i think we can't say for certain but if i had delay my bets and i like to lay bets i would lay my bets on the simpler non-parallel universe resolution another person who likes to lay bets is stephen hawking he and kip thorne have been betting on the nature of the universe for years this bet here is probably the most famous one it was a bit i made with stephen hawking in 1974 over whether or not cygnus x1 is actually a black hole and hawking after 16 years of this bet sitting here finally conceded stephen hawking and i have occasionally discussed making a bet over whether or not the laws of physics permit you to go backward in time uh we haven't been able to conclude a bit because almost all the time we're on the same side of the issue we both think it is probably impossible stephen hawking has been skeptical of time travel for years but even he cannot rule it out for sure to make time travel possible one seems to need both general relativity which describes the large-scale structure of the universe and quantum mechanics which governs very small scales these two theories are inconsistent with each other as they stand so we have to find a new theory that combines them cosmic relationships like the motions of planets are governed by einstein's theory of gravity general relativity but these laws do not work on the subatomic level just as the laws of quantum mechanics do not work on the larger scale so physicists are searching for a way to unite the two in a single theory called quantum gravity this is the holy grail of modern physics a unified description of the nature of space and time on all scales so i would like to discuss the big bang scientists are still in the process of crafting this theory and whether quantum gravity will permit time travel through a wormhole is unclear yet they can't resist speculating when you try to make a time machine out of a wormhole having discovered that it is possible according to general relativity to take a wormhole with two mouths and turn it into a time machine my students and i then searched for some way some mechanism to prevent this from really happening in the real universe uh because we thought it was ludicrous you shouldn't be able to do that uh and we actually found a mechanism uh that in fact we were able to show always intervenes whenever you try to make a time machine suppose that i have two wormhole mouths that i'm going to turn into a time machine of course i do it by taking that mouth and sending it off at high speed out into space it turns around and it comes back and just as it's on its way back it suddenly becomes a time machine and anyone can travel through it and get back before one started right as it's coming back and settling in just before the moment when it can become a time machine suddenly according to calculations we've done it explodes poof no more wormholes no more time machine question is why what made that happen what seems to happen is that particles can keep coming round again to the same event in space time this makes the energy density very large which warps space time so that travel into the past is no longer possible birth of the universe it seems that energy might pile up inside the wormhole until it self-destructs you get in fact but when thorn applied what is known of quantum gravity the picture changed again it appeared in dust that just as that mechanism was about to in fact make your time machine explode at the moment you're trying to activate it right at that moment our calculations would fail because the new laws of physics the new laws of quantum gravity would take over and the techniques we had of computing what happens with would no longer be any good so perhaps quantum gravity might allow the time machine to exist after all thorne decided to see what hawking made of this he uh got rather disturbed because stephen is a defender of the establishment he really believed that this really would be the mechanism to prevent you ever from making a time machine and said they've not correctly read the tea leaves of what quantum gravity is going to do ultimately after going back and forth with stephen uh several times we converged to an agreement that he was probably right we were wrong the explosion probably will destroy the time machine but we can't be absolutely sure we won't know absolutely for sure until we have the full laws of quantum gravity in our hands and we can explore just what they say about the end point of the explosion in the meantime hawking remains skeptical of time travel a belief he puts forward in what he calls the chronology protection conjecture the chronology protection conjecture would make the world safe for historians the most likely mechanism that would enforce hawking's conjecture is the explosion of the wormhole [Music] but this is just a conjecture it's a kind of informed guess which may be mistaken that's that's what a conjecture is and and the word conjecture in mathematics is often used as a challenge to future generations to try to prove or disprove it's not a principle it's not a theorem it's a conjecture because we do not yet know for certain that there is a mechanism in nature that will always enforce it well if you ask me do time travels really be possible in the future uh i must give the positive uh reply you see it's it will be possible i do not see a way to make time travel possible however if i heard that somebody else some other country was doing this i think i might plunge into the game i myself believe that there will one day be time travel because when we find that something isn't forbidden by the overarching laws of physics we usually eventually find a technological way of doing it i believe we will know in 10 to 15 years when we have the full loss of quantum gravity in our hands my best guess and i would be willing to lay a fairly heavy odds on this on a bet with hawking but he won't take the other side my best guess is that uh when we have those full laws in our hands they will say no you cannot make a time machine and go backward in time ever but until we have those full laws we just have to leave it as a possibility that remains a possibility i wouldn't take a bet against the existence of time machines my opponent might have seen the future and know the answer einstein showed that space is curved and time is relative on nova's website do a simple thought experiment and learn to think like the century's greatest scientist next time on nova the case that inspired the fugitive

Channel: Dan Burns

Views: 105,980

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Id: XWvd0OFtPA0

Length: 51min 18sec (3078 seconds)

Published: Sun Jul 26 2020