PROF. STEPHEN HAWKING: Time travel might be possible, but if that is the case, why haven't we been over-run by tourists from the future?
PROF. CARL SAGAN: If we could travel into the past, it's mind-boggling what would be possible. For one thing history would become an experimental science.
PROF. KIP THORNE: If that really can happen, then the very foundations of the laws of physics crumble out from under us and the enterprise that I have spent my entire adult life doing becomes an impossible enterprise.
PROF. IGOR NOVIKOV: My estimate is it could be, let's say 200 years.
NARRATOR (Jo Unwin): If a time traveller were ever to step into this moment in time the person they'd most want to visit is Kip Thorne. He's brought time travel to the attention of serious science, and it all began with a story. In the early '80s the astronomer Carl Sagan decided to write a science-fiction novel. He had no interest in time travel. His book was about making contact with aliens.
CARL SAGAN (Cornell University): In Contact, the heroine was a radio astronomer engaged in the search for radio signals from extraterrestrial intelligence. Well, she receives a signal and the signal after much decoding turns out to be a machine and the machine is a means of travelling great distances and there's something that looks like a chair.
READER: "This appears to be an ordinary armchair, perfectly configured for a human being. It's very unlikely that extraterrestrial beings would resemble us sufficiently to share our preferences in living-room furniture. Here, look at this close-up, it looks like something from my mother's spare room when I was growing up."
NARRATOR: An ordinary armchair that could carry its occupants safely across the universe, but what Sagan needed to find was the most realistic route.
CARL SAGAN: In the early 1980s there was a common misunderstanding that you might be able to travel
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 by nervous and it was for that reason that I contacted Kip Thorne.
NARRATOR: Kip Thorne was an old friend and an expert on black holes.
KIP THORNE (Professor, California Institute of Technology): I was a little upset because he had the heroine in his novel travelling 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.
NARRATOR: A black hole forms when a star collapses to an infinitely dense point with infinite gravitational pull. This warps space so severely that anything comes near is sucked into the gravity well. Nothing can survive the destructive power of a black hole, so they could never be used for realistic space travel.
CARL SAGAN: 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 phonecall that I had no anticipated.
KIP THORNE: Rather quickly I recognised that what he probably should do is replace the black hole as a means for a rapid inter-stellar 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.
NARRATOR: Wormholes were dreamed up in the 1950s by a leading physicist called John Wheeler.
JOHN WHEELER (Professor, Princeton University): I like to think of space and time as analogous 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. Then as one gets down closer to the surface one sees the waves breaking and the foam. I see no way to escape the conclusion that somewhere foam-like structure is developing in space and time.
NARRATOR: Wheeler had been Thorne's teacher. He was also part of the first wave of research into the nature of space and time. He thinks the space between atoms might be full of bubbles and that once in a while two bubbles might join together to make a tunnel.
JOHN WHEELER: There, one has what one can jokingly call a wormhole in space.
KIP THORNE: 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 over here and London is over there and it's thousands of miles from 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 us from Pasadena to London across that very short distance, and 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, in another galaxy or it may be near the star Vega or it may be in London.
NARRATOR: The short cut like this was just what Sagan needed for his armchair heroine, but there was even more to it than that.
KIP THORNE: If you have a wormhole so you can do this very rapid travel, 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 realised that. That's obvious.
NARRATOR: But even in fiction time machines have never been obvious.
ARCHIVE FILM: 1) Tardis appearing on station platform. Dur: 6"
2) Man in ice. Dur: 5"
MAN: "Oh, a man in a block of ice..."
3) Spaceship thru galaxy Dur: 3"
MAN: "We've broken the light barrier 22 hours early."
4) Spaceship. Dur: 7"
WOMAN: "Blazing lights in the sky."
NARRATOR: But Thorne's ideas were serious. It seemed that time travel might really be possible.
CARL SAGAN: 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 actualisation of time travel is, it just brings goose bumps.
KIP THORNE: 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, and 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.
NARRATOR: Thorne had started something big. But in the academic world some things simply don't do.
STEPHEN HAWKING (Professor, Cambridge University): A physicist working on the possibility of travel into the past has to be careful not to be labelled a crank, or accused of wasting public money on science-fiction fantasy. Nevertheless, it is an important question.
CARL SAGAN: 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.
PROF. MATT VISSER: Time is something which at a fundamental level we don't understand.
RAYMOND CHOW: Newton of course thought time was like a river that flowed.
KIP THORNE: Time is the thing out there that flows and I go with the flow.
JOHN WHEELER: Time is nature's way to keep everything from happening all at once.
CARL SAGAN: It is one of those concepts that is profoundly resistant to simple definition.
AUCTIONEER: "We continue with the sale of Albert Einstein's 1912 manuscript for The Theory of Relativity."
NARRATOR: One reason that Einstein's Theory of Relativity is valued so highly today is that within it Einstein defined what time is and how we relate to it.
AUCTIONEER: "...and I have a bid of $2 million to start the bidding on this, $2 million now. At $2 million, now bidding at $2 million, at $2 million, the bidding..."
NARRATOR: In this paper Einstein declared that the faster you move the slower time passes. An experiment in 1971 proved him right. Four atomic clocks were flown around the world and Joe Hafele and Richard Keating compared the time that passed for the moving clocks with that measured on the ground. When they began they weren't sure what to expect.
RICHARD KEATING (US Naval Observatory): 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.
JOE HAFELE (Professor, Christopher Newport University): Richard had told me that in all previous work he had done, he had never seen the effect of time
RICHARD: I hadn't, I'd never seen it. I knew what was being predicted and that, but it always seemed to me that the best proof is to measure it.
NARRATOR: Twenty-five years ago they had to fly right round the world to see how seconds stretch on an aeroplane. Today's atomic clocks are much more accurate, so Horizon asked a team from the National Physical Laboratory in London to join forces with Joe Hafele and see if they could measure the time warp on a transatlantic crossing.
MAN: Five minutes past four local time which is five minutes past nine, English time.
JOE HAFELE: 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 is.
MAN: Seem to be following an unbelievable...
NARRATOR: En route they collected data from the pilot, then put it into Einstein's equations to predict the time change.
MAN: I bring you the latest news from the cockpit. We have accumulated so far a change of 34 nanoseconds. Broadly speaking you know it seems to confirm the original...
The clock will have gained somewhere between 37 and 40 nanoseconds when we land in about half an
JOE HAFELE: Suppose you were to live for 100 years and you would spend your entire life on one of these aircraft, flying around the world, you could expect to beat people who did not do that by about one ten-thousandth of a second.
NARRATOR: At the speed of a 747 the warping of time is a small effect. To see exactly what happened on this trip, the clock had been lined up to the international standard timescale in London. Then it was compared to the same standard when it landed in the USA.
MAN: ...setting of the timescale of US...
MAN: I've got in here exactly what we measured at the other end.
MAN: If we put the cables in here and that changes by about 15 nanoseconds then, then it's OK.
NARRATOR: And it was. When the experiment was finished, the clock on the plane disagreed with the ones on the ground by 40 nanoseconds. Time on the plane had warped, by just what Einstein predicted.
AUCTIONEER: "...800,000, $2,900,000. Here at $2,900,000. At $3,000,000, $3,100,000... $3,200,000,
$3,300,000 is here. Don't wave back there. Fair warning now at $3,300,000."
NARRATOR: But time doesn't just stretch the clocks. Time would stretch for wormholes as well and that's why they might be turned into time machines.
KIP THORNE: There are several different ways to turn a wormhole into a time machine if you are a clever and infinitely advanced civilisation. By an infinitely advanced civilisation I mean, somebody who can do anything their heart desires except they can't violate the fundamental laws.
NARRATOR: What they could do is send one mouth of their wormhole on an interstellar holiday.
Travelling at close to the speed of light would cause time to slow down in that wormhole mouth. During the trip, time would slow just like it did on the 747, but far more because the speed is so much greater and the journey much longer. When the travelling mouth returned to Earth, less time would have passed for it than for the rest of the world, and for the other mouth that stayed behind. The wormhole would become a tunnel into the past.
KIP THORNE: If I now go into this wormhole mouth today, I will come out of that mouth yesterday.
NARRATOR: So in theory a wormhole could be turned into a time machine. But the practical details were something else again. How an infinitely advanced being might actually make one of these things was far from clear.
KIP THORNE: 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 anybody who tries to travel through it in that flash while it's open can't manage to get through. They get crushed in the pinch off.
NARRATOR: And there's another problem. Naturally wormholes are billions of times smaller than atoms, far too small to be useful. If a human ever wanted to travel through one the wormhole would need to be stretched up and held open.
KIP THORNE: What is it that you need to thread through the wormhole to hold it open long enough for somebody to travel through it? What you needed was something very exotic, some material that has negative energy.
MATT VISSER (Professor, Washington University): The bad news is that if you want a wormhole about one metre across, which is a really minimal requirement for something to put a human through, you need about minus 1 Jupiter's worth of this exotic matter.
NARRATOR: Ordinary matter, like armchairs, has positive energy, so you might think that needing exotic matter, stuff with negative energy, would rule out a realistic time machine, but in fact that's not the case. In a lab in Seattle, Steve Lamoreaux has shown that negative energy can be made.
STEVE LAMOREAUX (Professor, University of Washington): You can see here there's a, a tungsten wire comes out...
NARRATOR: Matt Visser is a theoretical expert on negative energy, but he's never seen it for real.
STEVE LAMOREAUX: ...so the whole triangular...
NARRATOR: Negative energy is made by squeezing energy out of a vacuum that they create in a tiny gap between two plates.
STEVE LAMOREAUX: Now when you bring two plates close together photons along wavelengths can't exist between the plates, so it excludes some of this energy from the system and when you do so the energy between the plates is lower than energy outside and so there's a force between these two plates.
MATT VISSER: 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.
STEVE LAMOREAUX: We just did it for fun and it's built with this junk we found around the lab here, and in fact we can measure an extremely tiny force with it.
NARRATOR: In fact, this is cutting edge science. Nowhere else do they measure negative energy.
MATT VISSER: The experiments from our point of view are a proof in principle that at least small amounts of exotic matter effectively negative energy do exist in the real world.
NARRATOR: So negative energy is real. In the future an infinitely advanced time tourist might be able to make enough of it to stretch a wormhole big enough and hold it open long enough to make a safe journey into the past.
With negative energy real in practice and wormholes real in theory, Thorne took the plunge and went public.
KIP THORNE: 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 my
research group's administrative assistant I do not return telephone calls from the press on this subject, I will talk about anything else but not time travel.
NARRATOR: But Thorne's work brought other scientists out into the open.
IGOR NOVIKOV (Professor, Copenhagen University): When I realised that my friend and extremely great scientist Kip Thorne publish it, I immediately called him and told thank you so much for that. Now I will publish, I will work on, on this subject also.
KIP THORNE: 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 then so can I.
NARRATOR: Igor Novikov had been working on time travel in secret for years. He was interested in the trouble that time travellers might cause if they went back and tried to change history.
KIP THORNE: Once it appeared that 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 causing the foundations of physics to crumble beneath us.
FILM EXTRACT: Time Traveller. Dur: 13"
HARVEY: "We're in focus now, Paul."
PAUL DRISCOLL: "Press the button Harvey. Press the button my friend, send me back into time."
STEPHEN HAWKING: Time travel would seem to lead to contradictions. If one was able to go back and
change the past...
CARL SAGAN: The grandfather paradox is a very simple science-fiction based, apparent inconsistency at the very heart of the idea of backwards time travel.
JOHN WHEELER: That cannon ball that knocked my grandfather unconscious in the Civil War battle, so he lay there three days before he came back. Suppose that I had deflected the cannon slightly and he'd really been killed.
CARL SAGAN: Where does that then leave you?
JOHN WHEELER: How do I then get here?
CARL SAGAN: Do you instantly pop out of existence because you were never made?
STEPHEN HAWKING: But then you couldn't have gone back, and so on.
KIP THORNE: Billiard balls provided us a way to study paradoxes with time travel without getting into the nasty business of freewill of human beings. If I don't have a time machine at all then billiard ball physics is very simple and very clear. If I have a time machine the story is quite different. In this case I have only one
billiard ball and I send that billiard ball into this mouth of the 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 'go back and kill my father before I'm conceived'.
IGOR NOVIKOV: Of course, this problem was discussed a lot in literature, in movies, in science fictions, but I am talking not about fantasies but real science.
KIP THORNE: In searching for a resolution of the paradox we were led by a principle introduced by Igor Novikov, which said that nature will only allow those behaviours that are absolutely self- consistent, so the question then was: can you find a solution of the equations where this ball maybe hits itself but does not produce any self-inconsistency?
NARRATOR: If a ball emerged in the past it would have to knock its earlier self into the time machine so that it's there to come out again, and knock itself back in again and so on. Only then could its behaviour be self-consistent. Novikov approached the equations from all angles and every time only the self-consistent solutions worked out.
IGOR NOVIKOV): 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 that strict mathematical proof that this principle is the consequence of the basic ideas of the physics.
NARRATOR: By the same token, any attempt a time traveller might make to rewrite history would be
FILM EXTRACT: Time traveller and Hitler.
IGOR NOVIKOV: What has happened has happened. It cannot be changed, it cannot be repeated twice in
MATT VISSER: Human beings are billiard balls and we might like to believe in freewill, 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.
IGOR NOVIKOV: I can have freewill to walk along this wall without special equipment. It's my freewill. Can I do that? No, I can't. Why? Because of a law of physics because of the law of gravity. It's forbidden.
NARRATOR: Gravity means you can't walk up a wall, so gravity already restrains our freewill, and if time travel ever does become possible other laws of physics would stop a traveller from changing the past.
NARRATOR: Those determined to change history might try to send a message backwards in time, but to do that we'd have to send information faster than light, and that's impossible. However, right now an international row is brewing because this man reckons he can send a signal faster than light into the realm where Einstein said time would run backwards.
PROF. GUENTER NIMTZ (University of Cologne): This signal is fitted 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.
NARRATOR: Guenter Nimtz splits a microwave signal. The half that goes through the air travels at the speed of light and is displayed on an oscilloscope. The half that hits the barrier should go nowhere, but that's not what seems to happen.
GUENTER NIMTZ: This is the oscilloscope where you see the signal and then we can see which one is
NARRATOR: The two humps on the screen are not in the same place because one signal got there faster than the other, and the faster one tunnelled through the barrier.
GUENTER NIMTZ: Only a 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.
NARRATOR: Nimtz believes that the faster signal uses a strange effect called quantum tunnelling to get past the barrier. Tunnelling depends on the fact that down at the quantum level, where particles are a lot smaller than atoms, the world is a totally random place. When a particle like a photon is here it also has a small but very real chance of being here, or here, or here, at the other end of the barrier. What Nimtz and his team did was to pick up the photons that appeared at the far end and then to measure how fast they got there.
GUENTER NIMTZ: And the news about this we did this for fun, and when we figured out that it's faster than the velocity of light we did not think about its importance.
NARRATOR: The leader in this field, Raymond Chiao, has misgivings about Nimtz's interpretations, but even he agrees with part of what Nimtz is saying.
PROF. RAYMOND CHIAO (University of California): In our experiments we have measured that a single photon can tunnel across a tunnel barrier at 1.7 times the speed of light.
NARRATOR: Chiao agrees that quantum mechanical tunnelling allows occasional random photons to break the light speed limit. What upsets him, and the rest of the physics world, is that Nimtz claims to have used it to send information faster than light. That really is taboo.
RAYMOND CHIAO: To have a genuine signal you really have to control the signal, but in, in quantum mechanical tunnelling it's a completely random process. Fundamentally we cannot, we cannot send information with this tunnelling particle.
GUENTER NIMTZ: Yeah, some colleagues are 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 a speed of 4.7 times the velocity of light and a distance of about 14 centimetre. Whether you can recognise Mozart 40 or not.
NARRATOR: Despite the random nature of the process, Mozart seems to have got through.
RAYMOND CHIAO: 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 Nimtz and I part company because we don't really have a rigorous definition of what is information at the quantum level.
GUENTER NIMTZ: Maybe that this is not information for American colleague, but for a German or a British colleague. I think Mozart 40 has some information in it.
NARRATOR: Whilst well established in his own field, Nimtz is new to tunnelling and claiming to send Mozart faster than light has brought him perilously close to being called crackpot. His critics cite signal fronts and carrier waves. He counters with a limited bandwidth, but so far he stands alone.
Right or wrong, this leads to an interesting thought experiment, a gerdanken experiment in German. What if you could tunnel a message to the other side of the universe? Going faster than light, the message would
seem to go backwards in time.
GUENTER NIMTZ: I came across a nice gerdanken experiment. There's the signal going to a far star which informations that you were born and 20 years later tunnel the signal at your age of 20 years and this will arrive before the signal comes to the star that you were born.
RAYMOND CHIAO: Yes, if we had a tunnel barrier that was, say, very wide from here to the next galaxy, then in principle yes, you could then in the tunnel effect advance the wave so much so that it, it begins to worry me that we have sent something really faster than the speed of light.
GUENTER NIMTZ: I consist... No, no, I not consist, I insist on it that we have and we can transmit signals faster than the velocity of light.
NARRATOR: Perhaps one day our infinitely advanced grandchildren will send messages back through time, or even use wormholes to travel back comfortably themselves, but that leaves one big question.
STEPHEN HAWKING: Time travel might be possible, but if that is the case why haven't we been overrun by tourists from the future?
CARL SAGAN: 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 the possibility that they're here, alright, but we don't see them. They
have perfect invisibility cloaks or something. If they're so smart, if they have such highly developed technology then why not? Then there's the possibility that they're here and we do see them, but we call them something else - UFOs or ghosts or...
STEPHEN HAWKING: 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 weirdoes who wouldn't be believed?
NARRATOR: But physics does put a limit on how far back any time tourist could ever travel.
CARL SAGAN: Relativity theory says in general that once you've made a time machine you can never use it to go backward in time before the period when it was made.
NARRATOR: Whatever else is allowed, relativity is firm on this: we can't go back because no-one has yet built a time machine.
KIP THORNE: 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 in the future one can change the future's past.
DR DAVID DEUTSCH (Oxford University): This is not an inconsistency. This is just a strange thing that would happen if time travel were possible and the classical laws of physics were true, but the whole thing's rather academic because in fact the classical laws of physics aren't true and quantum mechanics is a true description of nature.
NARRATOR: David Deutsch is convinced that time travel is possible, but he explains the lack of time travellers by turning to that random messy world of quantum mechanics. At bottom, the universe is riddled with quantum uncertainty. We're all made of quantum particles with uncertain positions and uncertain behaviours, but Deutsch's version of reality requires uncertainty on a much larger scale.
DAVID DEUTSCH: 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.
NARRATOR: This is a huge extrapolation of the uncertainty principle. Deutsch puts each quantum possibility into its own universe, and he's not talking fantasy. He has hard evidence for parallel universes. He finds it in a well-known experiment that's been taught to students of physics for years.
DAVID DEUTSCH: I first saw this experiment demonstrated when I was an undergraduate. In fact it's a very old experiment. It was first done in 1909.
SCIENTIST: This is our light source for the experiment. The light is being steered by these mirrors onto the slits here. There's two slits in this slide and that produces the young slit's interference pattern which we see on the camera.
NARRATOR: The interference pattern is the set of faint, dark stripes at the very centre of the screen. These only appear when two slits are open.