Time Trip (2003)

BBC Horizon

In 1963, Roy Kerr, a New Zealand mathematician, found a solution for the equations of Einstein for a rotating black hole, which had bizarre properties. The black hole would not collapse to a point (as previously thought) but into a spinning ring (of neutrons). The ring would be circulating so rapidly that centrifugal force would keep the ring from collapsing under gravity. The ring, in turn, acts like the Looking Glass of Alice. Anyone walking through the ring would not die, but could pass through the ring into an alternate universe. Since then, hundreds of other wormhole solutions have been found to the equations of Einstein. These wormholes connect not only two regions of space (hence the name) but also two regions of time as well. In principle, they can be used as time machines. Source: http://mkaku.org/home/?page_id=252
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Documentary Description

TIME TRIP

Time Trip - questions and answers

How widely accepted is the theory that we can travel in time?

The Future
According to Professor Paul Davies "Scientists have no doubt whatever that it is possible to build a time machine to visit the future". Since the publication of Einstein’s Special Theory of Relativity in 1905, few, if any, scientists would dispute that time travel to the future is perfectly possible.

According to this theory, time runs slower for a moving person than for someone who is stationary. This has been proven by experiments using very accurate atomic clocks. In theory, a traveller on a super high-speed rocket ship could fly far out into the Universe and then come back to Earth at a time hundreds or thousands of years in its future.

Another consequence of special relativity is that gravity slows time down. So, another way of time travelling to the future would be to go and sit next to a black hole or a neutron star, both of which are very massive and have huge gravitational fields. When you went back to Earth, it would have aged more than you.

The Past
Time travel to the past is more problematic, but there is nothing in the known laws of physics to prevent it. It is accepted that if you could travel faster than light, you could travel to the past. However, it is impossible to accelerate anything to a speed faster than light because you would need an infinite amount of energy.

But hope for prospective time travellers comes from Einstein’s General Theory of Relativity, considered to be the best theory of time and space that we have.

In 1948 Kurt Gödel worked with general relativity to produce equations suggesting the possibility of time travel to the past. He showed that a rotating universe, consistent with Einstein’s theory, would allow you travel back in time. Gödel knew that his model was unlike the real universe we inhabit and also that even if we did live in such a universe, time travel would be practically unachievable because you would need a hugely powerful rocket in which to cover astronomical distances. Despite this, Gödel’s work was firm evidence that time travel to the past is, at least in theory, possible.

Since then, numerous other scientists have come up with other solutions of general relativity that allow time travel to the past. Most rely on the prediction of the existence of 'closed time-like curves'. According to these scientists, there are ways of distorting space-time to make it curved in such a way that shortcuts through space-time exist allowing you to effectively travel faster than light and journey back into the past.

Not all scientists like this idea and there are some scientists, like Professor Stephen Hawking, who insist that there must be something that prevents it. In 1990, Hawking proposed a Chronology Protection Conjecture which says that the laws of physics disallow time machines. Basically, such scientists argue that nature will conspire to prevent the building of a time machine - one possibility is that runaway surges in quantum energy would generate massive gravitational fields and turn any time machine into mush. There are no clear answers to the issue because quantum physics and gravity do not sit well together and there is not yet a unified theory of quantum gravity.

Hawking and others have serious problems with the fact that time travel to the past would violate causality and this would have serious implications for our understanding of how the Universe works. A final answer to whether we really can travel back in time may have to wait until scientists find a way to bring quantum mechanics and general relativity together.

What are the different possible time machines we could build?

There are now a number of different proposals for time machines that have been put forward by well-regarded physicists, for example:

Professor Frank Tipler
In 1974, Professor Frank Tipler suggested that you could use an incredibly dense, spinning cylinder that was about 100 km long and 10 km wide. The cylinder would have to be incredibly strong and rigid so that it didn’t get squashed by its own gravity and so that it didn’t get torn apart by the centrifugal forces it would experience when spinning. Tipler pointed out these were 'just' practical problems which might be overcome by sufficiently advanced technology.

To use a Tipler Time Machine, you would leave Earth in a spaceship and travel to where the cylinder was spinning in space. When you were close enough to the cylinder, where the space-time is most warped, you would orbit around it a few times and then fly back to Earth, arriving back in the past. How far back in the past would depend on how many times you went round the cylinder. During your journey, your watch would always work as normal, going forward.

Tiplers work suggested that this could be done using a spinning black hole or neutron star. There are pulsars that have been observed which are spinning at a rate fast enough. However, the mathematics is not really conclusive as to whether such stars could be used for time travel or whether we would need to pile up a few of them on top of each other to form a cylinder.

Professor Richard Gott
Professor Richard Gott has shown that Cosmic Strings could be used for time travel. Cosmic strings are predicted to exist by about half the theories attempting to unify the different forces. They would be thin strands of high density material left over from the early universe. Cosmic strings have no ends so would be infinite in an infinite universe or be closed loops in a finite one. They should have a mass of about 10 million billion tons per cm and therefore they should warp the space-time round them. Gott has shown that if you have two such strings parallel to each other and moving past each other, they would warp space-time sufficiently to allow time travel to the past.

Professor Kip Thorne
Arguably the most likely method for time travel to the past is the wormhole time machine. This was invented by Professor Kip Thorne after he was asked to look into the idea by his friend Carl Sagan who used a wormhole as a plot device in his novel Contact.

If time machines are possible, why haven’t we built one?


Although the time machines suggested by physicists are theoretically possible, all of them would require massive amounts of energy and a level of engineering technology that we don’t have at the moment, and which we are unlikely to have for quite some time.

What about the paradoxes caused by time travel, like going back and killing your grandparents?

There are several problems that suggest that time travel is not possible. One of the arguments that is most frequently put forward is the so-called 'grandmother paradox': if you travel back to the past and kill your grandmother before your mother is born, you will not be born. Therefore, you could not have travelled back to the past to kill your grandmother, therefore you would be born!

Physicists have managed to come up with solutions to this. Some have proposed the Principle of Self-consistency: you can visit the past but are physically unable to change it. So, if you tried to shoot your grandmother, the gun would jam or you would be prevented in some other way from killing her. This is well illustrated in the film Bill & Ted’s Excellent Adventure. This seems to go against notion of free will but philosopher David Lewis made the point that free will does not allow you to do something logically impossible, such as instantaneously turning yourself into a tomato.

Another solution is suggested by Professor David Deutsch. He says that quantum mechanics tells us that parallel universes exist. So if you travelled back to past and killed your grandmother, you would simply end up being in a parallel universe where you had killed another version of your grandmother and were a time traveller.

One of the most famous arguments against time travel is that if time travel is possible, why haven’t we been visited by lots of time travellers from the future? Again, people have come up with ways round this objection: we may be inundated with time travellers and not be aware of it. Maybe that's what UFOs are. Perhaps civilisations don’t last long enough to develop the knowledge and technology required to build a time machine. And most convincing of all, general relativity says that you can only go back to the time a time machine was created. Since no one has built a time machine yet, no one can come back to this time.

Time Trip - programme summary

Horizon's Time Trip is a thrilling journey deep into the strangeness of cutting-edge physics - a place where beautiful, baffling ideas are sometimes indistinguishable from the utterly crazy.

On this journey, we meet a time-travelling pizza, a brilliant mathematician in a ski mask and even God. The journey ends with a strange and dark conclusion - one which calls into question our very existence.

Ever since Einstein showed it was theoretically possible, the quest to travel through time has drawn eccentric amateurs and brilliant scientists in almost equal numbers. The amateurs include Aage Nost, who demonstrates his time machine in front of the cameras. The professionals include the likes of Professor Frank Tipler of Tulane University. His time machine sounds good - but it would weigh half the mass of the galaxy.

There is, however, one way that time travel to the past could be possible. And it would be much more convenient. Future civilisations could use computers to create exact replicas of the past. Unfortunately that idea has physics trembling in its socks. Because if you can generate a perfect virtual reality version of the past, who's to say we are not one of the replicas?

Time Trip - transcript

NARRATOR (DILLY BARLOW): This is an unlikely tale about an unlikely quest. The attempt to find a way to travel through time. The cast is an unlikely one too. God, a man in a balaclava and a pizza with pretentions.

Prof RICHARD GOTT (Princeton University): This is a time machine.

NARRATOR (DILLY BARLOW): The quest would end in success, science has found a way to travel in time. But there’s a catch though, they’ve discovered you may not be real. For thousands of years time was thought to be so mysterious that no one could ever fathom it. No one that is but God, he could understand it, he could control it, he could even move through time at will, seeing the future before it happened and changing the past if he chose to. God was so good at controlling time he even managed to fit it in between his other jobs. For humans though time seemed impossible to understand.

Prof MICHIO KAKU (City University of New York): Time is the most mysterious object in the universe you can’t see it, you can’t smell it, you can’t touch it yet it’s everywhere, that’s the paradox of time.

Prof PAUL DAVIES (Macquare University): We feel it in here, it’s something that is part of our souls and any attempt to study time, to try to understand time brings bafflement.

Prof DAVID DEUTSCHE (Centre for Quantum Physics): Is the me that existed when I was a child the same me that exists now and do the things in the future already exist in some sense, that sort of question has been around from the beginning.

NARRATOR (DILLY BARLOW): Time was so mysterious scientists decided someone else should take charge of it. Someone other than God. Someone quite a lot like them.

Prof MICHIO KAKU: Scientists have long had a dream and that dream is to be a, a master of time, to be able to speed it up and slow it down and perhaps even stop time itself.

Prof FRANK TIPLER (Tulane University): The dream of time travel, what would it allow us to do? We could obviously go into the past and see what had actually occurred back then. We could also effectively go into the future and see what our ultimate destiny was. This question we could answer if we had time travel because we could go to the ultimate future and then return to the present with the answers.

NARRATOR (DILLY BARLOW): The quest to find a way to master time would take science on an adventure, deeper and deeper into strangeness and lead to a series of dark discoveries, each one more troubling than the last. The first revelation came with strings attached. Meet Sir Isaac Newton. The first attempt to understand time systematically began with him. His work astonished his contemporaries, they were less keen though on the starring role he gave to marionettes.

Prof MICHIO KAKU: Newton’s work was absolutely revolutionary, before Newton there was superstition and magic. After Newton we had three laws of motion and the laws of gravitation.

NARRATOR (DILLY BARLOW): Newton studied time for 30 years, until finally he announced he’d made sense of it. The universe he said was like a giant clock, time kept everything in order.

Prof PAUL DAVIES: He had the idea that there was simply one universal absolute time the same for everybody, everywhere, didn’t matter where they were it was the same throughout the universe.

NARRATOR (DILLY BARLOW): According to Newton time was fixed and it’s flow and pace never varied. So far so good. All this sounded rather sensible but what followed didn’t for Newton had a dark vision about time. He said time was so fixed, so ordered, that it must have all been planned from the beginning, our future was already decided upon and there was nothing we could do about it.

Prof MICHIO KAKU: There was a philosophical downside to Newton’s laws also. The future was pre-determined, we’re nothing but puppets reading from the script, reading from the book of destiny.

Prof FRANK TIPLER: We like to think that what we’re going to do tomorrow is undetermined except by what we decide to do but in the Newtonian view our detailed action tomorrow was determined in the beginning of time.

NARRATOR (DILLY BARLOW): Newton’s ideas were so influential they dominated science for 200 years. It was going to take something big to overturn them, something very big indeed. Einstein’s work is powerful baffling stuff. This explosion is one of the most beautiful sights ever seen by man. It did however lead directly to the nuclear bomb, which is a pretty convincing argument against those who think it is so strange it just can’t be true. His impact on our ideas about time was almost as devastating.

Prof PAUL DAVIES: It effectively demolished 200 years of understanding about the nature of space and time.

Prof MICHIO KAKU: Einstein’s theory is like a Trojan horse, on the surface it is magnificent, it is beautiful but inside that Trojan horse lurks goblins and demons.

NARRATOR (DILLY BARLOW): Newton had said there was just one time and there was therefore just one now. Einstein however begged to differ. He said time was much weirder than that. It was just like space, every bit of space exists right now and so every bit of time exists now too. Somewhere in the universe your descendents are already alive. Somewhere else your long dead ancestors are still living. The future and the past are out there waiting for us and Einstein even suggested a way to get there. He suggested the answer was all to do with speed. Until Einstein it was thought time flowed at the same pace everywhere, but he showed that wasn’t so, the pace of time can change. It differs depending on the speed you're travelling at. Bizarre as it may seem, the faster you travel the slower time will run for you.

Prof RICHARD GOTT (Princeton University): Einstein’s work completely transformed our view of time. He showed that time was bendable, all clocks did not tick at the same rate. If a clock was moving rapidly, near the speed of light it would tick very slowly.

Prof MICHIO KAKU: Time is relative, the faster you move the slower time beats and in fact a clock at the top of a building actually runs a little bit faster than a clock at the bottom of a building and for Gods sake near the surface of a neutron star or even at the center of a black hole time slows down and perhaps even stops.

NARRATOR (DILLY BARLOW): Einstein knew his theory had an astonishing implication, if time could be made to speed up or slow down then that means it could be manipulated and controlled. You could use that effect to leave your own time behind and go to someone else’s, time travel was possible. It’s easy to dismiss time travel as a fantasy but the reason it’s taken seriously at all is because it already happens. There are fully certified time travellers and they use Einstein’s predictions to do it. People like the cosmonauts on the Meer space station for example.

Prof RICHARD GOTT: The greatest time traveller so far is an astronaut, cosmonaut named Sergei Abediev who spent 748 days aboard the Meer space station travelling over 17000 miles per hour. And when he came back because of this he had aged a 50th of a second less than he would have if he had stayed home. In other words when he got back to the earth he found it to be a 50th of a second to the future of where he expected it to be, he has time travelled a 50th of a second into the future.

NARRATOR (DILLY BARLOW): Admittedly a 50th of a second may not sound dramatic but the cosmonauts of Meer could have travelled much further into the future, all they’d have needed to do was travel even faster.

Prof PAUL DAVIES: Imagine if I go off in a rocket ship at very close to the speed of light and perhaps I'm gone for about a year whizzing around our part of the galaxy and I come back to earth and I find that you're ten years older. I've been away one year but ten years have elapsed here on earth, and so in effect I've just nine years into your future.

Prof DAVID DEUTSCHE: This sounds like an extremely weird and unbelievable property of nature. It’s actually one of the best corroborated physical effects that we know of. People who build satellites have to routinely take into account the fact that time travels at a different speed in different states of motion.

NARRATOR (DILLY BARLOW): When Einstein first revealed this effect people were captivated. It seemed as if humans might master time at last, we would be able to travel through time at will. But disappointment though soon set in. For Einstein said his type of time travel was strictly limited. You could go forward to the future but you could never return to your own time. Which means if you travelled far into the future and then came back to earth the people you left behind and all your loved ones would be much more old or much more dead. There was a reason you couldn’t return to your own time, that would require travelling back in time and that was something which Einstein would not countenance. It was taboo for it broke one of the greatest rules in physics. To make time slow down you have to travel fast, but Einstein said to travel back in time you’d have to travel really really really fast. In fact you would have to travel faster than the speed of light. Unfortunately Einstein had also proved you just couldn’t do that.

Prof RICHARD GOTT: Einstein showed that if you could travel faster than light you could even go back in time. The trouble was that Einstein also showed that you couldn’t build a rocket ship that would travel faster than the speed of light.

Prof PAUL DAVIES: If by some magic you could travel faster than light then that could take you back in time, but travel faster than light was strictly proscribed, there is no way you could do it.

NARRATOR (DILLY BARLOW): This was Einstein’s golden rule and it seemed to make time travel to the past utterly impossible. Some people however refused to accept the golden rule. They would devise outrageous schemes to get round it and they would find a way. But first though, they’d have to sacrifice something important, something called common sense. In our every day lives we tend to think common sense is useful, physicists though have long been more sceptical. The universe just so happens is much too upmarket and exotic for our common sense.

Prof MICHIO KAKU: The universe consists of stars, supernovas or the freezing cold of outer space. That is the universe of Einstein with time speeding up and time slowing down. We are the odd balls, we are the exceptions. That’s why our intuition, our common sense fails us when we want to understand the universe. Some people complain that we physicists keep coming up with weirder and weirder concepts, the reason is we are actually getting closer and closer to the truth. So if we physicists keep coming up with crazier and crazier ideas that’s because that’s the way the universe really is. The universe is crazier than any of us really expected.

NARRATOR (DILLY BARLOW): The question is whether the universe is crazy enough to allow us to travel through time at will, to break Einstein’s golden rule and travel to the past. Some people think it is. It’s long been claimed that if people could travel through time at will then we’d be surrounded by tourists from the future. But how do we know we aren't? In the United States there are a number of people who claim to have travelled in time and they do it quite happily in both directions.

PATRICIA RESS: How many time travellers are there? I think there are probably thousands. I think it’s quite possible that there are thousands of people that are doing this.

NARRATOR (DILLY BARLOW): Patricia Ress has written a number of books about real life time travellers.

PATRICIA RESS: There’s two types of technology that go into time travel, one is the nuts and bolts where you would get into a machine and dial and say you want to go to a certain year and a certain place and the machine would take you, and it would work something like Star Trek where you would disassemble and then be reassembled at a different location. The other type of time travel it’s possible to use your mind to go through time portals. I’ve never time travelled because I might wander into a situation that I wouldn’t have the knowledge to deal with, for example if I ran into a dinosaur I don’t know how I would kill it or capture it and if I went ahead in the future and had a robot after me I don’t know what I would do about that.

NARRATOR (DILLY BARLOW): New Orleans seems somehow appropriate for Patricia’s research. A city which feels like it might be on the edge of one of her time portals, she’s here to meet a voyager in time called Aage, he claims to have his very own time machine.

AAGE NOST: In the beginning when I was working with this machine I couldn’t get anything to happen, but after I started manipulating my brainwave pattern at the same time I started getting flashes of light coming from the side into my field of vision and I found myself walking down the street and I walked up to a newspaper stand and I picked up a newspaper and I was able to read that newspaper and that newspaper was six months into the future. Some of the things that I saw in the future is very hard to believe, this country is in for some hard times. From about the Fall of 2004 the stock market and economy in this country is starting to take a dive. A year later in about the Fall of 2005 it gets to the point where we almost have an armed uprising, the military will have to step in and create order and install a new government.

NARRATOR (DILLY BARLOW): So how does his time machine work? Aggie aggrieved to demonstrate it.

AAGE NOST: And I will plug it in now. And here is the electromagnet and this is the time coil and then we have the electronic box with the chip in it, the stick plate and the dials for the fine tuning of the frequency itself. The time coil is put around your head, it goes right over here, right above your eyebrows around the middle of the head. Then I will bring my brainwave frequency down into the bottom of the alpha range and picture the time, the date and the place to where I want to go. This particular machine is a mind machine interface device. Without the mind I doubt it will work. I can feel the machine working because the magnet is really hot and I can feel a tingling right here in my solar plexus, from the direction of the brunt of the magnetic field.

MAN’S VOICE FROM BEHIND CAMERA: Is it working yet?

AAGE NOST: This time no, I’ve been talking too much, I don’t expect anything to happen here.

NARRATOR (DILLY BARLOW): If Aage had travelled into the future and then back he’d have proven it was possible to break Einstein’s golden rule the one that said travel back in time was impossible. His failure though doesn’t meant that the rule is safe, there are other people who say they can get round it, serious scientists. But their ideas make those of Aage seem positively mundane. The man who found a way around Einstein’s golden rule was a friend of his, he also happened to be one of the most eccentric figures in the history of science, which is saying something. Kurt Gödel spent most of his life convinced he was being followed and that there was a conspiracy to poison him. He was also one of the most brilliant mathematicians ever to have lived.

Prof MICHIO KAKU: In 1949 a bombshell was dropped.

Prof RICHARD GOTT: Einstein’s friend, Kurt Gödel, who was one of the most famous mathematicians of the 20th century found a solution to Einstein’s equations where time travel to the past was possible.

NARRATOR (DILLY BARLOW): Soon after he met Einstein, Gödel became obsessed with time travel and in particular the limits Einstein had placed on it. Initially he spent some time testing the idea that nothing could travel faster than light but it turned out Einstein was right. So Gödel decided to find another way to get to the past. He changed the nature of time itself. The problem he realised was that everyone assumed time flowed in a straight line but what if it didn’t? What if time could loop round like eddies in water? Gödel suggested if you could make time loop round like that then you could reach the past. You wouldn’t need to travel faster than the speed of light, you’d be able to take a shortcut and magically get there before the light did. There was only one way Gödel could think of to make time loop, it was to make everything else loop too. The universe would have to spin round.

Prof RICHARD GOTT: In Gödel’s universe if you stood absolutely still and weren't dizzy you would find that the stars and galaxies of the universe would be twirling, twirling around you. And you would find that if you went on a long enough trip away from home you could come back even before you started.

Prof PAUL DAVIES: Kurt Gödel found that in such a rotating universe not only could an observer go off on a trajectory and reach any given point in space they liked, they could also reach any given point in time, it would be possible to leave earth in a space ship and travel not only anywhere but any when too.

NARRATOR (DILLY BARLOW): Gödel’s idea of looping time was astonishing and breathtaking, it was also complete nonsense, for the universe we live in does not rotate. This however didn’t seem to matter for his idea made sense mathematically and so that meant it was inspirational.

Prof MICHIO KAKU: Gödel’s solution in 1949 opened up a Pandora’s box.

Prof DAVID DEUTSCHE: It broke the ice or perhaps the taboo.

Prof PAUL DAVIES: The genie was out of the bottle.

NARRATOR (DILLY BARLOW): Gödel was to inspire a whole new generation of physicists, and even the fact that he took to wearing a balaclava all the time wasn’t to put them off. And here we are back in New Orleans, the home of Mardi Gras, voodoo and by sheer coincidence an international famous pioneer of time travel and we’re not talking about Aage. Professor Frank Tipler has become the guru of all serious scientific time travellers. The first to take Gödel’s brilliant but crazy solution and turn it into something almost practical.

Prof FRANK TIPLER: I have always been interested in extending human power. That has been my motivation since kindergarten. I have a vivid memory of myself imagining rockets going up in the distance which I myself had designed. Ever since I have been interested in space travel and time travel.

NARRATOR (DILLY BARLOW): In the early 1970’s he heard about the work of Kurt Gödel and it set him thinking.

Prof FRANK TIPLER: Kurt Gödel had shown that if the universe happened to be rotating sufficiently rapidly it could give rise to time travel. And I thought that was a fascinating idea. Unfortunately we could not rotate the entire universe it’s not now rotating, it will never rotate. But I wondered to myself would it be possible to rotate a smaller object that might yield the same property of time travel?

NARRATOR (DILLY BARLOW): The first small object Tipler studied was smaller than the entire universe but it wasn’t exactly tiny. He wondered whether he could exploit the strange vast phenomena called black holes. They are spinning regions of the universe left by the collapse of stars. In black holes the normal laws of physics seem to have been reversed and it had been suggested that if you entered one you might experience time running backwards to the past. However, Frank found a rather practical problem with black holes.

Prof FRANK TIPLER: The problem with the black hole is it is surrounded by a surface so that if you go inside the black hole you can never get out again.

NARRATOR (DILLY BARLOW): So if a black hole couldn’t be a time machine, Tipler thought perhaps he could do the next best thing. He could make a black hole all of his own. He used mathematical equations to try and devise different rotating structures which would create the properties of a black hole without the disadvantages. There seemed to be just one structure which did the trick.

Prof FRANK TIPLER: Now it occurred to me another rotating body which would not have the surface of a black hole is a long extended cylinder. I was able to prove that inevitably if you were to spin up a cylinder sufficiently rapidly it would necessarily have a region around it that would permit time travel, permit you to go around the cylinder and return to your starting point far away from the cylinder before you left, true time travel.

NARRATOR (DILLY BARLOW): It had taken Tipler two years to work out the mathematical implications of this bizarre structure. Finally he was able to announce it really would work. If you travelled around the cylinder you could go back in time. The universe wouldn’t be rotating as Gödel suggested but the effect would be the same.

Prof FRANK TIPLER: I was so excited because I thought that this had at long last shown that time travel would be in some point in the future within human grasp.

NARRATOR (DILLY BARLOW): Frank Tipler had proved that a time machine might one day be built. The dream of moving around time at will it seemed could be achieved, humans would be able to go to the future or the past and return to the present day. And now others set out to build variations on Tipler’s theme.

Prof MICHIO KAKU: We now have a zoology of time machines. All these solutions that have been proposed for time machines do the same thing that Gödel did back in 1949 and that is warp and twist the fabric of space and time so much that space turns into time, time turns into space and you could literally go forwards in time until you come backwards in time.

NARRATOR (DILLY BARLOW): In this zoo there is a place for almost everything, even a pizza.

Prof RICHARD GOTT: You might wonder what pizza has to do with physics.

NARRATOR (DILLY BARLOW): Richard Gott brought two very disparate parts of the universe together, pizzas and time machines.

Prof RICHARD GOTT: This is time travel to the past, this is a time machine.

NARRATOR (DILLY BARLOW): It all started when physicists all over the world began investigating a curious phenomenon.

Prof RICHARD GOTT: In the mid 1980’s people became interested in cosmic strings, these were, I brought one here. These were thin strands of energy left over after the big bang that were predicted in about half the theories of particle physics at the time. They were narrower than an atomic nucleus, they had no end so either they were infinite in extent or they were large loops. These might be millions or billions of light years across, but very very thin.

NARRATOR (DILLY BARLOW): Perhaps, perhaps not, in fact cosmic strings have never been observed in the real world, they're entirely theoretical. They're mathematical inventions. So imaginary in fact that no one could find a way to describe them completely. Gott made his name by solving that particular problem.

Prof RICHARD GOTT: I was quite excited to find this solution and it was important to have an exact solution because it gives a way to look for the cosmic strings.

NARRATOR (DILLY BARLOW): His solution showed that space and time around a cosmic string were very strange indeed and now he wondered what would happen with an even more exotic phenomenon two cosmic strings passing in the night.

Prof RICHARD GOTT: Years later I found the solution for two moving cosmic strings and here’s, if you have two moving cosmic strings that are moving by each other, what does that solution look like?

NARRATOR (DILLY BARLOW): Gott’s mathematical equations predict that two cosmic strings would twist time out of shape completely, there would be a loop in time just like the one in Tipler’s cylinder.

Prof RICHARD GOTT: If the cosmic strings moved rapidly enough but still slower than the speed of light, the solution was sufficiently twisted so that you could start here, circle the two cosmic strings whilst they were passing and come back to an event in your own past.

NARRATOR (DILLY BARLOW): And if that’s not entirely clear then here comes the pizza again.

Prof RICHARD GOTT: Now you might wonder what pizza has to do with physics well if you wondered what the space around the cosmic string might look like, you might think that the string would come down through the center and the space around it would look like just a regular pizza, but actually in my solution surprisingly show that it would look like a pizza with a slice missing. So I’m just going to cut out a slice here and get rid of it, you can eat it if you like and then you fold the pizza up to make a cone and the circumference around the string is less than you might think. So this is what the geometry around a single cosmic string looks like. So if there's a second string here you take out a second slice of pizza here and then this is, this is folded up on both sides it kind of looks like a paper boat. Now, how does this make a time machine? Well suppose you're living over here on Planet A, you get a spaceship, you could send a light signal to a friend of yours on Planet B, go right through between the two cosmic strings but you could get in your spaceship and by jumping across this missing pizza slice, you could take a shortcut and get to Planet B quicker than the light beam. You could leave Planet B at noon here and travel over here to arrive back at Planet A at noon. In fact you could arrive back in time to shake hands with your younger self and wish yourself a nice trip around the cosmic string.

MAN’S VOICE FROM BEHIND CAMERA: So that’s a time machine?

Prof RICHARD GOTT: So that makes a time machine.

NARRATOR (DILLY BARLOW): Richard Gott’s proposal is only the latest suggestion for how a time machine might work. There are now many solutions which claim to allow an astronaut to go in both directions in time. Unfortunately though they all suffer from a common flaw. The only society that would be clever enough to try and build such machines lies thousands of years in the future.

Prof MICHIO KAKU: A civilisation that has galactic power that can play with star systems and black holes, they would be masters of space and time, however the energy necessary to drive a time machine is far beyond anything that can be harnessed by people on the planet earth.

Prof RICHARD GOTT: The time machine that I propose using cosmic strings, if you made that out of a loop of cosmic string and wanted to go back in time about a year, it would take half the mass of our galaxy, it would take a loop that big.

NARRATOR (DILLY BARLOW): And even if someone did manage to build these machines, they would be of limited appeal, for they all rely on creating loops in time and it’s now been realised that loops in time have a rigid rule of their own.

Prof PAUL DAVIES: All of the methods that have been discussed so far, whether it’s Gott’s cosmic strings or spinning black holes or any of these things all share the property that you can’t go back in time to before the time the thing was built. So if someone builds one in the year 3000 they can't come back and tell us about it. We can't build a time machine now and go back and see the dinosaurs, unless of course some friendly aliens across the galaxy made one hundreds of millions of years ago and would lend it to us.

DR WHO FOOTAGE – DARLEK: Exterminate, exterminate.

NARRATOR (DILLY BARLOW): You might have thought all this meant the whole idea of time travel had come to nothing. But you’d have reckoned without the imagination of physicists. The quest for a way to master time was about to lead to the darkest conclusion of all. Around five years ago a group of scientists and philosophers began arguing there might be another way to travel in time. But it wasn’t a machine in space and it wouldn’t involve moving galaxies around. It would be a computer and the pioneer of this unlikely approach was that unlikely pioneer of time machines, Professor Frank Tipler. From the 1970’s he’d been excited by a trend in computers.

Prof FRANK TIPLER: People realised that processing speed of computers was increasing exponentially, every year, every few years, every eighteen months the processing speed would double.

NARRATOR (DILLY BARLOW): If this trend was to continue unabated it would mean computers would get ever faster and ever more powerful. So much so that in the far future a super civilisation would have computers that make ours look puny.

Prof FRANK TIPLER: Imagine this occurring faster and faster. If that were to occur it would be possible to process an infinite amount of information.

NARRATOR (DILLY BARLOW): It was this infinite computing power Tipler said which would allow a future civilisation to travel in time. They’d do it using virtual reality. The images computers can create have improved hugely over the last 30 years. But many physicists believe eventually computers will be able to make virtual images which are so good they’d be indistinguishable from reality. They’d be exact copies of the real world down to the last particle.

Prof DAVID DEUTSCHE: In the distant future simulating physical systems with very high accuracy so that they look perfectly real to the user of the virtual reality will become common place and trivial.

NARRATOR (DILLY BARLOW): But what if a future civilisation wasn’t just able to create images of fictional events? What if instead they could create images of their own real past? Then they’d be able to make exact three- dimensional copies of events and places in our time. These simulations would be populated by beings identical to us. They’d feel like us, they’d have thoughts just like us. They’d even think they were us. Our future descendants could then use these images to find out about their past. They’d have a time machine, but they wouldn’t need to travel to the past, they’d bring the past to them.

Dr NICK BOSTRUM (Oxford University): So imagine an advanced civilisation and suppose that they want to visit the past it might turn out not to be possible to build a time machine and actually go back into the past, physics might simply not permit that. There is a second way in which they could get the experience of living in the past and that would be by creating a very detailed and realistic simulation of the past.

NARRATOR (DILLY BARLOW): The notion that computers might one day be used to create exact copies of the past might seem like a wild intellectual exercise but a new generation of believers in time travel have taken the idea to it’s own wild intellectual conclusion and they’ve pointed out that a super civilisation wouldn’t just make one perfect simulation of the past.

Dr NICK BOSTRUM: An advanced civilisation would have enough computing power that even if it devoted only a tiny fraction of one percent of that computing power for just one second in the course of its maybe thousand years long existence, that would be enough to create billions and billions of ancestor simulations.

NARRATOR (DILLY BARLOW): Every time science has thought it has found a way to control time it’s been forced to confront a worrying implication. This latest solution is no exception. Because it too turns out to have a bitter twist. One dictated by the laws of mathematical probability. If the computer of the future is churning out billions of simulations, how do we know we are living in the original real world and not one of the billions of copies? In fact the odds against us being real are billions to one against.

Dr NICK BOSTRUM: There would be a lot more simulated people like you than there would be original non-simulated ones. And then you’ve got to think, hang on, if almost everybody like me are simulated people and just a tiny minority are non-simulated ones then I am probably one of the simulated ones rather than one of the exceptional non-simulated ones. In other words you are almost certainly living in an ancestor simulation right now.

Prof PAUL DAVIES: The better the simulation gets the harder it would to be able to tell whether or not you were in a simulation or in the real thing, whether you live in a fake universe or a real universe and indeed the distinction between what is real and what is fake would simply evaporate away.

Prof FRANK TIPLER: Inside the simulation you can't tell any difference between the simulated environment, the virtual reality and the real environment. In fact this environment we need find ourselves in could be just a simulation.

NARRATOR (DILLY BARLOW): Three hundred years ago science set out on a quest to master time, to control it. People didn’t like time being controlled by a super intelligent superior being, we do it for ourselves instead. But every time we made a break through there was a downside. Now we’re told we may not even be real. Instead we may merely be part of a computer program, our free will as Newton suggested is probably an illusion. And just to rub it in, we are being controlled by a super intelligent superior being, who is after all the master of time.

Prof DAVID DEUTSCHE: From the point of view of science it’s a catastrophic idea, the purpose of science is to understand reality. If we’re living in a virtual reality we are forever barred from understanding nature.

Prof PAUL DAVIES: Our investigation of the nature of time has lead inevitably to question the nature of reality and it would be a true irony if the culmination of this great scientific story was to undermine the very existence of the whole enterprise and indeed the existence of the rational universe.

Prof MICHIO KAKU: The universe is not only queerer than we supposed, it is queerer than we can suppose.

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