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24 September 2014
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A future time machine?
BBC Two, Thursday 18 December 2003, 9pm
Time Trip
Coming Up
Horizon explores the relationship between science and chimpanzees. BBC Two, 8th January, 9pm.

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.

Professor Paul Davies
Time Loops
BBC Space
Time Travel
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