Original URL: http://www.theregister.co.uk/2013/11/13/time_travel_an_impossible_possible/

Want to BUILD YOUR OWN Tardis? First, get a star and set it spinning...

What science today says about how we might travel backwards and forwards in time

By Gavin Clarke

Posted in Science, 13th November 2013 15:03 GMT

Doctor Who @ 50 A joke was doing the rounds at CERN two years ago:

“Knock, knock.”

“Neutrino.”

“Who’s there?”

The inspiration for the gag were CERN boffins working on the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment. They had apparently found that a group of sub-atomic particles called neutrinos had broken a fundamental law of physics by travelling faster than the speed of light.

Albert Einstein’s 1905 Special Theory of Relativity says nothing with a mass can accelerate to go faster than the speed of light. Neutrinos have mass: not very much but enough to require a colossal amount of energy to accelerate one to a speed faster than that which light travels at. It’s hard to beat light. It has no mass and is constant wherever you go in the universe.

Special relativity is important: it’s one of the fundamentals of modern physics that forms our view of the universe. It sets limits on how quickly we can move, and makes time run more slowly the faster we go.

But the neutrinos the CERN folk had found had broken the rules. And for sci-fi fans and wannabe Time Lords that meant one thing: time travel.

Once you approach, or break, the speed of light causality begins to break down and an action can theoretically have consequences before it takes place. You might see a window break before a stone is thrown, or kill your own grandmother before you’re born and so on.

Hence the knock-knock joke doing the rounds at CERN.

“It was very, very surprising and of course we were all very sceptical,” CERN theoretical physicist Gian Francesco Giudice tells The Reg.

“The first reaction was amazement, then we immediately we began looking into what it could mean. If it were true, it changes completely our vision of how space-time works. It was very important.”

CERN Building 40. Copyright © CERN

Knock, knock - neutrino jokes at CERN
Source: CERN

Order was restored when further analysis proved the neutrinos hadn’t gone faster than light after all. OPERA’s results were attributed to a loose fibre-optic cable in the computer test rig that had sent the neutrinos on a 700km journey between Geneva and Gran Sasso in Italy.

The gaffe claimed the scalp of the experiment’s chief physicist, Antonio Masiero, who stepped down.

But don’t give up, Who fans. The idea of time travel isn’t as preposterous as it sounds. You just need a massive amount of power and the mathematical genius of somebody more clever than Professor Stephen Hawking or Albert Einstein to achieve it.

Albert first. Modern physics is based on two models from Einstein: special and general relativity. The General Theory of Relativity takes special relativity and adds mass to the equations. It reveals that objects of any size deform space-time. These distortions are perceived as gravity.

Breaking the light barrier

“Time flows slower the faster you move,” Giudice says. “For a particle moving at the speed of light, time is frozen. Then, by extrapolation, you must conclude that faster-than-light neutrinos would feel time flow backwards and you end up with all kinds of paradoxes about causality.”

But why is breaking the speed of light necessary for time travel?

There are three kinds of path between events in space-time, according to Einstein: time-like, space-like and light-like. Humans take a time-like path. Causality – the window breaking after the stone is thrown - is preserved. How do we know it’s preserved? Have you ever seen a window break before the stone responsible for breaking it was thrown. Exactly. The proof of the theory is self-evident in ordinary experience.

“Relativity tells us that, no matter how fast you run after a light beam, you always find that light has the same speed. Discovering that this is not true and that there is no absolute velocity would wreck our understanding of space-time,” Giudice says.

But despite this, general relativity does actually allow for the idea of time travel – it’s just making it happen that’s the hard bit.

There are at least two theories, both rooted in general relativity, which say time travel is possible.

Twisty-turny universe

Mathematician Kurt Gödel said time travel is possible in a rotating universe - a Gödel Universe, as we now call it. He said if we lived in a rotating universe, the universe could spin out loops and you could travel along them to meet yourself in the future.

Gödel’s theory is founded on one of Einstein’s core beliefs: that the universe is not expanding and is rotating at a constant speed.

Models have been created of a Gödel Universe but they don’t resemble what we see of the observable universe. Our space-time isn’t rotating at a constant speed, so the idea breaks down.

Another idea comes from theoretical physicist Miguel Alcubierre, who gave time-travel fans hope with his Alcubierre Warp Drive in 1996. Using the Alcubierre Warp Drive you create a bubble in space-time that contains you and your ship – let’s call it a Tardis. In front of the bubble you scrunch up space and behind you fold it flat. The upshot: you travel around faster than light does.

In October, physicists Benjamin Tippett and David Tsang published a paper outlining a conceptual Tardis that exploits the Aclubierre drive. Their Tardis – which stands for Traversable Achronal Retrograde Domain in Space-time, rather than Time and Relative Dimension in Space – would travel along a closed loop in space and time with a person safe within the bubble. You can read their paper here (non-technical version) and here (come wearing your propeller hat).

Space-time under general relativity is curved. It is curved because it’s distorted by objects with a mass. Imagine space-time is like a huge rubber sheet. Place a heavy ball in the middle of the sheet and the ball will create a dip.

Warp drive

Now scale that up. If you create a large enough gravitational force in space-time - swap the ball for a planet or a star - then you can bend space-time even further. If you can then rotate that object fast enough, up to and beyond speed of light, you can - potentially - travel in time through a trick called ‘frame dragging’.

This is presumably the theory of a time travelling Tardis: a ship with a tremendously powerful engine and energy source capable of distorting space-time enough to move faster than light, thereby letting an occupant travel in time. You can view the Tardis’ interior, held in a separate ‘dimension’ from the outside universe - a separate space-time, in other words - as the bubble of Alcubierre’s warp drive.

But could it ever generate enough power? Therein lies the rub and it’s the square-one Tippett and Tsang are pulled back to at the end of their paper.

“Unfortunately, just like the Alcubierre Warp Drive, generating the Tardis geometry would require exotic matter, violating the classical energy conditions. This matter would be gravitationally repulsive and would need to move faster than light,” they write.

Doctor Phil Bull, a post-doctoral research fellow at the University of Oslo, outlined the scale of the challenge for would-be time travellers. “You can can only get these extreme effects with really gigantic distortions in space-time. You’d need a planet to spin very close to the speed of light,” Bull tells The Reg.

The amount of energy required to even make a planet spin that fast would be huge. “Maybe you could set that up, but it’s still a gigantic engineering problem that’s out of our reach,” Bull says.

Exotic matter

Black holes are another option because they spin and they are hugely massive. But, Bull says, black holes don’t spin “in the right way to give us time travel”.

“Maybe in 200 years’ time space technology and the way we harness energy will have evolved to a point where we can manipulate things as big as stars and then we can start using general relativity to harness energy from black holes.”

Interestingly, this exactly what Rassilon, founder of Time Lord society, did to enable time travel. Just watch The Deadly Assassin.

Of course, all of the challenges outlined so far are put there by general relativity. But what if general relativity isn’t the all-encompassing framework that governs events in the universe that we think it is?

Indeed, general relativity is already under threat from the recent suggestion that there might be something out there in the universe which we now call ‘dark energy’.

This is the material theorised to be responsible for the continued and accelerating expansion of the observable universe. Einstein discounted the idea of an expanding universe and so, as a result, does general relativity. Einstein believed the universe to be static.

The missing 70 per cent

Dark energy was only postulated after it became possible to accurately count the number of galaxies and their position, and – therefore – to see how they are moving. This took place 20-30 years ago, long after Einstein’s time.

“Dark energy is a very, very, very big problem – it’s a crisis in cosmology. It’s a massive thing we don’t understand,” Bull admits.

Phil Bull

Phil Bull: dark matter a very, very, very big problem

The degree to which massive, galactic gravities bend beams of light - a distortion predicted by general relativity - is greater than the observable amount of matter in those galaxies. That suggests the presence of another, directly unobservable substance that accounts for the extra mass and thus the extra gravity. Cosmologists call this material ‘dark matter’.

The snag is that all the gravitational force this extra matter generates should be slowing down the expansion of the universe. Instead the universe’s expansion appears to be continuing. If anything, it’s increasing its pace. This is why we need dark energy to counter the extra gravitation induced by the dark matter and accelerate the expansion of the universe.

There’s a lot of this exotic material around: about 70 per cent of the universe is now thought to be made of dark matter, according to NASA. If the ‘dark stuff’ theory is correct, almost three-quarters of the universe can’t be seen directly.

Some scientists believe dark matter is made up of WIMPs: Weakly Interacting Massive Particles. Scientists are looking for them using detectors placed deep underground to shield them from cosmic rays and other particles. So far they have not had much success, which is a surprise given how ubiquitous dark matter is thought to be.

Einstein a-go-go

Or perhaps we have our sums wrong. There’s a possibility there might be a more a fundamental description of the universe beyond Einstein and general relativity.

“Some people think general relativity is sacrosanct. They think there’s a weird form of matter out there and we will be able to find out what it is - eventually. There are others who say we have to look at general relativity itself, and it’s the theory that’s wrong,” Bull said.

There is another unknown too: ‘quantum foam’.

It’s known that general relativity and quantum mechanics are not happy bedfellows. General relativity works on large objects like planets, stars and black holes but it doesn’t explain objects at the quantum level, where events are more unpredictable and described statistically rather than specifically.

Quantum mechanics allows for the possibility of virtual particles - pairs of sub-atomic entities popping in and out of existence. It’s this constant churn of particles appearing and disappearing that’s called quantum foam, and it has been suggested that this stuff is the fundamental fabric of the universe.

The problem with quantum foam as a theory is that it’s happening on a scale so small that even CERN’s atom smasher can’t it.

According to Bull, therefore, general relativity might be challenged but it still holds up for the universe at our scale. And that brings us back to neutrinos and time travel.

Counting neutrinos

Ben Still works on the Tokai 2 Kamioka Experiment in Japan that has found that certain neutrinos change from one flavour to another as they travel. T2K found that muon neutrinos actually become electron neutrinos, for instance. Like OPERA, T2K has also been firing neutrinos over long distances.

While OPERA found to its cost that neutrinos can’t travel faster than light, and so cannot travel in time, the results from T2K shows there are still too many unknowns about neutrinos to draw a final conclusion.

So Still doesn’t think faster-than-light neutrinos have been completely ruled out, which provides a potential way in for would-be time travellers.

“From these experiments we can say neutrinos can’t speed up to travel faster than light. But we might find that a neutrino is born faster than light and will continue to travel faster than light. But the question is, will we be able to see them,” Still says.

“We can only say what nature doesn’t do and I don’t think we are that a point where we can say what a neutrino does unequivocally.”

When it comes to time travel, physics isn’t a precise science. Neutrinos might be able to travel in time, but the operative word here is ‘might’. It’s a theory based on the fact we still know so little about these sub-atomic riddles.

On the grand scale of general relativity, time travel is a least possible - again, theoretically speaking. It’s just that the practical side of the story that gets in the way.

“The time travel thing, I’m not sure that's plausible,” Bull concludes. “But fitting more space into a smaller space like the Tardis. That’s something you can get out of general relativity.”

Don’t get us started. ®