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Wednesday, 3 February 2010

Is Time Travel Possible?
Science or Science Fiction?

'Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity.' A. Einstein

One of quite a few unsolved problems in physics is whether it is possible to time travel, but are there really any solved problems or it’s just an illusion? For most people time travel is a common plot device in science fiction, although I’ve always considered travel to the past a sort of Deus Ex Machina, since it doesn’t make much sense to me. Conversely, the possibility of time travel to the future is arguably deducible from special and general relativities based on the phenomenon of gravitational and time dilation linked to the speed of the observer.

An example of time travel, specifically a kind of twins paradox, can be found as early as in Hindu mythology, the Mahabharatha tells the story of the King Revaita, who travels to heaven to meet the creator Brahma and is shocked to learn that many ages have passed when he returns to Earth. The Japanese tale of "Urashima Tarō" first described in the Nihongi (720), and the Talmud story of Choni HaMeagel deal with a similar concept. Looks like Einstein’s idea wasn’t that new after all.

I wonder how scientists imagine actual time-travel to the future, but I think it’s just an abstract idea rather than a practical possibility. First of all, an astronaut would have to move nearly at the speed of light to create time dilation, but no living creature could survive the ride (at least those we know) since matter at such velocities will turn into energy according to the relativity. Even if he were able to travel at such a breakneck pace and come back to the Earth in one piece, he wouldn’t find himself in HIS personal future, but in the future of those who stayed behind.

How would time flow aboard the ship moving at the speed of light?

While the astronaut thinks his clock is ticking at a normal rate, from the physical point of view of the local observer (in their rest frame), for instance, the people on the Earth, the astronaut’s clock is slow.
The greater the spaceship’s relative speed, the slower the on-board clock, again for the local observer (Earth). Since any motion is relative for inertially moving systems, the clock on the Earth will also seem to be slow to the astronaut, so that he’ll see that in his frame of reference a hundred years have passed, but only one on the Earth. Who will look older to whom? A paradox?

While the astronaut have lived a year, a hundred have passed for the local observer (Earth), but the astronaut is always in his own present and measured off just one year of his life, his personal life span won’t be longer than that of an inhabitant of the Earth and he will age at the same rate from his point of view. By the way, how would he count his years? I guess only with a clock.

My take is that the one who makes an effort and creates the acceleration is really moving, at least from physical point of view. It’s like debating on what is moving, the train or the station. This is of course a layman’s approach, but that’s all I’m able to make of it right now.

However, if the astronaut accelerates, travels into space and then comes back to the Earth (as in the twin paradox), he will be the one who has aged less. It seems that he who accelerates has an advantage.

Satellite orbiting Earth
Satellite orbiting Earth
According to general relativity time dilation effects can also occur if one observer is deeper in a gravity well than the other, the deeper in the well the slower the clock; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it would make observers at different distances from a black hole age at a different rate. Still, all these phenomena hardly qualify for time travel in the strict meaning of the word.

In my book, written by my simple mind, travel into the future is meeting your older self or visiting the world where your descendants live so that you can either watch or interact with them.
And why would anyone want to travel to the future? The only benefit I can think of would be watching the ruck suffer pangs of envy at seeing you so much younger than them. That said, if time travel were possible, most people would rush either to the future or the past just because they can.

Now, time-travel to the past looks even weirder.
Assuming it is possible to travel back in time, I highly doubt the traveller would be able to change anything (like going back and killing your grandparents or other crooks, as many would love, otherwise why would anyone take the trouble to overtake light?), since we wouldn’t occupy any physical space and be as a kind of invisible observers. It would be like looking into the past through a one way transparent mirror. Since every object in the universe has a unique world line, therefore going back in time would mean bending the same object’s world line back on itself (precisely as they do in 'Lost'), or two objects would occupy the same point in the world line, which physicists say hardly makes any sense. In science fiction that sometimes leads to mutual annihilation.

On the other hand, there could emerge "an ensemble of parallel universes" every time the traveller kills the grandfather — the act took place in (or triggered the creation of) a parallel universe in which the traveller's counterpart will never be born as a result. However, his existence in the original universe remains unaltered. This makes much more sense to me, since it wouldn’t violate the causality — a fundamental principle of theoretical particle physics and.... sanity.

But what would be the way of winding up in the past? Those in the know say there are basically three methods of pulling it off successfully:

1. Travelling faster than the speed of light
2. The use of cosmic strings and black holes
3. Wormholes and Alcubierre drive.

Well, faster-than-light travel is a good choice since it will be seen as travelling backwards in time in some other frames of reference. Picture an object nearly on a collision course with you, travelling faster than light — you won’t see it coming, but you’ll see it leaving. Actually you’ll see two different images (tachyons) after it’s passed you by: one will be an approaching blue Doppler-shifted image, the other will be red-shifted light from the departing object — at least that’s how hypothetical tachyons would behave. Then again, whose past would you see? That of the object, of course, not yours —watching the past doesn't mean "being" in it. Anyway, it’s highly improbable a human eye can perceive any close to light speed motion.

Now imagine your space ship setting off with a superluminal speed and arriving at the point B. The light (or information) from your departure will arrive later, but what will you see when it does? An image of your departure — just like you would watch any past event’s video recording — which doesn’t actually mean getting into your past. You won’t manage to do anything other than passively observe your recent past. It’s like supersonic movement — if your plane travels faster than sound and then slows down to a subsonic speed, you’ll hear the sound of your departure without being able to muffle it.

Although special and general relativity rule out FTL speeds locally, non-local means might be available, which means moving with space instead of moving through space. This is what was called warp drive in Star Trek and Alcubierre drive in science. It would be quite a comfortable way to get around: all you’d have to do is to create a wave in the space that would travel faster than light and thus take you to your destination before a light beam travelling outside the warp bubble would. Therefore no superluminal speed would be actually involved — the ship would be but simply carried along with the region without suffering conventional relativistic effects such as time dilation.

But would you end up in the past? I wouldn't put my money on it.

Anyway, there’s no method to create such a warp bubble at the moment, unless we find an existing one, let alone how the ship would manage to get out of there.

According to general theory of relativity gravity can be illustrated in terms of curvature in space-time caused by mass-energy and the flow of momentum. There exist solutions to a system of field equations that permit what are called "closed time-like curves", and hence time travel into the past. It seems, though, it doesn’t fit in our universe. I wonder if mathematical solutions to equations always make sense in physics.

Another option is to create a sort of a spinning cylinder using cosmic strings, but none have been discovered as yet or seem possible to create. As for me, it sounds quite nonsensical.

Geometry of  wormholes
Geometry of wormholes
Wormholes, whose existence is allowed by general relativity, are my favourite and, to my eye, the most plausible (in theory) means of taking the shortcuts around the universe. A ship could travel through Lorentzian traversable wormholes to any part of that same universe very quickly, or even jump from one universe to another. Does this mean it would travel to the past?

Anyway, much as you try, Novikov self-consistency principle or the notion of branching parallel universes would prevent you from changing anything in the past, or at least that’s what most scientists believe.

Another argument against such possibility is that we haven’t been overwhelmed by tourists from the future so far (Stephen Hawking), unless they are disguising themselves (Carl Sagan).

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