Magazine 雜志：Wormholes

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by Linda Baxter

Have you ever read Carl Sagan's 1985 science fiction novel 'Contact'? Or have you seen the film of the same name starring Jodie Foster? If you have, then you will remember the scene near the end of the film, when the heroine travels to the star Vega, twenty-six light years away from Earth, through a wormhole in space. Just a story, you think. But is it?

The truth is that the scene described above is a serious scientific picture of travel through space-time using a wormhole. When Sagan was writing the book, he asked an American scientist, Kip Thorne, to give him an idea for a way of travelling long distances through space that was scientifically possible. Thorne thought about it for a long time and then had the idea of using a wormhole. He then looked again at Einstein's theories and the accepted rules of physics and was surprised to see that wormholes really could be used to travel through space.

But what is a wormhole?

A wormhole is a passageway between points in a 'folded' universe. The easiest way is to imagine a big piece of paper, with A written at one end and B written at the other. It will take a small insect a long time to walk from A to B. But now fold the paper in half, so that A and B are near to each other. Now connect A and B with a small tube pushed through the paper - the wormhole. The distance between A and B is now only a few millimetres, and the insect can walk along the tube and make the journey in a fraction of the time. So distances of many light years in space could become just a few metres.

So, did Kip Thorne invent wormholes?

No he didn't. Kip Thorne proved that there is nothing in the laws of physics and the general relativity theory to say that they're not possible. Einstein talked about 'folds' in the universe. He also thought that if these folds existed, then there might be passages, or shortcuts connecting them. He thought that there might be passages like these connecting two black holes, so that something could enter one black hole, travel through the wormhole and come out again at the other end. He called them 'bridges'. But for some reason, nobody thought very much about them until the 1980s.

But isn't it impossible for anything to escape from a black hole?

True. A black hole has such strong gravity that nothing, not even light, can escape from it. Everything is crushed at the centre point of the black hole. But mathematicians have proved that things are different if the hole is rotating. Then, it may be possible for a doorway to open at the centre point, where the wormhole begins. And some people believe that white holes exist too. These are the opposite of black holes. Instead of holding everything inside, a white hole pushes everything out. So you could enter a rotating black hole, travel through a wormhole, and then come flying out of a white hole at the other end.

But is it really possible?

Theoretically, yes. But there are a few problems. Wormholes are not very stable. They only exist for a few seconds before they collapse. And something travelling through one would probably make it break up anyway. And a wormhole would be full of x-rays and gamma rays, which would burn you. And of course, you would never be sure where you were going and almost certainly, you wouldn't be able to come back again the same way. So if we can find a natural wormhole, we then need the technology to control and stabilise it. Which is exactly what the advanced civilisation in 'Contact' were able to do.

Could we use wormholes to travel through time?

There are people who think exactly that. Einstein's equations treat space and time in exactly the same way. So a wormhole can link two different times as well as two different places. According to Einstein's theories, time travel would be difficult but not impossible. Scientists now take the idea very seriously. But there are some big problems of logic. For example, if you went back in time and killed your grandmother, you would then never be born. So you never existed, so you couldn't travel back in time because you didn't exist! To explain this problem (known as 'the granny paradox') people have invented theories to say that you can only travel back in time to the moment when the time machine was invented. This theory also explains why we don't have lots of 'time travel tourists' visiting us now.

So, in conclusion, wormholes have never been found in nature. No astronomer has ever seen things come flying out of one in the middle of space. But we do know that, according to the rules of physics that we use, they are possible. And if we could find one and control it, travel through space and maybe even time would be a real possibility.