Wednesday, 5 April 2006

Want to travel through time, or to the edge of the universe?

Imagine seeing the great wall being built, visiting your great, great, great grandchildren in the 23rd century, or instantly travelling to the other side of the universe.

Time travel and wormholes used to belong to the minds of science fiction writers. But, with a little inspiration, scientists have begun to show us that science fact can be just as strange as science fiction.

Einstein's special theory of relativity shows that time travel into the future is actually possible – there is no reason why we can’t do it. We just don’t know how to yet. Einstein’s theory suggests that the way we feel time going by is related to how fast we are moving. Objects travelling at speeds close to the speed of light (about 300 000 kilometres per second) age slower than objects which are not moving.

In 1975, Professor Carrol Alley tested Einstein's theory using two synchronised atomic clocks. Carrol loaded one clock onto a plane, which was flown for several hours, while the other clock remained on the ground. At the end of its flight, the clock on the plane was slightly behind the one that was left on the ground – that is, time had actually passed more slowly for the clock on the plane than the one on the ground. It had travelled every so slightly forward in time. So to travel into the distant future, somehow scientists need to come up with technology than can allow us to travel somewhere near the speed of light.

But what about time travel into the past? In theory, nothing in the laws of physics is stopping us from doing it, however, no one quite knows how to do it.

According to Einstein's theories, any object with mass will cause a warp in space-time, similar to a bowling ball sitting on a mattress. Because space and time has been stretched, clocks operate slower close to Earth than in the vast areas of space. Previous theoretical designs of time machines have used this concept of mass distorting space-time, however these theoretical machines require a tremendous amount of energy to work.

A professor of theoretical physics named Ronald Mallett came up with another idea. He believes that anything containing energy could warp space-time, and as a result, he has designed a time machine that uses light, rather than mass. His theoretical time machine consists of a ring of two intense beams of light, circling in opposite directions. By slowing the light down in an ultra-cold bath of atoms and increasing the intensity of the beams, he thinks space-time inside the ring would become warped. Eventually, space and time would become so distorted by the circling light that time would become a dimension similar to space - a dimension that you could move along! If you entered the ring and walked in the correct direction, you could walk backwards through time - maybe even passing yourself as you entered the ring!

However, there are lots of problems putting Ronald's theory into practice. The temperature of the ring would have to be close to absolute zero (-273°C), so humans would find it difficult to use. And it would also be impossible to travel back to a time before the machine was switched on. Perhaps this is why we’ve never met anyone from the future!

So, what about these wormholes? Can we travel across the universe instantly using one?

The universe appears as three dimensions in space (up-down, left-right, and forward-backward) and a fourth dimension known as time. Wormholes are connections between two different places in space and time. This is difficult to visualise in four dimensions, but it is easy to see in two. Imagine two points on an sheet of paper. You could travel between the points by following a line on the piece of paper, or you could fold the paper over so that the two points touch. By folding the paper, you are making a 'wormhole' in the two-dimensional paper world.

Although there is no experimental evidence for the existence of wormholes, theorists believe that they may exist. Wormholes first appeared possible in Einstein's theory of gravity, in 1913. However, physicists had almost forgotten about them until the eighties, when Carl Sagan included them in his novel Contact, in which the main character travels to another part of the universe to visit another civilisation.

To be stable, wormholes need lots of what’s called negative energy. Quantum mechanics suggests that it exists, but we haven’t found it yet, and we don't know whether the laws of quantum mechanics allow enough negative energy to be concentrated in such a way as to allow wormholes to exist.

One possible location for wormholes is at the centre of black holes. Travelling through one of these might prove extremely difficult however, since the wormhole would be so unstable that it would collapse as soon as a spaceship (or even a ray of light) entered it. This is because there would not be enough negative energy to hold it open.

So don't pack your bags for a trip to the other side of the galaxy just yet, or for a trip back in time. Scientists haven’t found any wormholes yet, or made a time machine. So we do know that a wormhole is not going to get you across Beijing in time for work after you’ve slept in.

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