The equations of physics suggest time should be able to go backwards as well as forwards. Experience suggests, though, that it cannot. Why? And is time travel really possible?

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Why does time pass? It is a question so profound that few people would even think to ask it. Yet its effects are all around. Human beings live in a perpetual present, inexorably sealed off from the past, but moving relentlessly into the future. For most people, time seems to be something that is just out there. A thing ticking away in the background - fixed, immutable. Time seems to go in one direction and in one direction only. But physicists see it much differently.

One of the great minds who changed the way science thinks about time was Albert Einstein. In 1905 he published his special theory of relativity. In it he demonstrated that time passes differently in different places depending on how those places are moving with respect to one another.

Einstein showed that the faster one travels the slower time goes for the traveler. At the speeds at which humans move this is imperceptible. But for someone traveling on a spaceship at speeds close to that of light, time would slow down compared with its passage for people on earth.

There was another important aspect of Einstein's theory which he didn't even realize when he published it. That time was woven into the very fabric of space itself. Einstein used this insight to help develop his general theory of relativity which incorporated gravity. He published it in 1915.

With the general theory of relativity he demonstrated that massive objects warped the fabric of space-time. It is this curvature that causes time to slow down near them. Time slows down in proportion to the gravitational pull of a nearby object so the effect would be strong near a black hole but milder near the earth. But even here it can be detected. Einstein's theories had to be taken into account when the GPS system was set up otherwise it would have been inaccurate.

One scientist who puzzled over the directionality of time was Arthur Eddington, a 20th century astronomer who defined the concept of the arrow of time, based on observations made by the 19th century physicist Ludwig Boltzmann. The arrow of time is based on the second law of thermodynamics which says the disorder known as entropy increases with time. For example, a building left untouched will slowly decay into its surroundings. It will disintegrate into a more chaotic state but it is highly unlikely that the building will become more orderly over time - this is because there are many more ways for a system to be disorderly than orderly. There can be many ways for something to break for instance but only one way for it to be put back together again. A system will be less disordered in the past and more disordered in the future. This is the arrow of time.

So how can the arrow of time be reconciled with Einstein's equations? If time can go forwards and backwards according to relativity does that mean it's possible to go backwards in time? The theory of relativity does allow time travel to the future.

Einstein's theories do allow for the formation of wormholes in space. These are shortcuts that link otherwise distant places in the space-time continuum.

Although wormholes are theoretically possible they're a highly implausible proposition. That's because the equations suggest enormous masses and energies would be required to create and manipulate one.

What remains then is a mystery. Theory fails to forbid traveling backwards in time but practice suggests it might just as well be forbidden. For now it would appear the arrow of time cannot be reversed. No one knows why time passes but it seems that no matter how people look at it, it goes in one direction in one direction only.

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