Not all scientists agree but according to Einstein and quantum theory, time travel could be possible.

Imagine sitting by the Statue of Liberty and watching dinosaurs play in Manhattan or walking on the headland round Botany Bay as Captain Cook arrives to claim the new land for King George and England. Would you keep quiet at the slave markets of Savannah or save Martin Luther King from the assassin's bullet? Time travel is everywhere in the media from Star Trek to Dr Who but what can science tell us about it? Is time travel possible?
Actually, time travel is happening all around us. Hold out your hand and every second a dozen or so tiny nuclear particles called cosmic ray muons will pass straight through you. These particles are too small to feel and sometimes do damage which your body repairs. Astronauts in space can die from too much cosmic ray damage which causes radiation sickness.
Cosmic ray muons are the debris from collisions high in the atmosphere. Stable nuclear particles from the sun and the stars collide with the atmosphere 20 km above the Earth. Traveling at the speed of light, about 300,000 kilometers per second, it should take these muons around seven millionths of a second to reach a person on Earth. The only problem is that muons only live for two millionths of a second and so should never reach someone standing on the ground. But Einstein's special theory of relativity claims that the muons travel through time to get to reach us.

According to Einstein's special theory of relativity, our lives pass more slowly if we travel close to the speed of light. He has also shown that we live longer if we go and live in an intense gravitational field. Einstein has thus opened up the future and shown that it is possible to slow down time for ourselves, leave the Earth and come back to meet our grandchildren or our great-grandchildren. But he has not shown that it is possible to come back! Physicist Steven Hawking has suggested that it is not possible to go back in time because you could kill your own grandmother before your mother or father were born and so make it impossible for you to be born.

But could it still be possible? It is impossible with our normal model of the world but some scientists have suggested that we inhabit a universe in which there are an infinite number of parallel worlds. So if you go back to the past and kill your grandmother, then you set off another parallel world in which you are not part of the future.

Others have suggested that if time travel into the past is possible then there will be time tourists around from the future visiting us in their past. No one has been known to have met such tourists. But that is not the whole story. There are lots of things that we do not understand that hint that time travel Star Trek fashion around the universe might be possible.

Entangled particles

If we plunge down into the tiny world of nuclear particles, we know that two or more nuclear particles can be entangled together, which causes all sorts of strange 'quantum' effects. Entanglement is a property of the world of the very small when particles emerge from the same event. Two particles of light - or photons - can become entangled and oscillate one way or another across their direction of travel. Electrons, the particles of electricity, can be entangled spinning one way or another. If they emerge from the same event at the same time they are probably entangled.