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 everysecond a dozen or so tiny nuclear
particles called cosmic ray muonswill pass straight through you. These particles are too small to feeland sometimes do damage which your body repairs. Astronauts in spacecan 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.Time is relativeAccordingto Einstein's special theory of relativity, our lives pass more slowlyif we travel close to the speed of light. He has also shown that welive longer if we go and live in an intense gravitational field.Einstein has thus opened up the future and shown that it is possible toslow down time for ourselves, leave the Earth and come back to meet ourgrandchildren or our great-grandchildren. But he has not shown that itis possible to come back! Physicist Steven Hawking has suggested thatit is not possible to go back in time because you could kill your owngrandmother before your mother or father were born and so make itimpossible for you to be born. Butcould it still be possible? It is impossible with our normal model ofthe world but some scientists have suggested that we inhabit a universein which there are an infinite number of parallel worlds. So if you goback to the past and kill your grandmother, then you set off anotherparallel world in which you are not part of the future.Othershave suggested that if time travel into the past is possible then therewill 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 thewhole story. There are lots of things that we do not understand thathint that time travel Star Trek fashion around the universe might bepossible.
Entangled particlesIfwe plunge down into the tiny world of nuclear particles, we know thattwo or more nuclear particles can be entangled together, which causesall sorts of strange 'quantum' effects. Entanglement is a property ofthe world of the very small when particles emerge from the same event.Two particles of light - or photons - can become entangled andoscillate one way or another across their direction of travel.Electrons, the particles of electricity, can be entangled spinning oneway or another. If they emerge from the same event at the same timethey are probably entangled.Individually, these tiny particles merge many of their properties together and we can't be sure of a particular value unless we measure it. The weird thing about entanglement is that each particle knows what is happening to the other instantly and this can happen right across the universe. So if we have a pair of entangled electrons with different spins but with one diring into space, the spin of either is defined by a probability, but when we measure the local one, we instantly know the spin of the other! Somehow the information has traveled instantaneously to define the spin value of the disappearing electron. This is instant travel around the universe and we don't understand why. Perhaps there are more than three dimensions through which quantum effects communicate and these could be used for time travel.Ina similar way, just after the Big Bang there was a period of inflationwhen the universe suddenly grew far faster than the speed of light andspread far and wide. Perhaps this had something to do with interactionsthrough extra dimensions, but we don't really know.Whatwe do know is that there are lots of hints that point to time travel.What can we do? Wait around for a time traveler to arrive from thefuture and tell us how to do it? Search for more advanced civilizationsthat are already time traveling and chat to them across the universe?Since a conversation might take decades you would be much better takingup physics and starting there. Who knows you may discover how to jumpinto a fifth dimension and take pictures of the dinosaurs in Manhattan.