Time dilation
What happens to time when travelling at extreme speeds? All About Space has asked a scientist to break it down in terms of twins
Martin and Carlos are identical twins. Well, almost. While Martin is a medical doctor, Carlos is an astronaut about to travel to Alpha Centuari, the nearest star system to our own, at speeds nearing that of light. Just before takeoff Martin and Carlos synchronise their watches, and Carlos reminds Martin that Einstein’s theory of special relativity predicts ‘time dilation’, that is, that watches moving at a constant high speed appear to tick at a slower pace.
This gets Martin thinking… With Carlos moving rapidly his watch will run slower, which means that he will be younger upon his return! But from Carlos’ perspective Martin will be the one moving at highspeed, so Martin’s watch will appear to run slower, and so it is Martin who should stay younger.
This is the famous ‘twin paradox’, which is not really a paradox! How is this possible? First, even in relativity the time and space intervals assigned by observers to pairs of events – like two successive ticks of a clock – do not necessarily match. There is an unambiguous map between the coordinates of events seen by different observers in relative motion. The key point in our example is that the events defining two successive ticks in Martin’s clock – which happen at the same point in Martin’s reference frame – do not happen at the same point in space from Carlos’ perspective, because he sees Martin’s clock in motion. So Carlos cannot compare directly the ticks of Martin’s watch to the ticks of his own watch, because they take place at different points in space.
But surely they could compare the ticks when they happen at the same point, for example just before and just after the trip?
For Carlos’ ship to return it has to change course in the middle, which implies the action of a force, and then special relativity does not apply. In this case it is Einstein’s general theory of relativity that tells us that Carlos will stay younger. Sorry, Martin!