What it's all about
Eminent physicists have stated that time travel is theoretically possible. The laws of physics as we know and describe them don't prevent time travel, but would it be possible to build an actual time machine, asks James Williams.
Stephen Hawking used to claim that time travel was impossible, but changed his mind, claiming it is theoretically possible. However, the energy required to create a time machine is so vast, and the machine would be so big, that nobody would have the technology to build one.
Physicists agree that time travellers live among us. Einstein showed, through his work on relativity, that the passing of time varies according to the speed at which an object or person is travelling. For an astronaut travelling to our nearest galaxy, at almost the speed of light, then returning to Earth, time would pass more slowly, relative to people left behind on Earth. The returning astronaut would age less than the rest of the population and have travelled into Earth's future, relative to the time spent travelling.
Light from distant stars travels at approximately 300,000kms and can take millions, even billions, of years to travel across the vast reaches of space.
In future, time travel may be possible and people may be able to travel back in time. But a limiting factor on how far back they can travel is the point at which the first time machine was switched on.
Introduce pupils to astronomy with Andrew Jackson's six lessons on space in one Prezi presentation. bit.lyMasterAstronomy
Professor Brian Cox explains how to measure light speed in this BBC Class Clips - Science video. bit.lytesBrianCox.