In 1733, Samuel Madden wrote Memoirs of the Twentieth Century. It is one of the first attempts at science fiction. Madden's future vision wasn't very radical and can't compete with modern science fiction, but the idea of time travel is not as absurd as you might think.
Many eminent physicists have stated that time travel is theoretically possible, including Albert Einstein and Stephen Hawking. The laws of physics as we know and describe them don't prevent time travel. Some physicists describe the possibility of time travel through "wormholes" in space, but would it be possible to build an actual time machine? In 1976, physicist Frank Tipler drew up blueprints for one, but this was an infinitely long cylinder rotating incredibly fast, warping space and time, to create a time loop, and impossible to build.
Physicist Professor Ronald L. Mallet believes that not only is time travel possible, it is also practical. He postulates that lasers that produce a single continuously circulating unidirectional beam of light could, in theory, create the foundation of a time machine.
Stephen Hawking used to claim that time travel was impossible. But Hawking changed his mind. Theoretically, he now claims, time travel is possible. The reason we haven't met a time traveller is simply that it's impractical. The amount of energy required to create a time machine is so vast, and the machine itself would be so big, that nobody would have the technology, now or in the future, to build one.
But physicists agree that time travellers live among us today, and of course anyone can see into the past. 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 closest neighbouring galaxy, at near 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 remaining population and have travelled into Earth's future, relative to the time spent travelling.
Look at the sky to observe celestial objects and you're looking into the past. 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. The farther into space we look, the further back in time we are looking.
Time travel evokes many "time paradoxes" - for example, could you accidentally kill your own grandfather? But one intriguing time-related paradox may help to answer the time-tourist dilemma. In the 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.
This could explain why Hawking has never met a true time traveller.
James Williams is a lecturer in science education at the University of Sussex's School of Education and Social Work
Introduce pupils to astronomy with Andrew Jackson's masterclass - 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.