The ability to jump forward and backwards in time has long fascinated science fiction writers and physicists alike. So is it really possible to travel into the past and the future?
Doctor Who is arguably one of the most famous stories about time travel. Alongside The Time Machine and Back to the Future, it has explored the temptations and paradoxes of visiting the past and voyaging into the future.
In the TV show, the Doctor travels through time in the Tardis: an advanced craft that can go anywhere in time and space. Famously, the Tardis defies our understanding of physical space: it’s bigger on the inside than it appears on the outside.
While time travel is fundamental to Doctor Who, the show never tries to ground the Tardis’ abilities in anything resembling real-world physics. It would be odd to complain about this: Doctor Who has a fairy-tale quality and doesn’t aspire to be realistic science fiction.
But what about in the real world? Could we ever build a time machine and travel into the distant past, or forward to see our great-great-great-grandchildren? Answering this question requires understanding how time actually works – something physicists are far from certain about. So far, what we can say with confidence is that travelling into the future is achievable, but travelling into the past is either wildly difficult or absolutely impossible.
Let’s start with Albert Einstein’s theories of relativity, which set out a description of space, time, mass and gravity. A key outcome of relativity is that the flow of time isn’t constant. Time can speed up or slow down, depending on the circumstances.