Physicists have come up with what they claim is a mathematical model of a theoretical "time machine" - a box that can move backwards and forwards through time and space. The trick, they say, is to use the curvature of space-time in the Universe to bend time into a circle for hypothetical passengers sitting in the box, and that circle allows them to skip into the future and the past. "People think of time travel as something as fiction. And we tend to think it's not possible because we don't actually do it," said theoretical physicist and mathematician, Ben Tippett, from the University of British Columbia in Canada. "But, mathematically, it is possible." Together with David Tsang, an astrophysicist at the University of Maryland, Tippett used Einstein's theory of general relativity to come up with a mathematical model of what they're calling a Traversable Acausal Retrograde Domain in Space-time (yep, the acronym is literally TARDIS). The physicists propose creating a kind of 'bubble' of space-time geometry, which carries whatever's inside it through space and time along a large circular path. If this bubble can hit speeds greater than the speed of light - something the pair say is mathematically possible - this would allow it to move backwards in time. "It is a box which travels 'forwards' and then 'backwards' in time along a circular path through spacetime," the researchers explained in their 2017 paper. "Delighted external observers would be able to watch the time travellers within the box evolving backwards in time: un-breaking eggs and separating cream from their coffee."