The past is the final frontier.

Traveling back in time isn’t necessarily science fiction, according to a new paper published in Classical and Quantum Gravity. The paper’s title, “Traversable acausal retrograde domains in spacetime,” creates the acronym TARDIS – the name of the fictional time machine in “Doctor Who.”

“People think of time travel as something as fiction,” Ben Tippett, the lead author of the study, told The University of British Columbia. “But, mathematically, it is possible.”

Specifically, the paper describes a spacetime “bubble” that would travel faster than the speed of light – thereby allowing it to move backwards. The idea that an object can travel through time if it reaches the speed of light is based on Albert Einstein’s theory of special relativity.

But in order to traverse the past, we need something that doesn’t exist yet. And Tippet isn’t sure if it’s something that ever will.

Described in the study as “exotic matter” this enigmatic material would be capable of slowing down time by bending the spacetime continuum.

“My model of a time machine uses the curved space-time — to bend time into a circle for the passengers, not in a straight line,” said Tippet. “That circle takes us back in time.”

Thanks to Einstein’s theory of general relativity, we already know that spacetime is curved. (If it wasn’t, all the planets and stars would exist in a straight line.) The 102-year-old theory states that the massive gravity of large objects forces spacetime to bend around them. It’s the same theory that predicted gravitational waves, which were detected in February 2016.

“Experts in my field have been exploring the possibility of mathematical time machines since 1949,” said Tippet. “And my research presents a new method for doing it.”