Good news, Doctor Who fans! A pair of physicists have proposed a spacetime geometry where Doctor Who’s TARDIS would actually be capable of traveling both forward and backward through space and time. This kind of spacetime could actually exist in our own universe, meaning the TARDIS — if such a craft was ever created — might actually have a scientific leg to stand on. In fact, given the scale of the universe, some semblance of the Doctor and his TARDIS might already exist, somewhere out there in the great wide expanse.
A spacetime geometry is when spacetime — i.e. the fabric of the universe where everything has happened, happens, and will happen in the future — is arranged into a certain shape. This is a very complex topic, but put simply, space consists of three dimensions (X, Y, Z), and time adds a fourth dimension. Spacetime tries to find a mathematical model/setting that accounts for scientific theories, such as general and special relativity. As such, there are lots of different proposed spacetime geometries — the most famous of which are Euclidean and Minkowski space, which are the mathematical settings for general and special relativity, respectively.