Time present and time past

This is where Amos Ori from Technion, the Israel Institute of Technology in Haifa, comes in. He says that according to Einstein's theories, space can be twisted enough to create a local gravity field that looks like a doughnut of some arbitrary size. The gravitational field lines circle around the outside of this doughnut, so that space and time are both tightly curved back on themselves. Crucially, this does away with the need for any hypothetical exotic matter.

Although it is difficult to describe what this would look or be like in real life, Ori says the mathematics reveal that every period of time between when the doughnut was created and the present moment would be somewhere in the vacuum inside the doughnut. All you need to do is work out how to get there.

In theory, it should be possible to travel back to any point in time after the time machine was built, reports Ori in Physical Review Letters1. One slight snag is that he has not worked out how to generate the gravitational doughnut, although he has some ideas. "It's wild speculation, but you may need to move large masses rapidly in a circular motion," Ori says.