Time Travel in Physics

Einstein's general relativity technically permits certain kinds of time travel. Physicist Kip Thorne showed that a sufficiently advanced civilization could, in theory, build a traversable wormhole connecting two distant points in spacetime. On its own such a wormhole is only a shortcut through space. It becomes a time machine only if the two mouths are pushed out of temporal sync — for instance by accelerating one mouth close to light-speed and bringing it back, or parking it in a strong gravitational field, so that it ages less than the other. Once the mouths differ in time, stepping through connects two different moments.

The engineering challenges are impossibly vast, but the physics does not forbid it. Stephen Hawking went further with his chronology protection conjecture — the idea that the laws of physics conspire to block time travel, even when individual equations seem to allow it.