I became interested in this question of whether you can build wormholes for interstellar travel. I realized that if you had a wormhole, the theory of general relativity by itself would permit you to go backward in time.

We have to have a combination of general relativity that describes the warping of space and time, and quantum physics, which describes the uncertainties in that warping and how they change.

'Closed timelike curve' is the jargon for time travel. It means you go out, come back and meet yourself in the past.

Gravitational waves will bring us exquisitely accurate maps of black holes - maps of their space-time. Those maps will make it crystal clear whether or not what we're dealing with are black holes as described by general relativity.

If Time travel were possible, the future would have already taught the present to teach the past how to do it.

Einstein, my upset stomach hates your theory [of General Relativity]—it almost hates you yourself! How am I to' provide for my students? What am I to answer to the philosophers?!!