Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a 'crank.'

If time travel is possible, where are the tourists from the future?

Even if it turns out that time travel is impossible, it is important that we understand why it is impossible.

Yeah, but what if you went back and killed your own grandfather?"

He stared at me, baffled. "Why the fuck would you do that?

Time travel used to be thought of as just science fiction, but Einstein's general theory of relativity allows for the possibility that we could warp space-time so much that you could go off in a rocket and return before you set out.

A zero-gravity flight is a first step toward space travel.