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.

Every man should marry. After all, happiness is not the only thing in life.

It would be very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended to create beings like us.

I don't think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I'm an optimist. We will reach out to the stars.

We are entering an increasingly dangerous period of our history. But I'm an optimist.

It is impossible to imagine a four-dimensional space. I personally find it hard enough to visualize a three-dimensional space!

We live in the most probable of all possible worlds.