If we find the answer to that [why it is that we and the universe exist], it would be the ultimate triumph of human reason—for then we would know the mind of God.

What is it that breathes fire into the equations and makes a universe for them to describe … Why does the universe go to all the bother of existing?

If we do discover a complete theory [of the universe], it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God.

The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.

Someone told me that each equation I included in the book would halve the sales.

A well-known scientist (some say it was Bertrand Russell) once gave a public lecture on astronomy. He described how the earth orbits around the sun and how the sun, in turn, orbits around the center of a vast collection of stars called our galaxy. At the end of the lecture, a little old lady at the back of the room got up and said: “What you have told us is rubbish. The world is really a flat plate supported on the back of a giant tortoise.” The scientist gave a superior smile before replying, “What is the tortoise standing on?” “You’re very clever, young man, very clever,” said the old lady. “But it’s turtles all the way down!”