Although I was first drawn to math and science by the certainty they promised, today I find the unanswered questions and the unexpected connections at least as attractive.

When people try to use religion to address the natural world, science pushes back on it, and religion has to accommodate the results. Beliefs can be permanent, but beliefs can also be flexible. Personally, if I find out my belief is wrong, I change my mind. I think that's a good way to live.

Physics has entered a remarkable era. Ideas that were once the realm of science fiction are now entering our theoretical — and maybe even experimental — grasp. Brand-new theoretical discoveries about extra dimensions have irreversibly changed how particle physicists, astrophysicists, and cosmologists now think about the world. The sheer number and pace of discoveries tells us that we've most likely only scratched the surface of the wondrous possibilities that lie in store. Ideas have taken on a life of their own.

You can be only a modest distance away from the gravity brane, and gravity will be incredibly weak.

There are many aspects of time we just do not understand. That’s the thing about writing a popular book: You realize the things you understand because for those you can give a really simple explanation. But some things about time I just don’t know how to give simple explanations for, even though I can tell you mathematically what’s going on.

You have to be careful when you use beauty as a guide. There are many theories people didn't think were beautiful at the time but did find beautiful later—and vice versa. I think simplicity is a good guide: The more economical a theory, the better.

All the normal matter in the Milky Way disc is denser than the dark matter that surrounds it.