Oh well, one tries to see what qualitative features there are. And nobody who deals with radiation or molecules can be unaware that in a molecule, say a hydrogen molecule, with two hydrogen atoms, even at the absolute zero temperature, those two hydrogen atoms are not sitting quietly at a fixed separation from each other, they're 'wiggle wagging'. The so-called zero point energy, a minimum, irreducible energy that can't be got rid of. And that same feature of minimum, irreducible activity, obtains for the electromagnetic field through space, and the gravitational field, this 'wiggle waggling' all the time. What we think of as smooth, simple space, is really a 'wiggly' business. I don't know any better image for it than the look of the ocean as one comes down from a plane high above the ocean, that seems to be a perfectly smooth surface. You come down closer, you see the waves, and as you get still closer you see the waves breaking and you see foam. I think it must be the same in the geometry of space, for all our everyday experience, the geometry of space is smooth and flat. But as we examine it more closely, it must show oscillations. And still more closely, it must show foam, a foam-like structure. And that means that down at the very smallest distances, this idea of before and after really lose their meaning. Very small distances means what? It's so interesting that Max Planck, the great German physicist who had done so much to understand radiation and set us on the track to the quantum, had, in the study of radiation, recognized a new constant of nature. And that constant, combined with the known constants of nature, the speed of light and the constant of gravitation, define a certain standard of length, certain standard of time and certain standard of mass. Planck's notation is not quite in tune with today's, but you tune it into accord with today's notation you find yourself led to a length and a mass and a time which I called the Planck length, the Planck Mass, the Planck Time. And it's at that enormously small scale, fantastically small scale, that space must have this foam-like character. Will we see any evidence of that in time to come? We surely will, but I'm not bright enough to see where the first key test will show up. I like the idea of Planck and of Karl Popper, the philosopher of science, Popper saying that the test of a scientific idea is: a) its surprise, and b) its success in surviving tests.