Cont:

We know that at rest, when the pendulum is changing direction, its velocity is zero. But the momentum, at low speeds at least, is equal to velocity multiplied by mass. However, if we multiply any quantity by a zero, we get zero. Thus, we have now established that the momentum of the pendulum at that point is zero, that is, we know its value very precisely: It’s zero. But we have said before that if we know precisely the momentum of a particle, then its position becomes diffuse and completely indefinite. That is, the pendulum can be just about any place, even at the end of the universe. Yes, but it has very little time to get there because this whole event occurs in zero time. So there we go again. The pendulum has to disappear in all directions at infinite velocity. It will have to expand very rapidly into space, like a balloon, and then collapse just as rapidly.

- Having done this, it comes back, picks up speed, and goes about its usual good-natured business as if nothing had happened. None of us would suspect the leisurely pendulum of doing such a wild thing when no one is looking! But then again, one cannot rely on appearances.