Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature

by Steven Weinberg

It is natural to describe an electron traveling through empty space as a wave packet, a little bundle of electron waves that travel along together, like the pulse of light waves produced by a searchlight that is turned on only for an instant. The Schrödinger equation shows that, when such a wave packet strikes an atom, it breaks up; wavelets go traveling off in all directions like sprays of water when the stream from a garden hose hits a rock. This was puzzling; electrons striking atoms fly off in one direction or another but they do not break up—they remain electrons. In 1926 Max Born in Göttingen proposed to interpret this peculiar behavior of the wave function in terms of probabilities. The electron does not break up, but it can be scattered in any direction, and the probability that an electron is scattered in some particular direction is greatest in those directions where the values of the wave function are largest. In other words, electron waves are not waves of anything; their significance is simply that the value of the wave function at any point tells us the probability that the electron is at or near that point.