Quantum physics predicts many astonishing physical effects that have been subsequently observed in the laboratory. Perhaps the most significant effect is the violation of Bell's inequality, which implies a failure of classical locality. But the most widely known bit of quantum magic is the experiment of Clinton Davisson and Lester Germer demonstrating interference effects for electrons. Richard Feynman said this is a phenomenon "which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery" (1963–1965, Vol. 3, p. 1-1). As we will see, Feynman somewhat overstates the case, but let us first try to get in his frame of mind.

The interference effect is illustrated by the two-slit experiment. If we send a plain water wave toward a barrier with two narrow slits in it, we find...