Six Impossible Things: The Mystery of the Quantum World

by John Gribbin

For example, probability. It was the German physicist Max Born who put the concept of probability, in the context of quantum mechanics, on a secure mathematical footing. But without going in to all the mathematics, we can get a feel for its importance using the example of electron spin (or tove gyre, as Eddington might have preferred). It is possible to describe, using the equations of quantum mechanics, an experiment in which an atom emits an electron that travels off through space (this is a real process called beta decay). In an idealized version of the experiment, the electron has a definite spin. It is either up or down. But there is no way to say in advance what it will be. There is a 50:50 chance of either possibility. If you do the experiment a thousand times, or simultaneously with a thousand atoms, you will find 500 electrons (plus or minus a few, maybe) with spin up, and 500 electrons with spin down. But if you catch a single electron and measure its spin you cannot tell which it will have until you look. Nothing surprising yet. But Einstein realized that something very surprising is predicted by the equations of the quantum theory for two electrons flying off in opposite directions.1 In certain circumstances, a conservation law applies, which says that the electrons must have opposite spin, one up and one down, so that, in effect, they cancel out. But the equations say that at the time the electrons are emitted from their source, they do not have a definite spi...