Spin

In the 1920s very sensitive experiments showed that individual electrons act like little magnets. At the time, it was known that the effect of a magnet was created whenever electricity went in a circular path (through a coil of wire, for example), so physicists developed the idea that the electron created its magnetic effect by spinning like a top. That idea  was quickly found to have problems. For example, the strength of the magnetic effect seems too high to be accounted for by spinning unless the electron was spinning so rapidly that its surface (assuming it has one!) was moving faster than the speed of light (which would break another set of rules called relativity). In truth quantum theory doesn’t provide a physical picture of what is going on with an electron to account for the magnetic effect, but we still use the word ‘spin’ as a shorthand for whatever it is.

We’ve since discovered that nearly all types of fundamental particle have the ‘spin’ property. Each type of particle has a certain spin, which can never be increased or reduced.

Spin seems to come in multiples of a half. Electrons, for example  have half a unit of spin, while photons have one unit. The other non-zero values that have been observed are one and a half, two, and two and a half (other multiple of a half are possible, according to the theory, but we haven’t seen them yet).

The ‘Higgs Boson’ (of which more later) is the only fundamental particle that has been found experimentally with zero spin.

Spins don’t add-up straightforwardly. For example a helium atom has no spin, even though it is made of protons and electrons with spin.

The most thought-provoking aspect of spin (to me) is that particles with spins that are not whole numbers (which are called Fermions) behave in a very different way from particles whose spins are whole numbers (which are called Bosons), and the difference is very striking. In fact, the difference is fundamental to the behaviour of matter, as we will see in the next couple of posts.