What is Spin?

Uhlenbeck and Goudsmit proposed many years ago a new property of elementary particles. Maybe it is time now to revisit the proposal.

Initially they were thinking literally. They thought that if particles were moving around their own axes, they were extended objects. This was then similar to any extended object which parts can rotate with respect to the axis. The problem with this metaphor was that an estimate of the radius produced superluminal velocities, i.e. some parts would move faster than light. This difficulty prompted these two, then young men, to keep an abstract proposal without a mechanical consistent analogy.

If they knew what was to come, maybe they would have worked harder in a successful mechanical model. As it happened, after this so-called Spin, an Iso-Spin was invented to fit the data, by leaving us, so to say, in the dark, about what degrees of freedom were we really talking about.

To save the idea of these two young men of yore, we have another young man to the rescue. Recently Garrett Lisi put together Geometry and all internal charges, or Iso-Spins, in one unified geometrical object. According to Lisi, all particles and forces we know, can be represented with octonions.

If we just understand why is it that octonions provide such economical description of reality, maybe we will finally know, What is Spin?