Myron Mathisson, Jan Weyssenhoff, and others

Theoretical concept of classical spinning particle has at least threefold origin.
1. It arises in classical limit of quantum mechanics of elementary particles with spin. The spin of the quantum particle is an observable related to unitary irreducible representation of the rotation group. In the classical limit this observable gives rise to classical degrees of freedom, called the classical spin.
2. Thrown stick or ring usually moves forward and quite often it also rotates. We easily notice the presence of rotation visually, but it is a nontrivial task when we want to quantify it. In the framework of nonrelativistic classical mechanics we know how to do this. On the other hand, it is formidably difficult task in the relativistic case. Here comes Myron Mathisson. His seminal paper from 1937 is appreciated until now. Anyway, often it is convenient and sufficiently accurate to model such a macroscopic body as a point particle with extra degrees of freedom related to the rotations, called the classical spinning particle as in the previous case. Notice that it has nothing to do with quantum mechanics. For example, it can be used to model a star orbiting around a black hole.
3. One can ad hoc generalize the theoretical concept of a classical point particle by adding degrees of freedom that have appropriate behavior under rotations of reference frame in the space. Such models usually have a purely theoretical motivation.
Large set of classical spinning particles can form a spin-fluid, first considered by Jan Weyssenhoff. Nowadays there are intense theoretical investigations of relativistic hydrodynamics of spin-fluids with its applications to quark-gluon plasma.
One of mysteries of the world of known fundamental quantum particles (quarks, leptons, gauge bosons, the Higgs scalar) is that their spins are equal only to 0, 1/2 or 1 - higher values of spin are absent.

Below there is the link to my talk at The 4th Conference of the Polish Society on Relativity held at Kazimierz Dolny in 2017. I was asked to recall the pioneering works of Myron Mathisson and Jan Weyssenhoff on the occasion of eightiest anniversary of publication by Mathisson of his seminal paper. I decided to briefly mention also other Cracow theoretical physicists who contributed to the subject of classical spin-particles and spin-fluids (the terms used by Weyssenhoff), including Antoni Raabe, Bronisław Średniawa, and others. The content of the talk goes well beyond the physics, and there are several interesting old photographs of the main figures. It is clear that the title of the talk - "Remarks (mainly historical) on the theory of classical relativistic spinning particles" - should be more specific.