### Re: Why a photon (or gluon) does 'NOT' have spin-3, 5, 7, 9 etc.?

Date: Thu Oct 9 00:17:23 2014
Posted By: Michael Wohlgenannt, PostDoc
Area of science: Physics
ID: 1410278187.Ph
Message:

Dear Questioner,

that's a very good question. The spin of a particle is related to the equation that "defines" the particle. In case of the photon, it is described by the Maxwell equations. These equations are for a spin 1 particle, i.e. a vector particle.
If the spin of the photon was different, physics and the behaviour of the particle would really change a lot, since the equations would change. The spin is an essential property of a particle. Let me try to explain in a few words why this is so.

In more mathematical terms (see e.g. the following wikipedia article (and try to find some other useful links starting from there): representation theory of the lorentz group,relativistic wave equations, particle physics and representation theory):
Symmetries are basic ingredients in physics. Symmetry means that an object looks the same when you act on it with the corresponding symmetry transformation, e.g. if you rotate the object by a certain angle. Particles obey some symmetries. The probably most fundamental is the so-called Lorentz symmetry. It's the symmetry of space-time (vacuum). For the particle to satisfy or obey this symmetry, means in mathematical terms that the particle needs to be in a certain representation of the symmetry. The symmetry imposes mathematical restrictions for the particles. The number of representations is also restricted. Each representation corresponds to certain kind of particles and to certain equations which the corresponding particles has to satisfy. The parameter, which characterizes the different representations, is the spin. Now, which particle observed in nature belongs to which representation has to be determined by experiments.

I hope this helps.
Best regards, Michael

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