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How is a photon its own antiparticle? As I understand it, the fundamental difference between matter and antimatter is the spin of the particle....

Fundamental difference between particle and antiparticle is NOT a difference in spin. An antiparticle can be thought of as a time reversed particle. If you start with description of a charged particle and time reverse, you get a particle with opposite charge and spin ( i mean that you get description of a particle with opposite charge and spin ). But if you start with a charge neutral particle like photon, you get the same particle with opposite spin. So we say that photon is its own antiparticle. This follows only because photon does not have any kind of charge.

jasjeet s bagla

might note that neutrinos also have no charge, and it is not at the moment certain whether an antineutrino is the same as a neutrino or not. If the neutrino is its own antiparticle, then that means the neutrino must have a mass (the argument is in fact based on spin, which might be where you heard that the difference between a particle and antiparticle is in the spin). Arthur Smith

Neutrinoes do not have electric charge but they have the Leptonic charge. I used the word charge in its generalised sense. I
Sorry for not making this explicit.

jasjeet s bagla

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