Number of different massless particle an odd number Why are there only three massless particles and not four? I thought for each kind of matter particle there is a corresponding particle. What makes this any different from everything else? 
 A: Electrons, protons, neutrons as well as their antiparticles are able to receive and to emit photons. The photon exchange is possible between each of this particles and antiparticles and this does not change the properties of photons.
Once emitted photons are the linear propagation of energy in the form of a oscillating electric and a oscillating magnetic field. Both fields are dipoles and in addition to the energy content there are only two possible states for this field called spin. In vacuum the two vectors of this fields are perpendicular to each over (and perpendicular to the direction of propagation).
Suppose one looks in the direction of propagation and see the electric vector at one moment vertical down. For half the photons the magnetic vector will be directed then to the right and for half the photons to the left. So the electric and magnetic vectors following each other clockwise ore anticlockwise. Energy content and spin - that's the full characteristic of photons. No net charge, no constant magnetic dipole direction. Photons are their own antiparticles.
A: Are you arguing that because particles come in pairs - particle and anti-particle - then there should be an even number of massless particles?
If so, the argument fails because the photon is its own antiparticle i.e. there is no antiphoton.
