““Protons and neutrons interact strongly with each other because their building blocks do so. The exchange particles of the strong color interaction are called gluons. Instead of the one photon that is the carrier of the electromagnetic interaction, there are eight gluons for the strong interaction-for reasons beyond our present discussion. But let's remember that there is more than one carrier particle for this interaction. We already know that the photon is electrically neutral-it does not carry the charge to which it is coupled. It has to be neutral, since the electromagnetic interaction has only this one carrier particle. If it were not neutral, there would have to be another carrier particle with opposite charge, the antiphoton. Every elementary particle, after all, has its antiparticle. As noted, however, the antiparticle of the photon is identical with the photon itself; the photon is its own antiparticle. For the gluons, this is not the case. The gluons themselves carry the color charges to which they couple. Thus gluons, in contrast to photons, can interact directly with one another.””
