>> For example, consider the case of the photon, the massless spin-1 particle of light and electromagnetism. For such a particle, the equation describing four-p

>> For example, consider the case of the photon, the massless spin-1 particle of light and electromagnetism. For such a particle, the equation describing four-particle interactions — where two particles go in and two come out, perhaps after colliding and scattering — has no viable solutions. >> Gluons are also massless spin-1 particles, but they represent the case where there are multiple types of the same massless spin-1 particle. Unlike the photon, gluons can satisfy the four-particle interaction equation, meaning that they self-interact. Newbie question: Shouldn't then theory need to explain why there are multiple typse of Gluons, but not of photons?