In particle physics, a massless particle is a particle whose invariant mass is theoretically zero. As of 2015 the two known massless particles were gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons. Neutrinos were, until recently, thought to be either massless or have a small mass.