In quantum physics, the particle with zero rest mass that travels at light speed in a vacuum is the photon.
A photon is the quantum of the electromagnetic field. It carries electromagnetic radiation, including radio waves, visible light, X-rays, and gamma rays. In a vacuum, every photon travels at the speed of light, commonly denoted c, although photons can move more slowly through materials because of interactions with matter.
Photons have zero rest mass and no electric charge. Each photon has energy E = hf and momentum p = h/λ, linking its frequency and wavelength to its quantum properties. The photon’s spin is 1, but because it is massless, only two physical polarization states occur for freely propagating photons.
Photons are not the same as all force-carrying particles. Gluons carry the strong interaction, while W and Z bosons mediate the weak interaction. Also, the speed of light in a material is not the same as the invariant speed reached by a photon in empty space.