Masselose

Masselose is a term used in physics to describe particles or fields with zero rest mass. The invariant mass m0 is zero, and in special relativity their four-momentum satisfies pμpμ = 0. They cannot have a rest frame and always propagate at the speed of light in vacuum.

The most prominent examples are massless gauge bosons, such as photons, which mediate electromagnetism, and the theoretical graviton for gravity. Gluons are massless in the underlying gauge theory of QCD, but because of confinement they are not observed as free particles. Neutrinos are not massless—their nonzero masses have been established experimentally. If detected, gravitons (hypothetical) would be massless spin-2 particles.

Kinematically, a massless particle has energy E and momentum p related by E = pc and E^2 = (pc)^2. For a photon, E = hν and p = hν/c, linking energy to frequency or wavelength. In media, photons behave as if massive due to interactions with the medium, characterized by a group velocity less than c.

In cosmology and astrophysics, massless particles such as photons carry information across large distances; their spectra are altered by redshift and scattering. Observations place stringent upper bounds on any nonzero rest mass for photons; current limits are exceedingly small.

Etymology: masselose is the German adjective for massless, used in physics literature and occasionally in translations. In English-language texts, massless is the standard term, but masselose may appear in multilingual discussions.