massless

Massless describes particles or fields that have zero invariant rest mass. In special relativity, a particle’s rest mass m0 is an intrinsic property; if m0 = 0, the energy-momentum relation becomes E^2 = (pc)^2, so E = pc for any momentum p. Consequently such particles travel at the speed of light in vacuum and cannot be brought to rest.

Many fundamental particles are massless in the standard model or at high energies. The photon, the quantum of the electromagnetic field, and the gluon, the carrier of the strong interaction, are described as massless in the absence of confinement or at sufficiently high energies. Other gauge bosons, such as the W and Z bosons, are not massless; they acquire mass through the Higgs mechanism, while photons remain massless. In practice, some particles have very small rest masses and may be treated as effectively massless when their kinetic energy far exceeds m0 c^2, as with high-energy electrons or neutrinos in certain regimes.

Massless particles carry energy and momentum and thus can interact gravitationally, despite having zero rest mass. Their behavior is governed by the principles of special relativity and quantum field theory, and their properties influence phenomena such as radiation, scattering, and cosmological backgrounds. The term “massless” can also describe idealized fields in theoretical models or regimes where the rest mass is neglected for practical purposes.