Massless particle
A particle with zero invariant mass. In vacuum, a free massless particle travels at the speed of light; photons and gluons are examples.
Energy–momentum relation: The relativistic equation relating a particle’s energy, momentum, and invariant mass. Setting invariant mass to zero gives energy equal to momentum times the speed of light.
Invariant mass: The mass obtained from a system’s total energy and momentum, independent of reference frame. Zero invariant mass is the defining property of this particle class.
Photon: The quantum of the electromagnetic field, carrying electromagnetic radiation. Photons are the best-known massless particles and transmit electromagnetic interactions.
Massive particle: A particle with positive invariant mass that travels below light speed in vacuum. Unlike a massless particle, it has a rest frame and cannot reach light speed.
Speed of light: The speed at which light propagates in vacuum, denoted by c. Every free massless particle in vacuum moves at this limiting speed.
Special relativity: The theory of space, time, and motion when gravity can be neglected. It supplies the relation between invariant mass, energy, momentum, and light-speed motion.
Gluon: The gauge boson of the strong interaction, carrying color charge. Gluons are massless in the Standard Model, though confinement prevents their isolation.
Rest frame: A reference frame in which an object’s spatial momentum is zero. A massless particle has no rest frame, because it always travels at light speed.
Four-momentum: The spacetime vector whose components encode a system’s energy and momentum. A massless particle has a lightlike four-momentum with zero squared norm.
Elementary particle: A particle not known to be composed of smaller constituents. Photons and gluons are elementary particles with zero invariant mass.