In particle physics, a massless particle is a particle whose invariant mass is zero. Currently, the only known massless particles are 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. Because neutrinos change flavour as they travel, at least two of them must have mass.
Read more about Massless Particle: Massless Particle Description, Special Relativity, Dynamics, Gravitons
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“The way to learn German, is, to read the same dozen pages over and over a hundred times, till you know every word and particle in them, and can pronounce and repeat them by heart.”
—Ralph Waldo Emerson (1803–1882)