Masslessness

Masslessness refers to the property of zero invariant mass, meaning a particle or field has m = 0. In special relativity, the energy–momentum relation is E^2 = (pc)^2 + (mc^2)^2. For a massless particle, this reduces to E = pc, so it always travels at the speed of light in vacuum and has no rest frame.

Most familiar massless quanta are gauge bosons. Photons are the massless mediators of electromagnetism, arising from U(1) gauge invariance. Gluons are massless in quantum chromodynamics, though they are not observed freely due to confinement. In the Standard Model, the W and Z bosons are initially gauge fields but acquire mass through the Higgs mechanism after electroweak symmetry breaking; the photon remains massless as a remnant of an unbroken gauge symmetry.

Massless particles have distinctive kinematics and polarization properties. Their energy is proportional to momentum, E = pc, and they carry no rest energy. For spin-1 massless particles like photons, only two physical polarization states exist (helicities ±1). Massless spin-2 particles, if they exist as gravitons, would also carry two helicities (±2). Gauge invariance removes unphysical degrees of freedom.

Experimental and observational implications include strict constraints on any photon mass (current limits are extremely small) and the prediction that light and other massless quanta propagate at c without dispersion in vacuum. Gravitational waves are consistent with massless gravitons, as predicted by general relativity.

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