Masslös

Masslös, meaning massless in English, describes particles or fields that have zero rest mass. In special relativity, rest mass is an invariant property of a particle and determines how it responds to forces. A zero rest mass implies the particle must travel at the speed of light in vacuum and cannot be at rest relative to any observer.

The energy–momentum relation is E^2 = (pc)^2 + (m0 c^2)^2. When m0 = 0, this reduces to E = pc and the particle moves at speed c for all observers. Massless entities thus carry energy and momentum without a rest frame. In the Standard Model, photons (electromagnetic quanta) and gluons (carriers of the strong force) are treated as massless. Neutrinos were long thought massless, but experiments on neutrino oscillations show they possess tiny but nonzero rest masses. The W and Z bosons are not massless; their masses arise through the Higgs mechanism, which breaks electroweak symmetry.

Massless particles have important roles in physics. They mediate fundamental forces, determine the structure of spacetime dynamics in general relativity (for example, light is deflected by gravity because energy and momentum gravitate), and influence the behavior of gauge theories where unbroken symmetries favor massless gauge bosons. In media, photons can exhibit an effective mass or reduced group velocity due to refractive effects, a separate phenomenon from intrinsic rest mass.

The term masslös is commonly used in physics discussions to denote zero rest mass and its implications for propagation, interactions, and symmetry in nature.