massattomat

Massattomat is a Finnish term used to describe objects or excitations that have zero rest mass. In physics, a rest mass of zero implies that all energy is kinetic or field energy, and the particle propagates at the speed of light in vacuum. The concept is central to special relativity and quantum field theory. In Finnish-language sources the term is used primarily in educational or popular-science writing; formal treatments typically discuss massless particles such as photons and, in the context of gauge theories, gluons.

In relativity, the energy-momentum relation E^2 = (pc)^2 + (m c^2)^2 reduces to E = pc when m=0, so massattomat particles travel at light speed and carry momentum proportional to their energy. They still interact with gravity through their energy and momentum and can experience gravitational redshift and lensing.

In condensed matter physics, similar behavior arises for massless quasiparticles, such as Dirac or Weyl fermions found in graphene and certain semimetals; they are described by linear dispersion relations near Dirac points.

Limitations: true masslessness is explained by gauge symmetries; some proposed beyond-Standard-Model scenarios include small but nonzero masses. The term massattomat is thus often used informally; in rigorous physics texts, the term "massless" or "zero rest mass" is preferred.

See also: mass, photon, gluon, Dirac and Weyl fermions, special relativity, general relativity.