restmassene

Rest mass, also called invariant mass or proper mass, is a property of a particle or system that remains unchanged under Lorentz transformations. It is a measure of the intrinsic mass of the object, independent of its motion relative to an observer. In relativistic physics the energy and momentum of a particle relate to its rest mass through the energy–momentum relation: E^2 = (pc)^2 + (m0 c^2)^2, where E is the total energy, p the momentum, c the speed of light, and m0 the rest mass. When the particle is at rest (p = 0), its energy reduces to E0 = m0 c^2, the rest energy.

Rest mass is invariant: all observers agree on m0, even though they measure different energies and momenta. Historically some physicists used the concept of relativistic mass γ m0 to describe inertia at high speed, but the modern view emphasizes rest mass as the fundamental, frame-invariant quantity.

Photons have zero rest mass, yet travel at the speed of light and carry energy and momentum. Particles such as electrons, protons, and neutrinos have nonzero rest masses; current experiments place neutrino rest masses in the sub-electronvolt/c^2 range.

For composite systems, the rest mass includes the internal binding energy: a bound system may have a rest mass smaller than the sum of its parts by the binding energy divided by c^2.

Rest mass is measured indirectly through kinematics and decay processes; in high-energy physics it is one of the key properties used to classify particles.