energimassa

Energimassa is a term found in Swedish and Norwegian physics literature that denotes the energy equivalent of mass as described by mass–energy equivalence, E = mc^2. The phrase can refer to two related ideas: the energy associated with a mass and, in older or pedagogical usage, the concept of relativistic mass, which increases with speed according to E = γ m0 c^2 and m rel = γ m0.

In modern relativistic physics, the standard distinction is between rest mass (invariant mass) m0 and total energy E. For a particle with momentum p, E^2 = (pc)^2 + (m0 c^2)^2, and the total energy equals the relativistic mass times c^2, E = m rel c^2, where m rel = γ m0. While the idea of relativistic mass can be useful for intuition about how energy and inertia change with velocity, many contemporary texts prefer to keep rest mass as the invariant quantity and describe energy, momentum, and stress separately.

Applications of the energimassa concept appear in contexts such as nuclear reactions, where changes in mass (mass defect) convert into released energy via E = Δm c^2, and in particle physics, where particles’ energy content depends on their motion. In cosmology and general relativity, the energy–momentum content of matter and fields governs gravitational dynamics through the stress-energy tensor, reinforcing the link between energy and mass in a broader physical sense.

See also: mass–energy equivalence, invariant mass, relativistic mass, E = mc^2.