Energytomass

Energy-to-mass, commonly discussed as mass-energy equivalence, is the principle that energy and mass are two forms of the same physical quantity. In special relativity, the energy–momentum relation E^2 = (pc)^2 + (mc^2)^2 ties total energy E, momentum p, and invariant (rest) mass m. For a particle at rest (p = 0), its total energy equals its rest energy E = mc^2. This means that energy can be stored as mass and, conversely, mass can be converted back into energy. In composite systems, changes in internal energy, such as binding energy or thermal energy, can alter the system’s effective mass.

In modern physics, mass typically refers to invariant rest mass, while kinetic energy contributes to total energy without changing the rest mass; the older notion of relativistic mass is not commonly used. Mass can be created from energy when sufficient energy is available, as in pair production where high-energy photons or particle collisions generate particle–antiparticle pairs. Conversely, mass can be converted into energy, as in particle–antiparticle annihilation, yielding photons with energy equal to the rest mass energy of the annihilating particles.

Applications of mass–energy equivalence include nuclear power, where small amounts of mass are transformed into large amounts of energy, and particle physics, where high-energy collisions produce massive particles. Medical imaging techniques like PET scans rely on annihilation photons produced by positron–electron annihilation. In cosmology and astrophysics, mass–energy dynamics influence the behavior of stars, supernovae, and the expansion of the universe, reflecting the fundamental link between energy and mass in nature.