MC2

mc2 refers to mass-energy equivalence in physics, most commonly illustrated by E = m c^2. Here m is the rest mass, E is energy, and c is the speed of light in vacuum. The relation implies that mass and energy are interchangeable, with c^2 acting as a conversion factor that is extremely large in practical terms.

At rest, the intrinsic energy of a mass is its rest energy E0 = m c^2. When moving, a body has total energy E = γ m c^2, where γ is the Lorentz factor 1/√(1 − v^2/c^2). The energy–momentum relation E^2 = (pc)^2 + (m c^2)^2 connects energy, momentum p, and mass.

Origin and significance: Albert Einstein introduced mass–energy equivalence in 1905 as part of the theory of special relativity. The concept became a central principle in modern physics, underlying descriptions of nuclear reactions, particle creation, and high-energy astrophysical processes.

Applications and implications: In nuclear fission and fusion, small changes in mass (mass defect) release large amounts of energy via E = Δm c^2. In particle physics, mass and energy can be exchanged in reactions, and particles can be created or annihilated in processes that conserve energy and momentum.

Limitations and clarifications: E = m c^2 refers to rest mass energy; objects with zero rest mass, such as photons, obey E^2 = (pc)^2 and still carry energy E = pc. c is a universal constant; mass–energy equivalence governs how mass and energy transform, not as a source of energy from nothing.