Mass–Energy Equivalence

Einstein's mass-energy equivalence (special relativity, 1905): E = mc².

1 kg of mass corresponds to 9 × 10¹⁶ J of energy — vast, but only nuclear reactions release a measurable fraction.

Atomic mass unit: 1 u = 1.66054 × 10⁻²⁷ kg. Energy equivalent: 1 u · c² = 931.494 MeV.

Mass defect in any bound system = (sum of constituent masses) − (system mass). Always positive for bound states.

Energy released = (mass defect) × c². For nuclear reactions, mass defect is detectable (~MeV scale).

Chemical reactions also have mass defects — but ~10⁶× smaller (eV scale) ⇒ undetectable with chemical scales.

Particle-antiparticle annihilation: m + m → 2γ. Each photon carries mc² of energy. Pair production: γ → e⁺ + e⁻ requires hf ≥ 2m_e·c² = 1.022 MeV.