Mass–Energy Equivalence

Einstein's most famous equation, $E = mc^2$, says mass and energy are two forms of the same thing — and because $c^2$ is enormous, a tiny amount of mass holds a colossal amount of energy.^[1] It isn't just theory: every time a nucleus splits, fuses, or undergoes radioactive decay, the products weigh very slightly less than what went in, and that missing mass emerges as energy — the power source of nuclear reactors, the Sun, and the atomic bomb. Particle accelerators run it in reverse, turning energy into brand-new particles of matter.