// physics › Relativity
Convert rest mass into its equivalent energy with Einstein's E = mc², or solve for the mass from a given energy.
E = m c²
A mind behind this: Albert Einstein 1879–1955
It means mass and energy are really the same thing in two forms. Any object, just by existing, holds energy locked inside its mass. The equation tells you how much: take the mass, multiply by the speed of light, and multiply by the speed of light again. Because the speed of light is so huge, even a crumb of matter holds a staggering amount of energy.
Because of the c² part. The speed of light is about 300 million metres per second, and you multiply by it twice — that is a 9 followed by sixteen zeros. So when you turn mass into energy, you are multiplying by that gigantic number. That is why one single gram of matter, fully converted, releases about the same energy as a small atomic bomb.
Sure. Take 1 gram, which is 0.001 kg. The formula is E = mass × c × c. Step 1: c × c = 299,792,458 × 299,792,458, which is about 9 × 10^16. Step 2: multiply by the mass: 0.001 × 9 × 10^16 = about 9 × 10^13 joules. That is roughly 21 kilotonnes of TNT — from one gram. A joule (J) is just the unit we measure energy in.
Yes — in principle. A 70 kg person is equivalent to a truly vast energy, far more than any power station produces in years. But we cannot release it: ordinary matter does not convert its mass into energy on its own. Only special processes like nuclear reactions or matter meeting antimatter unlock even a small fraction.
In the Sun and stars, where fusion turns mass into sunlight — our Sun loses about four million tonnes of mass every second. In nuclear power plants and weapons, where a small part of the fuel's mass becomes energy. And in PET medical scanners, where electrons meet positrons (their antimatter twins) and their entire mass turns into detectable energy.
It is not destroyed — it is converted. The total of mass-plus-energy is always conserved; mass just changes into an equivalent amount of energy. In most real processes only a tiny fraction of the mass converts. Complete conversion of all the mass happens only when matter meets antimatter and both annihilate into pure energy.
Kinetic energy is the energy of motion — energy a body has because it is moving. E = mc² is the rest energy — energy a body has just from existing, even when perfectly still. They are different pools of energy: one you get by speeding an object up, the other is frozen inside its mass from the start.