Albert Einstein's equation E=mc² is among the best-known equations of all time. The equation describes the relation between energy and mass:

For a technical account of the equation, see relativistic mass.

According to the equation, the total amount of energy obtainable from an object is equivalent to the mass of the object multiplied by the square of the speed of light.

It is important to note that practical conversions of mass to energy are seldom 100 percent efficient. One theoretically perfect conversion would result from a collision of matter and antimatter; for most cases, byproducts are produced instead of energy, and therefore very little mass is actually converted. In Einstein's equation, mass is energy, but for the sake of clarity, the word converted is used.

It is often cited that the equation explains how much energy is given off by the nuclear reactions in a nuclear weapon. This is true in a sense: the energy derived is the difference between the binding energies of the fuel and the fusion or fission products. The mass which is converted into energy is not the fissionable material itself (i.e. the uranium or the plutonium), but related to the strong nuclear force of the bonds which holds the large nucleus together.

The equation also proves one of the fundamental limiting factors of space travel - the inability for anything with mass to exceed (or even reach) the speed of light. As the speed of light squared is always constant, the two variables in the equation are energy on one side, and mass on the other.

Therefore, when energy (in this case velocity) increases, mass must increase. Conversely, the more mass an object has, the more energy it requires to accelerate even further. The speed of light (c = 299,792,458 meters per second) is the point where the energy required to reach it is infinite, and attaining such a speed becomes impossible.

It is a little known piece of trivia that Einstein originally wrote the equation in the form m = L/c² (with an "L", instead of an "E", representing energy).

A kilogram of mass completely converts into

• 89,875,517,873,681,764 joules or
• 24,965,421,632 kilowatt-hours or
• 21.48076431 megatons of TNT