Published in Annalen der Physik late in 1905, this three-page paper is a short sequel to Einstein’s work on special relativity earlier the same year. It takes a body at rest that emits light equally in two opposite directions, and compares the energy accounts kept by two observers in motion relative to one another.

Comparing the two descriptions shows that the body must lose mass in proportion to the energy it radiates, with the constant of proportionality being the square of the speed of light. Einstein concluded that the mass of a body is a measure of its energy content, and suggested that radioactive salts might offer a way to test the idea.

The relationship is now written as E=mc². It implies that a very small quantity of mass corresponds to a very large quantity of energy, a result central to later work in nuclear physics.