Background

Mass is a form of energy? In 1905, Albert Einstein (1873-1955) suggested that mass and energy are equivalent while developing his special theory of relativity. The famous mass-energy equivalence relation states that

where E is the energy equivalent or mass energy,

m is the mass of a body and

is the speed of light.

It is difficult to show a simple logical path through which Einstein came to his equation. It was merely an hypothesis made by him from special relativity and Maxwell's equations.

Energy gain/loss in everyday examples can, however, hardly show any noticeable change in mass. This is because the total mass of an object changes only by a tiny fraction. To check Einstein's equation, we need something with tiny mass, so that an appreciable change in total mass can be measured. Radioactive decay, a nuclear reaction, is a choice. In fact, Einstein tested his own equation with a lump of radium salt to see if it lost weight as it gave off radiation.

Today, the mass-energy equivalence relation has an important implication in nuclear industry.