5 - E = mc²

Summary

The most important result of a general character to which the special theory of relativity has led is concerned with the conception of mass. Before the advent of relativity, physics recognized two conservation laws of fundamental importance, namely, the law of conservation of energy and the law of conservation of mass; these two fundamental laws appeared to be quite independent of each other. By means of the theory of relativity they have been united into one law.

Albert Einstein, Relativity, 1916

Phlogiston and caloric

Before we look at Einstein's famous equation, we had better set the scene by describing how scientists had arrived at the concepts of energy and mass – the ‘E’ and ‘m’ in the equation. The gradual evolution of our present understanding of energy began with two ideas at least as curious as the infamous aether – ‘phlogiston’ and ‘caloric’. It provides us with some insight into the way that science progresses to look at why these two theories were invented and subsequently discarded.

Phlogiston was introduced towards the end of the seventeenth century by Georg Stahl, a German professor of medicine and chemistry, in an attempt to understand fire. Even in the latter part of the eighteenth century, many scientists still regarded fire as an element. Combustible materials were supposed to be made up of two parts -the calx, or ash, and the ‘phlogiston’. It was thought that, when a substance burned, the phlogiston was liberated, leaving the ash behind.