The principle of special relativity

At the end of the nineteenth century, it seemed that all the problems in physics had been solved and that scientists had a complete understanding of the laws of nature. Mechanics was triumphant, with daily applications in engineering as well as explanations of the motion of the heavenly bodies. Electricity and magnetism had been shown by James Maxwell to be two faces of the same thing (electromagnetism), thus unifying a large range of phenomena. The kinetic theory of gases provided an understanding of the workings of matter at the molecular level. Two problems defied solution, however, and they were about to bring down the entire structure of classical physics. The first was the attempt to understand the spectrum of the radiation emitted by a black body. The resolution to this problem came many years later. It involved the concept of a photon and the birth of quantum mechanics. The second problem had to do with the velocity of light: it is said that Albert Einstein, as a boy, asked himself what would happen if he held a mirror in front of him and ran forward at the speed of light.