One of the fundamental dichotomies in classical physics was that between energy and matter. By the late nineteenth century, most scientists agreed that matter is composed of tiny, discrete particles with measurable mass. At the time, the atom was considered to be the ultimate particle of which all matter consists. Energy, on the other hand, was thought to have no material basis, but corresponded to anything that was able to cause a change of position in some form of matter. Most forms of energy were thought to travel through space as waves. The duality between particles and waves was, therefore, a fundamental tenet of physical theory. By the turn of the century, however, the wave-particle duality began to come apart fairly rapidly. A critical factor in this change was Albert Einstein's analysis of the photoelectric effect. Einstein showed that the emission of electrons from a metal that has...