Albert Einstein
1905 papers proposed what is today called the special theory of relativity. At the time Einstein knew that, according to Hendrik Antoon Lorentz's theory of electrons, the mass of an electron increased as the velocity of the electron approached the velocity of light. Einstein also knew that the electron theory, based on Maxwell's equations, carried along with it the assumption of a luminiferous ether, but that attempts to detect the physical properties of the ether had not succeeded. Einstein realized that the equations describing the motion of an electron in fact could describe the nonaccelerated motion of any particle or any suitably defined rigid body. He based his new kinematics on a reinterpretation of the classical principle of relativity, that the laws of physics had to have the same form in any frame of reference. As a second fundamental hypothesis, Einstein assumed that the speed of light remained constant in all frames of reference, as required by classical Maxwellian theory. Einstein abandoned the hypothesis of the ether, for it played no role in his kinematics or in his reinterpretation of Lorentz's theory of electrons. As a consequence of his theory Einstein recovered the phenomenon of time dilatation, wherein time, analogous to length and mass, is a function of the velocity of a frame of reference. Later in 1905, Einstein elaborated how, in a certain manner of speaking, mass and energy were equivalent. Einstein was not the first to propose all the elements that went into the special theory of relativity; his contribution lies in having unified important parts of classical mechanics and Maxwellian electrodynamics.
The third of Einstein's papers of 1905 concerned statistical mechanics, a field of study that had been elaborated by, among others, Ludwig Boltzmann and Josiah Willard Gibbs. Unaware of Gibbs' contributions, Einstein extended Boltzmann's work and calculated the average trajectory of a microscopic particle buffeted by random collisions with molecules in a fluid or in a gas. Einstein observed that his calculations could account for brownian motion, the apparently erratic movement of pollen in fluids, which had been noted by the British botanist Robert Brown. Einstein's paper provided convincing evidence for the physical existence of atom-sized molecules, which had already received much theoretical discussion. His results were independently discovered by the Polish physicist Marian von Smoluchowski and later elaborated by the French physicist Jean Perrin.
He made important contributions to the quantum theory, but increasingly he sought to extend the special theory of relativity to phenomena involving acceleration. The key to an elaboration emerged in 1907 with the principle of equivalence, in which gravitational acceleration was held a priori indistinguishable from acceleration caused by mechanical forces; gravitational mass was therefore identical with inertial mass. Einstein elevated this identity, which is implicit in the work of Isaac Newton, to a guiding principle in his attempts to explain both electromagnetic and gravitational acceleration according to one set of physical laws. In 1907 he proposed that if mass were equivalent to energy, then the principle of equivalence required that gravitational mass would [next page]