Albert Einstein

The German-American physicist Albert Einstein, b. Ulm, Germany, Mar. 14, 1879, d. Princeton, N.J., Apr. 18, 1955, contributed more than any other scientist to the 20th-century vision of physical reality. In the wake of World War I, Einstein's theories--especially his theory of relativity--seemed to many people to point to a pure quality of human thought, one far removed from the war and its aftermath. Seldom has a scientist received such public attention for having cultivated the fruit of pure learning.


Einstein's parents, who were nonobservant Jews, moved from Ulm to Munich when Einstein was an infant. The family business was the manufacture of electrical apparatus; when the business failed (1894), the family moved to Milan, Italy. At this time Einstein decided officially to relinquish his German citizenship. Within a year, still without having completed secondary school, Einstein failed an examination that would have allowed him to pursue a course of study leading to a diploma as an electrical engineer at the Swiss Federal Institute of Technology (the Zurich Polytechnic). He spent the next year in nearby Aarau at the cantonal secondary school, where he enjoyed excellent teachers and first-rate facilities in physics. Einstein returned in 1896 to the Zurich Polytechnic, where he graduated (1900) as a secondary school teacher of mathematics and physics.

After a lean two years he obtained a post at the Swiss patent office in Bern. The patent-office work required Einstein's careful attention, but while employed (1902-09) there, he completed an astonishing range of publications in theoretical physics. For the most part these texts were written in his spare time and without the benefit of close contact with either the scientific literature or theoretician colleagues. Einstein submitted one of his scientific papers to the University of Zurich to obtain a Ph.D. degree in 1905. In 1908 he sent a second paper to the University of Bern and became privatdocent, or lecturer, there. The next year Einstein received a regular appointment as associate professor of physics at the University of Zurich.

By 1909, Einstein was recognized throughout German-speaking Europe as a leading scientific thinker. In quick succession he held professorships at the German University of Prague and at the Zurich Polytechnic. In 1914 he advanced to the most prestigious and best-paying post that a theoretical physicist could hold in central Europe: professor at the Kaiser-Wilhelm Gesellschaft in Berlin. Although Einstein held a cross-appointment at the University of Berlin, from this time on he never again taught regular university courses. Einstein remained on the staff at Berlin until 1933, from which time until his death (1955) he held an analogous research position at the Institute for Advanced Study in Princeton, N.J.


The 1905 Papers

In the first of three seminal papers published in 1905, Einstein examined the phenomenon discovered by Max Planck, according to which electromagnetic energy seemed to be emitted from radiating objects in quantities that were ultimately discrete. The energy of these quantities--the so-called light-quanta--was directly proportional to the frequency of the radiation. This circumstance was perplexing because classical electromagnetic theory, based on Maxwell's equations and the laws of thermodynamics, had assumed that electromagnetic energy consisted of waves propagating in a hypothetical, all-pervasive medium called the luminiferous ether, and that the waves could contain any amount of energy no matter how small. Einstein used Planck's quantum hypothesis to describe visible electromagnetic radiation, or light. According to Einstein's heuristic viewpoint, light could be imagined to consist of discrete bundles of radiation. Einstein used this interpretation to explain the photoelectric effect, by which certain metals emit electrons when illuminated by light with a given frequency. Einstein's theory, and his subsequent elaboration of it, formed the basis for much of quantum mechanics.

The second of Einstein's 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 ( Fitzgerald-Lorentz contraction). 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 seminal 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.

The General Theory of Relativity

After 1905, Einstein continued working in all three of the above areas. 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 interact with the apparent mass of electromagnetic radiation, which includes light. By 1911, Einstein was able to make preliminary predictions about how a ray of light from a distant star, passing near the Sun, would appear to be attracted, or bent slightly, in the direction of the Sun's mass. At the same time, light radiated from the Sun would interact with the Sun's mass, resulting in a slight change toward the infrared end of the Sun's optical spectrum. At this juncture Einstein also knew that any new theory of gravitation would have to account for a small but persistent anomaly in the perihelion motion of the planet Mercury.

About 1912, Einstein began a new phase of his gravitational research, with the help of his mathematician friend Marcel Grossmann, by phrasing his work in terms of the tensor calculus of Tullio Levi-Civita and Gregorio Ricci-Curbastro. The tensor calculus greatly facilitated calculations in four-dimensional space-time, a notion that Einstein had obtained from Hermann Minkowski's 1907 mathematical elaboration of Einstein's own special theory of relativity. Einstein called his new work the general theory of relativity. After a number of false starts, he published (late 1915) the definitive form of the general theory. In it the gravitational field equations were covariant; that is, similar to Maxwell's equations, the field equations took the same form in all equivalent frames of reference. To their advantage from the beginning, the covariant field equations gave the observed perihelion motion of the planet Mercury. In its original form, Einstein's general relativity has been verified numerous times in the past 60 years, especially during solar-eclipse expeditions when Einstein's light-deflection prediction could be tested.


When British eclipse expeditions in 1919 confirmed his predictions, Einstein was lionized by the popular press. Einstein's personal ethics also fired public imagination. Einstein, who after returning to Germany in 1914 did not reapply for German citizenship, was one of only a handful of German professors who remained a pacifist and did not support Germany's war aims. After the war, when the victorious allies sought to exclude German scientists from international meetings, Einstein--a Jew traveling with a Swiss passport--remained an acceptable German envoy. Einstein's political views as a pacifist and a Zionist pitted him against conservatives in Germany, who branded him a traitor and a defeatist. The public success accorded his theories of relativity evoked savage attacks in the 1920s by the anti-Semitic physicists Johannes Stark and Philipp Lenard, men who after 1932 tried to create a so-called Aryan physics in Germany. Just how controversial the theories of relativity remained for less flexibly minded physicists is revealed in the circumstances surrounding Einstein's reception of a Nobel Prize in 1921--it was awarded not for relativity but for his 1905 work on the photoelectric effect.

With the rise of fascism in Germany, Einstein moved (1933) to the United States and abandoned his pacifism. He reluctantly agreed that the new menace had to be put down through force of arms. In this context Einstein sent (1939) a letter to President Franklin D. Roosevelt that urged that the United States proceed to develop an atomic bomb before Germany did. The letter, composed by Einstein's friend Leo Szilard, was one of many exchanged between the White House and Einstein, and it contributed to Roosevelt's decision to fund what became the Manhattan Project.

However much he appeared to the public as a champion of unpopular causes, such as his objection in the 1950s to the House Committee on Un-American Activities and his efforts toward nuclear disarmament, Einstein's central concerns always revolved around physics. At the age of 59, when other theoretical physicists would long since have abandoned original scientific research, Einstein and his co-workers Leopold Infeld and Banesh Hoffmann achieved a major new result in the general theory of relativity.

Until the end of his life Einstein sought a unified field theory, whereby the phenomena of gravitation and electromagnetism could be derived from one set of equations. Few physicists followed Einstein's path in the years after 1920. Quantum mechanics, instead of general relativity, drew their attention. For his part, Einstein could never accept the new quantum mechanics with its principle of indeterminacy, as formulated by Werner Heisenberg and elaborated into a new epistemology by Niels Bohr. Although Einstein's later thoughts were neglected for decades, physicists today refer seriously and awesomely to Einstein's dream--a grand unification of physical theory.