The German-born American physicist Albert Einstein (1879-1955) revolutionized the science of physics. He is best known for his theory of relativity.
In the history of the exact sciences, only a handful of men—men like Nicolaus Copernicus and Isaac Newton—share the honor that was Albert Einstein's: the initiation of a revolution in scientific thought. His insights into the nature of the physical world made it impossible for physicists and philosophers to view that world as they had before. When describing the achievements of other physicists, the tendency is to enumerate their major discoveries; when describing the achievements of Einstein, it is possible to say, simply, that he revolutionized physics.
Albert Einstein was born on March 14, 1879, in Ulm, but he grew up and obtained his early education in Munich. He was not a child prodigy; in fact, he was unable to speak fluently at age 9. Finding profound joy, liberation, and security in contemplating the laws of nature, already at age 5 he had experienced a deep feeling of wonder when puzzling over the invisible, yet definite, force directing the needle of a compass. Seven years later he experienced a different kind of wonder: the deep emotional stirring that accompanied his discovery of Euclidean geometry, with its lucid and certain proofs. Einstein mastered differential and integral calculus by age 16.
Education in Zurich
Einstein's formal secondary education was abruptly terminated at 16. He found life in school intolerable, and just as he was scheming to find a way to leave without impairing his chances for entering the university, his teacher expelled him for the negative effects his rebellious attitude was having on the morale of his classmates. Einstein tried to enter the Federal Institute of Technology (FIT) in Zurich, Switzerland, but his knowledge of nonmathematical disciplines was not equal to that of mathematics and he failed the entrance examination. On the advice of the principal, he thereupon first obtained his diploma at the Cantonal School in Aarau, and in 1896 he was automatically admitted into the FIT. There he came to realize that his deepest interest and facility lay in physics, both experimental and theoretical, rather than in mathematics.
Einstein passed his diploma examination at the FIT in 1900, but due to the opposition of one of his professors he was unable to subsequently obtain the usual university assistantship. In 1902 he was engaged as a technical expert, third-class, in the patent office in Bern, Switzerland. Six months later he married Mileva Maric, a former classmate in Zurich. They had two sons. It was in Bern, too, that Einstein, at 26, completed the requirements for his doctoral degree and wrote the first of his revolutionary scientific papers.
These papers made Einstein famous, and universities soon began competing for his services. In 1909, after serving as a lecturer at the University of Bern, Einstein was called as an associate professor to the University of Zurich. Two years later he was appointed a full professor at the German University in Prague. Within another year and a half Einstein became a full professor at the FIT. Finally, in 1913 the well-known scientists Max Planck and Walter Nernst traveled to Zurich to persuade Einstein to accept a lucrative research professorship at the University of Berlin, as well as full membership in the Prussian Academy of Science. He accepted their offer in 1914, quipping: "The Germans are gambling on me as they would on a prize hen. I do not really know myself whether I shall ever really lay another egg." When he went to Berlin, his wife remained behind in Zurich with their two sons; after their divorce he married his cousin Elsa in 1917.
In 1920 Einstein was appointed to a lifelong honorary visiting professorship at the University of Leiden. During 1921-1922 Einstein, accompanied by Chaim Weizmann, the future president of the state of Israel, undertook extensive worldwide travels in the cause of Zionism. In Germany the attacks on Einstein began. Philipp Lenard and Johannes Stark, both Nobel Prize-winning physicists, began characterizing Einstein's theory of relativity as "Jewish physics." This callousness and brutality increased until Einstein resigned from the Prussian Academy of Science in 1933. (He was, however, expelled from the Bavarian Academy of Science.)
Career in America
On several occasions Einstein had visited the California Institute of Technology, and on his last trip to the United States Abraham Flexner offered Einstein—on Einstein's terms—a position in the newly conceived and funded Institute for Advanced Studies in Princeton. He went there in 1933.
Einstein played a key role (1939) in mobilizing the resources necessary to construct the atomic bomb by signing a famous letter to President Franklin D. Roosevelt which had been drafted by Leo Szilard and E.P. Wigner. When Einstein's famous equation E mc2 was finally demonstrated in the most awesome and terrifying way by using the bomb to destroy Hiroshima in 1945, Einstein, the pacifist and humanitarian, was deeply shocked and distressed; for a long time he could only utter "Horrible, horrible." On April 18, 1955, Einstein died in Princeton.
Theory of Brownian Motion
From numerous references in Einstein's writings it is evident that, of all areas in physics, thermodynamics made the deepest impression on him. During 1902-1904 Einstein reworked the foundations of thermodynamics and statistical mechanics; this work formed the immediate background to his revolutionary papers of 1905, one of which was on Brownian motion.
In Brownian motion (first observed in 1827 by the Scottish botanist Robert Brown), small particles suspended in a viscous liquid such as water undergo a rapid, irregular motion. Einstein, unaware of Brown's earlier observations, concluded from his theoretical studies that such a motion must exist. Guided by the thought that if the liquid in which the particles are suspended consists of atoms or molecules they should collide with the particles and set them into motion, he found that while the particle's motion is irregular, fluctuating back and forth, it will in time nevertheless experience a net forward displacement. Einstein proved that this net forward displacement of the suspended particles is directly related to the number of molecules per gram atomic weight. This point created a good deal of skepticism toward Einstein's theory at the time he developed it (1905-1906), but when it was fully confirmed many of the skeptics were converted. Brownian motion is to this day regarded as one of the most direct proofs of the existence of atoms.
Light Quanta and Wave-Particle Duality
The most common misconceptions concerning Einstein's introduction of his revolutionary light quantum (light particle) hypothesis in 1905 are that he simply applied Planck's quantum hypothesis of 1900 to radiation and that he introduced light quanta to "explain" the photoelectric effect discovered in 1887 by Heinrich Hertz and thoroughly investigated in 1902 by Philipp Lenard. Neither of these assertions is accurate. Einstein's arguments for his light quantum hypothesis—that under certain circumstances radiant energy (light) behaves as if it consists not of waves but of particles of energy proportional to their frequencies— were absolutely fundamental and, as in the case of his theory of Brownian motion, based on his own insights into the foundations of thermodynamics and statistical mechanics. Furthermore, it was only after presenting strong arguments for the necessity of his light quantum hypothesis that Einstein pursued its experimental consequences. One of several such consequences was the photoelectric effect, the experiment in which high-frequency ultraviolet light is used to eject electrons from thin metal plates. In particular, Einstein assumed that a single quantum of light transfers its entire energy to a single electron in the metal plate. The famous equation he derived was fully consistent with Lenard's observation that the energy of the ejected electrons depends only on the frequency of the ultraviolet light and not on its intensity. Einstein was not disturbed by the fact that this apparently contradicts James Clerk Maxwell's classic electromagnetic wave theory of light, because he realized that there were good reasons to doubt the universal validity of Maxwell's theory.
Although Einstein's famous equation for the photoelectric effect—for which he won the Nobel Prize of 1921— appears so natural today, it was an extremely bold prediction in 1905. Not until a decade later did R.A. Millikan finally succeed in experimentally verifying it to everyone's satisfaction. But while Einstein's equation was bold, his light quantum hypothesis was revolutionary: it amounted to reviving Newton's centuries-old idea that light consists of particles.
No one tried harder than Einstein to overcome opposition to this hypothesis. Thus, in 1907 he proved the fruitfulness of the entire quantum hypothesis by showing it could at least qualitatively account for the low-temperature behavior of the specific heats of solids. Two years later he proved that Planck's radiation law of 1900 demands the coexistence of particles and waves in blackbody radiation, a proof that represents the birth of the wave-particle duality. In 1917 Einstein presented a very simple and very important derivation of Planck's radiation law (the modern laser, for example, is based on the concepts Einstein introduced here), and he also proved that light quanta must carry momentum as well as energy.
Meanwhile, Einstein had become involved in another series of researches having a direct bearing on the wave-particle duality. In mid-1924 S.N. Bose produced a very insightful derivation of Planck's radiation law—the origin of Bose-Einstein statistics—which Einstein soon developed into his famous quantum theory of an ideal gas. Shortly thereafter, he became acquainted with Louis de Broglie's revolutionary new idea that ordinary material particles, such as electrons and gas molecules, should under certain circumstances exhibit wave behavior. Einstein saw immediately that De Broglie's idea was intimately related to the Bose-Einstein statistics: both indicate that material particles can at times behave like waves. Einstein told Erwin Schrödinger of De Broglie's work, and in 1926 Schrödinger made the extraordinarily important discovery of wave mechanics. Schrödinger's (as well as C. Eckart) then proved that Schrödinger's wave mechanics and Werner Heisenberg's matrix mechanics are mathematically equivalent: they are now collectively known as quantum mechanics, one of the two most fruitful physical theories of the 20th century. Since Einstein's insights formed much of the background to both Schrödinger's and Heisenberg's discoveries, the debt quantum physicists owe to Einstein can hardly be exaggerated.
Theory of Relativity
The second of the two most fruitful physical theories of the 20th century is the theory of relativity, which to scientists and laymen alike is synonymous with the name of Einstein. Once again, there is a common misconception concerning the origin of this theory, namely, that Einstein advanced it in 1905 to "explain" the famous Michelson-Morley experiment (1887), which failed to detect a relative motion of the earth with respect to the ether, the medium through which light was assumed to propagate. In fact, it is not even certain that Einstein was aware of this experiment in 1905; nor was he familiar with H.A. Lorentz's elegant 1904 paper in which Lorentz applied the transformation equations which bear his name to electrodynamic phenomena. Rather, Einstein consciously searched for a general principle of nature that would hold the key to the explanation of a paradox that had occurred to him when he was 16: if, on the one hand, one runs at, say, 4 miles per hour alongside a train moving at 4 miles per hour, the train appears to be at rest; if, on the other hand, it were possible to run alongside a ray of light, neither experiment nor theory suggests that the ray of light—an oscillating electromagnetic wave—would appear to be at rest. Einstein eventually saw that he could postulate that no matter what the velocity of the observer, he must always observe the same velocity c for the velocity of light: roughly 186,000 miles per second. He also saw that this postulate was consistent with a second postulate: if an observer at rest and an observer moving at constant velocity carry out the same kind of experiment, they must get the same result. These are Einstein's two postulates of his special theory of relativity. Also in 1905 Einstein proved that his theory predicted that energy E and mass mare entirely interconvertible according to his famous equation, Emc2.
For observational confirmation of his general theory of relativity, Einstein boldly predicted the gravitational red shift and the deflection of starlight (an amended value), as well as the quantitative explanation of U. J. J. Leverrier's long-unexplained observation that the perihelion of the planet Mercury precesses about the sun at the rate of 43 seconds of arc per century. In addition, Einstein in 1916 predicted the existence of gravitational waves, which have only recently been detected. Turning to cosmological problems the following year, Einstein found a solution to his field equations consistent with the picture (the Einstein universe) that the universe is static, approximately uniformly filled with a finite amount of matter, and finite but unbounded (in the same sense that the surface area of a smooth globe is finite but has no beginning or end).
The Man and His Philosophy
Fellow physicists were always struck with Einstein's uncanny ability to penetrate to the heart of a complex problem, to instantly see the physical significance of a complex mathematical result. Both in his scientific and in his personal life, he was utterly independent, a trait that manifested itself in his approach to scientific problems, in his unconventional dress, in his relationships with family and friends, and in his aloofness from university and governmental politics (in spite of his intense social consciousness). Einstein loved to discuss scientific problems with friends, but he was, fundamentally a "horse for single harness."
Einstein's belief in strict causality was closely related to his profound belief in the harmony of nature. That nature can be understood rationally, in mathematical terms, never ceased to evoke a deep—one might say, religious—feeling of admiration in him. "The most incomprehensible thing about the world," he once wrote, "is that it is comprehensible." How do we discover the basic laws and concepts of nature? Einstein argued that while we learn certain features of the world from experience, the free inventive capacity of the human mind is required to formulate physical theories. There is no logical link between the world of experience and the world of theory. Once a theory has been formulated, however, it must be "simple" (or, perhaps, "esthetically pleasing") and agree with experiment. One such esthetically pleasing and fully confirmed theory is the special theory of relativity. When Einstein was informed of D.C. Miller's experiments, which seemed to contradict the special theory by demanding the reinstatement of the ether, he expressed his belief in the spuriousness of Miller's results—and therefore in the harmoniousness of nature—with another of his famous aphorisms, "God is subtle, but he is not malicious."
This frequent use of God's name in Einstein's speeches and writings provides us with a feeling for his religious convictions. He once stated explicitly, "I believe in Spinoza's God who reveals himself in the harmony of all being, not in a God who concerns himself with the fate and actions of men." It is not difficult to see that this credo is consistent with his statement that the "less knowledge a scholar possesses, the farther he feels from God. But the greater his knowledge, the nearer is his approach to God." Since Einstein's God manifested Himself in the harmony of the universe, there could be no conflict between religion and science for Einstein.
To enumerate at this point the many honors that were bestowed upon Einstein during his lifetime would be to devote space to the kind of public acclamation that mattered so little to Einstein himself. How, indeed, can other human beings sufficiently honor one of their number who revolutionized their conception of the physical world, and who lived his life in the conviction that "the only life worth living is a life spent in the service of others"? When Einstein lay dying he could truly utter, as he did, "Here on earth I have done my job." It would be difficult to find a more suitable epitaph than the words Einstein himself used in characterizing his life: "God is inexorable in the way He has allotted His gifts. He gave me the stubbornness of a mule and nothing else; really, He also gave me a keen scent."
Further Reading on Albert Einstein
Numerous biographies of Einstein have been written. Three of the best are Philipp Frank, Einstein: His Life and Times, translated by George Rosen (1947); Carl Seelig, Albert Einstein: A Documentary Biography, translated by Mervyn Savill (1956); and Ronald W. Clark, Einstein: The Life and Times (1971). Einstein's illuminating "Autobiographical Notes" and bibliographies of his scientific and nonscientific writings can be found in P.A. Schilpp, ed., Albert Einstein: Philosopher-Scientist (1949; 2d ed. 1951). See also Max Born, Einstein's Theory of Relativity (trans. 1922; rev. ed. 1962); Leopold Infeld, Albert Einstein: His Work and Its Influence on Our World (1950); and Max Jammer, The Conceptual Development of Quantum Mechanics (1966).