“I was always interested in philosophy but only as a sideline. My interest in natural science was always essentially limited to the study of principles, which best explains my conduct in its entirety. That I have published so little is attributable to the same circumstance, for the burning desire to grasp principles has caused me to spend most of my time on fruitless endeavors”.
Albert Einstein letter to Maurice Solovine, October 30, 19241
Albert Einstein was born in 1879, the eighth year after Otto von Bismarck’s unification of Germany by “blood and iron”. He died in Princeton, New Jersey, in April 1955, two and a half years after a successful test of the world’s first thermonuclear weapon. His life spanned the two revolutions of 20th century physics, in both of which he was intimately involved, and two World Wars. His principal contributions to physical theory are the theories of relativity, the special theory (1905) and the general theory (1915). He was awarded the Nobel Prize in Physics in 1922 not for the theories of relativity but for “his services to physics, especially for the law of the photoelectric effect”, a tribute leaving oddly unmentioned his revolutionary – and in 1922 still controversial – “light-quantum hypothesis” of 1905 on which said law relies (see Chapter 3). His many contributions to the development of the so-called “old quantum theory” (1900–1925) show that he must be regarded as a father, though a stern and critical one, of quantum mechanics. A 1935 paper with Podolsky and Rosen (two assistants at Princeton, see Chapter 7) is recognized today as discovering quantum entanglement, a phenomenon endemic to quantum mechanics.
Einstein reached intellectual maturity around the turn of the century and can be treated legitimately as a 20th-century figure. By summarizing his life through his scientific contributions the account below follows his self-description, as one increasingly disengaged from the “merely personal” while striving for a “mental grasp of things”.2 There are roughly three periods following his childhood.
Einstein’s family moved to Munich shortly after he was born in the Swabian city of Ulm, on March 14, 1879. A Jewish medical student, Max Talmud (Talmey), befriended by the Einstein family, became something of a tutor to Einstein in philosophy and elementary natural science. After seven years at the Luitpold Gymnasium in Munich, where he was increasingly unhappy, Einstein withdrew at age 15 before graduating, joining his family then living in Milan. To avoid compulsory military service, he renounced his German citizenship in 1896 and remained stateless until 1901, when he was granted Swiss citizenship.
Though excelling in the math and physics parts, Einstein did not do well in the literature, social sciences, and zoology section, and so failed the entrance exam at the Polytechnic School in Zurich (renamed in 1910 as Federal Swiss Technical College, Eidgenössiche Technishe Hochschule or ETH); he then enrolled in secondary school at Aarau, near Zurich, where he passed the Matura exam (qualification for university entrance) in October 1896. He immediately enrolled as a student of physics and mathematics at the ETH. Among his fellow students were Marcel Grossmann, a lifelong friend who became a mathematician, and Mileva Mariç, a Serbian fellow student of physics. Unable to find a secondary school teaching position upon graduating in 1900 with a diploma for teaching mathematics and physics, he initially found work as a private tutor. His first scientific paper was published in 1901, the year he received Swiss citizenship.
In early 1902 he moved to Bern, where from June 1902 until October 1909 he worked as a patent clerk, a job he acquired with the assistance of Grossmann. These were immensely productive years. Three early papers on statistical thermodynamics in 1902–1904 were published in the leading German physics journal, the Annalen der Physik, edited then by Max Planck and Wilhelm Wien. Although Einstein later downplayed their significance, these are exceptional papers, generalizing the statistical methods of the kinetic theory of gases independently but along the same lines as the celebrated contemporaneous work of Ludwig Boltzmann and J. Willard Gibbs. In the last of them, in 1904, Einstein employed what he termed “Boltzmann’s Principle” (the connection of entropy and probability) to develop a theory of energy fluctuation phenomena. He would use this theory to probe quantum behavior in the theory of radiation (light) and the theory of matter (specific heats of substances, critical opalescence).
Together with friends Conrad Habicht and Maurice Solovine, for several years beginning April 1903 Einstein participated in a discussion group playfully named the “Olympia Academy”. Among their readings were books of philosophers (Spinoza, Hume) and philosophically themed works of scientists, such as Hermann von Helmholtz, Heinrich Hertz, Ernst Mach, and Henri Poincaré. Much later, Solovine recalled that the latter’s 1902 book La science et l’hypothèse, “held us spellbound for weeks on end”. Against the objections of both families, in January 1903 he married Mileva Maric; a year earlier, she had given birth to their first child, a daughter, at her parents’ home in Serbia. The child, whose existence was unknown to Einstein biographers until the 1980s, died of scarlet fever before her second birthday. A son, Hans Albert, was born in 1904; he would become a professor of engineering at the University of California, Berkeley. That year with the assistance of Grossmann, Einstein found a permanent job at the patent office in Bern.
At age 26, Einstein completed his doctoral dissertation in 1905 at the University of Zurich with a thesis “A New Determination of Molecular Dimensions”. He also published four groundbreaking papers. The first, in March, is a revival of the corpuscular theory of light, largely abandoned by physicists early in the 19th century. In an investigation related to Max Planck’s 1900 work on blackbody radiation, Einstein proposed that in certain circumstances radiation (light) behaved “as if” it had a particulate structure, like the molecules of gas. The so-called “light-quantum” hypothesis was indeed “very revolutionary” as Einstein characterized this paper in a letter to his friend Habicht in May;3 the idea was not widely accepted until the early 1920s (the term “photon” was coined by Berkeley physical chemist G.N. Lewis in 1926). In May followed a first paper on Brownian motion, the visible phenomena of irregular zigzagging motions of tiny particles suspended in a liquid or, as motes of dust, in the air of a closed up room. Though the phenomenon was recognized since the 1820s, Einstein provided the first theoretical explanation: the observable motions are due to statistical fluctuations in the motions of the invisible molecules of the containing liquid or air. Polish physicist Marian von Smoluchowski advanced a similar theory in 1906. Experiments conducted in Paris in 1908 confirmed the theories, after which the existence of atoms became universally accepted. On June 30, the Annalen der Physik received Einstein’s “On the Electrodynamics of Moving Bodies” initiating the special theory of relativity overturning Newtonian ideas of space and time. A follow-up paper in September attempted the first general proof of the famous equivalence between energy and mass, E = mc2. In the annals of science, perhaps only the “plague years” 1665–1666, when Newton discovered the theory of binomial series, the method of fluxions (differential and integral calculus), advanced the theory of colors and idea of universal gravitation, can compare to Einstein’s annus mirabilis of 1905.
The next year, 1906, Einstein published a critical examination of Max Planck’s derivation of his law of blackbody radiation. Beginning with classical electromagnetism and statistical physics, in 1900 and in years subsequent, Planck derived the experimentally determined graph of an energy density function relating the frequency of thermal radiation to its temperature. To fit the known experimental data, Planck had to introduce what he considered a stopgap assumption, effectively discretizing the energy spectrum of thermal radiation. Einstein pointed out the inconsistency between Planck’s starting point and the non-classical nature of his assumption, remarking that Planck had only succeeded by introducing the idea that there are discrete “atoms” of energy. Historians of physics, such as Thomas Kuhn, regard Einstein’s 1906 paper as marking the actual beginning of the quantum revolution.
Prompted by Johannes Stark, an experimentalist and Nobel Prize winner, to write a survey of special relativity, Einstein began thinking about the extension of the relativity principle from inertial to accelerating frames. He published the article in 1907 in the journal edited by Stark, Jahrbuch der Radioaktivitaät und Elektronik. An irony, since the paper is Einstein’s first step towards general relativity, the relativistic theory of gravitation that Stark, who would became a leading Nazi physicist, later attacked as exemplifying non-Aryan “Jewish physics”. Also in 1907, as he later recalled, occurred “the most fortunate thought of my life”, that a man falling freely (say, from a roof) would not feel his own weight.4 Following up this insight led to the principle of equivalence, the crucial heuristic step towards general relativity. Still, most of Einstein’s publications in 1907–1911 were devoted to the problems of understanding the increasing evidence of the quantum structure of matter that showed up in anomalies for classical theory. He proposed in 1907 a quantum modification that accounted for a well-known anomaly in the theory of specific heats. In 1908 he used the fluctuation theory developed in 1904 to advance a theory of critical opalescence, explaining the anomalous strong increase in the scattering (deviation in all directions) of light passing through a dense gas or liquid by the incidence of light on density fluctuations in the gas or fluid; he also showed that the scattering had a maximum near the material’s “critical” point, the temperature of a phase change to another state of matter. In a lecture in Salzburg in September 1909, Einstein presciently predicted that a future theory of light would be a kind of amalgam of the wave and particle theories, the first statement of the wave-particle duality that is a cornerstone of quantum theory.
Einstein acquired his first academic job when he was made associate professor of theoretical physics at the University of Zurich in October 1909, having received his first honorary doctorate from the University of Geneva in July. For a brief period, in 1911–1912, he became professor of theoretical physics at the German University in Prague, while in October 1911, he was the youngest physicist invited to participate in the first world physics (Solvay) conference in Brussels, joining luminaries such as Lorentz, Planck, Poincaré, and Marie Curie. He returned in 1912 to Zurich as professor at the ETH, where he remained until accepting a position in Berlin in the summer of 1914. From 1907 when he first formulated the equivalence principle until finally succeeding in November 1915, his principal theoretical activity lay in attempting to generalize the special theory of relativity to apply to accelerating systems, and so to gravitational fields. In Prague in 1911 he first calculated that the principle of equivalence implied an observable deflection of light rays passing near the surface of the Sun. Fortunately, since without the correct theory of gravity, he got the wrong value, the expedition to Russia to test this prediction was interrupted by WW I.
Einstein’s call to the Prussian Academy of Sciences and Berlin University in 1914 as a research professor without teaching duties placed him at the pinnacle of the scientific establishment in Germany. His greatest achievement, the general theory of relativity, was completed in outline in November 1915 (in something of a race with the mathematician David Hilbert). The final result is summed up in an apparently elegant, but highly complex, tensor equation. That equation stands for ten nonlinear field equations of gravity, the Einstein field equations, that must be given simultaneous solutions. For many years, very few exact solutions of this set of equations were known. The theory is inherently philosophical: It arose from a Machian-inspired (see Chapter 5) motivation to “relativize inertia”. Two of the three principles Einstein placed at the basis of the theory, the “principle of general relativity” or general covariance, and what he termed “Mach’s principle”, gave rise to a good deal of controversy and misunderstanding, some of it on Einstein’s part. A critical understanding of Einstein’s adherence to the principle of general covariance only emerged in the 1980s with the work of the historians and philosophers of physics, John Stachel and John Norton.5
In 1917, seeking to eliminate “un-Machian” solutions to his field equations of gravitation, Einstein created relativistic cosmology, today a central part of fundamental physical theory. Einstein essentially “abolished infinity”.6 In the belief that that a “consistent theory of relativity” required the inertial mass of a body to drop to zero in the absence of all other masses, Einstein projected the first relativistic cosmological model, a spatially finite, temporally infinite, static universe with a uniform distribution of stars. In order to insure the model’s stability against gravitational attraction between cosmic masses, he inserted a new “cosmological term” in his field equations that had the effect of a repulsive force to exactly balance against gravitational collapse. Almost immediately, Dutch astronomer Willem de Sitter produced an exact solution of the now-modified field equations that contained no matter at all, a universe possessing just the energy of empty space, as represented by the cosmological term. De Sitter’s model universe turned out to be expanding, though this was not clear for some time. In 1922 Russian mathematician Alexander Friedmann showed that the most natural solution of Einstein’s unmodified field equations resulted in a dynamic, not static, universe. Without knowledge of Friedmann’s work, Belgian Jesuit priest Georges Lemaître in 1927 created a general relativistic model with the cosmological term. The theoretical developments dovetailed with astronomical observations of American Edwin Hubble reported in 1929, agreeing that the geometrical structure of the universe is described by general relativity and is expanding. When convinced of Hubble’s results in the early 1930s, Einstein is reported to have called the introduction of the cosmological term his “biggest blunder”. Today, however, the so-called cosmological constant is a fundamental link between quantum field theory and general relativity while in Big Bang cosmology, de Sitter’s model for the matter-free early universe is widely adopted.
In the period immediately following completion of the general theory of relativity, Einstein showed considerable interest in attempts by philosophers to illuminate the theory’s philosophical significance. His approval of Moritz Schlick’s treatment was enthusiastic but short-lived. Schlick would go on to become the head of the Vienna Circle, the Olympus of logical positivism. Einstein’s own philosophical views about his theory were rather fluid, changing in tandem with his own pursuit of a generalization of that theory to incorporate the other known field at the time, that of electromagnetism, into space-time geometry. In the 1920s he increasingly came to emphasize, with perhaps questionable accuracy of memory, the predominate role that considerations of mathematical elegance played in guiding him to the correct field equations of gravity during the period 1913–1915. These considerations became a core, and controversial, methodological theme after his Herbert Spencer lecture at Oxford in 1933.
Less widely known, but worthy of emphasis, are contributions Einstein made to the development of the quantum theory during the Berlin period. These include the first quantum derivation of Planck’s radiation law in late 1916 that introduced the idea of “stimulated emission” of radiation, the theoretical basis of the laser and maser in the 1950s. In 1924–1925, spurred by works from two unknown physicists, by Louis de Broglie’s Paris doctoral thesis extending wave-particle duality to electrons, and by a paper by Satyendra Nath Bose in Calcutta that derived Planck’s radiation law by using an implicit assumption that light quanta are indistinguishable particles, Einstein initiated quantum statistics for gases. He also predicted the existence of a new state of matter, a so-called Bose-Einstein condensate in dilute gases, first produced in a laboratory seventy years later. These contributions built upon the “old quantum theory” but also signficantly influenced Schrödinger in the development of his wave equation in 1926. Nonetheless by the end of 1927 Einstein had turned against the quantum theory (linking its statistical character with its irrealism), proposed (and rejected) a “hidden variables” alternative, and initiated a decades-long debate with atomic physicist and founder of “complementarity”, Niels Bohr.
Conveniently visiting at Caltech in Pasadena, California when Hitler became Chancellor of Germany in January 1933, Einstein never returned; it was rumored that Nazi newspapers secretly placed a bounty of $5,000 (approximately $90,000 in 2016 dollars) on his head.7 World famous, he quickly became the most visible refugee from Hitler’s Germany. One of six initial faculty members of the Institute of Advanced Study, he worked tirelessly from 1933 to help émigré scientists and researchers. In August 1939, he signed the well-known letter to President Franklin D. Roosevelt regarding the possibility of an atomic bomb. Becoming a naturalized US citizen in 1940, he was a persistent vocal advocate for racial justice in the USA, for nuclear disarmament, and for universal human rights. In the McCarthy period of the early 1950s, he strongly and publicly supported those who refused to testify regarding their political views at government hearings. An offer from the new state of Israel to become its first president was turned down in 1952. His last signed letter, in April 1955, was to Bertrand Russell, lending his name to a joint manifesto urging the complete renunciation of nuclear weapons by all nations.
Philosophically, these years are of interest on account of Einstein’s sharpened critique of the quantum theory centering on several arguments that the theory is incomplete. Although that argument is imperfectly expressed in the Einstein-Podolsky-Rosen (EPR) paper of 1935, Einstein’s correspondence of the time (especially with Schrödinger) reveals its source in the twin philosophical principles of separability and locality, deployed in the quantum context of what Schrödinger dubbed “entanglement”. More than a generation of quantum physicists treated Einstein’s concerns as the philosophical prejudices of a young revolutionary turned old reactionary (if not the signs of scientific senility). Nevertheless, after Northern Irish physicist John S. Bell revived interest in the EPR paper in 1964, the ensuing study of entanglement continues to produce a rich harvest of significant results in the foundations of physics and currently underwrites the newly emerging field of quantum information theory. Einstein’s “other argument” that quantum mechanics is incomplete began with an example prior to, and analogous to, that of Schrödinger’s famous cat in 1935. That argument concerns quantum mechanical universality and so its characterization of the transition from the quantum realm, where the superposition principle of quantum mechanics is fundamental, to the macroscopic realm of classical objects, where it is irrelevant. The problem of the existence of stable macroscopic objects in a fundamentally quantum world remains today; it is the subject (“decoherence”) of an active branch of quantum research dealing with the coupling between macroscopic objects and their surrounding environment.
Einstein’s philosophical engagement with, and critique of, the quantum theory in these years should be considered in conjunction with his field-theoretic attempts to frame a comprehensive theory of what he termed the “total field”, from which quantum mechanics would emerge as a statistical approximation. That enterprise failed and in his stubborn pursuit of it, Einstein patiently suffered the ridicule of a younger generation of theorists eager to extend the quantum theory to the new physics of the nucleus and to quantum field theory. Though he defended himself by stating that he “had earned the right to make mistakes”, a more generous appraisal from the vantage point of more than sixty years is possible. The quest for unity in the foundations of physics that remained his lifelong passion was rekindled on a different, quantum, basis by superstring theory in the 1980s and continues to thrive. Will it, or any other such attempt, ultimately succeed? Einstein’s response, as fitting now as it was when given in 1950, is to answer that question with a smile.8
1Albert Einstein: Letters to Solovine. New York: Philosophical Library, 1987, pp. 62–3.
2“Autobiographical Remarks”, pp. 6–7.
3CPAE 5 (1993), Doc. 27.
4“der glücklichste Gedanke meins Lebens”, in Grundgedanken und Methoden der Relativitätstheorie in ihrer Entwicklung dargestellt (“Fundamental Ideas and Methods of the Theory of Relativity, Presented in Their Development”), draft of what is probably an article intended to appear in English translation in the British journal Nature but withdrawn by Einstein and never published. CPAE 7 (2002), Doc. 31.
5Stachel, John, “Einstein’s Search for General Covariance, 1912–1915”, in D. Howard and J. Stachel (eds.), Einstein and the History of General Relativity (Einstein Studies v. 1), Basel, Boston, Berlin: Birkhäuser, 1989, pp. 63–100. This paper is based on the written version of a talk circulated since 1980. Also Norton, John, “How Einstein Found His Field Equations, 1912–1915”, pp. 101–59 in the same volume.
6Eddington, Arthur, The Expanding Universe. Cambridge, UK: Cambridge University Press, 1933, p. 21.
7Vallentin, Antonina, The Drama of Albert Einstein. New York: Doubleday, 1954, p. 231. Vallentin was a friend of Einstein’s second wife, Elsa Löwenthal.
8“It seems to me a smile is the best answer”. “Physics, Philosophy, and Scientific Progress”, a speech delivered in English to the International Congress of Surgeons in Cleveland, Ohio in 1950. Text reprinted in Physics Today (June 2005), pp. 46–8; p. 48.
Clark, Ronald W., Einstein: The Life and Times. New York and Cleveland: World Publishing Company, 1971. Although superseded in many respects by more recent biographies, Clark’s is both well-written and full of interesting details later authors omit.
Fölsing, Albrecht, Albert Einstein: A Biography. Translated by Ewald Osers. New York: Viking Penguin, 1997. Unfortunately, the English edition of this biography omits a chapter on Einstein and philosophy in the 1993 German edition, entitled “Written in Honey” (“in Honig geschrieben”).
Hoffmann, Dieter, Einstein’s Berlin: In the Footsteps of Genius. Baltimore, MD: Johns Hopkins University Press, 2013.
Isaccson, Walter, Einstein: His Life and Universe. New York: Simon and Schuster, 2007.