Einstein, Albert

I INTRODUCTION

Einstein, Albert (1879-1955), German-born American physicist and Nobel laureate, best known as the creator of the special and general theories of relativity and for his bold hypothesis concerning the particle nature of light. He is perhaps the most well-known scientist of the 20th century.

Einstein was born in Ulm on March 14, 1879, and spent his youth in Munich, where his family owned a small shop that manufactured electric machinery. He did not talk until the age of three, but even as a youth he showed a brilliant curiosity about nature and an ability to understand difficult mathematical concepts. At the age of 12 he taught himself Euclidean geometry.

Einstein hated the dull regimentation and unimaginative spirit of school in Munich. When repeated business failure led the family to leave Germany for Milan, Italy, Einstein, who was then 15 years old, used the opportunity to withdraw from the school. He spent a year with his parents in Milan, and when it became clear that he would have to make his own way in the world, he finished secondary school in Aarau, Switzerland, and entered the Swiss Federal Institute of Technology in Zürich. Einstein did not enjoy the methods of instruction there. He often cut classes and used the time to study physics on his own or to play his beloved violin. He passed his examinations and graduated in 1900 by studying the notes of a classmate. His professors did not think highly of him and would not recommend him for a university position.

For two years Einstein worked as a tutor and substitute teacher. In 1902 he secured a position as an examiner in the Swiss patent office in Bern. In 1903 he married Mileva Marić, who had been his classmate at the polytechnic. They had two sons but eventually divorced. Einstein later remarried.

II EARLY SCIENTIFIC PUBLICATIONS

In 1905 Einstein received his doctorate from the University of Zürich for a theoretical dissertation on the dimensions of molecules, and he also published three theoretical papers of central importance to the development of 20th-century physics. In the first of these papers, on Brownian motion, he made significant predictions about the motion of particles that are randomly distributed in a fluid. These predictions were later confirmed by experiment.

The second paper, on the photoelectric effect, contained a revolutionary hypothesis concerning the nature of light. Einstein not only proposed that under certain circumstances light can be considered as consisting of particles, but he also hypothesized that the energy carried by any light particle, called a photon, is proportional to the frequency of the radiation. The formula for this is E = hν, where E is the energy of the radiation, h is a universal constant known as Planck’s constant, and ν is the frequency of the radiation. This proposal—that the energy contained within a light beam is transferred in individual units, or quanta—contradicted a hundred-year-old tradition of considering light energy a manifestation of continuous processes. Virtually no one accepted Einstein’s proposal. In fact, when the American physicist Robert Andrews Millikan experimentally confirmed the theory almost a decade later, he was surprised and somewhat disquieted by the outcome.

Einstein, whose prime concern was to understand the nature of electromagnetic radiation, subsequently urged the development of a theory that would be a fusion of the wave and particle models for light. Again, very few physicists understood or were sympathetic to these ideas.

III EINSTEIN’S SPECIAL THEORY OF RELATIVITY

Einstein’s third major paper in 1905, “On the Electrodynamics of Moving Bodies,” contained what became known as the special theory of relativity. Since the time of the English mathematician and physicist Sir Isaac Newton, natural philosophers (as physicists and chemists were known) had been trying to understand the nature of matter and radiation, and how they interacted in some unified world picture. The position that mechanical laws are fundamental has become known as the mechanical world view, and the position that electrical laws are fundamental has become known as the electromagnetic world view. Neither approach, however, is capable of providing a consistent explanation for the way radiation (light, for example) and matter interact when viewed from different inertial frames of reference, that is, an interaction viewed simultaneously by an observer at rest and an observer moving at uniform speed.

In the spring of 1905, after considering these problems for ten years, Einstein realized that the crux of the problem lay not in a theory of matter but in a theory of measurement. At the heart of his special theory of relativity was the realization that all measurements of time and space depend on judgments as to whether two distant events occur simultaneously. This led him to develop a theory based on two postulates: the principle of relativity, that physical laws are the same in all inertial reference systems, and the principle of the invariance of the speed of light, that the speed of light in a vacuum is a universal constant. He was thus able to provide a consistent and correct description of physical events in different inertial frames of reference without making special assumptions about the nature of matter or radiation, or how they interact. Virtually no one understood Einstein’s argument.

IV EARLY REACTIONS TO EINSTEIN

The difficulty that others had with Einstein’s work was not because it was too mathematically complex or technically obscure; the problem resulted, rather, from Einstein’s beliefs about the nature of good theories and the relationship between experiment and theory. Although he maintained that the only source of knowledge is experience, he also believed that scientific theories are the free creations of a finely tuned physical intuition and that the premises on which theories are based cannot be connected logically to experiment. A good theory, therefore, is one in which a minimum number of postulates is required to account for the physical evidence. This sparseness of postulates, a feature of all Einstein’s work, was what made his work so difficult for colleagues to comprehend, let alone support.

Einstein did have important supporters, however. His chief early patron was the German physicist Max Planck. Einstein remained at the patent office for four years after his star began to rise within the physics community. He then moved rapidly upward in the German-speaking academic world; his first academic appointment was in 1909 at the University of Zürich. In 1911 he moved to the German-speaking university at Prague, and in 1912 he returned to the Swiss National Polytechnic in Zürich. Finally, in 1914, he was appointed director of the Kaiser Wilhelm Institute for Physics in Berlin.

V THE GENERAL THEORY OF RELATIVITY

Even before he left the patent office in 1907, Einstein began work on extending and generalizing the theory of relativity to all coordinate systems. He began by enunciating the principle of equivalence, a postulate that gravitational fields are equivalent to accelerations of the frame of reference. For example, people in a moving elevator cannot, in principle, decide whether the force that acts on them is caused by gravitation or by a constant acceleration of the elevator. The full general theory of relativity was not published until 1916. In this theory the interactions of bodies, which heretofore had been ascribed to gravitational forces, are explained as the influence of bodies on the geometry of space-time (four-dimensional space, a mathematical abstraction, having the three dimensions from Euclidean space and time as the fourth dimension).

On the basis of the general theory of relativity, Einstein accounted for the previously unexplained variations in the orbital motion of the planets and predicted the bending of starlight in the vicinity of a massive body such as the sun. The confirmation of this latter phenomenon during an eclipse of the sun in 1919 became a media event, and Einstein’s fame spread worldwide.

For the rest of his life Einstein devoted considerable time to generalizing his theory even more. His last effort, the unified field theory, which was not entirely successful, was an attempt to understand all physical interactions—including electromagnetic interactions and weak and strong interactions—in terms of the modification of the geometry of space-time between interacting entities.

Most of Einstein’s colleagues felt that these efforts were misguided. Between 1915 and 1930 the mainstream of physics was in developing a new conception of the fundamental character of matter, known as quantum theory. This theory contained the feature of wave-particle duality (light exhibits the properties of a particle, as well as of a wave) that Einstein had earlier urged as necessary, as well as the uncertainty principle, which states that precision in measuring processes is limited. Additionally, it contained a novel rejection, at a fundamental level, of the notion of strict causality. Einstein, however, would not accept such notions and remained a critic of these developments until the end of his life. “God,” Einstein once said, “does not play dice with the world.”

VI WORLD CITIZEN

After 1919, Einstein became internationally renowned. He accrued honors and awards, including the Nobel Prize in physics in 1921, from various world scientific societies. His visit to any part of the world became a national event; photographers and reporters followed him everywhere. While regretting his loss of privacy, Einstein capitalized on his fame to further his own political and social views.

The two social movements that received his full support were pacifism and Zionism. During World War I he was one of a handful of German academics willing to publicly decry Germany’s involvement in the war. After the war his continued public support of pacifist and Zionist goals made him the target of vicious attacks by anti-Semitic and right-wing elements in Germany. Even his scientific theories were publicly ridiculed, especially the theory of relativity.

When Hitler came to power, Einstein immediately decided to leave Germany for the United States. He took a position at the Institute for Advanced Study at Princeton, New Jersey. While continuing his efforts on behalf of world Zionism, Einstein renounced his former pacifist stand in the face of the awesome threat to humankind posed by the Nazi regime in Germany.

In 1939 Einstein collaborated with several other physicists in writing a letter to President Franklin D. Roosevelt, pointing out the possibility of making an atomic bomb and the likelihood that the German government was embarking on such a course. The letter, which bore only Einstein’s signature, helped lend urgency to efforts in the U.S. to build the atomic bomb, but Einstein himself played no role in the work and knew nothing about it at the time.

After the war, Einstein was active in the cause of international disarmament and world government. He continued his active support of Zionism but declined the offer made by leaders of the state of Israel to become president of that country. In the U.S. during the late 1940s and early ‘50s he spoke out on the need for the nation’s intellectuals to make any sacrifice necessary to preserve political freedom. Einstein died in Princeton on April 18, 1955.

Einstein’s efforts in behalf of social causes have sometimes been viewed as unrealistic. In fact, his proposals were always carefully thought out. Like his scientific theories, they were motivated by sound intuition based on a shrewd and careful assessment of evidence and observation. Although Einstein gave much of himself to political and social causes, science always came first, because, he often said, only the discovery of the nature of the universe would have lasting meaning. His writings include Relativity: The Special and General Theory (1916); About Zionism (1931); Builders of the Universe (1932); Why War? (1933), with Sigmund Freud; The World as I See It (1934); The Evolution of Physics (1938), with the Polish physicist Leopold Infeld; and Out of My Later Years (1950). Einstein’s collected papers are being published in a multivolume work, beginning in 1987.

Contributed By: Samuel Glasstone

Einstein, Albert

b. March 14, 1879, Ulm, Württemberg, Ger.

d. April 18, 1955, Princeton, N.J., U.S.

German-American physicist who developed the special and general theories of relativity and won the Nobel Prize for Physics in 1921 for his explanation of the photoelectric effect. Recognized in his own time as one of the most creative intellects in human history, in the first 15 years of the 20th century Einstein advanced a series of theories that proposed entirely new ways of thinking about space, time, and gravitation. His theories of relativity and gravitation were a profound advance over the old Newtonian physics and revolutionized scientific and philosophic inquiry.

Herein lay the unique drama of Einstein's life. He was a self-confessed lone traveler; his mind and heart soared with the cosmos, yet he could not armour himself against the intrusion of the often horrendous events of the human community. Almost reluctantly he admitted that he had a "passionate sense of social justice and social responsibility." His celebrity gave him an influential voice that he used to champion such causes as pacifism, liberalism, and Zionism. The irony for this idealistic man was that his famous postulation of an energy-mass equation, which states that a particle of matter can be converted into an enormous quantity of energy, had its spectacular proof in the creation of the atomic and hydrogen bombs, the most destructive weapons ever known.

Early life and career

In 1880, the year after Einstein's birth, his family moved from Ulm to Munich, where Hermann Einstein, his father, and Jakob Einstein, his uncle, set up a small electrical plant and engineering works. In Munich Einstein attended rigidly disciplined schools. Under the harsh and pedantic regimentation of 19th-century German education, which he found intimidating and boring, he showed little scholastic ability. At the behest of his mother, Einstein also studied music; though throughout life he played exclusively for relaxation, he became an accomplished violinist. It was then only Uncle Jakob who stimulated in Einstein a fascination for mathematics and Uncle Cäsar Koch who stimulated a consuming curiosity about science.

By the age of 12 Einstein had decided to devote himself to solving the riddle of the "huge world." Three years later, with poor grades in history, geography, and languages, he left school with no diploma and went to Milan to rejoin his family, who had recently moved there from Germany because of his father's business setbacks. Albert Einstein resumed his education in Switzerland, culminating in four years of physics and mathematics at the renowned Federal Polytechnic Academy in Zürich.

After his graduation in the spring of 1900, he became a Swiss citizen, worked for two months as a mathematics teacher, and then was employed as examiner at the Swiss patent office in Bern. With his newfound security, Einstein married his university sweetheart, Mileva Maric, in 1903.

Early in 1905 Einstein published in the prestigious German physics monthly Annalen der Physik a thesis, "A New Determination of Molecular Dimensions," that won him a Ph.D. from the University of Zürich. Four more important papers appeared in Annalen that year and forever changed man's view of the universe.

The first of these, "Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen" ("On the Motion--Required by the Molecular Kinetic Theory of Heat--of Small Particles Suspended in a Stationary Liquid"), provided a theoretical explanation of. In "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt" ("On a Heuristic Viewpoint Concerning the Production and Transformation of Light"), Einstein postulated that light is composed of individual quanta (later called photons) that, in addition to wavelike behaviour, demonstrate certain properties unique to particles. In a single stroke he thus revolutionized the theory of light and provided an explanation for, among other phenomena, the emission of electrons from some solids when struck by light, called the photoelectric effect.

Einstein's special theory of relativity, first printed in "Zur Elektrodynamik bewegter Körper" ("On the Electrodynamics of Moving Bodies"), had its beginnings in an essay Einstein wrote at age 16. The precise influence of work by other physicists on Einstein's special theory is still controversial. The theory held that if, for all frames of reference, the speed of light is constant and if all natural laws are the same, then both time and motion are found to be relative to the observer.

In the mathematical progression of the theory, Einstein published his fourth paper, "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?" ("Does the Inertia of a Body Depend Upon Its Energy Content?"). This mathematical footnote to the special theory of relativity established the equivalence of mass and energy, according to which the energy E of a quantity of matter, with mass m, is equal to the product of the mass and the square of the velocity of light, c. This relationship is commonly expressed in the form E = mc².

Public understanding of this new theory and acclaim for its creator were still many years off, but Einstein had won a place among Europe's most eminent physicists, who increasingly sought his counsel, as he did theirs. While Einstein continued to develop his theory, attempting now to encompass with it the phenomenon of gravitation, he left the patent office and returned to teaching--first in Switzerland, briefly at the German University in Prague, where he was awarded a full professorship, and then, in the winter of 1912, back at the Polytechnic in Zürich. He was later remembered from this time as a very happy man, content in his marriage and delighted with his two young sons, Hans Albert and Edward.

In April 1914 the family moved to Berlin, where Einstein had accepted a position with the Prussian Academy of Sciences, an arrangement that permitted him to continue his researches with only the occasional diversion of lecturing at the University of Berlin. His wife and two sons vacationed in Switzerland that summer and, with the eruption of World War I, were unable to return to Berlin. A few years later this enforced separation was to lead to divorce. Einstein abhorred the war and was an outspoken critic of German militarism among the generally acquiescent academic community in Berlin, but he was primarily engrossed in perfecting his general theory of relativity, which he published in Annalen der Physik as "Die Grundlagen der allgemeinen Relativitätstheorie" ("The Foundation of the General Theory of Relativity") in 1916. The heart of this postulate was that gravitation is not a force, as Newton had said, but a curved field in the space-time continuum, created by the presence of mass. This notion could be proved or disproved, he suggested, by measuring the deflection of starlight as it traveled close by the Sun, the starlight being visible only during a total eclipse. Einstein predicted twice the light deflection that would be accountable under Newton's laws.

His new equations also explained for the first time the puzzling irregularity--that is, the slight advance--in the planet Mercury's perihelion, and they demonstrated why stars in a strong gravitational field emitted light closer to the red end of the spectrum than those in a weaker field.

While Einstein awaited the end of the war and the opportunity for his theory to be tested under eclipse conditions, he became more and more committed to pacifism, even to the extent of distributing pacifist literature to sympathizers in Berlin. His attitudes were greatly influenced by the French pacifist and author Romain Rolland, whom he met on a wartime visit to Switzerland. Rolland's diary later provided the best glimpse of Einstein's physical appearance as he reached his middle 30s:

Einstein is still a young man, not very tall, with a wide and long face, and a great mane of crispy, frizzled and very black hair, sprinkled with gray and rising high from a lofty brow. His nose is fleshy and prominent, his mouth small, his lips full, his cheeks plump, his chin rounded. He wears a small cropped mustache. (By permission of Madame Marie Romain Rolland.)

Einstein's view of humanity during the war period appears in a letter to his friend, the Austrian-born Dutch physicist Paul Ehrenfest:

The ancient Jehovah is still abroad. Alas, he slays the innocent along with the guilty, whom he strikes so fearsomely blind that they can feel no sense of guilt. . . . We are dealing with an epidemic delusion which, having caused infinite suffering, will one day vanish and become a monstrous and incomprehensible source of wonderment to later generations. (From Otto Nathan and Heinz Norden [eds.], Einstein on Peace; Simon and Schuster, 1960.)

It would be said often of Einstein that he was naïve about human affairs; for example, with the proclamation of the German Republic and the armistice in 1918, he was convinced that militarism had been thoroughly abolished in Germany.

International acclaim

International fame came to Einstein in November 1919, when the Royal Society of London announced that its scientific expedition to the island of Príncipe, in the Gulf of Guinea, had photographed the solar eclipse on May 29 of that year and completed calculations that verified the predictions made in Einstein's general theory of relativity. Few could understand relativity, but the basic postulates were so revolutionary and the scientific community was so obviously bedazzled that the physicist was acclaimed the greatest genius on Earth. Einstein himself was amazed at the reaction and apparently displeased, for he resented the consequent interruptions of his work. After his divorce he had, in the summer of 1919, married Elsa, the widowed daughter of his late father's cousin. He lived quietly with Elsa and her two daughters in Berlin, but, inevitably, his views as a foremost savant were sought on a variety of issues.

Despite the now deteriorating political situation in Germany, Einstein attacked nationalism and promoted pacifist ideals. With the rising tide of anti-Semitism in Berlin, Einstein was castigated for his "Bolshevism in physics," and the fury against him in right-wing circles grew when he began publicly to support the Zionist movement. Judaism had played little part in his life, but he insisted that, as a snail can shed his shell and still be a snail, so a Jew can shed his faith and still be a Jew.

Although Einstein was regarded warily in Berlin, such was the demand for him in other European cities that he traveled widely to lecture on relativity, usually arriving at each place by third-class rail carriage, with a violin tucked under his arm. So successful were his lectures that one enthusiastic impresario guaranteed him a three-week booking at the London Palladium. He ignored the offer but, at the request of the Zionist leader Chaim Weizmann, toured the United States in the spring of 1921 to raise money for the Palestine Foundation Fund. Frequently treated like a circus freak and feted from morning to night, Einstein nevertheless was gratified by the standards of scientific research and the "idealistic attitudes" that he found prevailing in the United States.

During the next three years Einstein was constantly on the move, journeying not only to European capitals but also to Asia, to the Middle East, and to South America. According to his diary notes, he found nobility among the Hindus of Ceylon (now Sri Lanka), a pureness of soul among the Japanese, and a magnificent intellectual and moral calibre among the Jewish settlers in Palestine. His wife later wrote that, on steaming into one new harbour, Einstein had said to her, "Let us take it all in before we wake up."

In Shanghai a cable reached him announcing that he had been awarded the 1921 Nobel Prize for Physics "for your photoelectric law and your work in the field of theoretical physics." Relativity, still the centre of controversy, was not mentioned.

Though the 1920s were tumultuous times of wide acclaim and some notoriety, Einstein did not waver from his new search--to find the mathematical relationship between electromagnetism and gravitation. This would be a first step, he felt, in discovering the common laws governing the behaviour of everything in the universe, from the electron to the planets. He sought to relate the universal properties of matter and energy in a single equation or formula, in what came to be called a unified field theory. This turned out to be a fruitless quest that occupied the rest of his life. Einstein's peers generally agreed quite early that his search was destined to fail because the rapidly developing quantum theory uncovered an uncertainty principle in all measurements of the motion of particles: the movement of a single particle simply could not be predicted because of a fundamental uncertainty in measuring simultaneously both its speed and its position, which means, in effect, that the future of any physical system at the subatomic level cannot be predicted. While fully recognizing the brilliance of quantum mechanics, Einstein rejected the idea that these theories were absolute and persevered with his theory of general relativity as the more satisfactory foundation to future discovery. He was widely quoted on his belief in an exactly engineered universe: "God is subtle but he is not malicious." On this point, he parted company with most theoretical physicists. The distinguished German quantum theorist Max Born, a close friend of Einstein, said at the time: "Many of us regard this as a tragedy, both for him, as he gropes his way in loneliness, and for us, who miss our leader and standard-bearer." This appraisal, and others pronouncing his work in later life as largely wasted effort, will have to await the judgment of later generations.

The year of Einstein's 50th birthday, 1929, marked the beginning of the ebb flow of his life's work in a number of aspects. Early in the year the Prussian Academy published the first version of his unified field theory, but, despite the sensation it caused, its very preliminary nature soon became apparent. The reception of the theory left him undaunted, but Einstein was dismayed by the preludes to certain disaster in the field of human affairs: Arabs launched savage attacks on Jewish colonists in Palestine; the Nazis gained strength in Germany; the League of Nations proved so impotent that Einstein resigned abruptly from its Committee on Intellectual Cooperation as a protest to its timidity; and the stock market crash in New York City heralded worldwide economic crisis.

Crushing Einstein's natural gaiety more than any of these events was the mental breakdown of his younger son, Edward. Edward had worshipped his father from a distance but now blamed him for deserting him and for ruining his life. Einstein's sorrow was eased only slightly by the amicable relationship he enjoyed with his older son, Hans Albert.

As visiting professor at the University of Oxford in 1931, Einstein spent as much time espousing pacifism as he did discussing science. He went so far as to authorize the establishment of the Einstein War Resisters' International Fund in order to bring massive public pressure to bear on the World Disarmament Conference, scheduled to meet in Geneva in February 1932. When these talks foundered, Einstein felt that his years of supporting world peace and human understanding had accomplished nothing. Bitterly disappointed, he visited Geneva to focus world attention on the "farce" of the disarmament conference. In a rare moment of fury, Einstein stated to a journalist,

They [the politicians and statesmen] have cheated us. They have fooled us. Hundreds of millions of people in Europe and in America, billions of men and women yet to be born, have been and are being cheated, traded and tricked out of their lives and health and well-being.

Shortly after this, in a famous exchange of letters with the Austrian psychiatrist Sigmund Freud, Einstein suggested that people must have an innate lust for hatred and destruction. Freud agreed, adding that war was biologically sound because of the love-hate instincts of man and that pacifism was an idiosyncrasy directly related to Einstein's high degree of cultural development. This exchange was only one of Einstein's many philosophic dialogues with renowned men of his age. With Rabindranath Tagore, Hindu poet and mystic, he discussed the nature of truth. While Tagore held that truth was realized through man, Einstein maintained that scientific truth must be conceived as a valid truth that is independent of humanity. "I cannot prove that I am right in this, but that is my religion," said Einstein. Firmly denying atheism, Einstein expressed a belief in "Spinoza's God who reveals himself in the harmony of what exists." The physicist's breadth of spirit and depth of enthusiasm were always most evident among truly intellectual men. He loved being with the physicists Paul Ehrenfest and Hendrik A. Lorentz at The Netherlands' Leiden University, and several times he visited the California Institute of Technology in Pasadena to attend seminars at the Mt. Wilson Observatory, which had become world renowned as a centre for astrophysical research. At Mt. Wilson he heard the Belgian scientist Abbé Georges Lemaître detail his theory that the universe had been created by the explosion of a "primeval atom" and was still expanding. Gleefully, Einstein jumped to his feet, applauding. "This is the most beautiful and satisfactory explanation of creation to which I have ever listened," he said.

In 1933, soon after Adolf Hitler became chancellor of Germany, Einstein renounced his German citizenship and left the country. He later accepted a full-time position as a foundation member of the school of mathematics at the new Institute for Advanced Study in Princeton, New Jersey. In reprisal, Nazi storm troopers ransacked his beloved summer house at Caputh, near Berlin, and confiscated his sailboat. Einstein was so convinced that Nazi Germany was preparing for war that, to the horror of Romain Rolland and his other pacifist friends, he violated his pacifist ideals and urged free Europe to arm and recruit for defense.

Although his warnings about war were largely ignored, there were fears for Einstein's life. He was taken by private yacht from Belgium to England. By the time he arrived in Princeton in October 1933, he had noticeably aged. A friend wrote,

It was as if something had deadened in him. He sat in a chair at our place, twisting his white hair in his fingers and talking dreamily about everything under the sun. He was not laughing any more.

Later years in the United States

In Princeton Einstein set a pattern that was to vary little for more than 20 years. He lived with his wife in a simple, two-story frame house and most mornings walked a mile or so to the Institute, where he worked on his unified field theory and talked with colleagues. For relaxation he played his violin and sailed on a local lake. Only rarely did he travel, even to New York. In a letter to Queen Elisabeth of Belgium, he described his new refuge as a "wonderful little spot, . . . a quaint and ceremonious village of puny demigods on stilts." Eventually he acquired American citizenship, but he always continued to think of himself as a European. Pursuing his own line of theoretical research outside the mainstream of physics, he took on an air of fixed serenity. "Among my European friends, I am now called Der grosse Schweiger ("The Great Stone Face"), a title I well deserve," he said. Even his wife's death late in 1936 did not disturb his outward calm. "It seemed that the difference between life and death for Einstein consisted only in the difference between being able and not being able to do physics," wrote Leopold Infeld, the Polish physicist who arrived in Princeton at this time.

Niels Bohr, the great Danish atomic physicist, brought news to Einstein in 1939 that the German refugee physicist Lise Meitner had split the uranium atom, with a slight loss of total mass that had been converted into energy. Meitner's experiments, performed in Copenhagen, had been inspired by similar, though less precise, experiments done months earlier in Berlin by two German chemists, Otto Hahn and Fritz Strassmann. Bohr speculated that, if a controlled chain-reaction splitting of uranium atoms could be accomplished, a mammoth explosion would result. Einstein was skeptical, but laboratory experiments in the United States showed the feasibility of the idea. With a European war regarded as imminent and fears that Nazi scientists might build such a "bomb" first, Einstein was persuaded by colleagues to write a letter to President Franklin D. Roosevelt urging "watchfulness and, if necessary, quick action" on the part of the United States in atomic-bomb research. This recommendation marked the beginning of the Manhattan Project.

Although he took no part in the work at Los Alamos, New Mexico, and did not learn that a nuclear-fission bomb had been made until Hiroshima was razed in 1945, Einstein's name was emphatically associated with the advent of the atomic age. He readily joined those scientists seeking ways to prevent any future use of the bomb, his particular and urgent plea being the establishment of a world government under a constitution drafted by the United States, Britain, and Russia. With the spur of the atomic fear that haunted the world, he said "we must not be merely willing, but actively eager to submit ourselves to the binding authority necessary for world security." Once more, Einstein's name surged through the newspapers. Letters and statements tumbled out of his Princeton study, and in the public eye Einstein the physicist dissolved into Einstein the world citizen, a kind "grand old man" devoting his last years to bringing harmony to the world.

The rejection of his ideals by statesmen and politicians did not break him, because his prime obsession still remained with physics. "I cannot tear myself away from my work," he wrote at the time. "It has me inexorably in its clutches." In proof of this came his new version of the unified field in 1950, a most meticulous mathematical essay that was immediately but politely criticized by most physicists as untenable.

Compared with his renown of a generation earlier, Einstein was virtually neglected and said himself that he felt almost like a stranger in the world. His health deteriorated to the extent that he could no longer play the violin or sail his boat. Many years earlier, chronic abdominal pains had forced him to give up smoking his pipe and to watch his diet carefully.

Einstein died in his sleep at Princeton Hospital. On his desk lay his last incomplete statement, written to honour Israeli Independence Day. It read in part: "What I seek to accomplish is simply to serve with my feeble capacity truth and justice at the risk of pleasing no one." His contribution to man's understanding of the universe was matchless, and he is established for all time as a giant of science. Broadly speaking, his crusades in human affairs seem to have had no lasting impact. Einstein perhaps anticipated such an assessment of his life when he said, "Politics are for the moment. An equation is for eternity."

Einstein's 1905 trilogy

In a few months during the years 1665-66, Newton discovered the composite nature of light, analyzed the action of gravity, and invented the mathematical technique now known as calculus--or so he recalled in his old age. The only person who has ever matched Newton's amazing burst of scientific creativity--three revolutionary discoveries within a year--was Albert Einstein, who in 1905 published the special theory of relativity, the quantum theory of radiation, and a theory of Brownian movement that led directly to the final acceptance of the atomic structure of matter.

Relativity theory has already been mentioned several times in this article, an indication of its close connection with several areas of physical science. There is no room here to discuss the subtle line of reasoning that Einstein followed in arriving at his amazing conclusions; a brief summary of his starting point and some of the consequences will have to suffice.

In his 1905 paper on the electrodynamics of moving bodies, Einstein called attention to an apparent inconsistency in the usual presentation of Maxwell's electromagnetic theory as applied to the reciprocal action of a magnet and a conductor. The equations are different depending on which is "at rest" and which is "moving," yet the results must be the same. Einstein located the difficulty in the assumption that absolute space exists; he postulated instead that the laws of nature are the same for observers in any inertial frame of reference and that the speed of light is the same for all such observers.

From these postulates Einstein inferred: (1) an observer in one frame would find from his own measurements that lengths of objects in another frame are contracted by an amount given by the Lorentz-FitzGerald formula; (2) each observer would find that clocks in the other frame run more slowly; (3) there is no absolute time--events that are simultaneous in one frame of reference may not be so in another; and (4) the observable mass of any object increases as it goes faster.

Closely connected with the mass-increase effect is Einstein's famous formula E = mc²: mass and energy are no longer conserved but can be interconverted. The explosive power of the atomic and hydrogen bombs derives from the conversion of mass to energy.

In a paper on the creation and conversion of light (usually called the "photoelectric effect paper"), published earlier in 1905, Einstein proposed the hypothesis that electromagnetic radiation consists of discrete energy quanta that can be absorbed or emitted only as a whole. Although this hypothesis would not replace the wave theory of light, which gives a perfectly satisfactory description of the phenomena of diffraction, reflection, refraction, and dispersion, it would supplement it by also ascribing particle properties to light.

Until recently the invention of the quantum theory of radiation was generally credited to another German physicist, Max Planck, who in 1900 discussed the statistical distribution of radiation energy in connection with the theory of blackbody radiation. Although Planck did propose the basic hypothesis that the energy of a quantum of radiation is proportional to its frequency of vibration, it is not clear whether he used this hypothesis merely for mathematical convenience or intended it to have a broader physical significance. In any case, he did not explicitly advocate a particle theory of light before 1905. Historians of physics still disagree on whether Planck or Einstein should be considered the originator of the quantum theory.

Einstein's paper on Brownian movement seems less revolutionary than the other 1905 papers because most modern readers assume that the atomic structure of matter was well established at that time. Such was not the case, however. In spite of the development of the chemical atomic theory and of the kinetic theory of gases in the 19th century, which allowed quantitative estimates of such atomic properties as mass and diameter, it was still fashionable in 1900 to question the reality of atoms. This skepticism, which does not seem to have been particularly helpful to the progress of science, was promoted by the empiricist, or "positivist," philosophy advocated by Auguste Comte, Ernst Mach, Wilhelm Ostwald, Pierre Duhem, Henri Poincaré, and others. It was the French physicist Jean Perrin who, using Einstein's theory of Brownian movement, finally convinced the scientific community to accept the atom as a valid scientific concept.

The Einstein-de Sitter universe

In 1932 Einstein and de Sitter proposed that the cosmological constant should be set equal to zero, and they derived a homogeneous and isotropic model that provides the separating case between the closed and open Friedmann models; i.e., Einstein and de Sitter assumed that the spatial curvature of the universe is neither positive nor negative but rather zero. The spatial geometry of the Einstein-de Sitter universe is Euclidean (infinite total volume), but space-time is not globally flat (i.e., not exactly the space-time of special relativity). Time again commences with a big bang and the galaxies recede forever, but the recession rate (Hubble's "constant") asymptotically coasts to zero as time advances to infinity.

Because the geometry of space and the gross evolutionary properties are uniquely defined in the Einstein-de Sitter model, many people with a philosophical bent have long considered it the most fitting candidate to describe the actual universe. During the late 1970s strong theoretical support for this viewpoint came from considerations of particle physics (the model of inflation to be discussed below), and mounting, but as yet undefinitive, support also seems to be gathering from astronomical observations.

Einstein and the photoelectric effect

In 1905 Einstein extended Planck's hypothesis to explain the photoelectric effect, which is the emission of electrons by a metal surface when it is irradiated by light or X rays. The kinetic energy of the emitted electrons depends on the frequency of the radiation, not on its intensity; for a given metal, there is a threshold frequency ₀ below which no electrons are emitted. Furthermore, emission takes place as soon as the light shines on the surface; there is no detectable delay. Einstein showed that these results can be explained by two assumptions: (1) that light is composed of corpuscles or photons, the energy of which is given by Planck's relationship, and (2) that an atom in the metal can absorb either a whole photon or nothing. Part of the energy of the absorbed photon frees an electron, which requires a fixed energy W, known as the work function of the metal; the rest is converted into the kinetic energy ¹/₂m_eu² of the emitted electron (m_e is the mass of the electron and u is its velocity). Thus, the energy relation is

If is less than ₀, where h₀ = W, no electrons are emitted. Not all the experimental results mentioned above were known in 1905, but all Einstein's predictions have been verified since.

Gravitation and the geometry of space-time

The physical foundation of Einstein's view of gravitation, general relativity, lies on two empirical findings that he elevated to the status of basic postulates. The first postulate is the relativity principle: local physics is governed by the theory of special relativity. The second postulate is the equivalence principle: there is no way for an observer to distinguish locally between gravity and acceleration. The motivation for the second postulate comes from Galileo's observation that all objects--independent of mass, shape, colour, or any other property--accelerate at the same rate in a (uniform) gravitational field.

Einstein's theory of special relativity, which he developed in 1905, had as its basic premises (1) the notion (also dating back to Galileo) that the laws of physics are the same for all inertial observers and (2) the constancy of the speed of light in a vacuum--namely, that the speed of light has the same value (3 10¹⁰ cm/sec) for all inertial observers independent of their motion relative to the source of the light. Clearly, this second premise is incompatible with Euclidean and Newtonian precepts of absolute space and absolute time, resulting in a program that merged space and time into a single structure, with well-known consequences. The space-time structure of special relativity is often called "flat" because, among other things, the propagation of photons is easily represented on a flat sheet of graph paper with equal-sized squares. Let each tick on the vertical axis represent one light-year (9.46 10¹⁷ cm) of distance in the direction of the flight of the photon, and each tick on the horizontal axis represent the passage of one year (3.16 10⁷ sec) of time. The propagation path of the photon is then a 45 line because it flies one light-year in one year (with respect to the space and time measurements of all inertial observers no matter how fast they move relative to the photon).

The principle of equivalence in general relativity allows the locally flat space-time structure of special relativity to be warped by gravitation, so that (in the cosmological case) the propagation of the photon over thousands of millions of light-years can no longer be plotted on a globally flat sheet of paper. To be sure, the curvature of the paper may not be apparent when only a small piece is examined, thereby giving the local impression that space-time is flat (i.e., satisfies special relativity). It is only when the graph paper is examined globally that one realizes it is curved (i.e., satisfies general relativity).

In Einstein's 1917 model of the universe, the curvature occurs only in space, with the graph paper being rolled up into a cylinder on its side, a loop around the cylinder at constant time having a circumference of 2R--the total spatial extent of the universe. Notice that the "radius of the universe" is measured in a "direction" perpendicular to the space-time surface of the graph paper. Since the ringed space axis corresponds to one of three dimensions of the actual world (any will do since all directions are equivalent in an isotropic model), the radius of the universe exists in a fourth spatial dimension (not time) which is not part of the real world. This fourth spatial dimension is a mathematical artifice introduced to represent diagrammatically the solution (in this case) of equations for curved three-dimensional space that need not refer to any dimensions other than the three physical ones. Photons traveling in a straight line in any physical direction have trajectories that go diagonally (at 45 angles to the space and time axes) from corner to corner of each little square cell of the space-time grid; thus, they describe helical paths on the cylindrical surface of the graph paper, making one turn after traveling a spatial distance 2R. In other words, always flying dead ahead, photons would return to where they started from after going a finite distance without ever coming to an edge or boundary. The distance to the "other side" of the universe is therefore R, and it would lie in any and every direction; space would be closed on itself.

Now, except by analogy with the closed two-dimensional surface of a sphere that is uniformly curved toward a centre in a third dimension lying nowhere on the two-dimensional surface, no three-dimensional creature can visualize a closed three-dimensional volume that is uniformly curved toward a centre in a fourth dimension lying nowhere in the three-dimensional volume. Nevertheless, three-dimensional creatures could discover the curvature of their three-dimensional world by performing surveying experiments of sufficient spatial scope. They could draw circles, for example, by tacking down one end of a string and tracing along a single plane the locus described by the other end when the string is always kept taut in between (a straight line) and walked around by a surveyor. In Einstein's universe, if the string were short compared to the quantity R, the circumference of the circle divided by the length of the string (the circle's radius) would nearly equal 2 = 6.2837853 . . . , thereby fooling the three-dimensional creatures into thinking that Euclidean geometry gives a correct description of their world. However, the ratio of circumference to length of string would become less than 2 when the length of string became comparable to R. Indeed, if a string of length R could be pulled taut to the antipode of a positively curved universe, the ratio would go to zero. In short, at the tacked-down end the string could be seen to sweep out a great arc in the sky from horizon to horizon and back again; yet, to make the string do this, the surveyor at the other end need only walk around a circle of vanishingly small circumference.

To understand why gravitation can curve space (or more generally, space-time) in such startling ways, consider the following thought experiment that was originally conceived by Einstein. Imagine an elevator in free space accelerating upward, from the viewpoint of a woman in inertial space, at a rate numerically equal to g, the gravitational field at the surface of the Earth. Let this elevator have parallel windows on two sides, and let the woman shine a brief pulse of light toward the windows. She will see the photons enter close to the top of the near window and exit near the bottom of the far window because the elevator has accelerated upward in the interval it takes light to travel across the elevator. For her, photons travel in a straight line, and it is merely the acceleration of the elevator that has caused the windows and floor of the elevator to curve up to the flight path of the photons.

Let there now be a man standing inside the elevator. Because the floor of the elevator accelerates him upward at a rate g, he may--if he chooses to regard himself as stationary--think that he is standing still on the surface of the Earth and is being pulled to the ground by its gravitational field g. Indeed, in accordance with the equivalence principle, without looking out the windows (the outside is not part of his local environment), he cannot perform any local experiment that would inform him otherwise. Let the woman shine her pulse of light. The man sees, just like the woman, that the photons enter near the top edge of one window and exit near the bottom of the other. And just like the woman, he knows that photons propagate in straight lines in free space. (By the relativity principle, they must agree on the laws of physics if they are both inertial observers.) However, since he actually sees the photons follow a curved path relative to himself, he concludes that they must be bent by the force of gravity. The woman tries to tell him there is no such force at work; he is not an inertial observer. Nonetheless, he has the solidity of the Earth beneath him, so he insists on attributing his acceleration to the force of gravity. According to Einstein, they are both right. There is no need to distinguish locally between acceleration and gravity--the two are in some sense equivalent. But if that is the case, then it must be true that gravity--"real" gravity--can actually bend light. And indeed it can, as many experiments have shown since Einstein's first discussion of the phenomenon.

It was the genius of Einstein to go even further. Rather than speak of the force of gravitation having bent the photons into a curved path, might it not be more fruitful to think of photons as always flying in straight lines--in the sense that a straight line is the shortest distance between two points--and that what really happens is that gravitation bends space-time? In other words, perhaps gravitation is curved space-time, and photons fly along the shortest paths possible in this curved space-time, thus giving the appearance of being bent by a "force" when one insists on thinking that space-time is flat. The utility of taking this approach is that it becomes automatic that all test bodies fall at the same rate under the "force" of gravitation, for they are merely producing their natural trajectories in a background space-time that is curved in a certain fashion independent of the test bodies. What was a minor miracle for Galileo and Newton becomes the most natural thing in the world for Einstein.

To complete the program and to conform with Newton's theory of gravitation in the limit of weak curvature (weak field), the source of space-time curvature would have to be ascribed to mass (and energy). The mathematical expression of these ideas constitutes Einstein's theory of general relativity, one of the most beautiful artifacts of pure thought ever produced. The American physicist John Archibald Wheeler and his colleagues summarized Einstein's view of the universe in these terms:

Curved spacetime tells mass-energy how to move;

mass-energy tells spacetime how to curve.

Contrast this with Newton's view of the mechanics of the heavens:

Force tells mass how to accelerate;

mass tells gravity how to exert force.

Notice therefore that Einstein's worldview is not merely a quantitative modification of Newton's picture (which is also possible via an equivalent route using the methods of quantum field theory) but represents a qualitative change of perspective. And modern experiments have amply justified the fruitfulness of Einstein's alternative interpretation of gravitation as geometry rather than as force. His theory would have undoubtedly delighted the Greeks.

Relativistic mechanics

In classical physics, space is conceived as having the absolute character of an empty stage in which events in nature unfold as time flows onward independently; events occurring simultaneously for one observer are presumed to be simultaneous for any other; mass is taken as impossible to create or destroy; and a particle given sufficient energy acquires a velocity that can increase without limit. The special theory of relativity, developed principally by Einstein in 1905 and now so adequately confirmed by experiment as to have the status of physical law, shows that all these, as well as other apparently obvious assumptions, are false.

Specific and unusual relativistic effects flow directly from Einstein's two basic postulates, which are formulated in terms of so-called inertial reference frames. These are reference systems that move in such a way that in them Newton's first law, the law of inertia, is valid. The set of inertial frames consists of all those that move with constant velocity with respect to each other (accelerating frames therefore being excluded). Einstein's postulates are: (1) All observers, whatever their state of motion relative to a light source, measure the same speed for light; and (2) The laws of physics are the same in all inertial frames.

The first postulate, the constancy of the speed of light, is an experimental fact from which follow the distinctive relativistic phenomena of space contraction, time dilation, and the relativity of simultaneity: as measured by an observer assumed to be at rest, an object in motion is contracted along the direction of its motion, and moving clocks run slow; two spatially separated events that are simultaneous for a stationary observer occur sequentially for a moving observer. As a consequence, space intervals in three-dimensional space are related to time intervals, thus forming so-called four-dimensional space-time.

The second postulate is called the principle of relativity. It is equally valid in classical mechanics (but not in classical electrodynamics until Einstein reinterpreted it). This postulate implies, for example, that table tennis played on a train moving with constant velocity is just like table tennis played with the train at rest, the states of rest and motion being physically indistinguishable. In relativity theory, mechanical quantities such as momentum and energy have forms that are different from their classical counterparts but give the same values for speeds that are small compared to the speed of light, the maximum permissible speed in nature (about 300,000 kilometres per second, or 186,000 miles per second). According to relativity, mass and energy are equivalent and interchangeable quantities, the equivalence being expressed by Einstein's famous equation E = mc², where m is an object's mass and c is the speed of light.

The general theory of relativity, as discussed above, is Einstein's theory of gravitation, which uses the principle of the equivalence of gravitation and locally accelerating frames of reference. Einstein's theory has special mathematical beauty; it generalizes the "flat" space-time concept of special relativity to one of curvature. It forms the background of all modern cosmological theories (see Cosmos: Relativistic cosmologies ). In contrast to some vulgarized popular notions of it, which confuse it with moral and other forms of relativism, Einstein's theory does not argue that "all is relative." On the contrary, it is largely a theory based upon those physical attributes that do not change, or, in the language of the theory, that are invariant.

Scientific papers

"Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt," in Annalen der Physik (1905); "Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen," in Annalen der Physik (1905); "Zur Elektrodynamik bewegter Körper," in Annalen der Physik (1905), the initial paper on special relativity; "Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?" in Annalen der Physik (1905); "Zur Theorie der Brownschen Bewegung," in Annalen der Physik (1906), translated separately as Investigations on the Theory of the Brownian Movement (1926); "Zur Theorie der Lichterzeugung und Lichtabsorption," in Annalen der Physik (1906); "Plancksche Theorie der Strahlung und die Theorie der spezifischen Wärme," in Annalen der Physik (1907); "Entwurf einer Verallegemeinerten Relativitätstheorie und einer Theorie der Gravitation," in Zeitschrift für Mathematik und Physik (1913); "Grundlagen der allgemeinen Relativitätstheorie," in Annalen der Physik (1916), on the general theory of relativity; "Strahlungs-emission und -absorption nach der Quantentheorie," in Verhandlungen der Deutschen physikalischen Gesellschaft (1916); "Quantentheorie der Strahlung," in Physikalische Zeitschrift (1917); "Quantentheorie des einatomigen idealen Gases," in Sitzungsberichte der Preussischen Akademie der Wissenschaften (1924 and 1925). Some of Einstein's important papers were collected in the joint work (with H.A. Lorentz and H. Minkowski), H.A. Lorentz: Das Relativitätsprinzip, eine Sammlung von Abhandlungen (1913; trans. as H.A. Lorentz: The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity, 1923). See also The Meaning of Relativity, which includes the generalized theory of gravitation (1953), the first edition of Einstein's unified field theory.

Other works

About Zionism: Speeches and Letters, Eng. trans. by Sir Leon Simon (1931); Builders of the Universe (1932); with Sigmund Freud, Warum Krieg? (Why War?, Eng. trans. by Stuart Gilbert, 1933); with Leopold Infeld, The Evolution of Physics (1938); The World As I See It (Eng. trans. by Alan Harris, 1949); Out of My Later Years (1950).

Bibliography

John Stachel et al. (eds.), The Collected Papers of Albert Einstein (1987- ), contains all his papers, notes, and letters, with companion translation volumes. Helen Dukas and Banesh Hoffman (eds.), Albert Einstein, the Human Side: New Glimpses from His Archives (1979), samples the letters of Albert Einstein to provide a good introduction to his personality and thought.

Studies of his life and work include Philipp Frank, Einstein: His Life and Times, trans. from German (1947, reprinted 1989), a scientific biography focusing on Einstein's early life and achievement; Antonina Vallentin, The Drama of Albert Einstein (also published as Einstein, a Biography, 1954; originally published in French, 1954), a personal story of Einstein's European years; Peter Michelmore, Einstein: Profile of the Man (1962), a popular, richly anecdotal treatment of Einstein as man and scientist; Ronald W. Clark, Einstein: The Life and Times (1971, reissued 1984), a distinguished, definitive, and well-illustrated work; Banesh Hoffman and Helen Dukas, Albert Einstein: Creator and Rebel (1972, reissued 1986), a significant biography, laced with a thorough but exciting interpretation of Einstein's scientific work; Jeremy Bernstein, Einstein, 2nd ed. (1991), a biography emphasizing the scientific theories; Cornelius Lanczos, The Einstein Decade: 1905-1915 (1974), a biography that includes detailed synopses of each Einstein paper written during the years covered; A.P. French (ed.), Einstein: A Centenary Volume (1979), a collection of essays, reminiscences, illustrations, and quotations--for the general audience; Abraham Pais, "Subtle is the Lord": The Science and the Life of Albert Einstein (1982), a scientific biography; Lewis Pyenson, The Young Einstein: The Advent of Relativity (1985), setting the development of his ideas in their social and cultural context; Peter A. Bucky and Allen G. Weakland, The Private Albert Einstein (1992), a chronicle of conversations and personal anecdotes as remembered by one of Einstein's friends; Michael White and John Gribbin, Einstein: A Life in Science (1994); and Denis Brian, Einstein: A Life (1996).

Studies of Einstein's impact on science and philosophy include Paul Arthur Schilpp (ed.), Albert Einstein: Philosopher-Scientist, 3rd ed., 2 vol. (1970), a discussion by eminent scholars; Lincoln Barnett, The Universe and Dr. Einstein, 2nd rev. ed. (1957, reissued 1974), a lucid exposition of Einstein's contribution to science; Thomas F. Glick (ed.), The Comparative Reception of Relativity (1987); and David Cassidy, Einstein and Our World (1995).