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Einstein was born at Ulm in Germany where his father was a manufacturer of electrical equipment. Business failure led his father to move the family first to Munich, where Einstein entered the local gymnasium in 1889, and later to Milan. There were no early indications of Einstein's later achievements for he did not begin to talk until the age of three, nor was he fluent at the age of nine, causing his parents to fear that he might even be backward. It appears that in 1894 he was expelled from his Munich gymnasium on the official grounds that his presence was disruptive. At this point he did something rather remarkable for a fifteen-year-old boy. He had developed such a hatred for things German that he could no longer bear to be a German citizen. He persuaded his father to apply for a revocation of his son's citizenship, a request the authorities granted in 1896. Until 1901, when he obtained Swiss citizenship, he was in fact stateless.

After completing his secondary education at Aarao in Switzerland he passed the entrance examination, at the second attempt, to the Swiss Federal Institute of Technology, Zurich, in 1896. He did not appear to be a particularly exceptional student finding the process of working for examinations repellent. Disappointed not to be offered an academic post, he survived as a private tutor until 1902, when he obtained the post of technical expert, third class, in the Swiss Patent Office in Bern. Here he continued to think about and work on physical problems. In 1905 he published four papers in the journal Annalen der Physik (Annals of Physics) – works that were to direct the progress of physics during the 20th century.

The first, and most straightforward, was on Brownian motion – first described by Robert Brown in 1828. Einstein derived a formula for the average displacement of particles in suspension, based on the idea that the motion is caused by bombardment of the particles by molecules of the liquid. The formula was confirmed by Jean Perrin in 1908 – it represented the first direct evidence for the existence of atoms and molecules of a definite size. The paper was entitled Über der von molekularkinetischen Theorie der Wärme gerförderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen (On the Motion of Small Particles Suspended in a Stationary Liquid According to the Molecular Kinetic Theory of Heat).

His second paper of 1905 was Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt (On a Heuristic Point of View about the Creation and Conversion of Light). In this Einstein was concerned with the nature of electromagnetic radiation, which at the time was regarded as a wave propagated throughout space according to Clerk Maxwell's equations. Einstein was concerned with the difference between this wave picture and the theoretical picture physicists had of matter. His particular concern in this paper was the difficulty in explaining the photoelectric effect, investigated in 1902 by Philipp Lenard. It was found that ultraiolet radiation of low frequency could eject electrons from a solid surface. The number of electrons depended on the intensity of the radiation and the energy of the electrons depended on the frequency. This dependence on frequency was difficult to explain using classical theory.

Einstein resolved this by suggesting that electromagnetic radiation is a flow of discreet particles – quanta (or photons as they are now known). The intensity of the radiation is the flux of these quanta. The energy per quantum, he proposed, was hν, where ν is the frequency of the radiation and h is the constant introduced in 1900 by Max Planck. In this way Einstein was able to account for the observed photoelectric behavior. The work was one of the early results introducing the quantum theory into physics and it won for Einstein the 1921 Nobel Prize for physics.

The third of his 1905 papers is the one that is the most famous:Zur Elektrodynamik bewegter Körper (On the Electrodynamics of Moving Bodies). It is this paper that first introduced the special theory of relativity to science. The term ‘special’ denotes that the theory is restricted to certain special circumstances – namely for bodies at rest or moving with uniform relative velocities.

The theory was developed to account for a major problem in physics at the time. Traditionally in mechanics, there was a simple procedure for treating relative velocities. A simple example is of a car moving along a road at 40 mph with a second car moving toward it at 60 mph. A stationary observer would say that the second car was moving at 60 mph relative to him. The driver of the first car would say that, relative to him, the second car was approaching at 100 mph. This common-sense method of dealing with relative motion was well established. The mathematical equations involved are called the Galilean transformations – they are simple equations for changing velocities in one frame of reference to another frame of reference. The problem was that the method did not appear to work for electromagnetic radiation, which was thought of as a wave motion through the ether, described by the equations derived by Maxwell. In these, the speed of light is independent of the motion of the source or the observer. At the time, Albert Michelson and Edward Morley had performed a series of experiments to attempt to detect the Earth's motion through the ether, with negative results. Hendrik Lorentz proposed that this result could be explained by a change of size of moving bodies (the Lorentz–Fitzgerald contraction).

Although Einstein was unaware of the Michelson–Morley experiment, he did appreciate the incompatibility of classical mechanics and classical electrodynamics. His solution was a quite radical one. He proposed that the speed of light is a constant for all frames of reference that are moving uniformly relative to each other. He also put forward his ‘relativity principle’ that the laws of nature are the same in all frames of reference moving uniformly relative to each other. To reconcile the two principles he abandoned the Galilean transformations – the simple method of adding and subtracting velocities for bodies in relative motion. He arrived at this rejection by arguments about the idea of simultaneity – showing that the time between two events depends on the motions of the bodies involved. In his special theory of relativity, Einstein rejected the ideas of absolute space and absolute time. Later it was developed in terms of events specified by three spacial coordinates and one coordinate of time – a space–time continuum.

The theory had a number of unusual consequences. Thus the length of a body along its direction of motion decreases with increasing velocity. The mass increases as the velocity increases, becoming infinitely large in theory at the speed of light. Time slows down for a moving body – a phenomenon known as time dilation. These effects apply to all bodies but only become significant at velocities close to the speed of light – under normal conditions the effects are so small that classical laws appear to be obeyed. However, the predictions of the special theory – unusual as they may seem – have been verified experimentally. Thus increase in mass is observed for particles accelerated in a synchrocyclotron. Similarly, the lifetimes of unstable particles are increased at high velocities.

In that same year of 1905 Einstein had one more fundamental paper to contribute: Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig? (Does the Inertia of a Body Depend on its Energy Content?). It was in this two-page paper that he concluded that if a body gives off energy E in the form of radiation, its mass diminishes by E/c² (where c is the velocity of light) obtaining the celebrated equation E = mc² relating mass and energy.

Within a short time Einstein's work on relativity was widely recognized to be original and profound. In 1908 he obtained an academic post at the University of Bern. Over the next three years he held major posts at Zurich (1909), Prague (1911), and the Zurich Federal Institute of Technology (1912) before taking a post in Berlin in 1914. This was probably due in part to the respect in which he held the Berlin physicists, Max Planck and Walther Nernst.

By 1907 Einstein was ready to remove the restrictions imposed on the special theory showing that, on certain assumptions, accelerated motion could be incorporated into his new, general theory of relativity. The theory begins with the fact that the mass of a body can be defined in two ways. The inertial mass depends on the way it resists change in motion, as in Newton's second law. The gravitational mass depends on forces of gravitational attraction between masses. The two concepts – inertia and gravity – seem dissimilar yet the inertial and gravitational masses of a body are always the same. Einstein considered that this was unlikely to be a coincidence and it became the basis of his principle of equivalence.

The principle states that it is impossible to distinguish between an inertial force (that is, an accelerating force) and a gravitational one; the two are, in fact, equivalent. The point can be demonstrated with a thought experiment. Consider an observer in an enclosed box somewhere in space far removed from gravitational forces. Suppose that the box is suddenly accelerated upward, followed by the observer releasing two balls of different weights. Subject to an inertial force they will both fall to the floor at the same rate. But this is exactly how they would behave if the box was in a gravitational field and the observer could conclude that the balls fall under the influence of gravity. It was on the basis of this equivalence that Einstein made his dramatic prediction that rays of light in a gravitational field move in a curved path. For if a ray of light enters the box at one side and exits at the other then, with the upward acceleration of the box, it will appear to exit at a point lower down than its entrance. But if we take the equivalence principle seriously we must expect to find the same effect in a gravitational field.

In 1911 he predicted that starlight just grazing the Sun should be deflected by 0.83 seconds ('') of arc, later increased to 1''.7, which, though small, should be detectable in a total eclipse by the apparent displacement of the star from its usual position. In 1919 such an eclipse took place; it was observed by Arthur Eddington at Principe in West Africa, who reported a displacement of 1''.61, well within the limits of experimental error. It was from this moment that Einstein became known to a wider public, for this dramatic confirmation of an unexpected phenomenon seemed to capture the popular imagination. Even the London Times was moved to comment in an editorial, as if to a recalcitrant government, that “the scientific conception of the fabric of the universe must be changed.”

In 1916 Einstein was ready to publish the final and authoritative form of his general theory: Die Grundlage der allgemeinen Relativitätstheorie (The Foundation of the General Theory of Relativity). It is this work that gained for Einstein the reputation for producing theories that were comprehensible to the very few. Eddington on being informed that there were only three people capable of understanding the theory is reported to have replied, “Who's the third?” It is true that Einstein introduced into gravitational theory a type of mathematics that was then unfamiliar to most physicists thus presenting an initial impression of incomprehension. In his theory Einstein used the space–time continuum introduced by Hermann Minkowski in 1907, the non-Euclidean geometry developed by Bernhard Riemann in 1854, and the tensor calculus published by Gregorio Ricci in 1887. He was assisted in the mathematics by his friend Grossmann. The theory of gravitation produced is one that depends on the geometry of space–time. In simple terms, the idea is that a body ‘warps’ the space around it so that another body moves in a curved path (hence the notion that space is curved). Einstein and Grossmann in 1915 succeeded in deriving a good theoretical value for the small (and hitherto anomalous) advance in the perihelion of mercury. The theory was put to an early test. Because of perturbations in the orbit of Mercury produced by the gravitational attractions of other planets, its perihelion (point in the orbit closest to the Sun) actually precesses by a small amount (9' 34'' per century). When these perturbation effects were calculated on the basis of Newtonian mechanics, they could only account for a precession rate of 8' 51'' per century, a figure 43'' too small. In 1915 Einstein, while completing his 1916 paper on General Relativity, calculated Mercury's perihelion precession on the basis of his own theory and found that, without making any extra assumptions, the missing 43'' were accounted for. The discovery, Einstein later reported, gave him palpitations and “for a few days I was beside myself with joyous excitement.”

The theory also predicted (1907) that electromagnetic radiation in a strong gravitation field would be shifted to longer wavelengths – the Einstein shift. This was used by Walter Adams in 1925 to explain the spectrum of Sirius B. In 1959 Robert Pound and Glen Rebka demonstrated it on Earth using the Mossbauer effect. They found that at a height of 75 feet (23 m) above the ground gamma rays from a radioactive source had a longer wavelength than at ground level. Physicists have been less successful, however, with the prediction in 1916 of the existence of gravitational waves. Despite an intensive search from 1964 onward by Joseph Weber and others, they have yet to be detected.

Einstein was less successful in applying his theory to the construction of a cosmological model of the universe, which he assumed to be uniform in density, static, and lacking infinite distances. He found himself forced to complicate his equations with a cosmological constant, λ. It was left to Aleksandr Friedmann in 1922 to show that the term could be dropped and a solution found that yielded an expanding universe, a solution that Einstein eventually adopted. He later described the cosmological constant as “my greatest mistake.”

By the early 1920s Einstein's great work was virtually complete. He wrote in 1921 that: “Discovery in the grand manner is for young people … and hence for me a thing of the past.” From the early 1920s he rejected quantum theory – the theory he had done much to establish himself. His basic objection was to the later formulation that included the probability interpretation. “God does not play dice,” he said, and, “He may be subtle, but he is not malicious.” He felt, like Louis de Broglie, that although the new quantum mechanics was clearly a powerful and successful theory it was an imperfect one, with an underlying undiscovered deterministic basis. For the last 30 years of his life he also pursued a quest for a unified field theory – a single theory to explain both electromagnetic and gravitational fields. He published several attempts at such a theory but all were inadequate. This work was carried out right up to his death.

Also, from about 1925 onward, Einstein engaged in a debate with Niels Bohr on the soundness of quantum theory. He would present Bohr with a series of thought experiments, which seemed undeniable even though they were clearly incompatible with quantum mechanics. The best known of these was presented in a paper written with Boris Podolsky and Nathan Rosen and entitled: Can Quantum Mechanical Description of Physical Reality Be Considered Complete? (1935). The EPR experiment, as it soon became known, assumed that, after interacting, two particles become widely separated. Quantum theory allows the total momentum of the pair (A, B) to be measured accurately. Thus if the momentum of B is also measured accurately, it is a simple matter, as momentum is conserved, to calculate the momentum of particle A. We can then measure the position of A with as much precision as is practically possible. It would therefore seem to follow that, without violating any laws of physics, both the position and momentum of particle A have been accurately determined. But, according to the uncertainty principle of Heisenberg, we are prevented from ever knowing accurately a particle's position and momentum simultaneously.

The paper troubled Bohr. He spent six weeks going through the text word by word and analyzing every possibility. Eventually he saw that the measurements of A's position and momentum are separate and distinct. The uncertainty principle insisted that nosingle measurement could determine a particle's precise position and momentum, and this central claim remained unchallenged by the EPR experiment. The EPR experiment has continued to trouble people interested in the fundamental principles of quantum mechanics. In 1964 the British physicist John Bell published an important theoretical paper indicating how the experiment might be done in practice. Einstein's correspondence with Bohr about quantum mechanics is published as the Bohr–Einstein letters.

Einstein was also involved in a considerable amount of political activity. When Hitler came to power in 1933 Einstein made his permanent home in America where he worked at Princeton. In 1939 he was persuaded to write to President Roosevelt warning him about the possibility of an atomic bomb and urging American research. He was, in later years, a convinced campaigning pacifist. He was also a strong supporter of Zionist causes and, on the death of Chaim Weizmann in 1952 was asked to become president of Israel, but declined.