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Albert Einstein (1879–1955)

Einstein remade the foundations of physics twice — first with special relativity (1905), which unified space and time and established the constancy of the speed of light, then with general relativity (1915), which recast gravity as the geometry of curved spacetime. His 1905 paper on the photoelectric effect — proposing that light comes in discrete quanta — extended Planck’s quantum hypothesis from matter to radiation and opened the path to quantum mechanics. His paper on Brownian motion the same year provided the decisive evidence for the atomic hypothesis that Boltzmann had defended and died without seeing confirmed. And his later critique of quantum mechanics — the EPR paradox, the insistence that the theory is incomplete — sharpened the conceptual questions that the interpretive debates of the century after him are still contesting. The physics of the twentieth century runs through Einstein at every turn: he contributed to the quantum revolution, built the relativistic framework, and then spent thirty years arguing that the revolution he helped start was fundamentally wrong.

Life

Born 14 March 1879 in Ulm, Kingdom of Württemberg, German Empire. Grew up in Munich, where his father Hermann and uncle Jakob ran an electrical-engineering company. The family was secular Jewish. Einstein attended the Luitpold Gymnasium but left without a diploma (accounts differ on the circumstances). Completed his secondary education at the cantonal school in Aarau, Switzerland, then studied physics and mathematics at the Swiss Federal Polytechnic (ETH) in Zurich (diploma, 1900). Could not obtain an academic position after graduation.

Patent examiner at the Swiss Federal Patent Office in Bern (1902–09). The patent office years were his most productive. In 1905 — his annus mirabilis — he published four papers that each transformed a branch of physics: on the photoelectric effect (light quanta), on Brownian motion (confirmation of atoms), on special relativity (the electrodynamics of moving bodies), and on mass-energy equivalence (E = mc²).

Professor at the University of Zurich (1909), then Prague (1911), then ETH Zurich (1912), then the Prussian Academy of Sciences and the University of Berlin (1914–33). General relativity was completed in November 1915 and confirmed by Arthur Eddington’s observation of light deflection during the 1919 solar eclipse — an event that made Einstein an international public figure. Nobel Prize in Physics (1921, awarded 1922) for the photoelectric effect, not for relativity.

Emigrated from Germany in December 1932, before the Nazi seizure of power. Never returned. Professor at the Institute for Advanced Study in Princeton (1933–55). Became a U.S. citizen in 1940. Signed the letter to President Roosevelt warning of the possibility of atomic weapons (1939), drafted by Leó Szilárd; did not work on the Manhattan Project. Spent the last decades pursuing a unified field theory that would combine gravity and electromagnetism — a programme that did not succeed. Died 18 April 1955 in Princeton, New Jersey.

Relativity

Special relativity (1905). Two postulates: the laws of physics are the same in all inertial reference frames, and the speed of light in a vacuum is constant regardless of the motion of the source or the observer. The second postulate follows from Maxwell’s equations but is incompatible with Newtonian mechanics. Einstein drew the consequence: it is Newtonian mechanics, not Maxwell’s electrodynamics, that must be revised. The result: space and time are not absolute but relative — they depend on the observer’s state of motion. Simultaneity is relative (events simultaneous in one frame are not in another). Time dilates and lengths contract at high velocities. Mass and energy are equivalent (E = mc²). The luminiferous aether — the medium physicists had assumed electromagnetic waves propagate through — is unnecessary and does not exist.

General relativity (1915). Gravity is not a force acting at a distance (Newton) but the curvature of spacetime caused by the presence of mass and energy. Matter tells spacetime how to curve; spacetime tells matter how to move. The mathematical framework is Riemannian geometry, applied to a four-dimensional spacetime manifold. The theory predicts: the deflection of light by gravity (confirmed 1919), the gravitational redshift of light (confirmed 1959), the existence of gravitational waves (predicted 1916, detected 2015 by LIGO), and the expansion of the universe (predicted by the equations, initially resisted by Einstein, confirmed by Edwin Hubble in 1929). General relativity remains the best-tested theory of gravity; it is the framework within which Penrose and Hawking proved the singularity theorems.

The quantum and its discontents

Einstein’s relationship to quantum mechanics is complex: he was one of its founders and its most persistent critic. His position was realist and deterministic: “I still believe in the possibility of a model of reality — that is to say, of a theory which represents things themselves and not merely the probability of their occurrence.” The belief shaped both his contributions and his objections.

The photoelectric effect (1905). Light striking a metal surface ejects electrons, but only if the light’s frequency exceeds a threshold — regardless of intensity. Classical wave theory cannot explain this. Einstein proposed that light itself comes in quanta (later called photons), each carrying energy hν (Planck’s constant times the frequency). The proposal was radical — it went beyond Planck, who had quantised the oscillators in a heated body but not radiation itself. Planck resisted Einstein’s step for over a decade. The photoelectric effect was the basis of Einstein’s Nobel Prize.

The EPR paradox (1935). Einstein, Boris Podolsky, and Nathan Rosen argued that quantum mechanics is incomplete. Their thought experiment: two particles that have interacted and then separated retain correlated properties. Measuring one particle instantaneously determines the state of the other, no matter how far apart they are. Either quantum mechanics is incomplete (there are hidden variables determining the outcomes in advance) or there is “spooky action at a distance” — instantaneous influence across space. Einstein considered the second option unacceptable and concluded that quantum mechanics must be incomplete.

John Bell showed in 1964 that no local hidden-variable theory can reproduce all the predictions of quantum mechanics — that the EPR correlations violate an inequality that any local realistic theory must satisfy. Alain Aspect’s experiments (1982) confirmed that nature violates Bell’s inequality: the quantum correlations are real, and local realism fails. Einstein’s preferred resolution — local hidden variables — is ruled out by experiment. Whether this vindicates “spooky action at a distance” or points to a deeper framework (such as Rovelli’s relational quantum mechanics, which dissolves the paradox by relativising the states) remains debated.

Where Einstein stops

Einstein spent the last three decades of his career pursuing a unified field theory — a single mathematical framework unifying gravity (general relativity) and electromagnetism (Maxwell’s equations). The programme did not succeed. The mainstream of physics pursued a different route: quantum field theory, the standard model of particle physics, and the eventual search for quantum gravity. Einstein’s unified-field programme was isolated from these developments; by the time of his death, it was widely regarded as a brilliant physicist’s misdirected final act. Whether the isolation reflected a genuine dead end or a premature judgment by a physics community that had moved on is occasionally revisited but has not been resolved in Einstein’s favour.

His critique of quantum mechanics — that it is incomplete, that “God does not play dice” — was principled, not merely stubborn. The EPR argument is a genuine contribution: it identified the entanglement phenomenon that Bell later proved has no local-realist explanation. But Einstein’s preferred resolution (local hidden variables) was eliminated by Bell’s theorem and the subsequent experiments. The deeper question Einstein raised — whether quantum mechanics provides a complete description of physical reality — remains open in the sense that the interpretation of quantum mechanics is unresolved. But the specific form in which Einstein posed the question (local hidden variables vs. completeness) has been answered against him.

The cosmological constant is Einstein’s most instructive error. He introduced it in 1917 to permit a static universe — the prevailing assumption. When Hubble showed the universe is expanding, Einstein removed it, reportedly calling it his “biggest blunder.” In 1998, observations of distant supernovae showed that the expansion of the universe is accelerating — a phenomenon best described by a positive cosmological constant. The constant Einstein introduced for the wrong reason turned out to be real, for a different reason. The episode illustrates both the reach of Einstein’s mathematics (the equations permitted what the observations later required) and the limits of his physical intuition (he preferred a static universe and could not foresee that the constant would find a different use).

Key works

See also: Maxwell · Planck · Rovelli · Relational quantum mechanics