Relativity

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In 1905 Albert Einstein was twenty-six, an examiner at the patent office in Bern because no university post had materialized, and in that single year he sent the Annalen der Physik four papers. The third, received on June 30 and published as Zur Elektrodynamik bewegter Körper — On the Electrodynamics of Moving Bodies — rebuilt space and time from two premises and algebra a strong student could follow. A three-page sequel received in September drew out of it the most famous equation in science. The apparatus throughout is homely — trains, clocks, measuring rods, light signals — and the conclusions are not. Within fifteen years the theory had been confirmed at a solar eclipse, its author had become the most famous scientist alive, and the physics was leaking into an occult conversation about the fourth dimension already forty years old.

Two postulates and a three-page sequel

Special relativity begins by surrendering an assumption so habitual it scarcely looks like one: that two distant events either happen at the same moment or do not, absolutely, for everyone. Einstein kept two principles instead. The first, the principle of relativity, holds that the laws of physics are identical in every uniformly moving frame of reference — no experiment performed inside a smoothly traveling ship can disclose that the ship is moving. The second he stated in a single clause: “light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.” The two looked irreconcilable, since a beam of light ought to be gained on, at least a little, by anyone chasing it. The 1905 paper dissolved the conflict by asking what simultaneity could mean in practice — what clocks, synchronized by light signals, could actually verify — and the price of consistency turned out to be simultaneity itself. Observers in relative motion disagree about which distant events happen at the same time, and nothing above the frames adjudicates. From that one concession the rest follows deductively: moving clocks run slow, moving rods contract along their direction of motion, velocities no longer simply add, and nothing carrying energy or information overtakes light. None of it is a trick of perspective. The muons born of cosmic rays high in the atmosphere reach the ground in numbers only time dilation allows; they survive the trip because their internal clocks genuinely run slower.

The September sequel, Does the Inertia of a Body Depend Upon Its Energy-Content?, ends with a sentence whose consequences took decades to arrive: “The mass of a body is a measure of its energy-content”. Mass and energy are interconvertible — E = mc², in the notation that settled later — and Einstein suggested the relation might be tested on radium salts, whose energy content measurably changes. It is the bookkeeping beneath nuclear binding energies and the light of the sun.

From geometry to gravity

Three years later the theory acquired its geometry. In September 1908, addressing a scientific congress in Cologne, Hermann Minkowski — who had taught Einstein mathematics in Zurich — announced that space by itself and time by itself were finished as independent things, and that only a kind of union of the two would keep an independent reality. In that union an event is a point fixed by four coordinates, a life is a worldline, and the theory’s real content is not that everything is relative but the opposite: the spacetime interval between events, like the speed of light, is invariant — the same for every observer, however moving. The four-dimensional arena itself, with its metric and its light cones, is the subject of spacetime. Einstein, at first unimpressed by the reformulation, took it up wholesale when gravity forced geometry on him.

The forcing began in 1907, at the patent office, with what he later called the happiest thought of his life: a person falling freely does not feel his own weight. Locally, gravity and acceleration are indistinguishable — the equivalence principle — and from it Einstein concluded that gravitation is not a force transmitted through space but a feature of spacetime itself. Eight years of work ended in Berlin in November 1915 with the field equations that bear his name: matter and energy curve spacetime, and freely falling bodies follow the straightest paths the curvature affords. The first vindication was already on the books. Mercury’s perihelion had been advancing beyond what Newtonian accounting could explain — an anomaly Urbain Le Verrier logged in 1859, refined by later measurement to about 43 seconds of arc per century, and never closed. Einstein’s November 1915 calculation yielded the missing 43 arcseconds with no parameter adjusted to fit.

A century of verdicts

The theory also predicted that starlight grazing the sun would bend by 1.75 arcseconds — twice the deflection a Newtonian argument allows — and testing that required a total eclipse. In May 1919 two British expeditions organized by the Astronomer Royal Frank Watson Dyson and by Arthur Stanley Eddington photographed the darkened sun from Sobral, in Brazil, and from the island of Príncipe off West Africa. The results, presented to a joint session of the Royal Society and the Royal Astronomical Society on November 6, 1919, favored Einstein, and the press coverage that followed — in Britain, then America, then Germany — made him an international celebrity within days and set off what historians of science call the “Einstein boom” of popular books. The data have an afterhistory worth telling straight. The principal Sobral telescope lost focus, its plates gave a value nearer Newton’s, and the published analysis leaned on the smaller, sharper instrument; later critics — most visibly Harry Collins and Trevor Pinch in The Golem — charged Eddington with keeping the data his theory wanted. A 1979 remeasurement of the Sobral plates with a modern plate-measuring machine, and the historian Daniel Kennefick’s reconstruction of the original analysis, have substantially cleared the expeditions: the discounted instrument was discounted for instrumental reasons, and the measured deflection stands.

The verdicts kept arriving for a century. Gravitational redshift — clocks deeper in a gravitational field running slower — passed from astronomical hints to precision in a laboratory program beginning in 1959, the Pound–Rebka experiment at its head; in 1976 Gravity Probe A flew a maser clock to ten thousand kilometers and confirmed the shift to 0.007 percent. Atomic clocks flown around the world in the Hafele–Keating experiment measured special- and general-relativistic time dilation together. The Global Positioning System is not an experiment, but it runs only because its satellite clocks are corrected for both effects; engineers on the first satellite doubted the correction would be needed, and the system itself ended the doubt. From 1974 the orbit of the Hulse–Taylor binary pulsar decayed exactly as energy loss to gravitational radiation requires; on September 14, 2015, the LIGO interferometers recorded the waves themselves, from the merger of two black holes — the detection designated GW150914, announced in February 2016 — and in April 2019 the Event Horizon Telescope published the first image of a black hole’s shadow, in the galaxy M87, in agreement with the theory’s prediction. A century of opportunities to fail, and the theory took none of them.

What it did to time

The deepest export was temporal. If observers in relative motion slice the four-dimensional whole into different presents, then no moment is the universe’s own present, and the question of whether the future already exists stops being idle. The argument from the relativity of simultaneity to eternalism — the view that all events, past and future, are equally real — and the serious resistance that argument has met are the business of the block universe entry. What fastened the idea to Einstein personally was a letter. In March 1955, weeks before his own death, consoling the family of Michele Besso — his oldest friend, the only person thanked in the 1905 paper — he wrote that Besso had departed this strange world a little ahead of him, and that it meant nothing: “For us believing physicists the distinction between past, present, and future only has the meaning of an illusion, though a persistent one.” It is a sentence of consolation in a private letter, not a theorem, and the distance between those two things is where the philosophical argument lives.

The famous twin paradox, meanwhile, is stated more often than it is stated correctly. A twin who travels out near the speed of light and returns is younger than the one who stayed; the outcome is measured physics, not appearance. The seeming symmetry — each twin regards the other as the one in motion — fails because the traveler turns around: one worldline is inertial throughout and the other is not, and relativity assigns less elapsed time to the bent path without contradiction. The clocks flown around the world aged unequally in just the way the geometry says.

The fourth dimension the occultists already had

The occult fourth dimension is older than the physical one, and it was a different thing. In November and December 1877 the Leipzig astrophysicist Johann Karl Friedrich Zöllner — holder of the university’s chair of astrophysics, with genuine photometric work to his name — held sittings at his home with the American medium Henry Slade, inviting witnesses of the rank of the physicist Wilhelm Weber and the psychophysicist Gustav Fechner. The design was explicitly dimensional: a being with access to a fourth spatial direction should be able to tie a knot in a closed loop of cord, or move an object out of a sealed space without breaching it, as easily as a hand lifts a coin out of a circle drawn around it on paper — the logic the later literature of teleportation would inherit. Zöllner published the sittings as Transcendental Physics (1878), partly translated into English in 1880, and counted them successes; Wilhelm Wundt, who attended, judged the controls unsatisfactory, and the long record of Slade’s career shows a practiced conjurer. The fiasco accomplished something durable anyway: it installed the fourth dimension in the séance room’s vocabulary as the address of the spirits. In the same years the English mathematician Charles Howard Hinton was domesticating higher space for general readers — the 1880 essay What Is the Fourth Dimension?, cube exercises meant to train four-dimensional intuition, the word tesseract, first used in his A New Era of Thought (1888), and a run of Scientific Romances in which higher space carried a moral charge: from a four-dimensional vantage, separateness looks like a defect of perception. By the new century the idea was common property in esoteric writing, and its most systematic heir was P. D. Ouspensky, whose Tertium Organum (1912) made the apprehension of time as a higher dimension the key to consciousness — three years before general relativity was finished. That doctrine, and the six-dimensional cosmos he later raised on it, are set out in Ouspenskian cosmology.

Minkowski’s fourth dimension is not that room. The occult fourth dimension was spatial — another direction to move in, adjacent to every point of this world, from which locked boxes open and knots untie. Relativity’s fourth coordinate is time, and it enters the geometry with opposite sign to the three directions of space; nothing in the theory supplies a hidden chamber beside the visible world. The two ideas share a number and a name and almost nothing else. After November 1919, the distinction never had a chance.

The absorption after 1919

The Einstein boom sold its books to a public schooled for four decades to expect revelations from the fourth dimension, and the absorption was immediate. Much of it was simple misprision: the theory’s name seemed to announce that science had ratified the relativity of all things, morals included, when its content is a set of invariants every observer shares — the slogan and the mathematics point in opposite directions. The philosophers’ scramble was its own spectacle, positivists and neo-Kantians and realists each claiming the theory as their vindication. But the durable esoteric purchase came from inside physics. Eddington, who had gone to Príncipe himself and became the theory’s great English-language expositor, set the physics inside a quasi-Kantian idealism he eventually named “selective subjectivism”: his Space, Time and Gravitation (1920) was a semi-popular best-seller, and The Nature of the Physical World (1928) drew from the new physics a case that the world’s substance lies closer to mind than to mechanism. James Jeans’s The Mysterious Universe (1930) went further down the same road, offering an enormous readership a universe nearer to a thought than to a machine. These were working scientists publishing under their own names, contested by colleagues in their own time, and they lent the mystical reception of modern physics a respectability it has drawn on ever since. The genre they seeded migrated, after the quantum revolution of the 1920s, from relativity to the quantum, where its modern descendants live; that account runs through quantum physics.

The paper trail

The primary documents are short, readable, and online. The two 1905 papers can be read in English translation at Fourmilab — On the Electrodynamics of Moving Bodies and Does the Inertia of a Body Depend Upon Its Energy-Content? — and Einstein’s own popular exposition of both theories, written in 1916 and translated in 1920, can be read complete at Relativity: The Special and General Theory. The documentary base of Einstein scholarship is the Einstein Papers Project’s Collected Papers of Albert Einstein, whose digital edition presents the volumes in the original German with English translation and annotation. Among the lives, Abraham Pais’s Subtle is the Lord (1982) is the scientific biography, written by a physicist who knew Einstein at Princeton and works through the calculations; Walter Isaacson’s Einstein: His Life and Universe (2007) is the standard general life. Daniel Kennefick’s No Shadow of a Doubt (2019) re-examines the 1919 expeditions and their data at book length. And the contest over what the theory meant — fought while the newspapers were deciding what it meant on their own — is surveyed in the Stanford Encyclopedia of Philosophy’s Early Philosophical Interpretations of General Relativity.

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