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Arthur Eddington (1882–1944)

Eddington named the arrow of time. In The Nature of the Physical World (1928), he identified the second law of thermodynamics — entropy increases — as “the only law of physics that distinguishes past from future” and gave it the phrase that stuck: “the arrow of time.” The observation was not new (Boltzmann had established the statistical foundations; Clausius had formulated the second law), but Eddington’s formulation crystallised the question that the subsequent century of physics would pursue: why does time have a direction when the fundamental equations are time-symmetric? Eddington was also the astrophysicist who confirmed Einstein’s general relativity through the 1919 solar eclipse expedition, established the theory of stellar structure, and — in his later career — pursued a speculative “fundamental theory” that was widely judged to have failed.

Life

Born 28 December 1882 in Kendal, Westmorland, England, into a Quaker family. His father, the headmaster of a Quaker school, died of typhoid when Eddington was two. Educated at Brynmelyn School in Weston-super-Mare, then Owens College (later the University of Manchester), then Trinity College, Cambridge. Senior Wrangler in the Mathematical Tripos (1904) — the youngest at the time. Fellow of Trinity (1907).

Chief assistant at the Royal Observatory, Greenwich (1906–13). Plumian Professor of Astronomy and Director of the Cambridge Observatories (1913–44) — positions he held for the rest of his life. Elected Fellow of the Royal Society (1914). Knighted (1930). Order of Merit (1938). Royal Medal (1928).

Eddington was a conscientious objector during the First World War — his Quaker pacifism was deeply held. His colleagues secured a deferment by arguing that he was engaged in essential scientific work; the 1919 eclipse expedition, which he led, was partly organised to prevent his being called up.

He was one of the first physicists outside Germany to understand and champion general relativity — his 1920 book Space, Time and Gravitation was the first comprehensive English-language account. His later philosophical writings (The Nature of the Physical World, 1928; New Pathways in Science, 1935; The Philosophy of Physical Science, 1939) argued for a form of idealism: the laws of physics describe the structure of the mind rather than of an external world. This position was contested by other physicists and remains outside the mainstream.

Died 22 November 1944 in Cambridge, of cancer.

The arrow of time

The Nature of the Physical World (1928) — based on Eddington’s Gifford Lectures at the University of Edinburgh — contains the passage that gave the concept its name:

“Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s arrow’ to express this one-way property of time which has no analogue in space.”

The formulation identifies entropy increase as the sole physical basis for the distinction between past and future. The fundamental equations of physics — Newton’s mechanics, Maxwell’s electrodynamics, Einstein’s field equations, the Schrödinger equation — are time-symmetric: they work equally well run backwards. Only the second law of thermodynamics introduces an asymmetry. Eddington elevated this from a technical observation in statistical mechanics to a question about the nature of time itself.

The 1919 eclipse expedition

The 1919 solar eclipse provided the first observational test of general relativity. Einstein’s theory predicts that light passing near a massive body is deflected by the curvature of spacetime — and that the deflection is twice the value predicted by Newtonian gravity. During a total solar eclipse, stars near the sun become visible, and their apparent positions can be compared with their positions when the sun is elsewhere in the sky.

Eddington led the expedition to the island of Príncipe (off the west coast of Africa); a second team, led by Andrew Crommelin, observed from Sobral, Brazil. The results, announced at a joint meeting of the Royal Society and the Royal Astronomical Society on 6 November 1919, confirmed the Einsteinian prediction. The announcement made Einstein an international figure overnight.

The reliability of the 1919 data has been debated. The Príncipe plates were affected by cloud; Eddington excluded some data points from the analysis. Critics (notably John Earman and Clark Glymour) have argued that Eddington’s analysis was influenced by his prior commitment to general relativity — that he selected the data that confirmed the prediction. More recent re-analyses of the original plates (Kennefick, 2019) have largely vindicated Eddington’s result: the data, properly analysed, do support the Einsteinian deflection. The episode illustrates both the social dynamics of theory confirmation and the resilience of good data under re-examination.

Stellar structure

Eddington’s astrophysical work, developed through the 1920s, established the theory of stellar interiors. The Internal Constitution of the Stars (1926) argued that stars are gaseous throughout (not liquid or solid in their interiors, as some had proposed), that they are in hydrostatic equilibrium (gravity pulling inward balanced by radiation pressure pushing outward), and that the source of stellar energy is subatomic — a prescient claim at a time when nuclear physics had not yet identified fusion as the mechanism.

The mass-luminosity relation: Eddington showed that a star’s luminosity is determined primarily by its mass — more massive stars are dramatically more luminous. The relation holds across a wide range of stellar types and became a foundational result in astrophysics. The Eddington luminosity (or Eddington limit) — the maximum luminosity a body can achieve before radiation pressure overcomes gravity — is named for this work.

Where Eddington stops

Eddington’s later career was devoted to a speculative programme he called the “fundamental theory” — an attempt to derive the values of the fundamental physical constants (the fine-structure constant, the ratio of the proton mass to the electron mass, the number of particles in the universe) from purely theoretical principles. The programme, developed through the 1930s and published posthumously as Fundamental Theory (1946), was widely regarded by physicists as numerology — the appearance of precise derivation without genuine theoretical justification. Chandrasekhar called it “unbelievable” that the same physicist who produced the mass-luminosity relation also produced the fundamental theory. The programme has not been taken up and is remembered as a cautionary example of a first-rate scientist pursuing a programme that exceeded the methods available.

The idealist philosophy — the claim that physics describes the structure of the mind rather than of an external world — was contestable in 1928 and has not been adopted. Eddington argued that the laws of physics are epistemological rather than ontological — that the constants and structures physicists discover are features of the observational procedure, not of nature itself. The position has affinities with later constructivist and anti-realist philosophies of science, but Eddington’s version was developed independently and was not taken up by the mainstream philosophy of physics. Whether the position is a genuine philosophical insight ahead of its time or a misreading of the relationship between observation and reality is debated in the small literature on Eddington’s philosophy.

The 1919 eclipse confirmation, while vindicated by recent re-analysis, raised methodological questions that remain relevant. The degree to which prior theoretical commitment influenced data selection — and whether such influence is a bug or a feature of scientific practice — is a live question in the philosophy of science. Eddington’s case has become a standard example in debates about theory-ladenness of observation, cited by both sides.

Key works

See also: Boltzmann · Einstein · Penrose