Max Born

Max Born: The Physicist Who Made Reality a Probability

On 4 December 1926, Albert Einstein sat down to write to a friend about the strange new physics coming out of Göttingen. "Quantum mechanics is certainly imposing," he conceded. "But an inner voice tells me that it is not yet the real thing... I, at any rate, am convinced that He is not playing at dice." The line, later compressed into the most famous sentence Einstein never quite wrote, was aimed at the man who had just introduced the dice. His name was Max Born, and five months earlier he had published the idea that the wavefunction does not tell you where a particle is — only how likely it is to be found there.

Breslau, and the Mathematicians of Göttingen

Born was born on 11 December 1882 in Breslau, then in the German Empire, now Wrocław in Poland, into a Jewish family with a strong scientific streak: his father, Gustav Jacob Born, was an anatomist and professor of embryology at the University of Breslau. His mother, Margarethe Kauffmann, came from a Silesian industrial family and died when he was four.

After the König-Wilhelm-Gymnasium he went to the University of Breslau in 1901, spending a summer at Heidelberg in 1902 and another at Zurich in 1903 before moving to Göttingen in April 1904. That move decided everything. At Göttingen he studied under Felix Klein, David Hilbert and Hermann Minkowski — the strongest mathematics faculty in the world — and absorbed a fluency in abstract algebra that most physicists of his generation simply did not have. His 1906 doctorate, on the stability of elastic wire, won the Philosophy Faculty Prize and a *magna cum laude*.

The Institute

He habilitated in theoretical physics at Göttingen in 1909, became professor extraordinarius in Berlin in 1914, took a full chair at Frankfurt in 1919, and in 1921 returned to Göttingen as director of the Institute of Theoretical Physics. For the next twelve years that institute was the centre of gravity of world physics.

The roster is almost comic in retrospect. Born's doctoral students included Robert Oppenheimer (1927), Max Delbrück (1930), Maria Goeppert Mayer (1931) and Victor Weisskopf (1931); two of them would win Nobel Prizes and one would run Los Alamos. His assistants included Werner Heisenberg, Wolfgang Pauli, Enrico Fermi, Edward Teller, Eugene Wigner, Pascual Jordan, Friedrich Hund, Gerhard Herzberg and Léon Rosenfeld. A conversation with the chemist Fritz Haber about what happens when metals react with halogens produced, in 1918, the Born–Haber cycle. Later work with Oppenheimer produced the Born–Oppenheimer approximation, still the standard tool for treating molecules; the Born approximation remains a workhorse of scattering theory.

Matrix Mechanics

On 9 July 1925 Heisenberg handed Born a paper full of peculiar arrays of numbers whose multiplication depended on the order in which you did it. Born recognised what he was looking at: matrices, the mathematical language he had mastered at Göttingen twenty years earlier and which most physicists then regarded as an obscure specialty. With Pascual Jordan, and then with Heisenberg, he transcribed and extended the theory into the first complete formulation of quantum mechanics, arriving at the commutation relation pq − qp = ih/2π — the equation that says position and momentum cannot both be sharp, written down before anyone knew what it meant.

The Born Rule

In July 1926 he published the interpretation that carries his name. Erwin Schrödinger had produced a wave equation; nobody could say what the wave was made of. Born proposed that ψ*ψ is a probability density — that quantum mechanics predicts not outcomes but the odds of outcomes. It was the moment physics stopped being deterministic. Everything downstream, from the uncertainty principle's meaning to the measurement problem to the century of argument that followed, runs through that one paper. Einstein never accepted it, and their correspondence, running from 1916 to 1955, became the most famous disagreement in modern science.

Born's own philosophical position was consistent with his physics. "I believe that ideas such as absolute certitude, absolute exactness, final truth, etc. are figments of the imagination which should not be admissible in any field of science," he wrote.

Statelessness

In January 1933 the Nazis took power and Born, of Jewish birth despite a 1914 Lutheran baptism undertaken partly to assimilate, was suspended from his Göttingen chair. He went to St John's College, Cambridge (1933–35), then to the Indian Institute of Science at Bangalore (1935–36), then to Edinburgh, where he held the Tait chair from 1936 to 1952. In November 1935 Germany revoked his citizenship, leaving him stateless; Göttingen then cancelled the doctorate it had awarded him with distinction in 1906. He became a naturalised British subject on 31 August 1939 — one day before Germany invaded Poland.

Why Max Is Called a Genius

Born's distinctive faculty was translation between two languages that had not yet been introduced: pure mathematics and physical intuition. When Heisenberg brought him a set of unfamiliar arrays, Born was almost uniquely equipped to see that they were matrices, and that the non-commuting multiplication was not an embarrassment but the physics itself. Then came the harder act, which was interpretive nerve rather than technique: proposing that the square of the wavefunction is a probability meant abandoning the assumption, unbroken since Newton, that a complete description of a system determines its future. He did it alone, in one paper. The Nobel committee eventually said so, honouring him in 1954 "for his fundamental research in quantum mechanics, especially for his statistical interpretation of the wavefunction."

The counter-case begins with that date. He waited twenty-eight years, while Heisenberg took the 1932 prize alone for the matrix mechanics they had built together — Heisenberg later expressed regret at not sharing the credit — and while a generation of his own students and assistants collected honours first. A sceptic can argue that Born's signature achievement was an interpretation of somebody else's equation, that the matrix work was Heisenberg's insight in Born's notation, and that his most durable role was institutional: he assembled and ran the room in which quantum mechanics happened. But that is also a description of a specific and rare talent — knowing which mathematics a new physics requires, and being willing to say what the mathematics means when everyone else is still calculating.

Legacy

He also won the Max Planck Medal in 1948 and the Hughes Medal in 1950, was elected a Fellow of the Royal Society of Edinburgh in 1937 and of the Royal Society in March 1939, and shared his Nobel with Walther Bothe. In 1952 he retired to Bad Pyrmont in West Germany, returning to the country that had made him stateless, and died at Göttingen on 5 January 1970, aged eighty-seven. His descendants supplied an unlikely coda: his granddaughter, through his daughter Irene, was the singer and actress Olivia Newton-John. The rule, though, is the monument. Every calculation in modern quantum theory ends with it.