On 24 October 1927, twenty nine physicists arrived in Brussels for the fifth Solvay Conference, titled Electrons and Photons. Before they finished, they were arranged in three rows outside the Institut de Physiologie in the Parc Léopold and photographed by Benjamin Couprie.
Seventeen of the twenty nine held or would go on to hold a Nobel Prize. It is routinely called the most intelligent photograph ever taken, which is the kind of phrase that gets in the way of looking at it properly.
Who Is Where, and Why
The arrangement is not random. It is a seniority diagram.
The front row is the establishment. Irving Langmuir, Max Planck, Marie Curie, Hendrik Lorentz, Albert Einstein, Paul Langevin, Charles Eugène Guye, Charles Wilson, Owen Richardson. Lorentz is at the centre because he chaired the conference. Einstein sits beside him. Planck holds his hat. These are the people who built the physics of the previous generation.
The middle row is the working core of the new theory. Peter Debye, Martin Knudsen, William Lawrence Bragg, Hendrik Kramers, Paul Dirac, Arthur Compton, Louis de Broglie, Max Born, Niels Bohr. Bohr is at the end of the row, which understates him considerably: the entire conference was in effect an argument about his position.
The back row is the future, and it is startlingly young. Auguste Piccard, Émile Henriot, Paul Ehrenfest, Édouard Herzen, Théophile de Donder, Erwin Schrödinger, Jules Émile Verschaffelt, Wolfgang Pauli, Werner Heisenberg, Ralph Fowler, Léon Brillouin.
Heisenberg was twenty five and had published the uncertainty principle that year. Dirac was twenty five and would write down his equation for the electron the following year. Pauli was twenty seven. Three of the men standing at the back, in the row you would give to junior staff, had already or were about to restructure physics.
Marie Curie is the only woman present. She is also the only person in the photograph with two Nobel Prizes, in two different sciences, a distinction still unmatched.
What They Were Arguing About
The conference had a specific job. Quantum mechanics had arrived in a rush between 1925 and 1927, in two apparently different mathematical forms, and nobody agreed what it meant.
Bohr, Heisenberg, Born and Pauli argued for what became the Copenhagen interpretation: that the theory gives probabilities and not trajectories, that complementary descriptions such as wave and particle are both needed and cannot be applied at once, and that asking what the electron is really doing between measurements is not a question physics can answer.
Einstein would not have it. His objection is regularly reduced to the remark about God not playing dice, which he did say in various forms, and which makes him sound like a man who could not accept probability. That is not what he was arguing.
His position was that the quantum description was incomplete. He accepted that the mathematics worked and produced correct predictions. He denied that a theory giving only probabilities could be the final account of an individual system, and he spent the conference producing thought experiments designed to show that you could, in principle, get more information out of a quantum system than the theory permitted.
Bohr answered each one, usually by the next morning, generally by showing that Einstein had neglected the uncertainty introduced by the measuring apparatus itself. The exchanges happened mostly at breakfast and dinner rather than in the formal sessions, with Paul Ehrenfest acting as interpreter between the two camps. Ehrenfest, who loved them both, wrote to his students that he had to side with Bohr, and that watching Einstein raise objection after objection and see them answered reminded him of the arguments against relativity.
Einstein did not concede in 1927, and he did not concede in 1930, and in 1935 he produced the sharpest version of the objection with Podolsky and Rosen. The EPR paper is the direct descendant of the conversations in that Brussels hotel, and it turned out to be the most productive wrong argument in the history of physics: it identified entanglement, which is now the working substance of quantum computing.
The Man at the Back on the Left
Auguste Piccard stands at the far left of the back row, and his later career is a reminder that the people in the photograph were not only theoreticians.
In 1931 he took a balloon with a pressurised aluminium sphere to nearly sixteen kilometres, the first person to reach the stratosphere, in order to measure cosmic rays. He then reversed the engineering and built the bathyscaphe, and his son Jacques took one to the bottom of the Mariana Trench in 1960. Hergé, who followed the balloon flights in the papers, gave Piccard’s face and build to Professor Calculus.
Louis de Broglie, seated in the middle row, is there because of a doctoral thesis of about a hundred pages proposing that matter has a wavelength, an idea so strange that his examiners sent it to Einstein to ask whether it was nonsense. Einstein said it was not, and de Broglie had his Nobel Prize by 1929.
What the Photograph Does Not Show
This is the last moment when all of this fitted in one garden.
Within six years, the institutions represented in the picture were broken up. The German laws of 1933 forced Einstein and Born out of Germany, Schrödinger left Austria, Debye eventually left, and the network of Göttingen, Berlin, Munich, Copenhagen and Cambridge that had produced quantum mechanics between 1925 and 1927 stopped existing as a single conversation. Paul Ehrenfest, the man who had mediated between Bohr and Einstein, took his own life in 1933.
Within eighteen years several of the men in the photograph had worked on nuclear weapons, on opposite sides, and Heisenberg’s role in the German programme is still argued about.
None of that is visible. What is visible is twenty nine people in good coats, looking calmly at a camera, at the point where they had just finished changing the description of matter and had not yet learned what the century would do with it.
Why It Became the Image It Did
Group portraits of institutions are an old genre. Guilds, boards, regiments and faculties have always commissioned them, and the conventions are stable: rows, formal dress, seniority at the front, direct gaze.
The Solvay photograph works so well because it follows those conventions exactly while containing something the genre was never designed to hold. It is composed like a company board and it happens to be a near complete inventory of the people who worked out how atoms behave.
It is also the last of its kind. Physics after 1930 grew too large to photograph. A modern experimental collaboration has several thousand authors, and the group portrait, as a way of representing who did the work, stopped being possible.
Our book Portraying Science traces four centuries of that impulse, the wish to put a face to knowledge, from sixteenth century oils through engraving to early photography. The Solvay picture sits at the end of that line: the moment when the portrait of science stopped being a painting of one person in a study and became a photograph of a group too large to paint and too small to be a crowd.
And as with any portrait, the interesting question is who is not in it. A collection of faces is always also a record of who was invited. Lise Meitner was in Berlin in 1927, working on the physics that would lead to nuclear fission. Emmy Noether was in Göttingen, having already proved the theorem that underlies conservation laws in every modern field theory. Neither was in the garden.
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