Few periods in the history of physics have received as much attention from scholars as the birth of quantum mechanics in the 1920s. That focus is well deserved: Between 1925 and 1927, a new theory was forged, one that laid the foundation for later advances and had tremendous philosophical consequences. Unfortunately, despite the vast scholarly literature on the quantum revolution—which easily comprises hundreds of books and articles—a mythologized version of the story remains prevalent in textbooks and popular literature.
The theorists Markus Fierz, Wolfgang Pauli, and Hans Jensen (from left to right) engaging in discussion over beers during a conference at Niels Bohr’s Institute for Theoretical Physics at the University of Copenhagen, sometime in the 1930s.
The theorists Markus Fierz, Wolfgang Pauli, and Hans Jensen (from left to right) engaging in discussion over beers during a conference at Niels Bohr’s Institute for Theoretical Physics at the University of Copenhagen, sometime in the 1930s.
Although every iteration of the mythologized quantum tale is slightly different, certain canonical elements are almost always present. One example is Werner Heisenberg’s summer 1925 trip to the island of Helgoland in the North Sea. Heisenberg made a crucial breakthrough in the development of matrix mechanics during that trip—at least according to his version of the story!—and quantum hagiographers typically mention it with a kind of mystical reverence. Another example is the portrayal of Niels Bohr, who hosted many of the quantum revolutionaries at his institute in Copenhagen. The Danish physicist is ordinarily depicted as a sage, who encouraged his young protégés to push the bounds of physics with the right word at the right time.
All the major players in the quantum revolution, including Erwin Schrödinger, Wolfgang Pauli, and Paul Dirac, usually are presented as larger-than-life demigods. But as the well-known historian of physics Alexei Kojevnikov demonstrates in his recent book, The Copenhagen Network: The Birth of Quantum Mechanics from a Postdoctoral Perspective, Heisenberg, Bohr, Schrödinger, Pauli, Dirac, and the rest of the quantum gang were undeniably human like you and me—complete with egos, feelings, and insecurities. Contrary to what many of them proclaimed later in their lives, the genesis of quantum mechanics was quite rocky.
Those looking for quantum gossip will find plenty of juicy nuggets here. Among other things, Kojevnikov reveals that in 1923 Bohr had a falling-out with the doyen of experimental physics, Arnold Sommerfeld, who felt that he deserved a share of Bohr’s Nobel Prize; that Heisenberg vehemently disagreed with Bohr’s early theories, contrary to what both claimed after World War II; and that Bohr possessed tremendous diplomatic skills but was a bad public speaker.
If Kojevnikov’s sole achievement in The Copenhagen Network was to dethrone the quantum demigods, his book would already be a valuable contribution to the ever-growing literature on the quantum revolution. But it’s much more than that. The book poses a question few have thought to ask: How was it that the Danish capital of Copenhagen, which at that time had essentially no history as a center of physics, became the global nexus for theoretical physics in the 1920s and 1930s?
In the traditional hagiographic story, the answer to that question is obvious: Bohr’s magnetic personality, brilliant ideas, and wise words attracted young luminaries to the new institute. But Kojevnikov rightfully notes that no matter how brilliant Bohr was, it was “a rather improbable task to make a country as small as Denmark the world leader in any major field of science.”
The main reason for Copenhagen’s emergence as a physics metropole, he writes, was not Bohr’s genius but the interwar political situation. Germany, the traditional powerhouse of the natural sciences, had been weakened by defeat in World War I and was subject to an international scientific boycott by the former Allies (see the article by Dirk van Delft, Physics Today, April 2022, page 30). Denmark had been neutral during the war and retained ties with Germany, which lent an air of impartiality to Copenhagen. It became a site where scientists from both sides could meet and exchange ideas.
The unstable economic situation in Germany and the Austro-Hungarian successor states also aided Bohr’s institute. Not only did Bohr attract postdoctoral scholars from the newly independent countries of Austria, Hungary, and Poland who might previously have studied in Germany, but economically stable Copenhagen began to look more appealing to young German scholars, including Heisenberg, Friedrich Hund, and Pascual Jordan, who would never have left home before the war. All three worked under Bohr via fellowships provided by the US Rockefeller Foundation.
As the subtitle suggests, The Copenhagen Network approaches the early years of quantum mechanics from the perspective of postdoctoral scholars like Heisenberg, Hund, Jordan, and Pauli. I do have to quibble with Kojevnikov’s ahistorical use of the word “postdoctoral,” which was employed only seldomly in the 1920s or 1930s. But his decision to center the book on the early-career experience is innovative. Kojevnikov rightly points out that senior scholars with permanent positions, like Bohr and Max Born, held incredible power over the quantum postdocs, who made crucial breakthroughs while holding temporary positions with minimal job security. (Sound familiar?)
I suspect the main reason the mythologized version of quantum history remains persistent is because many of the major players in the revolution—including Bohr, Heisenberg, Born, Dirac, and Jordan—remained active into the 1960s and 1970s. As elder statesmen of physics, they propagated a burnished version of the quantum origin story that became part of the discipline’s collective memory. But hagiography isn’t history. The Copenhagen Network admirably depicts the quantum revolution as the messy, uncertain reality it was.
Ryan Dahn is the Books editor at Physics Today. A historian of science, he is currently working on a biography of the German physicist Pascual Jordan.
