In one corner, we have a German-born theoretical physicist famous for his discovery of the photoelectric effect and his groundbreaking research on relativity theory. In the opposite corner, hailing from Denmark, we have a theoretical physicist famous for his transformational work on quantum theory and atomic structure. Albert Einstein and Niels Bohr frequently butted heads over the interpretation of quantum mechanics and even over the scope and purpose of physics, and their debates still resonate today.

Niels Bohr and Albert Einstein (photo by Paul Ehrenfest, 1925).

In a class on “Waves, Optics, and Modern Physics,” I am teaching my students fundamentals about quantum physics, and I try to incorporate some of this important history too. In the early 20th century, physicists gradually adopted new concepts such as discrete quantum energy states and wave-particle duality, in which under certain conditions light and matter exhibit both wave and particle behavior. Nevertheless, other quantum concepts proposed by Bohr and his colleagues, such as non-locality and a probabilistic view of the wave function, proved more controversial. These are not mere details, as more was at stake—whether one can retain scientific realism and determinism, as was the case with classical physics, if Bohr’s interpretation turns out to be correct.

Bohr had many younger followers trying to make names for themselves, including Werner Heisenberg, Max Born, Wolfgang Pauli, and others. As experimental physicists explored small-scale physics, new phenomena required explanations. One could argue that some of Bohr and his followers’ discoveries and controversial hypotheses were to some extent just developments of models that managed to fit the data, and the models needed a coherent theoretical framework to base them on. On the other hand, Einstein, Erwin Schrödinger, and Louis de Broglie were skeptical or critical about some of these proposals.

The debates between Einstein and Bohr came to a head as they clashed in Brussels in 1927 at the Fifth Solvay Conference and at the next conference three years later. It seems like all of the major physics figures of the day were present, including Einstein, Bohr, Born, Heisenberg, Pauli, Schrödinger, de Broglie, Max Planck, Marie Curie, Paul Dirac, and others. (Curie was the only woman there, as physics had an even bigger diversity problem back then. The nuclear physicist Lise Meitner came on the scene a couple years later.)

Conference participants, October 1927. Institut International de Physique Solvay, Brussels.

Einstein tried to argue, with limited success, that quantum mechanics is inconsistent. He also argued, with much more success in my opinion, that (Bohr’s interpretation of) quantum mechanics is incomplete. Ultimately, however, Bohr’s interpretation carried the day and became physicists’ “standard” view of quantum mechanics, in spite of later developments by David Bohm supporting Einstein’s realist interpretation.

Although the scientific process leads us in fruitful directions and encourages us to explore important questions, it does not take us directly and inevitably toward a unique “truth.” It’s a messy nonlinear process, and since scientists are humans too, the resolution of scientific debates can depend on historically contingent social and cultural factors. James T. Cushing (my favorite professor when I was an undergraduate) argued as much in his book, Quantum Mechanics: Historical Contingency and the Copenhagen Hegemony.

Why do the Einstein vs Bohr debates still fascinate us—as well as historians, philosophers, and sociologists—today? People keep discussing and writing about them because these two brilliant and compelling characters confronted each other about issues with implications about the scope and purpose of physics and how we view the physical world. Furthermore, considering the historically contingent aspects of these developments, we should look at current scientific debates with a bit more skepticism or caution.

Implications for Today’s Scientific Debates

In recent years, we have witnessed many intriguing disagreements about important issues in physics and astrophysics and in many other fields of science. For example, in the 1990s and 2000s, scientists debated whether the motions, masses, and distributions of galaxies were consistent with the existence of dark matter particles or whether gravitational laws must be modified. Now cosmologists disagree about the likely nature of dark energy and about the implications of inflation for the multiverse and parallel universes. And string theory is a separate yet tenuously connected debate. On smaller scales, we have seen debates between astrobiologists about the likelihood of intelligent life on other planets, about whether to send missions to other planets, and even disagreements about the nature of planets, which came to the fore with Pluto‘s diminished status.

Scientists play major roles in each case and sometimes become public figures, including Stephen Hawking, Neil deGrasse Tyson, Roger Penrose, Brian Greene, Sean Carroll, Max Tegmark, Mike Brown, Carolyn Porco, and others. Moreover, many scientists are also science communicators and actively participate in social media, as conferences aren’t the only venues for debates anymore. For example, 14 of the top 50 science stars on Twitter are physicists or astronomers. Many scientists communicate their views to the public, and people want to hear them weigh in on important issues and on “what it all means.” (Contrary to an opinion expressed by deGrasse Tyson, physicists are philosophers too.)

In any case, as scientific debates unfold, we should keep in mind that sometimes we cannot find a unique elegant explanation to a phenomenon, or if such an explanation exists, it may remain beyond our grasp for a long time. Furthermore, we should keep our minds open to the possibility that our own interpretation of a scientific phenomenon could be incomplete, incoherent, or even incorrect.