Einstein, Bohr and the war over quantum theory

Ramin Skibba explores a history of unresolved questions beyond the Copenhagen interpretation.
Ramin Skibba is an astrophysicist turned science writer based in San Diego, California.

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Black and white photo showing Bohr and Einstein sitting side by side in conversation.

Niels Bohr (left) with Albert Einstein in the late 1920s, when quantum mechanics was in its infancy.Credit: Emilio Segre Visual Archives/AIP/SPL

What Is Real?: The Unfinished Quest for the Meaning of Quantum Physics Adam Becker Basic: 2018.

All hell broke loose in physics some 90 years ago. Quantum theory emerged — partly in heated clashes between Albert Einstein and Niels Bohr. It posed a challenge to the very nature of science, and arguably continues to do so, by severely straining the relationship between theory and the nature of reality. Adam Becker, a science writer and astrophysicist, explores this tangled tale in What Is Real?.

Becker questions the hegemony of the Copenhagen interpretation of quantum mechanics. Propounded by Bohr and Werner Heisenberg in the 1920s, this theory holds that physical systems have only probabilities, rather than specific properties, until they’re measured. Becker argues that trying to parse how this interpretation reflects the world we live in is an exercise in opacity. Showing that the evolution of science is affected by historical events — including sociological, cultural, political and economic factors — he explores alternative explanations. Had events played out differently in the 1920s, he asserts, our view of physics might be very different.

Becker lingers on the 1927 Solvay Conference in Brussels, where 29 brilliant scientists gathered to discuss the fledgling quantum theory. Here, the disagreements between Bohr, Einstein and others, including Erwin Schrödinger and Louis de Broglie, came to a head. Whereas Bohr proposed that entities (such as electrons) had only probabilities if they weren’t observed, Einstein argued that they had independent reality, prompting his famous claim that “God does not play dice”. Years later, he added a gloss: “What we call science has the sole purpose of determining what is.” Suddenly, scientific realism — the idea that confirmed scientific theories roughly reflect reality — was at stake.

Quantum phenomena were phenomenally baffling to many. First was wave–particle duality, in which light can act as particles and particles such as electrons interfere like light waves. According to Bohr, a system behaves as a wave or a particle depending on context, but you cannot predict which it will do.

Second, Heisenberg showed that uncertainty, for instance about a particle’s position and momentum, is hard-wired into physics. Third, Bohr argued that we could have only probabilistic knowledge of a system: in Schrödinger’s thought experiment, a cat in a box is both dead and alive until it is seen. Fourth, particles can become entangled. For example, two particles might have opposite spins, no matter how far apart they are: if you measure one to be spin up, you instantly know that the other is spin down. (Einstein called this “spooky action at a distance”.)

Becker explains how these observations challenge locality, causality and determinism. In the classical world of billiard balls, projectiles and apples falling from trees, they were never problems.

Sifting through the history, Becker shows how Bohr, as an anti-realist, brought to his side many rising physicists, including Heisenberg, Wolfgang Pauli and Max Born. Einstein, however, persistently argued that the Copenhagen interpretation was incomplete. He conjectured that there might be hidden variables or processes underlying quantum phenomena; or perhaps ‘pilot waves’, proposed by de Broglie, govern the behaviour of particles. In 1932, mathematician John von Neumann produced a proof that there could be no hidden variables in quantum mechanics. Although mathematically correct, it was revealed to be flawed decades later. But the damage had been done: the potentially viable alternatives conceived by Einstein and de Broglie remained relatively unexplored. The Copenhagen interpretation had taken hold by the 1930s, and textbooks today state that Bohr’s view ‘won’.

Thus, the Solvay Conference can be seen as a stand-off between two mathematically equivalent but fundamentally different paradigms: Bohr’s instrumentalist view of quantum physics and Einstein’s realist one. In science, a dominant paradigm determines which experiments are done, how they’re interpreted and what kind of path a research programme follows.

But what if a field picks the wrong paradigm? Becker shows how, in the 1950s and 1960s, a handful of physicists dusted off the theories of Einstein and de Broglie and turned them into a fully fledged interpretation capable of shaking up the status quo. David Bohm argued that particles in quantum systems existed whether observed or not, and that they have predictable positions and motions determined by pilot waves. John Bell then showed that Einstein’s concerns about locality and incompleteness in the Copenhagen interpretation were valid. It was he who refuted von Neumann’s proof by revealing that it ruled out only a narrow class of hidden-variables theories.

The scientific community greeted Bohm’s ideas coolly. A former mentor, J. Robert Oppenheimer, said: “if we cannot disprove Bohm, then we must agree to ignore him”. And, as Becker shows, Bohm’s leftist views led to an appearance before the House Un-American Activities Committee, and subsequent ostracization.

Bohm’s contemporary, physicist Hugh Everett, delivered another challenge to the Copenhagen interpretation. In 1957, Everett set out to resolve the ‘measurement problem’ in quantum theory — the contradiction between the probabilistic nature of particles at the quantum level and their ‘collapse’, when measured, into one state at the macroscopic level.

Everett’s many-worlds interpretation posited no collapse. Instead, probabilities bifurcate at the moment of measurement into parallel universes — such as one in which Schrödinger’s cat is alive and another in which it’s dead. Although an infinite number of untestable universes seems unscientific to some, many physicists today view the theory as important.

The book has a few minor shortcomings. Becker gives too much space to recent applications building on Bell’s research, and too little to new developments in the philosophy of science. Yet he, like cosmologist Sean Carroll in his 2016 The Big Picture (R. P. Crease Nature 533, 34; 2016), does make an explicit case for the importance of philosophy. That’s a key call, with influential scientists such as Neil deGrasse Tyson dismissing the discipline as a waste of time.

What Is Real? is an argument for keeping an open mind. Becker reminds us that we need humility as we investigate the myriad interpretations and narratives that explain the same data.

Nature 555, 582-584 (2018)

doi: 10.1038/d41586-018-03793-2

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