Philosophers of science have been intrigued by this year’s Nobel Prize for Physics. That’s because Alain Aspect, John Clauser and Anton Zeilinger have been recognized for designing and performing a variety of ingenious experiments to demonstrate entangled particles. For philosophers, the work is fascinating because at its heart lies the challenge of understanding what quantum mechanics is all about.

This challenge has been around for as long the subject itself, with both Niels Bohr and Albert Einstein debating the implications of various thought experiments as far back as 1927. For Bohr, the experiments showed that the formalism of quantum mechanics, however strange, renders the world as it really is. For Einstein, they showed that quantum mechanics does not render the world as it really is – and therefore lacks any true meaning.

Einstein’s arguments culminated in the famous “EPR” paper that he wrote in 1935 with Boris Podolsky and Nathan Rosen, which supposedly proved that quantum mechanics could not represent reality (Physical Review 47 777). The EPR paper is unique among physics papers in that it begins by attempting to define reality. “A sufficient condition for the reality of a physical quantity,” the authors declared, “is the possibility of predicting it with certainty, without disturbing the system.”

If John Bell was disappointed by physicists who were apathetic about the meaning of quantum mechanics, he was even more irritated by physicists who were deferential to Niels Bohr

The paper is also significant in that “reality” was being approached for the first time in modern science as a testable hypothesis. Reality’s uncertainty sprang from what Erwin Schrödinger, in a post-EPR 1935 letter to Einstein, dubbed “entanglement” – or the condition that the quantum state of one particle in a system cannot be defined independently from all the others. Einstein and Schrödinger considered that a crime, and were shocked that their colleagues were not more shocked.

Getting real

One physicist who did care about the crime was John Stewart Bell. Born in 1928 after Bohr and Einstein had already begun their debates, he had no doubt that quantum mechanics was fine for all practical purposes. However, Bell felt that both the EPR paper and Bohr’s muddled reply skirted the fundamental issue, which had nothing to do either with the technical efficiency of quantum physics or with its accuracy as a theory.

If Bell was disappointed by physicists who were apathetic about the meaning of quantum mechanics, he was even more irritated by physicists who were deferential to Bohr. Many assumed he had successfully countered Einstein’s objections and had laid the fundamental issues to rest – somehow – in the “Copenhagen interpretation”. Conceptually vague, that interpretation involves something wave-like (either mathematical or real) collapsing in some unknown way into something particle-like.

In 1960, at a conference at CERN, Bell unexpectedly found himself sharing an elevator with Bohr, and tried to muster the courage to tell the 75-year-old grand old man of physics how flawed and irresponsible his interpretation of quantum mechanics was. Sadly, Bell lost his nerve. In fact, he wondered how many of his colleagues had been doing the same.

Then, in 1964 Bell devised a creative thought experiment that, if carried out for real, would show whether entanglement is caused by local “hidden variables” – properties that exist before taking a measurement. Initially, Bell thought Einstein was right, that hidden variables would be the answer to problems with quantum mechanics, and that he could show Bohr was wrong. But Bell increasingly saw that he couldn’t.

If Einstein had turned reality into a testable hypothesis, Bell also did something novel, which was to show that Einstein’s assumption of pre-existing variables could also be tested (Physics 1 195). For philosophers, Bell’s paper is fascinating in that it scrutinized the soundness of what Einstein had taken for granted; it asked what the microscopic world is really like and inquired into the consequences of that picture. What Einstein had taken for granted conceptually – the existence of definite properties of particles prior to measurement – could now be evaluated.

Read more

John Bell and the most profound discovery of science

What’s more, the paper showed that Bohr had not made clear why the existence of entanglement must exclude these pre-existing properties. In other words, Bell’s paper gave experimentalists a target that neither Einstein nor Bohr had considered. That work started in 1972 when Clauser did a “Bell test” with the late Stuart Freedman that offered the first hard evidence against local hidden variables. Aspect continued that work later.

As for Zeilinger, he had (unlike Bell) long embraced the quantum-mechanical description developed by Bohr. In 1989, the year before Bell died, he and his team demonstrated three-particle entanglement, removing the need for Bell’s inequality with a single measurement. Since then, Zeilinger has closed more and more potential loopholes to local hidden variable theories by designing more and more elaborate entanglement experiments.

The critical point

Bell’s philosophical sensitivity and physical subtlety has earned him a quasi-mythical status among reflective physicists and thoughtful philosophers of science. His re-engagement with the enigma of entanglement was sparked by philosophical concerns left unanswered by physicists who fought over the EPR paper. Bell could frame his qualms about the meaning of quantum mechanics in a way that appealed to physicists, highlighting conceptual differences via quirky and playful thought experiments.

But why weren’t scientifically sensitive philosophers first on the crime scene? Why hadn’t they recognized the challenge of EPR and pounced? One part of the answer, no doubt, involves their lack of technical training. Another is the restrictions of academic disciplinary life. Still, Bell’s rich philosophical conceptualization and ingenious metaphysical imagination should be more of an ingredient not only in mainstream theoretical physics, but in philosophy as well.

The physicist David Mermin once recalled a meeting in 1989 when Bell literally shouted at a fellow physicist, seeking to rally his colleagues to not let their imaginations atrophy in the shadow of conventional wisdom. This year’s Nobel prize highlights the audacity and openness of physicists who challenged the conceptual frame presumed by their equations and showed that philosophical questions about physics, at least, can often be made physically testable and rigorous.

Robert P Crease (click link below for full bio) is chair of the Department of Philosophy, Stony Brook University, US, and Gino Elia is a doctoral candidate in philosophy at Stony Brook University