John Stewart Bell

• 1. Introduction

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  John Stewart Bell was born in Belfast on the 28th of July 1928, into a working-class family where neither his parents nor any of his three older siblings had finished high school. By the age of 11, he had already decided he wanted to be a scientist. It was an ambition that his mother encouraged, and one that would lead him to overturn one of the most confidently held assumptions in the whole of modern physics.

  At 16, Bell graduated from Belfast Technical High School. Then, in what was described as an exceptionally rare occurrence for someone from his background, he enrolled at the Queen's University of Belfast. By 1949, he held two bachelor's degrees. One in experimental physics, one in mathematical physics. He went on to earn a PhD at the University of Birmingham in 1956.

  The questions this documentary sets out to answer are as fundamental as physics gets. Can the universe be explained by hidden rules we simply haven't found yet? Or is nature, at its core, irreducibly strange? Bell spent his career asking exactly those questions, and what he found changed physics forever.

• 2. The Theorem That Changed Physics

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  In 1964, after a year's leave from his usual post at CERN, Bell spent time at Stanford University, the University of Wisconsin-Madison, and Brandeis University. It was during that sabbatical that he wrote the paper titled "On the Einstein-Podolsky-Rosen paradox." That paper contained what became known as Bell's theorem.

  The Einstein-Podolsky-Rosen paradox had been a long-standing puzzle: a thought experiment suggesting that quantum mechanics was somehow incomplete, that particles separated by great distances could influence each other in ways that seemed to violate basic intuitions about causality and locality. Bell took Einstein, Podolsky, and Rosen's own reasoning and pushed it further. He derived an inequality, a mathematical boundary, that any classical, locally causal theory of the universe would have to obey. Quantum mechanics, he showed, predicts a violation of that inequality.

  The inequality wasn't just a theoretical curiosity. It was a test. A way to ask, with experiments rather than arguments, whether the universe plays by local rules or not. If quantum mechanics was right, the test would fail. And in 1972, the first experiment was conducted that, when extrapolated to ideal detector efficiencies, showed exactly that violation.

• 3. Einstein's Rational Man

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  Bell never hid where his sympathies lay. After the experimental tests of his inequality returned results that agreed with quantum mechanics rather than with local hidden variables, he was frank about how he felt. "It is so reasonable to assume," he said, "that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave."

  Bell called Einstein the rational man. He saw Einstein's insistence on local realism as logical, sensible, even obvious, even as history had sided against it. "So for me, it is a pity that Einstein's idea doesn't work," he said. "The reasonable thing just doesn't work."

  From the experiments, Bell drew a stark conclusion: non-locality is not a quirk or an artifact but is deeply rooted in quantum mechanics itself, and will persist in any completion of the theory. He also concluded that quantum theory is not locally causal and cannot be embedded in any theory that is. Bell grew resigned, over time, to the idea that future experiments would keep confirming quantum mechanics, writing that it was difficult for him to believe the theory would fail badly once detector efficiencies improved.

  Yet he remained alert to arguments from those who disagreed. Some physicists continued to hold out hope that loopholes, such as what was called the fair sampling loophole, might eventually rescue local hidden variables. Bell acknowledged the loopholes existed, even as most mainstream physicists remained sceptical that they would ultimately matter.

• 4. Taking on Von Neumann

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  Bell's theorem was not the only place where he challenged a settled consensus. Before writing that famous 1964 paper, Bell had been working to take seriously the hidden-variable theory developed by David Bohm. To do that rigorously, he had to confront a widely accepted mathematical proof by John von Neumann, one that most physicists took to show that hidden-variable theories were simply impossible.

  Bell addressed this in a paper called "On the Problem of Hidden Variables in Quantum Mechanics." Due to publishing delays, it did not appear in print until 1966, two years after his EPR paper. In it, Bell demonstrated that von Neumann's proof contained a flaw: it relied on an assumption about observable quantities that is not valid in quantum mechanics. That assumption was that the probability-weighted average of a sum of observables must equal the sum of their individual averages. Bell showed this was not required by the physics. He reportedly said of the proof: "The proof of von Neumann is not merely false but foolish!"

  What made the situation more striking is that Bell had not been the first to spot the flaw. The mathematician Grete Hermann had identified the same problem in 1935, but her finding had not entered common knowledge. Bell's rediscovery is what finally brought it to wider attention. In 2010, Jeffrey Bub published an argument that both Bell and Hermann had in fact misconstrued von Neumann's intent, suggesting it was the physics community that had misinterpreted the proof as applying universally. Bub's own conclusion was subsequently questioned in turn.

• 5. Beables and the Observer Problem

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  What bothered Bell most deeply about standard quantum mechanics was not just a technical objection. It was a philosophical one. He wanted physics to describe what actually exists, not just what an observer registers. He coined the word "beables" to describe what he was looking for. "The beables of the theory are those elements which might correspond to elements of reality, to things which exist," he explained. "Their existence does not depend on observation."

  Bell was openly critical of the vocabulary that dominated standard textbook treatments of quantum mechanics. He wrote that words like "system", "apparatus", "observable", "measurement", and "information" had no place in a formulation that claimed physical precision. The worst offender, in his view, was the word "measurement" itself. His objection was not semantic: he believed the standard formalism left a movable and undefined boundary between the quantum world and the classical apparatus used to probe it.

  Bohm's nonlocal hidden-variable theory appealed to him precisely because it did not require that boundary. Bell put it directly: what attracted him to the hidden-variable program was the possibility of a "homogeneous account of the world." He was also drawn to collapse theories late in his career, inspired by the Ghirardi-Rimini-Weber theory in 1987. He remained unimpressed by the Copenhagen interpretation, which he regarded as too subjective. His wife Mary reported that Bell was an atheist, a fact that sits naturally alongside his insistence that physics describe an observer-independent reality.

• 6. Special Relativity and the Spaceship Paradox

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  Bell's concerns about physical clarity extended beyond quantum mechanics. He also took a keen interest in how special relativity was taught, and he was not shy about saying that the standard pedagogical approach had problems. In 1985, he warned that Einstein's approach to special relativity was, in his opinion, pedagogically dangerous.

  By 1989, on the occasion of the centenary of the Lorentz-FitzGerald contraction, Bell wrote that a great deal of nonsense had been written on the subject. He preferred to understand Lorentz-FitzGerald contraction as a real, observable property of a material body, a view he shared with Einstein but felt Einstein's own presentation obscured. His collaborator Johann Rafelski later described Bell's position in detail in the 2017 textbook "Relativity Matters."

  To combat the confusion he saw around him, Bell adapted and promoted a thought experiment that became widely known as Bell's spaceship paradox. Though he left only one written report on the topic of teaching special relativity, titled "How to teach special relativity," his colleagues at CERN knew it as a subject he raised repeatedly and cared about deeply.

• 7. Death and Recognition

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  Bell died unexpectedly in Geneva on the 1st of October 1990, from a cerebral hemorrhage. He had not known that he had reportedly been nominated for a Nobel Prize that year.

  The Nobel Prize in Physics was eventually awarded in 2022 to Alain Aspect, John Clauser, and Anton Zeilinger for work on Bell inequalities and the experimental validation of Bell's theorem. The prize that Bell never received in life thus became a landmark in the history of physics through the work it had inspired.

  At CERN's Meyrin site near Geneva, a street called Route Bell was named in his honour. In Belfast, since 2015, a street has carried the name Bell's Theorem Crescent. The John Bell House, completed in 2016, houses over 400 students in Belfast city centre. The pedestrian entrance to the Olympia leisure centre, located about 200 metres from his childhood home, bears his full name. A physics lecture theatre at the Queen's University of Belfast is named for him, and two blue plaques mark his memory: one on the main campus of Queen's, and one at his childhood home on Tates Avenue.

  In 2008, the Centre for Quantum Information and Quantum Control at the University of Toronto created the John Stewart Bell Prize, awarded every other year for significant contributions first published within the preceding six years. The first award, in 2009, was presented by Alain Aspect to Nicolas Gisin for work on quantum nonlocality, quantum cryptography, and quantum teleportation. In 2017, the Institute of Physics commissioned composer Matthew Whiteside to write Quartet No 4 (Entangled) for performance at the 2018 Northern Ireland Science Festival, a piece inspired by Bell's work that went on to become the title track of Whiteside's second album.