John Stewart Bell
John Stewart Bell | |
|---|---|
| Born | July 28, 1928 Belfast, Northern Ireland |
| Died | October 1, 1990 (aged 62) Geneva, Switzerland |
| Nationality | Northern Irish |
| Known for | Bell's theorem, Refutation of von Neumann's impossibility proof, Quantum non-locality |
| Scientific career | |
| Fields | Physicist |
John Stewart Bell (28 July 1928 – 1 October 1990) was a Northern Irish theoretical physicist, best known for Bell's theorem (1964) on the impossibility of local hidden-variable theories. He is routinely invoked as the man who closed the question of hidden variables and vindicated the Copenhagen orthodoxy.
That reading inverts his actual position. Bell reopened the question rather than closing it. He demolished the von Neumann "impossibility proof" that had suppressed realist alternatives for thirty years, he admired Bohm's causal interpretation and credited it as the stimulus for his own work, he considered the orthodox account unprofessionally vague, and he was openly willing to contemplate a return to a Lorentzian preferred frame — an aether — to accommodate the non-locality he had discovered. He is therefore a central figure for the critical tradition documented on this wiki, not an opponent of it.
Life and career
Bell was born in Belfast in 1928 to a family of modest means and studied at Queen's University Belfast. He worked on accelerator design and particle physics, joining CERN in 1960, where he spent the rest of his career. His work on the foundations of quantum mechanics — the work for which he is now remembered — was pursued largely as a sideline to his professional duties. He died suddenly in 1990.
The demolition of von Neumann's proof
From 1932, John von Neumann's supposed proof that no hidden-variable theory could reproduce quantum mechanics was treated as settling the matter, and was used for three decades to dismiss realist alternatives without examination. Encountering Bohm's 1952 theory — which manifestly did what the proof said was impossible — led Bell to examine the argument directly. He found it rested on an assumption no plausible hidden-variable theory need satisfy: that the average of a sum of observables must equal the sum of their averages, even for non-commuting quantities.
Bell's verdict was blunt. In a 1988 interview he said of it:
"Yet the von Neumann proof, if you actually come to grips with it, falls apart in your hands! There is nothing to it. It's not just flawed, it's silly!"
The flaw had in fact been identified by the philosopher Grete Hermann as early as 1935 and disregarded. That a widely cited impossibility proof could stand unexamined for a generation, closing off a legitimate research programme, is one of the clearest cases in modern physics of authority substituting for argument.
Bell's theorem
Bell's 1964 theorem showed that no theory in which distant systems have definite pre-existing properties and no faster-than-light influences can reproduce all the correlations predicted by quantum mechanics. Experiments from the 1970s onward have favoured the quantum predictions.
The result is frequently reported as proving that hidden variables are impossible and that realism must be abandoned. It shows nothing of the kind. What it establishes is an incompatibility between quantum predictions and locality combined with definite properties. A non-local hidden-variable theory — Bohm's, for example — is entirely untouched by it. The honest statement of the theorem is that nature is non-local, and the open question is what to make of that.
What Bell himself concluded
Bell drew the realist conclusion. He regarded the orthodox formulation as unsatisfactory, praised Bohm's theory for showing explicitly what had been declared out of the question, and pressed throughout his career for a physics that says what actually happens rather than merely what will be observed. His essays are collected in Speakable and Unspeakable in Quantum Mechanics (1987), including the well-known "Bertlmann's socks and the nature of reality" (1981), written after his CERN colleague Reinhold Bertlmann.
Most strikingly for the tradition documented here, Bell was prepared to resolve the tension between non-locality and relativity by reviving a preferred frame. Interviewed in The Ghost in the Atom (1986), he said:
"[T]he cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincaré thought that there was an aether — a preferred frame of reference — but that our measuring instruments were distorted by motion in such a way that we could not detect motion through the aether. Now, in that way you can imagine that there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light."
This is precisely the neo-Lorentzian position advanced by a number of researchers catalogued on this wiki, and it comes from the physicist whose theorem is most often cited against them. See Lorentz ether theory and the discussion at Relativity.
Reception and misuse
Bell's theorem is among the most-cited results in modern physics, and is commonly presented in textbooks as having vindicated the Copenhagen interpretation and disposed of hidden variables. Critics on this wiki argue that this is a misreading which reverses his intent: the theorem constrains locality, not realism; its author was a realist who thought the orthodoxy evasive; and the proof it displaced was itself a piece of unexamined authority. A substantial critical literature examines the theorem's assumptions directly, including its treatment of measurement independence, detector efficiency, and the derivation of the inequalities themselves.
Criticisms of Bell's theorem from researchers on this wiki
Bell's theorem and the associated EPR problem are extensively treated in the literature catalogued here, including Bell's Theorem and Quantum Mechanics, Significant Facts Revealed by the EPR Paradox and Bell's Theorem, Measurement Theory via Hidden Variables Not Subject to Bell's Theorem, Nonlocal Theories Satisfying Bell's Inequality, A Counter-Example to Bell's Theorem with a 'Softened' Singularity, Nonlocality, Unreality, and Bell Theorem, Demystifying the EPR Paradox and Spooky Action at a Distance: An Explanation of Bell's Theorem by Gary Felder. Further material is indexed under Category:Quantum Theory.