John Stewart Bell

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John Stewart Bell, 1988

John Stewart Bell (June 28 1928October 10 1990) was an Irish physicist who worked in the field of particle physics at CERN, and who developed one of the most important theorems of quantum physics, Bell's Theorem.


  • To know the quantum mechanical state of a system implies, in general, only statistical restrictions on the results of measurements. It seems interesting to ask if this statistical element be thought of as arising, as in classical statistical mechanics, because the states in question are averages over better defined states for which individually the results would be quite determined. These hypothetical 'dispersion free' states would be specified not only by the quantum mechanical state vector but also by additional 'hidden variables' - 'hidden' because if states with prescribed values of these variables could actually be prepared, quantum mechanics would be observably inadequate.
    • "On the problem of hidden variables in quantum mechanics". Reviews of Modern Physics (1966)
  • Theoretical physicists live in a classical world, looking out into a quantum-mechanical world. The latter we describe only subjectively, in terms of procedures and results in our classical domain.
    • "Introduction to the hidden-variable question" (1971), included in Speakable and Unspeakable in Quantum Mechanics (1987), p. 29
  • The concept of 'measurement' becomes so fuzzy on reflection that it is quite surprising to have it appearing in physical theory at the most fundamental level... does not any analysis of measurement require concepts more fundamental than measurement? And should not the fundamental theory be about these more fundamental concepts?
  • A final moral concerns terminology. Why did such serious people take so seriously axioms which now seem so arbitrary? I suspect that they were misled by the pernicious misuse of the word ‘measurement’ in contemporary theory. This word very strongly suggests the ascertaining of some preexisting property of some thing, any instrument involved playing a purely passive role. Quantum experiments are just not like that, as we learned especially from Bohr. The results have to be regarded as the joint product of ‘system’ and ‘apparatus,’ the complete experimental set-up.
    • "On the impossible pilot wave" (1982), included in Speakable and Unspeakable in Quantum Mechanics (1987), p. 166
  • I am a Quantum Engineer, but on Sundays I Have Principles.
    • Opening sentence of his "underground colloquium" in March 1983, as quoted by Nicolas Gisin in an edition by J. S. Bell, Reinhold A. Bertlmann, Anton Zeilinger (2002). Quantum [un]speakables: from Bell to quantum information. Springer. p. 199. ISBN 3540427562. 
  • While the founding fathers agonized over the question 'particle' or 'wave', de Broglie in 1925 proposed the obvious answer 'particle' and 'wave'. Is it not clear from the smallness of the scintillation on the screen that we have to do with a particle? And is it not clear, from the diffraction and interference patterns, that the motion of the particle is directed by a wave? De Broglie showed in detail how the motion of a particle, passing through just one of two holes in screen, could be influenced by waves propagating through both holes. And so influenced that the particle does not go where the waves cancel out, but is attracted to where they cooperate. This idea seems to me so natural and simple, to resolve the wave-particle dilemma in such a clear and ordinary way, that it is a great mystery to me that it was so generally ignored.
    • "Six Possible Worlds of Quantum Mechanics" (1986), included in Speakable and Unspeakable in Quantum Mechanics (1987), p. 191

Against 'mesurement' (1990)[edit]

"Against 'mesurement'", Physics World (August 1990)

  • Surely, after 62 years, we should have an exact formulation of some serious part of quantum mechanics? By 'exact' I do not of course mean 'exactly true'. I mean only that the theory should be fully formulated in mathematical terms, with nothing left to the discretion of the theoretical physicist . . . until workable approximations are needed in applications. By 'serious' I mean that some substantial fragment of physics should be covered. Nonrelativistic 'particle' quantum mechanics, perhaps with the inclusion of the electromagnetic field and a cut-off interaction, is serious enough.
  • I agree with them about that: ORDINARY QUANTUM MECHANICS (as far as I know) IS JUST FINE FOR ALL PRACTICAL PURPOSES. Even when I begin by insisting on this myself, and in capital letters, it is likely to be insisted on repeatedly in the course of the discussion. So it is convenient to have an abbreviation for the last phrase: FOR ALL PRACTICAL PURPOSES = FAPP.
  • I expect that mathematicians have classified such fuzzy logics. Certainly they have been much used by physicists. But is there not something to be said for the approach of Euclid? Even now that we know that Euclidean geometry is (in some sense) not quite true? Is it not good to know what follows from what, even if it is not necessarily FAPP? Suppose for example that quantum mechanics were found to resist precise formulation. Suppose that when formulation beyond FAPP was attempted, we find an unmovable finger obstinately pointing outside the subject, to the mind of the observor, to the Hindu scriptures, to God, or even only Gravitation? Would that not be very, very interesting?
  • The concepts 'system', 'apparatus', 'environment', immediately imply an artificial division of the world, and an intention to neglect, or take only schematic account of, the interaction across the split. The notions of 'microscopic' and 'macroscopic' defy precise definition. So also do the notions of 'reversible' and 'irreversible'. Einstein said that it is theory which decides what is 'observable'. I think he was right - 'observation' is a complicated and theory-laden business. Then that notion should not appear in the formulation of fundamental theory. Information? Whose information? Information about what? On this list of bad words from good books, the worst of all is 'measurement'. It must have a section to itself.
  • The first charge against 'measurement', in the fundamental axioms of quantum mechanics, is that it anchors there the shifty split of the world into 'system' and 'apparatus'. A second charge is that the word comes loaded with meaning from everyday life, meaning which is entirely inappropriate in the quantum context.

Quotes about Bell[edit]

  • We must thank John Bell for having shown us that philosophical questions about the nature of reality could be translated into problems for physicists, where naive experimentalists can contribute.
    • Alain Aspect, "Bell's Theorem: The Naive View of an Experimentalist", in Quantum [Un]speakables (2002) edited by Reinhold A. Bertlmann and Anton Zeilinger
  • I had never met Bell, nor heard him lecture, but in my reading of his scientific papers I have developed a great admiration for him and his work. I have especially admired his attempts to dismantle the orthodox Copenhagen interpretation of quantum theory, written with such tremendous style and obvious enjoyment. Although in this book I have tried to present a balanced account - arguing one way and then another - I hope that I have done justice to Bell's superbly constructed criticisms. The debate over the meaning of quantum theory will certainly be poorer without him.
    • Jim Baggott, The Meaning of Quantum Theory (1992), Preface
  • In my opinion, John Bell performed an extremely important role then, and also later, in generally supporting - thereby making respectable - the apparently "fringe" activities of such people as Karolyhazy, Bohm, Pearle, Ghirardi, and many others (including myself) in suggesting schemes that go beyond standard quantum mechanics, in the intended direction of realism. No physicist could doubt the scientific credentials of John Bell. The fact that he was prepared to go out of his way to support research of this kind gave it a previously unaccustomed status.
    • Roger Penrose, "John Bell, State Reduction, and Quanglement", in Quantum [Un]speakables (2002) edited by Reinhold A. Bertlmann and Anton Zeilinger
  • It was John Bell who investigated quantum theory in the greatest depth and established what the theory can tell us about the fundamental nature of the physical world. Moreover, by stimulating experimental tests of the deepest and most profound aspects of quantum theory, Bell's work led to the possibility of exploring seemingly philosophical questions, such as the nature of reality, directly through experiments.
    And this was just Bell's "hobby".
    • Andrew Whitaker, "John Bell and the most profound discovery of science", Physics World (December 1998)

Alain Aspect, "Introduction: John Bell and the second quantum revolution" (2004)[edit]

Jeremy Bernstein, "John Stewart Bell: Quantum Engineer"[edit]

External links[edit]

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