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Making Sense of Bell’s Theorem and Quantum Nonlocality

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Abstract

Bell’s theorem has fascinated physicists and philosophers since his 1964 paper, which was written in response to the 1935 paper of Einstein, Podolsky, and Rosen. Bell’s theorem and its many extensions have led to the claim that quantum mechanics and by inference nature herself are nonlocal in the sense that a measurement on a system by an observer at one location has an immediate effect on a distant entangled system (one with which the original system has previously interacted). Einstein was repulsed by such “spooky action at a distance” and was led to question whether quantum mechanics could provide a complete description of physical reality. In this paper I argue that quantum mechanics does not require spooky action at a distance of any kind and yet it is entirely reasonable to question the assumption that quantum mechanics can provide a complete description of physical reality. The magic of entangled quantum states has little to do with entanglement and everything to do with superposition, a property of all quantum systems and a foundational tenet of quantum mechanics.

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Notes

  1. David Bohm, building on de Broglie’s notion of a pilot wave, had already created a nonlocal hidden variable theory [7] that reproduced all the predictions of non-relativistic quantum mechanics. Because of its nonlocality, it does not violate Bell’s inequality.

  2. However, because these correlations can only be interpreted in terms of a quantum mechanical statistical distribution of many such observations, no usable information can be transmitted from A to B in this way thereby preserving consistency with special relativity.

  3. Rather than embracing either nonlocality or solipsism, Mermin’s [4] more balanced position is simply that the experimental verification of the violation of Bell’s inequality provides direct evidence that excludes Einstein’s particular concept of an “independent existence of the physical reality.”

  4. As just mentioned, Bell and Wiseman argue that even if quantum mechanics is incomplete, the quantum mechanical violation of Bell’s inequality implies that it either violates the principles of relativity or objective reality does not exist.

  5. Mermin [19] provided a simpler version of this construction.

  6. Noncontextuality is the assumption that a hidden variable theory must assign to an observable a value that is independent of the complete disposition of the relevant measuring apparatus.

  7. The standard Copenhagen interpretation of quantum mechanics is counterfactually indefinite because one cannot make definite statements about the hypothetical results of measurements of quantities corresponding to non-commuting operators.

  8. Wigner [22] made essentially the same point.

  9. Bell’s 1964 paper has been cited more than 6000 times in the last 10 years compared to only a handful of citations of Furry’s 1936 paper in that time period.

  10. Also, these experiments proved to be seminal in the emerging field of quantum information and quantum computing.

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  1. Princeton University, Princeton, NJ, 08544, USA

    Stephen Boughn

  2. Haverford College, Haverford, PA, 19041, USA

    Stephen Boughn

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Correspondence to Stephen Boughn.

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This is an expanded version of a talk given at the 2016 Princeton-TAMU Symposium on Quantum Noise Effects in Thermodynamics, Biology and Information [1].

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Boughn, S. Making Sense of Bell’s Theorem and Quantum Nonlocality. Found Phys 47, 640–657 (2017). https://doi.org/10.1007/s10701-017-0083-6

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