This column is about a recent paper that appeared in the prestigious journal Nature Physics. The paper claims to prove mathematically that the causal influences exhibited by quantum nonlocality and entanglement can neither be considered to propagate slower than the velocity of light nor to propagate faster than the velocity of light (and therefore, presumably, must not propagate at all.) To understand the issues and context, let me begin this discussion by describing quantum nonlocality and quantum entanglement: what they are and where they come from. I note that what follows is my own view of entanglement and nonlocality, and it will not be found in any current textbook or popularization of quantum mechanics.
Quantum mechanics differs from the classical mechanics
On the other hand, quantum mechanics is nonlocal,
meaning that the component parts of a quantum system may continue to influence
each other, even when they are well separated in space and out of speed-of-light
contact. This characteristic of the
theory was first pointed out by Albert Einstein and his colleagues Boris
Podolsky and Nathan Rosen (EPR) in 1935, in a critical paper in which they held
up the discovered nonlocality as a devastating flaw in quantum theory.
Einstein called nonlocality "spooky actions at a distance".
followed up the discovery of quantum nonlocality by showing in detail how the
components of a multi-part quantum system depend on each other, even when
separated. Beginning in 1972 with
the work of Stuart Freedman and John Clauser, a series of quantum-optic EPR
experiments testing Bell-inequality violations and other aspects of quantum
systems have demonstrated that, like it or not, quantum mechanics and the Nature
it describes are indeed nonlocal. Einstein's
spooky actions at a distance are really out there in the physical world.
Causation in EPR:
"Quantum non-locality based on finite speed causal influences leads to superluminal signaling" by J. D. Bancal, S. Pironio, A. Achin, Y-C Liang, V. Scarani, and N. Gisin, Nature Physics 8, 867–870 (2012); http://arxiv.org/pdf/1110.3795.pdf .
Albert Einstein, Boris Podolsky, and Nathan Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", Physical Review 47, 777-780 (1935).
John S. Bell, "On the Einstein-Podolsky-Rosen paradox", Physics 1, 195-200 (1964);
John S. Bell, "On the Problem of Hidden Variables in Quantum Mechanics", Reviews of Modern Physics 38, 447-452 (1966).
Stuart J. Freedman and John F. Clauser, "Experimental Test of Local Hidden-Variable Theories", Physical Review Letters 28, 938-941 (1972).
Transactional Interpretation of Quantum Mechanics
"The Transactional Interpretation of Quantum Mechanics", John G. Cramer, Reviews of Modern Physics 58, 647 (1986); http://faculty.washington.edu/jcramer/TI/tiqm_1986.pdf .
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