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 of Newton that preceded it in one very important way. If a Newtonian system breaks up, each of its parts has a definite and well-defined energy, momentum, and angular momentum, parceled out at breakup by the system while respecting conservation laws.  After the component parts are separated, their properties are completely independent and do not depend on each other.

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".  Schrödinger 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.

How and why is quantum mechanics nonlocal?  Nonlocality comes from two seemingly conflicting aspects of the quantum formalism: (1) energy, momentum, and angular momentum, important  properties of light and matter, are conserved in all quantum systems in the sense that, in the absence of external forces and torques,  their net values must remain unchanged as the system evolves, while (2) in the wave functions describing emitted particles in a quantum system, as indicated by the uncertainty principle, the conserved quantities may be indefinite and unspecified and typically can span a large range of possible values.  This non-specifity persists until a measurement is made that "collapses" the wave function and fixes the measured quantities with specific values.  These seemingly inconsistent requirements of (1) and (2) raise an important question: how can the wave functions describing the separated members of a system of particles, which may be light-years apart, have arbitrary and unspecified values for the conserved quantities and yet respect the conservation laws when the wave functions are collapsed?

This paradox is resolved in quantum mechanics because the quantum wave functions of particles are entangled, a term coined by Schrödinger and meaning that even when the wave functions describe system parts that are spatially separated and out of light-speed contact, the separate wave functions continue to depend on each other and cannot be separately specified.   In particular, their conserved quantities in the components must add up to the values possessed by the overall quantum system before it separated into parts.

How could this entanglement and preservation of conservation laws possibly be arranged by Nature?  The mathematics of quantum mechanics gives us no answers to this question, it only insists that the wave functions of separated parts of a quantum system must depend on each other.  Theorists prone to abstraction have found it convenient to abandon the three-dimensional universe and describe such quantum systems as residing in a many-dimensional Hilbert hyper-space, with conserved variables forming extra dimensions, in which the interconnections between particle wave functions are represented as allowed sub-regions of the overall hyper-space.  That has led to elegant mathematics but is not much help in visualizing what is really going on in the physical world.  The two leading interpretations of the quantum formalism, the Copenhagen interpretation and the many-worlds interpretation (MWI), are both silent on how nonlocality and entanglement actually work.  Hugh Everett III, the originator of the MWI, labeled EPR nonlocality as a "false paradox" in his original paper and promised to address nonlocality in a later publication, which never appeared.  David Bohm's interpretation of quantum mechanics (which is really an alternative to the orthodox quantum formalism) attempts to  accommodate nonlocality by hypothesizing a "nonlocal field", but Bohm did not explain what precisely that was, how it operated, or how its superluminal aspects and preferred reference frame avoided conflicts with special relativity.

The only interpretation of quantum mechanics that, to my knowledge, adequately explains quantum nonlocality is the transactional interpretation (TI), which I originated in 1986.   The TI provides a tight and satisfying description of nonlocality by interpreting the "psi-star" complex conjugate versions of quantum wave functions, always present in the quantum formalism, as advanced waves that go backwards in space-time, back down the paths of particles wave functions "psi", and "handshake" with the emitter, meshing only when conserved quantities match, thereby permitting potential quantum events to emerge into reality only when the conservation laws are observed.  This allows the treatment of quantum wave functions as real waves traveling in 3-D space, without the need for casting them into some synthetic Hilbert hyperspace.  Thus in the TI the system evolves in space-time while preserving conservation laws by "feeling its way into the future" with multiple transactional handshakes.

In the October 28, 2012 online edition of Nature Physics, a paper entitled "Quantum non-locality based on finite speed causal influences leads to superluminal signaling" by J. D. Bancal, et al,  examine the nonlocality of quantum mechanics from another direction.  They consider Bell-type EPR experiments in which entangled pairs of photons are given entangled polarizations by the emission process (through angular momentum conservation) and their polarization states are measured in some selected polarization basis (H/V linear, ±45° linear, or L/R circular) by downstream detectors.  Quantum mechanics requires that whenever the detection bases of two such measurements match, the measured values must also match.  This requirement is called an "EPR correlation", referring to the work of Einstein, Podolsky, and Rosen.  (For a full discussion of such tests of quantum mechanics, see my AV column  "Einstein's Spooks and Bell's Theorem" in the January-1990 issue of Analog Science Fiction/Fact.)

The authors of the Bancal paper assume that they can replace orthodox quantum mechanics by some unspecified semi-classical process in which the "causal influences" have a well defined propagation velocity and travel between measurements to insure that the polarization correlations match.  It has already been well established through the work of J. S. Bell and others that any such causal influences traveling as velocities less than or equal to the speed of light cannot account for the EPR correlations observed in Bell-type EPR experiments.  The authors of the Bancal paper extend consideration to include causal influences traveling as velocities greater than to the speed of light. They show that causal influences traveling as velocities greater than to the speed of light can indeed account for EPR correlations, but the assumption of superluminal influences carries with it the inevitable consequence that signaling between observers at the superluminal speed of the causal influences becomes possible.

Special relativity forbids such signaling at well-defined superluminal speeds because its existence would allow the discovery of a preferred reference frame and would destroy the even-handedness with which relativity treats all inertial reference frames.  Thus, the authors concluded that no semiclassical explanation of quantum nonlocality and EPR correlations is possible, even when superluminal causal influences are allowed.

We note that extensions of the Many-Worlds interpretation have attempted to deal with quantum nonlocality by hypothesizing a traveling "split" between worlds, i.e., universes, that originates at the site on one measurement and propagates to the sites of other measurements, in order to arrange consistent EPR correlations between measurement results.  This moving split is just the kind of moving causal influence with a well defined propagation velocity that has been ruled out by the Bancal paper.

What's wrong with the Bancal paper and its arguments?  Nothing, really, in that it presents a hypothesis that some have seriously entertained and then demonstrates its unacceptable implications.   However, the basic approach, one that has been taken by many other works in the physics literature, seems intended to mystify and obscure quantum mechanics and nonlocality rather than to clarify and understand them.

The transactional interpretation, which is not referenced or considered in the Bancal paper, describes "causal influences", i.e., the wave functions psi and psi-star of the emitted entangled photons, as propagating in both time directions along the trajectories of the particles and handshaking to observe conservation laws by building in the observed EPR correlations.  The causal influences are not superluminal, but rather retro-causal.  Does this causal link imply that superluminal signaling is possible?  Not in the sense considered in the Bancal paper.  The lines of communication for the entangled EPR photons, as described by the transactional interpretation,  are all along light-like world lines that transform properly under the Lorentz transformations of relativity, favoring no preferred inertial reference frame and remaining completely consistent with special relativity.

Nonlocal signaling is not forbidden by the transactional interpretation, but it is also not required.  The question of whether quantum nonlocality and entanglement can be used for signaling between observers remains as an open question.  It would not be in conflict with special relativity, but it might lead to violations of causality.  It is widely believed in the physics community that nonlocal signaling is impossible, and indeed there are "no-signal theorems" in the physics literature that claim prove this mathematically.  However, there are also papers pointing out flaws in these "theorems" and demonstrating that they would also rule out well-established quantum properties like Bose-Einstein symmetrization.  Thus, from my own point of view, the possibility of nonlocal signaling is an open question that urgently needs to be tested experimentally.  

Followup Note (11/28/2014):  Nonlocal signaling appears to be blocked by the complementarity of one-particle and two-particle interference.  This is discussed in some detail in a paper that I and a colleague are publishing, which can be accessed at the online ArXiv at: http://arxiv.org/pdf/1409.5098.pdf .  This is also discussed in a chapter of my new book described below.

JGC

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Superluminal 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 .

The EPR Paper:

Albert Einstein, Boris Podolsky, and Nathan Rosen, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?", Physical Review 47, 777-780 (1935).

Bell 's Inequalities:

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).

EPR Experiments:

Stuart J. Freedman and John F. Clauser, "Experimental Test of Local Hidden-Variable Theories", Physical Review Letters 28, 938-941 (1972).

The 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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