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THE ALTERNATE VIEW
ENTANGLEMENT, SPOOKS, AND SUPERLUMINAL SIGNALS John G. Cramer | 1846 words
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 non-local? 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 non-locality 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 backward in space-time, back down the paths of particle 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, examines 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, http://www.npl.washington.edu/AV/altvw37.html.)
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 the speed of light. They show that causal influences traveling as velocities greater than 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 re
ference 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 of 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 to 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.
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;
•"Why Everettians Should Appreciate the Transactional Interpretation," R. E. Kastner and John G. Cramer, arXiv:quant-ph/1001. 2867; http://arxiv.org/abs/1001.2867.
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks and are available at: http://bookviewcafe.com/bookstore/?s=Cr amer&submit=Search.
Alternate View Columns Online: Electronic reprints of over 160 "The Alternate View" columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl.washington.edu/av.
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THE REFERENCE LIBRARY
Don Sakers | 2367 words
Science fiction has a very mixed record on issues of human diversity.
On one hand, much of SF celebrates diversity in a way no other literature can match. Science fiction taught us to respect intelligent life regardless of its outward form: robot or lizard-creature, microscopic entity or living planet, million-year-old vegetative consciousness or sapient rock, all deserve to be treated fairly.
On the other hand, SF was (and arguably still is) behind the curve when it came to human diversity. How could it be otherwise? Modern SF was born and came of age in the WASP-dominated world of the 1920s–50s; it was largely written and edited by and for white American males of the educated class. The few prominent women writers in the field went by initials (C.L. Moore) or used gender-ambiguous bylines (Leigh Brackett, Andre Norton), allowing squeamish male readers to convince themselves they were reading the work of other men.
When non-WASP characters appeared at all in SF stories of the period, they were treated stereotypically. Female characters, in general, conformed to the most sexist clichés—and the vanishingly few gay characters, always predatory villains, had it even worse.
These excesses were relatively few; much more frequently, non-male-WASP characters were simply absent... which was just a different level of chauvinism.
One can't fault the writers of the day; they were products of their culture. Especially in pre-war America, casual racism and sexism were pervasive in a way that's hard to believe today. Astounding/Analog, as the center of the SF universe at the time, was certainly complicit. Isaac Asimov, in his autobiography In Memory Yet Green, said of the legendary editor John W. Campbell, "He was a devout believer in the inequality of man and felt that the inequality could be detected by outer signs such as skin and hair coloring. Though he treated all men kindly and decently in his personal life... the fact is that, in theory, he felt that people of northwestern European extraction were the best human beings." Campbell was fairly typical in this.
Fortunately, time and the larger culture didn't stand still. As diversity gained traction in America, this progress was reflected in the pages of SF. With the civil rights movement, the women's movement, the counterculture message of nonconformity, and associated phenomena, science fiction became (a little) more diverse.
Occasional characters of color started to appear in SF, especially in the works of the field's leaders Arthur C. Clarke and Robert A. Heinlein—although many readers and critics missed them entirely. Strong, independent women also showed up in the pages of SF stories and novels. Gradually, character names took on more varied ethnic flavors.
Of particular note is a trilogy written by Mack Reynolds, one of the most popular SF writers of the time. The first two books, published as serials in Analog, are Black Man's Burden (December 1961 & January 1962) and Border, Breed nor Birth (July & August 1962). The third, The Best Ye Breed, was not published until 1978. This trilogy deals with a future independent, progressive North Africa and features many characters of color. To modern ears, parts of the stories border on offensive, but they were revolutionary for their time.
In many ways, Star Trek was a fine representative of the SF of this period, with its inclusion of a black woman as an integral part of the Enterprise bridge crew. These days, it's hard to appreciate what a breakthrough Lieutenant Uhura symbolized—just as it's hard to believe that Star Trek was the venue for television's first interracial kiss.
The broadening popularity of the field in the wake of Star Trek brought a greater diversity of readers (and writers). Also, society became more accepting of diversity—in fact, society
seemed almost to move more quickly than SF did. By the 1980s and 1990s, real-life space shuttle crews were more diverse than the average starship crew in SF.
So where are we today? We have characters (and writers) of color and lesbian/gay/transgender characters (and writers)—but they're largely obligatory tokens. Women appear as fully rounded characters, and women writers are well represented in the field—but many women SF writers still have trouble being taken seriously, and the SF community is convulsed over issues of sexism and sexual harassment. Science fiction, by history and (arguably) by its very nature, remains a product of Western culture, imbued will all the associated values—good and bad alike.
Let me share an illustrative anecdote. In 2012, there was a sensation involving the movie The Hunger Games. One beloved character, Rue, was played by an actor of col-or. (In the book, Rue is specifically and carefully described as being what we would call African-American.) A substantial number of viewers objected... vehemently. Apparently, they didn't like having this very sympathetic character portrayed as black. Another substantial number of viewers spoke out against this display of racism, feeling that Rue's skin color was a vital part of the character. Two competing worldviews in the arena of science fiction.
All things being equal, I think science fiction is still much more comfortable with the ideas of cultural and biological diversity than with their reality. We're much more likely to want to read about green-blooded insectoid aliens than about African Muslim women.
Yet I also believe that most of us are intellectually and emotionally honest enough to feel uncomfortable with our discomfort. When push comes to shove, we believe all that stuff about respecting all intelligent life in spite of its outward form... and so, more often than not, we choose to swallow our discomfort and expand our horizons.
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