In 1935 Albert Einstein and two colleagues, Boris Podolsky and Nathan Rosen (EPR) developed a thought experiment to demonstrate what they felt was a lack of completeness in quantum mechanics. This so-called "EPR paradox" has led to much subsequent, and still on-going, research. This article is an introduction to EPR, Bell's inequality, and the real experiments which have attempted to address the interesting issues raised by this discussion.

One of the principal features of quantum mechanics is that not all the classical physical observables of a system can be simultaneously known, either in practice or in principle. Instead, there may be several sets of observables which give qualitatively different, but nonetheless complete (maximal possible) descriptions of a quantum mechanical system.

These sets are sets of "good quantum numbers," and are also known as "maximal sets of commuting observables." Observables from different sets are "noncommuting observables." A well known example of noncommuting observables is position Q and momentum P. [P,Q] = PQ - QP = ih. You can put a subatomic particle into a state of well-defined momentum, but then you cannot know where it is -- it is, in fact, everywhere at once. It's not just a matter of your inability to measure, but rather, an intrinsic property of the particle.

Conversely, you can put a particle in a definite position, but then its momentum is completely ill-defined. You can also create states of intermediate knowledge of both observables. If you confine the particle to some arbitrarily large region of space, you can define the momentum more and more precisely. But you can never know both, exactly, at the same time.

Sarfatti Comment : The above discussion is based on the premise that the observing system is different from the observed system. We get a counter-intuitive result for a complex self-measuring device in which it is possible for that unique self- referential device to have arbitrarily precise simultaneous knowledge of special pairs of non-commuting observables. This has been shown by David Albert in his recent book The Quantum Mechanics of Experience. Albert calls this "photographs of other worlds".

Position and momentum are continuous observables. But the same situation can arise for discrete observables such as spin. The quantum mechanical spin of a particle along each of the three space axes is a set of mutually noncommuting observables. You can only know the spin along one axis at a time. A proton with spin "up" along the x-axis has undefined spin along the y and z axes. You cannot simultaneously measure the x and y spin projections of a proton.

EPR sought to demonstrate that this phenomenon could be exploited to construct an experiment which would demonstrate a paradox which they believed was inherent in the quantum-mechanical description of the world. They imagined two physical systems that are allowed to interact initially so that they subsequently will be defined by a single Schrodinger wave equation (SWE).

[For simplicity, imagine a simple physical realization of this idea - a neutral pion at rest in your lab, which decays into a pair of back-to-back photons. The pair of photons is described by a single two-particle wave function.]

Once separated, the two systems [read: photons] are still described by the same SWE, and a measurement of one observable of the first system will determine the measurement of the corresponding observable of the second system.

[Example: The neutral pion is a scalar particle - it has zero angular momentum. So the two photons must speed off in opposite directions with opposite spin. If photon 1 is found to have spin up along the x-axis, then photon 2 *must* have spin down along the x-axis, since the total angular momentum of the final-state, two-photon, system must be the same as the angular momentum of the intial state, a single neutral pion. You know the spin of photon 2 even without measuring it.]

Likewise, the measurement of another observable of the first system will determine the measurement of the corresponding observable of the second system, even though the systems are no longer physically linked in the traditional sense of local coupling. However, QM prohibits the simultaneous knowledge of more than one mutually noncommuting observable of either system.

The paradox of EPR is the following contradiction: For our coupled systems, we can measure observable A of system I [for example, photon 1 has spin up along the x-axis; photon 2 must therefore have x-spin down.] and observable B of system II [for example, photon 2 has spin down along the y-axis; therefore the y-spin of photon 1 must be up.] thereby revealing both observables for both systems, contrary to QM. QM dictates that this should be impossible, creating the paradoxical implication that measuring one system should "poison" any measurement of the other system, no matter what the distance between them.

[In one commonly studied interpretation, the mechanism by which this proceeds is 'instantaneous collapse of the wavefunction'. But the rules of QM do not require this interpretation, and several other perfectly valid interpretations exist.]

The second system would instantaneously be put into a state of well-defined observable A, and, consequently, ill-defined observable B, spoiling the measurement. Yet, one could imagine the two measurements were so far apart in space that special relativity would prohibit any influence of one measurement over the other.

[After the neutral-pion decay, we can wait until the two photons are a light-year apart, and then "simultaneously" measure the x-spin of photon 1 and the y- spin of photon 2. QM suggests that if, for example, the measurement of the photon 1 x-spin happens first, this measurement must instantaneously force photon 2 into a state of ill-defined y-spin, even though it is light-years away from photon 1.}

Sarfatti Comment: "... unless he is subtle and perspicacious, he cannot perceive the substance in intelligence reports. It is subtle, subtle!" Sun-tzu, The Art of War. Yes, the above argument by Blanton is elegant. It shows that one can violate the Heisenberg uncertainty principle if there is no faster-than-light action at a distance. In other words, we need faster-than-light quantum influences at the individual quantum event level in order to preserve the uncertainty principle. Heinz Pagels shows this using the original momentum-position wave function of EPR in his popular book, The Cosmic Code. Blanton's argument is the Bohm/Bell variation. Blanton's particular intriguing way of making the EPR argument seems to dispense with the necessity for "counterfactuality". There are some subtle hidden assumptions in Blanton's clever argument which a bit of history may show as follows: Assuming objective reality, and that the wave function applies to an individual pair of particles, and that what might have been measured but wasn't would have a definite result (counterfactuality), then, without a physically real faster-than-light influence connecting the two detections of the two particles in the same individual pair, it is possible to have simultaneous knowledge of two noncommuting observables of the same particle! The argument goes as follows. Let Sx(1) be the observable for particle 1 that measures the spin along the x axis. Similarly, Sy(1) measures the spin of particle 1 along the y axis.

Now these two observables for the same particle do not commute. In fact, Sx(1)Sy(1) - Sy(1)Sx(1) = iSz(1) Similarly for particle 2. But notice that Sx(1) commutes with Sy(2). Sx(1)Sy(2) - Sy(2)Sx(1) = 0 The original argument of EPR would then proceed as follows. Adam and Eve each have a detector and the geometry is such that the spacetime interval between the arrival of particles 1 and 2, respectively, for the same individual pair, at their respective detectors is spacelike. Suppose that Adam chooses to measure observable Sx(1) of particle 1 and finds that particle 1 has spin "up" along the x axis. Adam can correctly infer that the far away twin particle 2 has spin "down" along the x axis of space. Note, that we have already implicitly assumed that the wave function applies to the individual particle pair and is not some kind of idea only defined in terms of statistical ensembles. We are also considering an extreme case in which there is only one pair in the apparatus at any one time and we have ideal detectors. This is a gedankenexperiment.

Now we invoke the EPR criterion of reality which says, in Einstein-et-al's original words, that: If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity. Therefore, for that individual pair, particle 2 really had spin "down". Now comes "counterfactuality". Suppose, instead, by an act of free will, Adam decided to measure Sy(1) and found particle 1 to have spin "up" along the y axis of space. Therefore, he can correctly infer that particle 2 really has spin "down" along the y axis of space. Locality now enters the argument. Locality refers to the phrase "If without in any way disturbing a system".

There are actually four assumptions here. 1. The EPR criterion of reality. 2. The wave function describes the individual particle pair. 3. Counterfactuality: what could have been measured, but wasn't, would have a definite result. 4. Locality, i.e., absence of any faster-than-light influence from the detection of particle 1 that could disturb the detection of particle 2. Therefore, one can correctly infer, as EPR did, that particle 2 must have definite objectively real properties corresponding to the noncommuting observables Sx(2) and Sy(2) of the same particle 2. That is, in the particular individual nonseparable quantum event cited above, particle 2 both had spin "down" along the x axis simultaneous with it having spin "down" along the y-axis. Therefore, the Heisenberg uncertainty principle is violated.

One way out of the dilemma is to reject locality. Bohm's model of quantum mechanics demands this option. The many-worlds interpretations keep locality but reject counterfactuality. Whichever route one takes is strange compared to common sense. The only way to settle the dispute is to find a violation of quantum mechanics which selects one option over the others. For example, if one can construct a device to communicate faster than light over the quantum connection that would settle it. It would violate quantum mechanics today and it would violate the causality structure of relativity. There is evidence of such violations.

It is an experimental issue that cannot be decided by ideology and flaming. How do we reconcile the fact that photon 2 "knows" that the x-spin of photon 1 has been measured, even though they are separated by light-years of space and far too little time has passed for information to have travelled to it according to the rules of special relativity?

There are basically two choices. You can accept the postulates of QM as a fact of life, in spite of its seemingly uncomfortable coexistence with special relativity, or you can postulate that QM is not complete, that there *was* more information available for the description of the two-particle system at the time it was created, carried away by both photons, and that you just didn't know it because QM does not properly account for it.

So, EPR postulated that the existence of hidden variables, some so-far unknown properties, of the systems should account for the discrepancy. Their claim was that QM theory is incomplete; it does not completely describe the physical reality. System II knows all about System I long before the scientist measures any of the observables, thereby supposedly consigning the other noncommuting observables to obscurity. No instantaneous action-at-a- distance is necessary in this picture, which postulates that each System has more parameters than are accounted by QM.

Niels Bohr, one of the founders of QM, held the opposite view and defended a strict interpretation, the Copenhagen Interpretation, of QM. In 1964 John S. Bell proposed a mechanism to test for the existence of these hidden parameters, and he developed his inequality principle as the basis for such a test.

Using the example of two photons configured in the singlet state, consider this: After separation, each photon will have spin values for each of the three axes of space, and each spin can have one of two values; call them up and down. Call the axes A, B and C and call the spin in the A axis A+ if it is up in that axis, otherwise call it A-. Use similar definitions for the other two axes. Now perform the experiment.

Measure the spin in one axis of one particle and the spin in another axis of the other photon. If EPR were correct, each photon will simultaneously have properties for spin in each of axes A, B and C. Look at the statistics. Perform the measurements with a number of sets of photons. Use the symbol N(A+, B-) to designate the words "the number of photons with A+ and B-." Similarly for N(A+, B+), N(B-, C+), etc. Also use the designation N(A+, B-, C+) to mean "the number of photons with A+, B- and C+," and so on.

It's easy to demonstrate that for a set of photons (1) N(A+, B-) = N(A+, B-, C+) + N(A+, B-, C-) because all of the (A+, B-, C+) and all of the (A+, B-, C-) photons are included in the designation (A+, B-), and nothing else is included in N(A+,B-). You can make this claim if these measurements are connected to some real properties of the photons.

Let n[A+, B+] be the designation for "the number of measurements of pairs of photons in which the first photon measured A+, and the second photon measured B+." Use a similar designation for the other possible results. This is necessary because this is all it is possible to measure. You can't measure both A and B of the same photon. Bell demonstrated that in an actual experiment, if (1) is true (indicating real properties), then the following must be true: (2) n[A+, B+] <= n[A+, C+] + n[B+, C-]. Additional inequality relations can be written by just making the appropriate permutations of the letters A, B and C and the two signs.

This is Bell's inequality principle, and it is proved to be true if there are real (perhaps hidden) parameters to account for the measurements. At the time Bell's result first became known, the experimental record was reviewed to see if any known results provided evidence against locality. None did. Thus an effort began to develop tests of Bell's inequality.

A series of experiments was conducted by Aspect ending with one in which polarizer angles were changed while the photons were `in flight'. This was widely regarded at the time as being a reasonably conclusive experiment confirming the predictions of QM.

Aspect measured the time delays between detections of photon pairs. The critical time delay is that between when a polarizer angle is changed and when this affects the statistics of detecting photon pairs. Aspect estimated this time based on the speed of a photon and the distance between the polarizers and the detectors. Quantum mechanics does not allow making assumptions about *where* a particle is between detections. We cannot know *when* a particle traverses a polarizer unless we detect the particle *at* the polarizer.

In the 1970's Eberhard derived Bell's result without reference to local hidden variable theories; it applies to all local theories. Eberhard also showed that the nonlocal effects that QM predicts cannot be used for superluminal communication. The subject is not yet closed, and may yet provide more interesting insights into the subtleties of quantum mechanics.

Sarfatti Comment: As already mentioned above: see the new paper by Henry Pierce Stapp in the July, 1994 issue of Physical Review A on experiments showing violation of the statistical predictions of quantum mechanics by living matter and a model that permits communication on the nonlocal quantum connection that acts from the future to the past. This does not violate Eberhard's theorem because the latter assumes the validity of the statistical predictions of quantum mechanics as a premise. Furthermore, the above article does not point out that if one assumes that the quantum wave function applies to the individual quantum particle pair, as it does in Bohm's interpretation, and if one assumes counterfactuality, then violation of Bell's inequality implies a faster than light quantum action at a distance in violation of the "spirit" of the retarded causality postulate of special relativity. There is no violation of the "substance" of retarded causality because there is no local nonrandom probability shift at one detector of one particle in the pair that can be controlled by changing a parameter at the spacelike separated detector of the twin particle. This means that it is impossible to locally decode the superluminal message encoded by a willful changing of the relative orientation of the two spin-sensitive detectors from the "transmitter" detector side. One can only see the message in hindsight after correlating the locally random data from both detectors. This correlation does have a practical application in untappable quantum cryptography.

The various many-worlds interpretations violate counterfactuality and they need not have any real faster-than-light quantum action-at-a-distance. Murray Gell-Mann takes this position in his book, The Quark and the Jaguar where he cites a letter I helped to write as an example of "flapdoodle" in "The Story Distorted". In fact, it is Murray who is distorting the story because he is a biased advocate of his particular "decoherence" approach to the quantum measurement problem. Roger Penrose in his book, Shadows of the Mind takes the "flapdoodle" position that I do. Penrose also has a good discussion of the role of "counterfactuality".

REFERENCES:

1. A. Einstein, B. Podolsky, N. Rosen: "Can quantum-mechanical description of physical reality be considered complete?" Physical Review 41, 777 (15 May 1935). (The original EPR paper)

2. D. Bohm: Quantum Theory, Dover, New York (1957). (Bohm discusses some of his ideas concerning hidden variables.)

3. N. Herbert: Quantum Reality, Doubleday. (A very good popular treatment of EPR and related issues)

4. M. Gardner: Science - Good, Bad and Bogus, Prometheus Books. (Martin Gardner gives a skeptics view of the fringe science associated with EPR.)

Sarfatti Comment: The article "Magic and Paraphysics" in Gardner's book is mostly about my early immature ideas on the subject in the mid 70s. Gardner's book is totally out of date. My comments above reflect my current thinking modifed by the recent work of Roger Penrose with Stuart Hameroff, Brian Josephson and Douglas Home, Yakir Aharonov, Lev Vaidman and David Albert, Henry Pierce Stapp and the experiments by Ray Chiao's group at UCB.

Thanks to Carlton Caves and Mike Gallis for making explicit the need for a new kind of quantum mechanics that violates the unitarity postulate. This new quantum mechanics also violates the relativistic causality postulate in the strongest way possible. I believe it to be the essential physical signature of consciousness in the universe.

5. J. Gribbin: In Search of Schrodinger's Cat, Bantam Books. (A popular treatment of EPR and the paradox of "Schrodinger's Cat" that results from the Copenhagen interpretation)

6. N. Bohr: "Can quantum-mechanical description of physical reality be considered complete?" Physical Review 48, 696 (15 Oct 1935). (Niels Bohr's response to EPR)

7. J. Bell: "On the Einstein Podolsky Rosen paradox" Physics 1 #3, 195 (1964).

8. J. Bell: "On the problem of hidden variables in quantum mechanics" Reviews of Modern Physics 38 #3, 447 (July 1966).

9. D. Bohm, J. Bub: "A proposed solution of the measurement problem in quantum mechanics by a hidden variable theory" Reviews of Modern Physics 38 #3, 453 (July 1966).

10. B. DeWitt: "Quantum mechanics and reality" Physics Today p.30 (Sept 1970).

11. J. Clauser, A. Shimony: "Bell's theorem: experimental tests and implications" Rep. Prog. Phys. 41, 1881 (1978).

12. A. Aspect, Dalibard, Roger: "Experimental test of Bell's inequalities using time-varying analyzers" Physical Review Letters 49 #25, 1804 (20 Dec 1982).

13. A. Aspect, P. Grangier, G. Roger: "Experimental realization of Einstein-Podolsky-Rosen-Bohm gedankenexperiment; a new violation of Bell's inequalities" Physical Review Letters 49 #2, 91 (12 July 1982).

14. A. Robinson: "Loophole closed in quantum mechanics test" Science 219, 40 (7 Jan 1983).

15. B. d'Espagnat: "The quantum theory and reality" Scientific American 241 #5 (November 1979).

16. "Bell's Theorem and Delayed Determinism", Franson, Physical Review D, pgs. 2529-2532, Vol. 31, No. 10, May 1985.

17. "Bell's Theorem without Hidden Variables", P. H. Eberhard, Il Nuovo Cimento, 38 B 1, pgs. 75-80, (1977).

18. "Bell's Theorem and the Different Concepts of Locality", P. H. Eberhard, Il Nuovo Cimento 46 B, pgs. 392-419, (1978).