QUANTUM NONLOCALITY AND REALITY – 50 Years of Bell's Theorem

Fifty years ago John Bell published a ground-breaking paper, in which he suggested that quantum physics is nonlocal. In this highly technical book, physicists present their views on Bell's Theorem and what is meant by nonlocality. Several alternative versions of quantum mechanics are also discussed, in particular that of David Bohm.

The implications of Bell's theorem and non-locality are a hot topic not only among physicists but in New Age circles. Many people talk about the "quantum mind", multiple realities, about the nature of consciousness in the same breath as Bell's Theorem. It is often said that quantum mechanics implies the existence of consciousness, or that consciousness can affect quantum processes. Another view is that that "we create our own reality". Our own universe. Unfortunately, most of those views are unwarranted extrapolations of Bell's Theorem. The Theorem is extremely subtle, and has been misunderstood even by specialists in the field. It has far reaching implications in areas that are only now starting to be explored.

In standard quantum physics, it is said that the observer's presence changes the result of an experiment, through wave-function collapse. That the observer is separate from the apparatus. This view lies at the heart of the Copenhagen Interpretation, still regarded as the foundation of quantum physics. A photon for example, is said to behave like a wave or a particle, depending on the experiment that is being performed. Whether or not it is being observed. The circumstances in which we find the photon are key to its nature. Einstein disagreed with this view, just as he also disagreed with the statistical interpretation of the Schrodinger wave function which states that a particle does not have a well-defined path and momentum. Also, he could not accept the apparent separation of the observer and the experiment. Neither did John Bell, who felt that though the Copenhagen Interpretation gave the correct results, the theory lacked clarity. Whereas physicists generally adopt a view dubbed "For all practical purposes" (FAPP) that allows them to remain vague about what an electron is, Bell felt that we would gain additional results and insights if we understood the physics more clearly. Elementary particles are real objects. He called them beables. Beables are not depended on the observer. We know macroscopic beables such as an electric or magnetic field. What kind of beable is a quantum particle? Bell was convinced that we are dealing with a real entity and not a mathematical figment.

Early on in his career, Bell was drawn to the work of David Bohm. Bohm was also dissatisfied with the separation of the observer and the experiment. He viewed quantum behavior as deterministic and not random or statistical. In his paper, Sheldon Goldstein quotes Bell,

"But in 1952 I saw the impossible done…Bohm showed explicitly how parameters could indeed be introduced… with the help of which, the indeterministic description could be transformed into a deterministic one. More important, the subjectivity of the orthodox version, the necessary reference to the observer could be eliminated…"

Bohm's view was that the photon is simultaneously both a wave and a particle, a view also expressed by De Broglie. Its journey from point A to point B follows a definite trajectory, but one that is steered by its quantum potential. Quantum potential is an extra non-local term Bohm introduced into the Schrodinger equation. It does not depend on space or time. Its presence is responsible for the odd trajectories of photons as they pass through two slits in the Young's interference experiment, resulting in the commonly observed interference pattern. In Bohmian mechanics the photon does pass through either one slit or the other, not (as per the Copenhagen interpretation) through both. In short, Bohmian mechanics is deterministic but nonlocal. Where is the observer? He is inseparable from the experiment. As Bohm's friend J. Krishnamurti expressed it, "the observer is the observed." This is the fundamental difference between Bohmian mechanics and the Copenhagen Interpretation. As Basil Hiley explains in his chapter, a particle does have a defined position and momentum. This does not violate the uncertainty principle, as that principle refers only to the limits on measuring the position and momentum.

This view has been long dismissed by most physicists because it was viewed (incorrectly) as implying the existence of hidden variables. Many physicists from Von Neumann to Bell himself have proved that such variables do not exist. However, the quantum potential as described by Bohm does not imply hidden variables. Today, experiments to measure Bohmian trajectories have been successfully performed that confirm the theory's predictions, as a result of which more physicists are taking another look at Bohm's mechanics.

Bohmian mechanics is also nonlocal --- allowing for "spooky action at a distance". It was this feature that Bell described in his theorem, also known as Bell's Inequalities. What is an inequality? ¾ > 1 is an inequality. It is also an inequality violation because it is untrue. In his lucid chapter, quite readable by the non-specialist, Jean Bricmont describes Bell's Inequalities and why those lead to nonlocality.

Take two particles that are produced together with opposite spins in a magnetic field. According to quantum mechanics, if you measure the spin of particle A, you immediately know the spin of particle B, regardless of how far apart those particles are. Possible explanations are:

• The spin values, up or down, of both particles are predetermined.

• There is some form of instantaneous action between A and B regardless of their separation.

Bell proved that option (1) leads to an internal contradiction, assuming that the particles obey the rules of quantum mechanics. This implies that the particles do communicate instantly. Einstein found this even harder to believe. In a 1942 letter he wrote:

It seems hard to sneak a look at God's cards. But that he plays dice and uses "telepathic" methods (as the present quantum theory requires of him) is something I cannot believe for a moment.

However, Bell's Inequalities were borne out by experiments in the 1970s and 80s by Freedman and Clauser, Aspect, Orsay and others. Marco Genovese summarises those results, implying that nonlocality is real.

If particles can communicate instantly over large distances, what is the mechanism? Standard physics offers no explanation. Bohmian mechanics offers a mechanism, because quantum potential, an integral part of the theory, is already nonlocal. But Bohmian mechanics is not the only explanation for apparent nonlocality. Lev Vaidman and Travis Norsen describe the Many Worlds Interpretation. If we accept that events happen simultaneously in many worlds, we can explain Bell's result, and still avoid an unexplained action at a distance. However, the mathematical details of the Many World Interpretation are yet to be worked out.

Is a nonlocal world incompatible with relativity? The answer is by no means trivial. Even though a message cannot be conveyed instantaneously using entangled particles, information is transmitted --- faster than the speed of light. By measuring one particle's spin, the second particle's spin is instantly established. That counts as cause and effect. Shan Gao explores describes nonlocality and its apparent incompatibility with relativity. Does nonlocality mean that at some fundamental level there exists a preferred Lorentz frame of reference, analogous to the aether? Is there a possibility for superluminal signaling? Gao points out that if a preferred frame of reference does exist, it should be detectable by experiment. The Cosmic Microwave Background could be such a frame. What if an entangled microscopic particle has conscious awareness? Conceivably that particle could manifest its awareness, of its spin, by an action detectable by a measuring device. This could make superluminal signaling possible. While Gao admits that his hypothesis is highly speculative, he suggests that nonlocality should cause us to rethink the possible existence of a preferred frame of reference. One has to ask whether Bohm's Implicate Order could be such a frame of reference.

This reviewer is left with the impression that there is no consensus on the implications of Bell's Theorem. Everyone accepts the results of the experiments, and most feel that nonlocality is real. But there is no agreement on what that means. More conservative voices feel that standard quantum mechanics can accommodate nonlocality. That nonlocality has no practical meaning except to philosophers. But many others express a certain queasiness, that perhaps the Copenhagen Interpretation is limited or plain wrong. David Bohm may have been right all along. Perhaps some predictions of relativity must be questioned. The far reaching implications of Bell's Theorem clearly remain yet to be seen.