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The Guided Wave Theory of Louis de Broglie and David Bohm - Brief Article

Skeptical Inquirer, May, 2000 by Martin Gardner

There is little doubt that the mathematical formalism of quantum mechanics (QM) is accurate. No theory of physics has been more spectacularly successful in making predictions about the outcome of measurements. Some are accurate to many decimal places. Where experts disagree is not about the mathematics of QM but about how to interpret its equations. Even more than relativity theory, QM bristles with wild paradoxes that radically violate common sense and for which at present there are no agreed-upon resolutions.

The most notorious of these paradoxes is the EPR, named after the initials of Einstein and two associates, Boris Podolsky and Nathan Rosen. In 1935 they published an explosive paper in which they argued that their paradox proved that QM is incomplete, destined to be replaced or radically modified by a deeper theory.

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The EPR paradox has several forms, but the easiest to understand was proposed by the late American physicist David Jacob Bohm (1917-1991). It involves a mysterious property of particles called spin. Spin is roughly similar to the motion of a top because it has angular momentum that always takes one of two forms variously called left or right, plus or minus, up or down. Imagine a quantum reaction that creates two identical particles A and B which go off in opposite directions. In standard QM each particle has its left and right spins "superposed." When particle A is measured for spin, its "wave function" (a formula specifying the probabilities that certain values will be found when a particle is measured for a given property) is said to "collapse" (vanish). The particle at once acquires either a left or right spin with equal probability.

Now for the magic. To conserve angular momentum, after A is measured and so acquires a definite spin, B must acquire the opposite spin. Assume that A, measured in Chicago, has a left spin. (Remember, it does not have a definite spin until measured.) On a planet in a distant galaxy a physicist measures B when it gets there. It is certain to have a right spin. How does B "know" the outcome of the measurement of A? Does A send some kind of telepathic signal to B, either simultaneously or at a speed equal to or exceeding the speed of light? Einstein ridiculed this as "spooky action at a distance." He believed that his proposed experiment, then only a thought experiment, proved that QM was not complete. There must be local "hidden variables" giving definite spins to both particles before one is measured.

The standard Copenhagen interpretation of QM, based on the opinions of Niels Bohr, is that regardless of how far apart A and B get, they remain a single quantum system with a single wave function. When A is measured, the entire system's wave function vanishes and the two particles simultaneously acquire opposite spins. The particles are said to be "correlated," or in more recent terminology, "entangled."

Does this resolve the paradox? It does not. The mystery remains of how A and B can stay entangled when they are light-years apart unless there is some kind of connection between them that allows information to go from A to B.

All physicists agree that there is no possibility of sending coded messages by using the EPR phenomenon. The situation is like two persons, one in New York, the other in Paris, who simultaneously flip a penny. For reasons unknown, if one penny falls heads, the other must fall tails, and vice versa. If one could control the outcome of a flip in Chicago, then of course the Chicagoan could send a message in a binary code of ones and zeros. But there is no way to control the outcome of a measured spin. Like the heads and tails of a flipped coin, it becomes left or right with equal probability. The sequence of lefts and rights, at each end, is always a random, meaningless sequence. If it were otherwise, cipher messages presumably could be sent with a speed exceeding that of light, thereby violating a basic law of relativity.

The EPR paradox remained unconfirmed until recent years when the late Irish physicist John Stewart Bell thought of a brilliant way to test the paradox over short laboratory distances. The paradox has since been completely validated many times. The two entangled particles behave precisely as QM predicts.

Several conflicting efforts have been made to explain the EPR paradox. Here I confine my remarks to how it is resolved by what is called the pilot wave or guided wave theory (GWT). Obviously I'm no expert on QM, only a science journalist, so I haven't the foggiest notion of whether GWT will someday be confirmed. However, this theory, long ignored by physicists, is now gaining increasing support. [1] It deserves to be better known.

It was the French Prince Louis de Broglie (pronounced de Broy), one of the architects of QM, who first proposed GWT. He abandoned it after heavy ridicule by Copenhagenists, but took it up again after it was improved by Bohm. Now known as the Broglie! Bohm GWT, it has become the interpretation of QM favored by a raft of experts that include Bell, Jeffrey Bub, the French physicist Jean-Pierre Vigier, and many others. So far its predictions are exactly the same as those of the Copenhagen school, although there may be subtle ways it can be tested by difficult experiments not yet performed.

The Guided Wave Theory of Louis de Broglie and David Bohm - Brief Article

Skeptical Inquirer, May, 2000 by Martin Gardner

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