Holism and particlism in physics.

Physicists do not like the idea of instantaneous action at a distance. If one object influences another, the objects should be in contact. If they are far apart, some physical intermediary should go between them, and the intermediary should take some time to cross the distance. Physics is a pool table, and there is always a cue ball.

Lately, however, it seems there are cases of instantaneous action at a distance. Alain Aspect of the University of Paris-South at Orsay, France, and his collaborators have done a series of experiments that seem to show that phenomena known as quantum mechanical correlations do exist (SN: 1/11/86, p. 28). The conventional interpretation is that these correlations involve action at a distance. Opponents of action at a distance are interpreting them in a different way, and one of these opponents, Jean-Pierre Vigier of the Institut Henry Poincare in Paris, plans an experiment he believes will show his view to be correct.

Quantum mechanics predicts that under certain conditions an atom will emit a pair of particles with correlated properties -- for example, two photns with opposite polarizations or two neutrons with opposite spins. The prediction says the particles will maintain the correlation no matter how far apart they move in space and time. To maintain the correlation, it looks as if the particles have to be in constant (and therefore instant) communication with each other. The left always knows what the right is doing. This is what Aspect and his colleagues seem to have found.

Albert Einstein called such phenomena spukhaft (spooky). Those who follow his lead want to exorcise the spooks, and Vigier does so using a theory largely asociated with the name of David Bohm of Birkbeck College of the Univesity of London in England. This theory puts back the cue ball in the form of a quantum potential that mediates information between the two correlated particles. A potential is a condition of space that gives rise to physical effects.

Potentials are often used as intemediaries in relations over distances. For example, the moon feels certain forces and moves in a certain way because of the gravitational potential generated by the earth. Potentials do not exist instantaneously and everywhere in space. If a source is turned on, if a body suddenly becomes electrically charged, for example, an electric potential will propagate itself into space from the object.

The Bohm quantum potential comes from the basic equation of quantum mechanics, the Schrodinger equation. It is primarily a carrier of information. For example, it will tell a particle which of several possible trajectories to take. In the correlation case it supplies the informational contact that maintains the correlation. To do this it has to propagate itself faster than light, at 7.57 times the speed of light, to be precise.

Thus the quantum potential violates the precept of special relativity that nothing goes faster than light, but it saves a more important principle: the reductionism of physics. Niels Bohr said correlations indicate a certain wholeness in quantum mechanical phenomena, standing connections over long distances in time and space. If that is so, and Aspect's experiments seem to indicate it, physicists would no longer be sure they could analyze a given physical system into its parts and study the parts in isolation. This is the way scientific investigation has proceeded since the days of Demokritos of Athens, but it may now have reached a stop sign. The whole may have properties that do not result from the sum of the parts. This holism-reductionism crisis caused consternation of last week's meeting in New York on New Techniques and Ideas in Quantum Measurement, sponsored by the New York Academy of Sciences. Many physicists seem unsure what to do.

Vigier plans an experiment involving neutron interference, he told the meeting. Neutrons are usually thought of as particles, but they do behave like waves. If a beam of neutrons is split in two and sent on different paths and then recombined, the two halves will be in or out of phase with each other according to whether the two paths were equal or not.

The beam can be reduced until there is only one neutron in the apparatus at a time. Even so it interferes with itself. Which path--if any--did the single neutron take? The Bohrean view would say that the wholeness of things makes it impossible to talk about paths: Neutrons don't really move around like particles in space and take particular paths. The Einsteinian view says they do: The neutron takes one of the two paths even though it interferes with itself like a wave. The Bohm potential should tell which, and Vigier intends to show it by putting an electrical coil around one path. The coil will reverse the spin of the neutron, and in consequence the neutron will give the coil a tiny quantum of energy. Measuring that energy will tell if the neutron took that path or the other. Vigier plans to do the experiment at a reactor in Grenoble, France.