Religion in an Age of Science by Ian Barbour
Chapter 4: Physics and Metaphysics
I. Quantum Theory
4. Bell’s Theorem
Some recent experiments have thrown further light on the relation between the three classical assumptions -- realism, determinism, and reductionism. In 1935, Einstein, Podolsky, and Rosen (EPR) proposed a type of experiment that has become possible to carry out only in the last few years.18 In one version, a two-proton system splits up into two protons, A and B, which fly off in opposite directions, say left and right.
If the system initially had total spin zero, conservation laws require that the spin of B is equal and opposite to that of A. The spin of A has an equal probability of being oriented in any direction. If a directionally sensitive detector is placed perpendicular to the flight path at some distance to the left, one can measure a particular component of the spin of A. One can then predict the precise value of the corresponding
component of the spin of B (namely, equal and opposite), which can be measured with a second detector at the right.
Quantum theory describes each proton in flight as a mixture of waves, representing with equal probability various possible spin orientations.
Each set of waves collapses to a single value only when a measurement is made. But B will behave differently according to what one chooses to measure on A. How can B know which component of A’s spin one will choose to measure? Einstein argued that while in flight B’s spin must already have had a definite value, not a probability distribution.
Einstein made two assumptions: (1) classical realism (individual
particles possess definite classical properties at all times, even when we are not observing them), and (2) locality (no causal influence can be transmitted between two isolated systems faster than the speed of light, which we will see shortly is a limit set by relativity theory). Einstein concluded from his "thought experiment" that the probability
descriptions of quantum theory must be incomplete, and that there must be hidden variables in each of the traveling particles, determining a particular outcome.
Bohr replied that Einstein’s form of realism was misguided because we cannot talk about the property of a particle except in relation to a
measuring process. In particular, we must think of the two particles and the two detectors as a single indivisible experimental situation. The wave function encompasses both particles, even though they are distant from each other. We have seen that Bohr also asserted the inescapability of indeterminacy. Bohr and Einstein had protracted arguments over these and other proposed experiments, which strained their earlier friendship. Neither was able to convince the other.
In 1965 John Bell calculated the statistical correlation one would expect between the two detectors (as a function of their relative orientations) if Einstein’s assumptions are correct. Recent experiments by Alain Aspect and others (using photons rather than protons) have not been consistent with these expectations, indicating that one of Einstein’s assumptions is
incorrect. ma "delayed choice" version of the experiment in 1983, Aspect was able to switch the orientation of the left detector at the last minute while the photons were in flight -- too late for any signal to reach the right photon before it arrived at its detector.19 The photons behaved as if there were some communication between them, but they were too far apart to communicate in the time available. Classically realistic local theories seem to be ruled out by these experiments.
Most physicists conclude that we should follow Bohr here, giving up classical realism and keeping locality (the finite limit on the speed with which any influence can be transmitted). They insist that particles A and B originated in one event and must be regarded as a single system even when they are far apart. The quantum wave function must include both particles. Only after an observation can they be regarded as having separate identities and independent existence. But it is possible to maintain a critical realism concerning the probabilistic whole while abandoning classical realism concerning the separate parts. Thus the physicist Paul Davies concludes, "The system of interest cannot be regarded as a collection of things, but as an indivisible, unified
whole."20 Polkinghorne writes; "Quantum states exhibit an unexpected degree of togetherness. . . . The EPR experiment points to a surprisingly integrationist view of the relationship of systems which have once interacted with each other, however widely they may subsequently separate."21
Another option is to keep classical realism and give up locality. Among defenders of realistic nonlocal theories are Bell and David Bohm. We mentioned earlier Bohm’s idea that hidden variables could preserve determinism. He has developed the equations for a quantum potential that acts as a kind of instantaneous pilot wave guiding particles;
statistical variations arise from fluctuations of hidden variables. The quantum potential incorporates encoded information about both local and distant events and does not fall off with distance. Bohm holds that there is a holistic underlying implicate order whose information unfolds into the explicate order of particular fields and particles. One analogy he uses is a TV signal with information enfolded in an electromagnetic wave, which the TV receiver unfolds as a visual image. Another analogy is a holographic photograph, of which every part has three-dimensional information about the whole object photographed. If you cut the
hologram into small pieces, you can unfold the whole image by
illuminating any piece of it with laser light. The scheme is deterministic because entities in the explicate order are not self-determining but are
expressions of the underlying implicate order.22
Bohm’s scheme shows a dramatic wholeness by allowing for nonlocal, noncausal, instantaneous connections. Events separated in space and time are correlated because they are unfolded from the same implicate order, but there is no direct causal connection between them since one event does not itself influence another event. It is like two TV screens showing images of a moving object taken from different angles; the two images are correlated, but one image does not influence the other. The scheme does not violate the relativistic prohibition of signals faster than the speed of light, for there is no way to use it to send a signal from one detector to the other. (We can not control the orientation of particle A, which arrives at random. The statistical correlation only shows up in the later comparison of the records from the two detectors.)23
Most physicists acknowledge that Bohm’s view is consistent with these experiments, but they are reluctant to abandon Bohr’s view until there is experimental evidence against it. The development of quantum potential theory by Bohm and his coworkers may lead to distinctive testable predictions, but it has not done so to date.
In sum, Einstein’s classically realist, determinist, and local interpretation seems to be ruled out by the Aspect experiments. Bohm’s theory, with Its classical realism, determinism, and extreme nonlocal holism, cannot yet be experimentally distinguished from standard quantum theory. The instrumentalists claim that we cannot say anything about the world between observations and therefore questions about determinism and holism should be dismissed as meaningless. I have advocated a
combination of critical realism, indeterminacy, and a more limited form of holism, and I have suggested that Bohr himself was closer to this view than to instrumentalism.