David Peat

Quantum Uncertainty 21tion perturbs that system. However, by observing only one part, A, of asystem, when the other part, B, is located far away, no form of interaction—nomechanical force or field of influence—can possibly interferewith B.Bohr agreed that Einstein had ruled out any mechanical influenceon system B; nevertheless, he argued that “the procedure of measurement”has “an essential influence” on the very definition of the physicalvariables that are to be measured. 4With this argument Bohr felt that he had finally put an end to allobjections to his “Copenhagen interpretation” of quantum theory.There were no “independent elements of reality,” rather quantumtheory displayed the essential wholeness of the universe. It is not auniverse put together through a series of quasi-independent elementsin interaction; instead what we take for elements or “parts” actuallyemerge out of the overall dynamics of quantum systems. Properties ofa system do not exist “out there,” as it were, but are defined through thevarious ways in which we approach and observe a system. As Bohrpointed out, the intention or disposition to make a measurement—forexample, to collect the apparatus together—determines to some extentwhich sorts of properties can be measured. In this sense, although a“mechanical” interference between B and the apparatus used to measureA is absent, there is always an influence, to use Bohr’s term, onthose conditions that define possible outcomes and results.One interesting contribution to emerge out of this discussion ofthe EPR paradox was made by John Bell who pointed out that quantumwholeness means that the two parts of the system A and B willcontinue to be “correlated” even when they are far apart. In no sensedoes A interact with B; nevertheless (and loosely speaking) B “knows”when a measurement is being performed on A. Or rather, it would bebetter to say that A and B remain co-related. This co-relationship hassince been confirmed by very accurate laboratory experiments.Bohr felt that his refutation spelled the final death knell toEinstein’s dream of an independent reality. Einstein, for his part, was4If the reader finds this statement difficult to understand, that particular puzzlementis shared by deep thinkers from theoretical physics and the philosophy of science.