lew@ihlpa.ATT.COM (Lew Mammel, Jr.) (08/08/87)
[ The following is excerpted from Quantum_Theory_and_the_Schism_in_Physics by Karl Popper. The words capitalized for emphasis are italicized in the original. I'll be posting my comments on this in a separate article. I thought I'd post this first so people can think about it without being distracted by prejudicial commentary. - Lew Mammel, Jr.] ____________________________________________________________________ I wish to propose a simple experiment which may be regarded as an extension of the Einstein-Podolsky-Rosen argument. As originally formulated, the EPR 'thought experiment' is only an argument, not a real experiment. I wish to suggest a crucial experiment to TEST whether knowledge alone is sufficient to create 'uncertainty' and, with it, scatter (as is contended under the Copenhagen interpretation), or whether it is the physical situation that is responsible for the scatter. We have a source S (positronium, say) from which pairs of particles that have interacted are emitted in opposite directions. We consider pairs of particles that move in opposite directions along the positive and negative x-axis, towards two screens A and B, with slits whose width delta q_y is adjustable ( [see figure] ). Beyond the slits on both sides there are batteries of Geiger counters arranged in semi-circles. | | | | ( | | ) ( | | ) ( | | ) ( * ) ( | | ) ( | B S A | ) ( | | ) | | | | We assume that the intensity of the beam of the emitted particles is very low, so that the probability is high that two particles recorded at the same time on the left and on the right are those which have actually interacted before emission. Those particles that passed through the slits A and B will be counted by Geiger counters. These counters are coincidence counters; that is to say they are so wired that they only count particles that have passed at the same time through A and B. This should make it almost certain that only pairs of particles which have interacted are recorded. We now first test the Heisenberg scatter for both the beams of particles going to the right and to the left, by making the two slits A and B wider or narrower. If the slits are narrower, then counters should come into play which are higher up and lower down, seen from the slits. The coming into play of these counters is indicative of the wider scattering angles which go with a narrower slit, according to the Hiesenberg relations. Now we make the slit at A very small and the slit at B very wide. According to the EPR argument, we have measured q_y for both particles (the one passing through A and the one passing through B) with the precision delta q_y of the slit at A, since we can now calculate the y-coordinate of the particle that passes through B with approximately the same precision, even though its slit is wide open. We thus obtain fairly precise 'KNOWLEDGE' about the q_y position of this particle - we have 'measured' its y position indirectly. And since it is, according to the Copenhagen interpretation, our KNOWLEDGE which is described by the theory - and especially by the Heisenberg relations - we should expect that the momentum p_y of the beam that passes through B scatters as much as that of the beam that passes through A, even though the slit at A is much narrower than the widely opened slit at B. Now the scatter can, in principle, be tested with the help of the counters. If the Copenhagen interpretation is correct, then such counters on the far side of B that are indicative of a wide scatter (and of a narrow slit) should now count coincidences: counters that did not count any particles before the slit at A was narrowed. To sum up: if the Copenhagen interpretation is correct, then any increase in the precision of our MERE KNOWLEDGE of the position q_y of the particles going to the right should increase their scatter; and this prediction should be testable.