I have been invited to speak on ‘foundations of quantum mechanics’ – and to a captive audience of high energy physicists! How can I hope to hold the attention of such serious people with philosophy? I will try to do so by concentrating on an area where some courageous experimenters have recently been putting philosophy to experimental test.

The area in question is that of Einstein, Podolsky, and Rosen. Suppose for example, that protons of a few MeV energy are incident on a hydrogen target. Occasionally one will scatter, causing a target proton to recoil. Suppose (Fig. 1) that we have counter telescopes T1 and T2 which register when suitable protons are going towards distant counters C1 and C2. With ideal arrangements registering of both T1 and T2 will then imply registering of both C1 and C2 after appropriate time decays. Suppose next that C1 and C2 are preceded by filters that pass only particles of given polarization, say those with spin projection + ½ along the z axis. Then one or both of C1 and C2 may fail to register. Indeed for protons of suitable energy one and only one of these counters will register on almost every suitable occasion – i.e., those occasions certified as suitable by telescopes T1 and T2. This is because proton–proton scattering at large angle and low energy, say a few MeV, goes mainly in S wave. But the antisymmetry of the final wave function then requires the antisymmetric singlet spin state.