2 whereas previously the force varied over a

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( 2 ) Whereas previously the force varied over a continuous range with Q, it takes now just two values, +F, the sign bqiqg determined by whether the magnetic axis of the particle points more nearly in the direction of the field or in the opposite direction. No attempt is made to explain this change in the force law. It is just an ad hoc attempt to account for the observations. And of course it accounts immediately for the ap- pearance of just two groups of particles, deflected either in the direc- tion of the magnetic field or in the opposite direction. To account then for the Einstein-Podolsky-Rosen-Bohm correlations we have only to assume that the two particles emitted by the source have oppositely di- rected magnetic axes. Then if the magnetic axis of one particle is more nearly along (than against) one Stern-Gerlach field) the magnetic axes of the other particle will be more nearly against (than along) a paral- lel Stern-Gerlach field. So when one particle is deflected up, the other is deflected down, and vice versa. There is nothing whatever pro- blematic or mind-boggling about these correlations, with parallel Stern- Gerlach analyzers, from the Einsteinian point of view. So far so good. But now go a little further than before, and con- sider mn4arallel Stern-Gerlach magnets. Let the first be rotated away from some standard position, about the particle line of flight, by an angle a. Let the second be rotated likewise by an angle b. Then if fhe magnetic axis of either particle separately is randomly oriented, but if the axes of the particles of a given pair are always oppositely oriented, a short calculation gives for the probabilities of the various
possible results, in the ad hoc model, where "up" and "down" are defined with respect to the magnetic fields of the two magnets. However, a quantum mechanical calculation gives 1 P (up,up) = P (down,down) = 3 (sin e) ' 2 1 1 a-b P(up,down) = P(down,up) = 3 - 7 (sin T) Thus the ad hoc model does what is required of it (i.e., reproduces quantum mechanical results) only at (a - b) = 0, (a - b) = s/2 and (a - b) = n, but not at intermediate angles. Of course this trivial model was just the first one we thought of, and it worked up to a point. Could we not be a little more clever, and devise a model which reproduces the quantum formulae completely ? No. It cannot be done, so long as action at a distance is excluded. This point was realized only subsequently. Neither EPR nor their contempora- ry opponents were aware of it. Indeed the discussion was for long enti- rely concentrated on the points la - bl = 0, ~/2, and n. 3. Difficulty with locality.- To explain this denouement without mathe- matics I cannot do better than follow dVEspagnat /14, 15/. Let us re- turn to socks for a moment. One of the most important questions about a sock is "will it wash "?

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