Chapter 2 presents thought experiments in which electrons, after passing through a screen with two slits, display an interference pattern. Einstein claimed that, if we closely observe the recoil of the screen after each electron passes, we can determine through which slit the electron passed, and the interference pattern will remain. Bohr, however, insisted that quantum mechanics is consistent only if any procedure that reveals through which slit each electron passes also destroys the interference pattern. Bohr showed that Einstein's procedure for observing the recoil of the screen does, indeed, destroy the interference pattern. Since we claim that quantum mechanics is consistent, we must be able to show that any refinement or modification of Einstein's procedure would also destroy the interference pattern. We now describe two procedures, more sophisticated than Einstein's, to reveal through which of two slits an electron passes. Neither seems to destroy the interference pattern.
4.1 Two Paradoxical Procedures
In the first procedure, we put a capacitor next to the screen, halfway between the two slits. (See Fig. 4.1.) The flat plates of the capacitor are perpendicular to the screen and we assume that they are initially at rest. The plates, when charged, carry equal and opposite charges; each plate produces a constant electric field, perpendicular to the plates, but on either side of the capacitor these fields cancel. Thus, on either side of the capacitor, a passing electron feels no force. The electric field does fringe at the edges of the plates, but if we charge the capacitor only during the short time that the electron is on one side or the other of the capacitor, the electron never encounters this fringe field and never feels any force. However, the electron Figure 4.1: A two-slit experiment with a charged capacitor between the slits. An insulated spring keeps the plates apart.