You already know what will happen—each individual electron passes through both slits at once, as a wave. But no; each electron now passes through one slit or the other, randomly. The mere process of measuring, documenting what happens at the double-slit wall causes the electrons (and, as it turns out, streams of light, made up of photons) to stop acting as waves. The wave function “collapses,” and each electron passes through the double-slit wall as a singular particle. Thus, electrons and photons show particle/wave duality, with the process of measurement turning waves into particles.
