If we place a detector on one of the slits, then we should expect equal probabilities: 50 percent of the time we will detect the atom at the left slit and 50 percent of time we will detect it at the right slit. But—and this is the important bit—if we don’t try to detect the atom at the level of the first screen then the wave function flows through both slits without collapsing. Thereafter, in quantum terms we can talk of a wave function describing a single atom that is in a superposition: of its being in two places at the same time, corresponding to its wave function going through both the
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