went through slit $B$.*

\[...\] these two measurements cannot both be performed for the same photons, and so \[the figures\] cannot refer to the same experiment. Hence, in accord with the Copenhagen interpretation of quantum mechanics, there is no paradox. The complementarity principle does not prevent photons from behaving once as waves and once as particles. It only states that the same photon should not reveal this “split personality” in the same experiment. \[...\]

Despite the fact that we know with $99\%$ certainty the paths of the photons, they still have strong wavelike properties. \[...\] we have presented a result which, although not paradoxical, was nevertheless surprising (that is, that one can make a fairly precise determination of the slit through which each photon comes with only a slight disturbance of the interference pattern).

The finding loses its surprise in the light of the above considerations that suggest existence of the path but with intrinsic limitations on its observability.

For the original double-slit experiment, the arguments generally agree on the observation that there is no way to predict through which slit a particle [*will pass*]{}. However, it is possible to compute with high confidence through which a particle [*has passed*]{} 