Many physicists of the time - and probably most readers as well - wondered why the photons' paths, and thus which slit they entered, couldn't simply be measured.Two slits with a detector give two bars of light, leading researchers to believe the act of measurement had destroyed the interference effect. So imagine a photon detector placed on the slits. When physicists performed this experiment, they found that the interference pattern disappeared - two slits of light appeared on the plate! The act of measurement had interfered with the particles' trajectories, in effect forcing them to pass through one slit or the other. This is called reducing the wave-function because it takes what was a fundamentally uncertain, indeterminate quantity - which slit the particles passed through - and measured it. However, the act of measurement destroyed the experiment itself, so which slit the particle passed through not only cannot be known, but, according to wave functions or sum-over-paths, is not even a meaningful statement because the particle actually passed through both.