That's the attempt at figuring out which slit the photon goes through. It fails because those photons going through when one of the slits is closed act according to the probability distribution from the much smaller interference effect from the edges of the slit (nearly the same as if they were particles) and only those going through when both slits are only adds to the interference pattern of the main probability distribution.
Originally Posted by John Jaksich
The effect of blinking one of the slits is a pattern which is two (or three if both slits blink) different property distributions overlaid, with the relative intensity of the patterns proportional to the relative amount of time of each slit configuration.
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