In the double-slit experiment, quantum mechanics states that if you try to determine which slit the photon goes through, you won't have a resulting wave pattern.
But, knowing the time it took for the photon to go from the source to the observing screen, can you deduct the distance of the photon path and so which slit it passes through (except if the photon impacts the exact middle of the observing screen)?
Best Answer
... and indeed such information will make it impossible for fringes to appear.
Interference experiments use wavepackets that have a long duration, which makes it impossible to tell from timing information which slit the particle came through, eliminating the problem.