I'm trying to understand the implications of the double-slit experiment, and I have been considering what QM theory would predict as the outcome of the following experiment: Consider an electron gun firing electrons towards a detection screen two metres away (gun perpendicular to screen), and there is a blocking partition halfway between the gun and the detection screen. We shall consider the screen and the partition to be infinite planes, for simplicity. Now, we open up a slit in the partition, directly in front of the gun, whereupon we start to see electrons being detected on the screen. Now I know that if a second slit is opened up, the familiar diffraction pattern is seen, consistent with wave-like behaviour, and we can't really state which slit the electrons have travelled through. But what if the second slit is opened up, let's say, 500m along the screen from the first? I understand that any diffraction pattern would probably be almost unnoticeable, but let's say the instruments are extremely precise. My thinking is this: if the electrons "passing through" the second slit were having to travel sqrt(500^2 + 1) metres before getting to the screen, then at an average speed of, say 10^6 m s^-1, wouldn't we already notice some difference in arrival time? If the individual blips detected on the screen differed greatly in travel time, wouldn't this give us a clear indication of which slit the electron had travelled through? Thanks in advance, this subject is a little freaky to me!