# Timing in double slits experiment

In this setup the source, the slits and the screen are a lightsecond away from each other. I would like to know if there is any difference if I tempered with the slits before and after the photon is created from the source? Imagine a photon is being created and before it crosses the double slits and interfere with itself, I quickly close one of the slits.

Photons can be said to choose the best path considering all paths, this is another more modern interpretation as opposed to saying that "a photon interferes with itself".

If the slit closes before the photon reaches it then only a single slit pattern will be observed (from the other open slit), if the slit closes at the exact time the photon arrives then it will be based on probability whether single or double slit diffraction is observed. If the slit closes late then the normal double slit pattern will be observed.

All of the above assumes you will repeat the experiment a number of times to allow the pattern to build up as performed and shown in many single photon double slit diffraction experiments.

So you are saying that the slits and the screen are 1 lightsecond apart from each other. Let's assume there is an apparatus that will be able to temper with the slits this fast (faster then 1sec).

Now there are two cases that you mention:

1. you temper with the slits before the photon reaches the slits. In this case the photon's partial waves that cross the slits will not be able to interfere with themselves, and there will be no interference pattern. Though, the photon itself will still reach the screen, but when you repeat the experiment, you will not see any pattern as one slit will be closed every time before the photon would reach the slits

2. you temper with the slits after the photon reaches the slits. In this case, the photons' partial waves already passed the slits and interfered with themselves, so you will see a pattern.