We have the simple arrangement of the double slit, but with the addition of a precise clock. The screen is also substituted by a CCD camera, so one can check when there is an event (hitting of the photon on a camera pixel).

Now we concentrate on the first minimum (say left side). If we don’t watch the clock we must get the normal interference pattern. (e.g. no events in this segment of the camera). But if we watch the clock we should know where the particles come from (left or right slit) because the distance from the left slit is slightly closer to this minimum. So we get which way information and the interference pattern is destroyed – so we must get events (and even many such) in this segment.

There are some paradoxes appearing. Without any efforts I can figure out at least 3. Surely there are many more.

  1. How watching the clock on a distant spot is changing the physics on the slit panel (there seems to be no connection between them)?

  2. If we use a less precise clock we would get interference. How can it be that the precision of the instruments can influence the events?

  3. We could place a CCD camera above the first and not use clock with it. So on the first CCD there is 0 events and on the second CCD some events. The physics (on the slits) must be the same but it is not?

In order not to water up the discussion I would be thankful if your comments are concerned on the first item. Also please try to analyze this from the view of the orthodox interpretation of QM (Copenhagen or some variant).

  • $\begingroup$ Are you sure you can determine which branch by using a clock? the wave packet gets dispersed, so even with a high presicion clock you should not know if you detected the front of the long path particle or the rear of the short path particle $\endgroup$ – Wolphram jonny Oct 31 '18 at 13:59
  • $\begingroup$ It isn't a particle, the wave function traverses both slits and you see the result. No, you can't tell. $\endgroup$ – Jon Custer Oct 31 '18 at 14:14
  • $\begingroup$ If you are gating the CCD very precisely you will only observe the photons that have the correct timing, but many of the other photons will take different paths and will miss the gate. Fundamentally photons like to travel integer multiple of there wavelength and QM interaction with the slit results in the probability pattern. ( I don't like the word "interference" it is really a probability pattern) $\endgroup$ – PhysicsDave Oct 31 '18 at 15:55

Your basic assumption is just incorrect. There are videos of intereference patterns slowly building up, you will find them on the internet, so obviously you have a quite good information of when each spot appears on the screen, but the interference is not disturbed by that.

Your main mistake seems to be that you think that the presence of a measurement device (a clock or a which-way detector, for example), or even your knowledge, somehow magically does something with the wave function. But that is not the case. The wave function effectively collapses because of the interaction with the many particles that form a detector. You cannot obtain a good understanding of quantum mechanics by thinking that magical things may happen, and experiments show that they actually don't.

  • $\begingroup$ The time resolution in these videos and in all !! double slit experiments is far from neccessary to distinguish between two paths. I imagine the use of a very precise clock in order of femtosec (there are already such clocks). But this is a thought experiment!!! Now as I understand you state that there is no which way information principle? Then I dont think magical things can happen but point out that they will look magical (if you understand!). By the way Einstein thought that magic can not happent (ghost action on a distance) but it happens. $\endgroup$ – Mercury Nov 5 '18 at 12:58

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