Why do Young slits projecting via an intermediary mirror to a screen still shows the normal pattern of interference? This happens even though the mirror is so close that not all interference is constructed yet. The more dramatic experiment being to use a mirror for each slit so as to split and then using two other mirrors to recombine the light. It is my understanding from QM that the interference pattern only happens because there is no measurement between the slits and the screen. However, I don't see how the mirror avoids any measurement at all. Do mirrors somehow escape measuring light and if so, what is the justification for this exception? I find it even harder to accept this possibility considering the mechanical effect of radiation pressure on the mirror.

  • $\begingroup$ Maybe it'll be better if you add a diagram for better understanding? $\endgroup$
    – Eagle
    May 9, 2019 at 10:16
  • $\begingroup$ Maybe, but I cannot due to technical limitations here. $\endgroup$
    – Winston
    May 9, 2019 at 10:20
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    $\begingroup$ See physics.stackexchange.com/questions/119743/… $\endgroup$
    – alanf
    May 9, 2019 at 11:53
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    $\begingroup$ I would like to clear up that Youngs experiment with light does not need quantum mechanics, classical electromagnetic wave equations work fine.. It is only for massive particles and single photon double slit that quantum mechanics needs to be invoked . sps.ch/en/articles/progresses/… $\endgroup$
    – anna v
    May 9, 2019 at 13:59
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    $\begingroup$ Any interferometer based on semireflective mirrors will do. $\endgroup$
    – Winston
    May 9, 2019 at 15:55