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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.

Winston
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  • Maybe it'll be better if you add a diagram for better understanding? – Eagle May 09 '19 at 10:16
  • Maybe, but I cannot due to technical limitations here. – Winston May 09 '19 at 10:20
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    See https://physics.stackexchange.com/questions/119743/qm-why-is-reflection-of-a-photon-not-a-measurement/119749#119749 – alanf May 09 '19 at 11:53
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    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 . https://www.sps.ch/en/articles/progresses/wave-particle-duality-of-light-for-the-classroom-13/ – anna v May 09 '19 at 13:59
  • @anna v: the measurement problem is a QM one. – Winston May 09 '19 at 14:50
  • @alanf: thanks. Reading your post, it seems to me this is the same as saying measurement can be graded and have almost zero impact on light coherence in the context of reflection, right? I had imagine measurement as a binary process, either having place or not at all. I noticed you talking about probabilities though, thus it can still be a binary process yet happening with a graded probability. – Winston May 09 '19 at 14:54
  • You should state then that it is the specific measurement problem. Classical scattering has no problem with mirrors. In QM mirrors work with elastic scattering of the photons and the phases are retained., coherence is not lost, and as the mass of the mirror is very large with respect to the energy of the photon the effect of momentum transfer changes the frequency by a very small amount, not visible in colors. – anna v May 09 '19 at 14:59
  • You should give a specific diagram to see where you see a measurement problem. – anna v May 09 '19 at 15:01
  • @anna v: I did mention the measurement problem and the inability to provide a diagram. – Winston May 09 '19 at 15:11
  • well, if there is no diagram how can one answer? At the quantum mechanical level it is all about boundary conditions for the solution of the quantum mechanical set up. Do you have a link? where somebody has seen these interferences using mirrors? – anna v May 09 '19 at 15:49
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    Any interferometer based on semireflective mirrors will do. – Winston May 09 '19 at 15:55
  • There is a series of MIT videos which show the difference between interference and interaction among other things. https://www.youtube.com/watch?v=J4Ecq7hIzYU .photons interfere by superposition of their wavefunctions – anna v May 09 '19 at 16:35
  • Let us continue this discussion in chat. – Winston May 09 '19 at 16:50

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