Presumably light pressure would change the momentum of the beam-splitting mirrors in the interferometer version of Wheeler's delayed choice experiment. And it's not until the photon encounters the detector at the end that we consider it "observed", and presumably the probability field has collapsed (excuse my pop-science misunderstandings of the concept). But why does the photon's encounter with the splitter not count as a measurable, field-collapsing "observation", resulting in the photon either passing through or being reflected, instead of remaining as a probability and later interfering with itself?

  • $\begingroup$ Nice question! Here are some related ones: 1, 2, 3. $\endgroup$
    – knzhou
    Commented Nov 1, 2021 at 15:33
  • $\begingroup$ So, @knzhou, based on some of the answers in #3, is it basically because the initial momentum of a macroscopic object like a mirror can never be known ahead of time, such that we cannot determine the momentum of the photon that caused the change? And, by extension, does it mean that momentum-changing events are never sufficient to qualify as observation, whereas state-changing events (like the chirality of a photochemical) are observations? $\endgroup$ Commented Nov 1, 2021 at 17:26
  • $\begingroup$ First sentence -- yes, that's absolutely right! Second sentence -- no, the principle is just that observation requires a substantial change in state. For example, if a stray air molecule bounces off an electron (thereby changing the molecule's momentum significantly), that counts as an observation. But bouncing off macroscopic objects doesn't because the momentum uncertainty of a macroscopic object is much higher. $\endgroup$
    – knzhou
    Commented Nov 1, 2021 at 17:38
  • $\begingroup$ physics.stackexchange.com/questions/119743/… $\endgroup$
    – alanf
    Commented Nov 1, 2021 at 17:39


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