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I Delayed choice two slit experiment - photon

  1. Jul 27, 2016 #1
    Is it possible that the reason the delayed choice two slit experiment outcomes occur is because for the photon, travelling at the speed of light, the past, present and future are all as one, and therefore it "knows" what is going to happen and therefore always "makes the right choice" at the slit re adopting a wave or a particle state?
  2. jcsd
  3. Jul 27, 2016 #2


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    No. The delayed choice has nothing to do with retrocausal influence on the experiment but just with the possibility to choose subensembles of the full ensemble of measured photons due to the measurement protocol containing the appropriate information. It's a bit difficult to explain without a concrete example. One of the most simple ones is the "quantum eraser" experiment proposed by Scully et al and first realized experimentally by Walborn et al:


    I think, to understand the fascinating possibility of "postselection" due to quantum theory, it's good to read this paper. Then ask specific questions here in the forum when you have difficulties with it.

    More generally, you should also get rid of some unfortunately common misconceptions in popular-science writings about quantum theory. Many authors use outdated concepts from the socalled "old quantum theory" and use the corresponding old-fashioned concepts like "wave-particle duality". Theoretical physics is, however, all about finding mathematical intrinsically consistent models (or even theories) that describe as accurately as possible observed facts, and "old quantum theory" was not satisfactory even for its discoverers (among them Planck, Einstein, Bohr, and de Broglie). That's why after an amazingly short time modern quantum theory has been discovered around 1925/26, and not only once but already in three different equivalent forms: matrix mechanics (Heisenberg, Born, Jordan), wave mechanics (Schrödinger), and "transformation theory" (Dirac). The latter is the most general form and usually taught at universities today.

    Although a pleasure for experimentalists interested in the foundation of quantum theory for technical reasons, photons are among the most difficult concepts of the theory, because you need relativistic quantum field theory to accurately describe them. For no other elementary quantum field the idea to think about it in terms of classical particles as it is for photons. The classical limit are rather classical electromagnetic waves (i.e., coherent states in the quantum language). It's not even possible to define a position observable for them!
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