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What will happen in the following scenario?

  1. Apr 29, 2007 #1
    I am new to quantum mechanics and I am not much familiar with it. I have a doubt. Consider a scenario in which a photon is passed through an apparatus which is partly mirror and partly lens. The wave function of the photon is such that there is 75% chance of photon hitting the mirror thereby geting reflected back to the same side of the apparatus and 25% chance of photon hitting the lens thereby passing to the other side of the apparatus. My question is what will happen to the wave function of photon after interacting with the apparatus. Will the wave function be split in to either sides of the apparatus such that 75% of photon corresponds to one side of apparatus and 25% of photon corresponds to other side? Suppose we measure the photon after the interaction, will the photon be detected on one side of apparatus 75% of times and on the other side of apparatus 25% of times? In other words, will photon be measured to exist in different sides of the apparatus at diffeent times. If the same photon is measured after 1 year, will the photon be detected to exist at different locations which are 2 lightyears apart at different instants? Is there any misconception here? Please clarify my doubt. Thanks in advance.
    Last edited: Apr 29, 2007
  2. jcsd
  3. Apr 30, 2007 #2


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    To answer your question generally: the wave function is split as you imagine (25% & 75%) until the photon is actually determined to be one place or another. Once it is "narrowed down" then the collapse postulate applies - and it will be usually 100% one side or the other. A single photon will NOT be detected to exist in 2 places at once.
  4. Apr 30, 2007 #3
    Thanks for your reply. But suppose we try to detect the photon the second time, is it certain that it will be detected on the same side of the apparatus as the first time, or is it governed by probability i.e. 75% of times, it will be detected on one side of apparatus and 25% of times, it will be detected on the other side? In other words, once the photon is detected to be on one side of apparatus, is it a guarantee that it will be detected on the same side of apparatus for all the subsequent observations or is it probabilistic with different observations giving different positions for photon with respect to the apparatus?
  5. Apr 30, 2007 #4
    Once you have detected it once, you have collapsed the wave function, it now lies 100% on the side you first detected it on. Any further measurements will show it on the same side.
  6. Apr 30, 2007 #5
    Thanks a lot for the clarification.
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