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I The principle of least action and diffraction

  1. Apr 30, 2016 #1
    When reading through one of the feynman lectures (http://www.feynmanlectures.caltech.edu/II_19.html) there was a paragraph that said:

    "In the case of light we also discussed the question: How does the particle find the right path? From the differential point of view, it is easy to understand. Every moment it gets an acceleration and knows only what to do at that instant. But all your instincts on cause and effect go haywire when you say that the particle decides to take the path that is going to give the minimum action. Does it ‘smell’ the neighboring paths to find out whether or not they have more action? In the case of light, when we put blocks in the way so that the photons could not test all the paths, we found that they couldn’t figure out which way to go, and we had the phenomenon of diffraction."

    My question is how exactly diffraction result from light not knowing which is the path of least action. Is he saying that the light waves spread out and take every path? I thought that refraction would be more relevant to his example, when light hits a glass block it travels through it at an angle that results in the quickest travel time.
  2. jcsd
  3. Apr 30, 2016 #2


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    Too anthropomorphic, I think. Light behaves a certain way 'as if it knew' what to do. It has no motives or plans. We have invented laws and equations that predict (not explain) what it does.
    There are often alternative explanations for phenomena and people argue and worry to death about which one 'is correct' when what really counts is the 'accuracy' of each model.
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