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Do photons travel in straight lines?

  1. May 4, 2010 #1
    If a photon is detected at point B having been emitted at point A, can one deduce with near certainty that it must have travelled continuously in a straight line from A to B at velocity c?
     
  2. jcsd
  3. May 4, 2010 #2

    DrChinese

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    Not really. Photons do not travel like little point particles moving in a line. You are correct that they are usually observed to travel at c (assuming a vacuum) in a straight line (assuming you ignore spacetime effects). However, this is considered to be an average or net result of the various paths by which the photon could move. So it is "as if" it were as you describe in many ways, but in others ways it is quite different.
     
  4. May 4, 2010 #3

    mathman

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    Photon paths are deflected by gravity. 1919 Eddington observation was first confirmation of general relativity. It was based on the deflection of starlight by the sun, observable during a total eclipse.
     
  5. May 4, 2010 #4
    I think it depends on the scale of A, B, and the straightness of the presumed path, relative to the wavelength. If I block a laser beam crossing a room, I won't expect it to go around my hand and hit the aligned target anyway. But for a radio beam, it doesn't notice my hand at all.

    FM radio will ooze around buildings and under bridges; GPS signals won't. But while the shadow of a building, cast from a point-source of visible light would be sharp, the GPS signal would fade out over a few inches inside that shadow.
     
  6. May 4, 2010 #5
    No, photons do not travel in a straight line from point A to B. We can see this experimentally by noting what photons do when they travel through diffraction slits.

    Feynman has a layman's description of how photons travel in his book QED. Most of what he says in that book is explained in this nice video:
     
    Last edited by a moderator: Sep 25, 2014
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