Emulsion films (developed by F.Powell) for example show that elementary particles follow a straight trajectory staying in a film. The reason was clarified by Francis Mott in his famous 1929 paper about alpha tracks. It implies that at each weak interaction the wave collapses (decoherence) to a packet moving in a straight line. I don't understand why this can not be the same with a photon in thin film of weakly absorbing substrate on a plate (by example the film only rotating the polarization plane of the light). As far as I have been told this is a case of forward coherent scattering, which is some sort of interaction which doesn’t change the impulse (or at least its direction). So the first condition of a weekly interacting particle is met. The second is also met: the photons must be scattered exactly forward. Now I think that a photon can not follow a straight trajectory in a thin transparent film even provided the source has emitted the photon inside the film with impulse parallel to the film surface. (lambda greater than thickness). But according to the alpha tracks and the Mott analysis this must be so. Why?

  • $\begingroup$ @AccidentalFourierTransform no, you're completely wrong. The photon trajectory -- after ejection-- can be colloquially taken as the classical light-ray path, which depends on any variation in index of refraction within the medium. The relative angle of photon emission to photon absorption is a rather interesting subject area. $\endgroup$ – Carl Witthoft Apr 28 '16 at 19:34
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    $\begingroup$ @CarlWitthoft: If one wants to talk about light rays, one should talk about light rays. When we are talking about photons, we mean quantum field theory, which is really not necessary in this context. If, however, we are talking about particle tracks, then we need to talk about weak measurement theory because straight tracks are the result of the interaction of high momenta quanta with weakly interacting matter. See Mott's 1929 derivation of alpha particle tracks from wave mechanics. $\endgroup$ – CuriousOne Apr 28 '16 at 19:43
  • $\begingroup$ If coherent scattering does not change the direction of propagation of a photon I can see no physical reason why it doesn't follow a straight line. $\endgroup$ – Mercury Apr 28 '16 at 20:09
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    $\begingroup$ @Mercury: Because a photon doesn't move, at all. It's the result of a local measurement on a quantum field. The straight line approximation between any two measurements of "a photon" only exists in human minds. $\endgroup$ – CuriousOne Apr 28 '16 at 20:13
  • $\begingroup$ I can not understand you. Do you mean that light (photons) hasn't traveled from stars to us? $\endgroup$ – Mercury Apr 28 '16 at 20:28