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Why is Compton scattering thought to demonstrate light's behavior as a particle over that as a wave. I'm interested in the thoughts at the time of Compton, but also how it contradicts current theory of light-waves.

The closest I've found to an answer to my question are statements such as this: "[Compton's] data were inconsistent with the wave theory, which predicted that the scattered light would spread in all directions, like water waves radiating from a stone dropped in a pond."

Is this statement true, and is it saying that in Compton's time, wave theory predicted that a single light-wave would scatter in all directions...as in Huygens' "spherical" waves? Or am I not understanding the statement correctly?

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  • $\begingroup$ Compton scattering demonstrates, at most, the quantum behavior of light-matter interactions. One can, of course, try to formulate all these processes in pre-quantum mechanics language, but that is, at most, of historical interest. $\endgroup$
    – CuriousOne
    Apr 24, 2016 at 19:24
  • $\begingroup$ Are you saying that the claim that Compton scattering demonstrates light as a particle, and not as a wave, is basically baloney? $\endgroup$
    – David Reishi
    Apr 24, 2016 at 21:15
  • $\begingroup$ To talk about particles and waves in QM is, indeed, historical intellectual nonsense that wasn't necessary for almost a century, now. The correct answer is "it's neither, but a new, third type of entity, that in certain limits can have classical wave-like and classical particle-like behavior". $\endgroup$
    – CuriousOne
    Apr 24, 2016 at 21:20
  • $\begingroup$ "The correct answer is 'it's neither, but a new, third type of entity, that in certain limits can have classical wave-like and classical particle-like behavior.'" But might not that in 100, 20, or even 3 years, also be regarded as historical intellectual nonsense? I'm simply interested in the specific justification, from the time of Compton to now, used to claim that Compton scattering supports the notion of particle-like behavior over wave-like behavior. (Actual particles and waves aren't necessary in the discussion.) $\endgroup$
    – David Reishi
    Apr 24, 2016 at 21:29
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    $\begingroup$ Newtonian mechanics is now 350 years old, or so. We didn't change the content, even if we changed the language. QM is a far more universal theory than CM, so it's IMHO unlikely that we will change the basic content, either. However, what will probably change is how we calculate things (just like in Newton with introduction of Lagrange and Hamilton formalisms). The current calculation engine of QFT is woefully ineffective and some people seem to believe that this can be fixed. I don't know if that means we will have to fix the ontology and interpretation, or not. $\endgroup$
    – CuriousOne
    Apr 24, 2016 at 21:53

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I think this is an interesting question. Unfortunately, many hasty sketches of the history of physics, as they are taught, tend to draw somewhat biased conclusions for the sole purpose of avoiding delving into these types of questions (some people consider it to be a waste of time apparently).

As far as I can tell, the classical scattering theory at the time was essentially Thomson scattering theory which is basically an elastic scattering limit of the Compton scattering.

Since it is based on the assumption of an elastic scattering, it cannot explain what is observed for Compton scattering where, if anything, the frequency of the light is observed to change.

Now, to enter into a proper inelastic model one must try to figure out where goes the energy of the wave and propose an actual model for it, especially when using classical wave theory; and that's the difficult part.

In looking for references recently, I came across this paper by Raman (the same gentleman as for the Raman effect) who concludes his paper in a retrospectively surprising way by saying:

Incidentally, it becomes evident why the argument of the "triangle of momenta" by which Compton obtained his formulae gives the same result for the change of wavelength as the classical theory of the emission of spherical secondary waves by the electron.

Although there is no doubt that QED does a great job describing light-matter interaction better than any theory has done so far, it is thus safer to refrain ourselves from setting sharp demarcation lines with historical experiments allegedly signing the death warrant of classical theories; it is always much more complicated than we think.

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  • $\begingroup$ I find your answer satisfying, but I'm hoping I can milk you for a little bit more. You say, "Now, to enter into a proper inelastic model one must try to figure out where goes the energy of the wave and propose an actual model for it, especially when using classical wave theory; and that's the difficult part." Why is that the difficult part? Is it because, as Compton found, the newly-emitted light with the new wavelength is always emitted in one particular, definable direction? And if so, is that more or less the entire essence of why Compton won the day for particle theory? $\endgroup$
    – David Reishi
    Apr 24, 2016 at 21:11
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    $\begingroup$ Neither QM nor QED mean the death of classical theories. Newton is alive and kicking in its domain and that will never change. How this impacts non-theories like wave-particle duality and the historic development of the correct theory is not clear to me, though. There certainly have been plenty of false interpretations around and still are, but once a self-consistent interpretation is established, all of that falls by the wayside. $\endgroup$
    – CuriousOne
    Apr 24, 2016 at 21:26
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    $\begingroup$ "some people consider it to be a waste of time apparently" It's not so much a waste of time, but that the time simply isn't available if students are to be shepherded through the usual preparation in the usual number of terms at conventional loads. I consider it an advantage of ahistorical ordering in a modern physics course that you can pick and chose which transition arguments to present. Those that won't be covered in adequate detail simply aren't covered at all. Of course that replaces a superficial historical awareness with a deeper but patchy one, so you could reasonably disagree. $\endgroup$
    – dmckee --- ex-moderator kitten
    Apr 24, 2016 at 21:33
  • $\begingroup$ @CuriousOne. "There certainly have been plenty of false interpretations around and still are, but once a self-consistent interpretation is established, all of that falls by the wayside." Excellent statement, and superbly put. $\endgroup$
    – David Reishi
    Apr 24, 2016 at 21:35
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    $\begingroup$ @CuriousOne: "Neither QM nor QED mean the death of classical theories. " I am happy we agree on that point. I just disagree with the idea of sharp domains of utility of say Newton mechanics or classical EM. An alleged false interpretation can turn to be closer to te next best theory to come for a particular phenomenon than the best contemporary one and that's the reason why it is still valuable to talk about it. Even more so if it actually does the job quantitatively, as shown, with a specific model, by Raman. $\endgroup$
    – gatsu
    Apr 25, 2016 at 9:24

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