I Why we know average speed of single photon equal speed of EM wave?

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The discussion centers on the relationship between the speed of a single photon and electromagnetic (EM) waves, questioning if photons could have varying speeds or be massive. It is emphasized that experimental evidence consistently supports the idea that photons are massless and travel at the speed of light (c). The conversation touches on quantum electrodynamics (QED) and gauge symmetry, noting that the mass of a photon is defined as zero within this framework. Some participants mention conditions under which photons might appear massive, raising questions about the violation of gauge symmetry. Ultimately, the concept of a "point particle view" of a photon is dismissed as invalid.
fxdung
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Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
 
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Do you mean photons have variance in their speed ? I have not heard about it.
 
fxdung said:
Why we know that average speed of a single photon(in point particle view) equal the speed of EM wave?If average speed of a single photon smaller than c then there exist massive photons?
What we say we know is determined by experimental evidence and the success of various theories we construct to predict these results. All measurements known support the notion that the photon is massless. One cannot really say more.
 
It is U(1) gauge symmetry in QED(that say m of photon equal 0)(I only know this at just this moment with QFT book) .But I do not clear why the term contains m must be square of gauge field the Lagrangian?Does motion equation for gauge field to be able reduce to Klein-Gordon equation?
I hear in some condition photons are massive(?). Is in this case gauge symmetry violated by enviroment?
 
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fxdung said:
average speed of a single photon(in point particle view)
There is no such thing; there is no valid "point particle view" of a photon.
 

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