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

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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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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 environment?
 
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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.