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

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Discussion Overview

The discussion centers around the average speed of a single photon and its equivalence to the speed of electromagnetic (EM) waves. Participants explore the implications of this relationship, including the concept of massive photons and the theoretical frameworks that support the masslessness of photons.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the equivalence of the average speed of a photon to the speed of EM waves, suggesting that if the average speed were less than c, it could imply the existence of massive photons.
  • Another participant expresses uncertainty about the idea of photons having a variance in speed, indicating they have not encountered this concept before.
  • A participant emphasizes that the understanding of photons as massless is based on experimental evidence and the success of theoretical predictions, stating that no further conclusions can be drawn beyond this point.
  • Discussion includes a reference to U(1) gauge symmetry in quantum electrodynamics (QED), with a participant questioning why the mass term must be squared in the Lagrangian and whether gauge symmetry can be violated under certain conditions.
  • One participant asserts that the concept of a "point particle view" of a photon is invalid, challenging the framing of the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the nature of photons, particularly regarding their speed and mass. There is no consensus on the validity of the "point particle view" or the implications of potential variations in photon speed.

Contextual Notes

Some claims rely on specific theoretical frameworks and experimental evidence, which may not be universally accepted or understood. The discussion includes unresolved questions about gauge symmetry and the implications of mass in photons.

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 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.
 
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