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

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SUMMARY

The average speed of a single photon is conclusively equal to the speed of electromagnetic (EM) waves, as supported by experimental evidence and theoretical frameworks such as Quantum Electrodynamics (QED). The photon is established as massless, with U(1) gauge symmetry in QED confirming that its mass (m) is zero. Discussions surrounding the potential for massive photons arise from misunderstandings of gauge symmetry and the conditions under which it may be violated. The concept of a "point particle view" of a photon is invalid, as photons do not conform to classical particle descriptions.

PREREQUISITES
  • Understanding of Quantum Electrodynamics (QED)
  • Familiarity with gauge symmetry concepts
  • Knowledge of the Klein-Gordon equation
  • Basic principles of electromagnetic wave propagation
NEXT STEPS
  • Study the implications of U(1) gauge symmetry in QED
  • Explore the Klein-Gordon equation and its relevance to gauge fields
  • Investigate experimental evidence supporting the massless nature of photons
  • Examine conditions under which gauge symmetry may be violated
USEFUL FOR

Physicists, students of quantum mechanics, and anyone interested in the fundamental properties of light and electromagnetic theory.

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?
 
Last edited:
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.
 

Thread 'Two equivalent statements of time reversal symmetric Hamiltonian'

Time reversal invariant Hamiltonians must satisfy ##[H,\Theta]=0## where ##\Theta## is time reversal operator. However, in some texts (for example see Many-body Quantum Theory in Condensed Matter Physics an introduction, HENRIK BRUUS and KARSTEN FLENSBERG, Corrected version: 14 January 2016, section 7.1.4) the time reversal invariant condition is introduced as ##H=H^*##. How these two conditions are identical?
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