Why do we believe local gauge invariance (that tells us photons are exactly massless)?
47568
Is it experimentally proven that photons travel at speed $c$ in vacuum?
112366
Is mass an observable in Quantum Mechanics?
35409
Massless charged particles
88111
Why don’t photons interact with the Higgs field?
44239
Why the extra term $\frac{1}{2}(\partial_{\rho}A^{\rho})^2$ in the photon Lagrangian?
1537
Is a photon "fixed in spacetime"?
136143
Are gauge choices in electrodynamics really always possible?
64935
Using photons to explain electrostatic force
26806
Virtual photons, what makes them virtual?
158808
Do photons have relativistic mass?
128506
How do we know that the laws of physics are invariant in all inertial frames?
41466
Can the photoelectric effect be explained without photons?
7291
Gravity as a gauge theory
22842
The Interpretation of $|\psi|^2$ and the No-Clone Theorem
69983
Photon energy - momentum in matter
95125
Rest mass of the photon
9463
What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space?