How photons and static em feilds work

[MitchellW]

The photon as I understand it is a massless particle that "must" travel at the speed of light.

However I do not understand the idea that a photon is also the carrier of the em force.

If there is a simple ball of static charge and a test charge comes near do photons exchange between the two charge center back and forth, confining themselves to the two charge centers

 

[mfb]

You can describe this force via the exchange of virtual photons, but static fields are easier to visualize without photons.

 

[tom.stoer]

You can describe this force via the exchange of virtual photons, but static fields are easier to visualize without photons.

That's possible but missleading. Neither the potential nor the virtual photon are gauge invarian terms.

It's more appropriate to apply physical gauge fixing, i.e. setting A° = 0, solving the Gauß constraint G|phys> = 0 and therefore derive the physical Hamiltonian H. H contains physical, transversal photons NOT carrying the static Coulomb interaction, plus a term ~ ρ(x)ρ(y)/|x-y| where ρ are the charge densities (i.e. electron fields). This works both in classical electrodynamics and in full QED.

Therefore (virtual) photons are not required to explain the Coulomb force; they explain just dynamical fluctuations beyond the static approximation.

 

[Vanadium 50]

Tom, do you think the OP is wondering about gauge fixing? Or is it more likely he read something in a popularization like "quantum mechanics is just like regular mechanics, except that it has virtual particles, which can temporarily violate the conservation of energy."?

 

[tom.stoer]

I don't know. All I wanted to indicate is that the 0th (classical, static) order of the Coulomb field can be understood w/o mentioning virtual particles an w/o photons as force carriers.