Mediators and propagators of particle interactions

[valleyman]

hello I'm studying nuclear physics and I have a lot of questions which I can't figure out.
The first thing has been thrown in my mind by a colleague of mine about propagators of particle interactions. It has been said that, for example, photons are the mediators for the electromagnetic interaction: what does it mean exactly? Does a resting charge continuosly emit an infinite number of photons all over the surrounding space? And if so, as photons should bring energy, doesn't this mean that the charge is losing its energy? I know that this can't be true but this means I didn't understand well the point. Or does it emit photons just when another charge is present? Wouldn't that mean that the electric field is "activated" just in presence of other charges, when I thought it was a intrinsic property of the charge?

Thanks for the help, valleyman

P.S. I'd appreciate a lot some links to *simple/basic* articles or lessons talking about this argument (interactions, propagators, Feynamnn diagrams and so on..)

 

[tom.stoer]

For static charges a physical gauge like Coulomb gauge is most appropriate. In such a physical gauge you will find a static Coulomb potential. In the radiation gauge it looks like if static charges radiate, but is is not a physical process but a matehmatical artefact.

Perhaps it makes sense to merge this discussion with https://www.physicsforums.com/showthread.php?t=405604

 

[valleyman]

thanks for the help but I'm sorry to say that I still can't understand the answer to my question...

 

[Patrus89]

hello I'm studying nuclear physics and I have a lot of questions which I can't figure out.
The first thing has been thrown in my mind by a colleague of mine about propagators of particle interactions. It has been said that, for example, photons are the mediators for the electromagnetic interaction: what does it mean exactly? Does a resting charge continuosly emit an infinite number of photons all over the surrounding space? And if so, as photons should bring energy, doesn't this mean that the charge is losing its energy? I know that this can't be true but this means I didn't understand well the point. Or does it emit photons just when another charge is present? Wouldn't that mean that the electric field is "activated" just in presence of other charges, when I thought it was a intrinsic property of the charge?

Thanks for the help, valleyman

P.S. I'd appreciate a lot some links to *simple/basic* articles or lessons talking about this argument (interactions, propagators, Feynamnn diagrams and so on..)

The resting charge does not emit this, which is the whole point of quantization. Mediators are exactly what they sound like, bits of math to explain the processes of absorption and emission. The field itself is quantized at all points, and real photons do in fact deliver energy, but they are not what you friend was talking about. I think a Feynman Diagram would not be helpful here.

This may help: http://www.mathpages.com/rr/s9-10/9-10.htm

 

[tom.stoer]

Does a resting charge continuosly emit an infinite number of photons all over the surrounding space?

In terms of Feynman propagators in the radiation (= an unphysical!) gauge - yes, something like that. But these photons are only mathematical entities which are integrated over. The virtual photons are called "virtual" as they explicitly violate E² - p² = m².

... doesn't this mean that the charge is losing its energy?

No. At each vertex in a Feynman diagram the four-momentum is conserved.

Wouldn't that mean that the electric field is "activated" just in presence of other charges, when I thought it was a intrinsic property of the charge?

This is unfortunately the picture emerging from taking this picture too literally. But this is wrong.

Think about virtual particles as mathematical entities. Math in QFT is complicated enough. If explanations from popular books help to clarify what's going on, you are free to use them; if they confuse you, forget about them and restrict yourself to the calculation.

 

[valleyman]

Thanks for the clarification guys.
@Tom.stoer: I particularly appreciated ur explanation, very clear, but now what I'm wondering is: if mediator particles are just mathematical entities is the e.m. field compound by particles or is it a vector field? I heard that Newton said that a force acting between two bodies withouth anything mediating the propagation is kind of absurd, so what is it?