# Mediation of the electromagnetic force

## Main Question or Discussion Point

Hi

I have some problem to get insight into the mediation of the electromagnetic
force and hope someone here is able to help me.

If you look at the Standard Model there is the photon which acts on
electrically charged particles like electrons and quarks.
The electromagnetic force is pushing and pulling particles (compared to
gravity which only pulls).

But how is this pull/push information transmitted? The photon is
neutral charged. Is there any additional property that determines how
a photon acts on a negatively/positively charged particle?

Or I'm completely on the wrong path and the whole push/pull is only
a mechanism of the electromagnetic field not a single photon?

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Has anybody thought about that ? I did not find an explanation which would satisfy me. Any hint, anybody ?

George Jones
Staff Emeritus
Gold Member
A photon has no charge, but it does have momentum, so absorption or emission of ("virtual") photons by particles gives rise to a force (speaking very loosely) that may either be attractive or repulsive.

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html" [Broken] is an explanation.

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A photon has no charge, but it does have momentum, so absorption or emission of ("virtual") photons by particles gives rise to a force (speaking very loosely) that may either be attractive or repulsive.
Thank you for the response. I will go to Baez's site. I have the feeling that the problem is due to the assimilation of a Feynman diagram as a real process. The sign in the calculation appears at the vertices. The photon itself, with its propagator, does not "carry" information on the charge.

But diagrams are not real processes, so we must not worry about the minutes of those calculations...

Well, I do not find the answer completely satisfactory. I'll go read Baez :-) Thanks again !

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Well, thank you very much for pointing me to Baez's site. The answer is totally satisfying. As you said, the paradox lies in the fact that virtual particle exchange can lead to attraction, contrary to the classical picture. Although it is not stated exactly in this manner, I still think the OP should be advised not to think of Feynman diagrams as real processes. Read the link is definitely the best advice in any case.

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Hi

the link is indeed very good to understand the attractive force in QED.
thx you for that.

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If you do not want to go through Baez's discussion, the point I recall from it is the following : the information on +/- or -/- charges is not carried by the photon. In the calculation, it appears in the interference between the amplitudes at first order (one photon exchange) and zeroth order (no interaction). If you just look at the Feynman diagram, you only have one photon exchange and you can not get the attraction or repulsion.

It is very important from time to time to ask basic questions

I found an article tackling the problem for any spin of the exchanged boson :

Attraction/repulsion between like charges and the spin of the classical mediating field
Phys. Rev. D 33, 2475 - 2477 (1986)