# What is the QFT picture of forces?

1. Oct 14, 2009

### Gerenuk

According to QFT, how do two masses attract?
Is the action instantaneous? How is momentum/energy conserved? Is the action non-local?

2. Oct 15, 2009

### RUTA

See Zee, A.: Quantum Field Theory in a Nutshell. Princeton University Press, Princeton (2003), Chap I.4, From Field to Particle to Force, pp 24-27.

3. Oct 15, 2009

### alalaika

they interact as they always do. first, quantum field theory should in the limit of low energies always give the classical picture, and second, qft just means that you compute fields and quantizations of these fields are the known particles

4. Oct 15, 2009

### Staff: Mentor

Since you said "masses", are you asking specifically about gravity? There is no generally accepted QFT for gravity yet, as far as I know, so this is still an open question.

5. Oct 15, 2009

### Gerenuk

Yes, I did ask about gravity. But it would also be interesting to know how the Coulomb force acts through photons. At some point I will dive into QFT, but for know I was hoping to get a rough answer about what happens when two charges interaction. I mean where/when are there photons? How is momentum conserved?

6. Oct 16, 2009

### Mentz114

Look at the Hamiltonian of QED. It has three terms, one for the electron, one for the EM field, and an interaction term that is like $\mathbf{A} \cdot \mathbf{J}$. The virtual photons that exchange energy between the field and the matter live in the interaction term.

I don't know if this helps with the forces...

7. Oct 16, 2009

### RUTA

See Zee, A.: Quantum Field Theory in a Nutshell. Princeton University Press, Princeton (2003), Chap I.5, Coulomb & Newtown: Repulsion and Attraction, pp 30-34. Quote from bottom of p 33, "We are now ready to understand one of the fundamental mysteries of the universe: Why masses attract."

8. Oct 16, 2009

### Bob_for_short

In the Coulomb gauge where only physical (photon) variables are present, there is an instant Coulomb interaction term. But even in non-relativistic case the Coulomb interaction occurs bewteen de Broglie waves, not between point-like charges.