Charged particle and virtual photons

[xaratustra]

Imagine I am sitting on a lead ion and fly happily through the LHC tube . Suddenly I feel an strong force pushing me sideways to the left. A physicist sitting in the lab frame tells me later, that I have been going through a dipole magnet yoke with field lines from top to bottom.

How do I understand the force which is dragging me on the quantum level? Is it possible to describe this with collisions with virtual photos? If yes, then why to the left and not some other directions?

 

[Ken G]

Let's start classically to see something interesting. The magnetic field is described in the lab frame, but if you look for a description of what is happening to the particle in the particle frame, the relativistic frame change will alter the charge densities in the currents that cause the B field. In the particle frame, it sees a net charge on the left and right that pushes it left. Now you asked for a field theoretic answer, not a classical one, but my point is that the question is the same as asking for an interpretation of the source of electric field. That is indeed often described in terms of virtual photons, but the whole concept of virtual photons seems very contentious. On another thread, I asked exactly this question, and it seems that it depends on who you put it to as to whether or not the virtual photon concept is a good one here.

 

[xaratustra]

Thanks. Indeed I was interested in the field theoretical approach. How would QFT deal with the deflection of say an electron going through a dipole magnet?

The classical version seems to be well explained, using the Liénard-Wiechert and the E. M. Pucell explanation e.g. http://physics.weber.edu/schroeder/mrr/MRRtalk.html" .

 

[Ken G]

Right, that clarifies the role of frame changes. But frame changes are still relevant in QFT-- and so is the choice of a gauge. I believe that in the frame of the charge, and in the appropriate gauge, there are no virtual particles there-- just the Coulomb potential of the length-contracted charge density. But in a different gauge, the virtual photons show up as gauge bosons. So I'm not an expert, but it seems to me that the scenario you describe is not so different from two interacting stationary charges, and whether you treat that in a gauge where invoking virtual photons would be useful, seems to be up to you.