What types of forces are described in the standard minimal QED Lagrangian?

Click For Summary

Discussion Overview

The discussion focuses on the types of forces described in the standard minimal Quantum Electrodynamics (QED) Lagrangian, particularly how the electric and magnetic components of the Lorentz Force are represented through photon interactions. The scope includes theoretical aspects of QED and its relationship to classical electromagnetism.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that the electric part of the Lorentz Force in QED is realized through the absorption and emission of photons.
  • Others question how the magnetic part of the Lorentz Force is similarly realized via photon activity, suggesting that virtual photons may play a role.
  • One participant raises the issue of velocity dependence in the magnetic force, asking if this is represented in QED or if it only concerns the net force on particles.
  • Another participant challenges the notion of "force" in quantum mechanics, suggesting that the dichotomy between electric and magnetic forces may not be as clear-cut as assumed.
  • A later reply discusses the elegance of the relativistic description of electrodynamics, indicating that the split between electric and magnetic fields is frame dependent and may not be necessary in a formal QED context.
  • One participant references a more formal answer found in another thread regarding the standard minimal QED Lagrangian and its implications for forces.

Areas of Agreement / Disagreement

Participants express differing views on the representation of magnetic forces in QED and the relevance of velocity dependence, indicating that multiple competing perspectives remain without consensus.

Contextual Notes

Participants note that the understanding of "force" in quantum mechanics may differ from classical interpretations, and the discussion reflects unresolved questions about the relationship between classical and quantum descriptions of electromagnetism.

LarryS
Gold Member
Messages
359
Reaction score
33
TL;DR
How is the magnetic part of the Lorentz Force realized in Q.E.D. via photon exchange, etc.?
In Q.E.D., the electrical part of the Lorentz Force between unlike and like charged particles is realized through the absorption and emission of photons.

How is the magnetic part of the Lorentz Force realized in Q.E.D. via photon activity?

As always, thanks in advance.
 
  • Like
Likes   Reactions: Delta2
Physics news on Phys.org
referframe said:
In Q.E.D., the electrical part of the Lorentz Force between unlike and like charged particles is realized through the absorption and emission of photons.

How is the magnetic part of the Lorentz Force realized in Q.E.D. via photon activity?
Um, by absorption and emission of (virtual) photons?

Why do you think that is not a sufficient explanation for the magnetic part but it is for the electric part? What's the difference?
 
  • Like
Likes   Reactions: bhobba, vanhees71 and Delta2
The Lorentz Force is basically Classical EM, right? In that world, the magnetic force has a velocity dependence. Is that velocity dependence in the Q.E.D. world still represented somehow, or is Q.E.D. only concerned with the net force on the particle?
 
  • Like
Likes   Reactions: vanhees71
referframe said:
Is that velocity dependence in the Q.E.D. world still represented somehow, or is Q.E.D. only concerned with the net force on the particle?
"Force" isn't really a thing in QM. And this sentance seems to assume some dichotomy that isn't really there.
 
  • Like
Likes   Reactions: bhobba and vanhees71
Not sure which dichotomy you're referring to. Electrical vs Magnetic?
 
  • Like
Likes   Reactions: Delta2
referframe said:
Not sure which dichotomy you're referring to.
The one you posted and I quoted.
 
  • Sad
  • Like
Likes   Reactions: Motore and Delta2
The elegance of the relativistic description of electrodynamics (no matter whether classical or quantum) is that you don't need the artificial split of the electromagnetic field in electric and magnetic components anymore although this is of course important to understand the phenomenology, but the split indeed is frame dependent.

Writing down the standard minimal QED Lagrangian, i.e., a Dirac field minimally coupled to the em. field, contains all kinds of "forces", i.e., the standard Lorentz force ##q \vec{E}+q \vec{v} \times \vec{B}/c## as well as the "forces" due to the magnetic moment of the electron.

By chance we have a more formal answer in another recent thread:

https://www.physicsforums.com/threads/samalkhaiats-challenge-002.1006325/
 
  • Like
Likes   Reactions: bhobba, LarryS and PeroK

Similar threads

  • · Replies 1 ·
Replies
1
Views
865
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 87 ·
3
Replies
87
Views
8K
  • · Replies 5 ·
Replies
5
Views
6K
  • · Replies 1 ·
Replies
1
Views
1K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 7 ·
Replies
7
Views
4K
  • · Replies 2 ·
Replies
2
Views
2K