# Tagged Questions

Quantum-ElectroDynamics (QED) is the quantum field theory believed to describe the electromagnetic interaction (and with some extension the weak nuclear force).

450 views

### Coulomb interaction as virtual particles exchange?

I've been reading about virtual particle exchanges in physics books and in Physics SA posts, where a particle interpretation of gravity and Coulomb interaction is established. The Feynman Diagram ...
406 views

### accelerated charge in QED

Classical electromagnetism says an accelerated charge would radiate. Is this understood in QED? Is there QED derivation of Larmor's formula?
1k views

### Mass dimension of coupling constants in various dimensions

Just a quick question: Suppose I want to consider QED or YM in 4 dimensions we always say that the coupling constants are dimensionless and that the field then has a specific mass dimension. What ...
259 views

367 views

### Magnetic monopoles in spin ice and Dirac string comparison

In spin ice systems magnetic monopole-like excitations are sources or sinks of $H$, not the $B$ field, why is that? Is it because the strings carries magnetic moment $M$ and not solenoidal $B$ filed ...
437 views

### Determinant of Dirac operator in flat space?

How would you evaluate $$|iD\!\!\!\!/-m|$$ Where $D_{\mu}=\partial_{\mu}-ieA_{\mu}$. I have an idea of how to do this without the gauge field, because it's essentially \...
163 views

193 views

### LSZ theorem for photons

Is there any discussion on what the LSZ theorem for photons looks like? I would rather like a discussion of this in axial gauge $A_0 = 0$ (instead of the usual Lorenz gauge), but anything would do.
994 views

### Covariance of the Dirac equation and Infinitesimal Lorentz transformation

Okay, I've got really desperate now. I've spend 10 hours of work (for a few days) trying to prove 'trivial' equation in Quantum Electrodynamics. To anybody who want to write an answer for my questions ...
450 views

### Electron-Positron Scattering using the Feynman Rules - Integration Q

I'm doing independent studies on electron-positron scattering, specifically the annihilation diagram contribution to the M matrix in Bhabha scattering, and this is the equation I recovered with the ...
229 views

### Characteristic of photons for constant speed

What characteristic of photons causes them to travel with constant speed in all frames of reference? Till the topics I have studied, we always assume this but never got any ideas what can be in the ...
466 views

519 views

### Evaluation of QED amplitude with 1 external photon

I'm trying to compute the exact QED amplitude with one external photon. Suppose that the photon has 4-momentum $q$ and polarization $\varepsilon^\mu$. Peskin and Schroeder (p318) claim that ignoring ...
8k views

### How do we know photons have spin 1?

Electrons have spin 1/2, and as they are charged, they also have an associated magnetic moment, which can be measured by an electron beam splitting up in an inhomogeneous magnetic field or through the ...
879 views

### How does the Ward-Takahashi Identity imply that non-transverse photons are unphysical in QED?

Peskin and Schroeder say that the Ward Identity of QED proves that non-transverse photon polarizations can be consistently ignored, but I'm confused about the details. Setup One starts by ...
449 views

### How to introduce the electromagnetic field in Quantum Field Theory?

There are many ways to introduce the electromagnrtic field in Quantum Field Theory(QFT), such as canonical quantization method which introduces the creation and annihilation oprators by treating the ...
768 views

### Charge Renormalization and Photon Propagator

I'm trying to understand charge renormalization in QED. I know that one can write the full photon propagator as $$\frac{-i\eta_{\mu\nu}}{q^2(1-\Pi(q^2))}$$ where $\Pi$ is regular at $0$. Obviously ...
507 views

### How do fermion and scalar masses run with energy? Is the difference in their running the core of the hierarchy problem?

Do fermion and scalar running masses run in the same way? Specifically, what are the qualitative differences in the mass beta functions for, say, scalar $\lambda\phi^4$ field theory and the fermion ...
478 views

65 views

### Photon detection time in NMR rotating frame

I think of an NMR experiment, but with a single spin half nucleus initially set to the excited state. When the nucleus finally returns to its ground state, it will emit a photon. An observer in the ...
49 views

### Charge in terms of wavefunctions

For a charged quantum particle, say, an electron or a quark, how in the particle's wavefunction is the electric charge represented? Is it truly possible to represent electric charge using the wave ...
54 views

### If the measurement of the proton size is correct and it is lower that it was expected than what are the theoretical implications?

In the press release of the recent measurement of the proton structure, they barely mention various theoretical explanations for the effect. What is the most likely (not assuming now that the ...
10k views

### Can a light be bent by a magnetic field?

I'm struck with two competing ideas on the question in the title. Listing #1: http://van.physics.illinois.edu/qa/listing.php?id=2009 How far can a magnetic field bend light? A: Unfortunately, the ...
161 views

### Linear quantization in quantum electrodynamics?

This is a continuation of this question. What would be an example of linear quantization used on quantum electrodynamics? I ask this because QED is a nonlinear theory.
410 views

### Mass gap for photons

I am puzzled by the answers to the question: What is a mass gap? There, Ron Maimon's answer gives a clear-cut definition, which I suppose applies to any quantum field theory with Hamiltonian $H$, ...
2k views

### Photons, where do they come from? [closed]

Photons, where do they come from? What exactly is a photon? I've certainly heard how they get produced, but it doesn't seem to make sense that some sort of particle should need to be produced just to ...
3k views

### What are the limits of applicability of Coulomb's Law?

Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...
410 views

### Which is this formula Feynman talks about in the QED book?

I am reading the fantastic QED Feynman book. He talks in chapter 3 about a formula he considers too complicated to be written in the book. I would like to know which formula he talks about, although I ...