# Tagged Questions

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

712 views

### Feynman's path integral formulation and faster-than-light paths

In Brian Greene's book "The elegant Universe", he talks about the double slit experiment and Feynman's interpretation of Quantum Mechanics. According to the book, Feynman said that one vaild ...
127 views

### Convergence of light by light scattering amplitude

Perhaps I'm too exhausted to see the answer of why the photon-photon scattering should contain no divergences. In Peskin and Schroeder page 320 we find that because of the Ward identity the photon-...
2k views

### How many photons are needed to make a light wave?

What is the smallest number of photons needed to make a "light wave"? In other words, how many (coherent?) photons start to exhibit classical behavior? For example, how many photons are needed to get ...
41 views

### How Light interacts with Atoms? [on hold]

I think I have confused myself about how light interacts with matter, would somebody be able to clear these questions up for me? How does an atom reflect light? Can an electron just essentially '...
321 views

### Photon Mass Term in Schwinger's 2D QED Model

Why does the vacuum polarization in 2D massless Fermion QED, $$i\Pi^{\mu\nu}(q) = i(\eta^{\mu\nu}-\frac{q^\mu q^\nu}{q^2})\frac{e^2}{\pi},$$ have the structure of a photon mass term, as is ...
355 views

### Principle of Reflection on atomic level

The well observed phenomenon has besides several others has always been a fascination to me, we are well aware of several theories, experiments and practical applications of the well known phenomenon. ...
4k views

### Explain reflection laws at the atomic level

The "equal angles" law of refection on a flat mirror is a macroscopic phenomenon. To put it in anthropomorphic terms, how do individual photons know the orientation of the mirror so as to bounce off ...
109 views

### How is light related to photons? [duplicate]

This may seem like a duplicate but I do not understand other explanations. I have read that light is an electromagnetic wave (a fluctuation or disturbance in the electric and magnetic field). How ...
40 views

When we describe interaction of atom with EM field, we consider the interaction term in Hamiltonian as follows $$W(t) = -\frac{e}{2\mu c} (\hat{A} \hat{p} + \hat{p} \hat{A}) + \frac{e^2}{2\mu c^2} \... 2answers 349 views ### Why can't a real scalar couple to the electromagnetic field? If we have a complex scalar \phi we know that the gauge-invariant interaction with A is given by A^\mu J_\mu, where J is the Noether current of the U(1) symmetry of the Lagrangian$$ J_\mu\...
141 views

For massless fermion, the free propagator in quantum field theory is \begin{eqnarray*} & & \langle0|T\psi(x)\bar{\psi}(y)|0\rangle=\int\frac{d^{4}k}{(2\pi)^{4}}\frac{i\gamma\cdot k}{k^{2}+i\...
45 views

### Running of $\alpha$ and scattering amplitudes

Consider a QED scattering process $e^-+e^-\rightarrow e^-+e^-$. The scattering crosssection at the tree-level depends on the square of the fine-structure constant $\alpha$ (apart from the electron ...
134 views

### Electron-positron annihilation: Feynman diagram

Maybe my question is just a blunder. Consider Compton interaction: $e^{-}+\gamma \rightarrow e^{-}+\gamma$. There are two Feynman diagrams related to this process to the lowest order of $\alpha$. Now ...
47 views

### How do Rabi oscillations act on the relative phase of state kets

Suppose we have a system with a lower state $\left|{\downarrow}\right\rangle$ and and upper state $\left|{\uparrow}\right\rangle$, coupled by a pulsed electromagnetic field. I thought that from an ...
18 views

### Can someone please give some references on scattering in Proca theory?

Can someone please give some references on scattering in Proca theory? I am looking for scattering calculations with massive photons.
68 views

### Electric charge conservation in a superconductor

In a superconductor, $U(1)$ gauge symmetry is spontaneously broken. But $U(1)$ gauge symmetry is responsible for conservation of electric charge. Then it appears to me that the electric charge ...
281 views

### Explanation for Refraction of light (change of angle) and Reflection

Refraction: I want a qualitative Quantum Mechanical explanation of why do we see light rays -in the classical picture- bend when light goes from one medium to another. I read that it is due to ...
33 views

### Why is there a band gap in semiconductors but no band gap in conductors?

At this point, I understand that at least one fundamental difference between conductors and semiconductors is that in conductors, there is typically no band gap because the valence band and ...
18 views

### Helicity of photon as a symmetry group

I have troubles understanding how to derive E(2) representation of a photon symmetry group from the fundamental principles of Eelectrodynamics. I know that it has to do with the symmetries of ...
206 views

### Understanding the argument that local U(1) leads to coupling of EM and matter

I'm trying to better understand the argument that $U(1)$ local gauge invariance implies a coupling of EM and Dirac fields. I understand the math, but I'm not sure about the chain of logic. You start ...
218 views

### How are bound states handled in QFT?

QFT seems very well suited to handle scattering amplitudes between particles represented by the fields in the Lagrangian. But what if you want to know something about a bound state without including ...
122 views

### How can fields interaction give rise to particles?

We say light a matter-wave, meaning along with its wave property it shows particle nature. But how can fields interaction (electric and magnetic) give rise to particles (photon)? I wish someone could ...
56 views

### Thus, what mediates the electric interaction between two electrons? [duplicate]

We are all learning since kindergarten that the electric interaction is mediated by photons. then you learn this photons are not real, there are real photons, true, but these do not mediate the ...
30 views

### Energy loss of photons crossing a dielectric medium

How much energy do photons lose when crossing an "isotropic" dielectric medium? What would be possible mechanisms. I assume the straight forward answer would be that the photon loses zero energy and ...
1k views

### Why don't photons split up into multiple lower energy versions of themselves?

A photon could spontaneously split up into two or more versions of itself and all the conservation laws I'm aware of would not be violated by this process. (I think.) I've given this some thought, and ...
73 views

### What happens to the electric field lines when an electron and positron collide? [duplicate]

A month back I began my course in classical electrodynamics. I know about the field lines of moving charges. But I have this question that if an electron and a positron are situated some distance ...
2k views

### If electromagnetic fields give charge to particles, do photons carry charge?

As I understand these two statements: An electromagnetic field gives particles charge A photon is a quantum of electromagnetic field It must mean that a photon carries charge. But I guess it isn't ...
26 views

72 views

### Why is Dark Matter not simply explained by QED

My understanding is that Dark Matter needs to exist to explain the mass of the universe -- that is, all the observable matter in galaxies are not sufficient to explain the gravitational behavior -- ...
63 views

### How do physicists find the speed of neutrinos?

I have heard that there is evidence for neutrinos traveling close to the speed of light, but how is that done? Since neutrinos barely react with anything, and the only evidence for them is indirect (...
104 views

### Validity of the 'photon wavetrain' model of coherence

In some optics textbooks, coherence is introduced with the "photon wavetrain" model. In this model, we consider light generated by many atoms each making the same transition. During each transition, a ...
32 views

### Polarized Moller scattering cross section

When doing a computation of scattering cross sections of particles with spin, one usually averages over the initial spins and sums over the final ones. I'm a bit puzzled as to how to do the ...
47 views

### How frequently do accelerating electrons emit photons?

In the quantum 2-slit experiment with light, we know that as the source intensity is turned down, individual photons arrive at the detector. I would like to know if there is a formula which tells us ...
79 views

### Can the Biot-Savart law be derived from QED?

It's important that a new theory of physics contains the equations and results of previous related ones. Maxwell theory and QED both have explanations for electromagnetic phenomena so I'm wondering if ...