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

learn more… | top users | synonyms (1)

3
votes
3answers
210 views

Are electron fields and photon fields part of the same field in QED?

I know in classical field theory we have the electromagnetic field. And Maxwell's equations show how electromagnetic radiation can propagate through empty space. I also have been reading about QED ...
0
votes
0answers
33 views

How to go from Lorentz gauge to Coulomb gauge?

I want to know that if I find some solution, say a polarization vector in Lorenz gauge. Now I want to go to Coulomb gauge. There must be a transformation that links both the gauges. How would I find ...
5
votes
4answers
755 views

How is the path integral for light explained, or how does it arise?

In a Phys.SE question titled How are classical optics phenomena explained in QED (Snell's law)? Marek talked about the probability amplitude for photons of a given path. He said that it was ...
2
votes
6answers
145 views

Why does accelerating electron emits photons? [duplicate]

I have read that accelerating or oscillating electron emits photons. But why and how does it so? And why only photons? There are other bosons like gluons, W and Z bosons, so why does electron emit ...
3
votes
2answers
138 views

Electromagnetic reaction force?

The classical (retarded) Lienard-Wiechert scalar and vector potentials describe the electromagnetic field due to an arbitrarily moving electric point charge. Thus given the motion of electron $A$ one ...
12
votes
2answers
2k views

Using photons to explain electrostatic force

I am trying to understand the idea of a force carrier with the following example. Let's say there are two charges $A$ and $B$ that are a fixed distance from each other. What is causing the force on ...
4
votes
2answers
127 views

If proton spin emergence from quarks and gluons is mysterious, why is silver atom spin not?

A recent Scientific American article brought up an old issue, which is this: According to quantum chromodynamic models, the emergence of exactly 1/2 unit of spin in a proton (or a neutron, or any ...
1
vote
1answer
99 views

How do you visualize a quantized electromagnetic field?

Quantizing the EM field gives you the standard sum of all quantum harmonic oscillators as given by this hamiltonian: $$ \hat{H} = \sum_k \hbar\omega_k( \hat{a}^\dagger_k \hat{a}_k + 1/2) $$ ...
1
vote
2answers
64 views

Why is a transmon a charge qubit?

The classic charge qubit is the cooper pair box which is a capacitor in series with a Josephson junction. In my understanding, by changing the gate voltage at the capacitor, one can create a ...
1
vote
0answers
38 views

Can we quantize Maxwell-Chern-Simon Theory through Gupta-Bleuler approach?

In 3+1 QED we covariant quantize the Maxwell theory through Gupta-Bleuler method. But I have seen that MCS theory is explicitly covariant quantized using Nakanishi auxiliary field. Why cannot we take ...
6
votes
1answer
243 views

Peskin's book page 334 proof of $Z_1=Z_2$ to all orders in QED perturbation theory

Peskin in his QFT page 334 argued that $Z_1=Z_2$ to all orders in QED perturbation theory, but I couldn't understand his argument: ... With a generalization of the argument given there (section ...
3
votes
2answers
73 views

Sign of momentum in fermion propagator

Thinking of a process like Compton scattering, where we have an electron as a propagator, I would typically write down the propagator as $$i \frac{\not q+m}{q^2-m^2}.$$ If I were to replace the ...
3
votes
2answers
287 views

Is there a way to calculate the photoelectric effect in QED via a Feynman diagram?

The photoelectric effect is the historic origin of the quantum particle description of light. From it we learn that when light is shone onto a metal single photons interact with single electrons in ...
0
votes
1answer
58 views

Deriving Coulomb's law from quantum electrodynamics [duplicate]

Is it possible to derive the Coulomb's law using the principles of quantum electrodynamics? How?
10
votes
2answers
310 views

Renormalizing QED with on-shell fermions

When renormalizing QED, we calculate the 1 loop correction to the fermion-fermion-photon vertex using the diagram, $\hskip2in$ When doing the calculation we typically let the photon go off-shell ...
1
vote
1answer
117 views

Free Electromagnetic field in Lorenz gauge

To get rid of the extra term in the QED Lagrangian we need to redefine the electromagnetic four-vector: $A^{\mu} \rightarrow A^{\mu} - \frac{1}{c} \partial_{\mu} a(x)$ where $a(x)$ is the function ...
1
vote
2answers
68 views

Deriving solution of the Renormalization Group Equation

I am trying to follow Matthew Schwartz's renormalization group lectures (pdf or see Chapter 23 of QFT and the SM by Matthew Schwartz), but I am having trouble with Eq. (book 23.31/pdf 29). I ...
1
vote
1answer
85 views

How does exciting an electron's surrounding electromagnetic field cause 'electron excitation'?

In more meaningful words than the ones above, how does adding energy to the EM field cause the electron to to change orbitals or oscillate in a different pattern.
0
votes
0answers
19 views

Integration for quantum amplitude of the coupling between two molecules

I am trying to solve following expression for quantum amplitude of coupling between two molecules, (arriving from the second order perubation) $$\frac{1}{p}\nabla_{j}\int e^{ipR\cos(\theta)} dT=i\int ...
2
votes
1answer
79 views

Charge operator for Dirac spinor

In QED, the gauge transformation which acts upon a fermionic field $\psi$ is $$\psi'(x)= e^{i \alpha(x) Q}\psi(x)$$ where $Q$ is the charge operator. Most of the time it's just written as $$\psi'(x)= ...
0
votes
0answers
27 views

Massive vector field in curved spacetime

Setup Consider a massive vector field in anti-de Sitter space AdS$_{d+1}$ with metric $$ ds^2=\frac{1}{z^2}\left(dx_\mu dx^\mu+dz^2\right) $$ where $dx_\mu dx^\mu$ is the line element in d+1 ...
6
votes
2answers
219 views

Do virtual photons have a frequency?

Real photons do have frequencies, which is directly related to its energy. So, can virtual photons that take part in EM interactions have frequencies too? When my hand is pressed up against a glass ...
5
votes
2answers
365 views

How does light know which path is fastest?

We know from Fermat's principle of least time that light follows the fastest path. But how does light know which path is the fastest?
3
votes
0answers
52 views

Fastest path of light [duplicate]

Fermat's principle of least time says that light always takes the fastest path to any point. So how can light know which is the fastest path without taking all the paths first?
3
votes
1answer
116 views

Aren't $\phi^4$ composite operators?

I have this trouble with terminology. I wonder why authors introduce the concept of composite operators after they've already talked about eg phi four theory, it phi cubed. Aren't these operators ...
2
votes
0answers
45 views

Scalar QED perturbation theory [closed]

Where can I find material about scalar QED? Renormalization, scattering matrix, perturbation theory, etc...
10
votes
2answers
2k views

Bound states in QED

I am a beginner in QED and QFT. What is known (or expected to be) about bound states in QED? As far as I understand, in non-relativistic QM electron and positron can form a bound state. Should it be ...
0
votes
0answers
13 views

Associated photon field of a two state system

Given the time-dependent density matrix $\rho(t)$ of a driven two-state system $\mathcal{S}$. How do I calculate the time-dependent quantized photon field that would be generated by transitions in ...
1
vote
0answers
24 views

Correction to the residue in QED using $\overline{MS}$ contains IR divergence

I'm Calculating the next-to-leading orders in QED, but I'm using $\overline{MS}$ scheme, as known in $\overline{MS}$ the residue is no longer one and I have to calculate the correction to the residue ...
2
votes
1answer
59 views

What is the difference between QED and quantum optics?

What is the difference between QED and quantum optics? Once you have QED, you have not only the quantized EM field, but also the interaction between matter and the EM field. So, you should have ...
0
votes
1answer
72 views

Can photons travel faster than $c$? (Feynman Lectures)

I apologise for the very non-technical nature of this question. I am new to QED and perhaps am interpreting things in the wrong way, but I'll ask anyway, and hopefully someone can provide a ...
2
votes
0answers
33 views

Maximum Power transmitted using General Relativity waves - cf Schwinger limit

In Electromagnetism, QED says that the linearity of Maxwell's equations comes to an end when field strengths approach the Schwinger limit. Its about 10^18 V/m. What is the corresponding formula for ...
3
votes
1answer
55 views

Does Fermat's principle of least time apply to sound waves?

I am reading Feynman's presentation of Fermat's Principle of Least Time, which explains the behavior of light; does it apply to waves in general? for example sound waves or waves on the surface of ...
1
vote
0answers
46 views

Quantum mechanics question in derivation of Heisenberg-Euler Lagrangian in Schwartz “QFT” notes

In http://isites.harvard.edu/fs/docs/icb.topic1246957.files/IV-9-EffectiveActions.pdf (Page 20) Schwartz derives the Heisenberg-Euler Lagrangian using Schwinger's proper time method. To do so, he ...
2
votes
1answer
137 views

Photon polarization sum prescription in $e^-e^+\to{}2\gamma$

In calculating the amplitude for the process $e^-\gamma\to{}e^-\gamma$ the substitution $\sum\epsilon_{\mu}\epsilon^*_{\nu}\to-\eta_{\mu\nu}$ is useful to sum over photon polarizations. If we ...
4
votes
3answers
907 views

Why does positronium decay into 2 photons more often than into 3 photons?

I cannot find the answer to the above question. I know that para-positronium is created with a probability of $25\%$ and decays into 2 photons, while ortho-positronium is created with a probability ...
0
votes
1answer
43 views

Aharonov-Bohm experiment: A possible way to understand potentials?

I'm posting this question as a function of an another question I found here: What is potential energy truly? The answer of WetSavannaAnimal aka Rod Vance. As I understand the hole point is to see ...
0
votes
2answers
116 views

Why Lagrangian of electromagnetism with Lorenz Gauge evolve Klein Gordon equation?

Simply Lagrangian without a source for Maxwell equation is $$ L = -\frac{1}{4}F^{\mu\nu}F_{\mu\nu} $$ Also Lorenz Gauge condition is $$ \partial_{\mu}A^{\mu}=0 $$ and if so I can briefly add this ...
1
vote
1answer
64 views

First-order EM Feynman diagram?

Is there any 1st order electromagnetic Feynman diagram? I.e. a process whose probability is just $\propto \alpha_{EM}$? If not, is there any physical reason why? We always need at least two particles ...
2
votes
1answer
612 views

Does a quadrupole transition mean emission of one photon with spin 2?

If it's true and spin-2 photons do exist, could you please point to some literature that discusses spin-2 photons? If not, then how exactly does a selection rule for quadrupole transition make sense ...
5
votes
3answers
426 views

Why possibility for X-ray to excite inner electrons higher than outer electrons?

It seems X-ray absorption spectroscopy is usually ascribed to the interation between photons and inner electrons. Does it mean inner electrons are much preferred by X-ray photons to outer electrons? ...
5
votes
1answer
375 views

Why can't Compton scattering happen in leading order of perturbation theory?

Why is the matrix element of Compton scattering in leading order of perturbation theory equal to zero? Why can this process only be described in second order of perturbation theory, i.e. with exchange ...
2
votes
1answer
32 views

How do logarithms show up in the one loop calculation of the vacuum polarization in QED?

I am following Peskin with the computation of the vacuum polarization in QED and there is one thing I do not see. Equation (7.90) reads ...
0
votes
0answers
22 views

The Schwinger Limit and the Breit-Wheeler Process

The Breit-Wheeler process requires a two photon system to have a total energy of at least twice the rest mass of an electron, which is determined by the frequencies of the photons. The Schwinger ...
1
vote
0answers
39 views

Why do Lorentz invariance and Ward identities imply the structure $\Pi(k^2)(k^2\eta^{\mu\nu}-k^{\mu}k^{\nu})$ for the photon self energy?

In the first page of this link we can read that Ward identities and Lorentz invariance make the form if the photon self-energy be $$\Sigma^{\mu\nu}=\Pi(k^2)(k^2\eta^{\mu\nu}-k^{\mu}k^{\nu}).$$ Why?
4
votes
1answer
91 views

Why is tree-level interaction between neutral scalar and photons non-renormalizable?

I've read that the decay of a neutral scalar particle into two photons, i.e., $$ S(p+q) \to \gamma(p) + \gamma(q) $$ can't happen via tree diagrams and instead is caused by loop diagrams (such as a ...
2
votes
2answers
126 views

Propagation speed of photons when taking higher-order QFT corrections into account

In our group of experimental physicist who have nothing to do with and know very little about quantum field theory, we recently had a question concerning the propagation speed of photons in vacuum: ...
2
votes
2answers
167 views

Isn't the Coulomb interaction a photon interaction between two charges?

Isn't the Coulomb interaction a photon interaction between two charges? if yes then what does the following text mean? (Many-particle Physics by Gerald D. Mahan.)
2
votes
1answer
67 views

Vacuum has zero spin in Dirac theory

I have troubles trying to prove a statement made by Peskin-Schroeder in page 61, section 3.5 where he says that the "spin" operator $J_z$ given by the non numbered equation $$ J_z= \int d^3 x ...
13
votes
2answers
1k views

In an electron-positron annihilation, in what direction are the photons released?

I read that, in an electron-positron annihilation, at least 2 photons are produced, because of the law of conservation of momentum. my question is: in what direction are those photons released? and ...