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Questions tagged [quantum-electrodynamics]

Quantum electrodynamics (QED) is the quantum field theory believed to describe electromagnetic interaction. It is the simplest example of a quantum gauge theory, where the gauge group is abelian, U(1).

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How can I compute this one-loop Feynman diagram?

I have the following interaction lagrangian between a fermion $\psi$ with charge $q$ and a photon $A^\mu$, and between the fermion and a neutral heavy scalar boson $\phi$: $$ \mathcal{L_{I}} = -...
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Relation between the trace anomaly and the energy-momentum tensor being off-shell

Let's say we have a massless QED theory with a Lagrangian \begin{equation} L=i\bar{\psi}\not{D}\psi-\frac{1}{4}F_{\mu\nu}F^{\mu\nu} \end{equation} The symmetric energy-momentum tensor is \begin{...
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Can we create a random variable using QED effects?

Quantum Electrodynamics (QED) has some observable effects such as the lamb shift, which is mainly caused by the vacuum polarization and the electron self-energy. These effects contribute to the "...
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Is the direction of the Coulomb force at short distances indeterministic?

Suppose we have two electrons that apply a Coulomb force on each other. At large distances, we can consider the two electrons as point charges and the direction of the Coulomb force would be on the ...
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Is there any Bohmian approach to quantum electrodynamics?

I know that in some Bohmian papers (like https://arxiv.org/abs/quant-ph/0303156), electron-positron pair creation and annihilation is modeled by different methods like stochastic jumps in the ...
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Is the whole electromagnetic spectrum made of quanta?

My question is probably stupid (and maybe has already been answered on other posts), but I just started investigating quantum physics and I am struggling to understand the real meaning of "quantum". ...
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What is the physical interpretation of the derivative of a particle field?

I am learning quantum field theory, specifically the quantization of the electromagnetic field. We have this Laplacian $$ \mathcal{L} = -\frac{1}{2} \partial_\mu A_\nu \partial^\mu A^\nu -j_\mu A^\mu $...
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Sum over real photon polarizations. The minus sign

Ok for real photons there is the formula when summing over the polarizations: $$ \sum_{\lambda=\pm}\epsilon^{*\mu}_\lambda\epsilon^\nu_\lambda = -\eta^{\mu\nu}$$ But if I have a matrix element of ...
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Explanation about light reflection in Feynman's QED

In his book QED, Feynman explained how the probability of light reflecting from a mirror can be calculated by summing unit vectors, with their direction representing the phase of the wavefunctions. ...
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How does the collision between two atoms work?

Considering the quantum mechanical model for an atom, what exactly happens when two atoms (say, two Ca2+ ions in a Brownian motion) collide with each other? As I know, this collision is not like a ...
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Dimensional analysis of electron-positron spectrum

The theoretical formula for the numbers of particles per energy $\varepsilon$ with colliding photons with energies $\omega_{1}$ and $\omega_{2}$ is given by following expression (reaction $\gamma\...
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Quantum mechanical photon exchange [duplicate]

Background: While trying to understand the Standard Model I stumbled on a paper that explained it in very simple terms. I recognized that I don't even understand the quantum mechanical way of the ...
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Photo pair production: differential cross section

In a scientific paper, I need for my Bachelor thesis, the differential cross section for photo pair production $\gamma\gamma\to e^{+}e^{-}$ is given as follow: $\mathrm{d}\sigma = \frac{r_{0}^{2}\...
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What does 'coupling' mean? (Laser)

I assume this question is so basic that no YT-video or paper I found answers it. But in order to understand I have to know what it refers to. I'm talking about laser coupling to be exact. I have a '...
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Pair creation and annihilation and Wheeler's one electron universe

I stumbled upon Wheeler's idea that there could be only one electron. If I understood correctly, basically the idea is that one electron moves forward and backward in time such that it appears as if ...
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Variance of an interacting quantum field in its vacuum state

A non-interacting quantum field $\hat{\phi}(x)$ can be decomposed into $a_{\textbf{k}}$ and $a_{\textbf{k}}^\dagger$. This enables us to calculate the variance of a free field. For example, the ...
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What really enforces technical naturalness of electron mass?

Technical or 't Hooft naturalness A parameter $\theta$ in the Lagrangian of a field theory is said to be natural, if in the limit of vanishing $\theta$, the theory has some enhanced symmetry. If this ...
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Reference for canonical quantization of E.M field (for quantum optics purposes in the end)

I am looking for a nice book or online course treating in a rigorous way the quantization of the E.M field. My needs in the end are to do quantum optics, but I would like to see the proof in a nice ...
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$2s\rightarrow 1s$ transition in hydrogen

I would like to understand how the parity-violated interaction between electron and proton can provide the photon with circular polarization in the $2s\rightarrow 1s$ transition with single photon ...
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Dirac delta normalization of electromagnetic fields

Usually, to quantize electromagnetic fields we use box normalization and therefore the normalization constant contains the dimensions of Volume V of the box. But if we perform the Dirac-delta ...
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Feynman-Wheeler absorber theory

Firstly, I’m sorry if this is not a question that should be here. I would like to ask if anyone could link or guide me to where I could find resources (articles, books, etc.), about Feynman-wheeler ...
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Two photon vertex from tensor fermion bilinear

The fermion bilinears are $\bar{\Psi}\Psi$, $\bar{\Psi}\gamma⁵\Psi$, $\bar{\Psi}\gamma^\mu\Psi$, $\bar{\Psi}\gamma^\mu\gamma^5\Psi$ and $\bar{\Psi}\sigma^{\mu\nu}\Psi$, where $\sigma^{\mu\nu} = (i/2)[\...
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Does the Uehling potential have any observable effect?

Uehling potential is the correction made to the coulomb potential due to vacuum polarization (electron-positron pair creation and annihilation). How big is this correction compared to the coulomb ...
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How does photoluminescence spectroscopy work and how can it apply to quantum dots?

Does anyone have a clear explanation of how photoluminescence works? I know some of the basics, for example, that electrons are excited by photons to create excitons and when the excitons recombine, ...
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Why do we believe local gauge invariance (that tells us photons are exactly massless)? [duplicate]

There are experimental upper limits on the photon's mass, but they are finite. I heard that the reason we know that photons have exactly zero rest mass (and thus travel exactly at the universal ...
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QFT - QED total cross sections pair annihilation into photons

Reading on the book "peskin schroeder an introduction to quantum field theory" the cross section for the production of pairs, it is not clear to me why in the calculation of the total cross section it ...
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Compact QED in different dimensions

I would like to find papers, reviews or books where I can read more information about compact QED in different dimensions. I have read the chapter about compact QED in Polyakov's books but it is so ...
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Arbitrary function on the Faddev-Kulish dressing

On this paper the authors review the Faddev-Kulish dressing in QED which is a solution to the IR divergence problem. Given one electron momentum $\mathbf{p}$, They define the soft factor by $$F_\ell(...
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Order of Magnitude for Photon Induction

What does magnetic induction look like when we imagine just one photon passing by? I'm imagining some sort of coil wrapped around the edge of a waveguide or something so that it picks up some of the ...
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Vacuum birefringence

Many of the papers (e.g., this) dealing with nonlinear electrodynamics treat a theory's prediction of vacuum birefringence as undesirable, but don't explain why it would be undesirable. For example: ...
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Initial values of creation/annihilation operators

I have a question about creation/annihilation operators. For example, if I have an evolution equation for annihilation operator of photon $$ \frac{da_k}{dt} = -i \omega_k a_k$$ I obviously obtain $$...
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Reflection of a mirror concerning angle and wavelength

Mirror has a silver coating which helps it reflect all colors of light. From my classical understanding, silver atoms reflect light by absorbing it and then emitting it at the same angle to the normal....
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Single atom in an ion trap [duplicate]

I have seen this photo in a lot of articles which is described as picture taken of a single atom in an ion traphere's the link to one of them. I find this very fascinating, but i still can't ...
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Compton wavelength in massive vector field theory

What is the significance of Compton wavelength in massive QEDs? When we expand $A_{\mu}$ in Fourier modes, is there any significance for modes whose wavelength is less than Compton wavelength?
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Atomic shielding

Well the core of the atom has a positive net charge . For atoms with many electrons there is a negative electron cloud . How do valence electrons even exist? The electron cloud is closer to the ...
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Breit–Wheeler experimental evidence?

has there been any experimental evidence of the Breit–Wheeler process? Either in it's full or weak form, would you have some links to back this up? Can we say that all matter is merely energy ...
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Time varying electric and magnetic field due to oscillating or moving charge

Why does an oscillating or moving electric charge produces a time varying electric field is it time varying to some extent even in the state of rest.
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1-loop correction to photon propagator

(May be it is a duplicate). I do not understand clearly how should I write down 1-loop correction to photon propagator. I know what is $i\Pi_{\mu\nu}(k^2)$ (I need only this specific correction) and ...
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Why doesn't there exist a wave function for a photon whereas it exists for an electron?

A photon is an excitation or a particle created in the electromagnetic field whereas an electron is an excitation or a particle created in the "electron" field, according to second-quantization. ...
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The different limits of a scattering process

Let's assume I have a process of some sort ($e^-e^+\rightarrow e^-e^+$ / $e^-e^-\rightarrow e^-e^-$ / etc) and I calculated its unpolarized differential cross section using no assumptions at all (...
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Nonrelativistic Quantum Mechanics Results Implying Analogous QFT Results?

One particularly fascinating example of this I have found is the following. The delta function potential has no effect in nonrelativistic quantum mechanics in spatial dimensions greater than or equal ...
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Mechanism for a photon to impart momentum to an atom [duplicate]

Picture a simple hydrogen atom with one electron which is bound to a proton (nucleus). When trying to impart momentum to the atom, we may specify the photon wavelength to be $\lambda = 121.57$ nm to ...
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What is the minimum distance between an electron and a positron so that they do not annihilate? [duplicate]

What is the minimum distance between an electron and a positron so that they do not annihilate? I want to assume that the location of an electron can be determined with an accuracy of 0.004 Angstroms,...
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Lagrangian for fermions

I was trying to understand last term in the Lagrangian. $$\mathcal{L} =- \frac{1}{4} F_{\mu \nu}(x)F^{\mu \nu}(x) - \frac{1}{2} \alpha\Big ( \partial_\mu A^\mu(x)\Big)^2 +\sum_{f} \overline{\Psi}(x) ...
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What particle is created when a B- or E-field operator acts on the vacuum

I have some conceptual trouble understanding what the different field operators in QED do. According to Wikipedia, the field operators are given by $\mathbf A(\mathbf{r} )=\sum _{\mathbf {k} ,\mu }{\...
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Mechanism for radiating power when accelerating charged particles

On reading through selection rules constraining transitions from one state to another, it states: Thus, there is no E0 (electric monopoles) or M0 (magnetic monopoles, which do not seem to exist) ...
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Why Proca Term forbidden in Schwinger Model?

In my QFT Lecture we considered the Schwinger model with a Proca term. Solving the eom for the Stueckelberg field and plugging it back into the original Lagrangian, we receive an effective Lagrangian ...
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Is it possible to induce left-handed electromagnetic wave from positrons? [duplicate]

All observations of electromagnetic induction we have, are from the flow of electrons. The accelerated electrons in the antenna rod induce an EM wave with a right handed system of the electric field ...
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Quantising electromagnetic field in a box. Why can't I use the easy cartesian modes?

In this paper, the electric field eigenmodes of a box with perfect metallic walls are derived. They are (eqn. 2.39) But the author does not use these modes to construct the Electric field, and then ...
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Physics Meaning of Trace Technology in QED [closed]

As it pointed out on page 133 of Peskin and Schroeder, any QED amplitude involving external fermions, when squared and summed or averaged over spins, can be converted to traces of products of Dirac ...