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

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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 ...
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Derivation of photon propagator from EM Lagrangian

I am following Ryder's Quantum Field Theory. In chapter 7, in order to derive the photon propagator, he first derives eq. 7.4 $$\mathcal{L}=\dfrac{1}{2}A^\mu[g_{\mu\nu}\partial^2-\partial_\mu\partial_\...
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Does QED have engineering applications?

QED is certainly important for understanding the physical World; I'm wondering though, does it has any practical, engineering applications? For example, is it sufficient for development of lasers to ...
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What one would observe on extremely high voltages?

I am interested which physical processes generate the highest electrical voltage and what the highest measured voltage is. In everyday life, electrostatic charges e.g. while walking with socks on a ...
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35 views

The analytical result for free massless fermion propagator

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\...
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Understanding type of force interaction in particle decays

Are there any fundamental rules of thumbs that can be used to identify the type of force interaction (weak, electromagnetic, strong) in a particle decay without drawing the Feynman diagrams at the ...
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Fine Structure and Fine Structure Constant - intuitive relation?

How does the fine structure and fine structure constant relate to each other, intuitively? I've seen $\alpha$ extrapolated as a term in energy calculations for fine structure, but is there a ...
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Proton is made of 3 quarks of each spin 1/2, but the total spin of proton also 1/2, how it is possible?

Proton is made of 3 quarks (UUD) each has a spin of 1/2, so if you follow the constitution the total spin of the proton must be (3/2), but the spin of the proton is still 1/2. How it is possible? But ...
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Fundamental coupling constant equation - fine structure, etc [closed]

I'm trying to understand a bit more about the fine structure constant and how it relates to coupling constants in the fundamental forces. It's listed as a coupling constant for fundamental forces - I ...
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The singular term in the operator product of two fermion operators

Consider an operator product $\bar{\psi}(x+\epsilon)\psi(x)$ in which $\psi(x)$ is the fermion field operator in QED. We want to compute the singular term in $\bar{\psi}(x+\epsilon)\psi(x)$ in the ...
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Running coupling outside QFT

I'm reading about the running coupling in QCD. I understand the vacuum polarization and its consequences. Also I've read that you can find the same phenomenon on the strong interaction, giving us the ...
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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 ...
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Quantum field theory why is it a trace?

So in perskin when computing the amplitude of $e^+e^-\rightarrow\mu^+\mu^-$ summing up over spin they say \begin{align}\sum_{s,s'}\bar{v}^{s'}_a(p_2)\gamma^\mu_{ab}u^s_b(p_1)\bar{u}^{s}_c(p_1)\gamma^...
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In QED/Yang Mills, why do fermions contribute 4 times as much as scalars to vacuum polarization?

Consider a Yang-Mills theory in $4D$ over a gauge group $G$ $$ \mathcal{L} = - \frac{1}{4} F^{a\mu\nu}F_{\mu\nu}^a + \bar \psi i D_\mu \gamma^\mu \psi + (D_\mu \phi)^\dagger D^\mu \phi $$ where $\...
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Feynman propagator for photons and the actual propagation of photons

Reading some books of quantum field theory (c.f. LH Ryder. 'Quantum Field Theory') it seems that the concept of path integrals in quantum mechanics may be extended to the field theory using the ...
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Must the fine structure constant be an irrational number? [closed]

This question concerns a conjecture on the nature of $\alpha = e^2/4\pi\epsilon\hbar c$ in light of black hole holes. Let me consider the Reissnor-Nordstrom metric for a charged black hole $$ ds^2 = \...
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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 -- ...
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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 (...
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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 ...
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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 ...
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42 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 ...
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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 ...
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What does a one-photon state look like as a wave functional?

According to a post on a mailing list here: https://groups.google.com/d/msg/sci.physics.research/n7Ao7N9aeq0/wqgj20-jBQAJ In general, quatum states |Psi> of the electromagnetic field, no matter ...
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What is the size/mass of largest metal plate that was moved by casimir effect?

I learned about Casimir effect recently. Is it applicable only on very light/thin metal plates( in terms of micro/nano meters) or is the effect large enough to be noticed on mili meters scale? What is ...
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Implication of breakdown of scale invariance for problems with intrinsic length or time scales?

According to Wikipedia article on scale invarince, the equations for electric (and magnetic) fields : $$\nabla^2\vec{E}=\frac{1}{c^2}\frac{\partial^2\vec{E}}{\partial t^2}\hspace{0.3cm}\text{and}\...
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How to calculate the resonance peak of a NV center during a ESR/ODMR measurement?

I am thinking of making an magnetometer with ODMR measurement of a NV center in nanodiamond. But before I do the experiment, I want to estimate the sensitivity of my experiment beforehand ...
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Is there one wavefunction per field? [closed]

Is the big picture of quantum field theory that: There are fields (EM, electron, Higgs, gravity, etc.) A field can be described by a wavefunction indicating the probability density of 1 or more '...
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Does 'real' photon electromagnetically interact with matter?

Consider the standard description of Compton scattering - radiation is constituted of stream of photons (these are supposed to 'real' as contrasted to 'virutal' photons of the QED). One of these ...
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Sudakov double logarithm

I have calculated a few NLO corrections in QED and in the final result the Sudakov double logarithms have always canceled. So I thought that they have no physical meaning. On the other hand I have ...
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Homework question on the Poynting vector

I need to show what the Poynting vector becomes when i insert the definition of A through creation and annihilation operators, what i get is the expected answer plus a term containing two creation ...
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Two Loop QED Feynman Diagram

I'm trying to find the momentum space integral representation of the below Feynman Diagram, but am having troubles with the fact that there are two loops within the system. The main question I have ...
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Feynman graph, proportional charge of a vertex

It is defined that the contribution of a vertex in a Feynman Graph towards the probability amplitude $ M_{fi} $ is proportional to a charge $Q_f$. Yet I seem to cannot find any precise explanation how ...
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Evaluating the trace of an expression with gamma matrices

I am currently reading Srednicki's Quantum field theory Book and am having some troubles with evaluating the trace of some gamma matrix expressions. For instance in Equation 59.19 Srednicki defines ...
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Photon in the box electric field strength

If we put a photon in a box and suppose the resonant frequency is $f$. If we know the volume $V$, How do we approximate the electric fields at the center of the box?
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How does the Electric Field create a force? [closed]

If we have 2 charged particles which are a certain distance away from each other, they either attract of repel due to the electric field created by both of them. But I don't understand the mechanism ...
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Semileptonic decay amplitude

I'm trying to do past exam papers and I am absolutely failing understand some things. I would very much appreciate any insights/intuition you have. The part I'm struggling is here: In the first ...
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Photons when passing through matter

I am wondering whether there is a simple—or complicated—way to explain from a purely quantum mechanics (or QFT) viewpoint what happens to the photons when they go through some material whose index of ...
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Photon Propagator in QFT

Please forgive my temporary split-brain, but I'm a little thrown off by something when considering units at the moment. In QED (depending on the guage), the photon propogator is written as $<0|\...
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Why are photons massless (quantum field theory) [duplicate]

I'm really trying to understand Quantum field theory and gauge in-variance, I'd like to ask a question about this to aid my understanding. The QED lagrangian as below has a kinetic term for this gauge ...
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The true dimension of Dirac field

In natural units with $\hbar=1$ and $c=1$, as we know, the energy dimension of the Dirac field $\psi(x)$ in QED is $\frac{3}{2}$. But in cgs units, what is the true dimension of the Dirac field $\psi(...
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Invariant amplitude in QED in terms of Mandelstam variables [closed]

So I'm having a little trouble with this question: These are my workings thus far: I just can't seem to get certain terms to vanish. I have the 1/4 factor for going between $8e^4$ to $2e^4$. Is ...
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Why are there e.m. vacuüm fluctuations?

According to the rules of qft there are virtual photons in the vacuüm. But how can this be if for the production of photons you need an electric charge?
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What does it mean physically to set $\exp(ikr)=1$ in the dipole approximation?

In the dipole approximation one sets the $exp(ikr)=1+ik.r+...=1$. ($|<f|exp(ikr)\epsilon.r|i>|^2$). Which makes $k.r=0$. What does it mean physically? Is the plane transverse wave converted to ...
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Does an electromagnetic wave necessarily contain many photons? [duplicate]

I've often come across people saying from a quantum physics standpoint that an electromagnetic wave necessarily contains many photons. But doesn't the double-slit experiment conducted one photon at-a-...
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Deep Inelastic Scattering - electromagnetic current

When one tries to compute the deep inelastic scattering for the process: where $l$ is a lepton with incoming momentum $k$ and outgoing $k'$, $h$ is an hadron with momentum $P$, $q$ denotes some ...
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Plasmon quenching vs plasmon decay. What is the difference?

I'm reading an article "Quenching, Plasmonic, and Radiative Decays in Nanogap Emitting Devices" (pubs.acs.org/journal/apchd5). I don't understand the following: Counterintuitively, we evidence ...
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Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
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Crossing Symmetry in Bhabha scattering and Moller scattering

Given the amplitude for a particular process, it may be possible to obtain the amplitude for another similar process by a so called crossing symmetry. I know there is a $s \leftrightarrow u$ crossing ...
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Feynman graph of induced current

I'm self studying QFT from Peskin and Schroeder. In chapter 19 of this book, page 653 (Perturbation theory anomalies) the expectation value of the induced current is calculated. I'm confused with the ...
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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\...