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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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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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 ...
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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? [on hold]

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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Klein Gordon, Dirac, Proca [closed]

How do we know these equations : Klein-Gordon, Dirac, Proca is for spin 0, spin 1/2 , spin 1? How did people find Klein Gordon doesn't work for spin 1/2?
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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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41 views

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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37 views

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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Electromagnetic field produced by a charge?

I guess vibration of a charge particle produces vibrating EM wave and oscillation of a charged particle will produce oscillating EM wave. If charge is only accelerating (speeding up) what will happen ...
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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 ...
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88 views

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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51 views

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 ...
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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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76 views

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 ...
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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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24 views

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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42 views

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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51 views

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 $$ ...
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How can my window not scramble the image of my yard?

How can an image pass through a window if the atoms in the glass randomly emit photons in any direction? I've read that glass is transparent because the atoms don't readily adsorb visible light, so it ...
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Connection between statistical and quantum mechanics

I am aware of Gibbs measures, given the energy (Hamiltonian) of an arrangement, one can determine the frequency of the arrangement. Plug the energy level in the Boltzman equation and there you go. I ...
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Are there negative energy states in QED?

I was reading Weinberg I, when I came upon the following statement$^1$ (slightly edited by me): \begin{align} (\not p+m)u=ie\not A\\ (\not p-m)v=ie\not A \tag{1} \end{align} The minus sign on ...
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What is the QED model about half wave plate?

Surely classical electrodynamics (CED) has a good and well-known answer about the change of light polarization in a HWP (or Quarter wave plate). I tried to find how does this look like from point of ...
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Preference of Chirality

I was interested to see that , $$ \gamma^5 \psi = \psi_R - \psi_L $$ By the definition of chirality projection operator and that $\psi = \psi_R + \psi_L$. since $\gamma^5 \psi$ pops up a lot in ...
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How could we describe the electric bound state like hydrogen by QED? [duplicate]

We can solve the Schrodinger equation for the Hamiltonian operator from the classical Hamiltonian of hydrogen bound state, consisting of proton and electron attracting each other electrodynamically, ...
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Why have our eyes not evolved to see “gluons”? [closed]

Bit of a random question...photons are the propagators for QED, and we rely on photons to see the world around us. The gluon is the propagator in QCD. Why have our eyes not evolved to see gluons ...
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1-loop integration for self-energy

I am trying to calculate the following integral that corresponds to 1-loop contribution to electron self-energy for a specific component of the momentum: $$ \Delta_i(q) = \int \frac{dp_1 dp_2 \ldots ...
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Stimulated Emission in QED

The explanations of stimulated emission which I have found all describe the phenomenon in terms of non-relativistic quantum mechanics. How might you describe it in a field theory such as QED? In ...
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One particle infinite manifestation universe [closed]

I know how non-intuitive everything related to quantum physics is and I was thinking on some very unintuitive things like John Wheeler one electron universe theory ...
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What force prevents particles from penetrating other particles?

I understand that what prevents objects from penetrating each other is the electromagnetic force between the electrons in the respective objects. But if we don't have electrons, for example a proton. ...
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Divergent diagrams in QED

I was reading about how to choose divergent diagrams in QED by using the concept of Superficial degree of divergence. We have an empirical relation $$ D= 4-E_b -\frac{3}{2}E_f $$ where $E_b$ is number ...
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88 views

How does QED explain reflection? [duplicate]

By reflection of photons from a mirror, the mirror must get an impulse of 2p=h/pi.l for every photon. This means that the mirror receives kinetic energy. But the photon has the energy it has before ...
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Tree Level Feynman Diagrams of Electron Positron Interaction

Consider the interaction $$e^-+e^- \rightarrow e^-+e^-$$ The following is a tree level Feynman diagram for this: We can also make the paths of the two electrons on the right hand side cross over and ...
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Confusion over trying to understand spinor components

I've been reading about the quantisation of the Dirac field $\psi(x)$ and it is stated that the general solution to the Dirac equation $(i\gamma^{\mu}\partial_{\mu}-m)\psi(x)=0$ is given by the ...
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