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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If a photon's emission is detected is it real or virtual?

I understand that one can measure a single photon being absorbed using a photomultiplier tube or CCD. Can one measure a single photon being emitted by monitoring the current through an LED or the ...
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157 views

Quantum frequency vs classical frequency and Energy dependence

I recently realized what seems to be a contradiction in what is called "frequency" for light, and would enjoy some clarification of some more educated mind on the matter, thanking you in advance for ...
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138 views

Time reversal invariance and boundary conditions in electrodynamics

This is really several related questions... The equations of classical electrodynamics are time-reversal invariant. However, when we solve the equations for a particular system of charges it is ...
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175 views

Mass corrections to fermions proportional to the mass?

In this post regarding quantum corrections to a massless fermion field, the answerer stated that quantum corrections to the mass will always be proportional to the mass (at least in QED). This point ...
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558 views

How to get Hamiltonian of QED from lagrangian?

I have the QED lagrangian: $$ L = \bar {\Psi}(i \gamma^{\mu }\partial_{\mu} + q\gamma^{\mu}A_{\mu} - m)\Psi + \frac{1}{16 \pi}F_{\alpha \beta}F^{\alpha \beta} . $$ I tried to get hamiltonian by ...
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spread of fock state distribution and infinite revival time of rabi oscillation in spontaneous emission

In cavity QED for a 2-level atom, the revival time for oscillation b/w the states $\left|\ e\ 0\right\rangle$ and $\left|\ g\ 1\right\rangle$ (absorbing the same photon that is emitted) is said to be ...
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123 views

Why polarization vector $= (0,1,i,0)$?

I know from CED that one has e.g. polarization $$\vec{E}(z,t) = \begin{bmatrix} e_{x} \\ e_{y} \\ 0 \end{bmatrix} \; e^{i(kz - 2 \pi f t)}. $$ Why do Peskin&Schroeder define a polarization ...
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110 views

Angular momenta of photon

$A^\mu$ can have multipole expansions in classical electrodynamics. This gives rise to dipole photon, quadrupole photon etc. For dipole photon $j=1$ (In electrodynamics books they write it as $l=1$). ...
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131 views

What low-level process drives a frequency-doubling crystal?

I was reading about second-harmonic generation (SHG) crystals (or frequency-doubling crystals) used to produce green laser light from IR. What low-level process in the crystal is actually driving ...
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Equality of electric charges of all leptons

What does it precisely mean the often repeated statement that the electric charges of all leptons are the same. Let's consider QED with two leptons: electron and muon. The interaction part of the ...
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Why is electric charge conserved?

We have long been taught that electric charges are neither created nor destroyed. But somehow it is okay to destroy two oppositely charged particles at once ! Why is that so? Let's just take a look ...
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No non-trivial UV asymptotically free and IR free

How it could be proven that a non-trivial theory cannot be both asymptotically free and IR free (g=0 both in the UV and IR with some interpolating function in between)? This is of course contrary to ...
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Is the gauge fixing $\partial_\mu A^\mu + \gamma A_\mu A^\mu=0$ used in the literature and does it have a name?

In an exercise for a course on Gauge Theories, I was asked to derive the action of QED with the method by Faddeev and Popov, using the following gauge-fixing function: $$F(A) = \partial_\mu A^\mu + ...
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367 views

Mass gap for photons

I am puzzled by the answers to the question: What is a mass gap? There, Ron Maimon's answer gives a clear-cut definition, which I suppose applies to any quantum field theory with Hamiltonian $H$, ...
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885 views

Is forward scattering = no scattering?

What is forward scattering? If it is equivalent to no scattering, then why not call it "no scattering"?
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QED coupling constant at one loop

On page 257 in Peskin's QFT book a qualitative sketch of the QED coupling is given (see the picture below). Why should I expect such a behavior from QED? The QED beta function is ...
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183 views

What does this Lagrangian represents?

I came across this expression while doing exercises and I was wondering if it was a 'real' expression. $$\mathcal{L}=\frac{1}{2}\partial_\mu \phi \partial^\mu \phi -\frac{m^2}{2}\phi ^2 ...
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If the electron is point like, then what is the significance of the classical radius of the electron?

What is the physical meaning/significance of the classical radius of the electron if we know from experiments that the electron is point like? Is there similarly a classical radius of the photon? The ...
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Objects made up of electrons?

Say you have a neutral rod, and you bring a positively charged rod beside it (call the side the charged rod is brought near side A and the other side side B). The electrons from the side B will start ...
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649 views

Feynman rules for real scalar field interacting with electromagnetic field

I was wondering if anyone could help guide me in finding the Feynman rules for a real pseudoscalar field ($\phi$) interacting with the electromagnetic field $(F^{\mu\nu})$. The (effective) ...
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181 views

some questions regarding Doppler shifting versus absorption-emission

I just got in what I thought was a silly exchange where a self-identified physicist states that the difference between "red-shifting" (in the Doppler sense) and the re-emission of light at longer ...
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What starts the movement of a photon

Although a photon has no (rest) mass, it does have a measurable speed. Its movement can be altered by gravity. A photon "travels". If I turn on a flashlight, seen by someone at a distance, the photons ...
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Did the Goudsmit-Uhlenbeck analysis of spin consider relativity?

It's frequently mentioned in introductory quantum mechanics texts that Goudsmit and Uhlenbeck conjectured that the magnetic moment of an electron was due to angular momentum arising from the electron ...
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QED and Newton's 3rd law

According to some lecture notes on propagators (see bottom of page) the Feynman particle/anti-particle hypothesis states: The emission (absorption) of an antiparticle of 4-momentum $p^\mu$ is ...
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643 views

Canonical equal time commutation relations in QED

I understand that to quantize the classical electromagnetic field one needs to impose commutation relations and express the field in terms of creation and annihilation operators. I notice that the ...
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276 views

Photons traveling backwards in time?

Imagine that two widely separated charged particles $A$ and $B$ exchange a photon. Because they are far apart one can imagine that there is a major contribution to the photon propagator that travels ...
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What is the relation between electromagnetic wave and photon?

At the end of this nice video, she says that electromagnetic wave is a chain reaction of electric and magnetic fields creating each other so the chain of wave moves forward. I wonder where the photon ...
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Is quark electric charge also confined?

Since we will (maybe) never observe free quarks which has fraction charge, is it logically correct to say that the fraction (1/2,2/3 etc) electric charge is also confined. I.e. the charges that we do ...
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343 views

How do non-transverse photon polarizations cancel in Euclidean QED?

First, recall how to write scattering amplitudes in covariant fashion in Minkowskian QED. One starts by considering some process with an external photon whose momentum is chosen to be ...
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209 views

Derivative with respect to ${\not}{p}$

When studying renormalization of QED in standard textbooks, we typically encounter derivatives with respect to ${\not}{p}=p^\mu \gamma_\mu$, i.e., $\partial/\partial{\not}p$. As far as I understand, ...
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Does radio use virtual photons?

In radio communication each accelerated electron in the transmitter antenna interacts with an electron in the receiver antenna by exchanging a photon. Is that photon always a virtual photon as ...
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340 views

Quantum Electrodynamics

I was wondering if anyone could give a simple explanation of how light interacts with matter. From what I have read in QED, electrons will repel each other because of their ability to emit and ...
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424 views

Ward Identity makes QED logarithmic divergent?

Quick question regarding superficial degrees of freedom and Ward identities. For instance in Peskin and Schroeder it is stated that the photon-self energy is superficially quadratically UV divergent ...
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Photon propagator in terms of creation/annihilation?

As far as I understand it the photon propagator, $P(A\rightarrow B)$, described in Feynman's QED book, gives the amplitude that a photon moves from spacetime point A to spacetime point B. I was ...
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How can we derive the Feynman rule for the ordinary QED 3-vertex?

I have checked some Quantum Field Theory texts that include basic QED and they all include the Feynman rule that each vertex bring with it a factor of $$\pm i e \gamma^\mu$$ but I have yet to find a ...
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What does “Information” and Virtual Particles mean?

I've read that attraction and repulsion between particles is caused by the exchange of virtual photons, and that virtual photons carry information. I don't understand how a virtual photon actually ...
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How do you derive the Lagrangian for the Standard Model?

Is there a way to derive the Lagrangian for the Standard Model just like one would for Einstein's field equations for instance? Also, how do you derive the Laganrigans for QCD and QED? Is it possible ...
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417 views

gauge invariance of the Feynman amplitudes

When we calculate the photon polarization sums over amplitudes, $$X=\sum\limits_{r=1}^{2}|\mathcal M_r|^2=\mathcal M_\alpha\mathcal M_\beta^*\sum\limits_{r=1}^{2}\epsilon_r^\alpha\epsilon_r^\beta$$ ...
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104 views

Classical EM neglects electron recoil?

Imagine two electrons $A$ and $B$ at rest. Electron $B$ is at a vertical distance $r$ above electron $A$. Let us assume that the electrons are constrained to move on horizontal rails. At time $t=0$ ...
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Why and how, in QED, can excited atoms emit photons?

The quantum mechanics of the structure of atoms as determined by the electromagnetic forces inside them correctly describes the location and coupling of the different energy levels in essentially all ...
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Probability of absorption and emission of photons

Suppose you have a single electron in a box, and you shoot a single photon at it. How does one calculate the probability that the photon will be absorbed and the particle excited? Or that the photon ...
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Lorenz gauge in the Gupta-Bleuler Method

Greiner in his book Field Quantization page 180 & 181 wrote: As shown in (7.24) the Lorenz condition cannot be enforced as an operator identity. Instead we will use it as a condition for the ...
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How many photons can an electron absorb and why?

How many photons can an electron absorb and why? Can all fundamental particles that can absorb photons absorb the same amount of photons and why? If we increase the velocity of a fundamental ...
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319 views

Energy spectrum of a Dirac electron

How do you explain easily "The spectrum of an electron in a repulsive potential " and hence "bound state of charge conjugation" in Dirac hole theory ?
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101 views

What spin-statistics is a magnetic monopole expected to obey?

What statistics (or spin) is a magnetic monopole expected to have? Does it depend on the theory used?
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381 views

If time travel to the past is not possible why is this situation considered in Feynman diagrams?

I recently read R.P Feynman's QED:A Strange Theory of Light and Matter. It is believed that time travel to the past is not possible. Then why is particles going backward in time considered in the book ...
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How to understand the QED, QCD and standard model Lagrangians? [closed]

How do you read the QED, QCD and standard model Lagrangians? What do all the symbols and tensors represent? And, how can you derive them by yourselves?
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Photon as the carrier of the electromagnetic force

My physics background goes as "far" as reading popsci books on QM, Particle Physics, and Cosmology so pardon my ignorance in the below questions. I've read that the photon is the particle (quanta in ...
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Christoffel symbols and Dirac matrices mathematical similarities?

Maybe mine is a silly question, but are there mathematical similarities or common roots between the Christoffel symbols: $ \nabla - \partial = \Gamma $ and the Dirac matrices $ ( \gamma^\mu ...