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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29 views

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 ...
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51 views

u-channel in $gg \rightarrow u\bar{u}$

I've seen that for the QCD process $gg \rightarrow u\bar{u}$, where $g$ is a gluon and $u, \bar{u}$ are the up quark and the corresponding antiquark, there is s, t and u channels. I perfectly ...
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Why don't very high order Feynman diagrams contribute significantly?

In a particle physics lecture I had today it was stated that the magnetic moment, $g$, is not quite equal to 2, and the difference is accounted for by QED. Later it was stated that we can see this ...
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68 views

QFT Matter Fields and Anti-Matter Fields

In QFT is it the case that the electron matter field and anti-electron matter field (using the electron as a specific example) are truly distinct physical fields versus different excitation modes of ...
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What are Towsend Coeffiecents exactly? And are there any tables out there to look them up?

My question is expecially for the gamma coefficent, does it depend from the air? And further how the material of electrodes is involved in Paschen's Law (Electric Arcs)?
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The Speed of Light and Its Medium's Density

When reviewing a refractive index chart there is a relationship to the density of the medium and the velocity of electromagnetic radiation through the medium. The lower the density the greater the ...
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Touching is a Pauli exclusion principle or electrostatic force? [duplicate]

According to quantum field theory touching is an electrostatic repulsion between electrons or the Pauli exclusion principle? How can physicists distinguish these two phenomena if they give the same ...
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68 views

Accelerating charge in different frames

We know that accelerated charges radiate energy in form of electromagnetic radiations. Since acceleration is frame dependent, is the radiated energy also frame dependent? Is that why energy radiated ...
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LSZ reduction formula for massive vector bosons

What is the precise form of the LSZ reduction formula for massive vector bosons? The LSZ formula for scalar bosons, fermions, and photons is given e.g. in the textbook "Quantum field theory" by ...
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Gupta-Bleuler Condition

I am studying quantum field theory using David Tong's notes available at http://www.damtp.cam.ac.uk/user/tong/qft.html and I am stuck at page 135 eq. $6.56$ I fail to see how the following equation: $...
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Does the electro-dynamical lagrangian contain a (Dirac) wave-function?

Consider a lagrangian for quantum electro-dynamics. It contains the two fields: the vector $A$-potential inside $F_{\mu\nu}$ and the matter field $\psi$ (Dirac's spinor). A series of questions arise ...
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Quantization of EM field

Usually the first step to quantization of EM field is the Fourier expansion of vector potential: $$ A = \sum_k A_k e^{jkr} .$$ For example, in book "The classical theory of fields" by Landau, ...
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Localization of Electron Matter Field Excitation in Simple Electron QFT Model

I believe QFT represents a single free (stationary) electron as a an excitation of the electron matter field which then couples to the EM field to create a local 'attached' EM field - if this is ...
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42 views

what happens to an electrons orbital when it absorbs a photon

so I just learned about orbitals and hybridization theory through youtube and I have one question. if I have a hydrogen with one electron, the electron is in a 1s orbital, but then if that electron ...
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Check if total angular momentum is well-defined for a particular photon wave function

I have an exotic state, which can be expressed in a way: $$\pmb{A}(\rho, \phi, z; t)= \hat{e}_{\pmb k, \sigma} A_0 e^{i (k_z z - m \phi +\omega t)}$$ where $A_0$ is a scalar amplitude of the EM-...
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Absorption probability derivation in Feynman QED

In Feynmann QED (page 11), the tansition probability $$a_{lk} = \frac{4\sin^2(\Delta T/(2\hbar))}{\Delta^2}|u_{lk}|^2\,, \quad \Delta = E_l - E_k - \hbar\omega.$$ In the same page, there is an ...
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Muon catalysed fusion from cosmic muons

If thousands of muons are coming to earth. Why can't we use them for muon catalysed fusion which was proved legitimate
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Screening of the electric field in the Higgs phase

Dvali states in his paper on Three-Form Gauging of axion Symmetries and Gravity that “As usual, in the Higgs phase the electric field is completely screened in the vacuum,” and in another ...
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Furry's theorem and spontaneous parametric down-conversion

In spontaneous parametric down-conversion, a single photon interacts with a nonlinear medium to produce a pair of lower energy photons. It is observed that energy, momentum, and orbital-angular-...
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Why are there no radiative corrections to the electron charge for (an on-shell interaction)?

My question stems from the preliminary discussion of the electron vertex function found in section 6.2 of Peskin and Schroeder's Introduction to Quantum Field Theory. Specifically, they calculate the ...
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120 views

Why not anything like spontaneous absorption?

I understand that the idea of emission or absorption cannot be incorporated in the framework of non-relativistic QM. We need QFT or relativistic QM to explain the creation and annihilation of ...
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63 views

Are problems with self-energy of point charge in classical electrodynamics solved by field quantization?

Classical electrodynamics gives strange results when considering a moving charge in its self generated field (Abraham-Lorentz equation). Some 50 years ago there were many efforts and publications ...
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Calculating the ionization rate

here is a kind of silly question: in quantumn physics, we assume that the charge is reserved. So if we use a laser, which is strong enough to make ionization happen, to shot at the system, how can we ...
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95 views

Quantum tunneling in a capacitor

Ampere's law of Maxwell's equation includes displacement current as the Maxwell's correction. Suppose consider a capacitor with a thin distance of separation. For an applied voltage do they posses ...
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96 views

Explanation of Lenz's Law phenomena

If we drop a magnet through a copper pipe (without it touching any of the sides), it would fall slower than it would if there were no pipe. Having the pipe otherwise accelerate the magnet would be in ...
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167 views

Why Einstein's unified field theory is believed not to work?

In his later life, Albert Einstein was trying to extend his general theory of relativity to incorporate electromagnetism and other fundamental forces with it, something he himself called "The theory ...
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95 views

Why is particle number conserved in non-relativistic limit of QED?

I am trying to recover ordinary quantum mechanics from QED. One main feature of quantum mechanics is the conservation of particle and antiparticle number separately, i.e. $[N_{e^-},H] = [N_{e^+},H] = ...
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Scattering of light in Quantum Vacuum

The quantum mechanical vacuum (i.e. the vacuum of a typical QFT) viewed as the zero-point state of the system has an energy (of course, here enters the Vacuum Catastrophe and all the mess that follows)...
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Can particles popped into existence from the vacuum have electromagnetic effects on other particles?

I know my question might have problems, but I am curious about it. In quantum field theory, particle-antiparticle pairs continuously pop in and out of existence from vacuum. These particles have a ...
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Path Integral in Electric-Magnetic Duality

The action of electromagnetic field is $$S=\int\left(-\frac{1}{2e^{2}}F\wedge\ast F+\frac{\theta}{8\pi^{2}}F\wedge F\right),$$ where $F=dA$ is the curvature $2$-form, and $A$ is the connection $1$-...
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QED lagrangian: gauge fixing term

I have a question about the structure of the QED lagrangian, in particular the free photon lagrangian which is contained in it. My premise is: I only know how to exploit canonical quantization in ...
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Propagators in interaction with derivatives

Given a Lagrangian density containing an interaction with derivates, it's easy how to guess the Feynman rules for vertexes. However i was wondering about propagators: in S-matrix expansion it's ...
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Photons, light and electricity

Light is ultimately composed of photons. Photons are also force carriers of the electrical force. When an electric motor is turning it is photons which are turning it. What is the relation between ...
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51 views

Are phase and particle (photon) number in QED conjugated variables?

I found in A. Zee's book "QFT in a nutshell" (1.edition) the interesting relation (8) respectively (9) in chapter III section 5 (p.173) which states that in a collective of non-relativistic bosons the ...
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Calcluating the photon propagator with gauge fixing parameter

I'm trying to calculate the photon propagator via the functional integral, with lagrangian (plus source) $L = -\frac{1}{4}F^{ab}F_{ab} - \frac{\lambda}{2}\left(\partial^aA_a\right)^2 + J^aA_a $ ...
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When and why does the superposition principle of Coulomb's law fail to hold?

In this lecture, Professor Shankar Ramamurthi says that the superposition principle for force vectors of Coulomb's Law is experimentally observed and is not a product of logical analysis. In fact, the ...
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Electromagnetic current operator using Feynman rules

In the calculus of the electron anomalous magnetic moment some text books usually calculates the forms factors from $\Gamma_{\mu}$ in the expression bellow $\left<p',s'|J^{em}|p,s\right> = \...
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Virtual particles

I use Wikipedia to learn physics. In a recent search I found an ample description of virtual particles in terms of a discontinuous (in the present) and asymptotic distribution of these particles from -...
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Transition amplitude in scalar QED from a point-like charge

I have a problem for you to compute. I have a classical source, $A_\mu = \delta_\mu^0 \frac{Q}{r}$, representing the Coulomb potential generated by a point-like charge. I want to compute the ...
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238 views

Derivation of the complete Bethe Bloch formula

I can not find anywhere the explicit calculation for the complete Bethe Bloch formula including all the correction factors. I would like to have at least one idea of ​​the calculation that is actually ...
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Do longitudinal and scalar have anything to do with Faddeev-Popov ghosts?

In his this book, Hatfield calls ghosts the negative states appearing in the covariant (Gupta-Bleuler) quantization prescription of the electromagnetic field (page 89). When discussing Yang-Mills ...
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Gauge transformation on vacuum

Suppose we have two electromagnetic vector potential operator that differ by a gauge transformation in free field theory. $A'_{\mu}= A_{\mu}-\partial_{\mu}\alpha$ is the state $A_{\mu}|0\rangle$ ...
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Gauge transformation on operators

Suppose we have two electromagnetic vector potential operator that differ by a gauge transformation. $A'_{\mu}= A_{\mu}-\partial_{\mu}\alpha$ Now suppose we have $\partial_{\mu}\alpha=F(x)\hat ...
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Can the Hermitian conjugate of a column vector still be column vector?

From what I understand, contravariant vectors are represented by column vectors, and covariant vectors are row vectors. So for a QED current, say $j ^ { \mu } = \overline { \psi } \gamma ^ { \mu } \...
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Dirac equation and Hamiltonian for a collection of magnetic monopoles

I am trying to understand a mathematical comment by Eugene Wigner in some old lecture$,^{[1]}$ "The Hamiltonian of the Dirac equation for two oppositely charged monopoles is not self-adjoint." What ...
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38 views

Frequency gap between neighbouring spectral lines

According to classical theory the atomic line spectra are discrete and their frequencies quantized. Have the newer quantum theories changed anything since then, giving some other expressions for the ...
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75 views

Magnetic forces in physics

Could an accelerator be built in which all forces on the particles, for steering and for increasing speed, are magnetic forces? Why or Why not?