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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Why does $\zeta(3)$ appear in the expression for the anomalous magnetic moment?

The anomalous magnetic moment, to fourth order in $\alpha$, contains $\zeta(3)$. Is there a simple explanation for the appearance of this value of the Riemann zeta function?
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What light “with no apparent material source” is Feynman referring to at the start of his Nobel lecture?

Near the beginning of his 1965 Nobel lecture (end of page 2 in this pdf), Feynman justifies trying to remove the degrees of freedom of the electromagnetic field since in the end the field now is ...
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What are the deciding criteria for occuring of reflection, absorption, Emission, refraction and burning? [closed]

Why does some objects reflect, refract, absorb, emit and burn, when electromagnetic wave is thrown on them? I want to know all these things at atomic and molecular level.
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N00b path integral formulation question

I have a question about the path integral formulation used in QED etc. The path integral formulation implies to me that index of refraction (and reflection or anything also calculated using this ...
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Feynman’s QED: Photons travelling faster than $c$

I was reading though a Phys.SE thread on Feynman’s QED and FTL photon travel over short distances where the most popular answer explained how Feynman was not implying photons can travel faster than ...
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Schwinger's calculation of $\alpha$ [duplicate]

Could anyone shed some light as to how Schwinger first calculated the magnetic moment for the electron?
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Do γ gamma photons of pair production potential > 1.022MeV accelerate neutron decay? [duplicate]

If there are/have been studies testing this exact or the general version of this hypothesis, a reference link would be the preferred answer. Experimental phy. Leah Broussard describing current ...
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What happens to photon in photoelectric effect

In photoelectric effect, when the photon gives energy to the electron, then what happens to itself? Where does it go? For me the photon must take the place of electron after the electron escapes the ...
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Fermat's Principle of Least Time

I was going through the Feynman Lectures on optics when I came across an explanation for Fermat's Principle. I did not fully understand the explanation- can someone please break it down in a simpler ...
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The concept of force carrying photons

Are the force carrying photons really something physical? Do they really occur during electromagnetic interactions? Does this mean that in different mediums where the speed of light differ, the ...
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Is it a coincidence that the maximum number of protons in a stable nucleus is on the order of $\alpha_s / \alpha_{EM}$?

The equivalent to the fine-structure constant for the strong interaction is about $\alpha_s \approx 0.5$ at the proton rest mass energy (and it runs pretty slowly with energy, so it's on this order ...
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Wick Rotation and sign of the integrand in Weinberg's book

I'm studying from Weinberg's QFT volume 1, chapter 11. I have a problem with equation $(11.2.7)$. Starting from eq. $(11.2.5)$ $$ \begin{align} \Pi^{\rho\sigma} (q) = \frac{-ie^2}{(2\pi)^4} \int_0^...
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Is Lamb Shift the field correction of Darwin term?

Darwin Term comes from the low-energy approximation of the relativistic Dirac equation for hydrogen electron by Charles Darwin and the "Zitterbewegung" effect. Though apparently there is no ...
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What causes a light bulb to fluoresce?

When an incandescent light bulb or fluorescent light bulb is pumped up with energy from the power grid, presumably light is subsequently emitted via transitions from the molecular excited state to it'...
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Eliminating longitudinal photons

In Feynman's paper "Mathematical Formulation of the Quantum Theory of Electromagnetic Interaction" he writes that the semiclassical Coulomb interaction can be written by eliminating longitudinal ...
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Nature of electromagnetic absorption by quantum and classical systems

Forgive me for lack of formality and possibly incorrect understanding, but hopefully someone can both help to explain the intuition and also add mathematical formalism. In classical electrodynamics, ...
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Mean square values of vacuum fluctuations of the radiation field

It is well known that the mean square values of the electric and magnetic fields of vacuum fluctuations is given by(Welton, Phys.Rev.74, 1157) $${\langle {E^2}\rangle _{Av}}={\langle {B^2}\rangle _{...
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Quantum Electrodynamics explanation for refraction [duplicate]

I am trying to understand the fundamentals of Quantum Electrodynamics through the simple example of refraction. Let's start with a plane wave coherent state (or a number state) impinging on a boundary ...
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Can the electromagnetic field be defined for the Compton effect?

As far as I know, the Compton Effect can only be explained by using Quantum Electrodynamics, given an incoming electromagnetic wave there is a probability that a photon with different wavelength than ...
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Schwinger paper about charge in QED

I would like to find the paper by Schwinger where he discusses analytic properties of physical values in the limit $e\rightarrow 0$ and physical meaning of $e^2$ sign. I know only that this paper was ...
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Why are there two photons in pair production Feynman diagram? [duplicate]

Given I wonder why are there two photons entering in a) pair production? Isn't it one photon resulting in $e^+e^-$ ?
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Are photons emitted by a magnet?

If you put a photon detector near a magnet (with the magnetic field static in time), is there some probability that the photon detector will detect a photon? Does QFT not predict that a photon could ...
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How can we deduce that a hydrogen atom is stable in relativistic QED?

Consider relativistic quantum electrodynamics (QED) with three quantum fields: the electromagnetic field $A_\mu$, one fermion field $\psi$ for electrons/positrons, and one fermion field $\psi'$ for ...
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Need some literature recommendation - Quasi-Free electron scattering (Nuclear Physics)

I'm not sure if this is the right place for this kind of questions, but here it goes. I'm a last year undegrad and I'm currently working on a project with my professor. I just finished a smaller ...
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How Gupta-Bleuler condition implies $(a_p^3-a_p^0)| \phi \rangle=0$?

Gupta-Bleuler condition is $$\partial^\mu A_\mu^+ | \phi \rangle=0\tag{6.54}$$ where $$A_\mu^+= \int\frac{d^3\mathbf p}{(2\pi)^3 \sqrt{2|\mathbf p|}} \sum_{\lambda=0}^3 \epsilon^\lambda_\mu a_p^\...
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Could channeling effects explain cold fusion? [closed]

Anomalous QED effects related to channeling effects in  crystal lattices, could explain an enhanced fusion cross section, related to a wide class of fusion reactions. Following Schwinger , I ...
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Gamma radiation incident on material generating $e^+/e^-$ pair and Schwinger Limit

Does a gamma photon incident upon a material creating an electron-positron pair plus kinetic energy transfer momentum to an EM field and if so does the intensity approach the Schwinger Limit during ...
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Why does light come as quanta of the harmonic oscillator?

I've recently been learning the basics of Quantum optics and it seems to be a fundamental concept that light is best described in the framework of the Quantum Harmonic Oscillator. This lead to a ...
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Why are 2 gamma photons created? [duplicate]

When a positron and an electron come together they annihilate and produce 2 gamma photons $$e^+ +e^- \rightarrow 2\gamma$$ I can understand that they must be produced in pairs to conserve momentum. ...
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Does the concept of the photon as a particle exist in QFT in the path integral formalism? Does the concept of a particle exist?

In the second quantization approach to quantum field theory, how I understand it, the field is decomposed in components of definite momentum which are treated as non-interacting harmonic oscillators, ...
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Different statements of the Heisenberg uncertainty principle [duplicate]

I know that from the Heisenberg uncertainty principle, ∆x∆p=ℏ/2 . And I know that this equation can be rewritten as ∆t∆E=ℏ/2. From QED I also know that the equation ∆t∆E=ℏ/2 claims that some energy ...
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How to “transfer” indices from dot product to metric?

In this source, the author (Andrzej Pokraka, Solutions to problems from Peskin & Schroeder) is computing an integral related to scalar QED. In the step where the equation is labelled (29), the ...
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What does matter look like in the absence of photons?

Another question inspired by this one on alternative standard models. Greg Egan's story Wang's Carpets (or the similar segment of the larger novel Diaspora) sketchily describes a universe with no ...
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Evaluation of Wilson loop in QED

I'am trying to figure out the evaluation of the expectation value of the Wilson loop for QED. (Its actually the problem 15.3 in Peskin and Shroeder) Lets say the Wilson loop is $W(x) = exp(-ie\oint_P ...
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Can we remove IR divergences in continuous QFT?

During a QFT class, my professor briefly explored the fact that infrared (IR) divergences can be removed from QED by considering "soft" photons that have an energy inferior to the detector sensitivity,...
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What is the real problem in the free body problem?

Ions are produced when an EM neutral atom gains EM charge by gaining or losing electrons, by collision with other charged particles or photons. The study of such collisions is of fundamental ...
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Difference between Coulomb law and QED

According to Coulomb law, the electromagnetic force between point charges separated at zero distance is infinite. Does QED explains the same result, or there is any difference?
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Why is the spontaneous emission a big deal?

In spontaneous emission, an electron in an energetically higher state jumps to an energetically lower state. This seems a simple phenomenon. Why is it called a quantum mechanical effect? What was so ...
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Existence of Photon

I was reading Feynman lectures on physics (volume 1) when I encountered the following paragraph: Returning again to quantum mechanics and fundamental physics, we cannot go into details of the ...
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Confused about the anapole (toroidal) moment

I am a bit confused about the existence of the anapole moment. As far as I understand, in order to fully describe a charge-current distribution, one needs, beside the normal electric and magnetic ...
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Importance of $\alpha/\pi$

What is the physical meaning or importance of $\alpha / \pi$ ($\alpha$ being the fine-structure constant)? I have seen that it is the parameter in the expansion of the g-factor but I can not reach to ...
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What exactly does it mean to say that QED is of “universal strength and form”?

In a paper from 1961 (Gauge Theories of Vector Particles, Glashow and Gell-Mann), the authors describe quantum electrodynamics thusly: "It is of universal strength and form..." What exactly does ...
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Do electrons really perform instantaneous quantum leaps?

This is not a duplicate, non of the answers gives a clear answer and most of the answers contradict. There are so many questions about this and so many answers, but none of them says clearly if the ...
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What is the intuitive reason why matter and antimatter should be highly reactive?

Common knowledge has it that when an amount of matter and an amount of antimatter come anywhere near each other, they annihilate, leaving nothing but "pure energy". In more technical terms, maybe we ...
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Peskin and Schroeder Section 7.1 Mass Shift

I'm slowly reading my way through Peskin and Schroeder. Near the end of section 7.1 they compare the mass shift of the electron from QFT to the classical value, both of which are divergent but in ...
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Real space description/information of photons

recently I read a paper about some special tratment in QED (https://doi.org/10.1103/PhysRevLett.122.193603) and they used a lot the term "real space description" or "real space information" of photons....
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What are zero-energy Rindler photons?

In the discussion of the Unruh effect and Bremsstrahlung (e.g. here) I always come across "zero-energy Rindler photons". What exactly are these? Shouldn't a zero-energy photon correspond to a static ...
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Commutation relations in Gupta-Bleuler quantization

Quantization of the free electro-magnetic field has essential differences in comparison to quantization of say scalar or massive vector fields. In fact there are different approches to it. One of ...
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Reflectance and transmittance equations for metal thin film

The Fresnel's equations no long work when the thickness of the metal thin film is thinner than skin depth. For example, the reflectance of a metal thin film is always the same according to fresnel's ...
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Vector current conservation in vertex correction

Recently, I was calculating this observable: $\langle p s|\bar\psi(0)\gamma^{\mu}\psi(0)|ps\rangle$ Where we only consider the QED case. $\psi$ corresponds to massless Dirac fermion field, p is the ...