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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141 views

Why does the Dirac equation work for the hydrogen atom?

The Dirac equation works well for predicting the spectrum of the hydrogen atom, famously incorporating relativistic effects like fine structure. Yet, there seems to be a sense in which this is ...
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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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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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824 views

Most trivial neutral pion decay

Literature states neutral pion decay by QED cannot occur directly because the pion is uncharged. However, I cannot see why Photons are not allowed to interact with the Quarks directly. Please ...
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A question on IR cancellation calculation in Peskin & Schroeder

In Peskin and Schroeder Introduction to Quantum Field Theory book, above equation 6.64 on pg. 200, it was said that "to gain better understanding, of the divergence, let us evaluate the coefficient of ...
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170 views

Commutation of currents in QED

In an outline of a proof of the Ward identities in QED, the authors Green, Schwarz, and Witten in their book "Superstring theory", vol. I, Section 1.5.1, claim that in the QED the electromagnetic ...
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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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76 views

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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How are classical optics phenomena explained in QED (Snell's law)?

How is the following classical optics phenomenon explained in quantum electrodynamics? Reflection and Refraction Are they simply due to photons being absorbed and re-emitted? How do we get to Snell'...
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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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To which type of particles gauginos are supposed to couple?

I wonder about to which type of particles gauginos couple in general. I admit my knowledge in supersymmetry is very limited. Let's take an example: The photino. If it behaved similar to the photon, it ...
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Spatial wave-function of a single photon and its measurement

In the last decade there were several papers claiming that they've measured a "transverse quantum state" / "quantum wave-function" / "spatial Wigner function" of a single photon: Measurement of the ...
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133 views

Atom - light field coupling and emission process

Suppose a "2-state atom" and a light field are quantized with the following Hamiltonians, respectively: $$\hat{H}_A=\hbar\omega_{21}\hat{\sigma}^{\dagger}\hat{\sigma}$$ and $$\hat{H}_R=\sum_{\textbf{k}...
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148 views

Photon angular momentum, multipole transitions

Relying on many sources, I can summarize that somehow it is possible to show that a photon, in its plane wave expansion, can carry values of total angular momentum more than $\hbar$. Such photons can ...
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Can we derive the classical beam-splitter theory from the quantum beam-splitter Fock state picture?

A classical beam splitter divides incoming light into two parts according to the reflection and transmission coefficients. A quantum beam splitter on the other hand, can be modelled using Fock states,...
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142 views

Virtual photons around a Reissner-Nordström black hole

Say we have a Reissner-Nordström charged black hole. There is a static electric field around it and far away it is that of a point charge. This poses a puzzle because signals cannot travel outside a ...
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155 views

How much electrons can absorb a single photon?

In the one hand, energy can be emitted\absorbed only by a discrete portions. In the other hand - according to Coulomb law the higher distance - the lower force, because count of force lines per area ...
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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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Inverse pair production with hohlaraum as photon target; is this particular experiment going to be carried out?

The IFLScience article Scientists Work Out How To Make Matter From Light describes photon-photon collisions producing pairs of particles, and the Nature Photonics Letter A photon–photon collider in a ...
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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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Initial values of creation/annihilation operators

I have a question about creation/annihilation operators. For example, if I have an evolution equation for annihilation operator of photon $$ \frac{da_k}{dt} = -i \omega_k a_k$$ I obviously obtain $$...
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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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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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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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155 views

Commutation relation in quantized electromagnetic field theory

I have a question regarding a proposed problem (Problem 4.8) in Rodney Loudon's book "The Quantum Theory of Light". Let $U(t)$ be an operator defined by $$ U(t)=\exp\left\lbrace\frac{i}{\hbar}\int\...
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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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133 views

Hydrogen in an electromagnetic field bound states for full quantum electrodynamics

I am looking for a reference or derivation of the non-relativistic bound states of hydrogen in an electromagnetic field that include Zeeman effect, Stark shift, and Lamb shift. I am looking for a ...
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382 views

Observing a photon during flight

When I was reading about the double-slit experiment in quantum mechanics, everything seems to make sense in terms of the waves and the interference pattern, but if thinking more about this observation,...
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206 views

When will a quantum measurement occur?

I am confused about the nature of quantum measurements. In short: if a particle is coupled with a field, when will the wave collapse. A field might be coupled to a particle for a long time, and ...
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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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Semi-classical derivation of maximal magnetic field in the Universe

I'm looking for all (or most) theoretical semi-classical derivations of the maximal magnetic field intensity that there may be in the Universe. As an example, this paper evaluate a maximal field of ...
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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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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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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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158 views

Photon carries spin angular momentum of $\hbar$

I know from numerous articles and also wiki (http://en.wikipedia.org/wiki/Spin_angular_momentum_of_light) that a photon carries spin angular momentum of $\hbar$. How to prove it mathematically? What ...
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Why don't electron-positron collisions release infinite energy?

Questions of the form: An electron and a positron collide with E MeV of energy, what is the frequency of the photons released. quite often come up in my A Level course (for often fairly arbitrary ...
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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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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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Did the Feynman heuristic of “simple effects have simple causes” fail for spin statistics?

Someone here recently noted that "The spin-statistics thing isn't a problem, it is a theorem (a demonstrably valid proposition), and it shouldn't be addressed, it should be understood and celebrated." ...
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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 ...