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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3answers
161 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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1answer
41 views

Collapse operators in multi-level atomic systems?

I'm trying to understand the case of multiple ground states and excited states. Due to selection rules, usually we say there should be 3 collapse operators, namely C_+1, C_-1, C_pi, corresponding to ...
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Why are there no loop corrections to the photon mass? [duplicate]

I know that a question about why the photon is massless already exists, however it did not answer this question. First off, I do understand why the photon does neither have a bare mass term due to ...
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2answers
103 views

Is the magnetic field energy around a current carrying wire is similar to the electric field energy associated with an isolated charge?

What I mean by this question is, as we say that the electric field energy around a charge is the self energy of the charge, similarly can we say that the magnetic field energy around a current ...
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1answer
56 views

Is the electron field of QED similar to the Higg's field?

If I understand it (I probably don't...) the Higg's field is a field that permeates the universe, and its excitations are the Higg's bosons. It seems that this implies that the Higg's field is a ...
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82 views

Vector Spherical Harmonics and total angular momentum

In their book Akhiezer et al. give a definition of vector spherical harmonics (p.18 of Russian Edition) as $$\pmb{Y}_{j\ell m}(\pmb \Omega) = \sum_{m' \lambda} \langle \ell m' 1\lambda| jm \rangle Y_{...
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1answer
38 views

Wave energy in terms of classical and quantum mechanics [duplicate]

If there are two light waves of same amplitude but of different frequencies then their energy will be same or different? If same then how will you define the light waves as photons and will there be a ...
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1answer
68 views

Double photon emission, half photon absorption

Is it possible for atom with two electons to absorb one photon in this manner, $$|gg\rangle|n\rangle \to |ee\rangle|n-1\rangle$$ or for one electron to emit two photons like this $$|e\rangle|n\...
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What is interaction cross section of a photon and how to find it?

I encountered this term while reading about photon interaction with metal electrons. Is this equal to the scattering cross section or is it different? How to find it mathematically?
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1answer
66 views

What is the difference between photons in the E-Field and the H-Field?

It is well established that the photon is the propagator for the electromagnetic force. How are the set of photons used in the H-field behaving differently than those in the E-field? Is it different ...
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1answer
384 views

Why can there be only 3-particle vertices in Feynman diagrams? [closed]

Why can there be only 3-particle vertices in Feynman diagrams? For example, when a particle and its anti-particle annihilate to form two photons, why is its Feynmann drawn the way it is in diagram 2 (...
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2answers
158 views

How does QED explain friction between two bodies?

I was reading Brian Cox's "The Quantum Universe" and at one point he was taking the analogy of the ball resting at the bottom of a valley to explain the finite potential well or "electron in a box" ...
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199 views

Photon creation and quantum field theory

I remember learning the following familiar story in high school chemistry class: "an atom consists of a nucleus of protons and neutrons, and a collection of electrons orbiting around the nucleus ...
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1answer
113 views

Don't the magnets slip and turn in Stern-Gerlach experiment, if their orientation is not aligned with the applied magnetic field?

I was watching two videos on YouTube: In this video, there is an attempt to explain what spin is, using analogy of magnets. But here it is shown that magnets which are not aligned with magnetic field ...
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2answers
413 views

Simplify quantum electrodynamics expression

I have a quantum electrodynamics exercise on the one-loop electron self-energy correction in which I need to show that $$ \tag{1} ie^{2}\Sigma(p)=\frac{\left(-ie\right)^{2}}{\left(2\pi\right)^{4}}\...
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Contact terms in derivation of Ward identity

(This post is a bit long; the key question is between the horizontal lines, and an example follows it.) I am following the derivation of the Ward identity in Schwartz's QFT book, and there is a key ...
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1answer
100 views

Endless electron-positron annihilation/creation?

We start the process with gamma rays collision that produces positron electron pair, the pair then annihilates when hit each other and gamma rays are produced again and everything repeats. Sure, in ...
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How far can we take the analogy of particle exchanges as classical interactions?

In QED, electromagnetic interaction can be thought of as being a result of virtual photon exchanges between charged particles. In some books, it was claimed that to some extent, this interaction can ...
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2answers
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How can I do quantum chemistry with QED?

So I am a theoretical and computational chemist by trade and my usual zone of operation in the domain of quantum mechanics is Hartree-Fock and Density Functional Theories. I was thinking if there ...
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2answers
201 views

Simplification of an expression with gamma matrices [closed]

I am trying to understand the details of the calculations involved in determining the electron's anomalous magnetic moment to second order: the $\alpha/{2\pi}$ term. There is just one step, where an ...
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2answers
122 views

According to Coulomb law, why do fixed charges interact, if they don't emit the photons?

Why does it happens, if fixed charges don't oscillate the em field?
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49 views

Integral representation for inner product in the Gupta-Bleuler quantization

Following the Gupta-Bleuler method for the quantization of the electromagnetic field, we have that: $$[a_{k'\lambda'},\hat{a}^\dagger_{k\lambda}]=-g_{\lambda \lambda'}\delta(k-k')$$ And the vacuum $$\...
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1answer
72 views

Why there is a distance dependence in Coulomb's law if photons can travel to infinity?

Why there is a distance dependence in coulombs law if photons can travel to infinity? Why there is distance dependence at all?
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2answers
714 views

What do the arrows on this Feynman diagram represent? [duplicate]

The picture follows the convention of the time axis being on the vertical direction. I think this Feynman diagram represents Coulomb repulsion between two electrons with a virtual photon being ...
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1answer
482 views

Electric dipole moment of electron: about what point is the moment taken?

There is a lot of experimental research activity into whether the electron has an electric dipole moment. The electron, however, has a net charge, and so its dipole moment $$ {\bf \mu}= \int ({\...
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1answer
161 views

Classical Limit of coherent State in Jaynes Cummings Model

Im dealing with an exercise on the Jaynes Cummings model in a resonat single mode approximation. The interaction Hamiltonian in rotating wave approximation is $$H_{int}=g\, \sigma_+\,a\,+g^*\,\sigma_-...
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73 views

Expectation value of the electromagnetic field

According to wikipedia, the operator of the electromagnetic field is given by: $$E \propto \sum_k \sqrt{k}(a_k^{+}e^{-ikr}+a_ke^{ikr})$$ wouldn't that mean that the expactation value of $E^2$ is ...
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35 views

From a QED perspective, where is an electron on the surface of a charged conducting sphere before it is detected?

Classically, charges on a charged sphere is distributed on the surface as to minimize energy (and distribute like charges as far apart as possible). But in QED a single electrons has a probability ...
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1answer
985 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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1answer
161 views

Acceleration of photon

When light moves from one medium to another its speed changes due to the refractive index of the medium and so does its momentum given by $h/λ$. What is the acceleration of photon when it makes ...
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1answer
184 views

Is there any experimental proof for QED theory? [closed]

Are there any primary experiments used to try to prove the Quantum Electrodynamics theory? Were they all successful, and how were they performed? Also, I was wondering where Feynman's equations are ...
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1answer
56 views

Probability interpretation of electron charge

Consider the QED. When we write the covariant derivative of the theory and couple vector field of gauge boson and the spinor of fermion we think of electric charge of electron $e$ as a coupling ...
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External structure in BPHZ/Hopf algebraic renormalization of QED

I'm currently trying to understand and reproduce the Hopf-algebraic renormalization of QED presented by Walter D. Van Suijlekom. I don't understand why he chooses these external structures in (2), (7),...
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Is QED confining in 2+1 dimensions? Are there experimental realizations in condensed matter systems?

In 1d, QED is confining because the potential is linear and it costs an infinite amount of energy to separate two equal and opposite charges. I am guessing that the same is true in 2d, since the ...
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1answer
45 views

A doubt in Yang-Mills procedure

My question is this: I saw the next relation in a Yang Mills theory paper: $$L_{i}P^{\mu \nu}_{i}=P^{\mu \nu}$$ With $L_{i}$ a generator of su(2) and for any $P^{\mu \nu}_{i}$. But I can't ...
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2answers
109 views

Pair production near an electron

A high energy photon (E>~MeV) after interacting with an atomic nucleus produces an electron positron pair. The threshold of this process is the sum of the rest mass energies of these two leptons. My ...
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70 views

$Z_1=Z_2$ without Ward-Takahashi identity?

In the renormalization of QED, the way that $Z_1=Z_2$ is treated e.g. in Schwartz is by first giving a simple "heuristic argument" based on gauge invariance (in the beginning of section 19.5) before ...
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3answers
256 views

Are EM fields made of photons or are they fundamental?

I have read these questions: Virtual photon description of B and E fields How do virtual photon cloud produce discrete magnetic field lines in bar magnet? How virtual photons give rise to electric ...
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2answers
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Why aren’t photons re-absorbed into the electromagnetic field?

I understand that the quanta of electromagnetic field is a photon. Anyway, the electron field quanta is the electron. And the process of electron-positron annihilation releases two photons when the ...
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1answer
299 views

Correlators for space-like photon bouncing

Consider the diagram below of two detectors-emitters exchanging photons: Thin lines are null-paths between detectors. Detectors are also synchronized in such a way that measurements outside the blue ...
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160 views

Why don't atoms constantly emit light? [duplicate]

We know that accelerating charged particles emit electromagnetic radiation. We also know that electrons around nucleus have an angular momentum which means that electrons are revolving around the ...
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1answer
90 views

Second order correlation function of a quantized electric field

I'm analysing an open quantum system where the optical cavity is interacting with a quantum dot. I modelled the system using cavity QED and used Lindblad master equation to model the system. Now I ...
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1answer
103 views

Why does Jaynes Cummings not describe an atom in free space?

I'm a bit confused about the application of the Jaynes Cummings model and what exactly is meant by "one single mode": Usually, it is said that the Jaynes Cummings model describes a single atom in a ...
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0answers
49 views

Photon gravitational field traveling at speed c

If we assume that there is a timelike separation between the two photons, so if the two photons are not emitted together (as two entangled from a crystal), then we can assume that there is a certain ...
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3answers
345 views

Antenna Radiation

I am confused by something I have seen discussed but not with full clarity: it has to do with understanding antenna behavior at the quantum level for its own sake (yes, classical approach is easier ...
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What quantum mechanical property allows for reflection? [duplicate]

What are the quantum circumstances that manifest reflection? Also, to what precision does the wave reflect and is that verified or only speculated by model?
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408 views

Do we need Planck’s constant for second quantization?

The widely circulated folklore surrounding Planck’s constant $\hbar$ lends it an aura of importance. But could $\hbar$ be a constant of human convention which is dispensable? Does the unorthodox view ...
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1answer
104 views

Why is the fine structure constant the probability for photon emission by a charged particle?

I see in page 31 of Martin, Hanzel "Quarks and Leptons", that the fine structure constant is the probability for photon emission by a charged particle. Also I read from Lubos Motl's answer on this ...
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2answers
651 views

Photon spin and total angular momentun

I'm studying the photon spin and I assume that the angular momentum of spin is equal to 1 (from QED). In my book it's written, related to photons, that it doesn't make sense distinguishing between ...
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1answer
212 views

How does projective geometry related with Fourier transform via Lorentz transformation?

On one hand, the Lorentz transformation plays an important role in classical electromagnetism. On the other hand, each Lorentz tranformation is also a Möbius transformation of $\Bbb C$ (Needham, ...