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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How to count the number of modes/polarizations of a Gaussian field theory?

A Gaussian (free) field theory is described by a quadratic action of the field, e.g. $S=\int\psi^\dagger K\psi$ (or $S=\frac{1}{2}\int\phi^\intercal K\phi$ for real fields). Usually one just need to ...
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How are (semi-) static fields treated quantum-mechanically?

I am trying to understand a seemingly paradoxical assertion in this answer which argues that the magnetic field of an inductor corresponds to virtual photons because there is no net momentum ...
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Models of light

As far as I'm aware, there are two different (and almost contradictory) models that describe the behavior of light: light as a wave (EM), and light as a particle (QM). From what I've heard, depending ...
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Why most of physics is somehow related to light? [closed]

It seems that for the past 200 years, every physicist is concerned about light. For example : Newton's particle model, Young experiment, Photo-Electrict effect and Einstein's formula, Special ...
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How wrong are the classical Maxwell's equations (as compared to QED)?

Now, I don't really mean to say that Maxwell's equations are wrong. I know Maxwell's equations are very accurate when it comes to predicting physical phenomena, but going through high school and now ...
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How does the electric field operator change inside an optical cavity

In the free field, transverse electric field operator is given by the below expression; $$d^{\bot}(R)=i \sum_{p,\lambda}\Big( \frac{\hbar cq}{2V\epsilon_{0}}\Big)^{1/2} ...
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26 views

Which of the following produces more current? [closed]

In (1) photoelectric effect, (2) Compton effect, and (3) pair production, a beam of electrons are released. Out of Three Which produces More current? And How?
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Why can't photons have a mass?

Why can't photons have a mass? Could you explain this to me in a short and mathematical way?
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Electric current $j^{\mu}$ in standard QED vs. scalar QED

The expression for the 4-current $j^{\mu}$ in standard QED is $$ e\bar{\Psi}\gamma^\mu\Psi $$ and $$ \frac{e}{2 i}(\psi^\dagger D^\mu \psi - (D^\mu \psi)^\dagger \psi) $$ in scalar QED. I ...
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1answer
98 views

Can we measure the electron spin independently of its magnetic moment?

What experimental evidence do we have for the intrinsic angular momentum of the electron (its spin)? I am specifically interested in whether we have a value for this that is independent of the ...
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1answer
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Gauge invariance (QED)

In his book, the author says that according to the Feynman diagrams of this process in QED $$e^+ e^- \rightarrow \gamma \gamma,$$ gauge invariance requires that $$k_{1\nu}(A^{\mu\nu} + ...
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What would the collision of two photons look like?

Could someone explain to me what the collision of two photons would look like? Will they behave like, Electromagnetic waves: they will interfere with each other and keep their wave nature Particles: ...
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What do the tensors here equal to?

In the process $$e^+e^- \rightarrow \gamma \gamma$$ for which the amplitude can be written as: $M= \epsilon^*_{1\nu}\epsilon^*_{2\mu}(A^{\mu\nu}+\tilde{A}^{\mu\nu})$, where $\epsilon_i$ is the ...
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Electron-positron scattering: why is this the only diagram?

This is probably a very basic question, but I've been going through all my course materials and looking through various books to find a simple way of figuring this out. But consider the following ...
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1answer
51 views

Can an electron which is a bound in an atom absorb more than one photon at the same time? [duplicate]

Can an electron which is a bound in an atom absorb more than one photon at the same time ? In specific during photo-electric emission can an electron take in more than one photon if one photon doesn't ...
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3answers
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Could future experiments on “Gravitational Casimir Effect” confirm the existence of gravitons?

From Casimir effect, we know that when two plates are placed very close to each other in vacuum, they attract each other because the quantum fluctuations that press on the two plates' outer surfaces ...
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1answer
72 views

Is it possible to give photons an electric charge?

I know that photons have no electric charge and that they are stable, but is it possible to give them a positive or negative charge? If so how?
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Is there an expression for field charge analogous to field mass?

In electrodynamics, it is possible to derive an expression for the field momentum for a given moving charge distribution (e.g. a sphere with uniform velocity) and from that infer an "electromagnetic ...
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Is the elementary charge really a constant of nature? - Accuracy of QED

There are a couple of natural constants; examples are Planck's constant or the Speed of light in vacuum. The elementary Charge is the coupling factor to all Kind of electromagnetic interactions; this ...
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Are electron fields and photon fields part of the same field in QED?

I know in classical field theory we have the electromagnetic field. And Maxwell's equations show how electromagnetic radiation can propagate through empty space. I also have been reading about QED ...
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Physical meaning of the coupling matrix in Fermi golden rule

I am calculating the energy transfer rate using Fermi golden rule where the coupling matrix $M$ is obtained using second order pertubation method. $$ \Gamma_{tran}=\frac{2\pi}{\hslash}|M|^{2}\rho$$ ...
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Invariance of the QED Lagrangian under charge conjugation

Is it true that the QED Lagrangian $$\mathcal{L} = \bar{\psi}(i\gamma^\mu D_\mu-m) \psi $$ is invariant under charge conjugation? $$\begin{align} \psi &\mapsto -i(\gamma^0 \gamma^2 \psi)^T\\ ...
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62 views

How do photons mediate (or create) a force?

Is there a somewhat intuitive explanation as to why the exchange of a photon between two particles causes a force between those particles? Is there a difference in the way massless and massive ...
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Positronium energy level in QED

I'd like to know if it is possible to compute positronium mass and lifetime from a QED approach. I'm searching for some literature on how to treat resonances in QED (or general QFT) ; most of the ...
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Correction of arrow of particle direction

I have seen a stamp of Richard Feynman where Feynman hold the famous Feynman diagram. But is there any problem of the direction of arrow?
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Energy conservation if photon absorbed below resonance

Suppose I have some quantum system (like atom) with excitation energy $E_{exc}$ which is homogeneously broadened due to finite lifetime. I shine light with narrow spectrum centred around energy ...
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What are some practical things one can do with classical electrodynamics and QED?

Many basic types of physics have ready and obvious everyday applications. For instance, basic electromagnetism vector calculus can give great insights into how something as simple as a bar magnate ...
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Is frequency quantized in the black body spectrum?

I'm aware that there're some questions posted here with respect to this subject on this site, but I still want to make sure, is frequency quantized? Do very fine discontinuities exist in a continuous ...
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Can a photon get emitted without a receiver?

It is generally agreed upon that electromagnetic waves from an emitter does not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non ...
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1answer
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Basic QED - How are conserved charges expressions throught ladder operators derived?

I can't find this in similar questions, and I must be missing something very basilar since I can't find this in any textbook or online note: they just skip the passage. So, from my course's notes, we ...
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Is quantum uncertainty a function of how matter is distributed in the universe?

As an outcome of his PhD thesis work, Richard Feynman and John Wheeler wrote a series of papers on how the kickback on an electron as it emits a photon can be modeled accurately as the result of an ...
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1answer
185 views

Why quantum electrodynamics? [duplicate]

Most of the people seem pretty much content with classical electromagnetic theory .And most of the applications use classical EM theory .However, in such situations I would like to know what was the ...
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How the experimental charge $e=1.60217657 × 10^{-19} C$ has precisely this value?

The coupling constant that we measured in "arbitrarily" low energy is $e=1.60217657 × 10^{-19} C$. How this is presented in Renormalization Group flow in charge coupling space? Why the action of the ...
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QCD and QED with unlimited computational power - how precise are they going to be?

My question is about quantum algorithms for QED (quantum electrodynamics) computations related to the fine structure constants. Such computations (as explained to me) amounts to computing Taylor-like ...
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In the context of quantum field theory, what does it mean to “couple” something?

Suppose I have the following Lagrangian density \begin{equation} \mathcal{L} = - \frac{1}{4} F_{\mu\nu}F^{\mu\nu} \end{equation} The lecture notes I an reading suggest if I want to "couple to ...
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Converting mode expansion to an integral in an Cavity

Assuming the cavity length is large, can we convert the summation over cavity EM modes to an integral form? In that case is it reasonable to do below conversion? p is the mode number.
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1answer
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What are the electromagnetic fields of a photon?

I'm looking for expressions for the electromagnetic fields (preferably $E$ and $B$) of a typical photon which is localised in space to some extent (i.e. I'm not interested in the infinite plane wave ...
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1answer
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Conservation of energy and momentum in photoelectric effect

Sometimes it is shown that in a Compton scattering it is not possible that the photon transfers all it's momentum and energy to the electron, see for example here: If one assumes complete energy and ...
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1answer
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Why is the frequency bandwidth of the environment important for Markovianity?

In the derivation of Spontaneous Emission in two level systems in Quantum Optics (be it Wigner Weisskopf or a different approach, such as density operators to find the master equation), one makes ...
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1answer
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Linear and circular polarization in quantization of the EM field

I am going through the "Quantization of the EM field" in Chapter 7 of Sakurai's Modern Quantum Mechanics, which basically goes like: The vector potential satisfies wave function $\nabla^2\mathbf ...
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Why electrons can't radiate in their atoms' orbits?

It's an old-new question (I found only one similar question with unsatisfactory (for me) answer: Where did Schrödinger solve the radiating problem of Bohr's model?) It's strange for me how all books ...
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1answer
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The contraction of fermion field in 1+1-dimensional massless QED

My question comes from the textbook by Peskin & Schroeder, the integral (19.26): $$\begin{align} \int \frac{d^2 k}{(2\pi)^2}\! e^{- i k\cdot (y-z)}\frac{i \not{k}}{k^2} = -\not\partial ...
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If 2 photons collided head on, what would happen? [duplicate]

If 2 photons, in perfect synch (frequency, amplitude, etc. were all equal) and they collided head on, what would happen? Would they pass right through each other? Would they interfere, then go back to ...
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Is colour a purely quantum effect?

If the colour of an object is determined by the wave-lengths of light that is absorbs and reflects (?) then can colour be described as a purely quantum effect (i.e. without quantum effects an objects ...
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1answer
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Mølller scattering

I came across Mølller scattering today (which is just a fancy name for electron-electron scattering. I'm confused as to why there are two tree level Feynman diagrams for this process: Check out the ...
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1answer
507 views

Width of a photon. And its length

Everyone is always talking about photon's wavelength. But what about its dimensions? What is length and width of it? And does it even have a point to think about such things? Or those dimensions are ...
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0answers
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Thomson scattering on the elections does not produce any circular polarization?

All references on CMB polarization has this statement as if it is a trivial fact. But I have to admit that I completely don't understand what this sentence is telling us.
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Is diffraction affected by interaction between photons and electrons?

Suppose we take a sheet of ordinary metal, make a narrow slit in it, and shine a light beam through the slit onto a screen. The light beam will diffract from the edges of the slit and spread out onto ...
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Electro-magnetic wave propagation in vacum

I was wondering what will the EM wave propagation including virtual particle pairs existence look like. When electric field face virtual pair it should make the lowest state energy the most ...
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Counting d.o.f. and gauge fixing $A_{\mu}$ and $\psi$ in $D$-dimensions

Setup: Let us assume we are in $D$-dimensional Minkowski space-time where $D=d+1$. Consider a free Abelian gauge theory. Then the electromagnetic field will satisfy $$\partial_{\mu}F^{\mu \nu}=0 ...