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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Where did the gamma matricies disappear to?

In the discussion of external bremsstrahlung, the following amplitude is used: $$M=i\bar{u}_e(k')e \left(\gamma^\nu\epsilon_\nu \left[ \frac{i \gamma^\nu(k'_\nu+\omega_\nu) + ...
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Tadpole diagram in QED

In $\phi^4$ theory with the $\lambda \phi^4 / 4!$ interaction term gives rise to “tadpole” diagrams like this: If I have a standard QED interaction with $e \bar\psi \gamma^\mu \psi A_\mu$, can I ...
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156 views

How are bound states handled in QFT?

QFT seems very well suited to handle scattering amplitudes between particles represented by the fields in the Lagrangian. But what if you want to know something about a bound state without including ...
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50 views

Photon as a mediator of electric field

How can a photon (which has momentum) from one electrically charged particle to an oppositely charged particle cause these particles to be pulled toward each other - or how can a magnetic field cause ...
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163 views

Feynman diagram for attractive forces

I’m looking at Feynman diagrams for attractive forces and I'm thoroughly confused. Below are three diagrams from HyperPhysics: These all illustrate instances where the forces are attractive. ...
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164 views

u-Channel Matrix element for electron-positron annihilation

This one is a quantum related question. The calculation relates to the Matrix element for the annhilation of a electron-positron into two photons: $$ e^-e^+\rightarrow\gamma\gamma $$ I've recently ...
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177 views

Can an electron jump to a higher energy level if the energy is insufficient or exceeds the ΔE

Lets say we have an atom of hydrogen. It has one electron on E1 = -13.6 eV (E2 = -3.4 eV) energy level. I know that if we fire a photon with 10.2 eV energy the hydrogen atom will absorb it and the ...
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215 views

Explanation for Refraction of light (change of angle) and Reflection

Refraction: I want a qualitative Quantum Mechanical explanation of why do we see light rays -in the classical picture- bend when light goes from one medium to another. I read that it is due to ...
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55 views

How to understand photons in terms of EM force carrier? [duplicate]

Say, there are 2 stationary electrons placed at a distance. The result of observation would be both flying apart with the same speed and the opposite direction, which would obey laws of conservation ...
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180 views

How to Calculate Anomalous Dimensions in (Effective) QED

I am following the conventions here. Consider the (effective) QED Lagrangian ...
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24 views

Probability of having more than one multiplication per electron collision?

I am studying Gas Electron Multipliers (GEM) where is a common assumption that there is a one primary multiplication per electron collision: Sauli, Nappi and Knoll. However, I am unsure about this ...
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31 views

Is filming 25 micrometer from opposite sides possible QM event? [closed]

I have a Gaussian process where the mean is 0.381 and the standard deviation is 0.524. The process is simplifically the difference of diameters $d_1$ and $d_2$ where $d_1$ is taken from one side, ...
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Why is the term $m\delta_2\delta_m\bar{\psi}\psi$ ignored in the QED Lagrangian?

Consider the QED Lagrangian $$\mathcal{L}=\bar{\psi}_0(i\gamma^{\mu}\partial_{\mu}-e_0\gamma^{\mu}A_{0\mu}-m_0)\psi_0-\frac{1}{4}(\partial_{\mu}A_{0\nu}-\partial_{\nu}A_{0\mu})^2$$ where the 0 ...
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137 views

Which problem is Oppenheimer working on in this picture? [closed]

Does anybody recognize the equations on the blackboard? Above his hand, with the $\gamma_k$ term and its complex conjugate, it looks like a written out matrix representation of a Hamiltonian $U$, ...
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79 views

What is the exact meaning that QED perturbative series is only asymptotic and eventually diverges at very high orders?

When I read paper PRB89, 235431 about the effective field theory of graphene, there is a statement that QED perturbative series is only asymptotic and eventually diverges at very high orders (e. g. ...
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1answer
44 views

What is the exact mechanism in line broadening of transition frequencies?

I've been having trouble understanding why transition lines in unperturbed states are not infinitely narrow. I'll run through my reasoning and hopefully that helps find the flaw. I imagine that if we ...
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1answer
95 views

How to derive the vertex factor for the four-field interaction in scalar QED?

I tried to find the Feynman rules using path integral. For scalar QED, I have an interaction term $e^2\phi^{\star}A_{\mu}^2\phi$. Generating function then can be written as $$ ...
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62 views

Quadratic terms in QED lagrangian density

I recently learned that when we speak about a "free lagrangian", this actually means that the lagrangian is quadratic in the fields. When considering the Lagrangian density describing the coupling to ...
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44 views

spin direction of the electromagnetic field tensor

I guess the electromagnetic field strength tensor $F_{\mu\nu}$ can be considered as a spin-1 field. So, is it possible to derive $F_{\mu\nu}$'s spin direction based on the elements in the tensor (the ...
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Equal-time commutation relations, Feynman propagator for gauge parameter $\lambda = 1$, physical meaning

Classical electromagnetism (with no sources) follows from the actions$$S = \int d^4x\left(-{1\over4}F_{\mu\nu}F^{\mu\nu}\right),\text{ where }F_{\mu\nu} = \partial_\mu A_\nu - \partial_\nu A_\mu.$$The ...
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122 views

Maxwell equations and quantized electromagnetic field

When the electromagnetic field is quatized for a single mode, we first take Maxwell equations and proceed to write the electric and magnetic field as a stationary wave, since we consider the ...
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86 views

Would the detection of advanced waves be possible in curved space-time?

In the transactional interpretation of quantum mechanics reference 1, inspired by the Wheeler–Feynman absorber theory, a transaction is formed between the emitter and absorber by a superposition of ...
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3answers
138 views

Does orbital angular mometum has no meaning for single photons?

In the quantization of free electromagnetic field, it is found that the left-circularly polarised photons corrsponds to helicity $\vec{S}\cdot\hat p=+\hbar$ and right-circularly polarised photons ...
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How to describe time evolution in relativistic QFT?

I must confess that I'm still confused about the question of time evolution in relativistic quantum field theory (RQFT). From symmetry arguments, from the representation of the Poincare group through ...
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Lagrangian for $\mathcal{N}=4$ SQED in 3D

What is the Lagrangian for $3D$ $\mathcal{N}=4$ supersymmetric QED, with $N_f$ hypermultiplets? In particular, which is the form of the Fayet-Iliopulos terms, and the real mass terms?
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114 views

What are skeleton diagrams and what is their use in qft and many-body physics?

How does one construct skeleton diagrams from specific Feynman diagrams (e.g. for the electronic Green function in QED and in many-body gases, for the polarization function, for the vertex function, ...
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79 views

Interpretation of the electromagnetic field strength tensor as a spin-1 field

If I understand correctly the electromagnetic field strength tensor $ F_{\mu\nu}$ could be considered as a spin-1 field. In that case, what can one say about the total spin and the $z$-component of ...
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27 views

Does a Description with Photons as Force Carriers Apply for Non-Radiative Systems?

What is the full quantum mechanical description of the statement from classical electromagnetism "the electric field of a uniformly charged infinite flat plane is constant"? By "full" I mean that I'm ...
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48 views

Lorentz structure of coupling in QED

what is meant by the "Lorentz structure of the coupling in QED" as a possible test for QED?
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90 views

How photons can emerge quantized if their cause is continuous?

I know that photons are quantized, they are not continuous. But they are created by an accelerated charge. So how is it possible to have a quantized outcome from a symmetric continuous event? I mean ...
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85 views

Peskin-Schroeder, Unitarity of the S matrix, eq 9.61

I have a question regarding a derivation in Peskin and Schroeder's QFT book. On page 298, he is discussing a method for defining a gauge invariant S matrix. He does this by defining projection ...
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69 views

Can photons move in circles?

A photon can travel between two points A and B in a straight line. However, there is a chance it will go in a slightly curved path. Is there a chance the photon will turn around? Does this mean that ...
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If a photon doesn't necessarily travel in a straight line, doesn't it defy the law of cons. of momentum?

I just finished reading Richard Feynman's lectures on Quantum Electrodynamics (QED: The Strange Theory of Light and Matter) and it fascinated me. However, there's an unanswered question I have from ...
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What is going on in the photon-photon scattering Feynman diagram?

I understand the basic concepts of a Feynman diagram, but I don't get what is going on here. I have named the photons ABCD and the fermions 1234 for clarity. I'm going to assume that the vertical ...
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284 views

When does the concept of electric field in classical electrodynamics fail, and QED is needed?

It is really hard to find reference to when the traditional concept of electric wave, especially TEM wave, fails, and needs to be replaced by quantum electrodynamics. So when does the concept fail? ...
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30 views

Dipoles in an external rotating field?

Consider a bunch of magnetic dipoles in $x-y$ plane in an external magnetic field $B(t)=B_0 \hat{z}+B_1(\cos\omega t~\hat{x}+\sin\omega t~\hat{y})$. The dipoles are rotating around $z$ axis and of ...
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295 views

Polarization Sums in QCD for the calculation of parton model splitting functions

Before i state the actual problem, here's a premise. In the case of a Spin 1 massive particle it's possible to demonstrate that $$\sum_{\lambda=0,\pm1}\epsilon_{\lambda}^{* \ ...
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Electric field operator in 2D geomatry

In the free field (3D), transverse electric field operator is given by the below expression; $$e^{\bot}(\textbf{R}) =i \sum_{\textbf{p},\lambda}\Big( \frac{\hbar cp}{2V\epsilon_{0}}\Big)^{1/2} ...
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Why don't photons split up into multiple lower energy versions of themselves?

A photon could spontaneously split up into two or more versions of itself and all the conservation laws I'm aware of would not be violated by this process. (I think.) I've given this some thought, and ...
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QED+Classical Background Renormalization

I would like to ask a question related to quantum corrections and renormalization in QED. We have the QED vertex $\overline{\psi}[-ie \gamma^{\mu}(B_{\mu}+A_{\mu})]\psi,$ being $B_{\mu}$ a classical ...
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What happens to Hydrogen-Boron plasma at 3 billion Kelvin?

A recent popular report states (beware, I believe it is not peer reviewed and this slightly smells like a PR stunt) that a private company TriAlpha has made some notable progress towards ...
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Why do electrons and positrons exhibit opposite helical motion in a magnetic field?

When you throw an electron through a solenoid, it moves helically around the field lines, as per this schoolphysics illustration: © Keith Gibbs 2013 Then if we were to throw a positron through the ...
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How far do the propagation paths in QED go?

I am currently reading Feynman's popular book on QED "the strange theory of light and matter". I know classical optics quite well, know about Fresnel, Brewster angle and the like. I also am now used ...
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1answer
116 views

$U(1)$ local gauge invariance in QED [duplicate]

While constructing Lagrangian of QED, we don't add the mass term for photon $\dfrac{1}{2} m^{2}A_{\mu}A^{\mu}$ because gauge invariance does not allow. I want to ask, whether "$\bf{Theoretically}$", ...
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98 views

Two questions about Feynman propagator

Taking for example the meson propagator: $$ \Delta_F (x-y) = \int \frac{d^4k}{(2\pi)^4} \frac{e^{-ik(x-y)}}{k^2 - m^2 + i\epsilon}. $$ It describes a meson that propagate from a point of Minkowski ...
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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 ...
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Does a charged particle propagating in free space have a 'self-energy' like term due to it’s interaction with the fluctuations of the quantum vacuum? [closed]

Does a charged particle propagating in free space have a 'self-energy' like term due to it’s interaction with the fluctuations of the quantum vacuum? (particle-antiparticle pairs popping into and out ...
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What's actually going on when light interacts with matter?

There are many different theories but all of them fail to explain all 3 phenomena of light(absorption,emission,transmission). https://www.youtube.com/watch?v=CiHN0ZWE5bk ...
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241 views

Electromagnetic wave and quantum mechanics [duplicate]

I'm very new to physics. I studied and read about quantum mechanics and what the assumptions are (wave particle duality, uncertainty principle, observation, wave function collapse, etc.), but I also ...
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3answers
254 views

Why photons reflect off glass?

Why photon reflects and refracts through glass? Some photons pass through glass and some reflects.I know this is due to energy levels of electrons of glass, an incoming photon is unable to excite the ...