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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QED: Why the i disappear in eq.(65.7) in Srednicki? [closed]
In Srednicki QFT, section 65, he claims that we can get equation 65.6:
\begin{align}
i\Pi^{\mu\nu}(k) &= (iZ_1e)^2\frac{1}{i^2}\int\frac{d^4l}{(2\pi)^4}\frac{(2l+k)^\mu(2l+k)^\nu}{((k+l)^2+m^2)(...
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Can one formalise the Dirac field as a Grassmann-valued section of a $U(1)\times Spin(3,1)$ associated bundle?
We know that the Dirac field is both a spinor and also charged under the $U(1)$ bundle. How can one formalise this one a general manifold $M$?
One idea I had is that we take a $U(1)$ Principal bundle ...
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Question on a calculation in Weisskopf's 1939 paper on the electron self-energy
I'm trying to understand Weisskopf's 1939 paper 'On the Self-Energy and the Electromagnetic Field of the Electron'. Here Weisskopf demonstrates that in Dirac's positron theory, the self-energy of an ...
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Numerical results of precision tests in perturbative QFT
Where can I find a database or tables with the precisions/contributions that each Feynman diagram term adds to most common and famous numerical estimations for the standard model of particles (like $g$...
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Is it valid to keep $k_1$ and $k_2$ when considering a light propagating along $z$ axis?
If a mode function of the light is given by $\psi_{\mathbf k}(x^\mu)=ce^{ik_\mu x^\mu}$, where the degrees of freedom of polarization are suppressed, it can be normalized by requiring $\left <\psi_{...
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Computation of scattering cross section using Dirac equation
The differential cross section formula of the scattering of an electron on a nucleus are known to coincide when computed using either classical mechanics approach (Rutherford) or the Schroedinger ...
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How does the virtual photon get its mass?
I recently read that a virtual photon has mass, unlike a real photon.
"As a consequence, a real photon is massless and thus has only two polarization states, whereas a virtual one, being ...
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What are the units of a state in QFT?
Using Peskin+Schroeder as a reference. Bear with me, there may be multiple mistakes in my discussion. But the underlying question should be clear - it's really just the title.
By analyzing the ...
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Quantum vs classical behavior of monochromatic light
A polarized beam of monochromatic light in classical physics is described by something like
$$E = \hat z \cos(kx-\omega t),\quad B =\hat z\times E.$$
What happens if we aim this beam to a highly ...
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QED renomalization: Residue of the fermion propagator [duplicate]
I am trying to understand how one gets the residue of the fermion propagator and what its significance is.
I suppose that a distinction between the field in the free theory and in the interacting ...
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Quantizing EM field by imposing canonical commutation relations
As far as I understand, we can quantize electromagnetic field by simply imposing commutation relations on the components of electric and magnetic fields, acting here as conjugate variables (just as ...
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Product of Electric and Magnetic fields in QFT in an expanding universe
I am struggling a bit to understand the Quantum to Classical transition in QFT in an expanding universe. I want to compute the quantum expectation value of the classical quantity $\vec{E}\cdot\vec{B}$
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When physicists talk about light acting as a wave, are they referring to the wave function, or the EM field? What’s the difference? [duplicate]
Let me give an example to get to the heart of what I am asking:
In the case of the double slit experiment, an interference pattern shows up. Is this interference pattern the result of the wave ...
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Why the strong force is not ordinary magnetic force? [closed]
It is my understanding that naturally two protons cannot "stick" together even if they are within the strong force reach and beyond (i.e. even smaller distance) and this is possible only if ...
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How does string theory handle non-renormalizability of QED in $d>4$?
Quantum electrodynamics is non-renormalizable in more than four dimensions (see Peskin & Schroeder, chapter 10). This would seem to put it on similar footing as gravity for $d>4$ in the sense ...
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