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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Deriving non-relativistic potentials from QFT
Some systems, like atoms, are described well by quantum mechanics, where one just gives the Hamiltonian in the form $H=T+V$ and computes the eigenvalues and eigenvectors of this operator to figure out ...
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gauge invariance of the Feynman amplitudes
When we calculate the photon polarization sums over amplitudes,
$$X=\sum\limits_{r=1}^{2}|\mathcal M_r|^2=\mathcal M_\alpha\mathcal M_\beta^*\sum\limits_{r=1}^{2}\epsilon_r^\alpha\epsilon_r^\beta$$
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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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Path Integral on Feynman Hibbs: Interaction of EM field and matter, how can we get to equation (9.68) from (9.67)?
On Feynman Hibbs "Quantum Mechanics and Path Integrals", the equation (9.67) describes the transition amplitude of the matter (for example an atom) to go from the state $M$ to the state $M$ ...
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Integration & bremsstrahlung calculation
In this paper (relevant pdf section) that I'm reading, involving the calculation of bremsstrahlung in electron proton scattering (diagram below), the author calculates the integral over outgoing ...
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Do EM waves transmit spin polarization?
Suppose you have a normal dipole antennae (transmitter and receiver) . Spin polarized current (as opposed to normal current) is sent into the transmitter, it emits an EM wave and the Receiver receives ...
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QED lagrangian: gauge fixing term
I have a question about the structure of the QED lagrangian, in particular the free photon lagrangian which is contained in it. My premise is: I only know how to exploit canonical quantization in ...
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In QED/Yang Mills, why do fermions contribute 4 times as much as scalars to vacuum polarization?
Consider a Yang-Mills theory in $4D$ over a gauge group $G$
$$
\mathcal{L} = - \frac{1}{4} F^{a\mu\nu}F_{\mu\nu}^a + \bar \psi i D_\mu \gamma^\mu \psi + (D_\mu \phi)^\dagger D^\mu \phi
$$
where $\...
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How can QED by predictive if it diverges?
One of the tests of Quantum Electrodynamics is the value of the "Anomalous magnetic dipole moment".
The theoretical value is:
$$a_e = 0.001\ 159\ 652\ 181....$$
We say that QED "...
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Why are the propagators in old-fashioned QED oblique, while in modern QED they are horizontal (or vertical)?
In old-fashioned Quantum Electrodynamics, one can find diagrams such as these (probably Stückelberg was the first to use this notation, a kind of predecessor of Feynman diagrams):
In modern QED this ...
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How can QED have degenerate vacua without superselection?
This question is based on Andrew Strominger's lecture on the IR structure of field theories, in particular Section 2.11
The usual story with spontaneous symmetry breaking is as follows. You have ...
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Proof non-convergence of perturbation in QED
This is an attempt to ask separately about aspects of my previous question, which was closed as too broad. Note that I strongly prefer results that are or can be made mathematically completely ...
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Is it reasonable to interpret the Lamb shift as vacuum induced Stark shifts?
This is a pretty hand-wavy question about interpretation of the Lamb shift. I understand that one can calculate the Lamb shift diagrammatically to get an accurate result, but there exist "...
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Classical limits of Quantum Electrodynamics?
Quantum Electrodynamics is the theory that studies the interactions between matter and radiation (somewhat).
How would one explain for example the movement of an electron in a constant electric field ...
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Weinberg Vs Srednicki analysis for the electron self-energy
I am reading Srednicki's book on QFT. Specifically, I am reading about the loop corrections to the fermion propagator (Chapter 62). The relevant expression representing the one-loop and counterterm ...
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