✎ Quantum Electrodynamics [The Physics Travel Guide]

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====== Quantum Electrodynamics: $ \quad L = \bar {\Psi}(i \gamma^{\mu }\partial_{\mu} + q\gamma^{\mu}A_{\mu} - m)\Psi + \frac{1}{16 \pi}F_{\alpha \beta}F^{\alpha \beta} . $ ======

Quantum electrodynamics (QED) is the best theory of electromagnetic interactions that we have.

It accurately describes how elementary particles like electrons interact with each other and with light.

* The best layman introduction to quantum electrodynamics is Quantum Electrodynamics by Richard P. Feynman 
  
<tabbox Student>

Quantum electrodynamics is a [[theories:quantum_theory:quantum_field_theory|quantum field theory]] of electrodynamics. At its heart is a [[advanced_tools:gauge_symmetry|gauge symmetry]] called local $U(1)$ symmetry.

In practice, we use quantum electrodynamics to describe electrodynamical interactions between charged particles through elementary particles called photons.

Photons are the fundamental excitations of the electrodynamic field.

There are two frameworks to calculate things in quantum electrodynamics: either using [[advanced_tools:feynman_diagrams|Feynman diagrams]] (= the [[frameworks:hamiltonian_formalism|Hamiltonian framework]]) or using [[advanced_tools:path_integral|path integrals]] (= the [[frameworks:lagrangian_formalism|Lagrangian framework]]).

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See also https://en.wikipedia.org/wiki/Quantum_electrodynamics#Mathematics

The best resource for the correct **Feynman rules of QED** is [[https://arxiv.org/pdf/1209.6213.pdf|A resource for signs and Feynman diagrams of the Standard Model]] by Jorge C. Romao. The thing is that there are many different conventions and these authors develop a notation that allows us to map them all onto each other.

-->The QED Feynman Rules at one Glance#
[{{ :models:qedfeynmanrules.png?nolink |Source: Peskin, Michael; Schroeder, Daniel (1995). An introduction to quantum field theory}}]

<--

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**Recommended Resources**

* [[https://arxiv.org/pdf/hep-ph/0508242.pdf|Lectures on QED and QCD]] by Andrey Grozin

<tabbox Researcher> 
**Additional Conserved Charge**

In addition, to the usual electrical charge, which is conserved due to the global $U(1)$ symmetry, there is an additional __topological charge__.

This follows from the homogeneous Maxwell equations

$$ \partial_\mu \tilde{F}^{\mu \nu} =0 $$

which follow "trivially" from the definition of the dual field strength tensor $\tilde{F}^{\mu \nu} = \epsilon^{\mu \nu \sigma \rho} F_{\sigma \rho}. $ (They are simply the [[advanced_tools:bianchi_identities|Bianchi idenities]].)

They homogeneous Maxwell equations express a conserved current equation for each component $\nu$. For example, for $\nu=1$, we have

$$ \partial_\mu \tilde{F}^{\mu 1} =0  \quad \leftrightarrow \quad \partial_\mu j^\mu =0  ,$$

where I have defined $j^\mu \equiv \tilde{F}^{\mu 1} $.

When we have a conserved current, we have a conserved charge thanks to Gauss' theorem:

$$ Q \equiv \int_V dx^3 j^0 \quad \text{ with} \quad \partial_0 Q  =0 .$$

This charge generates an __additional global symmetry__.

<blockquote>The conserved charge is the magnetic flux associated to the U(1) gauge
symmetry.<cite>[[http://www.damtp.cam.ac.uk/user/tong/qhe/qhe.pdf|David Tong page 169]]</cite></blockquote>

Formulated differently:
<blockquote>[It] measures the magnetic flux of a line operator $H(C)$ (the "`t Hooft line operator") which is supported on a line $C$ which links the $S^2$. It corresponds to the worldline of a probe magnetic monopole, and $Q$ measures the magnetic flux of the monopole in the same way that $\int_{S^2} \star F$ measures the electric flux on the worldline of an electric charge. These are called 1-form global symmetries, because the charged operators are supported on lines.

The same story goes through in any dimension $d>2$. We obtain a $(d-3)$-form global symmetry, meaning the charged operators are supported on $(d-3)$-manifolds which link a 2-sphere over which we measure the charge $\int_{S^2} F$.

In 3 dimensions, $j=\star F$ is a 1-form, so this is an ordinary global symmetry. The 't Hooft operators are pointlike magnetic monopole operators, whose charge is again the magnetic flux.<cite>https://physics.stackexchange.com/a/391273/37286</cite></blockquote>

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**Recommended Resources**

* https://ncatlab.org/nlab/show/quantum+electrodynamics
  
<tabbox Experimental Tests>

* A great review of experimental tests of QED can be found [[http://edu.itp.phys.ethz.ch/hs10/ppp1/PPP1_6.pdf|here]].
  * Savely G. Karshenboim, D. I. Mendeleev, [[https://arxiv.org/abs/hep-ph/9411356|The Lamb Shift of Hydrogen and Low-Energy Tests of QED]]

<tabbox FAQ> 
  
<tabbox History> 
  * The notion “quantum electrodynamics” was invented by Paul Dirac in [[http://wwwhome.lorentz.leidenuniv.nl/~boyarsky/media/Proc.R.Soc.Lond.-1927-Dirac-243-65.pdf|The quantum theory of emission and absorption of radiation]]
</tabbox>

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