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5.6: The Predictive Power of QED

Last updated

Aug 9, 2024
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- 5.5: Inﬁnities and Renormalization
- 5.7: Problems

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It is hard to say that a theory has predictive power without comparing it to experiment, so let me highlight a few successes of QED.

One of those is the so-called $g$ factor of the electron, related to the ratio of the spin and orbital contributions to the magnetic moment. Relativistic theory (i.e., the Dirac equation) shows that $g=2$ . The measured value differs from 2 by a little bit, a fact well accounted for in QED. $\begin{array}{ll} {\rm experiment} &g/2 = 1.001 159 652 41 (20)\\ {\rm Theory} &g/2 = 1.001 159 652 38 (26)\\ \end{array} \nonumber$ Some of the errors in the theory are related to our knowledge of constants such as $\hbar$ , and require better input. It is also clear that at some scale QCD (the theory of strong interactions) will start playing a rôle. We are approaching that limit.

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