How can an asymptotic series give accurate results as in QED?

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The discussion centers on the nature of asymptotic series in Quantum Electrodynamics (QED) calculations, specifically regarding their convergence and accuracy. It is established that QED results are expressed as an asymptotic series in the fine structure constant, where adding too many terms can lead to divergence. The concept of "18 digits accuracy" is examined, emphasizing that one must cease adding higher-order terms once they exceed the magnitude of previous terms to maintain accuracy. The discussion also raises the question of whether the "reversal point" correlates with the optimal agreement with experimental results.

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shunra
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The results of QED calcualtions are in the form of an asymptotic series in the fine structure constant. This means that if one adds too many terms, the result diverges. So what is the meaning of "18 digits accuracy" of certain QED calculations, if they can be spoiled by adding MORE loops to the calculation or LESS ?
 
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In case of an asymptotic series you have to stop adding higher-order terms if these start to become larger than the previous term.
 
Thank you vanhees71.
In the case of QED, is the above "reversal point" also the point where the best agreement with experiment is achieved?
 

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