Theory of the Lamb Shift in Hydrogen and Light Hydrogen‐Like Ions.

Theoretical calculations of the Lamb shift provide the basis required for the determination of the Rydberg constant from spectroscopic measurements in hydrogen. The recent high‐precision determination of the proton charge radius drastically reduces the uncertainty in the hydrogen Lamb shift originating from the proton size. As a result, the dominant theoretical uncertainty now comes from the two‐ and three‐loop QED effects, which calls for further advances in their calculations. The present status of theoretical calculations of the Lamb shift in hydrogen and light hydrogen‐like ions with the nuclear charge number up to Z=5 is reviewed. Theoretical errors due to various effects are critically examined and estimated. [ABSTRACT FROM AUTHOR]