**The 1947 discovery of a small discrepancy in hydrogen’s atomic spectrum came at just the right time to push quantum theory forward.
In the second quarter of the 20th century, quantum theory faced some serious challenges, including unexplained details of atomic spectra and difficulties calculating basic properties of charged particles. In 1947 Willis Lamb and Robert Retherford of Columbia University discovered an unexpected detail in the hydrogen spectrum, later called the Lamb shift, that became an essential clue in solving both problems. The measurement agreed with new calculations and was the first indication that the theoretical approach called renormalization could resolve the mathematical infinities that had threatened to derail the progress of quantum mechanics.**

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