Lamb shift

The Lamb shift is the difference between two energy levels of the hydrogen atom that Dirac’s theory held should have precisely the same energy. Its resolution connected a discrepancy in hydrogen’s spectrum to the interaction of charged particles with electromagnetic fields, and became a key step in the development of quantum electrodynamics.

The discrepancy in hydrogen

In 1930, J. Robert Oppenheimer wrote a paper on the infinite nature of spectral lines using direct theory. A splitting in hydrogen’s spectrum suggested a small difference between the energy levels of two possible states of the atom, contrary to Dirac’s prediction that the states should be degenerate. Oppenheimer disagreed with that prediction, but his results did not settle the question.

Measurement and explanation

Years later, experimental physicist Willis Lamb, one of Oppenheimer’s doctoral students, resolved the issue. The difference between the two energy levels—known as the Lamb shift—was correctly attributed to self-interaction: charged particles interacting with electromagnetic fields. The contrast between Oppenheimer’s inconclusive theoretical result and Lamb’s experimental resolution marks the shift from a predicted equality to a measurable physical effect.

Significance

Lamb received the 1955 Nobel Prize in part for his precise measurement of the Lamb shift. That measurement was a key step in the development of quantum electrodynamics, giving the spectral discrepancy significance beyond the hydrogen atom itself.