In 1947, the Lamb shift was discovered: a small difference in the S 1 ⁄ 2 and P 1 ⁄ 2 levels of hydrogen, due to the interaction between the electron and vacuum. Lamb and Retherford experimentally measure stimulated radio-frequency transitions the S 1 ⁄ 2 and P 1 ⁄ 2 hydrogen levels by microwave radiation. An explanation of the Lamb shift is presented by Bethe. Papers on the effect were published in the early 1950s.