The Lamb shift is the quantum-electrodynamic shift of certain atomic energy levels caused by interactions between an electron and the quantized electromagnetic field.
Willis Lamb and Robert Retherford measured the effect in hydrogen in 1947. Their experiment showed that two levels predicted to have the same energy by the Dirac equation were actually separated by a small amount.
The shift arises partly from vacuum fluctuations and the electron’s interaction with its own electromagnetic field. It became an important early test of quantum electrodynamics, which combines quantum mechanics with special relativity.
The Lamb shift is often confused with the Stark effect, which is caused by an applied electric field, or the Zeeman effect, which is caused by a magnetic field. Lamb shared the 1955 Nobel Prize in Physics for this work.