In 1930s, following the initial development quantum electrodynamic theory (QED), American physicist J. Robert Oppenheimer (1904-1967) employed the new theory to calculate the energy levels of the hydrogen atom. The first such calculation had been performed by Niels Bohr in 1913, a full decade before the development of modern quantum theory. In the case of hydrogen (and only in the case of hydrogen) the Bohr model works, yielding the same values that are predicted by the full, but non-relativistic quantum mechanics. The measured values for other atoms, however, differed from expected values.

QED, like any relativistic quantum field theory, incorporates German-American physicist Albert Einstein's mass-energy relation, and thereby explains the creation and absorption of virtual particles. A propagating electron, for example, can spontaneously emit, and then almost instantaneously re-absorb a photon. This process contributes to what is called the electron self-energy, and will minutely shift the energy level values...