When it seemed that about hydrogen atom we knew almost everything in 1947 W.E. Lamb and R.C. Retherford decided to check results of Dirac. They used microwaves technique, available from the constructions of radar
The Lamb's shift*, a minimal difference in lowest energetic level of the excited hydrogen atom can’t be explained in any way without introduction of the absolutely new concept in Physics:Quantum Electrodynamics.
The essential of the QED is that vacuum is never empty, but filled with virtual particles – appearing suddenly and then quickly disappearing. Processes like those do not violate the energy conservation principle: the loan of energy from nowhere is very short, like it does the bank employee, who brings back in the morning the money borrowed in the evening.
Lamb's shift: a subtle structure of the n=2 level in hydrogen according to Bohr's,
Dirac's and QED with assumption Lamb's shift. *lamb, i.e. a small sheep. The song on Mary's lamb was
the first "phonograf" record, made by Edison, but this is a different
story. .
The Lamb shift removes the degeneration
due to quantum number j.
H. Haken, H.C.Wolf, Atomic Physics, 1996].