It is known that the Lamb shift is the consequence of the interaction of the electron with its virtual photon field. What is the meaning of virtual photon field? And how is it helpful to explain Lamb shift?
Lamb's experiment explanation had led to the creation of Quantum Electrodynamics, and so called Feynman diagrams techniques facilitating calculations and a graphical visualization of the interactions' physical pictures. So that Lamb and a group of the theoretical physicists (including Feynman) became Noble laureates. As the theory and experiments align well with each other, the 'virtual' photon is perhaps not so virtual. It was/is a helpful and natural physical idea to understand interactions of subatomic particles by including a 'virtual' photon as a participating in interaction(s) particle.
Please, look at Feynman diagram - Wikipedia
The ``virtual photon field'' describes the (quantum) fluctuations of the electromagnetic field. The reason these fluctuations explain the Lamb shift is (a) that for any classical electromagnetic field-that acts on a quantum electric charge-the shift is absent and (b) when taking into account the variation of the electromagnetic field that the quantum fluctuations of the electric charge produce, it is found that the energy levels of the electric charge in such an electromagnetic field are not degenerate anymore. The splitting, called ``the Lamb shift'', for the case of the electron, that's bound to the hydrogen atom, is consistent with what is measured. This is explained, for instance, here: http://hitoshi.berkeley.edu/221B-S02/QED.pdf
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