Complex sentence: According to Dirac, the ^ 2S--and ^ 2P--orbital should have same energies, but the interaction between the electron and the vacuum makes some energy shift on ^ 2S --.
Split into simpler sentences: According to Dirac, the ^ 2S--and ^ 2P--orbitals should have the same energies. However, the interaction between the electron and the vacuum causes a tiny energy shift on ^ 2S --.