  Quantum fluctuations of the electromagnetic field are known to produce the
atomic Lamb shift. We here reveal their iconic signature in semiconductor
physics, through the blue-shift they produce to optically bright excitons, thus
lifting the energy of these excitons above their dark counterparts. The
electromagnetic field here acts in its full complexity: in addition to the
longitudinal part via interband \textit{virtual Coulomb} processes, the
transverse part -- which has been missed up to now -- also acts via resonant
and nonresonant \textit{virtual photons}. These two parts beautifully combine
to produce a bright exciton blue-shift independent of the exciton wave-vector
direction. Our work readily leads to a striking prediction: long-lived excitons
must have a small bright-dark splitting. Although the analogy between exciton
and hydrogen atom could lead us to see the bright exciton shift as a Lamb
shift, this is not fully so: the atom shift entirely comes from virtual
photons, whereas the Coulomb interaction also contributes to the exciton shift
through the so-called "electron-hole exchange".
