  We show how the "proton radius puzzle" emerging from the measurement of the
Lamb shift in muonic hydrogen may be solved by means of a binding energy
contribution due to an effective Yukawian gravitational potential related to
charged weak interactions. The residual discrepancy from the experimental
result should be mainly attributable to the need for the experimental
determination of the gravitational radius of the proton. The absence of an
analogous contribution in the Lamb shift of electronic hydrogen should imply
the existence of generation-dependent interactions, corroborating previous
proposals. Muonic hydrogen plays a crucial role to test possible scenarios for
a gravitoweak unification, with weak interactions seen as manifestations of
quantum gravity effects at the Fermi scale.
