  Within the conventional theory of superconductivity superfluid carriers
respond to an applied magnetic field and acquire a speed according to their
effective (band) mass. On the other hand it can be shown theoretically and is
confirmed experimentally that the mechanical momentum of the supercurrent
carriers is given by the product of the supercurrent speed and the $bare$
electron mass. By combining these two well-established facts we show that the
conventional BCS-London theory of superconductivity applied to Bloch electrons
is $internally$ $inconsistent$. Furthermore, we argue that BCS-London theory
with Bloch electrons does not describe the phase rigidity and macroscopic
quantum behavior exhibited by superconductors. Experimentally the speed of the
supercurrent in superconductors has never been measured and has been argued to
be non-measurable, however we point out that it is in principle measurable by a
Compton scattering experiment. We predict that such experiments will show that
superfluid carriers respond to an applied magnetic field according to their
bare mass, in other words, that they respond as $free$ $electrons$, undressed
from the electron-ion interaction, rather than as Bloch electrons. This is
inconsistent with the conventional theory of superconductivity and consistent
with the alternative theory of hole superconductivity. Furthermore we point out
that in principle Compton scattering experiments can also detect the presence
of a spin current in the ground state of superconductors predicted by the
theory of hole superconductivity.
