  We point out that the Meissner effect, the process by which a superconductor
expels magnetic field from its interior, represents an unsolved puzzle within
the London-BCS theoretical framework used to describe the physics of
conventional superconductors, because it appears to give rise to
non-conservation of angular momentum. Possible ways to avoid this inconsistency
within the conventional theory of superconductivity are argued to be
farfetched. Consequently, we argue that unless/until a consistent explanation
is put forth, the existence of the Meissner effect represents an anomaly that
casts doubt on the validity of the conventional framework. Instead, we point
out that two elements of the unconventional theory of hole superconductivity,
namely that the charge distribution in superconductors is macroscopically
inhomogeneous, and the fact that spin-orbit coupling plays an essential role,
allow for a consistent explanation of the Meissner effect.
