  Alfven's theorem states that in a perfectly conducting fluid magnetic field
lines move with the fluid without dissipation. When a metal becomes
superconducting in the presence of a magnetic field, magnetic field lines move
from the interior to the surface (Meissner effect) in a reversible way. This
indicates that a perfectly conducting fluid is flowing outward. We point this
out and show that this fluid carries neither charge nor mass, but carries
$effective$ $mass$. This implies that the effective mass of carriers is lowered
when a system goes from the normal to the superconducting state, which agrees
with the prediction of the unconventional theory of hole superconductivity and
with optical experiments in some superconducting materials. The 60-year old
conventional understanding of the Meissner effect ignores Alfven's theorem and
for that reason we argue that it does not provide a valid understanding of real
superconductors.
