  The analisis of Pippard \cite{pip} for the growth of the normal phase into
the superconducting phase in the presence of a magnetic field $H>H_c$ is
applied in reverse to the case $H<H_c$ ($H_c=$critical magnetic field). We
carry out the analysis both for a planar and a cylindrical geometry. As the
superconducting phase grows into the normal phase, a supercurrent is generated
at the superconductor-normal phase boundary that flows in direction opposite to
the Faraday electric field resulting from the moving phase boundary. This
supercurrent motion is in direction opposite to what is dictated by the Lorentz
force on the current carriers, and in addition requires that mechanical
momentum of opposite sign be tranferred to the system as a whole to ensure
momentum conservation. In the cylindrical geometry case, a macroscopic torque
of unknown origin acts on the body as a whole as the magnetic field is
expelled. We argue that the conventional BCS-London theory of superconductivity
cannot explain these facts, and that as a consequence the Meissner effect
remains unexplained within the conventional theory of superconductivity. We
propose that the Meissner effect can only be understood by assuming that there
is motion of charge in direction perpendicular to the normal-superconductor
phase boundary and point out that the unconventional theory of hole
superconductivity describes this physics
