  To change the velocity of an electron requires that a Lorentz force acts on
it, through an electric or a magnetic field. We point out that within the
conventional understanding of superconductivity electrons appear to change
their velocity in the absence of Lorentz forces. This indicates a fundamental
problem with the conventional theory of superconductivity. A hypothesis is
proposed to resolve this difficulty. This hypothesis is consistent with the
theory of hole superconductivity.
