  Huygens' principle following from the d'Alembert wave equation is not valid
in two-dimensional space. A Schrodinger particle of vanishing angular momentum
moving freely in two dimensions experiences an attractive force - the quantum
anti-centrifugal force - towards its center. We connect these two phenomena by
comparing and contrasting the radial propagators of the d'Alembert wave
equation and of a free non-relativistic quantum mechanical particle in two and
three dimensions.
