Weinberg’s (1985) perturbative calculation of the dynamical friction by halos on bars predicts that bars will be slowed down very quickly. Previous numerical experiments (e.g. Hernquist & Weinberg 1992) have obtained results that are in agreement with Weinberg’s calculation.

Observations, however, suggest that most bars are rotating rapidly. (In this context, a bar is slow if the major axis Lagrange point is far outside the bar, while a fast bar ends at, or near, this Lagrange point.) Neither Weinberg’s calculation nor the numerical experiments have, to the present date, been fully self-consistent, so possibly important effects such as resonance capture and the back reaction on the bar were not included.