World-volume approach to absorption by non-dilatonic branes

We calculate classical cross sections for absorption of massless scalars by the extremal 3-branes of type IIB theory, and by the extremal 2-and 5-branes of M-theory. The results are compared with corresponding calculations in the world-volume effective theories. For all three cases we find agreement in the scaling with the energy and the number of coincident branes. For 3-branes, whose stringy description is known in detail in terms of multiple D-branes, the string theoretic absorption cross section for low-energy dilatons is in exact agreement with the classical gravity. This suggests that scattering from extremal 3-branes is a unitary process well described by perturbative string theory.