  We study how coincident Dirichlet 3-branes absorb incident gravitons
polarized along their world volume. We show that the absorption cross-section
is determined by the central term in the correlator of two stress-energy
tensors. The existence of a non-renormalization theorem for this central charge
in four-dimensional N=4 supersymmetric Yang-Mills theories shows that the
leading term at low energies in the absorption cross-section is not
renormalized. This guarantees that the agreement of the cross-section with
semiclassical supergravity, found in earlier work, survives all loop
corrections. The connection between absorption of gravitons polarized along the
brane and Schwinger terms in the stress-energy correlators of the world volume
theory holds in general. We explore this connection to deduce some properties
of the stress-energy tensor OPE's for 2-branes and 5-branes in 11 dimensions,
as well as for 5-branes in 10 dimensions.
