  A class of spherically symmetric spacetimes invariantly defined by a zero
flux condition is examined first from a purely geometrical point of view and
then physically by way of Einstein's equations for a general fluid
decomposition of the energy-momentum tensor. The approach, which allows a
formal inversion of Einstein's equations, explains, for example, why
spherically symmetric perfect fluids with spatially homogeneous energy density
must be shearfree.
