Abstract
The best evidence for black holes comes from dynamical studies of a few X-ray binary systems that contain massive (M>3M 0) compact stars. Unfortunately, the evidence is not decisive; in particular, this argument for black holes depends on the presumption that general relativity is the correct theory of strong gravity. If we hope to advance our position further, we must attempt to make clean quantitative measurements of relativistic effects that occur in the near vicinity of neutron stars and black holes. It appears unlikely that peculiar temporal or broadband spectral phenomena (e.g., ms flickering, steep spectra, etc.) can be used to test general relativity to prove that black holes exist; these phenomena are too confused by erratic magnetohydrodynamic effects. Two potential tests of general relativity are discussed: (1) The discovery of a 4.1 keV line from a neutron star suggests that strong-field spectroscopic tests of the equivalence principle and general relativity may be possible in the 1990s. (2) The polarization features of black-hole disk radiation can be affected strongly by general-relativistic effects; it may be possible to determine the metric of a black hole by measuring the change in polarization direction with energy. Our working hypothesis is that black holes exist and are described by general relativity; we are obliged to show that this hunch is correct.
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