Abstract
Abstract Early observations of black holes, before the LIGO/Virgo detection of gravitational waves, were made by observing electromagnetic processes involving atomic spectral lines. X-ray binary systems were observed consisting of a progenitor star such as a neutron star and a dark companion. X-rays emitted from the gas accreting the dark companion tells us whether it is a black hole. Evidence indicated supermassive black holes at the centers of galaxies. From observations of orbits of stars near the supermassive black holes, one could determine their masses, which proved they were black holes. Observations of quasars, among the brightest objects in the universe, showed they contain black holes. It is important to establish the existence of an event horizon with the black hole, as predicted by general relativity. The current evidence for the event horizon is circumstantial, based on controversial theoretical models about the accretion disks surrounding the collapsed dark objects.
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