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In general relativity, isolated black holes are invisible due to aninfinitely large redshift of photons propagating from the event horizon to theremote observer. However, the dark shadow (silhouette) of a black hole can bevisible on the background of matter radiation lensed by the gravitational fieldof black holes. The black hole shadow is the celestial sphere projection of thecross section of photon capture by the black hole. If the illuminatingbackground is far behind the black hole (at a distance much greater than theevent horizon radius), a classic black hole shadow of a maximal size can alsobe observed. A minimal-size shadow can be observed if the same black hole isilluminated by the inner part of the accretion disk adjacent to the eventhorizon. In this case, the shadow of an accreting black hole is a lensed imageof the northern or southern hemisphere of the event horizon, depending on theorientation of the black hole spin axis. A dark silhouette of the southernhemisphere of the event horizon is seen in the first image of the supermassiveblack hole M87* presented by the Event Horizon Telescope. The brightness ofaccretion matter is much higher than the corresponding one of the usualastrophysical stationary background in the form of numerous stars or extensivehot gas clouds. For this reason, it is improbable that a black hole shadow canbe observed in the presence of very luminous accretion matter.

Silhouettes of invisible black holes