In general relativity, isolated black holes are invisible objects due to their intrinsic physical properties. Nevertheless, the dark silhouettes of astrophysical black holes may be visible to a distant observer as a result of emitting matter lensing in the gravitational field of black holes. The most probable astrophysical conditions for the experimental detection of a dark black hole silhouette are either the stationary luminous background behind the black hole or nonstationary hot matter plunging into the black hole and having emissions approaching the black hole horizon. The black hole shadow is observed in the case of a stationary luminous background, which is a projection on the celestial sphere of the photon capture cross section in the black hole gravitational field. Meanwhile, in the case of nonstationary emitting matter in the inner part of the thin accretion disk adjoining the event horizon, a dark silhouette of the event horizon itself would be seen. This dark silhouette of the event horizon is seen in the first image of the supermasive black hole M87* presented by the Event Horizon Telescope international collaboration. The brightness of accretion matter is much higher than the corresponding one of the usual astrophysical stationary background in the form of numerous stars or extensive hot gas clouds. For this reason, it is improbable that a black hole shadow can be observed in the presence of very luminous accretion matter.

Keywords: gravitation theory, general relativity, black holes, event horizon, gravitational lensing PACS: 04.70.Bw, 98.35.Jk, 98.62.Js (all) DOI: 10.3367/UFNe.2020.01.038717 URL: https://ufn.ru/en/articles/2020/6/d/ 000563842900004 2-s2.0-85092023307 2020PhyU...63..583D Citation: Dokuchaev V I, Nazarova N O "Silhouettes of invisible black holes" Phys. Usp. 63 583–600 (2020) BibTexBibNote ® (generic)BibNote ® (RIS)MedlineRefWorks Received: 5th, November 2019, revised: 4th, December 2019, accepted: 15th, January 2020 Оригинал: Докучаев В И, Назарова Н О «Силуэты невидимых чёрных дыр» УФН 190 627–647 (2020); DOI: 10.3367/UFNr.2020.01.038717