Are black holes invisible
Pro Research Analysisby 
Searched over 200M research papers
Are Black Holes Invisible?
Black Holes and General Relativity: Event Horizons and Redshift
In the framework of general relativity, black holes are often considered invisible due to the extreme gravitational effects they exert on light. The event horizon of a black hole is a boundary beyond which nothing, not even light, can escape. This results in an infinitely large redshift of photons attempting to escape from the event horizon to a distant observer, rendering the black hole itself invisible Dokuchaev2019Wilson1973.
Black Hole Shadows: Visible Silhouettes
Despite the inherent invisibility of black holes, their presence can be inferred through the observation of their shadows. The shadow of a black hole is a dark silhouette that appears against a luminous background, created by the gravitational lensing of light around the black hole. This shadow is essentially the projection of the photon capture cross-section on the celestial sphere Dokuchaev2019Wilson1973. The size of the shadow can vary depending on the position of the illuminating background. A maximal shadow is observed when the background is far behind the black hole, while a minimal shadow is seen when the black hole is illuminated by the inner part of its accretion disk Dokuchaev2019Wilson1973.
Observational Evidence: The Case of M87*
The first direct image of a black hole's shadow was captured by the Event Horizon Telescope, showcasing the supermassive black hole M87*. This image revealed the dark silhouette of the southern hemisphere of the event horizon, surrounded by the bright emission from the accretion disk Dokuchaev2019Wilson1973. The brightness of the accretion matter significantly exceeds that of typical astrophysical backgrounds, making the shadow more discernible in such scenarios Dokuchaev2019Wilson1973.
Invisible Black Holes in Binary Systems
Black holes in binary systems can also be indirectly detected through their gravitational influence on companion stars. These systems often exhibit unique signatures such as large mass, small radius, and low luminosity, which are indicative of a black hole presence. For instance, Cygnus X-1 is a well-known binary system where the black hole's presence is inferred from its x-ray emissions and the orbital dynamics of its companion star .
Primordial Black Holes and Dark Matter
Primordial black holes, which may constitute dark matter, are another class of black holes that are effectively invisible. These black holes could span a wide range of masses and, if they have not completely evaporated, might exist as stable relics. Such relics are challenging to detect directly but could potentially be identified through their gravitational effects or interactions with other particles .
Conclusion
While black holes are fundamentally invisible due to their event horizons and the infinite redshift of escaping photons, their presence can be inferred through various indirect methods. The observation of black hole shadows, gravitational effects in binary systems, and potential primordial black hole relics all provide valuable insights into these enigmatic objects. The study of black holes continues to evolve, offering deeper understanding and new challenges in the field of astrophysics.
Sources and full results
Most relevant research papers on this topic