Edge of a black hole
Pro Research Analysisby 
Searched over 200M research papers
Understanding the Edge of a Black Hole
Defining the Inner Edge of a Black Hole Accretion Disk
Innermost Stable Circular Orbit (ISCO)
The concept of the "inner edge" of a black hole's accretion disk is multifaceted and varies based on the property defining the edge. For black holes with very low luminosities, the inner edge of the accretion disk, often referred to as the "radiation inner edge," coincides with the innermost stable circular orbit (ISCO). This is the smallest orbit in which matter can stably orbit the black hole without falling in 13.
Multiple Definitions of the Inner Edge
However, as the luminosity increases, the notion of a single inner edge becomes less clear. Different definitions of the inner edge, such as the "turbulence edge," "stress edge," "reflection edge," and "radiation edge," can be located at different radii. These edges are influenced by factors like magnetic field structure, dynamical contact with the outer accretion flow, and X-ray reflection features. These variations become more pronounced at higher luminosities, making the concept of a unique inner edge impractical 135.
The Photon Ring and Black Hole Shadow
Photon Ring
The image of a black hole, such as the one captured by the Event Horizon Telescope of the supermassive black hole in M87, reveals a bright, unresolved ring. This ring contains a series of increasingly narrow subrings, known as the "photon ring," which are formed by photons orbiting the black hole multiple times. These subrings approach the edge of the black hole's shadow and become exponentially narrower and weaker with each orbit .
Black Hole Shadow
The black hole shadow is a distinct visual feature that appears as a sharp-edged dip in brightness, coinciding with the projection of the black hole's unstable-photon region. This shadow is created by two key mechanisms: the blocking of rays that intersect the unstable-photon region and the path-lengthening of rays that travel along extended, horizon-circling paths. The shadow is a robust and model-independent observable for accreting black holes, especially in low-luminosity active galactic nuclei 47.
Edge States and Observables in Black Hole Physics
Edge States in Gravity
In the context of canonical gravity, the removal of a spatial region around a black hole leads to the appearance of edge states localized at the boundary. These edge states can contribute to the black hole's entropy and are accessible only to certain observers, highlighting a "complementary principle" in black hole physics. The coupling of these edge states to the bulk states of the gravitational field demonstrates their significance in understanding black hole entropy and dynamics .
Fractal Structure and Black Hole Area
Introducing a fractal structure to the horizon geometry of a black hole, such as a three-dimensional spherical analogue of a "Koch Snowflake," can significantly alter the black hole's area and entropy. This model suggests that the entropy of black holes and the universe could be much larger than previously thought, with implications for black hole thermodynamics and the Hawking lifetime of black holes .
Conclusion
The edge of a black hole is a complex and multifaceted concept, influenced by various physical properties and observational definitions. From the ISCO to the photon ring and black hole shadow, each aspect provides unique insights into the nature of black holes. Additionally, edge states and fractal structures offer deeper understanding of black hole entropy and dynamics, highlighting the intricate and evolving nature of black hole physics.
Sources and full results
Most relevant research papers on this topic