Physical observability of horizons

Abstract

Event horizons are (generically) not physically observable. In contrast, apparent horizons (and the closely related trapping horizons) are generically physically observable --- in the sense that they can be detected by observers working in finite-size regions of spacetime. Consequently event horizons are inappropriate tools for defining astrophysical black holes, or indeed for defining any notion of evolving}black hole, (evolving either due to accretion or Hawking radiation). The only situation in which an event horizon becomes physically observable is for the very highly idealized stationary or static black holes, when the event horizon is a Killing horizon which is degenerate with the apparent and trapping horizons; and then it is the physical observability of the apparent/trapping horizons that is fundamental --- the event horizon merely comes along for the ride.

V1: 4 pages; V2: added 4 references; no physics changes; now 5 pages; V3: added another 6 references; added 2 figures; reformatted; some rephrasing; no physics changes; now 6 pages

References in corpus (11)

• Advection-Dominated Accretion and the Black Hole Event Horizon
• Black hole evaporation: A paradigm
• Minimal conditions for the existence of a Hawking-like flux
• Hawking-like radiation from evolving black holes and compact horizonless objects
• Hawking-like radiation does not require a trapped region
• The region with trapped surfaces in spherical symmetry, its core, and their boundaries
• A Note on trapped Surfaces in the Vaidya Solution
• Black hole evaporation without an event horizon
• Holographic Space-Time Does Not Predict Firewalls
• Comment on the black hole firewall
• A Massless Firewall

Cited by in corpus (15)

• Foliation dependence of black hole apparent horizons in spherical symmetry
• Black Hole Complementarity in Gravity's Rainbow
• Cosmological black holes and white holes with time-dependent mass
• Do Black Holes Exist?
• Coarse graining Shannon and von Neumann entropies
• Do event horizons exist?
• Killing horizons and surface gravities for a well-behaved three-function generalisation of the Kerr spacetime
• Cosmology in Painleve-Gullstrand coordinates
• A Critical Assessment of Black Hole Solutions With a Linear Term in Their Redshift Function
• General class of "quantum deformed" regular black holes
• Multi-partite analysis of average-subsystem entropies
• Imaging fermionic dark matter cores at the center of galaxies
• The effective Tolman temperature in curved spacetimes
• From mindless mathematics to thinking meat?
• Alternative characterization of a black hole boundary