The event horizon

Assembled from 31 blocks · 3 evidence · 29 related

1. Story
2. The one-way surface of a black hole: the boundary of the region from which no signal, however energetic, can reach a distant observer. It is a feature of spacetime and not an object - there is nothing there to touch, no wall, no density change, and no local experiment an infalling observer could perform to detect the moment of crossing. The horizon is defined globally, by where light rays end up in the infinite future, which means its position depends on the entire future history of what falls in; a horizon can grow outward in anticipation of matter that has not arrived yet. For a non-rotating hole it sits at the Schwarzschild radius; for a rotating one it lies between half that and the full value, shrinking as spin increases. Hawking showed in 1971 that under classical general relativity the total area of horizons never decreases, a result whose formal resemblance to the second law of thermodynamics turned out not to be a coincidence. Not a place on Earth.
3. Knowledge
4. Schwarzschild radius
5. The event horizon
6. The Kerr solution and black hole spin
7. Connections
8. The Kerr solution and black hole spin
9. Schwarzschild radius
10. Black hole entropy
11. Schwarzschild solves the field equations for a point mass
12. Oppenheimer and Snyder compute continued gravitational contraction
13. Penrose proves that collapse to a singularity is generic
14. The term "black hole" enters use
15. GW150914 - two black holes merge and the signal reaches Earth
16. The event horizon
17. The no-hair result
18. Innermost stable circular orbit
19. Kerr finds the rotating solution
20. A mechanism for extracting energy from a spinning black hole
21. Karl Schwarzschild
22. The event horizon
23. The photon sphere and the shadow
24. Time dilation near a horizon
25. Tidal stretching near a black hole
26. Einstein presents the field equations of general relativity
27. Schwarzschild solves the field equations for a point mass
28. Evidence
29. The first exact solution of the Einstein field equations for the vacuum outside a spherical mass, containing the radius now named for its author. V55 VERIFICATION BASIS: the paper was NOT consulted in this session - WebFetch was refused by the network egress proxy for every host attempted and the WebSearch budget was exhausted before authoring began. Title, venue and page range are given from the author's recall of a canonical citation and must be checked.
30. Establishes that geodesic incompleteness follows generically from the formation of a trapped surface, without an assumption of symmetry. V55 VERIFICATION BASIS: not consulted in session; no research tool was available. Volume and page are from recall of a canonical citation and must be checked.
31. Relates the measured ring diameter to the mass of M87*, giving approximately six and a half billion solar masses and testing the shadow prediction of general relativity. V55 VERIFICATION BASIS: not consulted in session; no research tool was available. Volume and article number are from recall; the quoted mass uncertainty is not reproduced in this pack.