The photon sphere and the shadow

Assembled from 20 blocks · 2 evidence · 16 related

1. Story
2. The radius at which light itself can orbit. For a non-rotating black hole it sits at one and a half times the Schwarzschild radius, and it is unstable: a photon there falls in or escapes at the slightest perturbation. This surface, not the horizon, is what an image of a black hole actually outlines. Rays passing near it are bent so strongly that a distant observer sees a dark region whose angular radius corresponds to the square root of twenty-seven times GM over c-squared - a shadow whose diameter is about 5.2 Schwarzschild radii, roughly 2.6 times the diameter of the horizon inside it. The bright rim is a photon ring built from light that looped one or more times around the hole before escaping; successive sub-rings are exponentially thinner and fainter, by a factor close to e-to-the-pi for the non-rotating case. Because the shadow diameter depends only on mass and distance, measuring it measures the mass. Not a place on Earth.
3. Knowledge
4. Schwarzschild radius
5. The photon sphere and the shadow
6. Connections
7. Schwarzschild radius
8. The first image of a black hole shadow is published
9. The black hole at the centre of our own galaxy is imaged
10. A second epoch shows the ring persists and the bright spot moves
11. Karl Schwarzschild
12. The event horizon
13. The photon sphere and the shadow
14. Time dilation near a horizon
15. Tidal stretching near a black hole
16. Einstein presents the field equations of general relativity
17. Schwarzschild solves the field equations for a point mass
18. Evidence
19. 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.
20. Reports the first image of a black hole shadow, an asymmetric ring about 42 microarcseconds across, together with the array, observing campaign and imaging methodology. V55 VERIFICATION BASIS: not consulted in session; WebFetch to iopscience.iop.org and eventhorizontelescope.org was refused by the egress proxy and no search could be issued. Volume and article number are from recall of a canonical citation.