photon sphere

Definitions

Of a black hole, a spherical limit around a black hole at such distance that any photon meeting it tangentially is constrained to travel in a circular orbit; (more loosely) the region between this limit and the event horizon.

Equivalents

Examples

“The radius of the photon sphere is also the lower bound for any stable orbit.”

“For a Schwarzschild (non-spinning) black hole, the radius of the photon sphere is 1.5 times the Schwarzschild radius.”

“The redshift of light⟳ leaking outward from the photon sphere is #92;sqrt#123;3#125;-1#61;0.732. All light⟳ rays approaching a black hole closer than #92;sqrt#123;3#125; times the radius of the photon sphere spiral inwards and are captured (see⟳ Figure⟳ 13.5).”

“For a Kerr black hole though, one that has spin⟳, the situation is different for the orbits near the black hole. In particular, there are two photon spheres, in contrast with the one photon sphere around a stationary Schwarzschild black hole. The outermost sphere is for photons that are orbiting oppositely to the direction of rotation of the black hole (the ones we say⟳ are on retrograde orbits). Inside this is the photon sphere for photons travelling in the same sense⟳ around the black hole as it is rotating (on prograde orbits). For a very slowly rotating black hole that isn't so very different from a Schwarzschild black hole, these two photon spheres are very nearly co-spatial. For black holes of increasing spin⟳, these surfaces are increasingly further apart.”

“2017, Lori Gardi, The Mandelbrot Set⟳ as a Quasi-Black Hole, David de Hilster (editor), John Chappell Natural Philosophy Society: 2017 Proceedings, John Chappell Natural Philosophy Society, page 65, The lesser known region just outside the black hole is referred to as the photon sphere (outer grey region in Figure⟳ 9). The photon sphere is a region of space just outside the black hole where photons are forced to travel⟳ in complex orbits due to the extreme curvature of space within this region [2]. […] Finally, the boundary that exactly separates the black hole from the photon sphere is referred to as the event horizon (white circle surrounding the black region in Figure⟳ 9).”

“The region inside the photon sphere has a diameter of 1.5× Schwarzschild diameter D(s), but due to the curvature, this region gets magnified to 2.6× D(s).”

CEFR level