View Full Version : What is the path for light leaving a near black-hole object?

2015-Jul-03, 01:10 AM

Imagine a collapsing massive object just a tiny bit bigger that its Schwarzschild radius; maybe just a plank's length bigger, or whatever; anyway tiny!

Would this be anything like one of the paths for light leaving the massive object?
Or what would be the various paths be for light emitted at various angles from the surface?

2015-Jul-03, 03:53 AM
For light leaving radially away(straight out), it would be a straight line.

There would definitely be a critical angle(guessing) where the light ray would end up back on the star's surface. I say this because of the photon sphere, whose radius is 1.5x that of the Schwarzschild radius. Any ray of light must either exit that sphere to continue its existence, or stay within it doomed to hit the star's surface.

Jeff Root
2015-Jul-07, 07:14 AM
If a photon is rising, getting farther away from the black hole,
then it will escape. If a photon is inside the photon sphere and
falling, getting closer to the black hole, then it will fall in. Only
outside the photon sphere can a photon be falling yet eventually
start rising again and escape. All photons inside the event
horizon are falling and doomed. Photons can thus spiral in
close to the photon sphere, then spiral out again and escape,
but photons crossing downward through the photon sphere
from outside are doomed. Only photons emitted below the
photon sphere can rise upward through it.

If the mass of the body is such that it has a photon sphere but
no event horizon, then light emitted from the surface at a high
angle will escape, but light emitted at a low angle will never
move upward away from the surface at all.

-- Jeff, in Minneapolis