Spacetime Paths Near the Black Hole (3) ...
The adjacent animation by Robert Nemiroff
illustrates another mind-boggling aspect of the spacetime geometry
near a black hole.
The Photon Sphere
In this simulation, your spacecraft orbits at the photon sphere of the black hole.
This is defined
to be the radius at which light could be put into a circular orbit around the black hole. Thus, at the photon
sphere you should (with a large enough telescope) be able to see the back of your head, since light reflected
from the back of your head can orbit completely around
the black hole and strike your eyes from the front. (Such an experiment is impossible
to perform, of course, but the principle is correct!)
The photon sphere is not the same as the event horizon, which lies below it. For a spherical
black hole
the photon sphere lies at 1.5 times the radius of the event horizon.
You can (in principle) still escape the black hole from the
photon sphere, though you will need a lot of energy to do so.
In the above animation, the photon
sphere
lies at the apparent boundary between the black hole below and the sky above.
It is completely
dark because any light from distant stars coming from an angle that would cause the light to pass through the
photon sphere is sucked into the black hole.
Light at the Photon Sphere
It is clear from the animation that the sky appears to move in very strange ways at the photon sphere.
For example, if you look carefully you will see a thin line just above the photon sphere
that divides star images moving toward you from those moving away. This dividing line is called an
Einstein ring. The images above and below it are different copies of the same stars whose light
has orbited around the black hole different numbers of times. For example, you will sometimes see
a star approach you above the ring, and as it appears to pass you suddenly another copy of the image
is seen below the ring receding from you. (There is also an Einstein ring in the
earlier animation of circling a black hole. It
divides images revolving around the hole in one direction from copies of the same images
revolving in the opposite direction.)
Here is a more detailed
description of the photon sphere animation.
Animation:
photon paths near a black hole
