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The Photon Sphere
Mathematically speaking, the photon sphere occurs at 3/2 the Schwarzschild Radius.
The photosphere is a place where light rays can have unstable orbits around the black hole. To have a stable orbit the light ray has to travel at a high velocity (c= 3*10 ^8 m/s) as it gets nearer and nearer to the center of the black hole. As it gets farther from the center it doesn?t need such a high velocity. The farther the ray is from the center of the hole, the less the velocity of the ray. A light ray would have a hard time to keep its constant orbit, as there are other rays colliding with it and changing its velocity and trajectory, thus shifting it either into outer space or straight into the black hole. The idea is that in the photosphere nothing is stable: everything either is drawn at incredible speeds in the interior of the black hole or plunged into space but delayed for a few thousands of years.
Another interesting phenomenon is the movement of space. If one had a source of light, let?s say a reflector, at the black hole, then the light rays would start orbiting the hole. If the ray comes back after one rotation it would hit the reflector. Now if one would have a camera and try to move the camera towards the image forming in front of it, then the image would retreat and shrink. This is because the view of the camera is just like the view someone would have if they were standing behind it. If the camera stands behind the person and he would start moving forward, their image recedes.
Movie: circling the black hole from the photon sphere Explanation: The apparent position of the photon sphere is easy to spot - it is the apparent dividing line between black hole and sky. Stars approaching the other side of the black hole from your point are greatly magnified and move with high angular speeds. You can only see one star moving very fast and get very brighter, and then another one appears.
"After one passes the event horizon, the obvious question is what happens. At the center of the black hole (actually there is no real center) the singularity plays its role. It is the point where all the mass is condensed at infinite density and zero volume. All objects which pass through the event horizon are then inevitably drawn towards the singularity and they hit it in a very short amount of time."
"infinite density and zero volume"
The sound of one hand clapping
A Loud Egg
Brown is the new Black