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First, the phrase extremely massive objects such as black holes is misleading. Anything small enough to have a surface radius less than is likely to be a black hole, not because it is so massive, but because it is so compact.
Second, the numerical relations quoted hold only for a non-rotating black hole, which is modeled in gtr by the Schwarzschild vacuum solution (compare Kerr solution).
Third, within the photon sphere constant acceleration will allow a spacecraft or probe to hover above the event horizon is misleading in the context of reference to "spaceprobes" (compute the magnitude of acceleration required for a stellar mass black hole).
Fourth, the orbits (null world lines) in question are unstable (to see this, plot the effective potential, as in any standard textbook on gtr). This means that contrary to what the article implies, photons or radio pips cannot really be "injected" into such an orbit the way that a spaceprobe can be "injected" into a desired orbit around Venus, say.---CH 15:18, 14 April 2006 (UTC)
A bit of a conflict between information here and other sources (including other Wikipedia pages). It says "The photon sphere is located further from the center of a black hole than the event horizon and ergosphere."
For a non-rotating black hole, the photon sphere extends from the event horizon to a distance of 0.5 Schwarzschild radius from it, that is, the outer boundary of the photon sphere is at 1.5 Schwarzsfield radii from the singularity.
However, the Wikipedia page on the Ergosphere says "The ergosphere is ellipsoidal in shape and is situated so that at the poles of rotating black hole it touches the event horizon and stretches out to a distance that is equal to the radius of the event horizon." This page can be found here:
[[1]]
So according to this, the "equatorial bulge" of the ergosphere reaches to 2 Schwarzschild radii from the singularity, while the photon sphere only extends to 1.5 Schwarzschild radii. Obviously, the ergosphere extends farther than the photon sphere, contradicting the first statement, namely "The photon sphere is located further from the center of a black hole than the event horizon and ergosphere."
The situation is not clear cut, since the 1.5 Schwarzschild radius size of the photon sphere is for a non-rotating black hole, whereas the ergosphere is a feature of rotating black holes. So this needs some clarification. Is the photon sphere larger for rotating black holes? What is the nature of the photon sphere in rotating black holes that justifies the statement "The photon sphere is located further from the center of a black hole than the event horizon and ergosphere". —Preceding unsigned comment added by 67.176.200.23 (talk) 18:02, 20 June 2009 (UTC)
Was it me http://www.scienceforums.net/showthread.php?t=22400 I made that post quite some time ago, having not read this article. Alan2here 10:54, 30 September 2006 (UTC)
If there is a plenitude of black holes in a given volume of space, wouldn't a photon follow an erratic course around them, or between them?
Similarly, why is it assumed that the event horizon of a single black hole will be perfectly circular? It was my understanding that the event horizon was anything but perfectly circular, but rather vibrating and turbulent due to quantum effects.198.177.27.21 (talk) 07:17, 1 August 2008 (UTC)
I believe that if we are going to describe such object as black hole we shall use only GR as mathematical argument for our deductions. Unfortunately there is still "classical gravity" step in derivation of photon sphere radius, where it is found "speed of light on it's orbit". So I'm going to edit it in purely GR way. —Preceding unsigned comment added by Goykhman (talk • contribs) 09:47, 11 May 2009 (UTC)
"In the standard electromagnetic and gravitational resonator approach used the following photon radius:"
Someone know what this is supposed to say? LokiClock (talk) 17:45, 14 September 2009 (UTC)
... a general circular orbit can have a varying theta, as long as r remains constant. Right?
Suggest merging with the article on gravitational lensing; the photon sphere is really a special case of the general light-bending behaviour around a spherically symmetric mass. 130.56.71.50 (talk) 16:00, 23 September 2009 (UTC)
Isn't it true that, from the point of view of a photon, the entire universe looks like a singularity? A photon travels EVERYWHERE in the universe in ZERO time from its own point of view. So doesn't that mean that from a photon's point of view, the universe is a singularity? 68.200.98.166 (talk) 01:33, 25 December 2009 (UTC)
Mike Rosoft (talk) 09:47, 24 August 2010 (UTC)
From the statement: "There are no stable free fall orbits that exist within or cross the photon sphere. Any free fall orbit that crosses it from the outside spirals into the black hole. Any orbit that crosses it from the inside escapes permanently. "
How can you cross the orbit from the inside? That would require escaping from the event horizon.
Silenceisgod (talk) 23:52, 3 October 2015 (UTC)
The article currently states that photon orbits "are rarely stable in the long term." Well, no orbit goes on forever, and "long term" can mean different things in different contexts. I think the average term or term range, and aspects such as how the mass of the black hole affects these calculations, should be more precise. --Ayeroxor (talk) 14:18, 16 November 2016 (UTC)
The article currently says, "As photons approach the event horizon of a black hole, those with the appropriate energy avoid being pulled into the black hole by traveling in a nearly tangential direction known as an exit cone. A photon on the boundary of this cone does not possess the energy to escape the gravity well of the black hole." This is just wrong. The trajectory of a photon near a black hole has nothing to do with its energy. The geodesic equation has uniquely defined solutions for a given initial position and initial four-velocity. Two photons with different energies will still have the same trajectories if they start at the same position and with the same initial four-velocity. I've deleted the two sentences in question, since they're false, but that is going to disrupt the flow of the article. Maybe the word "energy" just needs to be replaced with "four-velocity," but I can't read the mind of the person who wrote this and tell what they had in mind or whether it made sense.--76.169.116.244 (talk) 17:43, 19 April 2019 (UTC)