Talk:Photon sphere

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[edit] Some corrections

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)

Maybe messive is ment to mean "Lots of mass" (as in weight) as opposed to "Lots of size"? Alan2here 15:13, 20 November 2006 (UTC)

Alan2here, the short answer is "no". For black holes, mass is size.

I've added a disputed tag to the article, CH is completely corect; this article has numerous faults. linas 00:57, 27 January 2007 (UTC)

I changed "small and massive" to "extremely compact". Thought that was better; micro black holes aren't massive, and galactic black holes aren't small. --Alvestrand 13:01, 2 December 2007 (UTC)

[edit] Who thought of this first?

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)

The idea of photons orbiting a black hole must be nearly a century old. Its a standard treatment in standard textbooks on GR. linas 00:59, 27 January 2007 (UTC)

TY for info :¬(