Talk:Photon sphere

From Wikipedia, the free encyclopedia

	This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.	Physics Portal
Stub	This article has been rated as Stub-Class on the assessment scale.
???	This article has not yet received an importance rating within physics.
Help with this template This article has been rated but has no comments. If appropriate, please review the article and leave comments here to identify the strengths and weaknesses of the article and what work it will need.

This article has been automatically assessed as Stub-Class by WikiProject Physics because it uses a stub template. If you agree with the assessment, please remove {{Physics}}'s auto=yes parameter from this talk page. If you disagree with the assessment, please change it by editing the class parameter of the {{Physics}} template, removing {{Physics}}'s auto=yes parameter from this talk page, and removing the stub template from the article.

[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)

[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)