Talk:Apparent horizon

This article needs attention from an expert in Physics.
"This article needs attention from an expert in Physics." As someone who is an expert in nothing, this is a somewhat disconcerting way to start a page when I have accessed it in pursuit of knowledge. Is there any way I (or anyone who wants to) can help? Else this statement seems like a relatively hopeless plea which has truly bummed me out on an already rock-bottom, sunny side up rotation of my planet. — Preceding unsigned comment added by 195.59.199.204 (talk) 13:44, 26 April 2016 (UTC)
 * As a physicist, but not an expert in the field I immediately recognize this as a candidate for a complete shit written by another baseball journalist. I don't need a warning, but you do. I am not sure, however. I came here because I met this term somewhere else and wanted to know the meaning. --Дядько Ігор (talk) 17:35, 23 June 2017 (UTC)

Errors
This article is wrong, or at least very incomplete/oversimplified. For example the Kerr solution has multiple horizons which have different meanings to different observers. These are not just "apparent" horizons, but have real meanings, and they are all a kind-of-like "absolute horizons"... except that the WP article on absolute horizon is incorrect in the same ways. linas 06:00, 13 January 2007 (UTC)
 * I splated on a disclaimer: See, however the articles on ergosphere, Cauchy horizon, the Reissner-Nordström solution, photon sphere, Killing horizon and naked singularity; the notion of a horizon in general relativity is subtle, and depends on fine distinctions. as a short term fix. linas 01:28, 25 January 2007 (UTC)

The article seems to be inaccurate at best. seems to provide a decent overview of the subject of alternative horizon definitions including alternative horizons. (TimothyRias (talk) 09:27, 3 April 2008 (UTC))

Re. the paragraph
 * "Now we look at light rays that are directed outward, along these normal vectors. The rays will either be diverging (the usual case one would expect) or converging. Intuitively, if the light rays are converging, this means that the light is moving backwards inside of the ball. If all the rays around the entire surface are converging, we say that there is a trapped null surface."

A Schwarzschild black hole being symmetrical, light rays directed along the normal vectors must all be radial and divergent. Taken with the above, this implies to me that there are no trapped null surfaces. I don't think this is what the author intended. --Dendropithecus (talk) 11:40, 22 March 2010 (UTC)

Reading this, and lacking any means of visualising the surface(s) referred to, several questions occur to me. One question is normal to what? The second question is "which way is outward". The simple answer (clearly not the right one) is to visualise a sphere with outwardly directed radial vectors. Another question is: what is meant by the phrase "all the rays are converging"? This is also difficult to visualise. I'm sure the author knows what s/he means, as may others here, but I think it unlikely that anyone unfamiliar with the territory could gain any insight from this article as it stands. This sorely needs a diagram or two and some rephrasing or expansion also. --Dendropithecus (talk) 02:53, 25 March 2010 (UTC)

Assessment comment
Substituted at 08:06, 29 April 2016 (UTC)

Removed "observer-dependent"
I spoke to two experts in general relativity, and neither of them understood exactly what it means for an apparent horizon to be "observer-dependent," so I removed those statements. If anyone adds them back in, please clarify what you mean.Ted.tem.parker (talk) 21:00, 3 April 2017 (UTC)ted.tem.parker

Bob Geroch quote
The phrase:

"Bob Geroch has pointed out that if all the stars in the Milky Way gradually aggregate towards the galactic center while keeping their proportionate distances from each other, they will all fall within their joint Schwarzschild radius long before they are forced to collide."

seems out of place. The appearance of the thought experiment would not be, as suggested, a denser galaxy. The Milky Way galaxy has a Schwarzschild radius of about a quarter of a light year. As the galactic matter gradually packs more and more densely, the gravitational lensing focuses, giving more and more the appearance of a supermassive black hole to a distant observer. Close to the limit of crossing the last stars into the Schwarzschild radius of the whole Milky Way, the extremely dense object would resemble one large near extremal neutron star. The final stage before the formation of a black hole would be the violent collapse of the interior as the condensed matter no longer supported itself.

Is there a quote on Bob Geroch stating this, can anyone find a source on this thought experiment explaining how exactly to form an event horizon without creating the appearance of a black hole? (Edit: I clarify that the definition of an apparent horizon is clear, what is unclear is that an event horizon may precede, in addition to the apparent horizon, the appearance itself of being near extremal to a distant observer) 85.31.132.229 (talk) 09:55, 20 April 2021 (UTC)