Talk:Event horizon: Difference between revisions

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Other types of event horizon

There needs to be a discussion of event horizons produced by the expansion of spacetime. As it is, the article is too specific. (--User:24.43.40.79, 1 June 2002)

Expanding FTL

When two objects expand faster than the speed of light relative to each other

Hmm? I thought this was impossible. All speed is relative, and nothing travels faster than light. Am I missing something? Evercat 22:46 16 May 2003 (UTC)

We don't actually know that nothing travels faster than c. We know that a body traveling at less than c with respect to another body cannot be accelerated such that its velocity with respect to the second object reaches c. It is quite possible that there are bodies which always travel faster than c; however this makes them fundamentally unobservable and thereby a moot point. - Che Nuevara: ^{Join the Revolution} 21:51, 18 May 2006 (UTC)[reply]

Relative to each other. So one is going half the speed of light in one direction and the other is going slightly more than half the speed of light in the other. The distance between them is expanding at more than the speed of light. J.S. Nelson 18:43, 22 Jul 2004 (UTC)

This is not quite correct; you can fire a bullet one way at 90% C, and a second the other way at 90% C, and a camera on one bullet will still see the other receding at less than C. The type of expansion that occurs within the hole is usually described to non-physicists as space itself expanding in such a way that the relative motion of two swatches of space exceeds C. A more accurate description would simply be to say that spacetime is curved in such a way that the future light cones produced from two points sufficiently separated will never intersect each other (space with very strong negative curvature). --Christopher Thomas 06:45, 21 Jun 2005 (UTC)

That section didn't explain why the future event horizon forms before the light behind. Does the accelerating particle create space? Would the particle then be invisible from a distance, even though the distance appears very finite? lysdexia 00:32, 20 Oct 2004 (UTC)

Sticking a hand through an event horizon

When he sticks out his hand, the tidal force (the difference between body and hand along his arm) also becomes infinitely high, so his hand will be chopped off before he manages to do so

According to the information found in Black hole this is not correct, as the distance from the singularity at which the tidal forces become lethal may as well be shorter than the Schwarzschild-radius given a large enough black hole! (--User:62.218.132.134, 26 Feb. 2005)

The infinite shearing force is a consequence of the fact that the astronaut is at rest with respect to a distant observer, rather than infalling. A freely falling astronaut would indeed experience little to no tidal forces for a sufficiently large hole. What the shearing example is actually trying to demonstrate is that it's not possible to remain stationary with respect to a distant observer while at the level of the horizon. --Christopher Thomas 06:45, 21 Jun 2005 (UTC)

Light emitted from inside an event horizon

"Light emitted from inside the event horizon will never reach a stationary observer outside the horizon, hence the name black hole" Um....This sentence just pops in out of nowhere. Nothing is discussed about it being in some way related to black hole. "[H]ence the name black hole" sounds more like a definition of "black hole" than "event horizon". I think somewhere before this sentence should be discussed how an event horizon is related to a black hole. I would, but I have very little knowledge on the subject. -- Jwinters | Talk 20:31, 21 Jun 2005 (UTC)

The article looks like it's been through an editing pass and is worse for wear. From what I can tell, it originally talked exclusively about black hole event horizons, and was expanded to discuss other types of event horizon. I'll take a crack at rewriting it, but not just yet. --Christopher Thomas 20:36, 21 Jun 2005 (UTC)

Pronoun choice

Why must the observer be male? Sorry to be such a PC nuisance. But women are just as able to witness an event by way of EM radiation detection. --njmayhall 04:45, 22 Dec 2005

No, they're not. The inherent inferiority of women deprives them of this ability. [Just kidding!!!] :-) Actually, this is the result of a defect (a fairly serious one, in my opinion) in the otherwise excellent English language, which is that there is no pronoun for a single person of undetermined gender. According to Strunk & White's The Elements of Style, the classic handbook on writing in English, the masculine pronoun (i.e., "he" or "his") should be used. I don't like it any better than you do, but until someone comes up with a suitable substitute, we're stuck with it. :-/

Naked singularities

The following text was added to the end of the "sticking your hand through an event horizon" section by 207.250.204.185 (talk · contribs). I've moved it here, as while it attempts to comment on Hawking's recent results, on closer inspection it doesn't actually make an analogy that makes sense. --Christopher Thomas 18:11, 12 January 2006 (UTC)[reply]

(Begin quoted text.)

A useful way of thinking of this can perhaps be construed by extrapolation from the saying that "Nature abhors a naked singularity..." and thus -clothes- it with an event horizon. Hawking's statement would appear to postualte that instead of -clothing- the singularity with an event horizon, Nature merely -blurs- it, much like modern media pixelates images of nudity.

(End quoted text.)

Article cleanup tag

I've added an "expert attention" cleanup tag to the article, for two reasons:

As noted in previous threads, this article has trouble deciding whether it's about black hole event horizons or the more general concept of an event horizon in GR. It needs to be substantially reorganized to present this in an orderly manner.
Recent edits by Supersexyspacemonkey (talk · contribs) added some material regarding escape velocities that I think is questionable (based on discussions at Talk:Black hole. Without singling out any particular edits, I think this article is overdue for a fact-check by one of the GR physicist lurkers, and adding the tag will get it on the "pages needing attention" list where it can be seen.

--Christopher Thomas 19:27, 26 April 2006 (UTC)[reply]

readable

Could we please make this article a little more understandable for the sake of those of us who don't know squat about event horizons? The definition of "event horizon" should be: "An event horizon is the point at which gravity starts pulling at a stationary observer." For example, every planet has an event horizon because there is a point where gravity will start tugging at passing objects. If you get close enough to the earth, you will eventually fall down to its surface because you have passed its "event horizon". Can we take off the "boundary in spacetime" stuff? Some users probably don't even know what spacetime is (and if you'd like to explain it please do so in a simple way and not as if you were talking to a fellow physicist or astronomer)! Scorpionman 16:25, 23 June 2006 (UTC)[reply]

Actually, this is not an accurate definition of event horizon. An event horizon is the boundary beyond which events cannot affect an observer. Cleaning up this article is on my to-do list, and with the cleanup tags it'll be showing up on PNA/Physics as well. Beyond that, it'll get cleaned up when someone has the free time to do a thorough fact-check and rewrite. --Christopher Thomas 20:03, 23 June 2006 (UTC)[reply]

I've started a rewrite on a scratch page. I'll post a link when it's ready for review. --Christopher Thomas 21:26, 23 June 2006 (UTC)[reply]

Proposed rewritten version

I've finished a draft of a proposed rewritten version of this article. It's at Event horizon/rewrite 200606. Please take a few minutes to read it over and respond (here!) with comments on it. I'm going to ping the WikiProject Physics people about it too, and if it looks like a decent starting point to everyone, we can replace the current article with it. --Christopher Thomas 05:47, 25 June 2006 (UTC)[reply]

Trivia: Firstly, secondly, thirdly should be first, second, third.

Make it clearer that black hole event horizons are a prediction of classical physics. If black holes evaporate then there will be no event horizon because all the formerly-trapped regions evaporate along with the black hole. (This may be what Hawking was talking about:I'd keep the Hawking quote) --Michael C. Price ^talk 06:35, 25 June 2006 (UTC)[reply]

My math isn't good enough to follow exactly what he's saying. A caveat of some kind is definitely in order (meant to add one to the introduction saying that event horizons as described are strictly a feature of the equations of general relativity). I'll deal with this tomorrow, after more comments come in and I've had a chance to sleep. Thanks for the feedback! --Christopher Thomas 07:14, 25 June 2006 (UTC)[reply]

I've (finally) tweaked phrasing of the introduction and the black hole sections to attempt to make this clearer, as well as dealing with the grammar fix you noted. --Christopher Thomas 06:42, 28 June 2006 (UTC)[reply]

Finished adding a section to replace "sticking your hand through an event horizon" from the original article. --Christopher Thomas 07:14, 25 June 2006 (UTC)[reply]

To make it more readable to the non-physicist, should we add a piece about how the event horizon of a general massive object is the radius at which the escape velocity exceeds c? --0SpinBoson 21:13, 27 June 2006 (UTC)[reply]

This is already covered in the first sentence of the section about black holes ("...a celestial object compact enough that no matter or radiation can escape"). Talking about escape velocities ends up being kind of misleading, as then you get people trying to invent relay-type schemes where you can use velocity addition to do it (which doesn't work, for reasons that aren't obvious if you're thinking about it in terms of escape velocities). Is there a part of this that you think should be better-phrased in the proposed rewrite?--Christopher Thomas 01:58, 28 June 2006 (UTC)[reply]

To a general reader (non-physicist) the second sentence seems to be gibberish. It's all correct, but it requires knowing what light-like paths are. I thought adding a section in terms of escape velocities would help provide a general understanding. You do raise a good point though, we should add a warning to that to make sure that readers know that Galilean transformations do not work in this context due to the velocities being too close to c (provide link to Lorentz transformations maybe?)

I suggest you mention somewhere that all event horizons radiate, described as Hawking radiation when the event horizon is around a black hole or Unruh radiation when seen by an accelerating observer. Possibly called the Hubble temperature when it's from the Hubble horizon.--Michael C. Price ^talk 20:24, 28 June 2006 (UTC)[reply]

These are non-GR effects, though. I'll have to think carefully about how to add discussion of them. --Christopher Thomas 21:38, 28 June 2006 (UTC)[reply]

@@ Line 64: / Line 64: @@
 :This is already covered in the first sentence of the section about black holes ("...a celestial object compact enough that no matter or radiation can escape"). Talking about escape velocities ends up being kind of misleading, as then you get people trying to invent relay-type schemes where you can use velocity addition to do it (which doesn't work, for reasons that aren't obvious if you're thinking about it in terms of escape velocities). Is there a part of this that you think should be better-phrased in the proposed rewrite?--[[User:Christopher Thomas|Christopher Thomas]] 01:58, 28 June 2006 (UTC)
+::To a general reader (non-physicist) the second sentence seems to be gibberish. It's all correct, but it requires knowing what light-like paths are. I thought adding a section in terms of escape velocities would help provide a general understanding. You do raise a good point though, we should add a warning to that to make sure that readers know that Galilean transformations do not work in this context due to the velocities being too close to c (provide link to Lorentz transformations maybe?)
 I suggest you mention somewhere that all event horizons radiate, described as [[Hawking radiation]] when the event horizon is around a black hole or [[Unruh radiation]] when seen by an accelerating observer.  Possibly called the Hubble temperature when it's from the Hubble horizon.--[[User:MichaelCPrice|Michael C. Price]] <sup>[[User talk:MichaelCPrice|talk]]</sup> 20:24, 28 June 2006 (UTC)