Talk: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)

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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)

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)

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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)

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"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)