Talk:Event horizon/Archive 1

message: :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)
replies: text: 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)
thread_title: Expanding FTL

message: :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! (--, 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)
thread_title: Sticking a hand through an event horizon

message: "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)
replies: text: 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)
thread_title: Light emitted from inside an event horizon

message: 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  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)
thread_title: Article cleanup tag

message: 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)
replies: text: 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)

text: 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)
thread_title:  readable