From optimist@u.washington.edu Mon Jul 27 02:34:25 1998 Path: news.wwa.com!news.voicenet.com!news.idt.net!feed2.news.erols.com!erols!wn4feed!worldnet.att.net!140.142.64.3!news.u.washington.edu!dante41.u.washington.edu!optimist From: "C. Hillman" Newsgroups: sci.physics,sci.physics.relativity Subject: Re: speed of gravity.... Date: Mon, 27 Jul 1998 00:34:25 -0700 Organization: University of Washington Lines: 70 Message-ID: References: <35A8B6C2.AD42768A@cern.ch><6o9e7b$8bq0$1@newssvr04-int.news.prodigy.com> <6ou094$k2h$1@news.ysu.edu><35B44CC7.9B50D61@cern.ch> <$P79iHA7H4u1Ew6M@jboden.demon.co.uk> <6ph3c1$nhv$1@sparky.wolfe.net> Reply-To: "C. Hillman" NNTP-Posting-Host: dante41.u.washington.edu Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Trace: nntp6.u.washington.edu 901524866 20152 (None) 140.142.17.38 X-Complaints-To: help@cac.washington.edu NNTP-Posting-User: optimist In-Reply-To: <6ph3c1$nhv$1@sparky.wolfe.net> Xref: news.wwa.com sci.physics:316331 sci.physics.relativity:67119 (Followup to sci.physics.relativity only, please!) On Sun, 26 Jul 1998, Russell Easterly wrote: > I often think so. If gravity travels at the speed of light then there > is no way gravity can get out of a black hole. This is a FAQ, and despite what another poster claims, the FAQ is perfectly accurate, as can be verified by consulting any gtr textbook (there are a half dozen suggested texts listed on my relativity page, see url below). > . I know space gets very bent near a black hole. But why does the effect > . of gravity get special treatment if gravity travels at the speed of > . light? > > The most common explanation that I have heard > is that gravity "escapes" BEFORE the black hole is formed. That's the right idea, more or less; I'd put it more like this: matter was gradually concentrated to form a star, thereby curving spacetime in the vicinity of the mass concentration. This happened sufficiently slowly that the "news" that "a mass-energy concentration is forming here!" had plenty of time to propagate out at a speed of light, in form of gravitational waves gradually modifying the far field as the matter continued to concentrate. Billions of years later the star collapses after a long and happy life. However, it turns out that spherically symmetrical collapse doesn't alter the exterior field; if you like, precisely because gravitational waves cannot escape from the event horizon, the "news" that the matter which originally caused the spacetime curvature has been crushed out of existence cannot affect the exterior field, which far from the black hole continues to look just like the field of the star which gave rise to the hole. [Needless to say, the details of realistic stellar collapse are quite intricate, involving a great deal of nuclear physics, bursts of strong gravitational radiation, and asymmetrical collapse. But the basic outcome is the same.] > The gravitational effects are cause by the mass that fell into the > black hole. No, after the star collapses, the exterior field continues to behave just the field of the original star, in particular if a large object falls in, you expect the Kepler "mass" of the hole to increase just as if this object had fallen on to the surface of the original star. (The principle is clear; the details are extremely tricky; see the Living Reviews site listed on my relativity page). > This mass is essentially "frozen" at the horizon of the > hole because of time dilation. Also false, nothing is frozen near the event horizon. These are all very common misconceptions which are addressed in the FAQ; see the link on my relativity page. Chris Hillman Please DO NOT email me at optimist@u.washington.edu. I post from this account to fool the spambots; human correspondents should write to me at the email address you can obtain by making the obvious deletions, transpositions, and insertion (of @) in the url of my home page: http://www.math.washington.edu/~hillman/personal.html Thanks!