Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site lanl.ARPA Path: utzoo!watmath!clyde!burl!ulysses!mhuxj!mhuxn!houxm!ihnp4!zehntel!hplabs!hao!seismo!cmcl2!lanl!jlg From: jlg@lanl.ARPA Newsgroups: net.physics Subject: Re: Re: "big bang" a big bust? Message-ID: <17266@lanl.ARPA> Date: Mon, 3-Dec-84 20:51:16 EST Article-I.D.: lanl.17266 Posted: Mon Dec 3 20:51:16 1984 Date-Received: Thu, 6-Dec-84 05:44:14 EST References: <127@sdcc12.UUCP> Sender: newsreader@lanl.ARPA Organization: Los Alamos National Laboratory Lines: 32 > Light can't escape from within a black hole, but information can. > > Two pieces of information escape from a black hole - its mass and angular > momentum. The mass can be found from the strength of its gravitational field. > As I remember (from a physics lecture) the angular momentum can be found by > sending a satellite around the black hole. The satellite will return rotated > as I recall, though I don't really understand why. It probably has something > to do with the curvature of space around the black hole. > > It would seem that information transmission is not necessarily bound by > the properties of light. Any comments? > Information does NOT escape from a black hole. The mass and angular momentum of the object are discerned by observing the properties of the space-time manifold in the neighborhood of the event horizon. The charge on the black hole is also discernable in a similar manner (observing the electric field in neighborhood of the event horizon). In each case the observation is of local phenomena that occur OUTSIDE of the black hole. Note also that light DOES escape from a black hole. A black hole radiates like a blackbody whose temperature is proportional to the inverse of the circumference of the event horizon (or is it the inverse of the area of the event horizon - I can never remember). This is a quantum mechanical effect. The implication is that all black holes shrink as time passes due to the loss of mass-energy. Of course, this effect is countered in the real universe since matter is always falling into the thing too. However, small black holes have a high blackbody temperature and a low accretion rate and will self-destruct over time. The blackbody temperature of a black hole can also be used to determine its mass, but no information about the internal structure of the black hole (if any) is carried by this radiation.