merrill@rex.DEC (11/29/84)
It only takes a couple of stars' worth of mass compressed together to create such a high gravitational field that bends space so much that even the speed of light does not exceed the escape velocity. In other words light cannot get out. You have undoubtedly heard this described as a "black hole." Apt description. Surly if "all the mass of the universe" were compressed to gether you'd have the ultimate black hole and since nothing can travel faster than light nothing could get out. Therefore the universe could not possibly have started from a single point "big bang." I have only ever heard of two ways to counter this using higher math and physics: 1) in the first 1E-?? seconds the laws of phisics were different than we now observe them to be (e.g. Planck's constant, wasn't) and 2) there was no "bang" just an infinitely long process of accrual (one of Fred Hoyle's cosmological contributions -- he's an astronomer as well as a writer.). Curious, Rick
robison@uiucdcsb.UUCP (12/01/84)
I can't answer the question, but it does bring up an interesting point. 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? Arch - uiucdcs
dn5@sdcc12.UUCP ({dn5) (12/03/84)
> 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? > > Arch - uiucdcs It is true that information may be transmitted without concern for the properties of electromagnetic radiation as there are other forces in nature. The question of the speed of information exceeding c is left untouched by your example, however, since a gravitational field must propogate at that velocity. [jc]
guy@rlgvax.UUCP (Guy Harris) (12/04/84)
> 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. Nope. The mass, charge (you forgot that one), and angular momentum of the black hole are merely the mass, charge, and angular momentum of the stuff that went into the black hole. You can't change any of the above from within the black hole - it's too late. As such, there's no "information" to be gained; an observer from the outside already could have figured out what the mass, charge, and A.M. of the black hole were if they knew what went into it. Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy
jlg@lanl.ARPA (12/04/84)
> 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.
gjk@talcott.UUCP (Greg J Kuperberg) (12/04/84)
> It would seem that information transmission is not necessarily bound by > the properties of light. Any comments? > > Arch - uiucdcs No, it's the other way around. Information transfer has certain limits; for example, that information cannot travel faster than c. Light, as a carrier of information, is also bound to this speed. A vacuum is a "perfect" medium for electromagnetic information transfer, so light in a vacuum travels as fast as it can---c. Since this is a good way to measure c, the quantity was originally defined this way. A good definition A.E. (after Einstein) would be: C (in meters/second) is the conversion factor between one meter and one second, just as 1.609 is the conversion factor between one mile and one kilometer. --- Greg Kuperberg harvard!talcott!gjk "Madam, there is only one important question facing us, and that is the question whether the white race will survive." -Leonid Breshnev, speaking to Margaret Thatcher.
gino@voder.UUCP (Gino Bloch) (12/05/84)
[information horizon] > The question of the speed of information > exceeding c is left untouched by your example, however, since a > gravitational field must propogate at that velocity. But the mass was there before it collapsed inside the event horizon, and the event horizon still has a gravitational potential boundary condition. Will some physicist tell me what I just said (if anything)? -- Gene E. Bloch (...!nsc!voder!gino) Mr Humility
bill@utastro.UUCP (William H. Jefferys) (12/05/84)
> 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. In fact, a Black Hole is the perfect destroyer of information. That is the point of John Wheeler's comment "A Black Hole Has No Hair", that is, once things are thrown into the black hole, we lose all information about them. One of the results of modern relativity theory is the deep connection between black holes and Thermodynamics, as worked out by Hawking, Beckenstein and others. -- "When evolution is outlawed, only outlaws will evolve" Bill Jefferys 8-% Astronomy Dept, University of Texas, Austin TX 78712 (USnail) {allegra,ihnp4}!{ut-sally,noao}!utastro!bill (uucp) bill%utastro.UTEXAS@ut-sally.ARPA (ARPANET)
sharp@noao.UUCP (Nigel Sharp) (12/05/84)
> Information does NOT escape from a black hole. Correct. The information was always there, locally. As the state of the object changes towards being a black hole, changes in the locally obtained information about that state propagate outwards at the speed of light. (Incidentally, the available parameters are mass, angular momentum, charge and [mathematically acceptable] magnetic monopole moment.) > Note also that light DOES escape from a black hole. A black hole radiates > like a blackbody whose temperature ..... NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO This is precisely the sort of remark which confuses people. The quantum mechanical effect by which a black hole radiates energy as a black body, and therefore loses mass, eventually disappearing in a puff whose properties noone has adequately calculated, is NOT THE ESCAPE OF LIGHT FROM THE HOLE. Using the word `escape' implies that something inside gets to the outside, whereas what is happening is very difficult to explain - energy transfer is happening by the addition of energy to external states and the removal of energy from internal states - in fact, the addition of negative energy states. I'm not explaining this very well: I have still to come up with an adequate way. The mathematics seems to make sense, and the radiated spectrum is black body, but there is no possibility of anything moving from the inside to the outside. The black body nature is a reflection of the destruction of information. > ................ but no information > about the internal structure of the black hole (if any) is carried by this > radiation. The original poster (post-person ? post-being ? :-) ) probably knows what he's talking about, and this is correct, but I have been trying for some time to persuade people to be very careful about their language when explaining any of a black hole's effects. It is difficult to translate mathematics, and we have more trouble explaining why, when x said y, he really didn't mean it, than we do explaining the phenomena. (`But Carl said ....' `Yes, but what he meant was ...' `Then why did he say ...' Sigh.) -- Nigel Sharp [noao!sharp National Optical Astronomy Observatories]
sean@ukma.UUCP (Sean Casey) (12/14/84)
- Can gravity propagate faster than c? The question I would ask is: Does gravity propagate? I think it depends on what you consider gravity to be. Is it a wave phenomena? It is a warpage of space? I seriously doubt we'll find the answers (facts, not theory) until we can measure it qualitatively. I once read a SF story about some astronauts that found an alien transmitter on Mars. It had a "microscopically small black hole" that was induced to vibrate by magnetic waves. According to the story, the gravitic disturbance created affected the entire space fabric of the universe simultaneously. One might ask whether the "fabric of space" has the same same properties as a piece of cloth, that is, if you pull on one edge, does the disturbance propagate or does it affect the entire sheet at once? One way that seems interesting would be to annhilate a certain weight of matter and measure the gravitic disturbance. The dis- tance of space could be used to create a delay that would be measurable. Then you'd know the speed of gravity. Does anyone know of any experiments, domestic or cosmic, that have in some way determined "the speed of gravity"? Sean Casey - Curiosity is the beginning of Wisdom
gino@voder.UUCP (12/18/84)
[how long did it take to eat this line, given its length of 16 cm?] > One might ask whether the "fabric of space" has the same same > properties as a piece of cloth, that is, if you pull on one edge, > does the disturbance propagate or does it affect the entire sheet > at once? Actually, the disturbance propagates throught the sheet at a speed v <= c (in some situations, <= speed of sound in the material). The speed of light is not just a good idea - it's the law. > One way that seems interesting would be to annhilate a certain > weight of matter and measure the gravitic disturbance. This was discussed here recently. The weight (mass) is not annihilated, it is transformed to another state (which may then depart at v == c). A better way is to accelerate a mass and look for gravity waves. -- Gene E. Bloch (...!nsc!voder!gino) Try to understand.