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Another black hole question

  1. Oct 26, 2007 #1
    so if the point of infinite density is infintessimally small, then it is a point right? If so, then wouldn't it be a link to another dimension like the tenth or zero dimension or something like that? idk if you believe in multiple dimensions but please if you can prove it right or wrong I would love it.
     
  2. jcsd
  3. Oct 26, 2007 #2

    Chris Hillman

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    Your question appears to bear little resemblance to the notion of black holes as treated in our gold standard theory of gravitation, gtr. Do you have another theory in mind? If not, can you clarify the question?

    (It might also be a good idea to say something about your math/physics background, i.e. if you are a high school student you should probably say so.)
     
  4. Oct 27, 2007 #3
    Yes, I am a high school student, but if you think I am confused on something please tell me, I want to get this straight, if you can.
     
  5. Oct 28, 2007 #4

    Chris Hillman

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    A Reading suggestion

    Hi, cooldude3122,

    Thanks for the indication of your level; this is very helpful in trying to give you answers/advice which I hope you will find useful!

    0. In future, I urge you to try to write complete sentences. "Blackberry style" posts are inappropriate for discussions of subtle concepts in math/physics!

    1. Physics deals with physical theories which are expressed in the language of mathematics and compared with experiment and astronomical observation. More precisely, theoretical physicists make specific quantitative predictions which experimentalists can then test. In this way, theories can be ruled out. In the case of our first "best theories" of gravitation, Newtonian gravitation and gtr, it would be more accurate to say that limits on the domain where a given theory is thought to be reliable can be changed by new evidence. Note that gtr is our current gold standard theory of gravitation and has been very extensively tested, with the result that we know a lot about where Newtonian theory breaks down (and where it is still safe to use), and we also know that any useful theory of gravitation must give predictions which are essentially indistinguishable from gtr in all conditions we have yet tested. Consequently, discussions of "gravity" are usually assumed to refer to gtr unless an alternative is specified. In any case some theory (or class of theories) must be specified in order to have a meaningful discussion.

    2. To prevent possible confusion I stress that under the conditions where it is known to be safe to use Newtonian gravitation, this theory gives predictions indistinguishable from gtr, but is much easier to work with, so it makes sense to use it wherever possible. If gtr is ever succeeded by a better theory, similar remarks should hold true for the domain where Newtonian gravitation breaks down, but gtr does not.

    3. Strong theoretical considerations strongly suggest that gtr should eventually be found to break down under certain conditions (sufficiently small scales, sufficiently strong gravitaitonal fields).

    4. It is true that in some ways black holes can be thought of as the analogs in gtr of the "point masses" of Newtonian gravitation. Nonetheless, a black hole, as this concept is understood in the context of gtr, is not a "point of infinite density" or even "infinitesimally small".

    5. I urge you to read an excellent book by Geroch, General Relativity from A to B, which is aimed at high school students (or equivalent) and which focuses at conveying by (good!) pictures an accurate intuition for some of the basic features of the black hole concept in gtr, such as the meaning of the "event horizon" in terms of "light cones" attached to each "event" in spacetime. (There are some other good nontechnical books, but this one is exceptionally good and specifically attempts to address the very issues which have confused you.)

    6. Some theories which have been proposed and which attempt to provide a candidate for the successor theory to gtr as our gold standard theory of gravitation---some of these, such as superstring theory, also attempt to unify gravitation with various other fundamental interactions--- do posit "extra dimensions". I recommend that you study the popular book by Geroch before trying to learn about superstring theory. However, a good popular book on superstring theory is Brian Greene, The Elegant Universe.

    7. Superstring theory has been criticized on the grounds that so far its advocates have apparently been unable to suggest any test feasible with currently forseeable technology which would enable experimentalists to compare it with gtr. Some critics charge that superstring theory advocates have been reduced to claiming the fact that string theory appears to recover gtr in an appropriate limit, just as gtr recovers Newtonian gravitation in an appropriate limit, as a successful "postdiction". IMO these critics have a point, but given the difficulty of the mathematical issues involved, I feel that string theorists have probably not been wasting their efforts. However, I tend to suspect that for the forseeable future string theory may be more valuable in pure mathematics than in physics (because if you extract the original physical motivation, the mathematics remaining is by common consent novel and beautiful), and I note that mathematics developed in order to pursue some "failed physical theory", e.g. knot theory, often turns out to be useful for unforeseen applications, and may even eventually turn out to have applications not very far from the original motivation!
     
    Last edited: Oct 28, 2007
  6. Nov 17, 2007 #5
    yeah, but i have trouble seeing just how a black hole could form a worm hole. Given that a black hole is (assuming that hawking radiation isn't enough to keep it from growing) gaining mass, it will keep on "bending" space-time. However there are two questions: 1, If a black-hole can't tear space-time, then you can't really have wormholes, and 2, if a black hole CAN tear space-time, then what happens next? it's in no-man's-land? in an area other than the universes? However, if hte BH is not in N-M-L then, "tearing" space-time, you probably could form a wormhole
     
  7. Nov 17, 2007 #6

    Chris Hillman

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    Come again?

    Hi, rubecuber, are you addressing me, and if so, are you the same person as cooldude3122? If not, same questions I had for him: can you briefly describe your math/sci background?

    You might have seen some recent posts in which I tried to carefully explain that there are many things which might be called "wormholes" with greater or lesser justification, and some of them are features of some models of black holes. There are several points here to explain so you might try the "search" tool to look for my posts.

    Hawking radiation is completely negligible for solar mass black holes, even more neglible for supermassive black holes. If a black hole is accreting matter, or even absorbing some fraction of background starlight, then yes, its mass is growing, perhaps very slowly.

    According to gtr, any concentration of mass-energy will change the curvature of spacetime, in a manner predicted by the EFE.

    Huh? Have you confused "tearing" as in "ripping paper" with what is supposed to happen in a wormhole? If so, that doesn't sound right, but note that wormholes are supposed to work a bit like "handles" in surface topology (think of a surface formed by gluing half a bagel onto a beach ball, and gosh I hope you get the right picture from this half-hearted description).

    Or are you confusing something like a "strong spacelike curvature singularity" with "tearing" as in "ripping paper"?

    You've completely lost me. I encourage you to seek out and read an excellent popular book which has almost no mathematical prerequisites but which can provide a geometrically accurate idea of the simplest black hole models, after which you'll be in a much better position to ask more questions. See Geroch, General Relativity from A to B.
     
    Last edited: Nov 17, 2007
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