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Black Holes

  1. Oct 3, 2009 #1
    I was wondering, novice that I am, whether there is any theoretical limit to the size of a black hole. I know there are supermassive black holes that are equivalent in mass to billions of stars. Could a black hole ever encompass a mass the size of our universe or perhaps even more mass? Could such an object have been responsible for the Big Bang? If so, what could have caused such a singularity to "explode?" I also know that when Black Holes lose mass (or Hawking radiation) they are supposed to heat up. Is this somehow connected to the Big Bang?

    Keep it simple guys. As you can see, I am limited

    :confused:
     
  2. jcsd
  3. Oct 3, 2009 #2
    The 'Black hole' concept is just a theoretical phenomenon as depicted by Stephen Hawking.
    It has not been proven to exist but the scientific method of theory and hypothesis formation lead it that way. It is just a concept the scientists hold to explain certain events.
    Is this true?^ Please experienced writers give a reply.
     
  4. Oct 3, 2009 #3
    I do not believe that you are correct. Black holes are far more than " a theoretical concept" by Stephan Hawking. They were first suggested by equations from General Relativity and then by Schwartzchild. Supermassive black holes have been detected in the center of virtually every galaxy, including our own Milky Way galaxy.
     
  5. Oct 3, 2009 #4

    Jonathan Scott

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    No, it's not that clear.

    Many instances have been observed of objects which are sufficiently massive that if the standard black hole theory is correct, they must be black holes. So far, there is no direct evidence that they are in fact black holes, and in at least some cases there is evidence which conflicts with the known properties of black holes (for example signs of an intrinsic magnetic field in a quasar).

    General Relativity is based on Einstein's Field Equations, which were discovered by both Einstein and Hilbert at around the same time. Even if those equations are completely correct, it is not certain that they necessarily predict black holes.

    Karl Schwarzschild was the first to find an exact analytic solution of Einstein's Field Equations for the case of a static central mass. Within that solution, there are some boundary conditions which are not specified by Einstein's theory. Schwarzschild assumed that a particular boundary condition made the most sense from a physics point of view. That condition does not lead to black holes, and Schwarzschild died (not very long afterwards) before anyone even suggested the possibility.

    Shortly afterwards, Hilbert reasoned that a different boundary condition was mathematically more obvious and therefore decided that it made sense to assume that condition on mathematical grounds. That assumption leads to black holes. It appears that at the time, few seriously questioned this assumption, or were even aware that it was at least to some extent an assumption, although Marcel Brillouin wrote a paper on the subject in 1923. Standard black hole theory is based on Hilbert's assumption, and his interpretation of Karl Schwarzschild's solution is what is now taught as the standard General Relativity result.

    It is apparently now considered heretical by many even to suggest that this difference does still appear to be an assumption rather than a proven fact. There is a 1989 paper by Leonard S Abrams called "Black Holes: The Legacy of Hilbert's Error" which calls attention to this point and asserts that Hilbert's assumption was arbitrary and unjustified and appears to have been a mistake, and there are very interesting papers by Salvatore Antoci and others (including translations of the original Schwarzschild and Hilbert papers) which support this point of view.

    From reading these papers and the quoted original papers, I agree that it appears that an unproven assumption has been made, but I can't see any way to determine the correct assumption from existing theory alone, and I suspect that we will either require experimental evidence or new theory to resolve this issue.

    There have been few responses to these controversial papers from the standard GR community, and what somewhat shocked me is that even from my level of understanding I see them failing to address the real questions, instead resorting to ridicule based on misinterpretations of what was being said. I feel that if standard GR is provably correct on this issue so there is no possibility of an alternative assumption, then I would like to be shown that convincing evidence. Instead, I get a tirade about the incompetence of the other side, and this leaves me very sympathetic to the case that the standard interpretation might be wrong.

    There are a few people such as Stephen Crothers who go further and assert that Hilbert's assumption is provably wrong and black holes don't exist. Although Crother's mathematics is extremely competent and helps illustrate the various points of view, I don't buy most of his arguments, especially as his confrontational approach makes it difficult to distinguish him from a crackpot. I will however agree with him that Schwarzschild's original model, without black holes, is at least more plausible from the point of view of the physics.

    There are many other ideas for ways in which black holes could be avoided; one example is the "MECO", Magnetospheric Eternally Collapsing Object, which claims to explain the observed quasar magnetic field. Others call on quantum effects to prevent them from happening.
     
  6. Oct 4, 2009 #5
    Evidence has been found for massive black holes at the center of virtually every galaxy studied...ours included. That was a surprise to astronomers!

    I believe the black hole in our Milky Way is believed to be about the size our our orbit about our sun...quite large!!!

    It has been shown that a close relationship exists between the mass of the black hole at the center of a galaxy and orbital speed of outermost stars...there seems to be a connection that until recently was not known. There is a one hour program on either History or Science channel BLACK HOLES in the title that explores some current astronomical research....they show during the program the plot of such data and a straight line best fit. I think they proposed that black holes somehow aid the "seeding" of stars in galaxies and at the same time spawn supernovas....My Yorkie distracted me and I missed some of that part!
     
  7. Oct 4, 2009 #6
    I've never seen anything about a theoretical upper limit...but some discussion about tiny size black holes..." primordial"...which might also be big....but some suggest evidence may appear in LHC experiments....
    see
    http://en.wikipedia.org/wiki/Primordial_black_hole for some discussion.....

    I just happened to be looking at one solution for black holes, Schwarschild, and in that case, as mass (M) gets larger so does the horizion and size of the black hole....a practical upper limit might be the gravitational "reach" (influence) of the black hole so it would seem maybe a black hole might eventually gobble up an entire galaxy...but adjacent galaxies have their own black holes so until those galaxies might happen to collide it seems like a loooooooooooooong time before they'd merge...along with their black holes...

    "If so, what could have caused such a singularity to "explode?""
    A black hole explodes because when it shrinks via Hawking radiation, and gets smaller, it also gets hotter...and eventually the thermal energy exceeds the gravitational energy and "boom"...
     
  8. Oct 4, 2009 #7
    I don't know what would satisfy you insofar as 'direct' evidence. So far, the only way (I know of) to detect a black hole is by it's gravitational effect on the motion of nearby stars or by gravitational lensing. Possibly, there may be some other phenomenon that would imitate the mass equivalent of millions of stars equal in mass to our own Sun, such as the one detected in the Andromeda Galaxy. Until that unknown specie is found, I prefer to stay with the simplest possible answer (Occham's Razor) which, at the present time, is a (supermassive) black hole.
     
  9. Oct 4, 2009 #8
    To Naty,

    Many thanks for your reply. That is exactly what I was looking for.
     
  10. Oct 4, 2009 #9

    Jonathan Scott

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    There's no "imitation". There's no dispute about the amount of mass involved, but only about whether it collapses into a black hole or not.

    If Schwarzschild was correct rather than Hilbert, the gravitational effect of the objects at the centers of galaxies and other super-massive objects such as quasars would be very similar to that predicted for a black hole, but it would simply be a very dense object similar to a neutron star. The same applies for other theories such as the MECO idea.

    There would be a few potentially observable differences from a black hole. For example, it could have a significant intrinsic magnetic field (unlike a black hole), and signs of this have been observed in the vicinity of a quasar (which is conventionally assumed to be a black hole because of its mass). It could also have a well-defined surface, emitting light and other frequencies (which could help explain the brightness of quasars), which would not be as redshifted as the lowest orbits of an accretion disk, so in some cases one could observe absorption of lower redshift light by higher redshift material; this feature has been observed in some quasar spectra.
     
  11. Oct 4, 2009 #10
    I don't mean to be flip with you but a supermassive object that doesn't emit light is, by definition, a black hole. It can be detected only by intense x-ray (or gamma-ray emission) and by gravitational effects on its "neighbors" or by lensing (previously mentioned.) Quasars and neutron stars, on the other hand, are usually readily observable (optically) although admittedly being very massive. If you know of some object or phenomenon that is equivalent to thousand (or millions) of stars the mass of our Sun and does not emit light, but is not a black hole, please let me know what it might be.
     
  12. Oct 4, 2009 #11

    Jonathan Scott

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    Er... most supermassive black hole candidates typically emit LOTS of light and other high-energy electromagnetic radiation, in fact it's difficult to account for how much they emit given the black hole model. The standard explanation is that the light is emitted from accretion disks, not from the central body, but as far as I know current observations are a long way from being able to resolve that difference to confirm the result either way.

    In any case, that's not the definition of a black hole, which is a gravitationally collapsed object from which no mass or energy can escape at all. If Schwarzschild's version of GR was right and there were no black holes, there would be no limit to the redshift that a massive object could have, and hence it could appear to radiate only in the far infrared or not produce any observable radiation at all.

    It is therefore difficult to confirm that an object is actually a black hole, but if it is not a black hole, there could be ways of detecting that.
     
  13. Oct 5, 2009 #12
    Would you be interested in my "flat-earth" theory?


    :rofl:
     
  14. Oct 5, 2009 #13

    russ_watters

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    I was thinking the same thing. The argument here is about the particulars of the theory of black holes. Whether there is a singularity or just a really dense ball of matter, either way those things are going to be called "black holes".

    ...This is a bit like the common question we get: "Is evolution a theory or a fact." And the answer is that it is both. Similarly, black holes are a fact: they are an observed phenomena with a name that goes with it. But they are also a theory and whether the theory is correct (partially, mostly or completely) is apparently still an open quesiton.
     
  15. Oct 6, 2009 #14

    tiny-tim

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    Abrams and others

    uhh? :confused: … once they've been debunked, there's not much point in anybody responding to them (or reading them :wink:) ever again. :smile:
     
  16. Oct 6, 2009 #15

    Jonathan Scott

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    Re: Abrams and others

    I'd agree if I found the "debunking" satisfactory. The arguments of Abrams, Antoci and others, and the original papers, especially those by Schwarzschild, are clear and fairly easy to understand, and if they are in fact wrong I want to see something which explains why in equally straightforward terms. Instead, I see papers which explain why something ELSE is ridiculously wrong, based on a misunderstanding of what they are saying. I've found this extremely frustrating.

    Anyway, the important point is simply to be aware that the scientific community now has a habit of simply referring to objects as "black holes" if they exceed the theoretical limit for gravitational collapse according to the standard (Hilbert) interpretation of GR, but so far there is little experimental evidence to confirm that they are actually black holes, and there is even a little evidence suggesting that they might not be. I guess that for the general public, it's easier to simply say "black hole" than to explain the limitations of this hypothesis, but I feel that scientists should not forget that gravitational collapse is still an unconfirmed theory.
     
  17. Oct 6, 2009 #16

    Wallace

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    Many many many many many people have worked on GR and black holes since the days of Hilbert and Schwarschild, and these issues have been thorughly settled. The entire field of numerical GR deals with the specifics of gravitational collapse to a black hole on a daily basis. If the problems you suggest really existed this would have come up long ago.

    There are occasionally some very bad papers that come out pushing the arguments you're pushing (I've seen a number of them in the last few years) but they are not 'ignored' in the sense of nobody reading them, just in terms of no one bothering to respond to them, because the arguements are wrong. They are wrong because they typically focus on the papers of Hilbert, Schwarzchild etc as if the almost 100 years since that time simply never happened. It is not as if the fundamentals of GR stopped being examined in 1930, we've been doing it non-stop since then.
     
  18. Oct 6, 2009 #17

    Jonathan Scott

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    The fact that the mathematics of standard GR works and is self-consistent should be obvious! That's not in dispute (at least not by me, nor as far as I know by Abrams and Antoci, although for example I suspect Crothers would dispute absolutely anything).

    The question raised by Abrams and Antoci is rather whether there is an alternative valid interpretation involving a different boundary condition for the spherical solution (such as that originally chosen by Schwarzschild), and if so whether there is any theoretical or experiment method of distinguishing which interpretation, if any, matches nature. They think that such an alternative exists (illustrated by Schwarzschild's original interpretation) and that the feature that it does not give rise to black holes is a strong argument in its favour, by Occam's razor, but obviously not proof.

    I have seen several attempts to refute the alternative interpretation which however appear to be circular (as far as I can see) because they implicitly rely on part of the Hilbert interpretation. I have also attempted to find flaws in either side myself, but I can't find anything in Einstein's equations or any other physical or mathematical considerations which would enable either interpretation to be confirmed or refuted. I've seen various appeals to "reasonableness" on either side, but that isn't proof.

    This is of course a very tricky subject because Schwarzschild's vacuum solution model involves a "point mass" which is seriously unphysical, so its properties cannot be described in a consistent way at all, and even assuming that it exists leads to contradictions. I originally hoped that the interior solution would help resolve the issue, but I found that the way it joins to the vacuum solution is determined by continuity with the boundary assumed for the vacuum solution. I have more recently been considering an alternative model involving thin spherical empty shells with flat space inside, where the shell is shrunk down towards the Schwarzschild radius. This is not exactly realistic but at least it doesn't involve the sort of infinite densities involved in a point mass, and it has made it easier to visualize some very odd properties of the Schwarzschild coordinate system.

    If you do know of any text book or paper which you think addresses this issue properly, I'd be very interested to see it. If you wish to discuss the technicalities about the coordinates and so on, it might be better to start a new thread over in the Relativity forum.

    Anyway, that's just one way in which black holes might not happen (and if the authors are right, their interpretation is still consistent with the GR field equations). The experimental evidence (for example relating to a quasar which appears to have an intrinsic magnetic field) also suggests the possibility that black holes might not happen, so even if you think GR definitely predicts them, that might hint that GR itself might be wrong or an incomplete explanation (as it is already in the dynamics of galaxies and the universe, where GR is now supplemented by the concepts of dark matter and dark energy).
     
  19. Oct 6, 2009 #18

    Wallace

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    The question has been asked... and has been answered. If you are still referencing 'as that originally chosen by Schwarschild' then you need to read a book, any book, on relativity written since circa 1960. As I said before, the self consistent dynamic collapse to a solution that matches the static black hole solutions (of which the Schwarzchild derivation is just one) would not work if what you are saying has any value.

    Again, you may as well argue that the Earth is the centre of the Universe, because some of Galileo's arguments are demonstrable wrong. The fact is we don't think black holes are a valid solution to GR because of 'Hilbert's interpretation'. You are ignoring everything written since the 1920's (save for the other literature that ignores everything written since the 1920's).

    GR is still the preferred theory of gravity. Dark matter and dark energy have in no way 'supplemented' it, they are just additional energy sources that act in accordance with the rules of GR.
     
  20. Oct 6, 2009 #19

    Jonathan Scott

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    Most of what I originally learned about GR was from MTW and in a more digestible form from Rindler's "Essential Relativity". The older papers just make it clear what Schwarzschild does to solve the equations and how Hilbert modified that slightly, and that leads directly to the question of what EXACTLY was wrong with Schwarzschild's original solution. For example, I've heard explanations that Schwarzschild "didn't realize" that there was a more general solution, and that of course (as everyone now knows) the singularity at r=2Gm is "only a coordinate singularity". However, none of that proves anything about the physics. I'm trying to understand why it has to be one way and not another; I want to know for myself. Those who simply assert that I have to respect the authority of the experts do not help me to understand - this is science, not religion.

    I don't know the details, but I can't see why you might expect the existence of an alternate set of viable assumptions about boundary conditions to affect the self-consistency of the standard GR solution.
     
  21. Oct 7, 2009 #20

    Mentz114

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    No Schwarzschild's vacuum solution does not assume a point mass. The exterior Schwarzschild solution assumes only that the source is spherically symmetric and not rotating.
     
    Last edited: Oct 7, 2009
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