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Various doubts about Black Holes

  1. Nov 21, 2014 #1
    Hi there! After giving a thought about this phenomena I came with some doubts and I thought that maybe it was a good idea to put them all together in one thread so I don’t star many discussions simultaneously and also because maybe their answers are related. So here I go:

    a) As they gravitationally pull matter and eventually end up absorbing everything that goes through the Event Horizon, I wonder: Are black holes in a continuously growing process?. By growing I mean getting heavier (“massier” if you allow me to invent that word)

    b) If they do actually keep growing: What could stop that process? Isn’t it possible that black holes end up absorbing all the matter in the universe?

    c) Also: Are there black holes that are in equilibrium? Like the one in the center of the Milky Way. Is every star and solar system orbiting this super massive BH in a closed steady orbit? Or does it happen that mass is sometimes falling into it?

    d) I think scientist have been successful at estimating their mass. I’ve heard for example that the one sitting in the center of our galaxy has a mass of about 4 million times the mass of our Sun. Do they also have a measurable diameter? When I refer to the diameter I’m talking about the sphere defined by the Event Horizon (I think it’s fair to picture it as a sphere).
    Thanks in advance.
    Best regards,
     
  2. jcsd
  3. Nov 21, 2014 #2

    Simon Bridge

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    (a) no - they can also lose mass by a mechanism called "Hawking radiation".
    Even if they didn't - they only capture objects that get close, same as any other mass. They would grow until all the local mass gets captured and then stop.

    (b) a black hole is not infinitely powerful

    (c) lets see ... in order asked:
    yes - it is technically possible to have a black hole that neither grows nor shrinks.
    no - stars move on a variety of paths and, on average, move away from the center.
    Presumably some mass will fall into the central black hole.

    (d) Yes. Once you know the mass, you also know the radius of the event horizon since this depends on the mass.

    Have you seen:
    http://blogs.discovermagazine.com/b...0/ten-things-you-dont-know-about-black-holes/
    "10 things you didn;t know about black holes" by Phil Plait
     
    Last edited: Nov 21, 2014
  4. Nov 21, 2014 #3
    Thanks for the reference to that article. I didn’t know it. It is very interesting and easy to read.

    Regarding answer (a)
    If they lose mass, is it possible that at some point the density would get low enough for light to escape the gravitational pull (thus, not being a black hole anymore)?
     
  5. Nov 21, 2014 #4

    Simon Bridge

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  6. Nov 21, 2014 #5

    Grinkle

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    I think I read this somewhere (Brian Greene maybe) but I don't recall exactly. It helped me develop my intuition along the lines of the questions you are asking.

    A black hole is a big mass, and we know from looking at the sky that space contains many observable big masses which do not often bang into and absorb each other - they mostly orbit each other or shoot past each other. If the sun were suddenly made dense enough and its mass were held constant it would become just as suddenly a black hole instead of a shiny star. The earth and the other planets would still orbit it - they wouldn't suddenly fall into it. Nothing would change except it would be very dark and cold, I suppose. Newton's laws would continue to predict that earth would stay in orbit around the darkened sun.

    I do wonder if there is a galactic (or local cluster or super cluster or etc) macro-density at which the black hole population will increase, decrease or remain stable. I imagine in denser parts of galaxies star populations would grow? Has anyone related matter density at some scale to the star demographics and changes in star demographics? Is expecting such a (simple) relationship reasonable?
     
  7. Nov 21, 2014 #6

    Ken G

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    Another thing to bear in mind is that Hawking radiation must hold some kind of record for the most widely held as true, yet least observable phenomenon in the history of science. Hawking radiation is so weak for any of the black holes that we actually have evidence of existing, that there aren't any such black holes that are losing mass, they are all gaining mass. Perhaps someday we'll discover tiny black holes, and can actually test for the existence of Hawking radiation, but today is not that day.
     
  8. Nov 21, 2014 #7

    Doug Huffman

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    See the Holographic Principle for interesting on point reading.
     
  9. Nov 24, 2014 #8
    Observation of Black Holes is naturally difficult if not impossible but observation of their effects is thankfully on the increase, as is computer modeling of such data. Some examples follow. The evidence of the existence and behavior of Black Holes is expanding fairly rapidly (often full of surprises) and while "today is not that day" we can hope that perhaps within a decade Hawking Radiation can have some actual evidence or be falsified since it's existence, or lack thereof, is such a crucial fundamental.

    http://www.space.com/27179-monster-black-hole-dwarf-galaxy.html

    http://www.space.com/27812-rogue-black-hole-strange-supernova.html
     
  10. Nov 24, 2014 #9

    Ken G

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    What is the way that Hawking radiation will be observed, do you think?
     
  11. Nov 24, 2014 #10
    I realize that it is faintly possible that LHC could contribute here but my hopes are pinned upon NASA's Fermi Gamma Ray Telescope.
     
  12. Nov 24, 2014 #11

    Ken G

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    But that requires there be a population of very small black holes. It's worth looking for, but if we don't see it, it's not going to falsify Hawking radiation. Hawking radiation is pretty darn hard to falsify, that's something that makes be a bit uneasy about how widely it is accepted as true. But if we see gamma-ray flashes all over, I agree that would be pretty clear support, and another coup for predicting what was not yet seen.
     
  13. Nov 24, 2014 #12
    Of course merely not finding it with this effort does not amount to falsification. While I'm convinced that the Universe is under no contractual obligation to make sense to humans, let alone me, I find it so troublesome and on such a fundamental level of Laws of Conservation that it seems to me that the odds that Hawking Radiation, or something akin to it, does exist. I admit some bias in expecting it to be found and I sincerely hope it is within my lifetime..
     
  14. Nov 24, 2014 #13

    Ken G

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    I think it's nice when we have something that we expect to be true, because we learn something important either when we verify it is true, or when we find it isn't. It's worth keeping track of our "shooting percentage" on things that we really regard as true. We do pretty well it seems, but there's also a tendency to kind of forget about the misses...
     
  15. Nov 25, 2014 #14
    I completely agree. It is more solid when we have direct observation but our skills at predictive Mathematics is honed pretty fine especially now with modeling computers relatively cheap. We have to watch we don't get off on a bad tangent since "garbage in, garbage out" still rules, but we get better at it.
     
  16. Nov 27, 2014 #15
    You talked about Hawking radiation.
    At first, i'd like to underline, that the following thing isnt my personal speculation.
    However on another forum i had a debate with someone, who claimed, that light can actually come out from the black hole, just it is redshifted so much, that we cant detect it.
    I dont think the solution is that easy, but the question bugs me, what should i say, why this claim is totally false?
    (Yes, i read the official description of Hawking radiation, somewhat odd, but not something i cant believe, still i could use some help in debunking the former. )
     
    Last edited: Nov 27, 2014
  17. Nov 27, 2014 #16
    I read the article but I didn’t found (or failed to understand) the following:
    What happens to the mass left after the “evaporation point”? I mean, if with the Hawking Radiation phenomena the BH gradually loses mass, and at a certain point the density is low enough for light to escape, what does the theory predicts to be left after that point? Some kind of very compressed matter like a neutron star or anything like that?
     
  18. Nov 27, 2014 #17

    Ken G

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    It sounds like they were talking about Hawking radiation. Officially, Hawking radiation isn't light "coming out of" the black hole, it is coming from the event horizon. But it does have the effect of removing mass from the black hole, so that does sound a bit like light coming out of the black hole. Most likely, everyday words like "coming out of" are insufficient to meet the technical requirements of how to say what is happening there.
     
  19. Nov 27, 2014 #18

    Ken G

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    The black hole is supposed to be a singularity, so it is always a black hole until it evaporates to nothing. The density is never low enough for light to "escape", but Hawking radiation is what causes the black hole to evaporate. Such is the idea anyway, there remains a lot of debate about what is really going on underneath an event horizon.
     
  20. Nov 27, 2014 #19
    Of course since "singularity" is a term that basically means "the math breaks down" which really means "Math still works but we are missing some fundamental part of the equations(s)" it seems to me very unlikely that we can ever know "what is going on underneath an event horizon". The only thing that gives me pause is supposing that is that until we actually observe Hawking Radiation I don't see how we can determine what, if anything, at the EH can reveal about what occurs beyond it.
     
  21. Nov 27, 2014 #20

    Ken G

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    The arguments among the experts does little to assuage our concerns there, as well.
     
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