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Is black hole a theoretical concept or a natural phenomenon?

  1. Sep 29, 2013 #1
    is black hole a theoretical concept or a natural phenomenon?
     
  2. jcsd
  3. Sep 29, 2013 #2

    phinds

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    As far as is known, it is a natural phenomenom
     
  4. Sep 29, 2013 #3

    UltrafastPED

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    It is a theoretical object - first predicted in 1939, but named much later - with strong experimental support.

    They seem to be very common, with at least one giant black hole in the center of most galaxies.

    See http://www.cfa.harvard.edu/seuforum/bh_reallyexist.htm
     
  5. Sep 29, 2013 #4

    D H

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    You are writing as if this was an either/or question. It's not. The answer is "yes".
     
  6. Sep 29, 2013 #5

    phinds

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    Good point.
     
  7. Sep 29, 2013 #6
    The existence of black holes is kind of a philosophical question since time dilation prevents the event horizon from existing until you actually fall in. Black holes do not actually exist in our frame of reference. The celestial objects we call black holes are really frozen by time dilation right on the verge of becoming black holes (I think of them as very very dim holes). Never-the-less we could, in principle, go to one of these objects and fall behind an event horizon. So, whether the thing actually exists depends on exactly what definition of existence you choose.
     
  8. Sep 29, 2013 #7
    I wrote like that because theoretical physics is understood by very few.Does any one know a verified blackhole in the sky{experimentally}
     
  9. Sep 29, 2013 #8

    mfb

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    Many
     
  10. Sep 29, 2013 #9

    SteamKing

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    Neutron stars were once theoretical constructs, first predicted in 1934, only a short time after the neutron itself was first discovered.

    The first observational evidence for neutron stars came from radiotelescope observations in the UK and USSR in 1967. Now, there are approximately 2000 neutron stars known.

    http://en.wikipedia.org/wiki/Neutron_star
     
  11. Sep 29, 2013 #10
    I think it is fair to call a region enclosed by an event horizon a "black hole". Even if all of it's mass appears on it surface.
    Also, I wouldn't be too certain about whether something, from our perspective, can cross that horizon or not. Take the situation where object A drops into the black hole. When A reaches what appears to be 1mm above the event horizon, planet B is dropped into the black hole causing the diameter of the event horizon to expand by more 1cm. Would object A still appear to us to be above the event horizon?
     
  12. Sep 29, 2013 #11

    atyy

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    But the article says "black hole candidates", and as of 2009 these authors http://arxiv.org/abs/0903.0100 wrote "Are the massive dark central objects in galaxies really Kerr black holes?", and considered "If the central object is not a black hole, but rather a boson star or something similar, then the inspiraling object will continue to emit long after it would shut off in Kerr (Kesden et al. 2005). This would be a clean and blindingly simple falsification of the central black hole paradigm." Kesden et al http://arxiv.org/abs/astro-ph/0411478 also write "the “smoking-gun” signature of an event horizon has yet to be observed. Until such a definitive determination is made, other candidates such as boson stars should continue to be considered."
     
  13. Sep 29, 2013 #12

    Chronos

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    Kerr black holes are torturously difficult to calculate and have their own problems. Boson stars are interesting, but, have little observational support. I'm beginning to suspect event horizons may be an illusion which might be resolved with a proper quantum treatment. There is too much 'magic' going on at event horizons [which fluctuate] for my comfort level. I would be interested in seeing a calculation showing the maximum possible mass density before an event horizon forms. That might prove difficult without a working theory of quantum gravity, but, might be interesting.
     
  14. Sep 30, 2013 #13

    mfb

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    Well, there are massive, very small objects which do not emit (significant) light on their own. That is certain.
    X-ray spectra of accretion disks have been measured, and the inner edges of those (determined by the redshift) are consistent with the predictions for the smallest stable orbit around black holes. This means the massive, very small object has to be smaller than a few times the Schwarzschild radius. And it has to be dark.
    Constraining Strong Gravity Using Iron Line Features in Black Holes
    A strong and broad iron line in the XMM-Newton spectrum of the new X-ray transient and black-hole candidate XTE J1652-453

    The precise metric of rotating black holes is a different question.

    The authors write that it is important to test the predictions of GR (it is always important to test existing theories), but they don't think that GR is wrong.
     
  15. Sep 30, 2013 #14

    atyy

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    I guess it depends on one's Bayesian prior and loss function. Nowadays consensus seems to have promoted Cynus X-1 from "black hole candidate" to "black hole". I've read that Hawking conceded his bet with Thorne in 1990. Yet the Chandra press release in 2011 http://www.nasa.gov/mission_pages/chandra/news/cygnusx1.html reports: "For forty years, Cygnus X-1 has been the iconic example of a black hole. However, despite Hawking's concession, I have never been completely convinced that it really does contain a black hole -- until now," said Thorne. "The data and modeling described in these three papers at last provide a completely definitive description of this binary system."

    In 2007 Rees http://arxiv.org/abs/astro-ph/0701512 is still indicating some degree of uncertainty in his language for the supermassive black holes.
     
  16. Oct 1, 2013 #15
    So there is no perfect example of a black hole yet??????
     
  17. Oct 1, 2013 #16

    mfb

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    What do you mean with "perfect example"? The best evidence you can get is falling into one, but that is not practical and you won't be able to share the results with others outside. Black holes are so small that a direct optical image of the accretion disk nearby is hard (but might be possible with future telescopes), and a direct observation of them is not possible as they are dark.
    Direct experiments with black holes are currently not feasible as we don't have one in our solar system and we don't have interstellar spaceships. So what is a "perfect example"? Which measurements have to be done, and in agreement with the predictions for black holes?
     
  18. Oct 1, 2013 #17

    Radarithm

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    No we haven't seen one yet, but they're believed to exist.
    What's interesting about a black hole is the singularity; infinitely deep and infinitesimally small.
     
  19. Oct 3, 2013 #18
    I agree with this. All so-called black holes, I think, must be called quasi-black holes. Furthermore, if a black hole is formed from a chunk of dust, I think, time dilation also prevents the dust from falling further before becoming a genuine black hole because time dilation is larger at the central region than at the surface. It must be observed as frozen in the air. As a result, singular point will never be formed. Following link shows further discussion.
    http://en.doppolaboratory.com/english-menu/my-simple-questions/can-a-black-hole-exist/
     
    Last edited: Oct 3, 2013
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