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Use of a black hole

  1. Feb 27, 2015 #1
    For example a black hole is stable. As in
    Ts=Tu
    then what would be the uses of the BLACKHOLE.
    Would it have any such use ???
     
  2. jcsd
  3. Feb 27, 2015 #2
    If you are planning to travel to a new universe, theoretically, a black hole would be of great use.
     
  4. Feb 27, 2015 #3

    Garth

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    Have you been watching 'Interstellar'? :wink:

    Don't believe the 'science' in that movie, there are so many mistakes - the tidal forces of the gravitational field approaching a Black Hole (BH) event horizon are so severe that you are much more likely to suffer from https://en.wikipedia.org/wiki/SpaghettificationSpaghettification.[/PLAIN] [Broken]

    It is true that the tidal forces of Supermassive Black Holes are more gentle and may not be fatal, but the accretion disc surrounding it is likely to be so luminous in the very high energy range of the electromagnetic spectrum (gamma rays and x-rays) that it would still be a very dangerous place to be.

    If you traveled through the event horizon there would be no guarantee that you would go anywhere beyond the 'singularity' at the centre.

    Apart from sending in an artificial probe, I would keep well clear of any BHs!



    Garth[/url]
     
    Last edited by a moderator: May 7, 2017
  5. Feb 27, 2015 #4

    PeterDonis

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    Not for a supermassive black hole. Saying that the tidal forces there "may not be fatal" is an understatement; they can be so small as to be undetectable near the horizon. Kip Thorne explicitly chose a mass for the hole in "Interstellar" for which this was true.

    This is true for many holes, including practically all the ones we can (indirectly) observe. But there's no reason it has to be true for all of them, particularly very large supermassive ones such as Thorne used as a model for the events in "Interstellar".

    This is actually connected to what I said about tidal forces above. Holes which are large enough to accrete significant mass from nearby space, but small enough to still have significant tidal forces near the horizon, will tear apart objects like stars that come close enough to them. That's how accretion disks are likely to form (at least it's one mechanism that is believed to be common).

    A hole of the size Thorne used in "Interstellar", however, is so large that, as I said above, tidal forces near its horizon are negligible. That means an object like a star that falls into the hole just gets swallowed whole; it doesn't start to break up until it is inside the horizon. So there's no accretion disk because there's nothing that breaks up infalling matter before it reaches the horizon.

    For a non-rotating hole (the one in "Interstellar" was rotating at near-maximal angular velocity), this is true--in fact you wouldn't even reach the singularity, because tidal forces would tear you apart first. Even if tidal forces are negligible at the horizon, they increase without bound as the singularity is approached in a non-rotating hole.

    For a rotating hole, things are different. First, the singularity in a rotating hole is timelike, not spacelike, so, unlike the singularity at the center of a non-rotating hole, it could be avoided by choosing your trajectory appropriately. However, that is only true in a spacetime that contains absolutely nothing besides the rotating hole. Any matter or radiation falling into the hole will be highly blueshifted as the inner horizon (not the event horizon--that's the outer horizon) is approached. The effect of this is to completely change the spacetime geometry inside the event horizon, so that (at least according to best current belief) the inner horizon never forms, and neither does the singularity.

    (The technical term is that the inner horizon and everything inside it, including the singularity, are unstable against small perturbations. Note that, strictly speaking, the "smooth" interior of Schwarzschild spacetime, the spacetime of a non-rotating hole, is also unstable against small perturbations; but unlike the rotating case, those perturbations do not eliminate the singularity--they just make the tidal forces you encounter as it is approached chaotically fluctuate.)

    (Also note that all of the above is classical--what the correct theory is when quantum effects are included is a hot topic of research, and nobody really knows the right answer at this point.)
     
  6. Feb 27, 2015 #5

    Garth

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    Some of the largest BHs, at the centre of quasars are very bright indeed, so you cannot count on a massive BH being benevolent.

    As we have been discussing in the Extremely large Black Hole discovered 900M years after BB thread, the paper about which can be read here: An ultra-luminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30, we have both an ultra luminous and Supermassive BH; not a nice place to be close to.....

    Whether rotating or not, or whether it has an accretion disc or not I would still advise keeping clear of them!

    Garth
     
  7. Feb 27, 2015 #6

    PeterDonis

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    Yes, but the BH that Kip Thorne used for the movie was a lot larger even than those--well over a trillion solar masses, IIRC. He was not trying to claim that such a BH actually exists; it's a movie, after all. :wink:

    I would too; I didn't mean to imply that what is portrayed in the movie is something I would recommend trying. Only that it is, in fact, consistent with GR, given the assumed conditions.
     
  8. Feb 27, 2015 #7

    Garth

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    I thought Gargantua was meant to be smaller than that, 108M, whereas the one at z=6.30 is ~1.2 × 1010M.

    From Space.com's The Science of 'Interstellar' Explained :
    interstellar-wormhole-travel-141107c-02.jpg

    Garth
     
  9. Feb 27, 2015 #8

    PeterDonis

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    Hm, yes, I see. I think I must have been remembering the figure from Thorne's Black Holes and Time Warps, where he also discusses a black hole called Gargantua (but not the same one he used for the movie).

    (Please note, btw, that this is not an endorsement of the space.com presentation in general. Just on a quick skim I see several bloopers.)
     
  10. Mar 3, 2015 #9

    mfb

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    I wonder how the accretion disk and the planet are supposed to co-exist. It looks like they are in the same radius range (although 2D images of 3D sytems are even harder to interpret if we actually have 4D systems) and the planet cannot be outside for the time dilation they gave.

    A black hole would be very useful: you could throw every type of waste in, and get useful radiation out of the accretion disk with a conversion efficiency of something like 20-30%.
    A very small black hole (~10^10 tons) would allow to get even more via Hawking radiation (not 100% as you get some neutrinos).
     
  11. Mar 4, 2015 #10

    Chronos

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    You would need a very dedicated crew to man any black hole creation project.
     
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