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Black Holes Disputed

Posted by michael on from the assuming-the-theory-holds dept.

JScarpace writes: "Researchers at the Los Alamos National Laboratory in New Mexico and at the University of South Carolina in Columbia have proposed the existence of "gravastars" which are bubbles of superdense matter. If they are correct, the idea of a black hole with a singularity at the center may be just a fantasy."

9 of 296 comments (clear)

  1. But if there are no black holes... by Oroborus · · Score: 5, Funny

    Where do all my left socks go from the dryer?

  2. Here's their paper by pepik_knize · · Score: 5, Informative

    4 pages in your choice of formats here.

    1. Re:Here's their paper by KjetilK · · Score: 5, Informative
      Thanks a lot! Saved me the trouble of searching! :-) However, it should be emphasized that this is a pre-print, it could have changed substantially when going through peer-review.

      Also, folks, don't slashdot the site unless you know a bit about cosmology (if you don't know what I'm talking about when I say "line element" forget it) - this is a site that is very important for physicists in their daily work.

      --
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  3. no singularity... by Cryptnotic · · Score: 5, Insightful
    No one ever said that there had to be a singularity for there to be a black hole. All a "black hole" means is that the mass of the object is so great that the escape velocity exceeds the speed of light. Nothing can go faster than light, so you'd never make it out of such a place. Since light can't be reflected off of it, it's called a "black hole".

    A "singularity" is a point at which the gravitational force is infinite. This logically doesn't even make sense, so it's no wonder that it's disputed.

    Cryptnotic

    --
    My other first post is car post.
    1. Re:no singularity... by harlows_monkeys · · Score: 5, Insightful
      Yeah, that's the physics geek explanation. When I posed this question to one of my physics professors at Caltech back in my student days, he came up with nothing better than that, either. Basically, as far as we could tell, (1) the "escape velocity is greater than speed of light, and nothing can go faster than light" explanation has nothing really to do with it, and (2) there is no simple intuitive explanation of why you can't get out of a black hole (where "simple intuitive" means comparable to the escape velocity explanation).

      This reminds me of the explanations in books for the general public on how airplanes work. The usual explanation is that because of the curve on top of the wing, when the airflow splits at the leading edge, the part that goes over the top has to go farther to meet up with the corresponding part that goes under the wing, and since it has to go farther in the same time, it has to go faster. We know from Bernoulli that a faster airflow has lower pressure, so we get a pressure differential, and the wing rises.

      Anyone see the problem with that? The first problem is that no reason is given for the airstream over the top to have to meet up with the airstreem under the bottom. Why can't it just flow straight back?

      The real reason wings work is that they cause vortices to be generated that, because of the shape of the wing, go down, and since vortices have momentum, by the usual laws of elementary physics, there is force upward on the wing.

      There is no simple explanation of why wings cause these vortices, but no one likes to say in their books for laymen that it takes a PhD in aerodynamics to understand how airplanes work, so we get the totally irrelevant Bernoulli explanation.

      It would be interesting to compile a list of various areas in science and engineering where there are explanations for the general public like this, that are basically wrong, and are so widespread that even scientists and engineers use them without thinking about them when writing for the general public.

    2. Re:no singularity... by alfredw · · Score: 5, Insightful

      because time / speed of light is 'apparently' slower in gravitational fields

      Indeed. This prediction is a result of trying to integrate a function through points where the value is infinite, and then dividing that result by another infinite number.

      The end is result is, to use the beautiful terminology of a Mathematician, "indeterminate."

      The conventional interpretation of this results is that the theory cannot reliably predict the behaviour of the universe at points like this. Given the total lack of experimental evidence in regards to these phenomenon, I'd say that's a safe bet.

      The same sort of thing happens when you try to calculate the total energy in a cavity (due to heat) using a classical theory. It tells you "infinity." What that means is "you should have used Quantum Mechanics."

      --
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  4. Re:gravastar? by nomadic · · Score: 5, Funny

    Fine, I shall rename them.

    From now on they will be referred to as Doom Spheres.

  5. Re:What is the difference? by BlueUnderwear · · Score: 5, Insightful
    A black hole is just a place where the escape velocity exceed the speed of light. There is no dispute over whatever such a thing can actually exist, as all you need is enough mass for it to happen.

    Escape velocity not only depends on the mass, but also on the distance from the center. In a black hole, escape velocity only exceeds the speed of light if you get closer than the event horizon.

    Now, if for some reason the necessary mass would not fit into the event horizon, no black hole could occur. This new theory stipulates that if you have such a huge mass, it will actually form a hollow sphere where much of the mass is actually concentrated outside of the event horizon. Now, a hollow sphere has the following "interesting" properties (or would have, in classical mechanics):

    1. outside the spere it is the same as for a point mass: mM/r^2
    2. within the shell it would be approximately linear in r: mM*(r-r_inner)/r_outer^2/(r_outer-r_inner)
      (approximative formula, for "thin" shells)
    3. inside, it would be zero
    Probably this is not 100% exact, as we're in a relativistic context here, rather than a classical one, but we can still presume that gravity inside the sphere would be much weaker than outside.

    This would basically mean that you would not have an escape velocity exceeding the speed of light anywhere:

    • not outside of the sphere, because you'd be outside of the event horizon
    • not inside either, because inside a hollow spere gravity caused by the sphere is basically zero.
    --
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  6. It's called the Kutta condition by Goldenhawk · · Score: 5, Insightful
    I'm an aerodynamic engineer.

    >Anyone see the problem with that? The first
    >problem is that no reason is given for the
    >airstream over the top to have to meet up
    >with the airstreem under the bottom. Why
    >can't it just flow straight back?

    See here for one of many explanations of the Kutta condition, one of the foundational principles of aerodynamics. This has nothing to do with an explanation for the layman. Basically, it states that the air MUST meet smoothly at the back of the wing.

    Logically, if you spend some time thinking about the flow, you cannot possibly construct a situation where the air above the wing somehow slips past the air below. Remember that a jet moves so fast that its wing is only passing through a portion of the air for fractions of a second - it's simply not possible to make the air move fast enough to slip like this.

    This principle has been demonstrated NUMEROUS times. You can demonstrate it very easily with a line of smoke through which a wing passes, among a zillion other simple experiments.

    --
    --Brandon / Split Infinity Music