Catch up on stories from the past week (and beyond) at the Slashdot story archive


Forgot your password?
Space Science

No Naked Black Holes 317

Science News reports on a paper to be published in Physical Review Letters in which an international team of researchers describes their computer simulation of the most violent collision imaginable: two black holes colliding head-on at nearly light-speed. Even in this extreme scenario, Roger Penrose's weak cosmic censorship hypothesis seems to hold — the resulting black hole (after the gravitational waves have died down) retains its event horizon. "Mathematically, 'naked' singularities, or those without event horizons, can exist, but physicists wouldn't know what to make of them. All known mechanisms for the formation of singularities also create an event horizon, and Penrose conjectured that there must be some physical principle — a 'cosmic censor' — that forbids singularity nakedness ..."
This discussion has been archived. No new comments can be posted.

No Naked Black Holes

Comments Filter:
  • by exp(pi*sqrt(163)) ( 613870 ) on Monday October 06, 2008 @12:42AM (#25269877) Journal
    The surface area of a black hole increases with its mass. And we expect the total area of all event horizons [] to increase over time (apart from a small amount of leakage from Hawing radiation).

    And the boom from a black hole is usually in the form of X-rays or gamma rays radiation and, in energetic terms, it's very loud.

  • Re:Cosmic Censor (Score:2, Informative)

    by Anonymous Coward on Monday October 06, 2008 @02:02AM (#25270197)

    the really funny thing about that, is that the word mass we use to refer to how much of an object there is, originates from the Christian meaning of it.

  • by The Master Control P ( 655590 ) <ejkeever&nerdshack,com> on Monday October 06, 2008 @02:26AM (#25270289)
    You appear to have no idea what's going on here. Okay, first of all, the Cosmic Censorship Hypothesis in question (short version): All singularities other than the one from the Big Bang are hidden behind event horizons.

    The equations of relativity, which were used to run the simulation, say nothing about cosmic censorship. The C.S.H. wasn't formulated until 50-odd years after general relativity because of a problem - relativity actually readily admits (physically-implausible) solutions that do have naked singularities, hence the censorship. Apparently, something always conspires to hide them.

    This simulation confirmed the hypothesis' prediction: Even in the most violent circumstances physically realizable, the singularity ended up behind an event horizon.

    Frankly, it's time we admitted it... the only way we're going to find a naked singularity is to go for a joyride in the direction of the Great Attractor in a sycamore-seed-shaped ship.
  • by D.A. Zollinger ( 549301 ) on Monday October 06, 2008 @02:35AM (#25270335) Homepage Journal

    A few years ago, I might have agreed with you. After all, on a basic level you are correct, if we program what we know into a simulation, the simulation will be based on what we know!

    Last semester I took a class in complex system, and it really opened my eyes about what computer simulations can do for us in providing unexpected behavior. Most of this is because we have a pretty good grasp on simple systems, and can take those simple systems and program them into a computer with rules of interaction to see how they will interact without human guidance.

    Let me give you an example: Most everyone here at one point of time or another have programed "Life []" into a computer. We understand the rules, we understand the program itself, and we understand how everything is going to work, but until you actually run the program, you would never have expected the results! How could you have predicted the formations that would develop? The stable formations, the chaotic formations, the moving formations? Much less how these formations would interact when they collide?

    I think in a way this is what was being simulated in the program mentioned above. We think we have a pretty good idea about the simple systems which make up a complex entity like a black hole. But how do these simple systems interact when they encroach upon another black hole? Assuming we really do understand these simple systems, and that they stay constant, I think this simulation gives us a reasonable expectation as to how black holes will react to a collision.

  • by earlymon ( 1116185 ) on Monday October 06, 2008 @02:54AM (#25270403) Homepage Journal

    Photons have no mass but do have momentum.

    The Lorentz transform causes a breakdown for E in E=mc^2/sqrt(1-v^2/c^2) where v=c. And when you have enough gravity to bend space such that it folds in on itself - light cannot escape, despite being massless but gaining momentum from the gravity well - you have a singularity.

    One can't just say that equations break down, but physics do not. The equations are the language used to express the known physics.

    So, there is a singularity, there needs to be, and it isn't magical - unless you mean magical in the sense of wonderful.

  • by meringuoid ( 568297 ) on Monday October 06, 2008 @03:16AM (#25270487)
    Why doesn't a black hole collapse onto itself and disappear since it attracts everything in its vicinity? IOW, why do black holes have sizes?

    All the mass of a black hole is compacted into an extremely small region at the centre - possibly infinitely small, but at the very least as small as physics allows matter to get. This is the singularity.

    When we speak of the size of a black hole, we're actually referring to the region around that central object from which nothing can escape. As you approach the black hole, the gravitational field gets stronger and stronger, and there's a point of no return at which the escape velocity reaches c, the speed of light. Nothing nearer the hole than this can ever escape. This we call the event horizon - because no events beyond the horizon can ever be observed from outside. The more massive the hole, the further out the event horizon: look up 'Schwarzschild radius' for the equation.

    The result of this is that any singularities in the universe are expected to be hidden behind event horizons, and cannot be seen. It's occasionally suggested that a naked singularity might form - for instance, a black hole might be spinning so fast as to counteract the effect of gravity and allow the singularity to be viewed from outside. This could have extremely bizarre results for the universe as a whole, so most physicists expect there to be some kind of 'cosmic censorship' principle that ensures that this does not happen. What we're looking at here is one way in which that might happen.

  • by Diamo ( 1364811 ) on Monday October 06, 2008 @05:30AM (#25270977)
    Time appears slow to the outside observer, for the object crossing the horizon it's business as usual, super fast acceleration, stretched out and sucked into oblivion. Lovely :)
  • by Diamo ( 1364811 ) on Monday October 06, 2008 @06:31AM (#25271197)

    ...and this might begin to answer your question but I still find it hard to understand! []

  • by uberdilligaff ( 988232 ) on Monday October 06, 2008 @06:58AM (#25271311)
    Remarkably well said.

    A black hole isn't some mystical thing unrelated to the other cosmological objects. Black holes are just stars that have consumed most of their fuel through fusion over billions of years, then collapsed. But consumed doesn't mean the mass is all 'burned up' and gone, but converted from hydrogen and helium into heavier elements that are harder to participate in further fusion reactions, resulting in decreasing internal pressure from energy being released by the star. If the conditions are right, the compacting force of gravity from all the 'star stuff' that's left exceeds the declining expansive pressure provided by the fizzling nuclear reactions inside the star, and it ultimately collapses into an incredibly small size. If the size is less than the Schwarzschild radius, it will become a black hole.

    But it's still just a lump of star stuff with mass like what the star had, but in a dramatically smaller package. It doesn't suddenly go on a cosmic rampage, marauding around and sucking up everything in sight. If something external has sufficient distance and velocity that it would have flown by or orbited the former star, then it will fly by or orbit the hole, as these parameters are solely determined by the masses of the star/hole and the external thing. If something would have fallen into the star, it will fall into the hole as well. Whether it falls into a black hole or a star, it's not coming back out.

    Astronomers infer the properties of black holes from what they can observe about the objects that are influenced by them, and from what they observe about the progression of stars throughout their lifetimes. Just because we can't see into black holes doesn't mean they are totally mysterious.
  • by Fyz ( 581804 ) on Monday October 06, 2008 @07:14AM (#25271371)
    A photon is not subjected to the flow of time at all since it travels at the speed of light, and thus has a time dilation factor of infinity compared with any other frame of reference.

    So pity not the photon, for even an eternity is less than a moment to it.
  • by Mr. Slippery ( 47854 ) <> on Monday October 06, 2008 @08:28AM (#25271779) Homepage

    Might there exist, in large black holes, ones with atomic numbers in the thousands?

    IANAA (I am not an astrophysicist) but IIRC before a collapsing star gets to the black hole stage it would (however briefly) go through a point where gravity was sufficient to collapse atoms - a neutron star. So I don't think there are any atoms in such a black hole. (Of course, that's theory, no one has made the observation to check!)

    However, not all black holes from from stellar collapse. I have no idea what the theory says about what the super-massive black hole at the galaxy's center might be made of.

  • by Almost-Retired ( 637760 ) on Monday October 06, 2008 @09:23AM (#25272267) Homepage

    First I'll correct your spelling by removing the spurious t from photons.

    Then there is the miss-use of the term black hole, at least according to my concept. From what you wrote, the proper term s/b "event horizon". You can see anything on this side of it, but whats inside it cannot be seen since the horizon diameter is in fact the distance from this object where the escape velocity equals C.

    Now here is a conjecture for you, an expansion of your idea if you will.

    Assume a large mass, whose gravity is so strong that its 'escape velocity' is within a few planks constants of C speed. We can still see it, and it is losing mass via its thermal radiation. Because its a large object, losing 1% of that mass via that mechanism would take many millions of years. During which time its gravity will be pulling in more material. This mass then sucks in another .000000001% of its own mass by gobbling up an ort cloud sized rock. Voila! An event horizon rises from its surface to that radius where the escape velocity is C. Then you have a black hole, because you cannot now see the object. This demarcation line might be only a fraction of a centimeter from the surface of the mass, which due to its gravity will be a perfect sphere, distorted only by the fattening effects caused by its rotation if any, and generally will be quite high according to the orbital mechanics, and determined by the transfer of the energy given it by the incoming material as its orbit decays.

    In any event, this large mass turning into a black hole by the formation of an event horizon might be an interesting show to watch, but I suspect not very spectacular, and over shadowed by this objects obvious effects on its environment, and gravitational lensing of distant objects is probably what would lead to our discovering it anyway. We may be able to see a star that is very bright in the far ultraviolet and x-rays, and watch it blink out forever in just a microsecond when the event horizon forms.

    Then you have a 'black hole'. And then you have the singularity because the only thing you can measure is the vector of its gravity, which will point to the exact center of the object, eg the 'singularity'.

Always leave room to add an explanation if it doesn't work out.