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First post a question about Black Holes and Gravity

  1. Dec 16, 2011 #1
    Hi everyone.

    I have a very keen interest in physics. Unfortunately at school I was made to do a combined qualification in science which didn't allow me to persue what I was interested in most.

    I still try and read and understand what I can about physics and cosmology... but one thing has bothered me for a while, maybe you can help answer...

    As far as I understand it, the mathmatical description of a black hole ends in singularity. In physics circles this is a bad thing as far as I know.

    We know black holes to exist, and from what we understand their mass is infinite, resulting in infinite gravity.

    Now, since gravity has unlimited range, why isn't a single black hole swalling up all mass objects from across the universe instantly? Surely infinite gravity + infinite range = no universe?

    Or is just because we don't understand the mechanics of black holes fully?

  2. jcsd
  3. Dec 16, 2011 #2
    This is incorrect.
    Whether an object is/becomes a black hole does not depend on its mass but on its density.
  4. Dec 16, 2011 #3
    i'm skeptical about black holes, a couple weeks ago they reported the dicovery of biggest BH yet, a 21 billion solar masses at a distance of 336 million light-years away, and yesterday they found that Milky Way Galactic BH is gulping up near cosmic cloud. these black holes pose clear and present danger to us in the long term (assume we survive in long term of course) according to singularity theory. they might find their theories are totally wrong about BH
  5. Dec 16, 2011 #4


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    Yes, "singularity" really just means "the math model breaks down and we don't know WHAT is going on".

    Yes, we do.

    Absolutely not. Their DENSITY may be infinite (won't know until we figure out what the "singularity" is) but the mass is most emphatically NOT infinite.
  6. Dec 16, 2011 #5


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    Well, I see two ways to interpret your post
    (1) You are lots smarter and have done lots more experiments than all those ignorant physicists who pretend to know what they're talking about.
    (2) You really need to study up on this stuff more.

    I can't be sure which, but I'm leaning towards #2.

    By the way, why in the world do you think black holes pose any danger to us even in the long term? Even the stars that are WAY closer to the black hole in the center of the Milky Way than we are just keep orbiting around it. What is the threat you see?
  7. Dec 16, 2011 #6
    Ah ok. So gravity is derived from mass not density? So its ok to be 'infinitely' dense. Or at least as far as the maths says so.
  8. Dec 16, 2011 #7


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    Yeah, but "infinitely dense" and "singularity" seem to be the same thing, so they are ALSO both "we don't know WHAT'S going on", or as you put it, " ... the math says so" (meaning, I assume, that this may NOT be what is REALLY going on)
  9. Dec 17, 2011 #8
    So a invariant mass is defined through it being at rest, also able to be the same if moved to any other 'frame of reference', although its weight can change. You can say that weight is always relative something else, but the invariant mass is expected to be what is left, taking all 'frames of reference' away, possibly :)

    One kg can be a sphere of one meter, or ten meters, or one decimeter, pick your choice. With the spheres scales defining different densities inside them.

    The radius from where you can expect a black hole to be formed in the simplest solution, for a non-rotating black hole, without a charge, in a empty isotropic space is called the Schwarzschild radius. In it Schwarzschild defines a 'place' where the density of that sphere becomes denser than is allowed by SpaceTime (Einsteins field equations in GR) relative its size, as you shrink that sphere. There the mass breaks down into something, defined by a event horizon, 'apparent' or not.

    That Event horizon will now define where light, or mass, stops being reflected. If you passed it all ways ultimately would lead you into its center as far as I know. If you ever looked at Newtons spheres you can see that in a sphere of a mass, Newton defined it as 'all mass' could be seen as existing inside its exact middle. Even though it not being the exact same definition from Relativity you can use it as a intuitive approach to that middle inside the Black Hole. And defined that way, you might say that the middle of that Black Hole indeed is of 'infinite mass' as that 'center' becomes infinitely small.

    But it also has to do with what people expect, some might assume that the mass still will 'take place', although I don't.

    But, what one need to remember here is that if defined through gravity, as you approach the event horizon, the Black Hole will behave exactly the same as it was before 'breaking down' into a black hole by its 'shrinking'. The mass it has for you will still be one Kg and its gravity will act that way, until you pass that event horizon, in where 'Terra incognito' takes over as no light, or mass, ever will be reflected/turning back to any observer outside the Event Horizon, at least not as long as we will exist.
    Last edited: Dec 17, 2011
  10. Dec 17, 2011 #9


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    Yes. A one kg sphere of lead is much smaller than a one kg sphere of aluminum.

    No, the location of the event horizon is simply where the curvature of space makes it impossible for light to escape. The gravitational force gradient determines how strong the different sides of an object are pulled. For solar mass black holes the gradient is severe, and most objects will be pulled apart as they fall in. For a supermassive black hole of millions of solar masses the gradient is much less severe, and a person could pass through the event horizon intact. (Although the event horizon is MUCH larger)

    Light is still reflected from any object that can reflect it. It is that the light can't get back out of the event horizon. Perhaps that is what you meant, I just wanted to clarify.

    No, this means that the mass of the sphere could be considered to be concentrated at a point in the very center when calculating the gravitational force between two objects at great distances from each other. The mass is never infinite.

    I have no idea what this means. Mass cannot "take place".

    Partially correct. If you compare a black hole to a star of equal mass, as long as you stay at least a distance equal to the star's radius away from the center of either the star or the black hole, then their gravitational fields are pretty much exactly the same. (Not quite, but nearly)
  11. Dec 17, 2011 #10


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    Keep in mind that black holes are a singularity, Physics stops at the event horizon (pt at which light cant escape). We don't have a theory to describe what happens beyond that point. We do have speculations though.

    Black holes do not have infinite gravity. As an example, if the sun were to turn into a black hole (with its existing mass) the gravitational effects we would feel would the same. In classical physics we treat massive objects as point masses which is a good approximation of its relativistic gravitational effect if you're far enough away from it. Based on that, the 21 billion suns BH is having the same effect on us now as it did before we knew of its existence.

    One other curious thing is that we will never see someone fall into a BH because as they approach the event horizon they appear to hover there forever. From their perspective, they approach and just fall in past the event horizon and we imagine they get stretched apart like taffy as the approach the singularity as they wonder why we are speaking so slowly to them over the radio, At least this is according to a StarGate episode where the gateway opens up to a planet being sucked in by a BH,
  12. Dec 17, 2011 #11


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    I think the problem is that physics stops at the singularity, not at the event horizon. It is by no means certain that singularities actually exist. It is possible that it is simply a result of our lack of knowledge about the subject.
  13. Dec 17, 2011 #12
    Drakkith, what's your beef with my writing?

    You just threw my words around.
    Rather cheap trick that doesn't impress me.

    Schwarzschild completed the first two exact solutions of the Einstein field equations in static isotropic empty space surrounding a massive body, as a black hole

    "General relativity predicts that as an object collapses to form a black hole, it will eventually reach a point of infinite density. What that really means is that the theory of relativity breaks down at this point, and no one knows what happens at the center of a black hole - we would need a viable theory of quantum gravity in order to understand this. "

    Btw: curvature of SpaceTime, not 'space'
    Last edited: Dec 17, 2011
  14. Dec 17, 2011 #13
    It is wrong, in my opinion, to think of black hole singularities as existing now.

    In the frame of a remote observer, the formation of a black hole appears to take an infinite time in classical general relativity - that is to say, if we could see one forming, every single particle would still appear to be outside the black hole for ever. This observation is not actually possible, but only because the material falling in becomes so red-shifted it cannot emit a single photon. Physicists sometimes say this effect is an illusion, but if so it is an illusion which predicts every single possible observation by a remote observer. That's good enough for me.

    A black hole is a region of space where we know that if we went there and headed towards the mass, we would eventually pass an event horizon and shortly after that reach a singularity. Note that all of this is in the future.

    It seems perfectly reasonable to say that black holes are places where time stops in the Universe. While there is a single starting point to the history of the Universe, at black hole event horizons one enters a sort of side branch of the future timeline which ends quickly and badly. These singularities do not exist in the main timeline of the Universe, you have to take a side branch through an event horizon to get to them.

    Whether there is any consequence of such singularities, no-one knows. But if you bear in mind that they are a region of space time that appear infinitely far in the future to all observers that remain at a safe distance from the event horizon, any consequences of such singularities can very likely have no influence on anyone who does not fall through an event horizon.

    With regard to other points in this discussion, black holes have finite mass, and a black hole of a particular mass is gravitationally just the same as any object of that mass, until you reach the event horizon. So being dragged into a supergiant black hole is no more imminent than being dragged into a passing galaxy. Which doesn't keep me awake at night.
    Last edited: Dec 17, 2011
  15. Dec 17, 2011 #14


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    Did I? Looking over it again I don't see how. Much of it was misleading and inaccurate.

  16. Dec 17, 2011 #15


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    I have to agree w/ Drakkith. I think his statements were much more clear (and correct) than yours. I have no beef with you and I don't know Drakkith except as another member of this forum, as you and I are, so you might consider that if two people feel this way about your statements, perhaps there's something there for you to consider.
  17. Dec 17, 2011 #16


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    I agree. The theory of general relativity works perfectly well both inside and outside the event horizon. The problem occurs only when we get very close to the centre. General relativity ignores quantum effects, but those effects can't be ignored close to the centre. Until someone successfully combines general relativity with quantum theory we don't really know what happens at the very centre.
  18. Dec 17, 2011 #17
    That's true for the singularity but not for the black hole itself.

    That would be true for a static black hole but due to the incoming mass the event horizon is expanding.
  19. Dec 17, 2011 #18
    I see what you are claiming, but this is contrary to my understanding. If you are remain at a remote point outside a forming black hole, my understanding is that no particle ever reaches a point from which the remote point is inaccessible (in the sense a photon could no longer be transmitted to it).
    Well, even if an event horizon had miraculously already been created (as observed by a remote observer) and mass known to have disappeared from the region from which a photon could reach him, any particles falling into it after this still appear to take an infinite time to reach the event horizon (in general relativity). Bear in mind the point that the event horizon cannot expand until a particle falls through it, and that gravitational effects respect the speed of light and hence cannot outrun photons.

    My understanding is that what we actually would see is the material falling in disappearing completely (very quickly no photons can get to us because the time dilation increases exponentially) but what we see next is probably a transition to the black body radiation predicted by Hawking due to the convergence of what we see towards a black hole - stuff falling in appears to get very very close to the event horizon, and after it is within the Planck length or whatever, the gravitational field is effectively indistinguishable from that of an observed black hole.

    I'll be glad to hear disagreement about these extrapolations of what I know from those who start off with the advantage of knowing more than I do!
    Last edited: Dec 17, 2011
  20. Dec 17, 2011 #19
    The event horizon will not be created miraculously but by exceeding the critical density of the mass inside.

    This is correct for a black hole with constant mass but the mass of the black hole increases due to the incoming particles. This expansion of the event horizon already happens before the incoming particles reach it. Therefore the particles does not reach the event horizon but the event horizon reaches the particles in finite time even from the view of a distant observer.
  21. Dec 17, 2011 #20
    [oops, posted twice - see next]
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