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Black Holes effect on each other

  1. Jan 28, 2009 #1
    I am fairly ignorant in the area of cosmology, but I was wondering what the effect black holes have on other black holes may be.
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  3. Jan 28, 2009 #2


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    Well that indeed is a really interesting question, except its quite hard (I should say impossible) to solve exactly and you normally need a computer to help you as well as making all sorts of potentially bad approximations to get any sort of handle on the situation.

    Anyway to first order, if they are far from one another you can think of them as point masses and they interact exactly like you would think from Newtons theory.

    If you get close to each other, they interact and well all sorts of hard to calculate things occur. The spacetimes start stretching and well you get a complete mess.

    It gets worse and worse as you try to be more and more exact (for instance quantum effects can become important, b/c they both emit Hawking radiation that is then absorbed which changes their equation of state and their effective horizon size, etc etc)

    In short, we think that to leading order you get something like a star on star interaction, and after that to compute GR and much later quantum corrections is an open problem.
  4. Jan 28, 2009 #3
    If they get too close they merge I would imagine, wait around for a few billion years and you might get to see it happen. I wouldn't know where to begin with the maths though.
  5. Jan 28, 2009 #4


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    There are computer simulations of what the gravitational field looks like, mapping how it behaves, during a merger. I remember finding some on the web a few years back. Short animation showing one spiraling in and merging, or a straight head-on merger.

    My memory is not clear about it, but I suspect if you do a google search for black hole merger simulation, or black hole merger computer animation, something like that, you might find something.

    They have to study this because it would produce gravity waves of a possibly distinctive character---IOW a type of signal to look for with the various gravity wave detectors planned and built.
  6. Jan 29, 2009 #5


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    Indeed, this is the case. Far away, they're just like any other masses. It's only when they start to get close together that interesting stuff happens. The basic effect of them getting close enough to one another is that they start emitting gravitational radiation as they orbit one another. As they emit gravitational radiation, their mutual orbit decays, and they get closer. The closer they get, the larger this effect, and more and more radiation is emitted.

    As you mention, of course, the precise details of how this occurs are rather difficult to calculate. However, one thing that we do know about black holes is that physicists have proven these slick theorems called "no hair" theorems. What they say is that black holes only have a few degrees of freedom: their mass, angular momentum, and charge. Once you specify those properties of a black hole, you have fully specified the black hole itself. This means that if two black holes merge, there are only a small number of variables that need to be used to describe said merger. This has tremendous implications for cosmology, because it means that it may be possible to estimate, to a very high degree of precision, the energy released during a black hole-black hole merger.

    Why is this interesting? Because the next generation of gravitational wave detectors should detect a significant number of these mergers every year as galaxies merge, and as a result we might be able to use these events as a new type of "standard candle": a standard candle is an object whose intrinsic brightness we either know or can estimate from other observables. Then by looking at how bright it appears from Earth, we can determine how far away it is. This is the principle that allows us to use certain types of supernovae to accurately measure the expansion of the universe. But because of the no hair theorems, black holes have the potential to be a vastly, vastly more accurate measure than supernovae.

    It only remains to actually detect them and see if there are enough things we can observe about these events to nail down their intrinsic brightness.
  7. Jan 29, 2009 #6
    I agree with Clanoth regarding the "no hair" theorems, but I would like to add another useful item.

    Black holes generally have accretion disks and polar jets, both which can be observed. From an observational POV, having two accretion disks, two sets of polar jets, two magnetic fields, etc. smacking into each other could be as interesting as observing the gravity interaction between two (or more!) black holes.

  8. Jan 29, 2009 #7


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    Well, from the standpoint of simple observation, it seems unlikely that such a merger will happen anywhere near close enough to us for us to get a good observational handle on those parts of the collision.

    However, it turns out that these non-idealities are pretty much essential for using such collisions as a distance measure: for us to use them as a distance measure, we need the redshift of the collision. And for that to occur, there needs to be an optical component of the collision. Naturally such an optical component could only come from the matter that surrounds the black holes, not the black holes themselves. So it stands to reason that we need two things:

    1. That there be enough matter outside the black holes involved in the interaction to provide a detectable visual signal.
    2. That the mass of that matter still be very small compared to the mass of either black hole, so that it doesn't interfere with the gravitational wave signal.

    The first point is simply unknown. We hope there will be a visible component, but since we've never detected such a collision we really can't say. The second point, I think, we can be pretty confident about, as normal matter just isn't going to approach black hole densities without, well, being in a black hole.
  9. Feb 4, 2009 #8
    Well im curious now about black holes. So if two black holes are headed towards each other, your saying it would attract each other even with huge magnetism racing about? Not necessarily north and south poles, but all over kind of the way the sun is. So who's not to say it would attract but at the same time repel and kind of just swing around each other to create like a central core of massive gravity? I can see it eating up the accretion disks of each other, but not the core itself cause of the huge magnetic fileds. I guess what im asking is this, why cant they swing around each other like binary suns? I'm not into physics or anything cause I believe that far in deep space it makes no difference what we calculate. (My opinion sorry) So take it easy, im just being curious.
  10. Feb 4, 2009 #9


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    If you've ever played with magnets, you know that they don't really repel one another. Even if you push the north poles of two magnets together as hard as you can, and then let go, they won't fly apart: instead they'll flip over and shoot together.

    But I sincerely doubt the magnetic fields of black holes is anything remotely as strong as their gravitational fields.
  11. Feb 4, 2009 #10
    OK I'll give you that one. For now. LOL Yeah I was reading about some of the gravitational forces of black holes, that is just amazing. So, even if the two cores were to come together or in close proximity, would there be enough force from one core to tear apart the other one if they were about the same size, that almost seems impossible considering the massive amount of gravitational pull on itself. I guess that's is where I was coming up with them acting like binary stars, the gravitation would be so massive and powerful they would just start to swing around each other, not in a big orbit, but in a tight knit orbit creating a more powerful black hole. Guess I have more reading, doubt I will find anything though.
  12. Feb 5, 2009 #11
    Now we're getting into something that interests me more. What does happen as the magnetic forces of two black holes approach each other? If the relativistic jets are caused by these great magnetic forces, what fireworks would be created as two jets from two great magnetic forces started to combine, twist, and curl upon each other!?! Who do I pay to see this light show?
  13. Feb 5, 2009 #12


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    Well, stay tuned. When we get some gravitational wave observations of such mergers, we may well also get some clue as to the optical component.

    Bear in mind, however, that a black hole only has a jet so long as it has matter that is falling into it. It is by no means clear that there will always be matter around for these black holes to feed upon as they collide.
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