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sage
2008-Feb-06, 07:10 PM
Hi! how close is the nereast balckhole to the earth? Is it possible for a black hole to consume another black hole? If its does what happens?

AndreasJ
2008-Feb-06, 07:46 PM
Unknown*. Yes. You get a bigger black hole.

*The nearest known black hole appears to be about 1600 light years away, but since black holes are usually only detected when they're gobbling up matter at a high rate (leading to X-ray emissions from the stuff falling in) there might be quiescent ones closer by that would be very difficult to find.

max8166
2008-Feb-06, 07:49 PM
I believe the closest Black hole would be in the the center of our galaxy.
------------------------------------------------------------
HOLLY: As it transpired, there weren't any Black Holes.
RIMMER: But you saw them -- you saw them on the monitor.
HOLLY: They weren't Black Holes.
RIMMER: What were they?
HOLLY: Grit. Five specks of grit on the scanner-scope. See, the thing
about grit is, it's black, and the thing about space, the defining thing about space is it's black so how are you supposed to tell the difference?

AndreasJ
2008-Feb-06, 08:02 PM
I believe the closest Black hole would be in the the center of our galaxy.

No, there are several stellar-mass black holes closer than that, the closest apparently being the one at 1,600ly (http://www.spaceref.com/news/viewpr.html?pid=579) I aluded to. :)

Neverfly
2008-Feb-06, 08:59 PM
Hi! how close is the nereast balckhole to the earth? Is it possible for a black hole to consume another black hole? If its does what happens?

Be careful with that word 'consume.'

The planet Earth 'consumes' a lot of space debris too.

A black hole merger simply creates a more massive dense black hole.

I'm sure that inside, the impact event is something rather extraordinary, under such extreme conditions. Sadly, we have no way of observing it directly to find out.

Kebsis
2008-Feb-07, 01:19 AM
I think that while the singularity would simply get denser, the event horizon of the black hole would increase, making what most people would consider to be a larger black hole. Correct?

tadj
2008-Feb-07, 02:02 AM
It may seem logical that two black holes merging would be simply the sum of the parts but Have we ever observed two black holes merging or colliding? Is there any evidence that this has happened? If it is conceivable that any two large bodies (not black holes) merging could be the first step in creating a different object, a black hole, then could two black holes merging give rise to some other, as yet unobserved, object? Any ideas, anyone?

Neverfly
2008-Feb-07, 02:05 AM
It may seem logical that two black holes merging would be simply the sum of the parts but Have we ever observed two black holes merging or colliding? Is there any evidence that this has happened? If it is conceivable that any two large bodies (not black holes) merging could be the first step in creating a different object, a black hole, then could two black holes merging give rise to some other, as yet unobserved, object? Any ideas, anyone?

Can some of the observed supermassive black holes be merged black holes?

Hornblower
2008-Feb-07, 02:26 AM
According to an article in Sky and Telescope, August 2006, p. 16, a merger of two supermassive black holes would emit a brief but colossal burst of energy in the form of gravitational waves. So far no such signature has been detected. Scientists studying the theoretical dynamics say that an array of spacecraft linked with lasers to form a giant interferometer would be needed to detect such an event.

publius
2008-Feb-07, 04:13 AM
They do give off an enormous burst of gravitational radiation (well, they are predicted to do so according to GR) -- the instantaneous radiated power supposedly can exceed the output of the whole visible universe.

But an even more fascinating thing is that burst of gravitational radiation is not isotropic, but directional, and gives the center of mass a large recoil kick. That is definitely not Newtonian. When both black holes are themselves spinning, well things get quite complex to say the least. It takes some humongous supercomputer processor farm running for ages to do those simulations.

Anyway, the merged black hole is predicted to kick a recoil kick of up to several hundred km/s with such events, and IIRC they've been looking for evidence of that. A merger could apparently kick itself out of the host galaxy. And that just strikes me as amazing.

And that also suggests a gravitational radiation rocket. Make directed gravitational waves and thrust yourself. There was recent paper exploring that very thing. It's basically an academic exercise, though.

To get any significant thrust, you'd have to be slinging around neutron star density material like it was nothing, requiring lord knows how much energy, not to mention the huge regular gravitional fields of that huge a mass anyway.

If you had the much energy and that much strength to generate the necessary forces, you'd do far better with other more traditional methods. But, as a theoretical exercise, it's pretty cool I think.

-Richard

publius
2008-Feb-07, 04:31 AM
Oh, and please, please, nobody ask me if you would feel a force riding a gravitational wave rocket. :lol:

Whether or not you feel the force of your own gravitational radiation reaction, I don't know, and that mess gets quite complicated. If you put a gun to my head and made me say yea or nay, I'd probably say yes, you do feel it. But this is well beyond my understanding. Well beyond.

If you are radiating as you free fall (which even a mass radially free falling in Schwarszchild does, actually -- and even that seemingly simplest case is a humdinger), you do end up getting pushed off your geodesic.

You can see it as a self-field interaction just like with EM, although a more complicated one. Your own gravitational field perturbs the geodesics of the larger field you're falling in, and you end up not following the resultant geodesic.

That is, that's how I think it works. And so, if you're deviating from your geodesic you feel a force, ergo you feel gravitational radiation reaction. But don't take that to the bank. :)

That sort of gives me an uneasy feeling about our old friend the Equivalence Principle, but I don't think there's a problem at all. In principle, the local tide is detectable, and so if this is the case, the radiation reaction would have to go "only as the tide" -- that is, the local reaction force we could measure with a local accelerometer could only tell us the tide (curvature of space-time). And I think that's how it works.

-Richard

FriedPhoton
2008-Feb-07, 04:36 AM
If you have a pair of binary stars and one is sucked into a black hole the other is flung out at ridiculously high speed, right? I think I read this somewhere. So what if you have two black holes orbiting each other and one is sucked into a third black hole, then you have a runaway black hole careening through the universe.

Imagine what would happen if one went zipping through our section of the neighborhood. It could cause some damage. And we'd never see it if it wasn't sucking down mass quantities of stuff. Would the only hope we'd have be seeing it occult a star?

AndreasJ
2008-Feb-07, 12:10 PM
I think that while the singularity would simply get denser, the event horizon of the black hole would increase, making what most people would consider to be a larger black hole. Correct?

I don't think that it makes sense to say that the singularity gets denser; density is mass/volume and that's undefined at the singularity. The event horizon indeed gets bigger*, as does, of course, the mass of the black hole.

* When two black holes merge, the area of the event horizon of the new hole is equal to or larger than the sum of the areas of the original holes.

Jeff Root
2008-Feb-07, 05:19 PM
I'm sure that inside, the impact event is something rather extraordinary,
under such extreme conditions. Sadly, we have no way of observing it
directly to find out.
A merger of two black holes would do something quite extraordinary to
the spacetime inside (as well as nearby) the resulting black hole, but
the matter would never collide, as everything falling into a black hole
is stretched infinitely in the radial direction. Everything gets pulled
farther and farther apart. Everything is spaghettified without limit.

This is almost certainly by design of the all-powerful FSM.

Well, almost all-powerful.
Even His meatballs wouldn't survive the fall into a black hole.



Can some of the observed supermassive black holes be merged black holes?
Probably most of them. Possibly all of them!

-- Jeff, in Minneapolis

John Mendenhall
2008-Feb-07, 05:51 PM
If you are radiating as you free fall (which even a mass radially free falling in Schwarszchild does, actually -- and even that seemingly simplest case is a humdinger), you do end up getting pushed off your geodesic. -Richard

I don't have the link at hand, but there's recent release about the anomalous
acceleration of gravity boosted probes being due to a horribly complex electromagnetic radiation courtesy of GR.

Kebsis
2008-Feb-07, 11:02 PM
I don't think that it makes sense to say that the singularity gets denser; density is mass/volume and that's undefined at the singularity. The event horizon indeed gets bigger*, as does, of course, the mass of the black hole.

* When two black holes merge, the area of the event horizon of the new hole is equal to or larger than the sum of the areas of the original holes.

Well I can't claim to know much about the math involved, but it seems logical to assume that if you add matter to something and it doesn't expand in overall size, it must have become more dense. If I am not mistaken, a singularity is considered a point in space which could not expand, although I'm not completely sure about that.

FriedPhoton
2008-Feb-08, 02:25 AM
A merger of two black holes would do something quite extraordinary to the spacetime inside (as well as nearby) the resulting black hole, but the matter would never collide, as everything falling into a black hole is stretched infinitely in the radial direction. Everything gets pulled
farther and farther apart. Everything is spaghettified without limit.

Hey, that's totally cool. I never heard about this lack of collision and stretching in the radial direction but it seems to make sense. Can you point me in a direction where I can read about these ideas?

alainprice
2008-Feb-08, 04:32 AM
The density of a BH cannot increase because it is already infinite with current models. You add mass and increase the density of infinity to....infinity. The end result is more mass.

Is there a difference between an event horizon merger and a singularity merger. Could enough energy need to be released(unlikely) from the singularity merger that it becomes measurable outside?

Jeff Root
2008-Feb-08, 04:58 PM
Hey, that's totally cool. I never heard about this lack of collision
and stretching in the radial direction but it seems to make sense.
Can you point me in a direction where I can read about these ideas?
One source is the book 'Black Holes and Time Warps - Einstein's
Outrageous Legacy' (1994) by Kip S. Thorne.

Search on some of these terms in combination:

spaghettification
black hole
singularity
Oppenheimer-Snyder
Finkelstein

-- Jeff, in Minneapolis

AndreasJ
2008-Feb-08, 07:47 PM
Well I can't claim to know much about the math involved, but it seems logical to assume that if you add matter to something and it doesn't expand in overall size, it must have become more dense.
It seems to me that the phrase "become more dense" cannot have any meaning when refering to an object that does not have a well-defined density in the first place.

alainprice
2008-Feb-08, 11:53 PM
Even if we use a superstring model so that the whole thing doesn't collapse to a point, one does not expect an increase in density. The maximum density should have been reached already.

FriedPhoton
2008-Feb-09, 08:00 AM
One source is the book 'Black Holes and Time Warps - Einstein's
Outrageous Legacy' (1994) by Kip S. Thorne.

You know, I was at the bookstore tonight and actually had that book in my hands and thought about buying it. Now all I have to says is :doh: