Message boards : LHC@home Science : Can a black hole pull matter out of another black hole?
Message board moderation

To post messages, you must log in.

AuthorMessage
Profile Alex

Send message
Joined: 2 Sep 04
Posts: 378
Credit: 10,765
RAC: 0
Message 13317 - Posted: 11 Apr 2006, 8:20:12 UTC
Last modified: 11 Apr 2006, 8:23:21 UTC

This is more of a theoretical question than a practical one.


Ok.. suppose you have two black holes that are spiralling around each other.
(like these ones http://www.cbc.ca/story/science/national/2006/04/06/black-holes-20060406.html )

Wouldn't there be a spot between the two black holes where the gravitational effect from each hole is cancelled out by the other black hole?

As two black holes spiral together, would there be the opportunity for material to be flung out from between the two spiralling holes?

What if you have two black holes, each 1 meter across, and as they overlap, you get a spot between them with the gravity cancelled out. Could material be spit out then?




I'm not the LHC Alex. Just a number cruncher like everyone else here.
ID: 13317 · Report as offensive     Reply Quote
Gaspode the UnDressed

Send message
Joined: 1 Sep 04
Posts: 506
Credit: 118,619
RAC: 0
Message 13318 - Posted: 11 Apr 2006, 8:35:20 UTC

What if you have two black holes, each 1 meter across, and as they overlap, you get a spot between them with the gravity cancelled out. Could material be spit out then?


I'm not sure what the real effect of an overlapping pair of black holes would be. The specific answer to your question is that the point at which the gravitational effect is cancelled out (assuming that it makes any sense inside a black hole) will still be within the event horizon of both black holes.

To imagine this, take two identical spheres and cut an identical slice from each. The flat surface exposed will be within the radius of the sphere. Place the two spheres together with the flat faces in contact - any point of equidistance is on the plane of the slice, and all within the original radius.

I suspect that the real effect of overlapping black holes this way would be more spectacular than a bit of matter being spat out. This could be a source of a Gamma Ray Burst, for example.


Gaspode the UnDressed
http://www.littlevale.co.uk
ID: 13318 · Report as offensive     Reply Quote
Nuadormrac

Send message
Joined: 26 Sep 05
Posts: 82
Credit: 112,933
RAC: 0
Message 13325 - Posted: 11 Apr 2006, 16:22:44 UTC
Last modified: 11 Apr 2006, 16:35:05 UTC

Eventually, they might merge together to become 1 object? I'd have to look to see if there are any suggestions/theories in place that might indicate one way or the other.

BTW, black holes might not be such that they eventually don't start to lose matter anyhow. At least based on some more recent

http://www.damtp.cam.ac.uk/user/gr/public/bh_hawk.html

We can try to describe the interaction of some quantum matter with gravity by quantising the matter on a fixed, classical gravitational background. That is, we can try quantising the matter, but not the gravity. This will work only if the gravity is weak. It should work outside a large black hole, but not near the singularity.

Using this approach, Hawking has shown that a black hole will radiate thermally. That is, if we study quantum matter fields on a classical black hole background, we find that, when the matter fields are initially in the vacuum (that is, there is no matter falling into the black hole), there is a steady stream of outgoing radiation, which has a temperature determined by its mass and charge.

This is an extremely startling discovery; classically, no radiation can escape from a black hole, but if we quantise the matter fields, we find there is steady flux of radiation coming out of the black hole! This outgoing radiation decreases the mass of the black holes, so eventually the black hole will disappear. The temperature goes up as the black hole gets smaller (unlike most things, which cool off as they lose energy), so the black hole will disappear abruptly, in a final flash of radiation.


Actually, not to throw controversy into this, but 2 more articles when I was searching about this matter I read about sometime ago... Some might find it of interest:

http://www.newscientistspace.com/article/dn8836-black-holes-the-ultimate-quantum-computers.html

This would likely prove highly controversal, and almost is of a matter we'd need more answers then anything else. Means by which the hypothesis could be tested are presented however...

http://www.newscientist.com/article.ns?id=mg18925423.600&feedId=online-news_rss20
ID: 13325 · Report as offensive     Reply Quote
Profile Alex

Send message
Joined: 2 Sep 04
Posts: 378
Credit: 10,765
RAC: 0
Message 13396 - Posted: 17 Apr 2006, 8:47:12 UTC - in response to Message 13318.  

What if you have two black holes, each 1 meter across, and as they overlap, you get a spot between them with the gravity cancelled out. Could material be spit out then?


I'm not sure what the real effect of an overlapping pair of black holes would be. The specific answer to your question is that the point at which the gravitational effect is cancelled out (assuming that it makes any sense inside a black hole) will still be within the event horizon of both black holes.

To imagine this, take two identical spheres and cut an identical slice from each. The flat surface exposed will be within the radius of the sphere. Place the two spheres together with the flat faces in contact - any point of equidistance is on the plane of the slice, and all within the original radius.

I suspect that the real effect of overlapping black holes this way would be more spectacular than a bit of matter being spat out. This could be a source of a Gamma Ray Burst, for example.



I meant to use Overlap as a verb, meaning that they approach each other in their spiral over time, finally overlapping.
If a black hole can pull material out of a star, then perhaps it can pull material out of another black hole while the spiraling is taking place, and perhaps the spiral motion would slingshot some material away.




I'm not the LHC Alex. Just a number cruncher like everyone else here.
ID: 13396 · Report as offensive     Reply Quote
River~~

Send message
Joined: 13 Jul 05
Posts: 456
Credit: 75,142
RAC: 0
Message 13403 - Posted: 18 Apr 2006, 6:59:00 UTC - in response to Message 13396.  
Last modified: 18 Apr 2006, 7:14:19 UTC

What if you have two black holes, each 1 meter across, and as they overlap, you get a spot between them with the gravity cancelled out. Could material be spit out then?


I'm not sure what the real effect of an overlapping pair of black holes would be. The specific answer to your question is that the point at which the gravitational effect is cancelled out (assuming that it makes any sense inside a black hole) will still be within the event horizon of both black holes.


I meant to use Overlap as a verb, meaning that they approach each other in their spiral over time, finally overlapping.
If a black hole can pull material out of a star, then perhaps it can pull material out of another black hole while the spiraling is taking place, and perhaps the spiral motion would slingshot some material away.





You make two assumptions, one is valid.

The good assumption is that there must be a neutral point between the two gravitational objects where a third "test object" would be equally attracted to either. Lagrange did the maths for this under Newtonian mechanics.

What is interesting is that General Relativity upholds many of the Newtonian results. As is well known, the size of a black hole is equal to the size of a Newtonian body with an escape velocity equal to c.

Think of our two black holes - what holds them apart?

Are they falling into one another without any sideways motion (Angular Momentum, AM) ? If so then the situation really will be transient. If the point on the line of centres is not already inside the event horizon, it soon will be. Not much scope for matter to be ejected.

There is very likey to be significant sideways motion. The smaller an object the harder it is to hit it - and for any given mass the black hole event horizon is pretty much the smallest possible target. This means that your two black holes are most likely in orbit around one another.

Now, consider how tides work. There is a tide that points from the Earth towards the moon, and another that points away.

(Due to the ocean's inertia, the ocean tides are delayed from the direct line between the centres, and some parts of the ocenas only experience one tide, or four tides, in a day. But here I am talking of the tidal force that causes the seas to move, and that force is along the line of centres, both directly towards the moon and on the other side, directly away from it).

This makes things feel lighter, though their mass is unchanged. It follows that the apparent accelaration of gravity is less along the line of centres than at right angles to that line. Escape velocity will therefore be less.

In a black hole, where there is no surface below you to stop you getting closer to the centre, you'd find that you could get back to the same escape velocity by getting closer to the centre.

So, without needing any maths at all, we can see that in the case you describe the event horizon, defined as the point where the Newtonian escape velocity is c, cannot be spherical in the case you describe. As there will be a tide on both sides, the event horizon is flattened slightly.

That was your invalid assumption, I'm afraid. But I am very glad you raised the question, as some of the implications are quite profound. As often happens in science a mistaken idea can be the route to new understanding.

How can a black hole not be spherical? Aren't all black holes spherical.

Actually, no. Einsteins equations of General Relativity are impossible to solve for the general case, so what theorists do is to find simple cases which they can solve.

The equations we usually think of as describing black holes were worked out by Schwarzchild. He produced a set of equations that solve Einsteins equations

(yes, GR is so complicated that the answer is not a number, like 42, but an equaltion that would itself give someone a heart attack!)

His equations are strictly speaking only true if

a) the central object is sperically symettrical
b) it is not spiining
c) it contains all the mass

This means that any spin, any irregularity, or any mass that is outside the central object, will perturb the results. When we think of Earth and Sun, the Earth is much less massive than the sun, and we can be confident the perturbation is very small. We can work out the black hole radius for the sun and find it is comfortably smaller than the real sul.

The sun spins. But again we can work out that the spin is small enough that it does not affect the results too much.

The sun is not symmetrical - it bulges rounnd the waist like someoneone on a SupersizeMe diet. Again we can work out the size of the bulge and it does not affect the results too much.

However, in your thought experiment, you have 50% of the mass outside the central object. The Schwarzchild equations will not apply, the event horizon(s) need not be spherical, which is good because we just "proved" intuitively that they can't be.

Someone called Kerr produced a solution to Einsteins equations. Kerr threw away restriction (b), to get the results for a black hole with angular momentum. In the Kerr solution, the event horizon is not spherical. If you are interested in so-called spiining black holes, do a Google on the "Kerr metric".

However the Kerr metric still has only one central object - you want two. Has someone produced such a metric? I don't know, sorry.

What we need to answer your question is a reworking of Lagrange's work but within General Relativity rather than within Newtonian mechanics. I don't know if this has been done, but for now let's call it the Alex metric ;-)

What is certain from General Relativity is that the Alex metric would not allow stuff to pop out of the event horizon - that only happens when we add quantum effects to GR. We'd either find that the event horizons had not yet met, and stuff could therefore pop out; or that the event horizons had met, and the stuff was permanently trapped inside.

For me the interesting questions would be where, firstly, as they approach each other, do the event horizons actually coalesce? If both objects were of a mass that would have an event horizon of 1 metre in empty space, how much close can we get them than 2m apart before the distorted event horizons meet?

Secondly, with the Earth-Moon system we have one escape velocity that allows escape from the Earth only to get to the Moon, and another escape velocity that allows us to get away altogether.

I am wondering if the Alex metric might predict that in the zone you describe matter is not yet inside the event horizon of either of the two centres, but nevertheless is inside a third communal event horizon surrounding both black holes. There is a proof that you canonly have a single event horizon, but if I remember right that proof only applies to the Schwarzchild metric, and not necessarily to other solutions to Einsteins equations.

Thirdly, is it true that, once inside the same event horizon, the two singularities from the parent black holes would inevitably have to meet? Could it be that their angular mementum would keep them apart? My guess is that, like in the Kerr metric, the angular momentum would not prevent coalescence. In that case, would the hole collapse into the Kerr metric, or somehow maintain some imprint of the fact that it had formed from a non-symmetric cofiguration?

Thank you for an interesting question. It leads to a lot o other interstng questions, and that is the sign of a good thought experiment.

~~gravywavy
ID: 13403 · Report as offensive     Reply Quote
River~~

Send message
Joined: 13 Jul 05
Posts: 456
Credit: 75,142
RAC: 0
Message 13404 - Posted: 18 Apr 2006, 8:00:04 UTC - in response to Message 13318.  

I suspect that the real effect of overlapping black holes this way would be more spectacular than a bit of matter being spat out. This could be a source of a Gamma Ray Burst, for example.


The only output from the overlapping of the black holes proper would be gravitiational radiation.

However, in the real universe (such as the links Alex gave in his original post) both black holes are likely to be surrounded by disks of accreting matter. As the material in each ring encounters the other accretion disk there are likely to be all sorts of energietic effects, and gamma ray bursts could well be among them.

In fact when astronomers talk of radiation from black holes, they mean from the accretion disk - the hole itself does not radiate appreciably unless it is too small to be of interect to astonomers.
ID: 13404 · Report as offensive     Reply Quote
Gaspode the UnDressed

Send message
Joined: 1 Sep 04
Posts: 506
Credit: 118,619
RAC: 0
Message 13415 - Posted: 20 Apr 2006, 0:50:33 UTC
Last modified: 20 Apr 2006, 0:52:29 UTC

However the Kerr metric still has only one central object - you want two. Has someone produced such a metric? I don't know, sorry.


It looks like Nasa has been working on this problem.

Here's a report from the BBC, and here are Nasa's pages on the subject.
Gaspode the UnDressed
http://www.littlevale.co.uk
ID: 13415 · Report as offensive     Reply Quote
Profile Alex

Send message
Joined: 2 Sep 04
Posts: 378
Credit: 10,765
RAC: 0
Message 13416 - Posted: 20 Apr 2006, 8:26:14 UTC
Last modified: 20 Apr 2006, 8:30:19 UTC

Thanks for all your responses. Interesting stuff.


Here's the wiki for the Kerr Metric

http://en.wikipedia.org/wiki/Kerr_metric


I'm not the LHC Alex. Just a number cruncher like everyone else here.
ID: 13416 · Report as offensive     Reply Quote
Profile Alex

Send message
Joined: 2 Sep 04
Posts: 378
Credit: 10,765
RAC: 0
Message 13417 - Posted: 20 Apr 2006, 9:43:55 UTC - in response to Message 13415.  
Last modified: 20 Apr 2006, 9:50:13 UTC

However the Kerr metric still has only one central object - you want two. Has someone produced such a metric? I don't know, sorry.


It looks like Nasa has been working on this problem.

Here's a report from the BBC, and here are Nasa's pages on the subject.



A lot of people have been working on it for years.

Nasa quotes equations used by 'BSSN'
BSSN is Baumgarte–Shapiro–Shibata–Nakamura
Another set of letters is KST or Larry Kidder (Cornell),
Mark Scheel (Caltech), Saul Teukolsky (Cornell)

http://arxiv.org/abs/gr-qc/0205064
http://www.ipam.ucla.edu/publications/pcaws3/pcaws3_5612.pdf
http://astrogravs.nasa.gov/conf/numrel2005/presentations/pfeiffer.pdf

A bunch of people have thought about binary black hole systems for a while.

Disclaimer: I'm not a mathie.


I'm not the LHC Alex. Just a number cruncher like everyone else here.
ID: 13417 · Report as offensive     Reply Quote

Message boards : LHC@home Science : Can a black hole pull matter out of another black hole?


©2021 CERN