# The Naked Scientists Forum

### Author Topic: What happens at the centre of mass of two identical colliding black holes?  (Read 1295 times)

#### chiralSPO

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##### What happens at the centre of mass of two identical colliding black holes?
« on: 23/10/2015 23:20:04 »
I have two closely related questions about colliding black holes:

1) Imagine two black holes on a direct collision course (no net angular momentum). For the sake of argument, let's say each is of sufficient mass to have an event horizon at a radius of 10000 km. Would a hypothetical observer (that is nearby but far enough to stay clearly outside of the event horizon landscape during the course of the collision) see evidence of the gravity change during whole process, or would the change cease for said outside observer once the event horizons merge? Phrased another way, can we see evidence of the mass distribution of stuff that is all inside the event horizon?

2) Now let's think about an observer that is at the center of mass of the system. Black holes are approaching on either side, completely symmetrically, so there is no net force on the observer throughout (ignoring spaghettification as the the proposed black holes have such large event horizons). What do they experience?

3) (BONUS QUESTION!!!!) As the event horizons approach, touch and merge, what happens to a beam of light that is passing through the center of mass, and is orthogonal to the motion of the two black holes?
« Last Edit: 24/10/2015 13:22:08 by chris »

#### evan_au

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##### Re: What happens at the center of mass of two identical colliding black holes?
« Reply #1 on: 24/10/2015 02:04:37 »
Simplification: The case of uncharged, non-rotating black holes is simpler to solve, since the event horizon can then be considered a sphere, and there is no frame-dragging involved.

It is already stated that these are massive black holes, so we can ignore Hawking radiation.

#### jeffreyH

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• The graviton sucks
##### Re: What happens at the center of mass of two identical colliding black holes?
« Reply #2 on: 24/10/2015 11:57:57 »
I have two closely related questions about colliding black holes:

1) Imagine two black holes on a direct collision course (no net angular momentum). For the sake of argument, let's say each is of sufficient mass to have an event horizon at a radius of 10000 km. Would a hypothetical observer (that is nearby but far enough to stay clearly outside of the event horizon landscape during the course of the collision) see evidence of the gravity change during whole process, or would the change cease for said outside observer once the event horizons merge? Phrased another way, can we see evidence of the mass distribution of stuff that is all inside the event horizon?

It all depends upon the spacial position in the system where the fields exactly cancel each other. Another consideration is time dilation. What happens to that in a net zero gravitational field?

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2) Now let's think about an observer that is at the center of mass of the system. Black holes are approaching on either side, completely symmetrically, so there is no net force on the observer throughout (ignoring spaghettification as the the proposed black holes have such large event horizons). What do they experience?

That's the 64 million dollar question.

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3) (BONUS QUESTION!!!!) As the event horizons approach, touch and merge, what happens to a beam of light that is passing through the center of mass, and is orthogonal to the motion of the two black holes?

If both forces are equal at all distances along a straight line path then there should be no curvature in the path of the light. Otherwise the path will curve. If you could 'freeze' the motion of both black holes at the instant that the event horizons are 2 Planck lengths apart what would happen to the photon? If time dilation is unaffected then this should be what an external observer sees I think. This should be the point at which light becomes motionless for a VERY short period of time..

#### Thebox

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##### Re: What happens at the center of mass of two identical colliding black holes?
« Reply #3 on: 24/10/2015 12:14:48 »
I have two closely related questions about colliding black holes:

1) Imagine two black holes on a direct collision course (no net angular momentum). For the sake of argument, let's say each is of sufficient mass to have an event horizon at a radius of 10000 km. Would a hypothetical observer (that is nearby but far enough to stay clearly outside of the event horizon landscape during the course of the collision) see evidence of the gravity change during whole process, or would the change cease for said outside observer once the event horizons merge? Phrased another way, can we see evidence of the mass distribution of stuff that is all inside the event horizon?

2) Now let's think about an observer that is at the center of mass of the system. Black holes are approaching on either side, completely symmetrically, so there is no net force on the observer throughout (ignoring spaghettification as the the proposed black holes have such large event horizons). What do they experience?

3) (BONUS QUESTION!!!!) As the event horizons approach, touch and merge, what happens to a beam of light that is passing through the center of mass, and is orthogonal to the motion of the two black holes?

1.  Why would a black hole be attracted to a black hole?
angular momentum is not existence of space so is irrelevant anyway.
and two black holes cant merge it would be against physics like two eddies don't merge.

2. if there is an equilibrium of force from either side the observer experiences a ''magnetic suspension'', Fn=0
However the observer suspended in the center of an x-axis will be drawn along a Y-axis to other forces if present.

3.  reflected back?  did not quite understand the question, do you mean like a pulsar?

#### lightarrow

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##### Re: What happens at the center of mass of two identical colliding black holes?
« Reply #4 on: 24/10/2015 12:28:49 »
1.  Why would a black hole be attracted to a black hole?
Why it shouldn't be?
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angular momentum is not existence of space so is irrelevant anyway.
I'd rather say that it's this phrase to be irrelevant.
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and two black holes cant merge it would be against physics
No, it would be against physics if they wouldn't do it (at zero impact parametre, of course).

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lightarrow
« Last Edit: 24/10/2015 12:31:39 by lightarrow »

#### evan_au

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##### Re: What happens at the centre of mass of two identical colliding black holes?
« Reply #5 on: 24/10/2015 22:42:46 »
Quote from: Thebox
Why would a black hole be attracted to a black hole?
Black holes have a considerable mass, and exert a considerable gravitational attraction on other massive objects (including other black holes), attracting nearby objects into an orbit.

A likely example of two black holes attracting each other in an orbit may be seen here: http://www.iflscience.com/space/two-supermassive-black-holes-merge

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two black holes can't merge it would be against physics
Most galaxies are thought to harbour a supermassive black hole.

The galactic jets seen shooting from some galaxies are thought to originate from a black hole at the center of the galaxy, emitted along the axis of rotation of the black hole.

When two galaxies merge, the black holes are attracted to each other, and will emit gravitational waves until they eventually merge. The axis of rotation of the merged black hole will be different from the two original black holes, so the direction of any gravitational jet will probably change. This has been observed in real galaxies: