Question of the Week

What happens when two black holes collide?

Sun, 20th Nov 2011

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Question

Clive Plum, Sutton Coldfield asked:

What happens when two black holes collide?

Answer

We posed this question to cosmologist and astrophysicist Professor Martin Rees, University of Cambridge.......

Martin -   One of the things weíve learnt in the last 10 years or so is that in the centre of every galaxy, there lurks a black hole which is as massive as a million, or even several billion, suns.  Itís not quite clear how this black hole formed, but we believe that its formation happened at the same time as the galaxy formed and that the black hole grew as the galaxy grew.  We also know that one way galaxies grow is by merging with each other - two small galaxies get close and eventually fall together and make one bigger galaxy.  Now, if two galaxies merge and each has a black hole at its centre, then the black holes will of course power towards the centre of the common system and form a binary orbiting around each other and the question then is what happens?  We think what happens is that various drag forces bring the holes close enough that so-called gravitational radiation then carries away more energy.  Gravitational radiation is something predicted by Einsteinís theory to happen whenever a gravitational field changes rapidly.  So when two black holes get close enough, then gravitational radiation from them carries away a lot of energy and that brings them closer and closer until they eventually merge into a single black hole.  And in the final coalescence, a huge burst of gravitational radiation is emitted and this is a challenge to detect.  But the biggest events of all would be the gravitational bursts from these super massive black holes which merge and there are plans to have arrays in space which could detect the slight jitter in space that happens when these black holes merge.

Diana -   The meeting of two black holes will produce a large one and it will also produce lots of energy in the form of gravity waves.  But that happens when the two black holes are of different size.

Martin -   So we believe that these black holes are merged and itís by this process that eventually, some of them accumulate masses as much as in some cases, 5 billion times the mass of the sun.  There's another interesting consequence.  Itís being possible for the last few years to actually do computations of what would happen when two black holes merge and when the black holes are of an equal mass, then you get a rather interesting effects namely recoil.  The" alt="Black Hole Merger" /> final merged black hole doesnít necessarily sit in the centre of the merged galaxy.  Itís given a kick and sometimes this kick could be big enough to expel the merged black hole from the galaxy itís in.  So, the final thought I leave you with is that it could be some of these huge black holes hurtling through intergalactic space, having been kicked out by the huge recoil speed from the galaxy in which they formed.

Diana -   So beware of a super massive black hole flying through space like a very dark, very heavy, juggernaut of the skies.  

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i have no idea but if i had to take a guess they would cancel each other out DAVID WOOD , Mon, 14th Nov 2011

THE RESULTANT BH IS SMALLER THAN EITHER OF THE ORIGINALS as the event horizon grows? CZARCAR, Mon, 14th Nov 2011

You'll have one big black hole i guess, which will be bigger than either of the originals, but smaller than just the sum of the two.

EDIT: The fact that we'll find out next week is a bit of a scary thought o_O Nizzle, Tue, 15th Nov 2011

maybe the new BH wont spin as much? CZARCAR, Tue, 15th Nov 2011



angular momentum will be preserved -  it was this problem with rotation that stopped me venturing an opinion on this question imatfaal, Tue, 15th Nov 2011



angular momentum will be preserved -  it was this problem with rotation that stopped me venturing an opinion on this question
centrifugal force is constrained by increased gravity? CZARCAR, Tue, 15th Nov 2011

I believe that they won't collide because of their gravitational interaction which causes them to orbit about a common center of mass. Daylights, Sat, 19th Nov 2011



I suspect you are right. Unless the BHs are on a perfect collision path, or one is much smaller than the other, they will probably go into orbit.

(I better fire up the geezimulator. Some smart alec like Matt erudite individual is bound to ask me to prove it.) Geezer, Sat, 19th Nov 2011

Maybe you will get a VERY ...... BIG BANG ! Dr Graham, Wed, 30th Nov 2011

I suspect you are right. Unless the BHs are on a perfect collision path, or one is much smaller than the other, they will probably go into orbit.

I think someone else has pointed out elsewhere that orbits are tricky-business.  That essentially for two objects that aren't locked in orbit to become locked in orbit, it requires a third object which can absorb kinetic energy of the system.  And, with black holes, this might mean either a very large star, or a third black hole.

One can still imagine many interesting scenarios happening as two equal sized black holes approached each other.  When considering two distant black holes, the center of mass in the 2-body system would be outside of their event horizons.  For distance objects, that might not mean much, but it could be a problem as they approached.

What if they approached so that the event horizons merged, but still  distant enough that the center of the black holes remained outside of the other's event horizon?

What if they approached on a tangent so that the center of each black hole just skimmed the other's event horizon, yet carrying with it an extraordinary amount of kinetic energy which one would anticipate could take them out of orbital range. CliffordK, Wed, 30th Nov 2011

I suspect you are right. Unless the BHs are on a perfect collision path, or one is much smaller than the other, they will probably go into orbit.

I think someone else has pointed out elsewhere that orbits are tricky-business.  That essentially for two objects that aren't locked in orbit to become locked in orbit, it requires a third object which can absorb kinetic energy of the system.  And, with black holes, this might mean either a very large star, or a third black hole.

One can still imagine many interesting scenarios happening as two equal sized black holes approached each other.  When considering two distant black holes, the center of mass in the 2-body system would be outside of their event horizons.  For distance objects, that might not mean much, but it could be a problem as they approached.

What if they approached so that the event horizons merged, but still  distant enough that the center of the black holes remained outside of the other's event horizon?

What if they approached on a tangent so that the center of each black hole just skimmed the other's event horizon, yet carrying with it an extraordinary amount of kinetic energy which one would anticipate could take them out of orbital range.


Clifford - what is required is some form of dispersion of energy - that can happen through the promulgation of gravitational waves, additionally it can happen through the heating and moving of dust/gas.  As black holes getting close are a prime example of rapidly changing gravitational there will be g waves produced.  And very few black holes are alone in the void - they tend to have a rapidly moving accretion disk of matter that has failed (most due to conserved ang mom) to fall into the black hole yet but is spiralling in (with slow radial movement) but very rapid angular movement. imatfaal, Thu, 1st Dec 2011

Assuming that an orbital capture occurs...  then there would be one of several possibilities.

Both black holes spinning in the same direction, also orbiting in the same direction.
Both black holes spinning in the same direction, but orbiting in the opposite direction.
Both black holes spinning in opposite directions.

One would assume the rate of spin is higher than the orbital speed.

In the first case (same spin, same orbit), it would seem to be similar to the Earth and Moon, and the two would slowly push apart, due to the tides slowly adding orbital velocity, rather than having a degrading orbit.

In the second case (orbit in anti direction), they would fight between the spin and rotation, and it would create a degrading orbit.

In the third case, opposite spins, I think it might depend on the mass distribution. CliffordK, Thu, 1st Dec 2011

Cliff - rather you than me trying to calculate the tidal forces on a potential singularity (none?) or on a kerr rotating blackhole donut (pass) imatfaal, Thu, 1st Dec 2011

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