View Poll Results: When blackholes collide,what happens to the singularity?  
Does gravity force them together?  5  33.33%  
Is there a cataclysmic explosion,and they are destroyed/  2  13.33%  
Do both add there gravity,don't merge,and become a bigger one?  8  53.33%  
Voters: 15. You may not vote on this poll 
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When blackholes merge. 
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#19
May203, 08:06 PM

P: n/a

Note that if two ordinary masses with radius r1 and r2 collide (say, two planets, or water droplets merge) then radius of resulting mass is NOT r1+r2, but rather (r1^3+r2^3)^1/3.



#20
May203, 08:19 PM

Astronomy
Sci Advisor
PF Gold
P: 23,236

In Wheeler's case of the two black holes (which get rid of massenergy by gravity waves as they coallesce) the new mass is sqrt (m^2 + M^2) and so the new radius is sqrt (r^2 + R^2). But with the two droplets the new mass is for all practical purposes m+M and as you point out the new radius is cuberoot(r^3 + R^3) 


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