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# When blackholes merge.

by chosenone
Tags: blackholes, merge
 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.
Astronomy
PF Gold
P: 23,274
 Originally posted by Alexander 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.
Thanks! that is a good way to see a way that the black holes differ from droplets!
In Wheeler's case of the two black holes (which get rid of mass-energy 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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