[ExI] paradox perhaps

BillK pharos at gmail.com
Thu Sep 15 12:11:22 UTC 2022

On Thu, 15 Sept 2022 at 05:44, spike jones via extropy-chat
<extropy-chat at lists.extropy.org> wrote:
<big snip>
> Regarding something I posted about a few weeks ago, why I thought these events would be so rare: I couldn’t (and still can’t) grasp where all that angular momentum is going.
> The following are not even BOTECs, no need to mess up an otherwise back of an envelope.  We can do these in our heads.  Imagine two black holes forming (somehow) at a sun/earth distance, one AU, so about 150 million km, so multiply by 2 pi, the circumference of the orbit is about a billion km, and it goes around in about 30 million seconds, so the earth goes about 30 km per second.  So now imagine increasing the mass of both to 30 solar masses, same distance apart, so the attraction is 60 times greater, so the orbit speed is close enough to 8 times faster, 240 km per second and we still having messed up an envelope.
> The momentum absorbed is the equivalent of (somehow) stopping an object of mass 30 solar masses going 250 km per second.  I run thru those equations regarding warping of spacetime, and I just don’t get how there could be so many of these events still happening.
> My visual picture of two black holes forming at 1 AU must be wrong, because if that really did happen (somehow) then it would require something weird like dark matter participating in drawing down the angular momentum.  Dark matter does interact gravitationally with ordinary matter, but I don’t understand how dark matter could come along and cause two black holes to fall together.
> There’s just too dang much I don’t know.
> spike
> _______________________________________________

Yes, it is very complicated. You need to solve general-relativity
field equations. And probably use supercomputers.  :)

Try this -
The first observation of stellar-mass binary black holes merging,
GW150914, was performed by the LIGO detector. As observed from Earth,
a pair of black holes with estimated masses around 36 and 29 times
that of the Sun spun into each other and merged to form an
approximately 62-solar-mass black hole on 14 September 2015, at 09:50
UTC. Three solar masses were converted to gravitational radiation in
the final fraction of a second, with a peak power 3.6×1056 erg/s (200
solar masses per second), which is 50 times the total output power of
all the stars in the observable universe. The merger took place
440(+160−180) megaparsecs from Earth, between 600 million and 1.8 billion years
ago. The observed signal is consistent with the predictions of
numerical relativity.

Orbital angular momentum can and does get transferred to spin, or
rotation, of the merged black hole.
(But some is also lost in gravitational radiation - see above).


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