[ExI] Gravitational Waves Detected By LIGO!
Anders Sandberg
anders at aleph.se
Sat Feb 13 13:04:04 UTC 2016
On 2016-02-13 11:50, Tomaz Kristan wrote:
> Interesting ... Still, what's bothering me is also the super-massive
> black hole in our Galaxy, five orders of magnitude closer and about
> five orders of magnitude as massive, orbiting by many massive stars
> ... but no gravity waves from there.
>
> That was my line of reasoning all along. If we can't gravitationally
> see this, how we could see something much smaller, so far away?
>
> I am not saying that it is entirely impossible, I am just hard to be
> convinced in such circumstances.
This is where the math really matters. Check the formula for power from
an orbiting pair (say at
https://en.wikipedia.org/wiki/Gravitational_wave ). It scales as r^-5
and m1^3 m2^3.
Saggitarius A has m1=4e6 sun masses, so for a m2=1 sun mass partner the
mass term is 6.4e19. The observed merger was m1=36 and m2=29, so the
mass term is about 1.1e9. Ten orders of magnitude difference in favor of
Sag A!
The closest star to Sag A is S2, with perimelasma (I always wanted to
used that word properly!) of 17 light hours (1.8e13 m). But the distance
of the merger went all the way down to zero. If we had the merging black
holes orbiting one AU apart the distance term would be 1.1e15 times the
Sag A distance term. And at one light second apart (still far away from
their Schwartzschlild radiuses) it would be 8.6e23 - totally
overwhelming Sag A.
I have no doubt Sag A can ring loudly when black holes merge with it.
But this time it was quiet.
--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University
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