<div dir="ltr"><div>Anders,</div><div><br></div>How many super-massive black holes like our own Sagittarius A we have inside the 10^9 ly radius sphere around us? From one hundred million to one billion, there about. Each having mass of about 10 million Suns or 1 million small black holes.<div><br></div><div>How often do small black holes rain onto those big ones? I don't know, but at least 10^14 of them have already fallen, apparently. Otherwise we would not have so many so big supermassives. That is at least 10000 per year for the entire history after the Big Bang.</div><div><br></div><div>10000 per year, but LIGO does not hear that noise? But caught a much smaller collision?</div><div><br></div><div>I find it difficult to believe this.<br><div><br></div><div><br></div></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Sat, Feb 13, 2016 at 2:04 PM, Anders Sandberg <span dir="ltr"><<a href="mailto:anders@aleph.se" target="_blank">anders@aleph.se</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"><span class="">
On 2016-02-13 11:50, Tomaz Kristan wrote:<br>
<blockquote type="cite">
<div dir="ltr">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.
<div><br>
</div>
<div>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?</div>
<div><br>
</div>
<div>I am not saying that it is entirely impossible, I am just
hard to be convinced in such circumstances.<br>
</div>
</div>
</blockquote>
<br></span>
This is where the math really matters. Check the formula for power
from an orbiting pair (say at
<a href="https://en.wikipedia.org/wiki/Gravitational_wave" target="_blank">https://en.wikipedia.org/wiki/Gravitational_wave</a> ). It scales as
r^-5 and m1^3 m2^3. <br>
<br>
<tt>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!<br>
<br>
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). </tt><tt>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. <br>
<br>
I have no doubt Sag A can ring loudly when black holes merge with
it. But this time it was quiet.<br>
<br>
<br>
</tt><span class="">
<pre cols="72">--
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University</pre>
</span></div>
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