
LIGO detects probable black hole merger event - apaprocki
https://twitter.com/ligo/status/1145405632553656326
======
astro123
With the caveat that I have nothing to do with ligo and so don't know much...

I _think_ that during this ligo run (it just turned back on in April-ish) they
expect to find roughly 1 event per week. So this will be fairly common! [1] is
a nice summary of what they have found so far.

For people here interested in the engineering side of LIGO, it is absolutely
mind blowing. The effect of the gravitational waves is tiny (fractions of the
size of a proton change in length) and so there are so many things that need
to be incredibly tightly controlled for. I went to a talk on this a year or so
ago, but can't find the slides... Here's a summary [2] and the wikipedia page
also has some info.

1 - [https://www.sciencenews.org/article/ligo-virgo-
made-5-likely...](https://www.sciencenews.org/article/ligo-virgo-
made-5-likely-gravitational-wave-detections-month) 2 -
[https://www.engineering.com/Education/EducationArticles/Arti...](https://www.engineering.com/Education/EducationArticles/ArticleID/14333/Engineering-
the-LIGO-Gravitational-Wave-Detectors.aspx)

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chrischen
> fractions of the size of a proton change in length

...less than a ten-thousandth the charge diameter of a proton.

~~~
badosu
I wonder if, as we get to smaller scales, do things start quantizing? Are we
far from it?

~~~
raattgift
LIGO is extremely far from quantum gravity.

The gravitational waves in question have no quantum properties, either because
they have so many quantum numbers in them that classical behaviour is
recovered, or because they are classical “all the way down”. LIGO, its
siblings, and their foreseeable successors are unlikely to help resolve this.
The results have already constrained the space of quantum gravity theories
from which General Relativity must be recoverable, but that is not the same as
preferring (much less indicating) any particular approach to quantum gravity
that will be consistent with LIGO et al.’s results.

That said, LIGO’s detector itself relies upon the details of quantum
mechanics, as it is also sensitive to quantum noise from its immediate
environment. However the quantum mechanics in question are purely local, and
have nothing to do with distant events like binary black hole mergers. (LIGO
is not sensitive to those distant quantum mechanics — it’s either outright
blind to them, or at best they blur into classicality — and it’s hard to
envisage a human-buildable detector that would be sensitive to their
gravitational influence within our solar system).

~~~
emiliobumachar
Is there such a thing as real phenomena that's classical “all the way down”,
as you have put it? If yes, which such phenomena is easiest to understand?

I was under the impression that all of classical physics was special cases of
quantum and relativistic physics, just like the relativistic equations of
motion "degrade" into classical equations of motion when we postulate that all
velocities involved are much lower than the speed of light.

~~~
jfengel
In this sense "classical" means "not quantum". General relativity is "classic"
in the sense that position and momentum are variables rather than operators.
They have real values rather than functions.

So it's distinct from "classical" meaning "Newtonian" (or, perhaps "Newtonian
+ Maxwellian"), which is the sense in which "all of classical physics was
special cases of quantum and relativistic physics".

~~~
raattgift
Yes, all true, but we have the Hamiltonian formulation (e.g.
[https://arxiv.org/abs/1505.01403](https://arxiv.org/abs/1505.01403) which is
a nice book chapter) which “… is also the starting point for the canonical
quantization program, which constitutes one main approach to the as yet
unsolved problems of Quantum Gravity. In this approach one tries to make
essential use of the Hamiltonian structure of the classical theory in
formulating the corresponding quantum theory.” From there you could jump to
[https://en.wikipedia.org/wiki/Canonical_quantum_gravity](https://en.wikipedia.org/wiki/Canonical_quantum_gravity)
which has references.

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jeanlucas
Ok, their twitter humor is on point:
[https://twitter.com/LIGO/status/1145427537629208577](https://twitter.com/LIGO/status/1145427537629208577)

edit: actually all their replies are hilarious.

~~~
mehrdadn
Wow yeah, these are hilarious.

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DiogenesKynikos
This will sound cynical, but at this point, it's no longer news unless it's a
binary neutron star merger. If there's an electromagnetic counterpart, then
it's _big_ news.

If you have a gravitational wave detection with an EM counterpart, you can get
a redshift and luminosity distance, which means that you can measure the
Hubble constant (in a way that is completely independent from Type Ia
Supernovae). Binary black hole mergers are not expected to give off any EM
radiation.

There should be a few binary neutron star mergers this run, and with some
luck, there may be one with an EM counterpart.

~~~
cperciva
Presumably if the location could be determined accurately enough and it was
close to a galaxy, we could conclude that it was most likely _in_ that galaxy,
and thereby get a useful measurement though?

~~~
DiogenesKynikos
Not with gravitational waves alone.

People do want to correlate large numbers of binary black hole mergers with
galaxy positions and come up with a statistical measurement of the Hubble
constant, but no single BBH merger will be clearly identified with one galaxy
under that scheme. You need lots of mergers, and the Hubble-constant
determination is more model-dependent.

Binary neutron stars with EM counterparts are much more straightforward.

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gnufx
Yes, the experimental details are stunning, particularly with an experimental
physics background.

As someone who had a narrow escape from a PhD working on resonant bar-type
detection attempts, I've never understood how those were ever thought to be
sensitive enough -- whether decent calculations just weren't available or
what. Unfortunately, you're typically not in a good position to evaluate such
things before starting the work.

~~~
effie
Weber's initial calculations and experiments with bar detectors were a
reasonable effort, because gravitational waves were something that nobody
believed in. Bar detector has an advantage - it is cheap. A strong enough wave
could excite it and it would have been great and cheap discovery. But later
when nothing was being reliably detected, people turned to more expensive
interferometric detector. Today it is believed the bar type detector could
only detect extreme events nearby, which are very unlikely.

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civilian
Can anyone explain how I should be reading that graph?

I'm surprised that the gravity anomaly is coming from a large region of space.
And it's a curve. Is that expected? Why is it happening?

A black hole merge is coming from a single point in space, so I would have
expected there to be a single point. But I'm definitely missing something.

~~~
petschge
It does come from a single point but they can not tell where inside the shaded
area of space the single point was.

As far as I can tell from their website the event was seen by Virgo and only
one of the Ligo detectors (Livingston), which makes localization in the sky
rather uncertain. No clue at the moment why the second Ligo detector (Hanford)
did not see it.

~~~
turndown
From what I understand a lack of detection can also tell you something about
where the point of origin is.

~~~
petschge
If you had a working detector taking good data than not hearing it also tells
you something. But if the detector was not taking good data for whatever
reason (local seismic event, high surface winds, technical glitch), then it
doesn't help you. Also afaik (and I am not an expert) the first automatic
pipeline doesn't use non-detections.

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7373737373
How can black holes merge from our perspective, if something needs an infinite
time to reach the event horizon from our perspective?

~~~
fsh
Because it doesn't take infinite time from our perspective. The object falling
into the black hole experiences strong time dilation due to the extreme
gravitational field, but that doesn't affect a remote observer.

~~~
7373737373
In the twin experiment, the observer sees his twin age slower, so it _does_
have a remote effect. From the perspective of the object falling into the
black hole, the local conditions do not change, time is progressing normally
etc.

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Odenwaelder
I'm fascinated about the degree of automation. It seems that all the necessary
computations are made and presented on that website. I would love to learn
more about this and their tech stack.

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choeger
Was there not some severe doubt on the whole signal inference? I recall an
article that critized them for some changes in the protocol and also
questioned the statistical significance of their findings. How is the current
state of that discussion?

~~~
KnightOfWords
Pretty much dismissed by later analysis I believe. Also, please bear in mind
LIGO/VIRGO pinned down a neutron star merger with an optical counterpart, so
there is independent confirmation the detectors can lift signal out of the
noise.

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peter303
19 candidate events past 92 days since the improved system was turned on-
about every five days.

I am not sure what elevates a candidate to a confirmed event.

Candidate events are posted quickly in hopes of observing associated photons.

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perlgeek
I love how that data is public, and viewable by anybody.

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idlewords
This is so incredibly cool.

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Geenirvana
Is there anywhere that I can read up to understand the graphs?

