
Astrophysicists do a 3-day long AMA about the discovery of gravitational waves - yread
https://np.reddit.com/r/IAmA/comments/45g8qu/we_are_the_ligo_scientific_collaboration_and_we/
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pinko
I'm part of the LIGO team responding to the AMA. If anyone on HN sees a good
question that hasn't been answered, link it here and I'll sure we get to it!

~~~
ghayes
Again, asking here since it's easier, but sorry if that wasn't the intent.

To get a better understanding of what LIGO is observing, does every massive
object create a gravitational wave as it moves? Do gravitational waves act
similarly as to how moving a charged particle creates magnetic flux? Do we
know from LIGO if gravitational waves propagate at exactly the speed of light?

~~~
pacifist
Not on the team. Not even close. My under standing from other comments from
the team is yes and yes. The reason we needed a collision of black holes was
to get the signal up to where we can detect it. And they definitely said that
this proves that gravitational waves propagate at the speed of light.

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mirimir
In the reddit AMA, DW wrote:

> Myself and one of my supervisors had a conversation about this a few weeks
> ago. We did some calculations which suggest that if you were in a space-ship
> close to the merging black holes you would feel a force which was pretty
> comparable to the force you feel by standing next to a loudspeaker at a
> music concert. You'd feel a vibration travelling through your body, but we
> were pretty confident it wouldn't hurt you!

Maybe the gravitational waves wouldn't hurt, but the gammas etc would be
painful. I'm reminded of Greg Egan's _Diaspora_.

~~~
asgfoi
Assuming no accretion disks, there wouldn't be any em radiation as far as I
know. Where would it come from?

~~~
mirimir
Thanks. I see now that it's merging binary neutron stars that create the huge
gamma spikes.[0]

[0]
[https://www.astro.cf.ac.uk/research/gravity/tutorial/?page=4...](https://www.astro.cf.ac.uk/research/gravity/tutorial/?page=4blackholecollisions)

Edit: That tutorial also explains that black holes are very simple objects, so
the chirps and ringdowns are likewise very simple. So black hole masses can be
calculated very precisely. Also, amazingly:

> Just as optical radiation and radio waves, the luminosity of gravitational
> radiation falls off in inverse proportion to the square of the distance from
> the source. This makes binary black hole inspirals standard sirens: if we
> know what the masses of the two black holes are then we can infer the
> distance to the source by measuring its apparent luminosity. We can
> precisely measure the masses because the rate at which the frequency and
> amplitude of an inspiral increases depends only on the masses.

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ThePhysicist
As a physicist and programmer I just love that Python was used both for
controlling the actual experiment and for analyzing the data! Six years ago
when I started my PhD I also moved all the code for my experiment from
proprietary software (Matlab/Mathematica/Labview) to Python (+matplotlob,
pyvisa, PyQt/PySide). Many of my colleagues and supervisors were rather
skeptical about this back then as Python was still considered quite new and
the tooling was not as good as it is today. It seems though that Python is
finally becoming the de facto standard for experiment control and data
analysis, especially for more complex use cases that require a good software
architecture (which is hard to implement e.g. with Labview). This is really
great news for me as it makes sharing code and data much easier (if they had
done their analysis in Matlab it would be hard to reproduce for people that
don't have access to that software, but with the iPython notebook everything
you need to reproduce the results is open-source). Great stuff, let's keep
pushing Python forward :)

~~~
agnivade
wow, that is a very interesting piece of news. Any chance you can cite the
original reference to this ?

~~~
ThePhysicist
They mention it in the AMA, have a look at the "How big is Python in the
scientific community?" question!

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febed
A fantastic discovey for the ages. Makes you wonder how many civilisations out
there were technologically advanced enough to hear that chirp.

~~~
dps1879
Zero

~~~
dps1879
Hahaha truth hurts b-otches? The truth can hurt ya or the truth can change ya.
What will truth do to you.

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pjdorrell
Should we building a 3rd or maybe 4th LIGO so we can know where the events
actually come from?

What's the best timing precision possible on the different reception times, to
get better triangulation (if there was at least a 3rd LIGO)?

What's the odds of seeing one event when starting it up, and none since then?

How long will it take for any new events to be announced?

~~~
jessriedel
> What's the odds of seeing one event when starting it up, and none since
> then?

They have seen other events, this was just the biggest.

~~~
InclinedPlane
[citation needed]

~~~
jessriedel
What's the MLA format for word of mouth? :)

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nonbel
Thanks for doing this. I am concerned about the base rate fallacy[1] and would
like to know whether/how the probability that this signal was a false positive
has been estimated.

I read much discussion regarding the probability of a false alarm given the
baseline model.[2] However, without an estimate of the probability of
detecting a true signal during the same time frame, we cannot calculate the
probability this is a false positive. Such an estimate would obviously require
some prior estimates of the rate at which these black hole mergers occur and
the percent that can get detected, etc.

[1]
[https://en.wikipedia.org/wiki/Base_rate_fallacy](https://en.wikipedia.org/wiki/Base_rate_fallacy)

[2]
[https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.11...](https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102)

~~~
topquark
It sounds like you've already read about how we estimate the false-alarm rate
for signals, so I'll just add that the estimated rate of BBH mergers was
highly uncertain before this observation; the error bars spanned three orders
of magnitude. See, for example, Fig 5 of
[http://arxiv.org/abs/1111.7314](http://arxiv.org/abs/1111.7314), which
compares the previous LIGO-Virgo upper limits on similar events to the
expected rate from population synthesis models and observations of high-mass
X-ray binaries, known BNS systems, etc.

The rate inferred from GW150914 is on the high end of the rate estimates from
astronomers, but it's completely consistent with prior observations.
Certainly, if we had seen ten events in the first 16 days of data, it would
not have made sense! But one event is well within expectations.

~~~
nonbel
A lot of this information seems to be available, for example figure 4 here[1].
According to that, the horizon distance for "a binary black hole system with
the same observed spin and mass parameters as GW150914 for optimal sky
location and source orientation and detected with an SNR of 8" was 1.5-2 Gpc.
Now we only need an estimate of the rate at which events with these properties
occur, not using the data from GW150914. I presume this is somewhat (a few
orders of magnitude) less than the rate at which the mergers occur in general.

[1] [http://arxiv.org/abs/1602.03844](http://arxiv.org/abs/1602.03844)

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Cogito
I couldn't see it in the post, but I'm interested in how much this specific
event, and future events, allow us to tighten the bounds on our models.

It's mentioned a few times that the size of the black holes/other model
parameters were "within expectations" and so were a good fit.

What are some of the specific quantities that these measurements will allow us
to refine? Previous astronomical measurements have allowed us to put better
and better estimates on things like the size of the sun, or the gravitational
constant - do we have any idea what estimates this technology will allow us to
improve?

~~~
topquark
In [http://arxiv.org/abs/1602.03840](http://arxiv.org/abs/1602.03840) we
describe the various parameters that we were able to measure for this signal,
and in [http://arxiv.org/abs/1602.03841](http://arxiv.org/abs/1602.03841) we
compare the results to the predictions from general relativity.

In Figs 6 and 7 of the second paper you can see the constraints from GW150914
on what are known as the "post-Newtonian" expansion terms of Newtonian
gravity. Previously, terms beyond first order were only loosed bound, mostly
from observations of the Double Pulsar system J0737-3039.

Just from this one event, we can also constrain the mass of the graviton to an
order of magnitude less than the previous best measurement.

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dps1879
The announcement is the climax of a century of speculation, 50 years of trial
and error, and 25 years perfecting a set of instruments so sensitive they
could identify a distortion in spacetime a thousandth the diameter of one
atomic nucleus across a 4km strip of laserbeam and mirror.

The phenomenon detected was the collision of two black holes. Using the
world’s most sophisticated detector, the scientists listened for 20
thousandths of a second as the two giant black holes, one 35 times the mass of
the sun, the other slightly smaller, circled around each other.

At the beginning of the signal, their calculations told them how stars perish:
the two objects had begun by circling each other 30 times a second. By the end
of the 20 millisecond snatch of data, the two had accelerated to 250 times a
second before the final collision and a dark, violent merger.

The observation signals the opening of a new window on to the universe.

A century of speculation, no wonder people are starving. Come on are u guys
reading this? Are you just closing your eyes opening your mouth and letting
them stick in what ever they want?tf? firstly i think there is bs going on but
if i do accept this bs then we are way far behind in our spiritual growth.
Dont forget what sting said... we are spirits in the material world....better
get balanced soon or the world will do it for you. Or is that what you need?

