
Strange Noise in Gravitational-Wave Data Sparks Debate - digital55
https://www.quantamagazine.org/strange-noise-in-gravitational-wave-data-sparks-debate-20170630/
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maxander
Do they propose a mechanism for how there could be simultaneous correlations
between the two detectors that _aren 't_ gravitational waves? There's few
enough forces that can act through kilometers of solid earth that I'd be
willing to take it as a given that they're uncorrelated, absent a signal...
but then, I'm not a physicist.

Even more I'm-not-a-physicist-y speculation; could the additional correlation
be some secondary gravitational wave effect hidden under the main signal,
pointing to another aspect of the phenomenon?

~~~
spuz
I think the argument is that any correlation in the noise must be simply
coincidental. In other words, the detection could be explained by chance
rather than some true physical phenomenon measured by both detectors.

But, LIGO ran their detectors for 8 years at a lower sensitivity with no
detections. Almost as soon as they turned on the upgraded detectors in
September 2015, they detected the first event (GW150914) which appeared at a
level that would have been undetectable previously. Also, they have made two
other detections in the 1.5 years since. My guess is that if these events were
due to flawed analysis, then we should have seen something during those first
8 years.

~~~
mannykannot
While I think it is unlikely that there is a problem here, one has to consider
that upgrading the sensitivity could introduce or reveal a systematic error.

Also, it is not just a correlation that has been observed - it is a
correlation that has a specific form, a rising fundamental frequency that
happens to in line with what we would expect from merging black holes. I know
there's an element of circularity in the argument, but what are the chances of
that particular sort of signal arising from a spurious correlation?

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dnautics
“The only persons qualified to analyze this paper are in the LIGO Scientific
Collaboration”

What? That's an incredibly dangerous position for a scientist of any stripe to
take.

~~~
kurthr
I think the end of the sentence is interesting as well... _" , said Robert
Wagoner, a theoretical physicist at Stanford University who is not affiliated
with LIGO. “They are the only ones who have had access to the raw data.”_

That kind of gives the lie to the conspiracy that many would like to
believe... And before you go demanding that they release the raw data (in
addition to the processed data), I hope you are aware of how many disk drives
you're going to need to fit into that station wagon.

~~~
dnautics
I mean yeah there's probably lots of instrument telemetry in the data, but
let's give LIGO the benefit of the doubt and say that the instrumentation has
consistent performance through null control regions of the timeseries and the
"interesting" regions of the timeseries (bloop #1, in this case, but
apparently now bloops #2, and #3 which were not analyzed by the person doing
the criticism).

the 'truckloads' of data are basically all the uninteresting parts of the
timeseries. Of course there is statistically going to be random events where
the noises correlate at the level seen by the critique, and maybe the
truckloads of data show that those are frequent enough that say 1/3 of the
bloops should have that correlated noise superimposed on the 'actual signal';
and maybe that bloops 2 and 3 do not have the correlated noise if you apply
the same analysis to it.

How much more 'raw data' do you need to reach a conclusions that refutes the
criticism?

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smortaz
If you want to look at the data and run their code w/o installing any
software, check out their ready to view/run Jupyter notebooks:

[https://notebooks.azure.com/losc/libraries/tutorials](https://notebooks.azure.com/losc/libraries/tutorials)

1\. click on a notebook to see the html rendering 2\. or clone+run to
execute/edit the code (need to create login)

------
justifier
code links:

original criticism: [http://www.nbi.ku.dk/gravitational-
waves/correlations.html](http://www.nbi.ku.dk/gravitational-
waves/correlations.html)

response:
[https://github.com/spxiwh/response_to_1706_04191/blob/master...](https://github.com/spxiwh/response_to_1706_04191/blob/master/On_the_time_lags.ipynb)

definitely click through the quanta article's links.. fascinating reads all
the way down

~~~
hcrisp
It is fascinating, and I learned that one of the events was first detected by
an _unmodeled_ search for strange events in the signals:

"Finally, LIGO runs 'unmodelled' searches, which do not search for specific
signals, but instead look for any coherent non-Gaussian behaviour in the
observatories. These searches actually were the first to find GW150914, and
did so with remarkably consistent parameters to the modelled searches..."

[http://www.preposterousuniverse.com/blog/2017/06/18/a-respon...](http://www.preposterousuniverse.com/blog/2017/06/18/a-response-
to-on-the-time-lags-of-the-ligo-signals-guest-post/)

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unlikelymordant
I have done lots of signal processing. You cant just subtract out a noisy
signal and complain the residual is correlated, there will be bits remaining.
If they want to show correlated noise, why cant they use any other part of the
signal?

~~~
yodon
The correlated remainder is only in the vicinity of the signal event.

The 2nd team is arguing the presence of residual correlations after the signal
is removed means the total correlation isn't well described by a gravity wave
signature.

This is experimental data, so there's clearly going to be noise and there's
clearly going to be complexities in the detector response to the event. The
question boils down to whether the residuals are significant enough and
distinct enough from expected detector response behaviors to call the
detection into question or suggest something unexpected is contained inside
the detected signal.

~~~
chmike
It all boils down to what they subtract. It is said that they subtract the
wave signal. Is this signal known for sure or very predictable ? It also
depends on how they subtract it. Is it in the fequency domain or the space
doman ? I didn't read the arXiv article and don't plan too. Just commenting
and asking.

~~~
yodon
Think of it as a matched filter tuned to the impulse response of a gravity
wave. If that filter matches the observed signal then it's a detection, if it
doesn't then it's not. The debate is over the significance of the inevitable
remainder after subtracting the model of the signal. Is it an expected amount
of remainder or an unexpectedly large amount of remainder.

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snarfy
Is it really noise or just more signal?

Cosmic background radiation is noise but it can be mapped out into a pattern.
Could the correlation be a view of a type of cosmic background gravitational
waves?

~~~
dboreham
The pattern is in a different dimension than the noise (time vs spatial). A
pattern by definition isn't noise.

~~~
mirimir
Well, except:

> The main claim of Jackson’s team is that there appears to be correlated
> noise in the detectors at the time of the gravitational-wave signal. This
> might mean that, at worst, the gravitational-wave signal might not have been
> a true signal at all, but just louder noise.

If that's an accurate characterization, "louder noise" does seem a lot like
signal.

Couldn't the "correlated noise" just be a superposition of numerous small
gravitational-wave signals?

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PhantomGremlin
Obviously the simplest solution is to have a third detector, located far away
from the other two. I'm surprised the article didn't even mention that.

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sleavey
Let's wait and see if their article gets through peer review in a similarly
respected journal to the three published detection papers'.

------
shamaku
where there's smoke...

------
raverbashing
Wait

Scientists keep saying and repeating how correlation doesn't mean anything to
the point it's become a joke

Now they are dabbling about a spurious correlation in a small section of the
data? For real?

No, really, if anyone would be trying to prove anything based on that
correlation alone they would have been laughed and dismissed by the majority
of the scientific community

But suddenly when it's to reject a result then it's important?

> points out that Jackson’s team could have misused a common data-processing
> technique called the Fourier transform... The error, Harry writes, has to do
> with the technical assumption that the input data signal be “cyclical,”

Correct. Not to mention the sample is too short (and the even is short as well
and there's noise and the sampling rate is not too big).

If you play with FFT on these conditions you start seeing things that "aren't
there"

~~~
phreeza
Scientists don't say correlation doesn't mean anything. What they do say is
that correlation doesn't imply causation.

If the basic model underlying the original LIGO finding would predict no
correlation between the recordings, then a significant amount of correlation
should lead scientists to revise the model, and reexamine the findings. Very
much standard scientific practice.

~~~
raverbashing
There are several possible sources of correlation on the "impulse response" of
the LIGO detectors (the whole detector, from building construction to the
electronics and detectors in itself) so it is not surprising that might
happen. Or maybe it's something in the source of the signal

But the signal is still stronger than the noise

Besides the "poking the data in weird ways until it stops meaning anything"
issue

~~~
yodon
No. The word "Correlation" as used in the article to discuss correlations in
the data generated by two remote detectors has a very specific technical
meaning. It's not simply one of the words in the trope "correlation does not
imply causation."

Correlation as used here and put simply means that when the signal coming out
of one the detectors increases, the signal coming out of the other detector
(which is physically separated from the first detector by a very large
distance) also increases, and when the signal coming out of the first detector
decreases the signal coming out of the second detector also decreases.

The existence of that mathematical correlation between those two signals
coming out of those two remote detectors absolutely implies there is a
causation to the correlation (yes, mathematical correlation can imply a
causation, even while it alone is not able to indicate what that causation
is).

The question at hand is not whether there is a correlation or whether there is
a causation. The question is what is the causation for the correlation and
whether that causation is a gravity wave or something else.

The odds are extremely good that that causation is a gravity wave, but it's a
big enough experiment and a big enough result that it's worth looking very
closely at the analyses that argue in favor of it being a gravity wave (and
those that argue against it).

~~~
catamorphismic
... what? This is precisely the correlation meant in the aphorism you
mentioned and is also the only sense of the word correlation of which I am
aware of. Signals can be similar (correlated) purely by chance and this cannot
be taken to imply a causative link between them.

~~~
phreeza
If a correlation is consistent, some sort of causation is quite likely. There
are of cause spurious correlations but in physics experiments we can mostly
get rid of them by repeating the experiment several times, unlike in economics
for example.

The more important point, however, is that a correlation between A and B does
not imply that A causes B or B causes A. It could also be that unknown factor
C causes A and B.

~~~
catamorphismic
Agreed, though repeated measurements weren't mentioned so I thought it was
important to emphasize that finding a correlation in one measurement cannot be
taken to imply causation, which sounded like what OP was saying.

Your point is a very good one, though. If we view causation through a causal
graph, such that vertices are variables and direct causal links are arrows
(directed edges), a statement "correlation doesn't imply causation" can be
taken to mean several things:

    
    
      * From correlation, we cannot determine the direction of the arrow.
      * From correlation, we cannot claim there *is* a (direct) arrow between two vertices.
      * From correlation, we cannot claim there is even a path from one vertex to the other.

~~~
phreeza
Indeed. I think it is also important to note that the unknown factor can be
something very abstract. For example if you measure the oscillations of two
pendulums with the same properties, they will also be correlated, but simply
because they are driven by the same underlying physics...

