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Requested Informal LIGO Design Critique
Someone asked for my LIGO design critique so I provided it.
My post was titled LIGO Background for a Prediction
The first comment:
How do you know all these?
LIGO underwent several upgrades before discovering their recent g waves. Many have called LIGO's infinitesimal results absurd and shouldn't convince the awardment of the Nobel Prize. But here are you predicting LIGO. How?
my response:
I listed the few steps in the post.
I suppose I left out some details like a few months ago I read detailed descriptions of their pattern matching algorithm.
Once one knows how the LIGO software works it is easy to see why it declares a detection during a full moon.
I suspect few wish to read my critique of the LIGO design. I thought important just to get to the connection to moon and sun events.
His response:
No! I will like to see it. A lot of people including physicists were asking doubtful questions about LIGO's triggers but it went unanswered. What transmitted the G waves remained unanswered.
my response included my critique:
I apologize in advance. These opinions were only for me so it is cynical. Perhaps any or all of my personal opinions are wrong but writing this many months ago helped my memory.
Perhaps something is useful to you.
LIGO Design Critique
LIGO has a generic hardware design with the expectation whatever it detects can be interpreted as what the scientists are looking for.
I see these steps in the LIGO design sequence. Quotes are usually from ligo.org site.
1. Define its objective.
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General Relativity predicts that a change in gravitational field will travel through the universe at the speed of light. It is exactly these changes in gravitational field that are gravitational waves.
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My interpretation:
A change in a gravitational field occurs whenever a body's mass changes either through addition, like a merger, or through subtraction, like a fission or collision.
This event could occur anywhere in the universe, without being certain of the details such as the mass of the fragments before or after the event.
2. Design an instrument that can detect that wave.
from LIGO site:
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LIGO's sensitivity and makes it capable of detecting changes in arm-length thousands of times smaller than a proton.
In a telescope, these [background] vibrations are unwelcome, but LIGO is designed to feel them.
LIGO's arms can readily magnify the smallest conceivable vibrations enough that they are measurable.
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My interpretation:
Make the instruments so sensitive they can detect the smallest conceivable vibration or literally anything and everything.
3. Define how to find a wave.
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LIGO has been analyzing data since 2002 in an effort to detect and measure cosmic gravitational waves. LIGO's L-shaped detectors uses laser beams and mirrors in hopes of detecting changes in distance between its test masses as small as one-hundred-millionth of the diameter of a hydrogen atom. That change would indicate a wave's presence.
Gravitational waves have a finite speed and are expected to travel at the speed of light. This will induce a detection delay (up to about 10 milliseconds) between the two LIGO detectors. Using this delay and the delay between LIGO and its international partners will help pinpoint the sky location of the gravitational wave source. Multiple detectors also help sort out candidate gravitational wave events that are caused by local sources, like trees falling in the woods or even a technician dropping a hammer on site. These events are clearly not gravitational waves but they might look like a gravitational wave in the collected data. If a candidate gravitational wave is observed at one detector but not the other within the light travel time between detectors, the candidate event is discarded.
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4. Define how to find the wave details in the data.
'
Searches for gravitational-wave signals from the merger of compact binary systems were carried out by two independent search algorithms, named "PyCBC" and "GstLAL", that compare the observed data with the theoretical signal predicted by General Relativity using a technique called "matched filtering". In addition, another generic search algorithm, named "cWB", that does not assume a specific, theoretical model for the gravitational-wave signal, was also used. Improvements in these search algorithms and an extension of the search, in terms of the properties of the astrophysical objects being searched for, motivated the reanalysis of data from O1. Similarly, the application of a "data cleaning" procedure, to remove some of the detector noise and improve the sensitivity, has also motivated re-analysis of the O2 data.
Each search method produces a list of candidate events which are ranked in terms of their signal strength with respect to the detector's noise — a quantity called the "signal-to-noise-ratio" (SNR) — and their statistical significance, quantified by the false alarm rate (FAR), i.e. the rate at which one might expect such a candidate event to have occurred by chance, due simply to the noise characteristics of the detector data mimicking an actual gravitational-wave detection. By setting a FAR threshold of less than 1 per 30 days (about 12.2 per year) in at least one of the two matched-filter analysis algorithms, we restricted the list of candidate events and eliminated many candidate signals that are very likely to have been simply artefacts of the detector noise: within these candidates we found 11 events with a probability larger than 50% of having an astrophysical origin, rather than being instrumental noise. These candidates are labeled with the prefix 'GW' followed by the date of the detection (i.e. GW150914). The other candidates are considered as 'marginal' events, unlikely to be of astrophysical origin.
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My interpretation:
Having designed instruments to record everything including background vibrations or noise, the signal to noise ratio is critical.
from wikipedia:
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In signal processing, a matched filter is obtained by correlating a known delayed signal, or template, with an unknown signal to detect the presence of the template in the unknown signal.
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My interpretation:
Their analysis will find their 'known' signal in this recorded noisy data.
They had to develop a 'list of candidate events' for a reference. Detected events not in the list were considered 'marginal.'
5. Celebrate success.
The initial celebrated event detections involved proposed mergers with black holes and neutron stars, as described by a mathematical model to provide such a template.
Immediate success under these conditions should have been unlikely, with no testing with real events.
5a. This claim with a black hole is not justifiable.
A black hole is mass compressed into a geometric point, a singularity where there is no volume to hold mass to exert a gravitational field. A neutron star is an invisible source of electromagnetic radiation, usually a pulsar. That oddity of only neutrons is impossible.
This claimed merger scenario is preposterous.
7b. Even when ignoring the unlikely black hole event, I have no confidence in a design that requires noise in the recorded data.
I question whether they really could detect what they claim.
An alternative approach for detecting gravitational waves:
1. Define the event to detect.
2. Just before it occurs start recording.
3. Check that each detector in the system responded as predicted. If the system detected the predicted event this confirms the prediction.
4. Celebrate after other predictions are confirmed.
5. If the event was not detected then either a) the predicted event did not occur or b) the system failed to detect it.
If the reason is (a) try again for another prediction.
If the reason is (b) the system must be improved.
The obvious problem for such black hole merger events is they are not where we are looking.
If the system looked for binary star mergers maybe that flare up could be observed before sampling. That might be more likely than finding either BH or NS.
this data sampling must be repeated many times to verify specific predictions with the instruments.The theory and its predictions must be repeatable.
My comment today:
With a matched filter approach they are looking for their known signal rather determining what the signal is first and then matching.
This is not like: first see if it's a square wave or sine wave. If wrong discard, If right filter further, so there is not a detection until many criteria are met.
This filter technique is almost go/nogo as an initial step. The filter match will reach a conclusion even when the overall signal is wrong.
That possibility is confirmed when earth tide events are GW detections.
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