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.
This critique is long.
I see these steps in the LIGO design sequence. Quotes are usually from ligo.org site.
1. Define its objective.
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.
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:
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.
Make the instruments so sensitive they can detect the smallest conceivable vibration or literally anything and everything.
3. Define how to find a wave.
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.
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.
Having designed instruments to record everything including background vibrations or noise, the signal to noise ratio is critical.
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.
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. Ignore the more likely wave events at first.
I would have expected with so many binary stars in the universe some of them might merge, to provide more frequent events to detect.
Could the system detect Sirius merging with its smaller binary companion?
That none of the more likely events are reported is suspicious.
6. Stay positive
LIGO site mentions no failures before a success. They mention only equipment upgrades.
I would have expected an announcement of unexplained events first, because of simple reasons like they were too far away or the masses were too small for a measurable field change or the combination was not in their templates. Were there any other significant events? None are reported.
7. 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.
There have been no reports of events too faint or too unusual.
7a. Is this claim with a black hole even 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. That oddity has an unknown mass.
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 probably rare or are not where we are looking. The universe is huge.
If the system looked for binary star mergers maybe that flare up could be observed before sampling.
this data sampling must be repeated many times to verify specific predictions with the instruments. The theory and its predictions must be repeatable.
Right after posting I added this comment [because my item 6 is misleading]:
I was not fair: 'Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves.'
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