Have the interferometers been "zero" tested?

LivingDog
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Topic 195176

Some time ago I learned that anytime one does an experiment one should always do what I call a "zero" test. (Maybe there's a more professional/standard nomenclature?) It's a kind of calibration - to make sure your equipment is working as it should.

Basically all you do is measure a known quantity. E.g. if you build a new voltmeter you could connect the leads to measure the potential difference. If you see anything other than 0 you know something is amiss with the equipment/design/whatever.

The reason I ask is that it seems impossible. How can you test for gravity waves when no one has ever _measured_ gravity waves???

So has any one done a "zero" test of the interferometers?

Thanks in advance for any replies...

-LD
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Mike Hewson
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Have the interferometers been "zero" tested?

There are hardware and software 'injections' on a regular basis. A hardware injection is when the optics are deliberately wiggled so as to mimic a signal of some form. A software injection is done by adding in numbers to some data set. Many of our work units will contain them, and crunching should reveal them. So this is a test of the whole analysis pipeline.

More generally, the interferometers are 'null' devices. When correctly configured and without other disturbances ( which is a huge challenge ), then it should read 'zero' in the absence of a passing gravitational wave. In addition, since we have never 'heard' a wave to date, the redundancy of several such gadgets worldwide ought increase our confidence if many/most/all agree in their responses at some time. Of additional assistance is the particular geometry of the positions of the interferometers around the globe, if four or more respond to a wave then one could deduce if a consistent direction ( of the source ) was being indicated.

You are quite right, it is a bit like the first voltmeter ever made ! :-)

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

LivingDog
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RE: ... if four or more

Message 98378 in response to message 98377

Quote:
... if four or more respond to a wave ...

Four or more? How many are there? I thought there were only 2 - LIGO and GIGO.

Any future plans to put one on the moon? Seems like an ideal place since there are no vibrations other than the low tidal forces from the Sun/Earth system.

-LD
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tullio
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There is VIRGO in Tuscany and

Message 98379 in response to message 98378

There is VIRGO in Tuscany and another in Japan, plus a European project and LISA in space, also a project.
Tullio

Bikeman (Heinz-Bernd Eggenstein)
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Though Lisa isn't started

Though Lisa isn't started yet.

So let's count:

LIGO has three detectors on two different sites
VIRGO: Tullio mentioned that one already
GEO 600: small but sophisticated :-), near Hannover, Germany

In Australia there ia AIGO, but I'm not sure what the exact status is and if it's opearting in a science mode already.

EDIT: I forgot TAMA, the Japanese interferometer. Sorry.

Those are the GW interferometers. There are a few GW detectors that use resonant mass, but I guess in this thread we are focused on the interferometers :-)

CU
HB

ML1
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RE: [...] VIRGO: Tullio

Message 98381 in response to message 98380

Quote:

[...]
VIRGO: Tullio mentioned that one already
GEO 600: small but sophisticated :-), near Hannover, Germany

... TAMA, the Japanese interferometer


What are they using for their data analysis? Have they got their own dedicated compute clusters?

Quote:
... There are a few GW detectors that use resonant mass...


I thought they had long ago been discredited...

Keep searchin',
Martin

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tullio
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The GEO600 site lists 6

The GEO600 site lists 6 resonant detectors, 2 of them Italian, one at Legnaro and the other at CERN, this one cryogenic. They are not "discredited". So far they have found no certain signals, despite Joe Weber's claims, but so have the interferometers. I would not discredit them, I have followed Weber's attempts since 1967 and the Rome group attempts, headed by such a figure as Edoardo Amaldi. Amaldi took inspiration from Weber and, with his group, developed cryogenic detectors at Frascati and CERN. He believed in this approach and he was not an easy person to convince, having worked with Fermi, Segre', Rasetti, etc.
Tullio
Virgo has its own cluster. They just had a meeting in Hannover with LSC but the program is not accessible to us. I wish someone could give us some information on these meetings.

Mike Hewson
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RE: I thought they had long

Message 98383 in response to message 98381

Quote:
I thought they had long ago been discredited...


There was a fair kerfuffle with them for Mr Weber and his approach to the topic some time ago. Amongst various issues, it seemed to other scientists at the time that he didn't pay enough attention to fault modes. Whether technically right or wrong, he is at least revered for putting the GW detection idea on some experimental footing. His work certainly highlighted the essential problems of the field, and specific difficulties to be overcome. He's paid the price which is typically felt by pioneers, alas .....

It's worth re-iterating that if, by whatever method, no gravitational waves are found ( with a certainty that they would have been if they were there to be detected ) then there is no failure of science. We will have simply found something true about the universe - that gravity doesn't propagate in waves. The Michelson-Morley experiment comes quickly to mind as what a tremendous boon it was to have that null result. Do not theorists panic when experimenters grin about some new finding ?? :-)

But bars are still in the running. The resonant bar idea is to have it tickled by a fairly narrow range of GW frequencies ( that for which a given bar is tuned ). It suffers many of the local environmental noise problems that IFO's do, however a key difference is that a detection depends far more sensitively on the recent history of the bar. So instead of having photons run laps amongst mirror arrangements, they look at the behaviour of distorting metallic lattices.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Bikeman (Heinz-Bernd Eggenstein)
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Indeed, to be fair, you have

Indeed, to be fair, you have to say that both technologies (resonant mass and ifo) have their own strengths and weaknesses.

I'm not a physicist, but from literature I understand that

- the understanding/consensus of the GW "cross-section" is maybe better for laser interferometers than for resonant mass detectors

- resonant mass detectors need cryogenics to reach comparable sensitivities to "room temperature" interferometers. Cryo-stuff adds complexity and is in general hazardous to the duty cycle statistics (warming up, cooling down again for major madifications, fixes etc)

- ifos would allow to have a clearer image of the actual waveform of a GW if detected.

- resonant mass detectors require less lab space (well, less real estate) , which is nice if you want to place them underground (no fun doing excarvations for a N km long ifo)

- some spherical resonant mass designs (not all are "bars") would allow better detection of the *direction* from which a GW is radiated.

- ifos have a wider range of frequency covered

I think it's fair to say that if a detection claim was made by a resont mass detector, but the event was not seen by any of the ifos, it would encounter more scepticism than if it were the other way round. But the dream scenario, of course, would be a detection by detectors of both technologies!

CU
HB

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