In some science show on last night they said that in seeking evidence of gravity waves they were measuring distances of the order of km to a precision of 1/billionth the diameter of a proton.
Is that correct?
If so, how can they possibly do that?
If not, what precision do they work to?
This:
http://labcit.ligo.caltech.edu/LIGO_web/about/brochure.html
says 1/1000, not 1/1000000000.
Maybe I misheard last night.
But even with 1000000 times less precision, the questions remain the same.
https://en.wikipedia.org/wiki/Gravitational-wave_observatory
https://en.wikipedia.org/wiki/LIGO
The Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment aiming to directly detect gravitational waves. Cofounded in 1992 by Kip Thorne and Ronald Drever of Caltech and Rainer Weiss of MIT, LIGO is a joint project between scientists at MIT, Caltech, and many other colleges and universities. It is sponsored by the National Science Foundation (NSF). At the cost of $620 million, it is the largest and most ambitious project ever funded by the NSF.
Initial LIGO operations between 2002 and 2010 did not detect any gravitational waves. This was followed by a multi-year shutdown while the detectors were replaced by much improved “Advanced LIGO” versions. As of February 2015, two such advanced detectors (one in Livingston, Louisiana and the other in Hanford, Washington) have been brought into engineering mode. On September 18, 2015, Advanced LIGO became fully operational and began formal science operations at twice the sensitivity of the initial LIGO interferometers
If these detectors find nothing after many years of operation do we conclude gravity either don’t exist or are way beyond our current technogical levels to detect.
b. gravity waves are predicted by GR. we know GR has stood the test of time, even though it isn’t a complete theory of the universe, so we are pretty confident that gravity waves should exist.
JudgeMental said:
b. gravity waves are predicted by GR. we know GR has stood the test of time, even though it isn’t a complete theory of the universe, so we are pretty confident that gravity waves should exist.
Not really. It is perfectly possible for simplified theories to give good matches to most observable quantities, but be way off in others, and thus require revision.
yeah, really.
JudgeMental said:
yeah, really.
Care to tell us why, really?
because any new theory will include GR.
JudgeMental said:
because any new theory will include GR.
Not necessarily. It may include something that is not GR, but gives the same results as GR in those areas where GR works; in exactly the same way that GR gives the same results as NG in many areas, but is not NG.
that will depend on how important gravity wave prediction is to gr. maybe if this new theory doesn’t predict them then some other part of gr would be incorrect. and that other part may be backed with evidence of being correct.
JudgeMental said:
that will depend on how important gravity wave prediction is to gr. maybe if this new theory doesn’t predict them then some other part of gr would be incorrect. and that other part may be backed with evidence of being correct.
I don’t know what that means.
would gr stand up without gravity waves. or are they dependant on other predictions in gr. so, if no gravity waves then what other predictions of gr would be wrong. and, if other predictions would be wrong, are these currently supported by observation and thus lead to further problems.
JudgeMental said:
would gr stand up without gravity waves. or are they dependant on other predictions in gr. so, if no gravity waves then what other predictions of gr would be wrong. and, if other predictions would be wrong, are these currently supported by observation and thus lead to further problems.
How can the existence or not of gravity waves affect how well the other predictions work?
If a prediction of GR matches observations to some level of precision, then it will continue to match observations to that precision, unless the observations are found to be wrong, or the mechanism by which gravity works changes. Either way, the discovery or not of gravity waves will not affect it.
I have asked on The Science Forum.
http://www.nature.com/news/has-giant-ligo-experiment-seen-gravitational-waves-1.18449
gravitational waves were eventually discovered when the endurance took measurements when around the super massive black hole Gargantua
Michael V said:
http://www.nature.com/news/has-giant-ligo-experiment-seen-gravitational-waves-1.18449
Chad Hanna, a physicist at Pennsylvania State University in University Park who leads LIGO’s search for mergers, told me earlier this year that he has already got permission from his wife to keep his mobile phone on at night, waiting to receive alerts. “When the signal of a merger arrives, we’ll know 30 seconds later,” he says.
———
Wonder what happens if she says, “Chad, you’re not going off to play LIGO at this hour of the night.”
“Each observatory supports an L-shaped ultra high vacuum system, measuring 4 kilometers (2.5 miles) on each side. Up to five interferometers can be set up in each vacuum system.”
“The primary interferometer at each site consists of mirrors suspended at each of the corners of the L; it is known as a power-recycled Michelson interferometer with Gires–Tournois etalon arms.” (My emphasis.)
https://en.wikipedia.org/wiki/LIGO
See also:
https://en.wikipedia.org/wiki/Michelson_interferometer
https://en.wikipedia.org/wiki/Interferometry
There’s no dearth of respectable sources supporting the notion that LIGO can detect changes around the scale of an attometre.
As to how, I don’t know.
dv said:
There’s no dearth of respectable sources supporting the notion that LIGO can detect changes around the scale of an attometre.As to how, I don’t know.
I’d better hope they said 1/billionth on the ABC then, so I can maintain my outrage.
Measuring to 1 attometre in 4000 m is pretty impressive though. I’d expect the variation in the position of the mirror surface due to random vibrations and thermal movement to be way more than that.
The Rev Dodgson said:
dv said:
There’s no dearth of respectable sources supporting the notion that LIGO can detect changes around the scale of an attometre.As to how, I don’t know.
I’d better hope they said 1/billionth on the ABC then, so I can maintain my outrage.
Measuring to 1 attometre in 4000 m is pretty impressive though. I’d expect the variation in the position of the mirror surface due to random vibrations and thermal movement to be way more than that.
It is a little misleading. They can’t measure the distance (circa 4 km) to a precision of 1 attometre. But they can detect changes in the distance around 1 attometre.
The Rev Dodgson said:
dv said:
There’s no dearth of respectable sources supporting the notion that LIGO can detect changes around the scale of an attometre.As to how, I don’t know.
I’d better hope they said 1/billionth on the ABC then, so I can maintain my outrage.
Measuring to 1 attometre in 4000 m is pretty impressive though. I’d expect the variation in the position of the mirror surface due to random vibrations and thermal movement to be way more than that.
They said it was isolated from outside influence but I wonder if it still gets false positives
you can see the exhaust of the rockets going backwards, doesn’t slow aircraft
The Rev Dodgson said:
In some science show on last night they said that in seeking evidence of gravity waves they were measuring distances of the order of km to a precision of1/billionth1/thousandth the diameter of a proton.Is that correct? If so, how can they possibly do that? If not, what precision do they work to?
I wish I knew how they could possibly do that. I do know that they adjust distances on a very fine scale using piezoelectricity. In piezoelectricity each atom is slightly squeezed when an electric charge is applied, and this allows accurate squeezing to much less than the diameter of an atom.
An article on the limitations on the accuracy of positioning can be found here https://ligo.caltech.edu/page/research-development
I don’t claim to fully understand it, but it refers to concepts of:
“More than thirty different control systems are required to hold all of the lasers and mirrors in proper alignment and position”.
“Gravitational wave interferometric detectors are the most sensitive position meters in existence, aiming to measure strain (∆L/L) sensitivities of the order of 10 -23. In order to achieve such incredible sensitivities, the LIGO detectors employ 4 km long Fabry-Perot cavities in the interferometer arms”
“Thermal fluctuations in mechanical systems (“thermal noise”) are a limiting noise source in current gravitational wave detectors, in the best frequency references, and in the field of macroscopic quantum measurement. Thermal fluctuations can be limiting, despite having a magnitude typically of order 10 -18 m”
“At frequencies below 200 Hz LIGO’s detector is limited by the Brownian motion noise”
“The fluctuating radiation pressure from the photons will result in fluctuations in the positions of the mirrors. This quantum radiation pressure noise (RPN) limits LIGO’s ability to measure accurately the strain of a passing gravitational wave” … “this quantum noise is more rigorously described by the interaction of quantum vacuum fluctuations, which enter the readout port of the interferometer with the circulating laser light.”
“The variances in orthogonal quadratures of a vacuum state are governed by a Heisenberg-like-inequality. One can reduce the variance in one quadrature, mitigating the corresponding noise term at the expense of heightened noise of a different type. The result is called a ‘squeezed vacuum state’, with the act itself known as squeezing.”
Even the article http://arxiv.org/pdf/1205.5643v1.pdf doesn’t say much about the incredible accuracy obtained, and it says nothing aboiut using piezoelectricity. Instead, the optic position is controlled by something called a “coil driver”, which is incredibly simple, see Figure 5. The position is controlled by a magnet (cylindrical rod) attracted by a coil. An extension of the magnet (flag) partly blocks the light path between an LED (with lens to give parallel rays) and a photodiode. The amount of light detected by the photodiode tells the software by how much to change the current through the coil, and that’s about it, all the rest (tuned suspension, viscous damping) is largely window dressing.
mollwollfumble said:
An article on the limitations on the accuracy of positioning can be found here https://ligo.caltech.edu/page/research-development
Thanks for that. Very interesting.
This is supposed to be the position control mechanism for LIGO. As I say, amazingly simple. Peak actuation forces are up to about hundred mN per actuator. The coil design is 800 turns of polyimide coated 32-AWG wire and the magnets are 10 mm×10 mm Nd-B-Fe cylindrical magnets.
Position accuracy quoted for gravitational wave detectors tends to be given not in m but in m/sqrt(Hz)
For example, of order 10 -10 m at 1 Hz from Figure 7 in the linked article.
Now, 10 -10 m is one Ångström, which is about the diameter of a hydrogen atom, which is definitely not “one thousandth of the diameter of a proton”.
So in summary – I don’t get it.
http://www.thescienceforum.com/astronomy-cosmology/51928-gravity-waves.html
see Janus’ post.
JudgeMental said:
http://www.thescienceforum.com/astronomy-cosmology/51928-gravity-waves.htmlsee Janus’ post.
It’s an interesting comment, but it doesn’t answer the question.
i know and have addressed that now. but it shows that this particular prediction is probably correct.