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Gravity waves not seen on 2nd try (LIGO)

  1. May 8, 2005 #1

    marcus

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    Posted 6 May 2005

    http://arxiv.org/abs/gr-qc/0505029

    Upper limits on gravitational wave bursts in LIGO's second science run

    Authors: LIGO Scientific Collaboration
    23 pages, 14 figures, to be submitted to Phys Rev D

    "We perform a search for gravitational wave bursts using data from the second science run of the LIGO detectors, using a method based on a wavelet time-frequency decomposition. This search is sensitive to bursts of duration much less than a second and with frequency content in the 100-1100Hz range. It features significant improvements in the instrument sensitivity and in the analysis pipeline with respect to the burst search previously reported by LIGO. Improvements in the search method allow exploring weaker signals, relative to the detector noise floor, while maintaining a low false alarm rate, O(0.1) microHz. The sensitivity in terms of the root-sum-square (rss) strain amplitude lies in the range of hrss~10^{-20} - 10^{-19}/sqrt(Hz) No gravitational wave signals were detected in 9.98 days of analyzed data. We interpret the search result in terms of a frequentist upper limit on the rate of detectable gravitational wave bursts at the level of 0.26 events per day at 90% confidence level. We combine this limit with measurements of the detection efficiency for given waveform morphologies in order to yield rate versus strength exclusion curves as well as to establish order-of-magnitude distance sensitivity to certain modeled astrophysical sources. Both the rate upper limit and its applicability to signal strengths improve our previously reported limits and reflect the most sensitive broad-band search for untriggered and unmodeled gravitational wave bursts to date."

    Since LQG is basically an explicit quantization of General Relativity (with some quantum corrections/higher-order terms allowed), results that confirm expectations based on Gen Rel can be seen as favorable. Or at least the opposite as unfavorable.

    On the second run, they tried for 10 days to detect waves, and they did not see any. From a quantum gravity viewpoint it would have been nice, but it didnt work out. maybe next time.

    thanks to Lubos for calling attention to this report
     
  2. jcsd
  3. May 9, 2005 #2

    ohwilleke

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    What sort of gravity wave activity does theory predict? Page 19-20 of the paper seems to indicate that the search is being undertaken in the right Hz range and that this looks pretty bad for existing gravity wave theoy.
     
    Last edited: May 9, 2005
  4. May 9, 2005 #3

    marcus

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    I agree it looks bad, ohwilleke, to the extent that I can judge. You may have more reliable expertise here----what I am going on is that people who know what they are doing designed it (to be in the right range and have adequate sensitivity) so they could reasonably expect to see results.

    I am still in wait-and-see mode. It is still early and I depend on hearing some interpretation from experts. Clifford Will seems to be a leader in experimental GR---he and others should eventually have something to say.
    If you hear anything or have some insight, please share it.
     
  5. May 9, 2005 #4

    wolram

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    I have searched the net for some mechanism that could, "damp out", or
    "cancell", Gravitational Radiation but with no luck, i wonder if there is
    such a thing?
     
  6. May 9, 2005 #5

    Chronos

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    I think it's too early to draw any conclusions. My interpretation of that study is they have merely constrained the frequency of detectable events given the set up used in that study. It seems nearly certain gravity waves are real. It would be difficult to explain pulsar orbital decay [e.g., PSR B1913+16] without them.
     
  7. May 9, 2005 #6

    Kea

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    Hi Chronos

    Don't you mean: The inference of GR from a distant vantage point explains orbital decay? This is not exactly the same thing as direct observation of gravitational waves.

    But yes, we must wait and see.
    Cheers
    Kea
    :smile:
     
  8. May 9, 2005 #7

    Stingray

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    It would have been very surprising if LIGO had already detected gravitational waves. The expected sources are not thought to be strong enough with the current sensitivity (or more precisely, we might all be dead by the time something large enough was likely to blow up/collide/...).

    This does not mean LIGO is useless. There are upgrades planned for the next few years that will improve the sensitivity enough for us to see something (theoretically...). The experiment was too complex to be constructed from the start with the more advanced design, so the plan was to build it in a two-step process. We're only in step 1 right now.
     
  9. May 9, 2005 #8

    Chronos

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    Hi Kea! Of course you are right. Detection of gravity radiation is a daunting prospect. LIGO is going to be hard pressed to isolate any specific events - at least for the time being. Right now, I think they would be thrilled with any decent signal.
     
    Last edited: May 9, 2005
  10. May 9, 2005 #9

    ohwilleke

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    Fools rush in where angels fear to tread, but I'll rush in anyway. I'm not a wait and see kind of guy.

    Contrary to Stringray's statement, I think that detection of gravitational waves was expected because as one of the charts at the end of the paper shows, hundreds of different proposed gravitational wave sources should have been visible with the level of accuracy at this stage (and this appears to already be a second stage report, they mention a prior less accurate effort). As a result, I would disagree with Chronos that "It seems nearly certain that gravity waves are real."

    Are any kind of gravity waves ruled out by this experiment? No. But, the truth of the matter is that while we understand solar system class gravity and strong graviational fields rather well, our understanding of weak gravitational fields is far less empirically based. For example, the only solid confirmation of gravity waves so far, PSR J0737-3039 involves a very strong field. http://en.wikipedia.org/wiki/Gravitational_wave Yet phenomena like dark energy point to our weak understanding of very long range weak fields. We also don't have any really solid evidence on the speed of gravity waves, although we have good reasons to think that they should go at the speed of light.

    LQG and Sting Theory suggest that there are strong parallels between gravity and QCD which suggest that there may be some form of range dependence in gravity as there is in QCD, through terms that have a negligable effect in most cases. This would in turn throw a spanner into models to estimate gravity wave strength from straight GR.

    Could there be quantum or dark energy effects that cause us to not see gravity waves? Sure. One could also trot out any number of other possibilities. One fellow has shown that a slightly massive graviton (with a relaxation of one of GR's subtler assumptions ill tested by experiment) could create long range effect. Dark matter could have some sort of unanticipated effect. Gravity could, rather than being an infinite range force, be a medium range force that only seems infinite due to holographic like processes (with near objects pulled by gravity, and farther objects pulled by near objects). The GR equations could have higher order terms not previously known (after all, IIRC, the equations were chosen to have the smallest number of derivatives necessary to fit known evidence). All of these are highly speculative, but this experiment does up the ante that someone has a piece of the puzzle wrong.

    Baez has a nice discussion of where the moving parts are in GR gravity, and where we are pinned down by experiment: http://math.ucr.edu/home/baez/RelWWW/tests.html
     
    Last edited: May 9, 2005
  11. May 10, 2005 #10
    This is a great posting, and one that gets the mind racing with anticipitation, LIGO has a role to play, it is searching, it is an experiment where one can hopefully test Gravity (our presumed) understanding that is.

    I think your post is a good overview, but there is a subtle inkling of detection?..if I may pose a question:How does one detect GW if there are 'two' kinds?..are there waves that are heading 'towards and away'..how does one really distinguish..Ill ask the same question in an original format that I posed when I heard about the LIGO originally..so her goes:I am standing on Earth, I feel Gravity's 'pull' close range below my feet, but I know that at the top of my Head there is also a slight 'Pull' moving away from my feet, in the opposite direction..how does one detect those wave that are moving away?..do I need two kinds of detectors?
     
  12. May 10, 2005 #11

    ohwilleke

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    Gravity waves are perpendicular to the direction of motion, not parallel. To paraphrase a website that I didn't cut and paste, suppose that a gravity wave is staring you in the face, a really strong one. Your head to toe direction might expand from 6 feet to 8 feet, and you side to side width might squish from two feet to six inches, then you might squish down to four feet and stretch from side to side to four feet, and then you'd return to normal, aside from the fact that you'd be dead. Of course, we are looking for tiny effects, not huge ones, in these tests.

    You wouldn't know the direction (was it staring you in the face or coming at you from the rear) without additional data. But, a simultaneous gamma ray burst, or a second detector, like the one in Italy, might help clue you in.
     
  13. May 10, 2005 #12
    Spot on!, Thanks, this is almost a word for word reply that I had some years ago when originally posted..but as then, I ask..how does the detector in Italy filter out its OWN, 'squishing'..stretching..expanding..from that of the nearest City..or, dare I say it the nearest "in-your-face" Human that ventures close to the apparatus?

    Thanks for the reply.
     
  14. May 10, 2005 #13

    ohwilleke

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    It works in principle basically like noise cancellation headphones. Local infererence is filtered out through a really clever mechanism.
     
  15. May 10, 2005 #14

    Stingray

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    Are you talking about Fig 13 (regarding supernovae)? If so, you should read it more carefully. Those dots represent events where all orientations are optimized. Much more importantly, they are also assumed to occur 100 pc away. That is EXTREMELY close. In fact, the nearest supernova ever observed in modern times (i.e. since the invention of the telescope) was SN1987A at 50,000 pc.

    By the way, many of the people I work with are heavily involved in LIGO. Although I'm not an expert myself, I was relaying the general consensus in my previous post. Nobody seriously expects anything out of the current version of LIGO. That is especially true with this data, which was taken when they hadn't even gotten near design sensitivity yet (it's close now).
     
  16. May 10, 2005 #15

    Stingray

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    LIGO itself is actually two detectors. One is in Louisiana, and the other in Washington.
     
  17. May 10, 2005 #16

    ohwilleke

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    WRT Fig 13, as I read the caption it says that is where we would expect data from 176 different possible supernova at 100 pc, and the line shows what the reads were. I took those as likely expectations, but I certainly could be wrong about that. Certainly, there were no supernova in the expected range during the test.

    As far as the location, is LIGO really two detectors? Or, is it a network that makes up one detector at two widely seperated locations. I thought, perhaps incorrectly, that the two worked together to make one detection, while the Italian set up was an independent operation.
     
  18. May 10, 2005 #17

    Stingray

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    No. They tried to fit to 176 different waveforms. These were derived just from plugging different numbers into various calculations that have been done before. The line is a measure of the minimum signal strength that can be detected (roughly - it of course depends on many different things), so those 176 waveforms were scaled up until most of them were above the line. That required the signal to originate at 100 pc.

    I'm not sure what you mean. Each of LIGO's locations can operate independently of the other, but they were designed to share data. I don't think the noise level would be low enough if they didn't.

    As far as I understand, all of the detectors around the world are eventually going to be linked together once their individual bugs are worked out. I don't know if the Italian detector is really expected to be sensitive enough on its own. It seems more likely that they only had enough money for one detector, but knew that the American, German, and Japanese detectors would be able to lend a hand.
     
  19. May 10, 2005 #18

    marcus

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    More reports appeared today

    http://arxiv.org/abs/gr-qc/0505041
    Search for gravitational waves from galactic and extra--galactic binary neutron stars
    LIGO Scientific Collaboration: B. Abbott, et al
    20 pages, 12 figures, to be submitted to Phys. Rev. D

    "We use 373 hours (approx. 15 days) of data from the second science run of the LIGO gravitational-wave detectors to search for signals from binary neutron star coalescences within a maximum distance of about 1.5 Mpc, a volume of space which includes the Andromeda Galaxy and other galaxies of the Local Group of galaxies. This analysis requires a signal to be found in data from detectors at the two LIGO sites, according to a set of coincidence criteria. The background (accidental coincidence rate) is determined from the data and is used to judge the significance of event candidates. No inspiral gravitational wave events were identified in our search. Using a population model which includes the Local Group, we establish an upper limit of less than 47 inspiral events per year per Milky Way equivalent galaxy with 90% confidence for non-spinning binary neutron star systems with component masses between 1 and 3 solar masses."


    http://arxiv.org/abs/gr-qc/0505042
    Search for Gravitational Waves from Primordial Black Hole Binary Coalescences in the Galactic Halo
    LIGO Scientific Collaboration: B. Abbott, et al
    7 pages, 4 figures, to be submitted to Phys. Rev. D

    "We use data from the second science run of the LIGO gravitational-wave detectors to search for the gravitational waves from primordial black hole (PBH) binary coalescence with component masses in the range 0.2 - 1.0 solar masses The analysis requires a signal to be found in the data from both LIGO observatories, according to a set of coincidence criteria. No inspiral signals were found. Assuming a spherical halo with core radius 5 kpc extending to 50 kpc containing non-spinning black holes with masses in the range 0.2 - 1.0 solar masses, we place an observational upper limit on the rate of PBH coalescence of 63 per year per Milky Way halo (MWH) with 90% confidence."
     
    Last edited: May 10, 2005
  20. May 11, 2005 #19
    Thanks Marcus, I am about to be placing a number of questions based on L Smolins recent statement of:Asking the right questions
    from a recent article in NS on the basis of T.O.E

    the above link you provided:Search for Gravitational Waves from Primordial Black Hole Binary Coalescences in the Galactic Halo, is just what I have been waiting for!

    As soon as I have finished reading the pre-print, and actually another pre-print that deals with a relevant obstacle, I will be posing the Question, but I want to finish search online to see if the Question has been allready posed, I am assuming it 'has', allready been asked but in a different format as a number of recent pre-prints have been quite close to an 'new', explanation for the far off Luminocity Function of early primordial Universe.
     
  21. May 11, 2005 #20

    marcus

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    Hi Spin Network,

    I do not have a subscription to the New Scientist. If it is not too much trouble for you, could you copy some bits and pieces of the NS article into your post? That way I and other readers can know what Lee Smolin and the other scientists said in this article, and we can better understand what you say as well.

    Probably the NS article you refer to is this:
    http://www.newscientist.com/article.ns?id=mg18624971.500

    "The theory of everything: Are we nearly there yet?
    30 April 2005
    Stephen Battersby

    The hunt for the theory of everything is turning into a road trip from hell - and don't even ask who's reading the map..."

    but that is about all I can read for free. to read more I must pay five dollars to NS.

    EDIT: Here is a link with a few quotes from that article, including something that Smolin said:
    "If you look back over the last 200 years, every decade or two there's a dramatic advance, people always understand something new that couples theory and experiment... I suspect there is some right question that we're not asking."
    http://www.math.columbia.edu/~woit/blog/archives/000187.html


    =====================

    About LIGO and gravity waves, in this thread you can see that Sting Ray has very good information. He is a grad student at Penn State, where much important gravity research is done, and he personally knows people there who are involved with gravity experiments such as LIGO. My advice to you is to listen carefully to what the knowledgeable people in the thread have to say about LIGO and not to try to guess too far out ahead. there may be surprises that none of us (at least not me!) can anticipate.
     
    Last edited: May 11, 2005
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