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Gravity Wave Detectors

  1. Mar 17, 2007 #1
    1) Do earth-based gravity wave detectors exist?

    2) If so, how effective are they?

    3) Has any gravity wave ever been detected?

    4) Where are there such devices operating currently?

    5) What is involved in establishing an earth-based gravity-wave detector?

    FWIW, an Australian physicist has devised a test for MOND involving a gravity wave detector. See Experiment sets the ultimate test for Newton's laws.
  2. jcsd
  3. Mar 17, 2007 #2
    Yes, for instance LIGO and GEO600. They are all "earth-based "gravitational waves detectors .Unfortunately,so far they all have no strong direct evidence for the existence of gravitational waves . In the near future,a "space-based" gravitational waves detector-LISA will be put into space.
    Last edited: Mar 17, 2007
  4. Mar 17, 2007 #3


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    As far as I know, those detectors have not yet found any evidence for gravitational waves. (It would be big news if they had!)

    With LIGO at least, I think I read that they're still working on improving its sensitivity to the point where they might have a reasonable chance of detecting something. They hadn't expected to find anything by this date.
  5. Mar 17, 2007 #4

    George Jones

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    As others have said, thus far, no direct evidence for gravitational radiation has been found. Indirect evidence for gravitational radiation, however, won Taylor and Hulse the 1993 Nobel Prize in physics.

    General relativity predicts that a system in relative orbital motion will emit gravitational radiation. Since this radiation carries away energy, the period of the orbit will change. In 1974, Taylor and Hulse found a pulsar in orbit, and used the Doppler shift on the pulse frequency to find the period of the orbit. They found that the period of the orbit changes in precisely (to QED-like accuracy) the way that general relativity predicts.
  6. Mar 17, 2007 #5
    Uncovering a Wave Event

    Having now read a little more about LIGO, TAMA, GEO and VIRGO, I understand these instruments to be something slightly larger than a tabletop device. Therefore, the test of MOND outlined by Alex Ignatiev from Australia's Theoretical Physics Research Institute in Melbourne does not appear practical. The instruments needed at the two global locations could never be stabilized or controlled sufficiently to detect a theoretical gravity wave.

    In fact, as I learn more about what such a wave would entail, I wonder if any of the ongoing experiments could succeed. Tides caused by the sun and moon, subduction of tectonic plates, vibrations from distant airborne aircraft, all appear to generate sufficient noise to drown out any gravity wave.

    • That aside, observations obviously generate data. From that data, how do the researchers actually uncover a wave event?
    • And why haven't they already found one?
    There are numerous Gamma Ray Bursts detected each month, comprising some of the universe's largest explosions. Granted the cause of GRBs is not yet fully understood, and GRBs are likely to be the result of several different phenomena.

    • Do not at least some types of GRB's generate gravity waves?
    • What type of event must occur in the universe for any of the current observatories to detect its gravity wave?
    Last edited: Mar 17, 2007
  7. Mar 17, 2007 #6
    - a complex series of computer algorithms scan through the data to try to identify statistically significant events amongst the noise. At some point the process involves a comparison with data from microphones, seismometers, atronomical observations, and other laser interferometers.

    - as you note, there is a lot of (loud) noise, which makes it really hard to detect any possible signals.

    - yes, various astronomical events are expected to produce both gravitational and electromagnetic waves (the latter being much easier to observe).

    - At the current sensitivity, only very unlikely (eg. big and nearby) events will produce signals big enough to detect.
  8. Mar 17, 2007 #7
    I believe these instruments are waiting for a close, powerful supernova or similar; or anything that involves vast, abrupt acceleration of a large amount of matter. The idea is that accelerating mass produces gravitational waves just as accelerating charge produces electromagnetic waves. Th ething is, gravitational waves are so faint and the background so noisy, that you wouldn't be able to isolate a particular signal unless you were expecting it and identified a corresponding 'blip'.
  9. Mar 17, 2007 #8

    Meir Achuz

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  10. Mar 17, 2007 #9
    Me thinks something more than simple Fourier analysis needs to be at work. Do you have any insights as to what mathematical methods are in play?

    Do you have any idea as to what the current observatories' detection sensitivities are?

    What kind of sensitivity is needed to absolutely, positively identify a wave?
  11. Mar 17, 2007 #10
    Do you feel gravity waves do not exist?

    Or do you simply feel the effort and resources would be better allocated and more productive on other endeavors?

    I am troubled by the paucity of observational evidence for gravity waves. At least when it came to the cosmic microwave background radiation, there was evidence of its existence before costly orbiting detectors were built to scan the skies.
    Last edited: Mar 17, 2007
  12. Mar 17, 2007 #11
    How close is close or big is big? Or is this a case of 'we will know it when we see it'?

    Would Supernova 1987A been a candidate? Or was that not large enough?

    Does theory hold that the waves would be isotropic? 1987a was thought to have become a pulsar, but no lighthouse effect was observed, possibly because the beam is not aligned with the earth.
  13. Mar 17, 2007 #12


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    About 30 years ago, low temperature experimentalist Bill Fairbank and his grad students built the most sensitive of the first generation of gravity wave detectors. He noted once at a conference that theorists expected events close enough and large enough for detection would occur in the range from every year or two to every millenium or two. He said, given our limited knowledge of the cosmos and the difficulty of the calculations, that three orders of magnitude span in the estimates was pretty good. Unless you were a grad student studying gravity waves and waiting for an event to appear for your thesis, in which case it wasn't good at all!
  14. Mar 18, 2007 #13

    Meir Achuz

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    Any reasonable signal would be too weak to discern from background, unless the event also destroyed us, and yes to better endeavors.
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