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Gravity Waves

  1. Sep 30, 2013 #1
    I am trying to learn GRT so I can answer questions for myself. But I might croak first, so I’ll ask here. That gravity wave interferometer they are building out in Richland, Washington - I obviously haven’t read all the technical papers on their web site, but I am pretty sure one I did read showed a spectrogram or PSD with search frequencies of 40 Hz and above. I understand that the behavior of the Taylor-Hulse binary is the only empirical evidence we have so far of gravity waves - and its orbital period is around 8 hours. Can anyone explain why they are looking at such high frequencies? Shouldn’t we be looking for ultra-low frequency waves with periods like 8 hours (or maybe half that for these type waves)? Do we expect stuff falling into a black hole to emit broad-band high frequency gravity waves? Thanks for any enlightenment.
     
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  3. Sep 30, 2013 #2

    Vanadium 50

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    Because the light is better here.

    It's difficult to build a gravity wave detector at such low frequencies. If you half the frequency, you double the wavelength, so your instrument wants to be twice as big.
     
  4. Sep 30, 2013 #3

    phyzguy

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    The gravitational waves emitted by a binary like the Hulse-Taylor binary are much too weak to measure. The best hope for measuring gravitational waves is to measure two orbiting neutron stars or a neutron star-black hole binary just before they coalesce into a black hole. In the last phase of their coalescence, they are orbiting each other extremely rapidly, and their orbital periods are measured in milliseconds. So 10's of Hertz is about right to see these events.
     
  5. Oct 1, 2013 #4
    Wow. How long might such an intense phase last?

    Can you recommend a good textbook? Grad schools are too far away for me.
     
  6. Oct 1, 2013 #5

    HallsofIvy

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    The recent "Scientific American" had an article on how much we can learn by detecting gravity waves. We have been looking for gravity waves for, what, 40 to 50 years, now? And no one has ever detected gravity waves! Yes, the general theory of relativity predicts gravity waves but is there any real evidence they exist? And if they exist but we have not detected them yet do we really know how to detect them?
     
  7. Oct 1, 2013 #6

    phyzguy

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    On the order of 0.1 seconds. This link has a good description of the waveform, and links to other sources.
     
  8. Oct 1, 2013 #7

    phyzguy

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    The Hulse-Taylor binary is, IMHO, irrefutable evidence that GWs exist. The Nobel prize committee must agree, since they awarded Hulse and Taylor the Nobel prize for their work. Do we really know how to detect them? I guess we'll see. If the "Advanced LIGO" experiment doesn't detect them in the next five years or so, something is seriously wrong. But I would bet that they will detect them.
     
  9. Oct 1, 2013 #8

    HallsofIvy

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    I will hold my breath!
     
  10. Oct 1, 2013 #9

    Bill_K

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    Detection of gravitational waves has been "in just a few years" ever since the 1960's.
     
  11. Oct 1, 2013 #10

    phyzguy

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    So has practical production of fusion energy. So we should give up trying? Put things in perspective. How long has it taken us to understand the night sky? Thousands of years, and we're still learning. Suppose Kepler had said "I've been working for decades to understand the planetary orbits and I still don't get it. I give up!"
     
  12. Oct 1, 2013 #11

    Bill_K

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    I don't know what I said that you interpret as a suggestion to 'give up'. That seems like a large overreaction. What I do say (and maybe this has changed) is that gravitational wave experiments have in the past harmed their credibility by a lack of candor. A failure, perhaps, to be up front about the difficulty of detection, and the time scale required for success. "In just a few years" should have been more plausibly stated, "in just a few decades".

    An example of this was a talk I once attended, at which the following exchange (paraphrased) took place between audience member and speaker:

    Q. "Why does your experiment deserve funding, when its sensitivity is two orders of magnitude less than a signal that could be produced by any imaginable astrophysical source?"

    A. "Well, yes. But if we did see a signal, just think how remarkable that would be!"
     
  13. Oct 1, 2013 #12

    phyzguy

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    Bill_K,

    I agree completely. You're right, I overreacted, for which I apologize.
     
  14. Oct 4, 2013 #13
    Thanks for the replies.

    Is there any directionality to the emitted gravitational waves, with respect to the orbital plane of the bodies? And how about the polarization of the waves?
     
  15. Oct 4, 2013 #14

    Bill_K

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    Remember that gravitational waves are quadrupole in nature, so instead of a polarization vector like in EM, they have a polarization tensor. The effect of a wave is an alternating compression and extension along axes perpendicular to the propagation. Instead of the two polarization modes being inclined at a 90-degree angle from each other as in EM, the two polarization modes in a gravitational wave are inclined wrt each other at a 45-degree angle.

    For a binary star source, for emission in the equatorial plane the polarization tensor is in the latitude-longitude direction. That is, parallel and perpendicular to the equator. For emission in the polar direction, the polarization tensor rotates with the two stars, i.e. circular polarization.
     
  16. Oct 5, 2013 #15
    Yes, I called those "n = 2 shell modes" back in engineering. Does anyone know the natural frequency of them in the earth’s crust? And what might be their effective damping? It seems to me that we might be able to detect gravitational waves by looking for global responses of those modes near resonance.

    I’ve read that the magnitude of the crust’s response to lunar tides is about 30 inches (not sure if that is amplitude or range - factor of 2). It seems that the damping of such small deflections should be very small, and thus some dynamic amplification of harmonic forcing might be significant.

    I’ve also read that, "from direct observation", the lobe of that mode facing the moon does not exactly align with the moon, but lags by 2 to 3 degrees. (What type of observation could detect that with such precision?) This lag information and the natural frequency would allow one to calculate the potential dynamic amplification.

    If seismologist can detect effects from major disturbances circling the globe, or the earth "ringing" after some earthquakes, it seems they could also be looking for near resonant responses to some harmonic forcing from distant gravitational wave sources.

    It would require multiple detectors with very accurate time stamps, and then post-processing to look for correlations for source directions, polarizations, etc. And it would of course be complicated by the earth’s rotation, solar orbit, lunar and solar tidal responses, etc.
     
  17. Oct 6, 2013 #16
    I asked this once before, but I'll ask once again: why can't GPS satellites be used to detect gravity waves?

    GPS satellites have a good clock source, they track themselves by lasers and they are far away from each other (in fact, the size of the GPS constellation is much bigger than any laboratory we could ever build on Earth).

    I'm not speaking about using the GPS geographical location determination. I'm speaking about measuring deformations of the GPS constellation.

    We could install laser interferometers on GPS satellites to measure distances between them. This would allow to determine the size and shape of the constellation down to nanometer precision (and it would also have technical and commercial advandages). Then we could check the whole constellation for periodic ellipsoid-like deformations.

    I wonder if I'm not missing something here, but I think the GPS constellation is the best gravity wave detector we've already built.
     
  18. Oct 6, 2013 #17

    phyzguy

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    This is the concept behind the LISA gravitational wave mission. You say "We could install laser interferometers on GPS satellites to measure distances between them" How would you do this? Mount a special mission to fly to all of the GPS satellites to install these interferometers? How much do you think this would cost? Also, some of the GPS satellites can't see each other because the Earth is in the way, and their orbits are distorted due to the non-uniform gravitational field of the Earth. Far easier and cheaper to mount a dedicated mission in an orbit where the Earth doesn't interfere. This is what LISA is.
     
  19. Oct 6, 2013 #18
    OK, I accept you answer, but I still think GPS is a better option. Installing interferometers would have not only scientific applications, but also technological and commercial. It could be done by gradually replacing old satellites with upgraded ones. I also would argue if LISA approach is cheaper.

    From the GPS' satellite perspective Earth is very small and would cover only one or two satellites at once.

    It doesn't matter, since we look only for periodic changes of specific frequency and shape. Does Earth emit some disturbances that could be mistaken for gravity waves for orbit experiments? How is LISA free from that?
     
  20. Oct 6, 2013 #19

    phyzguy

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    Doing it by gradually replacing old satellites might be feasible.

    You're probably right on this.

    LISA would go in a trailing heliocentric orbit, so Earth would be extremely distant (about 1 AU). I think the reason they chose this orbit is to avoid the gravitational influence of any nearby bodies like the Earth or moon. You need to be able to sense changes in position on the order of 1E-12 meter on time scales of 100's to 1000's of seconds. I think the distortions of the orbits caused by the Earth's non-uniform gravitational field would swamp these signals, but I'm not really sure.
     
  21. Oct 6, 2013 #20
    I've wondered the same thing about our GPS satellites, but using their very accurate altitude measurements above the earth's surface - they are already installed. As with ground based observatories, you would be looking for phases between the satellites' (or gravimeters' on the surface) response to the incident gravity wave and the earth's response to the same wave. I recognize that the sources are probably very weak, so that's why I think we should be looking for situations where some dynamic amplification could exist. I.e., very small deflections, thus very small damping, and HARMONIC forcing near a resonance. Polar regions would be important observation sites, as I think tidal variations there due to lunar, solar, other planets' influence are small.
     
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