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Earth Science

The Earth As a Gravitational Wave Detector 70

Posted by Soulskill
from the we'll-never-get-funding-to-build-it dept.
b30w0lf writes "Gravitational wave detection — i.e. the detection of propagating ripples in spacetime — is a hot subject these days, with ground-based interferometer experiments like LIGO active, and hopes for a space interferometer like LISA. However, physicist Freeman Dyson proposed back in 1969 that the earth itself could be used as a gravitational wave detector. The idea is behind the approach is that gravitational waves impact the earth's crust, causing potentially detectable seismic waves. Using Dyson's approach, Physicists at Harvard and NINP, Florence were able to put an upper limit on the intensity of gravitational background radiation based on a year of observational seismic data (abstract, full pre-print). The upper limit they found improved currently laboratory upper limits by 9 orders of magnitude."
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The Earth As a Gravitational Wave Detector

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  • Noise is "easy" enough to account for, but what about other things that could generate similar signals. like constant gas expansion in the mantle, or whatever? I'd be fascinated to know if they have some tools to help with that.

    • Re:Not just noise (Score:5, Informative)

      by geoskd (321194) on Friday March 14, 2014 @03:36PM (#46486119)
      Thay're not saying that gravity waves are creating the signals. What they are saying is that if gravity waves are creating any signals at all, the size of the signals being measured limits the possible size of the gravity waves to smaller than a certain size. It is putting an upper bound on the possible size of gravity waves. This is important, because the previously determined upper bound on gravity wave size was 9 orders of magnitude bigger than it is now that we have these experimental results.
    • Re:Not just noise (Score:4, Informative)

      by K. S. Kyosuke (729550) on Friday March 14, 2014 @03:48PM (#46486235)

      but what about other things that could generate similar signals

      What "other things"? Electromagnetism is the only thing that has comparable speed to gravity waves, and unlike gravity waves, it only penetrates Earth-sized solid matter with exponential falloff at short distances. Forget possible seismic causes for the kind of measurements they must be looking for. Just recalling my high school physics 101, you know...

      Also, from TFA:

      Gravitational waves are the last untested prediction of Albert Einstein's General Theory of Relativity.

      I'd argue that this is debatable, seeing that we've already measured the decay of at least one binary pulsar that wonderfully corresponds to the predicted gravity-wave-mediated energy loss.

  • The Guardian link is reporting on a likely confirmation of gravitational waves. But the seismograph study seems to be ruling them out, at least near the frequency of 1 hz. They made a new, more sensitive detection mechanism and found nothing, thus ruling out the existence of any wave above a very low energy level. Am I missing the point?
    • Neither. (Score:5, Informative)

      by tlambert (566799) on Friday March 14, 2014 @04:17PM (#46486529)

      Neither.

      What they did is say is basically "We now have a detector 10^9 times more sensitive, which is capable of detecting gravitational waves up to 10^9 times smaller than previous detectors, if there are waves. We didn't see any waves with this detector. Therefore if they exist, they are smaller than what our new detector can detect".

      In other words, if there are gravitational waves, they are smaller magnitude than they are able to detect with the new detection system. This doesn't rule them out, it just blacks out a potential energy/amplitude range in which they might have existed before nothing was seen in that search band.

      They've more or less reduced the probability set, and pissed in a number of esoteric theories cheerios, but not done a lot else to prove or disprove gravity waves.

      It's the difference between having to look for a lost item in an entire warehouse, or having to look for it in a crackerjack bix sized area of the warehouse - albeit it'll take a lot more expensive and redesigned equipment to even look in part of the crackerjack box.

      Frankly, if we threw 4 ten ton spheres into relatively deep space (e.g. solar orbit), arranged them at the vertices of a tetrahedron, and then used laser interferometry between the spheres, and then threw another ten ton sphere across the solar system at a non-trivial speed, and through the tetrahedron, not intersecting a face or the body center, we could pretty much say once and for all if there were gravity waves or not, based on delay (or non-delay) of the effect of the moving sphere being "not there, then suddenly there, then suddenly not there", at least to about 1/2 the wavelength used by the interferometers (hence the need for a "non-trivial speed" for what is, in effect, a gravitational probe inserted into the system, to do the experiment).

      Doing the more or less definitive experiment would be expensive (as in "on the order of the cost of the LHC").

      • by hubie (108345)

        and pissed in a number of esoteric theories cheerios

        I'm not inclined to think this is the case. I think their result was many orders of magnitude over the previous measurement because there was only one other measurement made in this frequency range. The previous experiment [aps.org] was a torsion bar experiment done in a modest-sized lab. According to the LISA folks [nasa.gov]:

        In general, a ground-based interferometer is limited to frequencies above about 10 Hz because of seismic noise

        I didn't read the torsion bar paper t

      • It doesn't rule them out, but holy cow, a nine-order-of-magnitude sensitivity leap is huge. This must be a devastating wake-up call to the theorists who were predicting amplitudes that LIGO-style detectors have a snowball's chance of finding.

        The extreme sensitivity of this approach means that nobody will invest in another LIGO-style detector, correct?

      • Frankly, if we threw 4 ten ton spheres into relatively deep space (e.g. solar orbit), arranged them at the vertices of a tetrahedron, and then used laser interferometry between the spheres, and then threw another ten ton sphere across the solar system at a non-trivial speed, and through the tetrahedron, not intersecting a face or the body center, we could pretty much say once and for all if there were gravity waves or not, based on delay (or non-delay) of the effect of the moving sphere being "not there, then suddenly there, then suddenly not there", at least to about 1/2 the wavelength used by the interferometers (hence the need for a "non-trivial speed" for what is, in effect, a gravitational probe inserted into the system, to do the experiment).

        That doesn't sound easy to get right, even with a ton of money. That's a tough engineering problem there.

  • Resonant Detector (Score:5, Informative)

    by mbone (558574) on Friday March 14, 2014 @03:01PM (#46485757)

    The crucial thing is that they improved the limits in the narrow frequency band where the Earth is a resonant detector :

    in the frequency range 0.05 Hz – 1 Hz

    This is very cool, but note that it is at a frequency where there are not a lot of expected sources (stellar-mass binary black hole coalescence is up in the kHz range).

    The announcement on Monday [theguardian.com] about inflationary gravitational waves is likely to get a good deal more scientific attention.

    • Re:Resonant Detector (Score:4, Informative)

      by mbone (558574) on Friday March 14, 2014 @03:15PM (#46485901)

      Actually, scratch the above. Reading their paper and the Dyson paper [harvard.edu], the frequency limit is set by the seismometers, not by the normal modes of the Earth.

    • by bware (148533)

      [...] note that it is at a frequency where there are not a lot of expected sources

      There are sources [nasa.gov] in that range, thus LISA [nasa.gov]. Galactic black holes merging, inspirals of stellar mass objects by galactic black holes.

      LISA was a high pick in the DOA Astro2010 Decadal, now sacrificed on the altar of HSF and JWST.

      • by photonic (584757)
        The LISA project has a long history, with several iterations of down-sizing its costs, and at some point the Americans pulled out of the project completely. The latest version of the project is called ELISA [elisascience.org], which was recently approved as ESA's L3 mission in 2034. A bit late, but better than nothing ...
        • by bware (148533)

          '95-ish? That's the first meeting I remember where there was a realistic attempt to propose something to NASA. I don't think there wasn't much research funding before that. Not that long a history.

          Mostly the cost estimates have gone up, especially after the scrutiny brought by JWST overruns brought more honest costing. It was always going to be a flagship mission. We could debate whether eLISA is actually going to save that much money over the combined US/ESA LISA proposed in Astro2010, which we cast

    • by Saija (1114681)
      My god the words "resonant" and "Freeman" on the same subject... something it's not going to end well to us
  • I know Slashdot takes a lot of heat for reposting old stories, but this one is truly impressive. Its 45 years old! Most Slashdot readers weren't even born yet. The Beatles were still together. I was 2.

    Its like the editors went, oh old stories huh? You think these are old stories? I'll show you old stories, mofo!

    Tomorrow: Jules Verne in 1877 suggested it might be possible to bootstrap space navigation by hitching a ride on a passing comet [wikipedia.org] (tagged: screwyouwhiners)

    • Re: (Score:3, Insightful)

      by Netdoctor (95217)

      Upper Limit on a Stochastic Background of Gravitational Waves from Seismic Measurements in the Range 0.05–1 Hz

      Michael Coughlin and Jan Harms
      Phys. Rev. Lett. 112, 101102 (2014)
      Published March 13, 2014

  • Use the Moon instead (Score:5, Interesting)

    by avandesande (143899) on Friday March 14, 2014 @03:35PM (#46486101) Journal

    Since the moon is much more stable than the Earth, would it be a better detector? Have seismic readings been taken on the Moon?

    • Yes, during the Apollo missions.
    • If gravity waves have actually been detected then we should get data on frequency and variations in intensity if any exist. I think they are simply using geek speak to say that they now have a capacity to detect even less energetic events or isolate specific events of small intensity. They have not detected any actual wave. Isn't it a stretch to think that gravity travels in waves or exists in standing waves? Maybe it simply exists and is all about us and has no movement at all. Gravity may simply be
  • by fluffy99 (870997) on Friday March 14, 2014 @04:19PM (#46486547)

    They've sunk over a billion into the Hanford and Livingston observatories. The LIGO observatories from 2002 to 2010 were only operational for a very small fraction of the time, plagued by equipment problems, never acheived the design sensitivity, and NEVER detected anything useful. Most of their data was contaminated by local noise, including the highway a few miles away. They blindly collected terabytes of raw data that has never been fully analyzed and they have minimal local data analysis capability.

    Now NSF is pouring even more money into it in the hopes they can improve the sensitivity and actually detect something? At best they might record a perturbance that is correlated between multiple sites (they also partner with an Australian site I believe), of which the value of that data is still debatable.

    I wish the NSF would pull the plug on this waste of resources and invest in something more useful like cleaner nuclear power.

    • by iggymanz (596061)

      cleaner nuclear power is solved problem, we already know how to make reactors that can burn "spent fuel" and leave waste that decays in decades rather than millenia. smarter countries are pursuing that, but the USA just extended life on its 2nd gen reactors that were designed in the late 1950s, can melt down if not externally cooled, and make waste that will last longer than any civilization has.

      • Reactors can not "burn spend fuel".
        The resulting products of fissioned Uran etc. can not be made to fission another time.
        And reactors have nothing to do with gravity wave detection anyway, so what is your point?

        • And why did some idiot mod me troll, again?

          • by iggymanz (596061)

            because you stupidly make your own definition of "spent fuel" ignoring what the commercial nuclear power industry calls "spent fuel". The spent fuel in cooling ponds and casks is 96% fissionable and/or breedable fuel that can be used in a properly designed reactor.

            • Breedable yes, fissionable: no.
              Americans usually mix up 'waste' with spent fuel, obviously you did not, my appologizes.

        • by iggymanz (596061)

          You are very ignorant.

          "Spent fuel" from U.S. commercial reactors contains 94 percent U-238, 0.9 percent U-235 and 0.8 percent pu-239. ALL of that can be "burned" in a properly designed reactor, and moreover long-lived isotopes from those reactions can be converted to short lived ones.

          • That is not spent fuel. It is 'part of the fuel'.
            That is the reminder in a fuel rod, after a bit of the real fuel is 'spent'. The spent fuel is what is left over after a U-235 atom is split.

            You are also very ignorant. The stuff you mention can not be 'burned' in a 'properly designed reactor'. It only can be 'bread' into a fissionable form.

            There are no short half life forms of U or Pu, and if there where, what would be the point of : and moreover long-lived isotopes from those reactions can be converted to s

    • by Anonymous Coward

      I have ever quite understood why an interferometer would work for detecting gravity waves. If the wave was a distortion between two space dimensions then OK, but if its between a space dimension and the time dimension it seems to me that the effects would cancel out.

      Perhaps someone with a a better grasp of general relativity would care to explain.

      • By analogy, a black hole is a distortion in space time, but it can bend light. The curved surface of the earth can be measured entirely by measuring the distance between points along the surface.

        I believe you are right that you could construct space=time distortions that would not affect the travel time of light, but you can also construct those that do - and gravity wave distortions do change the travel time.

        • by mikael (484)

          Does a large gravitational object then bend gravity waves? If they are ripples in the space-time continuum then wouldn't they be refracted? Would there be S-type and P-type gravity waves which are like shear and pressure waves?

          • A heavy object would bend gravity waves - they propagate the same way light does.

            My memory is that there are 2 polarizations of gravity waves (sort of the way there are 2 polarizations of light), but they are carried by spin-2 particles not spin-1 so the polarizations look somewhat different. They look vaguely like sheer waves, I do not believe there is an equivalent of pressure waves in standard general relativity.

      • by Anonymous Coward

        You are essentially correct. The only mode that would not cancel out is the tidal mode, which causes the beam to curve, and no experiment is set up to detect that.

    • by joe_frisch (1366229) on Friday March 14, 2014 @06:18PM (#46487873)

      LIGO is enormously more sensitive (~12 orders of magnitude), than this seismic measurement but in a different frequency band (~100Hz), so both are valuable measurements sensitive to different types of GW sources .

      LIGO itself is a phenomenally difficult project, but with big payoffs. There is the basic physics of understanding how gravity works, but there are also technology spinoffs. The extremely low loss mirror technology developed for LIGO is not being used for other applications, including telecom. The high Q optical cavities are used in commercial measurement devices for measuring tiny concentrations of materials in gasses . There are likely many other spin-offs from the project.

      • by fluffy99 (870997)

        LIGO is enormously more sensitive (~12 orders of magnitude), than this seismic measurement but in a different frequency band (~100Hz), so both are valuable measurements sensitive to different types of GW sources .

        LIGO itself is a phenomenally difficult project, but with big payoffs. There is the basic physics of understanding how gravity works, but there are also technology spinoffs. The extremely low loss mirror technology developed for LIGO is not being used for other applications, including telecom. The high Q optical cavities are used in commercial measurement devices for measuring tiny concentrations of materials in gasses . There are likely many other spin-offs from the project.

        Near as I can tell, most of the technology flow (at least recently) is in the other direction, i.e. now that extremely low loss mirrors, etc are available they are upgrading LIGO to use them. Obviously they have a special use case and deserve kudos for developing their own fabrication techniques and applications of the technology.

        The "big payoff" hasn't happened yet and isn't clearly defined. What exactly would the payoff be? I can see how correlating an observed perturbance as measured by this large sc

        • by joe_frisch (1366229) on Friday March 14, 2014 @08:34PM (#46488851)

          I was on a LIGO review committed years ago (and worked on the precursor to the project many years before that). At the time of the review, LIGO had worked with a vendor to produce extremely low loss coatings. Based on that technology that vendor was able to move into the (at that time) rapidly expanding telecom optics business - and actually refused to make the parts LIGO needed) because the technology was more valuable to them for telecom. LIGO really was driving the optics business back then.

          I believe there have also been spinoffs from their stabilization and vibration isolation work, and possibly from their ultra- stable frequency laser work (Maybe someone from the project will respond.... Stan???).

          The value of basic physics like verifying, or disproving general relativity is of course much more difficult to measure. What is the value of understanding the large scale structure of the universe, or physics at very high energies? I don't know the coin to use to measure that. There was a time when number theory, quantum mechanics and relativity all seemed pretty esoteric and useless. That doesn't mean that all basic science is valuable, but there is no way to know in advance what is.

    • by Anonymous Coward

      The LIGO observatories from 2002 to 2010 were only operational for a very small fraction of the time, plagued by equipment problems, never acheived the design sensitivity, and NEVER detected anything useful.

      Holy shit, theoretical science on the very limits of engineering ability is hard and may not result result in a Hollywood-style monitor going "beep, signal detected, beep, signal detected!" when the bright red ON button is pressed?

      It was known in advance that the strain sensitivity of LIGO would probably

  • by peter303 (12292) on Friday March 14, 2014 @04:44PM (#46486857)
    Here [stanford.edu] is the null result. I presume there are new analytical techniques every few years.
  • by Anonymous Coward

    Gravitational Waves [tikalon.com].

  • I don't know the answer to this, but looking at some LIGO charts (http://www.ligo.caltech.edu/advLIGO/images/refdes03.gif) they seem to be looking at 10-100Hz (roughly). Are there interesting or even expected sources in the frequency band investigated in this paper?
    • by SeanDS (1039000)

      I don't know the answer to this, but looking at some LIGO charts (http://www.ligo.caltech.edu/advLIGO/images/refdes03.gif) they seem to be looking at 10-100Hz (roughly). Are there interesting or even expected sources in the frequency band investigated in this paper?

      Gravitational waves are emitted at a wide spectrum of frequencies by different astronomical bodies. LIGO's frequency range is limited mostly by seismic activity at the low end and radiation pressure noise (essentially the momentum imparted by photons hitting mirrors) at the high end. It's about as well as we can do on Earth, currently. Indirect detections via astronomical techniques can avoid the issue of seismic activity disrupting measurements, and so it is possible to look at much lower frequencies. Thes

  • Coincidence? (Score:4, Interesting)

    by uassholes (1179143) on Friday March 14, 2014 @09:16PM (#46489049)
    December 27, 2004 at 21:30:26 UT, a burst of gamma rays from SGR 1806-20 passed through the Solar System. The burst was so powerful that it had effects on Earth's atmosphere, at a range of about 50,000 light years.
    At 00:58:53 UTC on Sunday, 26 December 2004, an undersea megathrust earthquake occurred in the Indian Ocean which caused a tsunami which killed 250,000 people.

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