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Space Science

Initial Tests Fail To Find Gravitational Waves 553

Posted by timothy
from the check-my-belly dept.
eldavojohn writes that though gravitational waves are "predicted to exist by Einstein's Theory of General Relativity, the initial tests run by the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration (LIGO) failed to find anything. It doesn't disprove their existence although it does rule out a subset of string theory. From the article, 'For example, some models predict the existence of cosmic strings, which are loops in space-time that may have formed in the early universe and gotten stretched to large scales along with the expansion of the universe. These objects are thought to produce bursts of gravitational waves as they oscillate. Since no large-amplitude gravitational waves were found, cosmic strings, if they exist at all, must be smaller than some models predict.' The scientists working in Washington and Louisiana (in tandem to rule out flukes) will now move on to Advanced LIGO which will analyze a volume of space 1,000 times larger. If they don't find any gravitational waves in that experiment, the results will be more than unsettling to many theorists."
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Initial Tests Fail To Find Gravitational Waves

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  • by tygerstripes (832644) on Friday August 21, 2009 @08:02AM (#29144877)
    Have they tried turning it off & back on again?
  • Linearization (Score:2, Interesting)

    by Anonymous Coward

    As far as I remember from my course on general relativity, gravitational waves follow from a linearization of Einstein's field equations. Thus, if they failed to find them, it wouldn't falsify the theory as a whole but only the linear approach to the field equations.

    • Re:Linearization (Score:5, Interesting)

      by geekgirlandrea (1148779) <andrea+slashdot@persephoneslair.org> on Friday August 21, 2009 @08:54AM (#29145201) Homepage
      No, there are exact gravitational radiation solutions [wikipedia.org], and you can also predict gravitational radiation from weak-field situations where the linearized approximation is very, very accurano te (the h^2 term would be less than 10^-15 for the sun's gravitational field at Earth's orbit, for example). The decay of orbits due to gravitational radiation has been observed indirectly in PSR B1913+16 [wikipedia.org], and matches the theoretical prediction. If no gravitational radiation is observed at the expected amplitudes for things like that, it will throw a lot more than just string theory into question, and would raise the obvious conservation of energy question about that pulsar.
    • Re:Linearization (Score:5, Interesting)

      by SleepingWaterBear (1152169) on Friday August 21, 2009 @08:56AM (#29145211)

      As far as I remember from my course on general relativity, gravitational waves follow from a linearization of Einstein's field equations. Thus, if they failed to find them, it wouldn't falsify the theory as a whole but only the linear approach to the field equations.

      This isn't exactly right. The equations describing gravitational waves do result from a simplifying approximation of Eintstein's equations, but it's the sort of simplifying approximation that really has to be quite accurate in many circumstances. If they don't find gravitational waves of a certain magnitude then either Einstein was wrong or, more likely, the sorts of astronomical phenomena that could create the waves don't exist.

      • by Lumpy (12016)

        and why do we think we can detect them this deep inside a Gravity well?

        honestly, looking for something like that needs to be outside the gravity well of the sun.

        Just a bit of brain farting in the morning... I haven't had my 2nd cup of coffee yet.

        • Re:Linearization (Score:5, Insightful)

          by Idarubicin (579475) <allsquiet@hotmail3.14.com minus pi> on Friday August 21, 2009 @11:06AM (#29146449) Journal

          and why do we think we can detect them this deep inside a Gravity well?

          honestly, looking for something like that needs to be outside the gravity well of the sun.

          There's a pebble on top of Mount Everest. Using my trusty ruler, I measure the pebble as being 1.3 inches tall.

          "Aha!", says my colleague. "Now we know that the top of the pebble is exactly 6 miles, 1.3 inches high!"

          "No, silly!", says my other colleague. "The only way that we can measure the height of the pebble precisely is by bringing it down to sea level! Being on a mountaintop confounds any precision measurement!"

          Oddly enough, the pebble turns out to be 1.3 inches tall. A most remarkable coincidence, I'm sure.

      • Re:Linearization (Score:5, Insightful)

        by Brain-Fu (1274756) on Friday August 21, 2009 @10:55AM (#29146325) Homepage Journal

        Of course Einstein was wrong.

        He was, at best, as right as any human could have been given the evidence available at the time. If he was as true a scientist as the world portrays him, then he expected to have his model refined over time as new evidence comes to light, eventually being completely replaced by something much more accurate.

        Whatever new theory we build based on this new evidence will also be wrong, for the exact same reasons.

        But it will be right enough to be useful as a stepping-stone to an even righter theory. That is how science works, and that is also why find science zealots to be even more annoying than religious zealots...science zealots have accepted as absolute truth a model that is just a stepping-stone, in direct contradiction of the very methods that they proclaim to be the ultimate determiners of truth.

        • Re: (Score:3, Interesting)

          by timeOday (582209)
          Physics is never perfect because measurement is imperfect, and we're never sure the analogy between the math and reality is quite correct. But math itself is different. I don't think any facts widely accepted as "proven" in math have ever been overturned.
  • by Vinegar Joe (998110) on Friday August 21, 2009 @08:05AM (#29144893)

    Gravity sucks.

  • Success! (Score:5, Insightful)

    by benwiggy (1262536) on Friday August 21, 2009 @08:06AM (#29144899)
    An experiment is only a failure if you don't learn anything from it.
  • by spike1 (675478) on Friday August 21, 2009 @08:09AM (#29144911)

    Doesn't mean the gravitational waves aren't there.
    Maybe they've just got the detection method wrong.

    • Re: (Score:3, Insightful)

      by 4D6963 (933028)
      That's the wrong way to look at it, when you fail to detect something that SHOULD have been detected using what you used, that means that things just actually aren't quite as you expected them to be. Sure there still may be some be gravitational waves, but this proves that they're nothing like we thought/nowhere as strong, if they exist at all.
      • Re: (Score:3, Informative)

        things just actually aren't quite as you expected them to be

        This goes along with some of the greatest scientific discoveries .... "HUH!?!?!?!?! THATS ODD?!?!?!"

        It is these moments of Whiskey Tango Foxtrot that often give us the greatest insights. The ramifications of these types of discoveries can take decades to fully decode and understand.

        I love it when experiments have unexpected results, because those are the most exciting to a scientist.

  • by Gazzonyx (982402) on Friday August 21, 2009 @08:13AM (#29144937)
    Will these tests apply for open, closed, or both so far as strings are concerned? IIRC, open and closed string models are mutually exclusive and should each have a different 'signature' that could be tested for.
    • Re: (Score:2, Informative)

      by horace (29145)

      This comes down to how long it is. A cosmic string isn't the same thing as a string in string theory. A cosmic string is very long macro scale topological feature of the universe while a string theory string is a model for subatomic particles. However you can investigate cosmic strings in string theory leading to the theory of stringy cosmic strings of Vafa et al..

    • by Lumpy (12016)

      And can you test for it if you are inside one of those strings?

      What if our solar system is inside one of those loops?

  • They exist. (Score:5, Informative)

    by Anonymous Coward on Friday August 21, 2009 @08:20AM (#29144975)

    It should be noted that the existance of gravitational waves is pretty much certain - measurements of pulsars like the Hulse-Taylor binary match up perfectly with the predictions of GR.

    What LIGO is about is trying to observe them directly, rather than just observing the effects of them.

    • Re:They exist. (Score:4, Insightful)

      by aicrules (819392) on Friday August 21, 2009 @09:26AM (#29145471)
      Pretty much certain? Yes, a lot of observations have fit the theory of gravitational waves, but this one in particular went against it. The observation method may be flawed in some way, but it COULD mean that the other observed effects are actually attributable to something else. Whether flawed or not, this observation did not disprove or prove the existence and/or nature of gravitational waves. It only served to potentially better define them.
      • Re:They exist. (Score:5, Informative)

        by photonic (584757) on Friday August 21, 2009 @12:54PM (#29148017)
        This result does not contradict 'the theory of gravitational waves'. As mentioned by the OP, there is indirect evidence for their existence, for which Hulse and Taylor got the physics Nobel prize in 1993. The result published now sets a new upper limit on the strength of certain types of signals. This excludes some of the more exotic (stringy) models for the astrophysical generation of GWs (under the assumption that LIGO does indeed have the sensitivity it claims). It did in no way disprove the existence of GWs in general, or rule out some of the less exotic models, which predict much lower levels.
  • by blirp (147278) on Friday August 21, 2009 @08:22AM (#29144985)

    This is obviously because gravity does not exist, but the observed effect is a result of an higher intelligence pushing things down.

    http://www.theonion.com/content/node/39512

    • I always though the expansion of the universe is pushing us upwards at an increasing rate, to give the impression to us that we were always being pulled down. Of course simulations of that theory doesn't seem to account for orbital patterns.

  • Of course... (Score:2, Informative)

    by chill (34294)

    Gravity is related directly to space, which in turn is directly related to time. Time, as we know, is an illusion. Lunchtime, doubly so. Therefore, gravity is an illusion. Q.E.D.

  • their equipment is not sensitive enough to detect the gravity waves, you're talking about billions of years ago when they started and billions of light years of distance traveled since the universe began with a big bang...
  • by damburger (981828) on Friday August 21, 2009 @08:47AM (#29145161)
    Whilst scientists, being human, sometimes form attachments to a particular theory, the failure to find predicted gravity waves can only possibly be good for physics. It is also an exciting time for physicists; failures of existing theories to explain observations provide the kind of mystery a scientist can make a name for himself or herself by solving.
  • I'd be very curious to see how many career paths this experiment just derailed. How large is this subset of string theory that just got wiped out? Also, what does it mean if the bigger version of this test doesn't find gravitational waves? Does it poke a big fat hole in relativity?

  • The LISA mission (Score:5, Interesting)

    by fulldecent (598482) on Friday August 21, 2009 @09:40AM (#29145591) Homepage

    Please see the LISA mission:

    http://lisa.nasa.gov/ [nasa.gov]

    LISA can be thought of as a giant Michelson interferometer in space. The spacecraft separation sets the range of GW frequencies LISA can observe (from 0.03 milliHertz to above 0.1 Hertz).

  • by MrKevvy (85565) on Friday August 21, 2009 @09:48AM (#29145665)

    My own "pet theory" for this was that they would never be detected because although they do exist, they perturb the measurement device to the same degree that they do everything else, ie a gravity wave may perturb one arm of a LIGO detector, but it also correspondingly perturbs the waves of the laser beam passing through it. As a result it isn't detected.
    An analogy: It would be like measuring everything in a room with a ruler, then scaling the whole room including the ruler up or down. You wouldn't see a change with the same scaled ruler; you'd have to bring one in from outside.

    I bounced this idea off a few physicists (including Bruce Allen who runs the Einstein@Home project on LIGO) but they don't seem to like it. :^) Maybe it will turn out to be correct, who knows. It certainly seems to be turning out to be more difficult to detect gravity waves than was initially predicted.

    • Re: (Score:3, Informative)

      by MrScience (126570)

      This is why the laser is split, and sent down two perpendicular paths. Sure, a wave might stretch the spacetime of the X axis... but that same stretching wouldn't effect a similar increase in the return time of the Y axis. This very stretching of the measuring device itself against one axis(thus modifying the round-trip time of the split laser as compared to a perpendicular path) is the very thing that they are measuring.

      Your pet theory, and their experiment, match. :)

  • by GodfatherofSoul (174979) on Friday August 21, 2009 @10:15AM (#29145945)
    I proposed the "really tiny strings" theory long ago that said that a really tiny string is attached between the gravitational bodies like the earth and the moon. Sure, some laughed and countered with their silly "spooling paradox" argument, but sometimes it takes decades to appreciate a true genius.
  • by bjorniac (836863) on Friday August 21, 2009 @11:56AM (#29147189)

    Disclaimer: I don't work on LIGO, but I work with people who do.

    LIGO didn't expect to see a signal above the noise here. What it has done, is largely rule out a lot of 'exotic' sources - sources with equations of state that don't fit the normal matter we see, but some of the more ambitious parts of string theory thought might be possible. What they have achieved is a phenomenal reduction in their 'noise curve' - the background above which a signal must register to be considered real. So far it's only been a one-way test - just ruling out exotic sources, but nothing that we think should necessarily be there.

    LIGO primer and vast oversimplification:

    LIGO is an interferometer. The way it works is that a laser is split into two parts, each of which goes down an equal length tunnel, at right angles to one another. If the light went the same distance, when it is reflected back, it should still be in phase, and should interfere constructively (think back to intro physics and the way waves on a string add). If a gravitational wave which had the right polarization passed through the region in the time of detection, one tunnel will have been 'shorter' due to the contracting geometry caused by the wave, and hence the beams will no longer be in phase when they return, so will not interfere constructively in the same way.

    So why is it so hard to see waves? Well, all kinds of things (drilling, trucks going by, someone sneezing!) can cause a minute wobbling of any part of the equipment and thus will cause the waves to interfere in the wrong way. What LIGO looks for is a specific 'signature' measured at three sites concurrently, the signature being the waves predicted to occur from certain galactic events (two black holes spiraling into one another, for example). They do some pretty impressive data processing to look for this, but so far have only found that they can't see anything above the noise. We've ruled out some of the less likely things that could be going on - types of matter that some string theories allow, but certainly aren't predicted to exist by established theories (like GR).

    However, over time with a few additions to 'advanced' LIGO, or the amazing LISA project we should have a two-way test: Either we'll see the wave that GR predicts to exist from standard black hole collisions, or theoretical physicists have a lot of explaining to do.

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