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Space Science

LIGO Fails To Detect Gravity Waves 357

Posted by kdawson
from the listen-harder dept.
planckscale writes "Last weekend, LIGO (the Laser Interferometer Gravitational-Wave Observatory) did not detect gravitational radiation in association with a gamma ray burst (GRB). The non-detection was actually a valuable contribution, as it helped to distinguish between competing models for what powers GRBs. The detector is due to be upgraded this year for even more accurate measurements. The interferometer is constructed in such a way that it can detect a change in the lengths of the two arms relative to each other of less than a thousandth the diameter of an atomic nucleus."
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LIGO Fails To Detect Gravity Waves

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  • by gnalre (323830) on Wednesday January 23, 2008 @06:19AM (#22151038)
    This is another failiure in the long history of trying to detect gravity waves.

    As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?
    • by rucs_hack (784150) on Wednesday January 23, 2008 @06:53AM (#22151202)
      As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?

      There will be less for spectators to do when gravity scores?
      • by jstott (212041)

        As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?

        Gravity waves are predicted by Einstein's theory of general relativity. Thus, if gravity waves do not exist, then general relativity is fundamentally flawed (hence the interest in testing this particular prediction of the theory).

        -JS

    • by Anonymous Coward on Wednesday January 23, 2008 @07:05AM (#22151258)
      Of course it doesn't exists, there is only Intelligent Falling.
    • Bummer (Score:5, Informative)

      by tqft (619476) <ianburrows_au AT yahoo DOT com> on Wednesday January 23, 2008 @07:11AM (#22151288) Homepage Journal
      1) General Relativity as formulated by Einstein (and a lot of other similar derivates - are there many?) would be in serious doubt. An exam question I had was take GR and show gravity waves exist - you basically show how the wave equation falls out of the formulas and these things carry momentum out of a system.

      2) You then need to explain stuff such as Mercury's orbit precession and observed effects of double Neutron stars slowing down - the FSM stirring his planetary meatball lunch slower?
      • GR is only one of a large number of possible theories that all make similar predictions for the kinds of phenomena we have actually been able to observe. GR happens to be the one that was first written down, and as long as it worked, there has been no reason to consider any of the others.
        • by rucs_hack (784150)
          GR is only one of a large number of possible theories that all make similar predictions for the kinds of phenomena we have actually been able to observe. GR happens to be the one that was first written down, and as long as it worked, there has been no reason to consider any of the others.

          There are others? I've never heard of any. At least not as an overall theory. I know there are theories as to the causes of specific phenomena, but those are, to my knowledge, within the General Relativity realm.
      • by Ironsides (739422)
        Is it possible for them to carry out an experiment that will directly prove the existence of gravity waves? Or that gravity waves travel at the speed of light? Surely there has to be some way to do this directly without waiting for a close enough cosmic event.
    • by BlackGriffen (521856) on Wednesday January 23, 2008 @07:16AM (#22151304)
      It would be a serious blow to the picture in General Relativity of gravity warping space-time itself if we go for too long without detecting gravitational waves using length measurements as an interferometer does. This is especially true if we ever improve the accuracy of our measurements to the point where we can predict that we should observe gravitational waves but don't.

      What we would replace it with that could explain all of the observations that GR predicts I don't personally know, but it's a good day in physics when a theory is proved wrong because it means that we've done our job.
      • What we would replace it with that could explain all of the observations that GR predicts I don't personally know, but it's a good day in physics when a theory is proved wrong because it means that we've done our job.

        Not only does it mean we've done our job, it's also a whole lot of fun. Suddenly there's a whole new theory (or even better, lack of one) to test. Lots of new experiments to do. More hours to spend in basement labs...

        ID'ers just don't know the fun they're missing.

    • by master_p (608214) on Wednesday January 23, 2008 @07:30AM (#22151358)
      Other types of waves (e.g. sound waves, energy waves etc) are composed of particles. What is a gravitational wave composed of? of gravitons? gravitons are not proven to exist. If a gravitational wave has energy (as well as momentum and angular momentum) then what kind of energy is contained in the wave? where does this energy come from?
      • by dlevitan (132062) on Wednesday January 23, 2008 @07:42AM (#22151420)

        Other types of waves (e.g. sound waves, energy waves etc) are composed of particles. What is a gravitational wave composed of? of gravitons? gravitons are not proven to exist. If a gravitational wave has energy (as well as momentum and angular momentum) then what kind of energy is contained in the wave? where does this energy come from?
        Theoretically, gravitational waves are gravitons, just as light/EM waves are photons. Gravitons have not been detected and there is no solid theory for them, but to be consisted with the rest of particle physics, they need to exist. One of the ways GWs are generated are by inspiraling binary neutron stars or black holes. As they circle each other, GWs are produced and the rotational energy of the binary is sent out in the GWs. This is not a significant effect until in the vest last stages of a merger, at which point it will cause the system to lose enough energy for the two objects to collide.

        We have seen binaries losing energy in a manner consisted with GW predictions, so there is a good chance the theory of GWs is correct.
        • Re: (Score:3, Interesting)

          by Chemicalscum (525689)

          Gravitons have not been detected and there is no solid theory for them, but to be consisted with the rest of particle physics, they need to exist.

          One of the results of string theory, that the proponents of string theory point out as one of its greatest successes, is the prediction of the existence of the graviton. They were not trying to derive the graviton from the theory, they found that the theory predicted an unexpected particle. When they looked at it closer they then realized that it had the expe

          • by ameline (771895)
            This brings to mind the question: WHICH string theory? (of the roughly 10^500 possible ones).

            Does it make any testable predictions? What? No? Oh well then.

      • Re: (Score:3, Informative)

        > What is a gravitational wave composed of? of gravitons?

        General relativity predicts that gravity waves exist. Quantum mechanics predicts that all energetic wavelike phenomena can be thought of as made up of particles. So putting the two together suggests that gravity waves can be thought of as being made up of particles, and a good name for these particles is 'graviton'. But there are big problems with combining general relativity and quantum mechanics and there isn't a very good theory of gravitons.

    • This is another failiure in the long history of trying to detect gravity waves.

      "Failure" isn't really the correct word. An experiment such as this has a certain sensitivity, meaning that they would detect gravitational waves greater than such-and-such magnitude, in a certain frequency range, etc. In scientific lingo, what you call "failure", we call a null result. Of course, it seems like I'm just renaming it with a fancier, nicer-sounding name. But really it's more than that. From a scientific standpo
      • Re: (Score:2, Insightful)

        by demallien2 (991621)
        That would be true, provided that we knew that LIGO actually worked. As it has to date failed to detect any gravity waves, we can not eliminate the possibility:
          - that gravity waves don't exist (ie that GR is wrong)
          - that the calculated sensitivity of LIGO is wrong by orders of magnitude

        As a result, this study really doesn't tell us very much at all.
        • "As a result, this study really doesn't tell us very much at all."

          TFA proposes a third explaination, ie: the burst was not in Andromeda, it was from a galaxy, far, far, away.
    • Re: (Score:3, Interesting)

      by IWannaBeAnAC (653701)

      As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?

      They do exist. There have been measurements done of the slowing down of a rotating binary pulsar, which is a prediction of Einstein's theory of General Relativity, where the system will emit gravitational radiation and slowly lose energy. This was the subject of the 1993 Nobel prize in Physics [nobelprize.org].

    • They could not find an anticipated ether-drag. Their result nicely dove-tailed with special relativity. However Einstein started from first principles and not their result.
      Perhaps the gravity result suggests a replacement for general relativity.
    • by TeknoHog (164938)

      Einstein's theory of General Relativity predicts gravitational waves from certain kinds of astrophysical events. If these events take place but there are no g-waves, the theory is wrong.

  • diameter? (Score:3, Insightful)

    by bwd234 (806660) on Wednesday January 23, 2008 @06:25AM (#22151062)
    "...of less than a thousandth the diameter of an atomic nucleus."

    Would that be a hydrogen nucleus... a uranium nucleus? Please be more specific.
    • Re: (Score:3, Informative)

      by KiloByte (825081)
      This is a press release, for general public who need illustrations they can somehow comprehend. Those of us who understand more know where to look for more accurate data, which would be both useless and confusing for the broader audience.
    • Re:diameter? (Score:4, Informative)

      by ars (79600) <assd2.dsgml@com> on Wednesday January 23, 2008 @07:03AM (#22151246) Homepage
      The article says proton, not atom, so hydrogen I guess.
      • by QuickFox (311231)

        The article says proton, not atom,
        From the article, as quoted in the comment that you replied to (but with my emphasis):

        "...of less than a thousandth the diameter of an atomic nucleus."
      • by radtea (464814)

        In physics parlance, the "diameter of the nucleus" means ~10^-15 m, which is the diameter of a proton. Because nucleons are close-packed, nuclear diameters are less variable than atomic diameters. The cube root of 240 is only about 6, so the heaviest nuclei are less than 10 times the diameter of the lightest.

        That said, this is not a blow for GR. We do know (in the perfectly ordinary sense of "know", the same way you know you had at least one great-great-grandfather) that gravitational waves exist, based o
  • by MobileTatsu-NJG (946591) on Wednesday January 23, 2008 @06:26AM (#22151066)

    LIGO Fails To Detect Gravity Waves
    Well that's one Yo Mama joke used against me that can finally be laid to rest.
    • by gardyloo (512791)

      Well that's one Yo Mama joke used against me that can finally be laid to rest.
      So can Yo Mama.
  • by Rik Sweeney (471717) on Wednesday January 23, 2008 @07:04AM (#22151252) Homepage
    They're little plastic blocks that kids build things with. Granted, you can make things like the Millenium Falcon with them but it can't actually fly.

    Oh wait, you said LIGO, nevermind.
  • by heikkile (111814) on Wednesday January 23, 2008 @07:06AM (#22151262) Homepage
    There is no gravity - Earth sucks!
  • The non-detection was actually a valuable contribution, as it helped to distinguish between competing models for what powers GRBs.

    Alternately, since no one can really say where gravity itself comes from, the concept behind LIGO could simply fail to account for how gravity really works.

    Who can say that the same shortening of one side compared to the other doesn't affect the speed of light proportionately to the change in length? In that case, we could just as well have a black hole buzz our solar syste
    • by boot_img (610085) on Wednesday January 23, 2008 @07:38AM (#22151402)

      ... but I would call this simply "bad" science - You can't use one poorly-understood phenomenon to explore another.
      You are incorrect. Gravitational waves (the phenomenon) are a very clear and very well understood prediction of the theory of General Relativity. So I would say that this is as far from "bad" science as you can get. If, ultimately, gravitational waves are not detected by LIGO and its successors that would prove GR was incorrect. And that would be a huge scientific advance.
      • by pla (258480)
        Gravitational waves (the phenomenon) are a very clear and very well understood prediction of the theory of General Relativity.

        Yeah yeah, I understand that. So why haven't we ever seen one of these flying pink unicorns?


        If, ultimately, gravitational waves are not detected by LIGO and its successors that would prove GR was incorrect.

        Quantum phsics already "proves" GR as wrong. We just can't articulate how.

        In any case, I disagree. The wrongness of GR doesn't necessarily follow from a lack of gravi
        • by dlevitan (132062)

          Yeah yeah, I understand that. So why haven't we ever seen one of these flying pink unicorns?

          Because they have almost no effect on space time. The fact that we didn't see neutrinos for a long time didn't meant hey didn't exist.

          Quantum phsics already "proves" GR as wrong. We just can't articulate how.

          In any case, I disagree. The wrongness of GR doesn't necessarily follow from a lack of gravity waves - We simply don't
          know
          how gravity works. It could have an underlying mechanism totally outside the scope of GR, thereby not disproving
          GR but requiring a small modification to it, just as GR didn't "destroy" Newtonian physics. We still use the classic kinetics
          laws juuuuust fine in day-to-day calculations. Instead, it extended the older work into a realm that Newton had virtually
          no knowledge of. I don't see why the same idea can't apply here.

          Quantum physics does not prove GR wrong. GR is a valid theory and will continue to be so. However, for certain circumstances (inside black holes, for example) we need both general relativity and quantum mechanics, and it is at that time that we cannot say what happens. The two theories address completely different problems and a theory like string theory is the attempt to i

  • by dlevitan (132062) on Wednesday January 23, 2008 @07:36AM (#22151382)
    For all the people arguing over whether or not this is a failure of LIGO or not...it doesn't really say much at all. Initial LIGO (which is currently running) is more of a proof of concept sold as a viable project. But if you look at the expected rates of detection, the absolute high end for all binary sources is less than one event/year. The low end is between 4 events every 10000 years and 4 events every 100 years. The other source types are not any better.

    This article basically says that because LIGO is known to not be sensitive enough to measure past a certain distance from Earth (which encompasses the Andromeda galaxy, in whose direction this burst occurred) and because no detection was seen, the burst was not caused from a source in the Andromeda Galaxy.

    I suppose that after spending all this money its not a bad thing that LIGO can actually produce some useful results (though I doubt they were amazingly useful). Advanced LIGO should be able to do the job - but not for another 5-6 years. At that point, the minimum event rate is supposed to be around 1/year and we should finally get some sort of positive detection.

    Personally I'm hoping Advanced LIGO does work, because otherwise all this money will have gone to waste and the field of gravitational wave astronomy will be even more damaged than it already is. The thing is, many people in astronomy who are not affiliated with LIGO are not excited by it. Maybe that interest will be rekindled when Adv LIGO actually works, since right now its more of an engineering problem than an astronomy or physics problem. More people are interested in LISA which (if it ever launches) should have many more interesting sources. Its amusing seeing LIGO people try to point out the flaws of LISA while trying to explain why LIGO doesn't work, but then maybe I'm biased since I am working on LISA (though I have worked on LIGO in the past).
    • by gnuman99 (746007)
      Wouldn't the failure of "Advanced LIGO" be significant for the underlying theory, GR? If LIGO fails, it would be one of the best thing to happen to Physics since the photoelectric effect. Einstein's explanation gave rise to a whole new branch of physics and LIGO may end up propelling physics into new areas instead of the same old, same old 'ok, verified theory to x ppm'. I'd say experiments are most successful when they disprove a theory!

  • Go LISA (Score:3, Informative)

    by Zoxed (676559) on Wednesday January 23, 2008 @07:40AM (#22151412) Homepage
    Perhaps the LISA [wikipedia.org] (NASA/ESA) project will have more luck (2015+).
  • and merely an incremental increase in knowledge. It either means that gravity waves are either smaller than "1/1000 of the diameter of a nucleus" (whatever that means) which would require a re-write of the theory because they were predicted to be large enough to detect or LIGO doesn't work which would require a re-write of the theory of gravity because according to that theory LIGO should detect gravity waves. It's a nice result but nothing definitive.
  • LIGOs? (Score:2, Funny)

    by artdwpmt (1222908)
    At the risk of opening up some sensitive old wounds here, if you had more than one of these...

    would they be called LIGOs?
    • Actually we do have more than two of these. I used to work for a lab at the University of Florida making components for the cavities; I was also with them the day they went live! Part of me wished I stayed with the research, but it was a complicated racket and I had other interests. Anyway, one of the interferometers is out in Washington State [caltech.edu] and the other is just up the road from me now outside Baton Rouge in Livingston Parish [caltech.edu]. I got off I-10 one day helping a friend get to some wreckage yard out in t
  • by sapphire wyvern (1153271) on Wednesday January 23, 2008 @07:57AM (#22151480)

    I looked at the Wikipedia article about LIGO and noticed this interesting question in the discussion. No one has answered it there. Apparently it's from some forum somewhere. Maybe someone here can explain the solution to this "conundrum" for me?

    Just getting back to LIGO for a while (sorry if this isn't strictly on topic), I understand that two long laser beams, at 90 degrees to each other, split from one laser source originally by a semi-silvered mirror, are re-combined at a sensitive detector to see whether their wave forms are cancelling or reinforcing. A passing gravity wave will sequentially lengthen and shorten the wavelength of only one of these light beams because the space-time continuum is distorted in only the direction of travel of the gravity wave. This, it is assumed, will cause the interference of the two laser beams to vary also - causing a variation in the light level measured at the detector. I still don't see why LIGO will work because a gravity wave is indiscriminate in the way it distorts things.

    Everything is embedded in our 4-space, including the laser light waves lying along the direction taken by the gravity wave. As the gravity wave compresses and then dilates space-time, the LIGO tube and the laser beam within it will compress and dilate in perfect synchrony. Even the human observers' heads will compress and dilate as the gravity wave passes! The number of light waves per unit length of the LIGO tube (the laser wavelength) will appear unchanged because the actual physical length of the tube will shorten and lengthen as the light waves do, and as the eyeballs of the experimenters do too. If the waves of the re-united beams were re-inforcing peak-to-peak before the gravity wave arrived, they will remain peak-to-peak as the gravity wave passes through also. This alteration in the length of the tube, or arm, of the LIGO experiment, together with the variation in the wavelength of the laser beam, will be completely undetectable for that reason.

    It's not a case of the gravity waves being too weak to detect, their influence is universal within our frame of reference and therefore cannot be directly detected .. by definition! The above is the way I see the situation. But dozens of scientists have spent billions of dollars designing LIGO, so I have to conclude I'm completely incorrect in my reasoning. Can anyone tell me how you can measure a distortion of space-time (4-space) if you, and every tool you use to measure the distortion, including light, are part of the same space-time being distorted?

    I'd be fascinated to see what's wrong with the reasoning here!

    • by JohnFluxx (413620) on Wednesday January 23, 2008 @08:18AM (#22151596)
      > As the gravity wave compresses and then dilates space-time, the LIGO tube and the laser beam within it will compress and dilate in perfect synchrony.

      This part isn't correct. The laser beam will be redshifted and change its wavelength, however it will still travel at the speed of light, c. Since the distance between the two ends is less, it will travel that distance in a shorter time.
    • Re: (Score:3, Informative)

      by dlevitan (132062)

      Everything is embedded in our 4-space, including the laser light waves lying along the direction taken by the gravity wave. As the gravity wave compresses and then dilates space-time, the LIGO tube and the laser beam within it will compress and dilate in perfect synchrony. Even the human observers' heads will compress and dilate as the gravity wave passes! The number of light waves per unit length of the LIGO tube (the laser wavelength) will appear unchanged because the actual physical length of the tube will shorten and lengthen as the light waves do, and as the eyeballs of the experimenters do too. If the waves of the re-united beams were re-inforcing peak-to-peak before the gravity wave arrived, they will remain peak-to-peak as the gravity wave passes through also. This alteration in the length of the tube, or arm, of the LIGO experiment, together with the variation in the wavelength of the laser beam, will be completely undetectable for that reason.

      Basically, as a gravitational wave passes through a section of space time, that section will dilate and contract. To the light between the mirrors (which is not affected because light always travels at the same speed in a vacuum) there is now an additional distance to go. It may make more sense to imagine the light as a stream of photons that have zero volume and thus can't be stretched. When it hits the mirror, it will now be out of phase with the Fabry-Perot cavity and will exit, thus generating a signal

      • by ps236 (965675)
        So, what you're effectively saying that light would appear to slow down if space-time dilates?

        If it travels 300000000 m/s in a vacuum, it would suddenly be travelling at 299999999.999 dilated-m/s.

        So, to the observer it would appear that light goes slower?

        Would the converse apply, that if space-time is compressed, light would appear to go faster than the speed of light?

        So, if it were possible to have a permanently "compressed" area of space-time, light would travel faster than the speed of light in a permane
      • by bodan (619290) <bogdanb@gmail.com> on Wednesday January 23, 2008 @12:14PM (#22154156)
        In fact the length of the space between the mirrors (and any length whatsoever) is _defined_ as the time light spends traveling between the two. This is the definition of distance in GR. It works because the speed of light is constant for everyone everywhere (in GR); the same thing causes all the other funny effects of relativity, for instance the same object having different lengths for different observers.

        So, by the same definition, a piece of space is lengthened or shortened _iff_ light spends a longer or shorter time traveling it. The speed of light never changes, but due to conservation laws its _frequency_ changes.

        Very approximately, imagine the pulse of light starting at the far mirror. The EM wave makes (say) 100 oscillations in 100 seconds (totally out of scale with the real experiment, but that's not important). If the length between the mirrors is constant, the 100 wave peaks will hit the close mirror in 100 seconds. But if the distance between mirrors changes (eg, due to a gravity wave compressing space) _during_ the 100-pulse emission, the last peaks will have less space to travel than the first peaks. This means that the close mirror will be hit by 100 peaks in, say, 90 seconds. So the frequency of the wave went from 1 Hz to 10/9=1.1Hz. The waveform was deformed (compressed), but its speed was constant. (Note that the effect happens _only_ if the space changes shape _during_ the pulse. If it changes, say, between two 100-oscillation pulses spaced apart, you'll still get the travel time difference, but not the frequency shift. LIGO uses continuous lasers, though.)

        The LIGO can't actually measure the change because it's much smaller than in this example. So it sends the lasers in perpendicular directions, and reflects them back. Because gravity waves stretch space differently in each direction (except if their direction happens to exactly bisect the angle between the arms), a passing gravity wave will force the two beams to go slightly out of phase. The difference between the two beams is (barely) detectable for big waves.
    • Gravity also travels at the speed of C (light in a vacuum), if a gravity wave hits one of the end points of LIGO before the others, their will be a minute shift that would put the laser out of phase and be able to be detected from the interference patterns. My guess to why we haven't detected Gravity waves as yet is because that gravity is leaked out to other membranes http://en.wikipedia.org/wiki/Membrane_(M-Theory) [wikipedia.org].
  • The gravitational wave detector doesn't detect gravitational waves?

    "Hooray, I'm useful. I'm having a wonderful time." --Dr. Zoidberg
  • Seriously, how can you trust CDE [wikimedia.org] users [wikimedia.org] ?
  • by mbone (558574) on Wednesday January 23, 2008 @10:17AM (#22152620)
    There have been no direct detections of gravitational waves so far. There have been indirect detections (most robustly with the various binary millisecond pulsars, whose orbits slowly decay due to their radiating energy away in gravitational waves), but no direct detections. However, this was not really seen as an issue, as gravitational wave searches before LIGO suffered from the problem that there were no known sources strong enough for them to detect with good probability. You have to start somewhere, and there is always the chance of either good luck, say a close supernova, or some unknown source that is stronger than expected, but I believe that this is the first actual event whose gravitational waves, by a reasonable model, had a chance of being detected with existing equipment. One such non-detection means nothing - maybe the Gamma Ray Burst occurred way behind the Andromeda Galaxy, for example. If this is consistently repeated, we will eventually conclude that there is something wrong with our physics or our astrophysics, but it is much too soon for that.
  • by rotenberry (3487) on Wednesday January 23, 2008 @10:45AM (#22153022)
    Many persons have implied that not detecting gravitational radiation will somehow invalidate General Relativity. Unless I am mistaken, every theory of gravitation that requires that

    1. Forces due to massive bodies (gravity) to propagate at the speed of light, and
    2. Energy to be conserved

    must also have gravitational radiation. Information propagates at infinite speed in Newton's theory of gravity, so there is no gravitational radiation.

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