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Space Science

Gravity Wave Detector Ready For Business 53

Arthur Embleton writes "The BBC has an article about a Gravity Wave Detector. There are two L shaped set-ups. One in Washington, the other in Louisiana. They've got a Laser pointing at a mirror 4km away, watching for the reflection and if it is distorted then it shows that there has been a gravitational pulse, possibly by two Black Holes colliding. The detectors are apparently so accurate they can measure to one-thousandth of the width of a proton! How did they test that it works?"
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Gravity Wave Detector Ready For Business

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  • Really.. (Score:4, Insightful)

    by NegativeK ( 547688 ) <> on Tuesday February 18, 2003 @11:37PM (#5332163) Homepage
    How did they test that it works?

    I think that's the problem. These detectors should work in theory, but gravitation waves are so minute when they get to us that it's _really_ hard to be able to get a reading on them. My bet is the first to provide fairly solid evidence of gravitational waves gets a Nobel.
    • Re:Really.. (Score:3, Insightful)

      by Anonymous Coward
      I betting that's why they build two. You get the same event at the same time on both then say, "What else could it be." I think the hardest problem they will have is that they also made the worlds most sensitve seismic detector. Luckily seismic waves travel much slower than gravity waves.
    • My bet is the first to provide fairly solid evidence of gravitational waves gets a Nobel.

      My vote goes to Kip Thorne. He recommended building these things a long time ago and is still a key member of the project.

      Black Holes and Time Warps [] is a very good book (for the layperson) on the topic.
    • My bet is the first to provide fairly solid evidence of gravitational waves gets a Nobel.

      You win your bet. Russell Hulse and Joe Taylor won in 19xx for their work on the slowing down of a binary pulsar. The slowdown matches the expected orbital energy loss for dissipation by gravity waves. But you knew that, didn't you?

      I believe gravity waves also make their presence known in the large angle fluctuations of the cosmic microwave background.

      Some people will argue that those effects could be caused by something else, but such a coincidence seems as least as large as a simultaneous* jiggle in two places. What excites me about gravitational wave telescopes is that they are just that...telescopes that use something other than light. Black holes and supernovae tend to be dense and opaque, so gravity waves should reveal things that light (or even neutrinos) cannot.

      * with the appropriate semblance of simultaneity, of course.

  • by Anonymous Coward
    Start looking that closely at the fabric of the universe and things are likely to start jumping up at you.

    Kind of a serendipity type situation. Hell, maybe there's some sort of unanticipated effect of some sort of technology we have on earth that will be picked up by the thing. Oscillations from the sun. Maybe the earth is ringing sympathetically with the gravity waves that pass through it.

    Too bad they can't rig this thing up with a bunch of elements. Maybe generate images of a sort.
  • by lingqi ( 577227 ) on Wednesday February 19, 2003 @12:18AM (#5332383) Journal
    1) here [] is an excellent presentation about how it works, etc. something about the sensitivity is on the order of measuring saturn moving toward the sun by the distance of the diameter of a proton.

    2)here [] toward the bottom of the page you can LOG IN to their system and view all the logs. the password and login is blatantly displayed on the site. we should all email [mailto] the site admin to have this changed.

    3) I hope they figured it out for 300 million dollars, but wouldn't changes in gravity wave stretch / compress the tubes AND CAUSE REDSHIFT / BLUESHIFT in the lasers therefore cancelling out the effect?

    • for number one above, from here []:

      Observing this fantastically tiny effect is equivalent to detecting the motion of Saturn if it were to move closer to the sun by the diameter of a single hydrogen atom

      so it's not the diameter of a proton but the diameter of a hydrogen atom. A lot better, but well, still pretty small.

    • If I'm correct, that is the purpose of the two axes. You should get a different component of redshift/blueshift depending on where the gravity wave comes from, and you can detect this.
      • no that's not right. the point of the two axis is so that since the gravity stretch / compress effects are vertical / horizontal, you will get different differences between the tubes, therefor costing the light a different amount of time to return. If the light is at constant frequency, this would cause phase shifts between the beams, something you can measure. (this is from their "how it works" website, by the way)

        However, I am proposing that since there will be corresponding red/blue shifts in the two shafts, the lights will oscillate the same amount of cycles before merging again, therefore nullifying any potential phase difference, and hence eliminating the possibility of obtaining any result.

        I'd think that building these tubes parallel (with a lot of distance between) would be better, because then you would really get phase-shifts from the finite speed of the gravity waves, a positive shift and then a negative shift between the two beams, as the wave affects them sequentially.

        Of course, I am just armchair researching - like I said, sure hope they got this right for 300 million dollars.
        • Placing the tubes at a right angle makes perfect sense... because you don't know which way the gravity wave will come from.

          What you are saying makes sense only if there were some way a gravity wave could make one tube shorter and one tube longer the same amount at the same time, AND the same photon had to travel through both tubes.

          However, it seems that blue/red shift issues are moot anyway, since the device works by basically measuring a relative shift in arrival time between two photons, not my measuring spectrum shift.
        • I'd think that building these tubes parallel (with a lot of distance between) would be better...

          You mean like this []?

          Incidentally, they need two perpendicular beams at each location because they don't know in advance from which direction the gravity waves will be coming.

        • They are betting on theories that predict a quadrupole gravitational wave, not a dipolar one as EM; this is the reason for the two axis. A red or blue shift does not change the speed of light (and in any event will be negligible as the laser will NOT be accelerated to any appreciable fraction of lightspeed in relation to the receiver); simply the measured distance in one tube will shrink as the other enlongates as a gravitational wave passes (or so theory predicts).
    • the sensitivity is on the order of measuring saturn moving toward the sun by the distance of the diameter of a proton

      With that sort of sensitivity, just imagine what CowboyNeal getting up and going to the fridge might do to it...

    • They could launch a mirror into outer space. Send it off into the edge of the solar system. Then calculate where it 'SHOULD' be. They can do their lazer experiment by shooting a lazer at it from a sattelite and timing how long it takes to get back. They could shoot another sattelite off at 90 degrees to get their L shape and have as many of the sattelites as they need to get rid of noise ( which should be much less in space )
      • But then again the gravity pull from the sun is MUCH weaker than the pull of the earth on objects on earth so I'd guess that the variations on earth would be easier to detect than those in space...

        And the price of launching something into space would probably be so huge that it would be smart to test the setup on earth before launching it anyway ;-)

      • ... it's called LISA [].

      • See []. They are planning one -- 5 million kilometer arms, sensitivity to 10^-23 and at much lower frequencies than LIGO. The instrumentation requiremts for this are pretty scary: The distance between proof masses on spacecraft separated by five million kilometers will be measured with an accuracy of better than ten picometers to sense the passage of gravitational waves.
  • by mike_stay ( 631250 ) on Wednesday February 19, 2003 @12:32AM (#5332456) Homepage
    John Baez has some really good info about LIGO in several of his "This Week's Finds in Mathematical Physics" columns. week198 [] is the most recent to mention it. Baez is a great place to start if you like understanding connections between all kinds of different areas of math & physics (which, of course, includes everything else :)
    • by Anonymous Coward
      Baez is a great place to start if you like understanding connections between all kinds of different areas of math & physics (which, of course, includes everything else :)

      And what a versatile person; folk music AND advanced maths..

      Oh, wait.. you said JOHN Baez?
  • virgo [] in italy, GEO [] in Hung^H^H^Hannover, and TAMA [] in Japan. There is talk of building one in Australia, too.

    All of them I approve, but what's up with Japan? Japan gets some 1,200 minor earthquakes per DAY. how in the world do they expect to overcome the seismic noise floor (pun somewhat intended)?

    • Japan also has several semiconductor fabrication plants. These plants require really really low amounts of vibrations to get profitable yields. The foundations for these things are typically 1-2 miles deep, such that they are anchored in the second layer of the earth's crust. And then there's all sorts of vibration damping material everywhere. I figure the gravitational wave detectors use some similar methods, only more so. Besides, all they have to isolate is their laser source and detectors, not the whole vacuum chamber.

      And there's this page [] on the site to which you linked.
  • Interference (Score:3, Interesting)

    by spin2cool ( 651536 ) on Wednesday February 19, 2003 @03:37AM (#5333207)
    But the weak nature of gravity means these disturbances are unimaginably small. . . One of the major tasks for engineers has been to insulate the installations from vibrations - from passing lorries and earthquakes - that might swamp the real data

    As a physics student, I know of many who question the reliablility of such instruments, especially when they're on the surface of the Earth. The earth's crust is composed of constantly moving, shifting layers of rock that create almost constant imperceptible geologic disturbances. It's nearly impossible to completely negate these.

    The scientists responsible for the experiments claim that the non-proximity of the two locations will negate any interference, but there is plenty of seismic data that shows that even the smallest tremors can be picked up by delicate equipment on the other side of the globe!
    • Re:Interference (Score:2, Informative)

      by MindStalker ( 22827 )
      Yes, but gravity waves travel at light speed, seismic waves travel, at a much smaller known speed (about twice the speed of sound is it??) As long as you keep perfect time sync between the stations its easy to measuse how fast the wave traveled and tell which it was.
      • by Anonymous Coward
        Made me grin.

        Seismic waves are sounds, so they travel at the speed of sound. But you are still right because we are talking about sounds waves in rock and rock is more dense than air, so the seismic waves (in rock) are faster than sound (in air).

    • Yeah. But there's not much else we could do about it, except maybe put it in geosynchronous orbit with Earth, and that would be an entirely new ballgame.
  • More info (Score:5, Informative)

    by crapulent ( 598941 ) on Wednesday February 19, 2003 @03:42AM (#5333233)
    Here are the slides [pdf] [] from the Oct 2002 NSF review. Lots of pictures, graphs, technical details, etc. for anyone interested. In a nutshell they are aiming to measure strain on the order of 10^-21 over the frequency range of 100Hz - 1kHz. Using two facilities separated by 3000km allows them to search for correlated events and weed out localized noise. IANAP.

    More slides here. []

    LIGO home page. []

  • The installations must have (as the article hints) excellent insulation from vibrations like trucks (there's no lorries in the USA 8-) ) and earthquakes. I wonder how exactly they are doing this - what kind of technology can be used to hold two things 4km away at precisely (give or take a few thousandths of the width of a proton) the same relative position all the time.

    Any reader here who can explain?

    • by Frans Faase ( 648933 ) on Wednesday February 19, 2003 @05:58AM (#5333546) Homepage
      The idea is to find a place where there are not many fibrations and to make the system rigid. Movements parallel to the detector axis are (theoretically) not noticed. The remaining vibrations will simply make the instrument temporarily blind. That is no problem as long as they do not occur continiously. It is possible to distinguis between vibrations and changes in lenght. Using multiple instruments all over the world also helps distinquishing between local vibration events and globally caused changes in length due to gravational waves.
      • but the instrument is L-shaped, i.e. there are no movements parallel to the axis, as there are two axes.

        And how would one go about making a 4km-long thing rigid, even if only in one axis?

        • I thought we lived in a three dimensional world? If the axis of the L are horizontal, than vertical movements would not interfere.
          • did you mean perpendicular instead of parallel, then? 8-)

            But in any case, while vibrations almost perpendicular to both the axes would not interfere, that's only a very small percentage of the vibrations, and how many vibrations do you get coming from the air above or from the center of the earth? Remember, you've got the installation basically on the ground and most vibrations travel along the ground.

    • by stevelinton ( 4044 ) <> on Wednesday February 19, 2003 @08:16AM (#5333881) Homepage
      <blockquote>I wonder how exactly they are doing this - what kind of technology can be used to hold two things 4km away at precisely (give or take a few thousandths of the width of a proton) the same relative position all the time.</blockquote>

      They don't. They damp out a certain amount of vibration via clever mountings, etc.
      Then they make sure that all the rest happens at very specific frequencies. You can think of a guitar string. When you jolt a guitar, the string will "sing" at its tuned note. I think the LIGO mirror supports are incredibly precisely tuned.

      Now they only look for gravity waves at other frequencies, mainly ones away from where seismic noise mostly is.

      Finally, they compare respoonses from two remote detectors and look for "matching" events separated by the speed of light, instead of the speed of seismic waves.

    • What really is going on is that, the scientists use both locations of the "sensors"(one located in washington, the other in new mexico I think). Were they found places that are relativelly quiet motion wise, use some really ingeneous ways of dampening out local noise, and when they think a gravity wave has moved the laser they check the other one that is located a couple states away and if this one has also moved in the same maner they have most likelly witnessed a gravity wave passing by. Also they obtain this precission from the rigidity of the laser mounts, they make these so that the lasers don't move at all but the supports absorb all the local outside motion of the earth (trucks going by, earthquakes, etc..) and theoretically a gravity wave supercedes all these and will be able to move the lasers therefore allowing a measurement. hope that helped
    • (there's no lorries in the USA 8-) )
      Right, but I think they meant "passing lorries in London". Two passing lorries, and the relativistic effects as they approach each other, may throw these things completely out of whack!
  • by Twylite ( 234238 ) <twylite@cr[ ] ['ypt' in gap]> on Wednesday February 19, 2003 @09:30AM (#5334137) Homepage
    How did they test that it works?

    As everyone is well aware, a gravaton pulse has a 78.2% probability of overloading the power conduits leading to microfractures in the dilithium chamber and a chain reaction that causes a rift in the space-time continuum.

    Basically, Seven of Nine appeared briefly, bad mouthed someone about something they may do one day in an alternate future, recalibrated the sensors not to detect her, and never appeared in the first place.

    Scientists analysing the situation need only to look for a slight seemingly-random deviation in the operational parameters and one operator who feels insulted for no particular reason, in order to prove this theory.

  • How did they test that it works? Easy. You or I could probably do it. Just pick up a proton and measure it with a yardstick, and compare that measurement to that of the gravity wave sensor.
  • hmm... (Score:2, Insightful)

    by C21 ( 643569 )
    wouldn't this work better in deep space. Failing that, wouldn't tossing it up in low earth orbit be better? I can't imagine how theyre going to get past the incredible amount of vibrations, tweaks, tilts that the earth provides.
  • I've been following LIGO for a long time and am pretty excited about the sort of data that is bound to come out of it. Having yet another type of observatory viewing different aspects of celestial events is ok in my book. The one thing I've not been able to understand about the detector is how it is aimed to look at a particular celestial object. I've got a decent understanding of interferometers being a laser loving lad but I'm at a loss figuring out how an individual event can be focused on.

    I've been thinking maybe a particular object is flagged out of an optical catalogue then the total data chunk is parsed for waves that should be coming from that particular object. I'm imagining the detector as like a dipole antenna and having two of them a particular object is tracked using a sort of gravitational Doppler shift between the two (soon to be three) sites. Am I close or do I need a few more physics classes? Can anybody in the know shed some light (pun intended) on this problem for me?
    • The one thing I've not been able to understand about the detector is how it is aimed to look at a particular celestial object.

      At this point, there is no way to look at a particular celestial object as I understand it; the basic objective is just to prove the existence of gravity waves.

      • They're confusing the hell out of me then. All over the LIGO sites they're claiming to be looking at particular celestial objects, supernovae and black holes and such. Unless I read their stuff wrong they're trying to say they intend to look at particular types of objects to find gravity waves coming from them. Then again I don't see how any particular object can be focused on with these things so maybe they do indeed just want to see if they can find any gravity waves coming from anything.
        • Basically they have a good idea what a gravity wave will look like when it comes from a particular object. They are simply looking for a similar 'signature'.

          You can't actually focus the detectors for this, they just interpret what is detected.

          If the fact that there are more than one of these detectors confuses anyone... They are simply (or not so simply) used to filter out the vibrations coming from our own planet. The detectors should see the waves coming from space the same way. Any differences (above a tiny margin) are considered to be coming from earth.

          So, I guess you could say that they focus, but rather than focusing on a particular object in space, they focus on all objects outside our planet.

          • Basically they have a good idea what a gravity wave will look like when it comes from a particular object. They are simply looking for a similar 'signature'.

            To be less confusing:

            Basically they have a good idea what a gravity wave will look like when it is caused by a particular kind of event (black hole coalescence, supernova, neutron star starquake, etc.)
  • There are detectors being built which use a different idea... a gravitational wave which enters the detector modifies the sizes of a 0.6m diameter ball of copper. This ball is in lifted position, all stable from outside vibration and supercooled (~20 mK). They detect the variation in size of the ball and say a grav wave came in.

    A guy in Brazil wanted to build a detector like this (3m in height) for US$ 2million. Let's see what he can do with that... there would be hundred of those in the USA for that amount.

BLISS is ignorance.