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How is it that the gravitational wave occurred exactly so as to compress one tunnel and expand the other one( as is inferred from the explanations they Caltech gives)?

Or is LIGO built in a way that all gravitational waves will cause this effect? Can you elaborate on this if it is so?

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    $\begingroup$ A gravitational wave causes strain in one direction and stress in the perpendicular direction. How that projects on the detector arms depends on the direction of the wave in the coordinate system of the detector. That's why, ultimately, more detectors are needed. There are other detectors in Europe that are being upgraded, right now, and under construction in Japan, which together with LIGO will make the detection of the direction of these waves much more precise. $\endgroup$
    – CuriousOne
    Commented Feb 20, 2016 at 2:23
  • $\begingroup$ so we were just lucky enough to construct our apparatus just right to detect this one eh?@CuriousOne $\endgroup$
    – Sidarth
    Commented Feb 20, 2016 at 2:26
  • $\begingroup$ I think you are having difficulty with geometric projection, which simply gives you a factor of 1/2 on average (for one angle, I would have to think about it what it does for a full angle in 3d) for a single detector and less (loss of signal) if you have multiple detectors operating at different angles in parallel. Luck has nothing to do with it, it's just simple geometry. $\endgroup$
    – CuriousOne
    Commented Feb 20, 2016 at 2:30
  • $\begingroup$ I cannot understand anything you just said...care to elaborate? $\endgroup$
    – Sidarth
    Commented Feb 20, 2016 at 2:32
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    $\begingroup$ Possible duplicate of Can LIGO measure anything? $\endgroup$
    – Kyle Kanos
    Commented Feb 20, 2016 at 11:50

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Gravitational waves are transverse waves. That means if the source of the waves is at some distance in the x-direction, you get the best effect by aligning your interferometer arms in the y- and z-direction.

Since the gravitational waves can come from any direction you theoretically would get the best results if your interferometer had 3 arms - 2 perpendicular to each other on the ground and one pointing upside (or down), then you would get an effect on at least 2 of the 3 arms, no matter from which direction the wave is coming from. Then it would also be easier to triangulate the exact position of the source.

Practically you can only cover 2 axes per location, because it is not possible to build a 4 km hight tower, but in the worst case you still get the effect on at least 1 of the 2 arms (if the wave source is located in the x-direction and you have the arms aligned on the x- and y-axis you still get an effect on the y-arm).

Therefore one builds more detectors located on different positions on the globe. If you have a 2 arm detector at longitude 0° and one on longitude 90° you can have x, y and z covered.

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This is seen simply in the animation from wikipedia

gravwave

The effect of a plus-polarized gravitational wave on a ring of particles.

It shows the way space is distorted by how a ring of particles would behave as the wave passes. It is seen that a detector with two arms ( L shape) measuring the diameters of the ring , would get a maximum effect by the arms being in a perpendicular direction to each other.

The LIGO arms by being perpendicular measure the space distortion at a maximum possible geometry. Think of a detector in the animation to measure two perpendicular distances, the signal would fit the same function whichever way it was oriented to the detector.

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  • $\begingroup$ "It shows the way space is distorted by how a ring of particles would behave as the wave passes" ........will we actually see this?.....since gravitational waves and light travel at the same speed, by the time the light from the above deformation reaches us, the g wave would have reached us as well......will we be able to see this deformation? $\endgroup$
    – Sidarth
    Commented Feb 20, 2016 at 7:33
  • $\begingroup$ LIGO is the way we see it, because it is very very weak, that is why 4 kilometers are needed $\endgroup$
    – anna v
    Commented Feb 20, 2016 at 10:56
  • $\begingroup$ no...not that..read my comment anna...I did not ask if it was hard or not..... $\endgroup$
    – Sidarth
    Commented Feb 20, 2016 at 14:38
  • $\begingroup$ Are you asking if the distortions were high enough in amplitude could we see them? Can we see the tide coming? We would see them on the distortions of matter, even our body. as the amplitude is tiny, we need LIGO $\endgroup$
    – anna v
    Commented Feb 20, 2016 at 15:48
  • $\begingroup$ ok...but the thing is, i ask "(forget LIGO, imagine your eyes are powerful enought o see those effects) since gwaves and light travel at the same speed,if there was a ring of matter in front of us, and undergoes the above demonstrated warping, the light from that warped matter and the g wave itself (that was travelling)would reach the observer( standing in front of the above demonstrated ring) at the same time, will we still be able to see the warping?" $\endgroup$
    – Sidarth
    Commented Feb 21, 2016 at 0:37

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