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Henri Garcia
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LIGO is most sensitive to a GW from directly above/below. As a transverse wave hits an arm why don't the laser source and the mirror move in unison -- thereby covering up the distorted motion?
Henri Garcia said:As a transverse wave hits an arm why don't the laser source and the mirror move in unison
PeterDonis said:Because the wave doesn't move them in unison. It's a wave of tidal gravity; it alternately stretches and squeezes the arm.
Henri Garcia said:Is the displacement for a GW completely within the 4 spacetime axes, or are the effects in spacetime, but the displacement is in a non-existent 5th dimension?
PeterDonis said:I don't understand the question. How can you have a displacement in a non-existent dimension?.
All of general relativity works with four dimensions. You are thinking of "embedding" spacetime in some higher dimensional space, like a piece of paper (2d, or close enough) in 3d space. Although this kind of thing is sometimes drawn (the infamous rubber sheet model, for example) the reality is that there's no evidence of anything outside spacetime in which it could be embedded. And there's no need for it in theory.Henri Garcia said:. But I thought perhaps that the curvature effects in spacetime could be viewed *as if* they resulted from such a displacement. Clearly that is not the case.
In some senses, neither mirror nor laser is moving (if you strap an accelerometer to them, you'll find that they never accelerate). The distance between them is just changing.Henri Garcia said:Similarly, why doesn't the transverse GW not move the laser and mirror together as one? I promise this is my last query on this! :-)
Henri Garcia said:why doesn't the transverse GW not move the laser and mirror together as one?
Henri Garcia said:LIGO is most sensitive to a GW from directly above/below. As a transverse wave hits an arm why don't the laser source and the mirror move in unison -- thereby covering up the distorted motion?
Mister T said:You do understand that the mirror is mounted in such a way that it's free to move towards or away from the laser?
Henri Garcia said:Why should it matter if it is free to move or if it is rigidly held 4 km from the laser?
Justin Hunt said:My understanding is that when neutron stars or black holes collide and create gravitational waves that the event would propagate in the form of expanding spheres, so that after the millions or billions it takes to get here the curvature is essentially flat.
Justin Hunt said:Which means they waves would be equivalent to planes moving through the earth. I would imagine anything that lines up symmetrically would look the same to both arms.
Justin Hunt said:It would be more of a solid passing through the earth.
Justin Hunt said:Not an instantaneously event.
Justin Hunt said:wouldn’t detectors on different parts of the earth, the moon, or even on Mars detect the same “shape”?
Justin Hunt said:Shouldn’t it be symmetrical in all directions it propagates?
Justin Hunt said:I was imagining the wave fronts being planes and it being a longitudinal wave. After reviewing the article and researching I see that gravitational waves are transverse with a transverse tensor rather than vectors and have two polarizations.
Justin Hunt said:I can’t admit to fully understanding what that means.
Justin Hunt said:you indicated the orientation of the dector will effect the “shape” of the gravity wave that it sees.
Justin Hunt said:Does this cause an issue with multiple detectors confirming a result?
Justin Hunt said:If their orientations are different due to their location, wouldn’t the waves look different and make it difficult to pick out in the background noise?
Justin Hunt said:Couldn’t a certain orientation cause both arms to be effected equally so that nothing is detected?
Justin Hunt said:two arms have a plane of symmetry.
Justin Hunt said:3 arms would have a line of symmetry. You need need 4 arms to guarantee no symmetry.
Justin Hunt said:even with two arms and having a plane of symmetry, it would be very improbable for any incoming propagation ray to fall entirely inside that plane of symmetry.
Henri Garcia said:I thought that the delicate suspension of the mirror was only to isolate it from vibrations.
Henri Garcia said:Now I am led to believe that the unconstrained mirror responds more to the stretching. The solid pipe is rigid by electromagnetic forces that fight against the stretching.
Considering only motion induced by the GW, are you implying that the unconstrained mirror moves more than if it were fixed to the Earth? (Let's neglect noise for the sake of argument)Mister T said:You do understand that the mirror is mounted in such a way that it's free to move towards or away from the laser?
Henri Garcia said:Considering only motion induced by the GW, are you implying that the unconstrained mirror moves more than if it were fixed to the Earth?
Mister T said:It's a necessary part of the design. Without it the device wouldn't work.
Huh?PeterDonis said:The time it takes light to travel 4 km is a lot longer than the time it takes the gravitational wave to move atoms
mfb said:Light needs about 13 microseconds for that distance. That is very short compared to the period of gravitational waves where LIGO is most sensitive (1 to 10 ms).
mfb said:... it would only make the background worse.
That's not really what LIGO detects. What it detects is a difference in flight times of light down the two arms. Usually this is interpreted as a changing difference in the length of the arms. In principle it's doing nothing more complex than racing light pulses down the two arms and watching for anything other than a draw. The practice is rather more complex because the signals are so weak. (Edit: there's more than one way of describing what LIGO does, but that's one way).Justin Hunt said:Similar to how gravity effects the trajectory of photons, gravity waves also effect the photons traveling down the arms by distorting space time.
Gravitational waves (not gravity waves, by the way - those are a kind of surface wave on water) cause changes in distances perpendicular to the direction of propagation of the wave. Say the wave is propagating in the z direction. In one direction (call it x) distances stretch and then squish; in the other (y) direction distances squish then stretch. You are correct that if a wave happens to impinge on LIGO with its x and y directions exactly at 45° to the arms the wave will not be detected. That's one of the reasons for building multiple detectors with different orientations, because each detector is blind to gravitational waves from some parts of the sky. You also need multiple detectors for triangulation and speed estimates and probably other things.Justin Hunt said:I did and still don’t understand the mechanics of GW themselves, but it would be feasible that gravity waves with propagation rays that lie on the plane of symmetry would have identical effects on the photons traveling down the arms and thus have a zero net effect on the detector.
As an example, Virgo was close to this orientation for the observed neutron star merger, and had a very small signal as result.Ibix said:You are cotrect that if a wave happens to impinge on LIGO with its x and y directions exactly at 45° to the arms the wave will not be detected.
PeterDonis said:Because the wave doesn't move them in unison. It's a wave of tidal gravity; it alternately stretches and squeezes the arm.