When a gravitational wave passes, do we always have the same amount of stretching and compression?
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3$\begingroup$ I don't understand the question. Please clarify. What do you mean by "squeeze and stretch"? Do you mean perpendicular and parallel to the direction of the waves? Do you mean Do you mean the difference in strain between the crest and the trough of the wave? $\endgroup$– James KMay 10 at 21:32
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1$\begingroup$ +1 for a very interesting and answerable question! In some cases, pretty much, yes. But for fast merging events they're usually close but not quite equal." But I think you should edit the question and add a clarification, perhaps something like "...do the largest positive and negative excursions of the strain wave have the same absolute value?" $\endgroup$– uhohMay 10 at 22:36
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2$\begingroup$ From what I remember, gravitational observatories detect waves when the two “arms” of the experiment are deformed differently; I think it’s even one of the telltale signs that it is indeed a gravitational wave and not something else. But I may be wrong, hence a comment instead of an answer. $\endgroup$– Pierre PaquetteMay 11 at 4:28
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1$\begingroup$ @uhoh yes that's what i meant, but i need some formula if there exist or any source which dive into this subject... $\endgroup$– Hamidreza AbdollahiMay 11 at 10:14
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1$\begingroup$ @PierrePaquette yes we know that , here i want to explore that : length of change in any direction always same? as they pass they effect in any direction , as called squeez and stretch effect. what i asked is ; both effect on strain is same when GW pass through.? $\endgroup$– Hamidreza AbdollahiMay 11 at 10:16
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2 Answers
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I suspect you still haven't clarified your question sufficiently.
Gravitational waves are transverse so there is no stretching and compression in the direction they are propagating.
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Consider this graph of the "stretch and squeeze" at Ligo, Hanford of GW150914 (the first gravitational wave detection, and one of the clearest). This shows the numerical relativity model that was fit to the observations, and so looks "cleaner" than the actual observations.
You may think that the part of the graph above the centre line represents a stretch (of about 10^{-21} metres per metre) and the part below is a squeeze.
As the wave train passes each subsequent stretch is a bit more than the previous squeeze, and each squeeze is more than the previous stretch. This is because the intensity of the radiation is increasing.
But overall there is no tendency for the stretches to be more than the squeezes, and after the gravitational wave has passed, there is no residual stretch. Space returns to its neutral metric, just as after an electromagnetic wave has passed, the electromagnetic field returns to its neutral level.
Note that these stretches and squeezes are in a direction that is perpendicular to the direction of propagation of the wave
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$\begingroup$ thanks for your explain great notes in your answer, my exact Q: if you look at very first moment of your graph; some region(below center line) completely we have only squeeze regime for many moment! and we do not have any Stretch in that timeline moment? (it is sound like squeeze and stretch are not a rhythmic scenario for some moments ) $\endgroup$ May 13 at 22:38
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$\begingroup$ I wouldn't look too closely at the start. Theres not much signal at the start, look at the actual signal in the middle $\endgroup$– James KMay 14 at 5:54