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I have been reading an article on gravitational waves here. There, it is written that the gravitational wave, unlike the electromagnetic waves, interact very weakly with matter. The principle of LIGO detectors also support this point.

Question 1: But, if the gravity itself arise due to mass and energy distributions in spacetime, why the ripples in spacetime has a little effect on matter?

I mean, the gravitational radiations are caused by violent cosmic events and hence they should carry enormous energy. But that energy seems to have a little effect on matter. Why is this so? Also, is there any possibility of some entity in spacetime that it could interact with? Do GTR predicts any such entities?

Question 2: Is the spacetime actually a medium for gravitational waves?

I'am asking because the NASA's gravity probe b experiment reveals that the warping of spacetime is a reality. Also, we say that gravitationl waves are ripples in spacetime. The ripples are real. So does the medium too? If yes, is it the same medium which Michelson and Morley searched for (and called as aether)?

Could someone point me in the right direction?

Thanks :)

UKH
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2 Answers2

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Gravity, in general, couples very weakly to matter --- that's why it is often called the 'weakest force'. You can see this by examining the 'coupling constants'---where gravity is $10^{37}$ times weaker than electromagnetism, or comparing how much 'stuff' you need to get equivalent forces---where gravity is about $10^{32}$ times weaker.

Your second question is largely philosophical. But, basically, yes --- spacetime is the medium by which gravity is felt, and the medium through which gravitational waves travel.

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DilithiumMatrix
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Re: "I mean, the gravitational radiations are caused by violent cosmic events and hence they should carry enormous energy. But that energy seems to have a little effect on matter. "

An enormous amount of energy is involved - perhaps a few solar masses worth of mass is converted into gravitational waves when two large staller black holes merge. But the energy of those waves then spreads out into the three-dimensional universe. LIGOs detections are of mergers billions of years ago, billions of light years away. The waves have spread out and weakened by a factor of those billions... cubed. So the gravitational waves LIGO is detecting are almost... undetectable. At billions of LYs distance, the space-time wobble is too weak to bend the concrete under LIGO's arms because the electromagnetic forces holding it together are far stronger than those gravitational waves. But the space-time inside the arms does bend, and the freely suspended mirrors therefore move - allowing the detection of the waves by interference caused by variation in light travel time between the 2 arms. I would think that if LIGO were on a planet orbiting the merger, perhaps the spacetime wobbling /would/ be strong enough to overcome the EM forces, and solid matter would bend and break. I mean, it must happen, at some distance I suppose? But I don't know at what distance that would be. But out here, billions of LYs away, the waves are too weak to break things - they can barely be detected (wobbling a 4km length by less than 1/1000th the width of a proton).

I'm not a professional btw; above is just my understanding as an interested layman having read the excellent LIGO newsletters (https://www.ligo.org/magazine/) over recent years.

LIGO Observing period O4 is planned to start in March 2023. Lots more detections coming! # :)

Gordon Panther
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