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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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  • Look at the ground below your feet... that's a lot of mass there, just to keep you from escaping into space. To do the same with electromagnetism would require only a tiny amount of charge. You can turn the scale question around and ask... how comes we don't see this humungous power of the electromagnetic force in everyday life? Because it is self-shielding. Over long distances electromagnetism is irrelevant and gravity is the only force that matters (even though it is not even a force). The release of a few solar masses in gravitational energy is pretty small, by universal standards, btw. – CuriousOne May 11 '16 at 20:43
  • @Curious one: Universal standards are chosen by us. Of course this helps us to compare and contrast things. But, something that causes very large distortion is spacetime send waves that are feebly detected. Why is it so? Also, is there some kind of entity that these waves could interact? – UKH May 12 '16 at 00:21
  • This means that radiant energy has a little effect on spactime than mass has – UKH May 12 '16 at 00:22
  • I am just observing that the universe is capable of an enormous range of scales and within that range both gravity and electromagnetism have their scales. One can't say that one phenomenon is stronger than the other because as a result of effective interactions they switch sides. The electromagnetic force wins in the small, gravity wins in the large. They also depend on each other. Gravity, without the ability of matter to stick together electromagnetically, would never become strong... mass-energy would stay dispersed in the universe. – CuriousOne May 12 '16 at 00:26
  • What's your opinion on question no.2) – UKH May 12 '16 at 00:28
  • Media break Lorentz invariance and define rest systems. I would not use that term in connection with the propagation of either gravity or electromagnetic waves. I know that it is tempting to think of the physical vacuum as a continuation of the aether, but reality is more complicated than that. We don't know what causes the structure of spacetime and pretty much everything that has been said about it, so far, is pretty unsubstantiated (pun!) conjecture. We have a low energy limit (the standard model), but how gravity fits into it all is virtually unknown. – CuriousOne May 12 '16 at 01:38
  • The Michelson Morley excluded the luminiferous aether, i.e an absolute inertial framework . Lorenz invariant frameworks can be thought as a type of aether, as long as one does not think of them as an absolute inertial frame. QFT for example, which fills space with fields of electron, (photon etc) , and electrons are excitations on this field. – anna v May 12 '16 at 15:39
  • @anna v: Thankyou for your attention. Yeah. Considering spacetime as an absolute frame is nonsensical. But, it can be possibly as you said, based on the constraints. But I still have one unanswered question. is there any possibility of some entity that could interfere the GW. Sorry, if I'am asking anything nonsensical – UKH May 12 '16 at 16:16
  • Do you mean "interact'? yes it will interact with very low probability due to the small coupling constant, graviton photon, graviton gluon, graviton atom etc. – anna v May 12 '16 at 16:56

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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  • Thank you, I have done those calculations (comparing electric and gravitational forces) myself and knew the huge order at which they differ. But dense objects like black holes bend spacetime so brutally and the energy coming from such a cosmic body doing some violent event is very poor!!!!. In the case of electromagnetic waves, they could impart a significant force on charges and since charges comprise matter, they interact with matter. But, why gravitational waves behave so differently? – UKH May 11 '16 at 17:12
  • @Unnikrishnan GW dont behave any different than normal gravity. they both couple weakly. That's the only explanation there is, no one can explain "why does gravity couple weakly". – DilithiumMatrix May 11 '16 at 17:13
  • Suppose we have some cosmic entity with very high density of electric charge. I'am speaking about the electric analogue of a black hole. This could wipe out most portion of the universe around it by the same analysis on the magnitude of the two forces as you said. But, I don't understand the radiant energy originated by mass and energy distributions are weakly affected to the same :( – UKH May 11 '16 at 17:16
  • I'm sorry, I don't understand what you're asking. – DilithiumMatrix May 11 '16 at 23:40
  • More on weakness of gravity: https://physics.stackexchange.com/a/570443/226902 – Quillo Mar 27 '23 at 16:48
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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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