Are gravitational effects instantaneous?by jacksonb62 Tags: general relativity, gravity, instantaneous, speed of light, waves 

#1
Dec2111, 08:32 PM

P: 21

My friend tried to tell me that the effects of gravity are instantaneous and I assured him that he was wrong because nothing can move faster than the speed of light. Can someone verify that statement because the discussion got me thinking




#2
Dec2111, 08:33 PM

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What test would you propose to measure the speed of propagation of gravitational influence?




#3
Dec2111, 09:01 PM

P: 21

There are many topics in modern physics that cannot be tested per say. I'm talking theoretically. If tomorrow, our sun were to explode, would we notice the gravitational effects of no longer having a sun prior to seeing the light from the supernova? And actually come to think of it there are devices for detecting gravitational waves. I am merely wondering if gravitational effects are instantaneous




#4
Dec2111, 09:07 PM

P: 3,015

Are gravitational effects instantaneous?
The predicted speed of propagation of gravitational waves by General Relativity is equal to the speed of light in vacuum. However, this is not what you ask in your posts.




#5
Dec2111, 09:08 PM

P: 21

That was my intended question. thank you for verifying




#6
Dec2211, 04:50 PM

P: 446

Jackson, the effect of gravity propagates at the speed of light, c.
It is hard to move large masses around at near the speed of light. I do not know of any experimental results that measure the speed of propagation of gravity. 



#7
Dec2211, 07:47 PM

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Gravitation is indeed instantaneous in Newtonian mechanics. On the other hand, relativity theory says nothing (including gravitation) can go faster than the speed of light. This suggests adding a lighttime correction to Newton's law of gravitation. Here's the problem: This would result in something far worse than pretending that gravitation is instantaneous. So how does Newtonian mechanics work so well for predicting the behavior of the solar system? One way to look at it is "massenergy tells spacetime how to curve; the curvature of spacetime tells massenergy how to move." When some small object moves in a region subject to the gravitational influence of a larger mass, the curvature of spacetime from the larger mass is already present in the region into which the object moves. Gravitation appears to be instantaneous. Well, almost instantaneous. Instantaneous gravity cannot explain the precession of Mercury, for example. Another way to look at it is that general relativity says a finite transmission speed is only one of many subtle effects of gravitation. Some of these other effects nearly cancel the effects of a finite transmission speed. The end result is that gravitation appears to be (almost) instantaneous, at least for objects moving at relative speeds that are much slower than the speed of light. Instead of a lighttime correction, you get a 1/r^{4} correction to Newtonian gravity (for "slow" moving objects). 



#8
Dec2211, 08:13 PM

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FAQ: How fast do changes in the gravitational field propagate?
General relativity predicts that disturbances in the gravitational field propagate as gravitational waves, and that lowamplitude gravitational waves travel at the speed of light. Gravitational waves have never been detected directly, but the loss of energy from the HulseTaylor binary pulsar has been checked to high precision against GR's predictions of the power emitted in the form of gravitational waves. Therefore it is extremely unlikely that there is anything seriously wrong with general relativity's description of gravitational waves. Why does it make sense that lowamplitude waves propagate at c? In Newtonian gravity, gravitational effects are assumed to propagate at infinite speed, so that for example the lunar tides correspond at any time to the position of the moon at the same instant. This clearly can't be true in relativity, since simultaneity isn't something that different observers even agree on. Not only should the "speed of gravity" be finite, but it seems implausible that that it would be greater than c; based on symmetry properties of spacetime, one can prove that there must be a maximum speed of cause and effect.[Ignatowsky, Pal] Although the argument is only applicable to special relativity, i.e., to a flat spacetime, it seems likely to apply to general relativity as well, at least for lowamplitude waves on a flat background. As early as 1913, before Einstein had even developed the full theory of general relativity, he had carried out calculations in the weakfield limit that showed that gravitational effects should propagate at c. This seems eminently reasonable, since (a) it is likely to be consistent with causality, and (b) G and c are the only constants with units that appear in the field equations, and the only velocityscale that can be constructed from these two constants is c itself. Highamplitude gravitational waves need *not* propagate at c. For example, GR predicts that a gravitationalwave pulse propagating on a background of curved spacetime develops a trailing edge that propagates at less than c.[MTW, p. 957] This effect is weak when the amplitude is small or the wavelength is short compared to the scale of the background curvature. It is difficult to design empirical tests that specifically check propagation at c, independently of the other features of general relativity. The trouble is that although there are other theories of gravity (e.g., BransDicke gravity) that are consistent with all the currently available experimental data, none of them predict that gravitational disturbances propagate at any other speed than c. Without a test theory that predicts a different speed, it becomes essentially impossible to interpret observations so as to extract the speed. In 2003, Fomalont published the results of an exquisitely sensitive test of general relativity using radar astronomy, and these results were consistent with general relativity. Fomalont's coauthor, the theorist Kopeikin, interpreted the results as verifying general relativity's prediction of propagation of gravitational disturbances at c. Samuel and Will published refutations showing that Kopeikin's interpretation was mistaken, and that what the experiment really verified was the speed of light, not the speed of gravity. A kook paper by Van Flandern claiming propagation of gravitational effects at >c has been debunked by Carlip. Van Flandern's analysis also applies to propagation of electromagnetic disturbances, leading to the result that light propagates at >c  a conclusion that Van Flandern apparently believed until his death in 2010. W.v.Ignatowsky, Phys. Zeits. 11 (1911) 972 Palash B. Pal, "Nothing but Relativity," Eur.J.Phys.24:315319,2003, http://arxiv.org/abs/physics/0302045v1 MTW  Misner, Thorne, and Wheeler, Gravitation Fomalont and Kopeikin  http://arxiv.org/abs/astroph/0302294 Samuel  http://arxiv.org/abs/astroph/0304006 Will  http://arxiv.org/abs/astroph/0301145 Van Flandern  http://www.metaresearch.org/cosmolog...of_gravity.asp Carlip  Physics Letters A 267 (2000) 81, http://xxx.lanl.gov/abs/grqc/9909087v2 



#9
Dec2311, 06:11 AM

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#10
Dec2411, 07:12 AM

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#11
Dec2411, 07:51 AM

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#13
Dec2411, 09:32 AM

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#14
Dec2411, 10:19 AM

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George: thanks for the reference..a good read
my notes: 



#15
Dec2411, 10:44 AM

P: 3,015

There is one point about Einstein's equations that I've read in textbooks (see Landau Lifgarbagez vol.2) that makes them quite different from Maxwell's equations.
While in EM, you can solve for the fields if the motion of the charges is given (as long as the continuity equation is satisfied), or, conversely, solve for the motion of the charges if the "external fields" are given (as long as they obey the sourceless Maxwell's equation in the region where the charges are), this is generally quite impossible for Einstein's equations, even in principle. One must solve simultaneously for the metric as well as the geodesic motion of the particles. I never quite understood why this is so. Perhaps because of the nonlinearity of Einstein's equations. Maybe someone could illuminate this issue further. 



#16
Dec2411, 10:45 AM

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#17
Dec2411, 10:57 AM

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If you mean that all of mass of the sun instantaneous converts into energy (e.g., some hitherto undiscovered process in quantum mechanics), no problem. It is energy that gravitates. The intrinsic mass of all of those photons moving away from the sun at the speed of light is equal to the (former) sun's mass. That collection of photons will gravitate just as the sun did. 



#18
Dec2411, 11:04 AM

P: 2,889

I think wrt a empirical verification of what really happens we are stuck as Dickfore says in #2 with the impossibility or at least great difficulty to come up with a practical test given the limitations that c imposes to signal sending. So the only answer is purely theoretical. 


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