Speed of gravity
in [Relativity]
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From: OG on 28 Feb 2010 19:33 "Art" <null(a)zilch.com> wrote in message news:r38lo5572ammeul343f0skafhbktuq92qk(a)4ax.com... > On Sun, 28 Feb 2010 06:55:06 -0800 (PST), Darwin123 > <drosen0000(a)yahoo.com> wrote: > >> Here is my conjecture on what you may have read concerning >>gravitational instability. >> If the speed of gravitational waves is a finite value, including >>c, all orbits are unstable. This is because the orbiting body emits >>gravitational waves. > > No, the reason given has nothing to do with gravitational waves. > Here's a article on the subject: > http://metaresearch.org/cosmology/speed_of_gravity.asp > and here's a quote from the article: > <begin quote> > anyone with a computer and orbit computation or numerical integration > software can verify the consequences of introducing a delay into > gravitational interactions. The effect on computed orbits is usually > disastrous because conservation of angular momentum is destroyed. > Expressed less technically by Sir Arthur Eddington, this means: "If > the Sun attracts Jupiter towards its present position S, and Jupiter > attracts the Sun towards its present position J, the two forces are in > the same line and balance. But if the Sun attracts Jupiter toward its > previous position S', and Jupiter attracts the Sun towards its > previous position J', when the force of attraction started out to > cross the gulf, then the two forces give a couple. This couple will > tend to increase the angular momentum of the system, and, acting > cumulatively, will soon cause an appreciable change of period, > disagreeing with observations if the speed is at all comparable with > that of light." (Eddington, 1920, p. 94) See Figure 1. > <end quote> > > Art Under the general theory of relativity, what we describe as gravity is not an attractive force in the way you describe; rather it is a geometric effect caused by matter (or matter/energy) distorting the 'metric' of spacetime. A 'metric' is the series of equations that describe how position and distance are related. Within the theory, this means that the imagined 'lines of force' between the sun and the planet does not exist, which I think is the point that Eddington was making. With regard to the 'speed of gravity', this is the speed with which perturbations in the speed of gravity are propagated.
From: OG on 28 Feb 2010 19:35 "OG" <owen(a)gwynnefamily.org.uk> wrote in message news:7v0gacFcn2U1(a)mid.individual.net... > > "Art" <null(a)zilch.com> wrote in message > news:r38lo5572ammeul343f0skafhbktuq92qk(a)4ax.com... >> On Sun, 28 Feb 2010 06:55:06 -0800 (PST), Darwin123 >> <drosen0000(a)yahoo.com> wrote: >> >>> Here is my conjecture on what you may have read concerning >>>gravitational instability. >>> If the speed of gravitational waves is a finite value, including >>>c, all orbits are unstable. This is because the orbiting body emits >>>gravitational waves. >> >> No, the reason given has nothing to do with gravitational waves. >> Here's a article on the subject: >> http://metaresearch.org/cosmology/speed_of_gravity.asp >> and here's a quote from the article: >> <begin quote> >> anyone with a computer and orbit computation or numerical integration >> software can verify the consequences of introducing a delay into >> gravitational interactions. The effect on computed orbits is usually >> disastrous because conservation of angular momentum is destroyed. >> Expressed less technically by Sir Arthur Eddington, this means: "If >> the Sun attracts Jupiter towards its present position S, and Jupiter >> attracts the Sun towards its present position J, the two forces are in >> the same line and balance. But if the Sun attracts Jupiter toward its >> previous position S', and Jupiter attracts the Sun towards its >> previous position J', when the force of attraction started out to >> cross the gulf, then the two forces give a couple. This couple will >> tend to increase the angular momentum of the system, and, acting >> cumulatively, will soon cause an appreciable change of period, >> disagreeing with observations if the speed is at all comparable with >> that of light." (Eddington, 1920, p. 94) See Figure 1. >> <end quote> >> >> Art > > Under the general theory of relativity, what we describe as gravity is not > an attractive force in the way you describe; rather it is a geometric > effect caused by matter (or matter/energy) distorting the 'metric' of > spacetime. A 'metric' is the series of equations that describe how > position and distance are related. > > Within the theory, this means that the imagined 'lines of force' between > the sun and the planet does not exist, which I think is the point that > Eddington was making. > > With regard to the 'speed of gravity', this is the speed with which > perturbations in the speed of gravity are propagated. Let's rephrase that, With regard to the 'speed of gravity', this is the speed with which perturbations in the metric are propagated.
From: funkenstein on 28 Feb 2010 20:14 On Feb 28, 2:05 pm, Art <n...(a)zilch.com> wrote: > Has this question been settled yet? I've read that Einstein assumed > gravity travels at c. But I've also read that certain orbits are > iunstable unless gravity travels >> c. > > Art Well the short answer is no: it's never been actually measured. The late Tom van Flandern presented quite well the argument that the "speed of gravity" must be much greater than c, due to aberration making orbits unstable, which dates at least as far back as LaPlace. The Speed of Gravity--What the Experiments Say, by Tom Van Flandern, Physics Letters A, 250 (1998) 1-11. However there is an argument from GR which also predicts stable orbits as presented by Steve Carlip: Carlip, S. (2000). "Aberration and the Speed of Gravity". Phys. Lett. A 267: 8187. Enjoy -
From: BURT on 28 Feb 2010 22:57 On Feb 28, 5:14 pm, funkenstein <luke.s...(a)gmail.com> wrote: > On Feb 28, 2:05 pm, Art <n...(a)zilch.com> wrote: > > > Has this question been settled yet? I've read that Einstein assumed > > gravity travels at c. But I've also read that certain orbits are > > iunstable unless gravity travels >> c. > > > Art > > Well the short answer is no: it's never been actually measured. > > The late Tom van Flandern presented quite well the argument that the > "speed of gravity" must be much greater than c, due to aberration > making orbits unstable, which dates at least as far back as > LaPlace. > > The Speed of Gravity--What the Experiments Say, by Tom Van Flandern, > Physics Letters A, 250 (1998) 1-11. > > However there is an argument from GR which also predicts stable orbits > as presented by Steve Carlip: > > Carlip, S. (2000). "Aberration and the Speed of Gravity". Phys. Lett. > A 267: 8187. > > Enjoy - When mass moves gravity moves with it. Mitch Raemsch
From: carlip-nospam on 1 Mar 2010 12:53
Art <null(a)zilch.com> wrote: > Has this question been settled yet? I've read that Einstein > assumed gravity travels at c. But I've also read that certain > orbits are iunstable unless gravity travels >> c. It depends what you mean by "settled." General relativity predicts that gravity propagates at the speed of light, in the sense that if you change the matter configuration in some finite region, the gravitational effects of that change don't reach distant regions until after the light-travel time to those regions. I wouldn't say Einstein "assumed" this -- it was not put into the derivation of the field equations of general relativity, but is, rather, a conclusion. There's a rigorous proof in Low, "Speed limits in general relativity," Class. Quant. Grav. 16 (1999) 543, on line at arxiv.org/abs/gr-qc/9812067. It's also true that if you start with *Newtonian* gravity and stick in a finite propagation speed, orbits become dramatically unstable. This does *not* happen in general relativity, though; in GR, there are additional velocity-dependent interactions that almost (but not quite) cancel the instability. The lack of exact cancellation leads to slow changes in the orbits of binary neutron stars ("gravitational radiation reaction"), which are observed and agree very precisely with prediction. This cancellation was, again, not put into the derivation of the field equations of general relativity, but comes out as a conclusion. It's discussed in my paper, "Aberration and the speed of gravity," published in Phys. Lett. A267 (2000) 81, on line at arxiv.org/abs/gr-qc/9909087. As for the experimental/observational question, we have no direct evidence. Gravity is too weak an interaction for the difference between an infinite propagation speed and the GR prediction of a finite speed plus velocity-dependent interactions. But a Newtonian theory with infinite propagation speed would give the wrong results for binary pulsars, unless some additional radiation reaction terms were stuck in by hand. It's also worth noting that the same issue occurs in electromagnetism. Almost everyone accepts that the electromagnetic force travels at the speed of light. But if you look at the force exerted by a charge moving at a constant velocity, it points towards the "instantaneous" position of the charge, not the retarded (light-travel-delayed) position. This is discussed in the Feynman Lectures, Vol. II, chap.21 -- you can see very explicitly that the effects of finite propagation speed are canceled to lowest order by additional velocity-dependent interactions that effectively "extrapolate" the position of the moving charge. Steve Carlip |