Speed of gravity
in [Relativity]
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From: Art on 28 Feb 2010 08:05 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
From: Androcles on 28 Feb 2010 08:14 "Art" <null(a)zilch.com> wrote in message news:r2qko59fd9jcn17iefd44ipe1t1pg21lks(a)4ax.com... > 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 What's the speed of wetness when you dive into a pool? I've read that some nym-shifting moronic idiots are so fuckin' stooopid they imagine a fixed field has a speed. But I've also read that certain fuckwits are "iunstable" unless kept under lock and key. Where did you escape from?
From: Darwin123 on 28 Feb 2010 09:55 On Feb 28, 8:05 am, 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 General relativity predicts that gravitational waves travel at velocity "c". However, I never read your second statement. 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. The orbiting body accelerates. The orbiting body has mass. This takes energy out of the system. In a two body system, the orbiting body spirals into the object that attracts it. The radius of the orbit eventually gets smaller. The larger the centripetal acceleration, the faster the collapse. I read that the hypothetical gravitational lifetime of the orbit of earth is many trillions of years. The tidal forces are much larger than the force due to gravitational wave emission. The orbital radius of our earth is actually increasing due to effects like tides, solar wind, etc. The effect caused by gravitational wave emission is not measurable in our solar system. However, there is some experimental evidence of this effect. Neutron stars that orbit close to other objects have a very large centripetal acceleration. Furthermore, their tidal forces are damped because of their composition. Neutronium doesn't deform very well, so the tidal forces don't cause expansion in the same way as for a gaseous star. Supposedly, the change in orbital speed of some pulsars has been measured and matches theory. I conjecture that the orbits of these neutron stars are the "unstable orbits" you were talking about. Because the gravitational emission is so big, these pulsars will merge with their companion in a "reasonably short" geological time. The orbit of the earth is relatively stable compared to these neutron star orbits. If "c" was larger, the neutron star orbits would be more stable. As "c" increases, the emission of gravitational waves decrease. If "c" was infinite, these neutron stars orbits would be stable. This may be what your reference was talking about.
From: Art on 28 Feb 2010 12:06 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
From: dlzc on 28 Feb 2010 12:32
Dear Art: On Feb 28, 6:05 am, Art <n...(a)zilch.com> wrote: > Has this question been settled yet? Yes. Changes in gravitation must propagate at c, in GR. But changes in gravitation are not orbiting bodies. Closest I can figure would be what happens to mass as it crosses the event horizon of a black hole, and how fast is it seen to be "at center". > I've read that Einstein assumed gravity > travels at c. *Not* assumed, it falls out of the equations. > But I've also read that certain orbits are > iunstable unless gravity travels >> c. No. Imagine that the curvature of spacetime the corresponds with the body, orbits with the body and has since the body coalesced. No need for FTL gravitation to stabilize orbits... David A. Smith |