Speed of gravity
in [Relativity]
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From: Urion on 3 Mar 2010 16:24 Gravity is invisible and dark energy is invisible. So these two could be related. Both are subnuclear invisible forces.
From: BURT on 3 Mar 2010 16:28 On Mar 3, 1:24 pm, Urion <blackman_...(a)yahoo.com> wrote: > Gravity is invisible and dark energy is invisible. So these two could > be related. Both are subnuclear invisible forces. There is no dark energy because any regular energy moving through it will gather it to itself. Since energy does not snowball while things move there can be no dark energy. Mitch Raemsch
From: carlip-nospam on 3 Mar 2010 19:30 Art <null(a)zilch.com> wrote: > On Mon, 1 Mar 2010 17:53:55 +0000 (UTC), > carlip-nospam(a)physics.ucdavis.edu wrote: > >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." > Settled by experience, as Einstein used to say. There was > controversy over the Kopeiken and Fomelot result back > in 2002, for example: > http://www.csa.com/discoveryguides/gravity/overview.php Unfortunately, the general consensus is that Kopeikin-Fomelont measurement didn't test the speed of gravity. It's a little tricky. We know what the speed of gravity is in general relativity, and the observations agree with the GR prediction. But to make this a real test of the speed of gravity, we also need to compare the outcome to the predictions of some other class of theories in which the speed is not the same as the speed of light. You can't just reach in and change the speed of gravity in GR -- if you just adjust the relevant parameter, you find that the speed of light changes too, so the two still agree. Instead, you need a more general class of theories in which the two speeds can differ. That requires some arbitrary choices. But in the simplest class of "two speed" theories, the prediction for the Kopeikin-Fomelont observation depends, to lowest order, only on the speed of light; the speed of gravity drops out. So while the observation excludes *some* theories in which the speed of gravity differs from c, many others remain viable. To get a cleaner test, you can look at purely gravitational processes that depend on the speed of gravity. For example, in GR the rate of decay of binary pulsar orbits due to gravitational radiation depends on (v/c)^5, where v is the orbital velocity and c is the speed of gravity. So the observations show that within GR, the speed of gravity is equal to the speed of light to very good accuracy. But this is still, in some sense, theory-dependent: to really give a certain answer, we would need to compare the predictions of theories other than GR in which the two speeds differ. Steve Carlip
From: Tom Roberts on 3 Mar 2010 19:50 carlip-nospam(a)physics.ucdavis.edu wrote: > Unfortunately, the general consensus is that Kopeikin-Fomelont > measurement didn't test the speed of gravity. In a seminar at Fermilab a few months ago, Kopeikin said explicitly that the speed of gravity is c. The room certainly interpreted it as repudiating earlier claims. Tom Roberts
From: Uncle Al on 3 Mar 2010 19:55
Urion wrote: > > Gravity is invisible and dark energy is invisible. So these two could > be related. Both are subnuclear invisible forces. The speed of gravity is 6. As this is a unitless constant it must be universally true. idiot -- Uncle Al http://www.mazepath.com/uncleal/ (Toxic URL! Unsafe for children and most mammals) http://www.mazepath.com/uncleal/qz4.htm |