On 03/03/2010 10:27 AM, Stephen A. Lawrence wrote: > I don't really understand how tidal and retarded effects can cancel. > > Tidal effects are only detectable on extended bodies, and the "tidal > force" generally (always?) has nonzero divergence.
Actually I think that's wrong -- I don't think it's *divergence* that I mean here; the tidal forces "diverge vertically" and "converge horizontally" and the result is the net divergence stays zero. Just fished around a bit on the Web. You've got tidal forces when the Weyl tensor is nonzero. See, for instance: http://www.answers.com/topic/weyl-curvature http://en.wikipedia.org/wiki/Congruence_%28general_relativity%29 That's from GR but the same general concept is going to apply to Newtonian gravity as well, since the theories are essentially identical anyplace where conditions are mild enough for humans to live comfortably (surface of the Sun, or anyplace inside the orbit of Mercury, are examples of places where it's not "comfortable"). I don't think retarding the gravity field results in a nonzero Weyl tensor, and in "small field" conditions these tensors are going to be combining more or less linearly, so, again, I don't see how the effects of retardation can cancel tidal forces. > > On the other hand, the retarded gravity effect typically manifests > itself as a rotation of the acceleration vector versus what you'd > calculate given the (calculated) current position of the gravitating > body. It's detectable by its action on point particles, which don't > accelerate as Newtonian theory would predict. > > But the divergence of retarded gravity is still zero where the mass > density is zero. > > So, again, I don't see how they can cancel. > > I also question this assertion: > > [DJ:] >> If either of these effects are acting on a body its orbit will become >> unstable. > > I don't understand what you mean by this. Tidal forces act on all > planets, without exception, if they're in orbit. But as long as the > planet's own gravity is larger than the tidal forces acting on it, that > doesn't result in instability in its orbit. In fact the effect of tidal > forces can be to "lock" a planet into a particular orbit and rotation > rate -- in other words, tidal forces can make the orbit *more* stable. > > > On 03/01/2010 04:13 PM, David Jonsson wrote: >> Hi >> >> My own posting today on Usenet. >> >> I would be glad if someone could help me with how to calculate this for >> some examples to see if there is any reason in it. >> >> David >> >> David Jonsson, Sweden, phone callto:+46703000370 >> >> >> ---------- Forwarded message ---------- >> From: *David Jonsson* <[email protected] >> <mailto:[email protected]>> >> Date: Mon, Mar 1, 2010 at 6:41 PM >> Subject: Can retarded gravity be counteracted by tidal acceleration? >> To: [email protected] <mailto:[email protected]> >> >> >> Could the accelerating tidal effects >> http://en.wikipedia.org/wiki/Tidal_acceleration >> be cancelled by retarded gravity effects >> http://en.wikipedia.org/wiki/Speed_of_gravity ? >> >> If either of these effects are acting on a body its orbit will become >> unstable. Tidal forces are almost always accelerating and the retarded >> gravity effect is decelerating an astronomical body. Maybe there are >> conditions when the two effects balances each other and maybe these >> conditions form the structure known as the Titius series which is >> purely empirical and not yet explained >> http://en.wikipedia.org/wiki/Titius–Bode_law >> . >> Exoplanets give new cases to test this idea. Moons, rings and pulsars >> could also be tested against this idea. >> >> David >> > >
