On the "Meaning of the "Speed of Gravity""

Just a few days ago you said gravity would cause effects not observed in redshift surveys did you not? You said there would be an some kind of spectral line shift not seen. Why the sudden reversal on this?

Jim,

Whether you misunderstood or that wasn't me <img src=icon_smile.gif border=0 align=middle>

It is quite possible I misunderstood your statement about redshift and its cause being gravity. The question was about alternate ideas to the cause of redshift being velocity. I suggested gravity as a cause of the observed redshift of quasars and other structures where redshift was observed. This may not have anything to do with the link you left with your new comment about redshift since I may be making assumptions about this you have not intended.

I'd like to return to discussing the Kopeikin's project. Here's my understanding of the matter:
Gravity is a phenomenologically instantaneous central force. In the GR formalism, such instantaneous force is reconstructed through retarded potentials; the residual non-instantaneity results in radiation of gravitational waves (in case of point masses).
Kopeikin opts to understand the "speed of gravity" exactly as the "speed" of those retarded potentials. He doesn't even consider the speed of gravitational force for the simple reason that the standard "geometrodynamical" GR formalism does not consider gravity as force.

So what could that experiment possibly "measure"??? Since they analyse in the GR formalism, they'll get a finite speed close enough to c for those potentials which tells us nothing of the speed of the central force. What can sane people infer from their "result"? Any deviation from c would mean that light is losing/gaining energy in interaction with Jupiter's field through some mechanism unaccounted for in the initial analysis of experimental scheme. We could also get an estimation of the effectiveness of residual red/blue shifts by the means of fields of moving objects.

Any differing opinions?

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>[AB]:Gravity is a phenomenologically instantaneous central force. In the GR formalism, such instantaneous force is reconstructed through retarded potentials; the residual non-instantaneity results in radiation of gravitational waves (in case of point masses). Kopeikin opts to understand the "speed of gravity" exactly as the "speed" of those retarded potentials. He doesn't even consider the speed of gravitational force for the simple reason that the standard "geometrodynamical" GR formalism does not consider gravity as force. So what could that experiment possibly "measure"??? Since they analyse in the GR formalism, they'll get a finite speed close enough to c for those potentials which tells us nothing of the speed of the central force. What can sane people infer from their "result"? Any deviation from c would mean that light is losing/gaining energy in interaction with Jupiter's field through some mechanism unaccounted for in the initial analysis of experimental scheme. We could also get an estimation of the effectiveness of residual red/blue shifts by the means of fields of moving objects. Any differing opinions? <hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

I think you have it exactly right. Kopeikin will measure only the speed of gravitational waves (v_gw = c), but nothing about gravitational force (v_gf >> c). Yet he seems determined to announce it as an experimental determination of the "speed of gravity" in November. Is this science, or politics? -|Tom|-

<BLOCKQUOTE id=quote><font size=2 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
Kopeikin will measure only the speed of gravitational waves (v_gw = c), but nothing about gravitational force (v_gf >> c).
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

He won't even *measure* it, he will calculate it in a formalism that he intends to prove. It's called "circular reasoning" in my vocabulary.

I actually have something to say on the very matter of forces-to-potentials relation. If there is a potential field, one can get force from it as a gradient of the potential (be that retarded one), that's OK. BUT(!!!), if we start with the force, the inverse task has non-unique solutions. Consider a mathematical example - say, we have gradient field that's smooth over a given spacetime area and we have a potential field solution for that gradient field; if the gradient field is arbitrarily modified in only one (or more finite number of) spacetime point(s), the former solution for the potential field stays valid in its unmodified form. But the point-like spacetime events can communicate non-zero impulse/momentum. Furthermore, force is an observable even in a point-like area, the potential field is not.
Now a more physical example - say, we have a test body in a potential field, we get force by the gradient, then we get a sudden extraneous delta-pulse of force impulse - momentum is communicated instantly, but energy is communicated to the potential field LATER as the test body actually starts to move through the field.
Thus the gravitational interaction can be easily constructed of two components - instantaneous transfer of pure momentum (no energy!) through instantaneous scalar gravitons, then energy and momentum redistribution/exchange through slow retarded potential field by lightspeed-slow gravitons. So we have GR working as prescribed and we have instantaneous central force, all without contradictions.

Such mechanism can be easily modelled in analog hydrodynamic schemes...