From: tjroberts137@sbcglobal.net   
      
   On 3/22/17 3/22/17   9:49 AM, Nicolaas Vroom wrote:   
   > Current main? opinion is that the speed of gravity is the same as the   
   > speed of light.   
      
   Hmmmm. This depends in detail on what one means by "speed of gravity". See my   
   recent post in this thread for details.   
      
   > Is the speed of light constant?   
   > IMO the answer is no. The speed of light is influenced by gravity.   
      
   In GR the LOCAL speed of light (in vacuum) is not influenced by   
   gravity. Over non-local paths one can certainly measure values   
   different from c, but that's not a very useful measurement because   
   it depends in detail on how it is made, and standard techniques   
   won't hold (in particular, how to synchronize clocks and KEEP them   
   synchronized).   
      
   > What I myself have done is to simulate the movement of the planet   
   > Mercury (including many more planets) assuming that the speed of   
   > gravity is not infinite.   
   > The problem is when you use cg = c than such a simulation is not possible.   
   > When you use much higher speeds such simulations become realistic.   
      
   Well known. Simply modifying Newtonian gravity with a finite speed   
   is a non-starter. You have to use the post-Newtonian approximation   
   to GR, which is much more complicated, but accurate.   
      
   Also see my earlier post in this thread -- in the solar system the   
   approximation holds, and even though gravity "propagates" with speed   
   c, the "gravitational force" essentially extrapolates objects'   
   positions to "now" so it is essentially as if gravity "propagated   
   instantaneously".   
      
   	I'm speaking loosely here; see that post for details.   
      
   > I have frequently asked in this newsgroup about results to do the same   
   > using GR. There is not much responds.   
   > My own experience with GR involves one object the Sun and the planet   
   > Mercury as a point mass. No other planets. The mathematics involved   
   > using GR is very tricky.   
      
   Well known. Doing a full-up simulation of the 2-body problem in GR is HIGHLY   
   non-trivial, and using more objects is even more difficult. Here there be   
   dragons, and non-experts are unlikely to succeed. But the post-Newtonian   
   approximation might be accessible to a person skilled in simulations and   
   software development.   
      
   Tom Roberts   
      
   --- SoupGate-Win32 v1.05   
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