On 22 Feb, 19:52, Eric Gisse wrote:
On Feb 22, 7:27*am, Ian Parker wrote:





On 22 Feb, 13:40, "Robert Karl Stonjek"
wrote:

Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.

There is always the question about the validity of assumptions.
Instantanious travekl for gravity can be defended on the basis that
the errors in that approximation are of the order of rotation
velocities relative to c.

Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the
instantaneous travel of gravity can be justified. The velocities of
the stars relative to each other are lower still.

Uh, no. Gravitational effects travel exclusively at c in GR, and this
has been shown to be consistent with reality in observation.

I never said they did not. What I DID say was that the error involved
is proportional to relative velocities/c. (assuming travel at c). If a
lot of calculation is involved you may assume instantaneous travel for
orbital velocities of 600km/s and relative velocities lower still.


- Ian Parker