Questions on the enigmatic rotational curve of spiral galaxies

"Ian Parker" wrote in message
...
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

RKS:
If gravity effects travel at c and the Milky way is 100,000 light years
across then at the outer edge the gravity from the centre of the galaxy
takes 50,000 years to reach the outer edge. That is no problem because the
gravity is uniform. But the pull of the object on the central hub also
takes 50,000 years, and in that time the object has moved.

If I imagine a two dimensional spacetime sheet between a very massive object
such as the centre of the galaxy and a less massive object at a great
distance, such as the sun, then I would expect to see a trough between the
two where the sun's gravity meets the central objects gravity. For an
instant gravitational effect this trough would lead in a straight line
between the sun and the centre, but for a gravitational effect at c we would
expect it to form a helical groove.

Is the sun attracted to the lowest point in this two dimensional space-time
sheet and if so, wouldn't that lowest point be at an angle other than that
of an instantaneous gravity model?

What I am thinking here is that the sun tugs on the centre of the galaxy no
matter how big the difference in the two masses, so the sun must make a dent
in space time all the way to the centre of the galaxy - straight from the
sun to the centre in an instant gravity model but helically in a delayed (by
c) gravity model.

But it might make no difference at all, hence I ask

Robert