# Velocity of rotation at the edge of the Galaxies

1. Apr 15, 2007

### talksabcd

I found an interesting explanation for rotational speed of the galaxies in the below link at section 7.1

http://www.ag-physics.org/gravity/

Can anyone comment on this ?

2. Apr 15, 2007

### Wallace

The first sentence of that link goes like this:

This is wrong. I assume the rest relies on this so I didn't bother reading it.

3. Apr 15, 2007

### cesiumfrog

That particular statement isn't completely wrong: it is common and reasonable to model the space-time curvature that way, treating the geometry as flat but assigning refractive index around mass. Kinda like the whole PPN approach.. But then it can be taken too literally..

No doubt it's easy to come up with (other) phenomena that do require curvature.. at least in order to be explained in a simple and consistant manner.

..just realised, my choice of the word "simple" may be disputable. Funny how differential geometry seems so difficult to those who haven't learnt the math yet, and seems so elegant to those who've spent time using it.

Last edited: Apr 15, 2007
4. Apr 15, 2007

### talksabcd

What he meant was reduction in velocity of light is just relative not absolute.

Example:
Suppose we are doing two experiments to find the velocity of speed of light
on Earth and Moon then both will get the same value C but velocity of light on the earth measured from moon will be lower due to gravitational length contraction and time dilation on earth and vice-versa from the earth.

Further explanation:

As there would be gravitational time dilation and length contraction on earth
relative to moon, velocity of light should be reduced as time is running slow by
a factor of x and length is contracted by a factor x on earth when observed from moon. So the apparent velocity of light on earth observed from moon will
be C(changed) = Changed length/Changed time = d x^2/t. So apparent velocity of light on earth when observed from moon would be reduced by a factor of x^2.

5. Apr 17, 2007

### Chronos

Just picking the low hanging fruit from the article:

"2. the basic model of matter. That means that elementary particles are built by sub-particles, which orbit each other at the speed of light c"

A surprising number of particle physicists would have issues with this assertion.

"And there is a lot of evidence that the "Dark Matter" which is assumed to be necessary to explain the stability of rotating galaxies is in fact the stream of photons emitted by every hot star."

What evidence? It is customary to provide credible, published, reference sources.

6. Apr 17, 2007

### SpaceTiger

Staff Emeritus