Also spiral arms are something of an illusion. What happens is that spiral arms are places where there are more star formation, and areas with more star formation have lots of blue stars. So when you take a picture with instruments that are particularly sensitive to blue light, you see lots of blue stars in the arms.
In fact it's not true that there is "nothing" between the arms. It turns out that with current models of galactic structure that the areas between the arms have as nearly as much "stuff" as the arms. It's just that the "stuff" between the arms are dark bits of gas and dust that don't show up in picture.
Ok, I understand that the arms are areas of a type of condensation and that material is more uniform.
You mentioned that the objects (stars?) dont move but that the gravity field rotates instead. I thought I once heard about the orbit time or our star around the galaxy (60 mill years?) so is this true?
Also, the split arms would indicate a kind of offset gravity interference. I also notice strange concentric circles around supernovae.
Anyways, I appreciate the responses and this cool forum. Thanks.