Yes, I agree galaxies are not solid - which is the essence of my question. Bars appear to rotate like rigid bodies, not independent masses orbiting the center. I could see the LaGrange point analogy if a large, say black hole or cluster formed in the outer part of the galaxy, but isn't the resonance a POINT, not a bar? Also, how could that generate such a symmetrical bar on the opposite side of the core? I can understand resonances, is there an analogy to ocean waves? Saturn's rings are a prime example of resonances. Something has to be creating the sweet spot for the stars to prefer. Some cataclysmic event? We see high energy jets all the time, but those are along the axis of rotation, not in the plane.
The other part of this is the abrupt end of the bars. A wave would not end so distinctly. Obviously, there is an evolution from barred spiral to rings. you can see some that are just starting, others that are nearing the first complete revolution and full rings.
I'm not trying to be argumentative, I'm just trying to understand and I can't picture the mechanism.
Thanks for your inputs - please keep trying