Jerry Sellwood (Rutgers) gave a talk today at AMNH in which I learned a huge amount about spiral structure. He showed that disks have various kinds of linear and nonlinear instabilities to create spiral structure, and the structure tends to be transient. Nonetheless, a transient spiral pattern has an effective pattern speed and can drive resonant evolution at an inner and an outer resonance. These drive stars to eccentric orbits and plausibly cause some of the kinematic structure we see in the disk. After the talk, Sellwood, Bovy, and I talked about our differences and agreements in this literature. It turns out we don't really have differences at this point.
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