In our semi-regular Dynamics Meeting at Flatiron, Gus Beane (Flatiron) showed some simulations in a realistic potential that suggest that the actions we compute for the Milky Way might not be even close to being invariant. That caused a big fight to break out! Some were arguing that the actions shouldn't be called actions but rather “orbit labels”. Some were arguing that the effects he is seeing are exactly what you expect. Some argued that the variations he was seeing are way too large and there must be a bug! And some were arguing that it might be problems as fundamental as the reference frame: If you have the axes or origin slightly wrong, you compute everything wrong! But all these things are possibly happening in our analyses of the Milky Way, so Beane stepped on a very important issue for everything that is going on now in Milky Way dynamics with Gaia. As my loyal reader knows, I don't like action space: There aren't necessarily actions at all, and anything you compute using them might be extremely misleading, especially if you implicitly assume that they are invariant, or close.
In another conversation, Katie Breivik (CITA), Adrian Price-Whelan (Princeton), and I discussed a possible Decadal Survey science white paper about binary stars, population synthesis, and interdisciplinarity within astrophysics. That's a good project, but it requires a group effort from a large community; how to organize that?
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