unwinding a spiral

A lot of conversations in the Dynamics group at Flatiron recently have been about spirals: Spirals in phase space, spirals in the disk, even spirals in the halo. In general, as a perturbed dynamical system (like a galaxy or a star cluster) evolves towards steady-state, it goes through a (or more than one) spiral phase. We've (collectively) had an interest in unwinding these spirals, to infer the initial conditions or meta-data about the events that caused the disequilibrium and spiral-winding. Jason Hunt (Flatiron) discussed these problems with Adrian Price-Whelan (Flatiron) and me today, showing some attempts to unwind (what I call) The Snail. That led to a long conversation about what would make a good “loss function” for unwinding. If something was unwinding well, how would we know? That led to some deep conversations.