The fluid dynamics of spin

Michael Baer is known for his many achievements in the field of non-adiabatic quantum dynamics and the theory of reaction dynamics. Among his main tools of inquiry is the study of two-state quantum systems. I was fortunate enough to collaborate with him on such models in the first part of the previous decade and the last years of the previous century. Here, I return to the original two-state system, that is Pauli's electron with a spin and show how this system can be interpreted as a vortical fluid. The similarities and difference between spin flows and classical ideal flows are elucidated.