Internal waves and synchronized precession in a cold vapor

Exchange in a Boltzmann gas of bosons with several internal states leads to collective transport of internal polarization. The internal dynamics can be understood as Larmor precession in the presence of a torque induced by atoms on each other via exchange coupling. A generalized Bloch equation that includes interatomic exchange effects as well as orbital motion in the gas is derived and used to interpret recent experiment by Lewandowski et al. as an excitation of a collective wave of internal state polarization. It is shown that exchange leads to formation of domains in which precession frequencies are synchronized.