Picture a pot of boiling soup, but every grain in it is not just a point but a tiny top spinning around its own axis. That's the world of spin-1 particles. For example, some short-lived mesons born in accelerator collisions. When such particles are numerous and hot, their collective behavior can be described as the flow of a special fluid, with one important twist: each particle carries its own rotational momentum.

The authors built on a formalism devised by Wigner back in the 1930s. It allows describing quantum particles as probability clouds in phase space. For spin-1 particles (unlike familiar electrons with spin 1/2), this cloud is more complex. But the scientists managed to construct an equilibrium distribution function that accounts for polarization — the preferred direction of rotation. It turned out that, to second order in polarization, the energy-momentum tensor and spin current look exactly the same as for spin-1/2 fermions or particles obeying classical Boltzmann statistics.

This result matters not only for fundamental physics but also for practice. The equations governing this spin fluid belong to 'divergence-type theory.' This means signals in it do not travel faster than light, and the flow itself is stable: it doesn't break apart from the slightest perturbation. In the future, these equations could be used to model matter under extreme conditions, such as inside neutron stars or in the first moments after the Big Bang. Then the picture of boiling soup made of spinning particles will gain even sharper contours.