On freeze-out problem in relativistic hydrodynamics

Understand

A finite unbound system which is equilibrium in one reference frame is in general nonequilibrium in another frame.

• This is a consequence of the relative character of the time synchronization in the relativistic physics.
• This puzzle was a prime motivation of the Cooper--Frye approach to the freeze-out in relativistic hydrodynamics.
• Solution of the puzzle reveals that the Cooper--Frye recipe is far not a unique phenomenological method that meets requirements of energy-momentum conservation.