Relaxation-time approximation
= Relaxation-time approximation
{title2=$I_{\mathrm{coll}}[f]\simeq-\delta f/\tau$}
Approximation in which a small departure from a specified equilibrium distribution relaxes exponentially with time constant $\tau$. Different observables can have different effective relaxation times; momentum relaxation is not automatically the same as every microscopic scattering lifetime.