# Mean free time

Scattering is a random process. It is often modeled as a Poisson process, in which the probability of a collision in a small time interval ${\displaystyle dt}$ is ${\displaystyle dt/\tau }$. For a Poisson process like this, the average time since the last collision, the average time until the next collision, and the average time between collisions are all equal to ${\displaystyle \tau }$.[1]