Mean free time

The mean free flight time or rush hour in a many-body system is the average time between collisions of a particle (eg atom, molecule, ion, or electron).

The mean free flight time is associated with the mean free path length through the medium speed:

The mean free flight time is particularly used in solid state physics, because you can not argue with the mean free path in considerations of the Fermi sphere in momentum space. These two concepts of free flight ends with a classic edge to another particle. Therefore, they are conceptually separate from the relaxation time, fall back in, for example, electrons from the conduction band higher electronically into an energetically lower state.

Generalizing such a shock processes are to be regarded as a kind of friction or damping. In many physical theories, therefore, a peak time as a parameter for inevitable losses of the mechanism is introduced, see for example the harmonic oscillator or the Drude theory.

Mean free flight time of electrons in solids

Mean free flight time, average speed, and the mean free path of conduction electrons of the Fermi - surface is calculated from the conductivity and the electron density in accordance with:


  • Electron mass
  • Elementary charge.


  • Statistical Physics