Astrophysics (index)

Mean Free Path

(average length that a particle travels before interaction)

The term Mean Free Path is used in the physics of moving particles (atoms, electrons, photons, etc.) to indicate the average length they travel between interactions. When performing calculations in a substance where some type of particle moves around, estimates of the consequences can be calculated by assuming the particle always moves the same distance between interactions, using the mean free path as that distance. The mean free path can be determined from density, and the radius at which the particles interact.

Some situations where it is used:

  • Distance a molecule travels in a gas before encountering another molecule, say, some specific gas at some specific Temperature and pressure.
  • Distance a photon travels in space until Absorption or scattering, e.g., before and after Recombination. Optical Depth is distance divided by the mean free path (in the case where the latter is constant over the distance).
  • More specifically, the distance a photon travels within a star before encountering an atom or electron (Radiative Transfer).
  • Rate of nuclear reactions in the central region of a star: one factor in Fusion rate is how often nuclei hit each other.


Referenced by:
Cross Section
Optical Depth (τ)