Note on Becker's Theory of the Shock Front

Becker, in a classical paper, concludes that weak shocks are many free paths thick and may be treated by ordinary hydrodynamics including the effects of viscosity and thermal conductivity of the medium; that moderately strong shocks have a thickness of the order of a free path and must be treated by a direct attack on the relevant Boltzmann equation; while violent shocks have a thickness small compared with a free path and even the Boltzmann equation is no longer applicable. It is shown here that this last conclusion rests on an oversight; the thickness of a shock front is always at least of the order of magnitude of a free path, and it is to be expected that the Boltzmann equation can be applied even for the most violent shocks.