The Discontinuous Shock—Fact or Fancy?

Abstract

There is scientific interest in the use of shock waves to generate material conditions that are extreme states of matter. In the strongest shock waves commonly generated in the laboratory, pressures of hundreds of gigapascals and corresponding temperatures of an electron volt or two may be reached. In porous materials pressures are usually lower but the temperatures can be significantly higher. In some cases it has been argued, on the basis of empirical evidence and induction, that certain processes measured in shocked systems would be most easily explained if the shocks were essentially discontinuous changes in the state of the material, i.e., mathematical and physical discontinuities. Later in this section a practical definition of a “physical” discontinuity is provided. Clearly, in a material made up of atoms, one must pick a scale that is satisfactory to the notion of “physically” discontinuous for the problem at hand. It is easy to see that there is a huge difference in striking a diatomic molecule impulsively on one atom in a direction along the axis connecting the atoms and in pushing on the same atom in the same direction gently over a longer period of time to get the molecule to the same total center of mass energy. This conceptual difference lies at the heart of the interest in the structure of shock waves. Are shocks catastrophic (impulsive on the scale of atoms or molecules) or not? If shocks can be catastrophic, how does it happen, how is the structure maintained, and what is the dissipative mechanism if there is one? Finally, is the state at the end of the shock process actually an equilibrium state or does one simply hope that it is?