Dynamic high pressure: Why it makes metallic fluid hydrogen

•In 1935 Wigner and Huntington predicted solid H2 would transition to a metal.•Predicted conditions were a pressure above 25 GPa and “very low temperatures”.•In 1996 degenerate fluid H was observed under dynamic compression (DC) at 140 GPa.•Static compression has yet to make metallic solid H at pressures up ~400 GPa.•We discuss why DC makes metallic fluid H, while static compression has yet to do so.

Metallic fluid H has been made by dynamic compression decades after Wigner and Huntington (WH) predicted its existence in 1935. The density at which it was made is within a few percent of the density predicted by WH. Metallic fluid H was achieved by multiple-shock compression of liquid H2, which is quasi-isentropic and thermally equilibrated. That is, the compressions were isentropic but for enough temperature and entropy to drive the crossover to completion from H2 to H at 9-fold compression. The metallic fluid is highly degenerate: T/TF≈0.014. The basic ideas of dynamic compression, also known as supersonic, adiabatic, nonlinear hydrodynamics, were developed in the last half of the Nineteenth Century in European universities. Today dynamic compression is generally unfamiliar to the scientific community, which impedes general understanding as to why fluid H becomes metallic at a pressure observable in a laboratory. The purposes of this paper are to (i) present a brief review of dynamic compression and its affects on materials, (ii) review considerations that led to the sample holder designed specifically to make metallic fluid H, and (iii) present a brief inter-comparison of dynamic and static methods to achieve high pressure relative to their prospects for making metallic H.