Matter in bulk has both a microscopic and macroscopic description, with the latter going back to the very earliest days of thermodynamics. Of the common thermodynamic variablespressure (p) and temperature (T), it is temperature that has played by far the most prominent role in probing condensed matter and in our fundamental understanding of it. However, even as far back as 1660, Robert Boyle declared in his famous treatise commonly known as Touching the Spring of the Air that “perhaps the pressure of the air might have an interest in more phenomena than men have hitherto thought.” More than three centuries later, we can see how right he was, as systematic use of pressure has led to considerable insight into the properties of matter, especially its electronic properties. As A. Jayaraman noted when he was at AT&T Bell Laboratories, of all physical variables,pressure possesses one of the greatest ranges—over 60 orders of magnitude. At the high end, the pressures are those of the interiors of neutron stars; at the other, they gauge the conditions of the remotest vacua of outer space. And as Claude Berthelot demonstrated in early experiments on simple fluids, the pressure is not even obliged to be positive. (Negative pressure is created by, for example, pulling on the surface of a solid or on a wall of a sealed vessel full of fluid.)
Experimenters can now change the densities of condensed matter by upward of an entire order of magnitude, and thereby impart dramatic changes in physical and chemical properties of materials.