The first ever “diamond anvil cell,” displayed here in the NIST museum of Gaithersburg ©Gasper J Piermarini, National Institute of Standards and Technology

Normality is notoriously subjective, but from a cosmic perspective it looks nothing like our own experience. Most of the observable matter in our universe is either much more tenuous or dense than our surroundings, and much colder or hotter. This leaves us more parochial in our preconceptions than the most insular of Little Englanders (however difficult that might be to imagine).

The past several weeks have delivered a couple of stark reminders. One report in the journal Science presented us with an image (literally) of hydrogen not as the invisible, lightweight gas that held the Hindenburg aloft on its fateful transatlantic voyage in 1937, but as a shiny metallic solid. Another, in Nature Chemistry, shattered the reputation of helium (hydrogen’s safer replacement in buoyant balloons) as the most chemically inert element by showing it as a component of a chemical compound of sorts, in a union with sodium.

What both of these discoveries had in common was that they transformed chemical intuitions by subjecting the materials to extremely high pressures. Of course it’s all relative: the squeezing achieved by the researchers who made these things was feeble indeed compared to, say, the pressure at the centre of the sun, let alone the surreal compression inside neutron stars that squeezes atoms themselves out of discrete existence. All the same, making hydrogen dense enough to turn it into a solid metal pushes the capabilities of high-pressure science to its limits.

Physics and chemistry at tremendous pressures might…

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