Numerous physicists over the years have claimed they can turn hydrogen into a metal by squeezing it extremely hard – but none so far have managed to persuade sceptical rivals. Now researchers in France reckon they have finally found convincing evidence for the transformation, having built new devices for pressurizing and observing tiny samples of hydrogen. Yet others in the field remain doubtful, arguing that infrared data reported by the French group does not on its own constitute adequate proof – and that what is needed are measurements of conductivity.

Experts have little doubt that hydrogen should become a metal when subject to very high pressures. Theory tells us that pressure frees electrons from the confines of individual atoms or molecules, allowing them to freely propagate through the material. Indeed, many insulators have been observed making the transition – molecular oxygen, for example, was shown about 20 years ago to become a metal at around one million times atmospheric pressure (about 100 GPa). “Indisputably, metal hydrogen should exist,” write Paul Loubeyre, Florent Occelli and Paul Dumas of the French energy agency CEA in a paper recently uploaded to arXiv.

Metallic hydrogen could have many striking properties including being a superconductor at room temperature. Studying its behaviour could also lead to new insights about conditions inside Jupiter and other gas-giant planets, given its predicted abundance there.

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