Chemistry

Metallic Hydrogen Is The Holy Grail Of High-Pressure Physics, And One Team Says They've Made It

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For more than 80 years, physicists have dreamed of the ability to produce metallic hydrogen. In 2016, one team claimed to have finally done it. To understand why science wants metallic hydrogen so badly—and why the 2016 announcement caused so much drama—you have to first understand the potential of this elusive material.

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To Infinity And Beyond

We all know hydrogen: it's the first element on the periodic table, and the most abundant in the universe. It appears most commonly as a gas. If you cool it to very low temperatures, as rocket scientists are wont to do, it becomes a liquid. Liquid hydrogen makes great rocket fuel because it's light and it burns with extreme intensity. Specifically, when you combine it with something like liquid oxygen, liquid hydrogen yields the highest specific impulse—efficiency, basically—of any rocket fuel.

You can imagine, then, what you could accomplish if you squeezed hydrogen with enough pressure to turn it into a metal. High-pressure researchers first predicted this was possible in 1935. They theorized that not only would metallic hydrogen conduct electricity (that's what metals do, after all) but it might do it without resistance, which would mean it was a superconductor. What's more, it could do that at room temperature—no supercooling necessary for this supermaterial! But that's not all it could help accomplish, as Nature's Davide Castelvecchi points out: "By making metallic hydrogen, physicists might also be able to explore planetary science at their lab bench: gas-giant planets such as Jupiter are theorized to have metallic hydrogen in their cores, which would perhaps explain how they can sustain a magnetic field."

More Like Metallic Hy-Drama

In October of 2016, two physicists announced that they had actually squeezed hydrogen between diamonds at such low temperatures and high pressure that it turned metallic. As Gizmodo reports, "As the scientists cranked up the pressure, they observed transparent hydrogen turn black. Finally, at a pressure 5 million times our own air pressure, the hydrogen turned reflective. The researchers presented this as proof that the hydrogen atoms had arranged into a regular, 3D structure like a metal." Other physicists did not take this lying down. "I don't think the paper is convincing at all," French physicist Paul Loubeyre told Nature. "We express a doubt that [the physicists] were even in a close vicinity of the claimed pressure," wrote Carnegie Institute staff scientist Alexander Goncharov in a response paper.

Why are other scientists so skeptical? There are a few reasons. For one thing, there's little evidence that the material was even hydrogen in the first place; it could have been the aluminum oxide that coats the tips of the diamonds themselves. The researchers also took just a single measurement of the sample at its very highest pressure, which makes it hard to see how the pressure changed as the hydrogen turned metallic. Worst of all, further testing led them to lose the sample. "Basically, it's disappeared," team leader Isaac F. Silvera told ScienceAlert. "It's either someplace at room pressure, very small, or it just turned back into a gas. We don't know." And you thought losing your keys was rough.

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