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21 February 2020

Metallic Hydrogen, a High-Pressure Race to Bag the ‘Unicorn’ of Physics

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2020 has got underway with some scientific news that, if confirmed, could permit human technology to take a great leap towards the future. A group of French researchers say the have turned the simplest gas (hydrogen) into a metal, an achievement eagerly awaited by NASA’s rocket designers who plan trips to other worlds, by the theoretical astronomers who study the giant planets and by engineers waiting for the ultimate superconductor, which will bring us huge global energy savings. So numerous are the interests and so great is the pressure to succeed that in recent decades various teams have declared themselves winners of this hotly contested scientific race —although none have finally been credited for bagging this unicorn of physics: metallic hydrogen.

In light of this, this time round Nature has announced the achievement more cautiously than ever before. The title of the scientific article speaks of “evidence of the probable transition to metal hydrogen,” while in its informative note the magazine announces that “a milestone in the hunt for metallic hydrogen” has been achieved and then details that: “An optical study of cold solid hydrogen at extreme pressures indicates that electrons in the material are free to move like those in a metal. This suggests that the long-sought metallic phase of hydrogen might have been realized.” Each word is measured with the same precision as the announced experiment, and these results could lead to a Nobel Prize for one of the two teams leading this race, which started back in 1935.

It was then, more than eight decades ago, that Eugene Wigner and Hillard Bell Huntington became the first to realize that it was theoretically possible for hydrogen to become a metal, so it would behave like the rest of the elements in its group in the periodic table. After doing the calculations, they concluded that the only thing that would be necessary for solid hydrogen to exhibit metallic properties such as lithium, sodium or potassium, was to apply sufficient pressure. That prediction was not made by just anyone —Wigner would win the Nobel Prize in 1963, along with Maria Goeppert-Mayer, for his theories to understand the atomic nucleus —and if it “simply” needed high enough pressure, why has nobody achieved it yet and why have the attempts been so contentious?

Higher pressure than in the Earth’s inner core

To begin with, Wigner and Huntington fell quite short in their calculations of the very high pressure needed. In 1935 they estimated it would be at 25 gigapascals —the equivalent of 250,000 times the atmospheric pressure on the Earth’s surface— when hydrogen molecules would be so compressed that their atoms would be compacted in a solid lattice and the electrons would move freely throughout them (instead of each electron being attached to their own atom). This is the typical structure of a metal, and what makes them such good conductors of electricity. Today the consensus is that this frontier is in the vicinity of 400 gigapascals —that is, four million times the pressure on the Earth’s surface, and even higher than that in the inner core of our planet, where the pressure can reach 360 gigapascals.

Therefore, finding metallic hydrogen on Earth would be impossible. The eyes of scientists are directed at other worlds such as Jupiter, Saturn and some exoplanets, which are mostly composed of hydrogen gas. Different studies have indicated that in the nucleus of these gas giants, hydrogen could exist in its metallic form. Thus, producing metallic hydrogen in the laboratory would give astrophysicists a fundamental clue towards understanding how these huge planets form.

Diagram of Saturn, where metallic hydrogen is believed to exist in the core’s section marked in pink. Credit: Kelvinsong

However that would be just the most basic usefulness of metallic hydrogen. Throughout the more than 80 years since its existence was predicted, its hypothetical properties have continued to be studied. The most promising scientific belief at the moment is that it would be a superconductor at room temperature, turning one of the great ambitions of materials science into a reality. The most common gas in the universe could also become the ultimate metal.

A game-changing superconductor and fuel

As if this were not enough, being able to store hydrogen in a solid form (in much less space) would make it an even more useful alternative fuel and, according to NASA, “a game-changing rocket propellant” necessary to explore our solar system. Tremendously light and capable of storing a huge amount of energy, metallic hydrogen would be the fuel of a new space age… besides being the dream superconductor, and the key to great advances in astrophysics.

All these fantastic properties, that could revolutionize science and technology, make it nothing less than a unicorn. And so far it has eluded scientists who hunt for it as if it was a mythological animal. The great challenge has been to achieve the required super high pressure: the first time that human technology managed to approach 400 gigapascals was in 1998, when hydrogen atoms were compressed between the tips of two extremely sharpened diamonds. Since then, that has been the main method used by the two great rivals in this scientific race: the team led by Paul Loubeyre in the laboratories of the Commission for Atomic Energy (CEA) of France, and the one started by Ranga Dias and Isaac Silvera at Harvard University in Massachusetts.
To the difficulties in applying these otherworldly pressures to a tiny sample of hydrogen is added the technical challenge of demonstrating that once that high pressure has been achieved, the hydrogen has actually been transformed into metal. The great advance of the recent study that Loubeyre published in Nature is that, for the first time, a research group seems to have provided evidence that its experimental hydrogen is metallic (after applying 425 gigapascals of pressure). However, we will still have to wait for other researchers to be able to replicate their results in order to accept that metallic hydrogen is real and not a mythological creature of science. The race appeared to have been won in 2017, when Dias and Silvera made a similar announcement in the journal Science, but weeks later they lost their microscopic sample of metallic hydrogen (created at 495 gigapascals) while attempting to do further analysis. The competition between the two teams has ended up overturning all the previous announcements and, at the same time, encourages them to present the most convincing evidence possible that they have finally achieved the impossible.

Francisco Doménech

@fucolin

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