Hydrogen is the most abundant element in the universe — and we know it mainly as a gas, not a metal. But in 1935, the physicists Eugene Wigner and Hillard Bell Huntington theorized that under high enough pressures, hydrogen could actually become metallic.
Since then, scientists have tried all sorts of techniques to create metallic hydrogen. Now, reporting in the journal Science, researchers at Harvard University say they’ve squeezed hydrogen between two diamonds — and made metal happen.
Isaac Silvera, a physicist and co-author of the study, says the metallic hydrogen could be a groundbreaking superconductor, useful in everything from our electrical grid to hospital MRI machines. But first, the researchers’ material has to pass a few more tests.
Silvera explains that to make their sample — which, at a micron thick and less than a human hair wide, is not enough to pass around — they cooled hydrogen gas down to just below -430 degrees Fahrenheit. As it chilled, the hydrogen became a liquid, then a solid. Then, the researchers began compressing it between special diamond anvils.
“When you start out at very low pressure, the two atoms in the molecule are very close together,” Silvera says. “And the molecules themselves are very, very far apart.” But he explains that as you continue pressing, the molecules get closer together. “And finally, they get so close that a proton in one molecule can't decide ‘should I stay bonded to this other one, or should I bond with my neighbor?’”
Silvera and his co-author, postdoctoral fellow Ranga Dias, squeezed their sample to just under 5 million atmospheres of pressure — greater than the pressure at the center of the Earth. “What happens is when you get them really close, the molecules dissociate and you form an atomic solid,” Silvera explains. “And now the electrons are free to run through the system. They no longer hold the molecules together, and it's a metal.”
They tested the sample’s reflective properties, and according to Silvera, it passed with flying colors: “It's not enough to measure reflection at one wavelength or one color of light. But we measured from the green, blue, red and into the infrared. And we find exactly the behavior that you would expect for a metal.”
But the study has drawn criticism from skeptics who want to see more evidence that the shiny sample is, in fact, metallic hydrogen — and not contaminated, for example, by the aluminum oxide that coats the diamond anvils. Silvera says he and Dias plan to test the material further, but he’s confident about its veracity.
“You know, it's sufficient to show reflection,” he adds. “Reflection comes from high-frequency electromagnetic waves. And light is an electromagnetic wave, and that makes electrons respond. They move back and forth, and they reflect the light. So, we're actually measuring the very high-frequency electrical conductivity.”
Right now, the speck-sized sample remains under high pressure in its diamond vise. But a number of beguiling predictions have been made about metallic hydrogen’s properties, and Rivera is excited to eventually test them. For one, it’s thought that metallic hydrogen could be metastable.
“That means that if you get it up to the high pressure, it becomes a metal. And [when] you release the pressure, it will stay in the form of a metal,” Silvera explains. “And if that happens, that would be terrific.” Metastability is naturally occurring — he points out that a diamond is a metastable form of carbon: “You take the pressure and the temperature off and you still have diamond.”
Silvera says it’s also been predicted that metallic hydrogen is a superconductor at room temperatures, meaning that it conducts electricity without dissipation. If so, the material could revolutionize our electrical infrastructure.
“Imagine that it's metastable, you take the pressure off and it stays in the metallic phase,” he says. “If you could now develop a technique to scale it up, that is, make lots of it — and I've got ideas on how to do that — then you could make electrical wires out of it.”
Silvera estimates that in our current electrical power grid, about 15 percent of energy is lost to dissipation in the copper wires. “But if they were superconducting wires, you would have no loss, no dissipation in the system. So, that would be a great advantage.”
What’s more, today’s commercial superconductors only work at very cold temperatures. That means superconducting magnets, like the ones used in hospital MRI machines, have to be cooled with liquid helium, Silvera says. Metallic hydrogen superconductors could change that. “You could have these magnets any place in the world — places where they didn't have helium,” he adds.
To test superconductivity, Silvera says they’ll have to make another sample, carefully inserting electrical leads into the hydrogen before it’s pressurized. But determining whether metallic hydrogen is metastable is an easier — if no less nerve-wracking — test. Sooner or later, Silvera and Dias will unscrew their sample from the vise.
They hope to find that it remains a solid metal, even when it’s no longer under high pressure.
“We'll do it together, and perhaps pray a little and maybe have something to settle our hands — a little drink to settle our hands as we turn it down,” Silvera says.
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