Hydrogen is most commonly found as a gas floating in our atmosphere, but it can take many forms. Many rocket engines, for instance, used ultra-cold liquid hydrogen as a propellant, and cooling the hydrogen even further than that will get it to freeze into a solid form. But some physicists believe that it's possible to get hydrogen to take another form, a metallic solid, by subjecting it to extremely high pressures. One team of physicists claims to have done this, but many experts are doubtful.
While it's relatively easy to get solid hydrogen by cooling it enough, what physicists are after is a solid form of hydrogen that displays the properties of a metal, like being shiny and conducting electricity. It is hypothesized that metallic hydrogen could be stable even at low pressures, as long as it's initially created at high pressure. If such a stable form exists, it could potentially be used as an ultra-dense rocket fuel or a superconductor.
While the existence of metallic hydrogen has never been proven, there may be some evidence of it. Some scientists believe that metallic hydrogen exists in the center of Jupiter due to the extremely high pressures there. The electrical properties of theoretically metallic hydrogen could explain Jupiter's extremely powerful magnetic fields.
However, creating Jupiter-level pressures in the laboratory is extremely difficult, which is why nobody has personally observed metallic hydrogen before. Pressures at this scale, nearly 72 million pounds per square inch, can only be achieved with a powerful diamond anvil. But at that pressure, even the ultrahard diamonds can still shatter if they're not perfectly smooth and virtually flawless.
But one team from Harvard claims they've overcome this hurdle thanks to a new polishing technique, and have used their apparatus to produce real metallic hydrogen. Their research was published in the journal Science.
The team subjected a tiny drop of liquid hydrogen to intense pressures from their diamond anvil. The hydrogen, which was initially clear, turned dark before becoming shiny at around 72 million psi. This, the team says, is a clear sign the hydrogen has turned metallic.
However, not everyone is convinced. Many other physicists are skeptical that the Harvard team managed to reach a pressure 20 percent higher than anyone else in the world on their first try. Others are concerned that the team didn't repeat its experiment before publishing.
Further, the only evidence the team has that the hydrogen is metallic is its shiny appearance. They never tested to see if it conducts electricity or had any other hallmarks of metallicity. And the team never established that it is the hydrogen itself that is metallic. It could also be the aluminum oxide that coats the diamonds. More tests are planned, but the researchers wanted to release what data they had before doing anything that might destroy the sample. According to Nature:
"We wanted to publish this breakthrough event on this sample," [Silvera] says. To preserve the material, he and Dias have kept it in the cryostat; the lab has only two cryostats, and the other is in use for other experiments, he says. "Now that the paper has been accepted, we're going to do further experiments."
Of all of these concerns, the lack of a second measurement is the most damning. Without some kind of confirmation of the experiment there's no way to know if this was some kind of fluke, no matter how rigorous the results might otherwise seem. Even once the sample is retested, there's room for doubt. Until these results are repeated, either by the Harvard tea—or preferably someone else—metallic hydrogen remains an exciting but unconfirmed possibility.