If proven, lab-created metallic hydrogen could revolutionize physics.
In 1935, Princeton University physicists Eugene Wigner and Hillard Bell Huntington predicted that if hydrogen was subjected to high enough pressure, it would solidify. For decades, scientists in a few top facilities across the world have been attempting to do just that, bombarding hydrogen with lasers and electrical pulses and squeezing it between diamonds. They are motivated by more than just curiosity. If metallic hydrogen could be produced and kept stable at room temperature, it may be an extremely useful material. For starters, experts believe it would be a superconductor, allowing electrons to pass through it with no energy loss. When heated, it would unleash a massive quantity of energy, making it a potentially game-changing rocket propellant.
But is there a drawback to metallic hydrogen? Nobody has been able to produce it - at least, not until today. Harvard University researchers believe they have generated metallic hydrogen in the laboratory for the first time. Natural science professor Isaac Silvera and research fellow Ranga Dias describe compressing hydrogen at low temperatures between specially treated synthetic diamond anvils and observing it transition through stages — from transparent to black, and eventually to a substance that reflects light — in an article published in the journal Science. They conclude, "The properties are those of an atomic metal."
Corresponding author Silvera didn't answer to an email, but in a Harvard YouTube video, he says: "We have developed a new substance. It's a material that never has been on Earth before." Others also consider the discovery as monumental. "This study is likely to be one of the most important in physics for several decades since it answers a key outstanding problem experimentally," says Jeffrey M. McMahon, an assistant professor of physics at Washington State University who was not involved in the research, via email. "This is especially true if metallic hydrogen exhibits the extraordinary qualities anticipated," he says, adding that metallic hydrogen might become "possibly the most powerful rocket fuel known."
Silvera and Dias began by compressing liquid hydrogen — the element liquefies when cooled to minus 423 degrees Fahrenheit in an anvil composed of two synthetic diamonds — in an anvil built of two synthetic diamonds. Then they spun a steel screw, increasing the pressure on the liquid hydrogen. The hydrogen was transparent at nearly 2 million times the pressure of Earth's atmosphere. However, when the force was increased to 4 million atmospheres — more intense than the pressure inside Earth's core - the hydrogen became opaque and black. Tightening the screw even more, the hydrogen sample reflected almost 90% of the light that was shone on it.
The discovery by Harvard scientists comes more than 80 years after metallic hydrogen was originally conceived. "It has taken so long because the pressure at which it turns metallic is higher than Wigner and Huntingdon initially expected," University of Illinois professor David M. Ceperley, an expert in condensed matter physics, explains via email. "Diamonds become particularly sensitive to breaking at pressures around 300 atmospheres, especially when hydrogen is involved; many other experimentalists are shocked that Dias and Silvera were able to reach up to the pressures they report successfully."
Ceperley warns that more research is needed to validate the discovery. "Not even the Harvard gang has done it yet," he claims. "They need to be sure that what's in their cell is pure hydrogen and that the gasket material didn't dissolve and turn it metallic, and they need to detect more than just optical reflectivity." However, in a recent paper published in the magazine Nature, some scientists expressed skepticism about the discovery, stating that they needed further proof that metallic mercury had been generated.
Rivera told The New York Times that he intends to conduct additional measurements on the hydrogen using a technique known as Raman scattering, which employs laser light. He also intends to transport the sample to the United States government's Argonne National Laboratory in Illinois, where it will be examined using X-rays.
Ceperley is similarly skeptical that metallic hydrogen will become a game-changing rocket fuel. He doesn't believe it will remain stable after the pressure is released, as a diamond does. Instead, he expects it to return to a molecular state that is only stable at low pressure. "I believe the most likely application is to teach us how to construct better superconductors out of hydrogen-containing compounds," he says, "since pure metallic hydrogen by itself will be at too high a pressure to be helpful."