Though it has the potential to transform life on Earth, metallic hydrogen could also play a key role in helping humans explore the far reaches of space, as the most powerful rocket propellant yet discovered, researchers said.
"It takes a tremendous amount of energy to make metallic hydrogen," Silvera said.
"And if you convert it back to molecular hydrogen, all that energy is released, so it would make it the most powerful rocket propellant known to man, and could revolutionise rocketry," he said.
The most powerful fuels in use today are characterised by a "specific impulse" - a measure, in seconds, of how fast a propellant is fired from the back of a rocket - of 450 seconds. The specific impulse for metallic hydrogen, by comparison, is theorised to be 1,700 seconds.
"That would easily allow you to explore the outer planets. We would be able to put rockets into orbit with only one stage, versus two, and could send up larger payloads, so it could be very important," Silvera said.
To create the new material, Silvera and Dias turned to one of the hardest materials on Earth - diamond.
Rather than natural diamond, Silvera and Dias used two small pieces of carefully polished synthetic diamond which were then treated to make them even tougher and then mounted opposite each other in a device known as a diamond anvil cell.
"Diamonds are polished with diamond powder, and that can gouge out carbon from the surface. When we looked at the diamond using atomic force microscopy, we found defects, which could cause it to weaken and break," Silvera said.
The solution, he said, was to use a reactive ion etching process to shave a tiny layer - just five microns thick, or about one-tenth of a human hair - from the diamond's surface.
The diamonds were then coated with a thin layer of alumina to prevent the hydrogen from diffusing into their crystal structure and embrittling them.
The research was published in the journal Science.
