On October 5th 2016, Ranga Dias and Isaac F. Silvera of Lyman Laboratory of Physics, Harvard University released the first experimental evidence that solid metallic hydrogen has been synthesized in the laboratory.
It took 495 GPa pressure to create. The sample is being held in the cryostat in liquid nitrogen.
If as predicted by theory the metallic hydrogen remains metastable when the extreme pressure is removed then the world will eventually be greatly changed.
Even if the metallic hydrogen had to be kept at liquid nitrogen temperatures, it could still be handled and used for the expected amazing properties.
Liquid (molecular) hydrogen-oxygen used in modern rockets has an Isp of ~460s; metallic hydrogen has a theoretical Isp of 1700 s! Detailed analysis shows that such a fuel would allow single-stage rockets to enter into orbit or carry economical payloads to the moon.
Researchers have ideas on how electron injection could reduce the pressure needed to initially create the metallic hydrogen.
Then the engineering challenge will be creating enough of the metallic hydrogen for applications.
But the key is having it exist after the pressure is taken off.
If as predicted by theory the metallic hydrogen is a room temperature superconductor [and the pressure was not needed] then even relatively small amounts [kilograms per week] of the metallic hydrogen would have very useful applications.
Picture of the tiny amount of metallic hydrogen created in the lab
Superconductors can make engines smaller and more powerful. Room temperature superconductors would be easier to handle than superconductors that need to be cooled down to liquid nitrogen temperatures. Ceramic superconductors have had a lot of problems for applications because of brittleness and other issues.
Room temperature metal superconductors with very high critical current could make superstrong magnets. Powerful magnets could be used to solve challenges getting to commercial energy generation from nuclear fusion.
HyperMach Aerospace is about halfway through a two-year program to validate crucial technologies [including superconducting parts] for a mach 5 supersonic commercial passenger plane. Robust metallic room temperature superconductors could make this area of superconductor application more successful.