The Pursuit for Metallic Hydrogen, the “Holy Grail” of Physics
We all know hydrogen — it has an atomic number of 1 on the periodic table, it is the most abundant element in the universe, and it appears most commonly as a gas. Knowing these three facts may get you your marks on a science test, but the sheer potential of the seemingly simple element is rarely discussed. In fact, hydrogen’s technological potential is so great that it may revolutionize everything from space travel to the energy grid.
To begin, there is a myriad of uses for hydrogen gas — most of which you may already be familiar with. These uses can be found in things like electrical generators, in weather balloons, in the food and chemical industries, and in numerous other industrial processes and products. Once hydrogen is cooled to a very low temperature and turns into a liquid, it can then be effectively used as a rocket fuel. Since it yields the highest specific impulse — efficiency, basically — of any rocket fuel reacted with an oxidizer, it is light and burns with extreme intensity. Furthermore, its combustion does not produce any pollutants: the only by-product is water. Due to these “clean burning” properties, liquid hydrogen is often referred to as the fuel of the future.
Now, what could you accomplish if you exerted enough pressure on hydrogen to turn it into a metal? Scientists first theorized in 1935 that not only would metallic hydrogen conduct electricity, evidently, but it might do so without resistance. One prediction that is very important is that metallic hydrogen is meta-stable. This means that once pressure is released, it will stay metallic, similar to the way diamonds form graphite under intense heat and pressure, but remains a diamond when that pressure and heat are removed. This would make metallic hydrogen the only known room-temperature superconductor in existence.
Thus, the applications of the substance are immense — particularly for the electric grid, which suffers energy loss through heat dissipation. It could also facilitate magnetic levitation for futuristic high-speed trains; substantially improve the performance of electric cars; and revolutionize the way energy is produced and stored. Metallic hydrogen could enable rockets to go into orbit in a single stage and perhaps even allow humans to explore the outer planets. Since it requires a tremendous amount of energy to make metallic hydrogen, when all of it is released, it would be most powerful rocket propellant known to man.
It comes to no surprise that physicists have been in the pursuit to create metallic hydrogen for more than 80 years. In 2016, physicists Isaac Silvera and Ranga Dias announced that they had compressed a hydrogen sample between diamonds (called a diamond anvil cell) at such low temperatures and high pressure that it turned metallic. More specifically, hydrogen was squeezed at at 495 gigapascal, or more than 71.7 million pounds-per-square inch — greater than the pressure at the center of the Earth. In those extreme conditions, solid molecular hydrogen — which consists of molecules on the lattice sites of the solid — breaks down, and the tightly bound molecules dissociate to transform into atomic hydrogen, which is a metal.
“As the scientists cranked up the pressure, they observed transparent hydrogen turn black. Finally, at a pressure 5 million times our own air pressure, the hydrogen turned reflective. The researchers presented this as proof that the hydrogen atoms had arranged into a regular, three-dimensional structure like a metal,” as Gizmodo reports.
Although metallic hydrogen only exists in a diamond anvil cell at high pressure, it is still a very fundamental and transformative discovery. Several other groups of researchers have come close to turning hydrogen into a metal, continuously reaching for ever-higher pressures.
Credit: Philip Dalladay-Simpson and Eugene Gregoryanz
The quest continues, propelled by a handful of hydrogen-obsessed scientists. As participating physicist Gilbert Collins says,
“We all love hydrogen. It has the essence of being simple, so that we think we can calculate something and understand it, while at the same time it has such a devious nature that it’s perhaps the least understandable material there is.”
