Metallic Hydrogen: 80 years in the making

Ashley Carley

Rocket fuel, lightning-fast supercomputers and levitating trains are just three uses of the newly discovered metallic hydrogen – if, the Harvard scientists say, everything goes to plan.

Hydrogen is the lightest and most abundant of all the elements. It forms two thirds of every drop of water, and almost 75% of the gas in the Sun’s core. Alone, hydrogen is most often found floating around in its gaseous phase, but it has been predicted a metallic form may exist when exposed to intense pressure.

Two physicists at Harvard University claim to have isolated this incredibly rare form for the first time, in a paper published this week. By squeezing solid hydrogen between two diamonds at temperatures well below freezing, the researchers created pressures larger than those found at the centre of the Earth. In these conditions, the hydrogen atoms began to share their electrons. Using this new electron cloud, they could conduct electricity.

Isaac Silvera, who made the discovery alongside his colleague Ranga Dias, recognises the importance of his achievement, calling it the “holy grail of high-pressure physics.”

This breakthrough has been a long time coming; it has been over 80 years since Eugene Wignar and Hillard Bell Huntington made the first predictions about metallic hydrogen. Since then the goalposts have continually shifted. Estimates of the pressure required to make the substance have been continually revised upwards, from 25 gigapascals (GPa), 250,000 times above atmospheric pressure, in 1935, to the most recent estimate of 400-500 GPa.

Each time the prediction changed, it moved out of the range scientists were capable of recreating in a lab environment, making it somewhat of a carrot on a stick for researchers in the field. Jeffrey McMahon, theoretical physicist at Washington State University, told New Scientist that if the results were reproducible, the recent experiments had solved “one of the major outstanding problems in all of physics.”

It wasn’t easy – the synthetic diamonds had to be flattened, polished and heated to remove any imperfections that could result in cracking. They were then covered in alumina, an extremely hard material made from aluminium and oxygen that hydrogen could not leak through. The two diamonds were then crushed together with great force, and Dr Dias watched as the hydrogen between them turned from clear to black, until it began to shine. The force required was 495 GPa – higher than the pressure at the Earth’s core. Dr Dias then called Professor Silvera, and they took the measurements that would confirm their discovery.

The next step is to see if it retains its structure when compression is relaxed. Some predictions suggest it will be too unstable to survive at room temperature, and will gradually decay, although others have more hope. Graphite forms diamonds under high pressures and temperatures, but when the sources of compression and heat are taken away – the diamond remains. Scientists are hoping metallic hydrogen could act the same way once released from its diamond vice.

If it does, its potential applications are exciting. If the amount of energy used to create the metallic hydrogen can be released by breaking it down again, it could become the most powerful rocket fuel ever made. “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,” Professor Silvera says. Electronic systems would also be revolutionised, as “superconductors” could be made which reduce energy wastage in wires.

When Professor Silvera is asked what thinks will happen next, he responds “I don’t want to guess, I want to do the experiment.” After an 80-year wait, perhaps the suspense is great enough.

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