Physicists make metallic hydrogen in the lab for the first time


It's the most abundant element in the universe, making up around 74% of the Milky Way galaxy's mass. But squeeze hydrogen under immense pressures and it starts to look a little different. Vishnu Varma reports.



It’s been 80 years in the making.
US physicists have successfully created a metallic form of hydrogen in the lab – but it took pressures more than 20 times higher than predicted.

Ranga Dias and Isaac Silvera from Harvard University squeezed a sample of hydrogen at extraordinary pressures and at temperatures close to absolute zero. In a paper published in Science, they found that at these conditions, hydrogen takes on a metallic sheen.

You’re probably most familiar with hydrogen as a gas – or maybe liquid, if you're into rockets. But the search for metallic hydrogen, which conducts heat and electricity, began in 1935. It was then that theoretical physicists Eugene Wigner and Hillard Huntington proposed the idea that at pressures around 25 gigapascals, hydrogen would become metallic.

And it’s since been predicted that liquid metallic hydrogen might make up a large part of gas giants such as Jupiter and Saturn.

To try to make this mysterious form of hydrogen, Dias and Silvera knew they had to squash hydrogen under incredible pressures. They did this using what’s known as a diamond anvil cell – where two diamonds sandwich a very small sample – capable of applying pressures of up to a few hundred gigapascals.

And at pressures between 465 and 495 gigapascals – much higher than predicted by Wigner and Huntington – the hydrogen started to turn black and eventually reflective.

Image of diamond anvils compressing molecular hydrogen. At high pressures, the sample converts to metallic hydrogen, as shown on the right.
R. Dias and I.F. Silvera

While the researchers think that the metallic hydrogen created is solid, there is no evidence yet to show if it is a solid or a liquid.

Dias and Silvera believe that solid metallic hydrogen may be “metastable at room temperature when the pressure is released”. This means that after solid metallic hydrogen is created at extreme conditions, the hydrogen could remain solid after the pressure and temperatures are returned to normal.

Some theoretical physicists proposed that metallic hydrogen could be a high-temperature superconductor – a material that allows electricity to flow with no energy loss.

And while physicists are yet to uncover just how stable metallic hydrogen is at higher temperatures, Dias and Silvera think that metallic hydrogen could be “important for solving energy problems and can potentially revolutionise rocketry as a powerful propellant”.

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Vishnu Varma R Vejayan is a physics student from Queen Mary University of London with an interest in scientific writing and research in physics. He interned at Cosmos in early 2017.