All That Is Shiny May Not Be Metallic Hydrogen

Scientists aren't sure if an intriguing feat performed at Harvard University – turning hydrogen into a metal – is legitimate because they suspect some inaccurate assumptions may have crept in.

If one university press release is to be believed, there’s more than one Holy Grail of physics, and the latest is supposedly a form of hydrogen that is a metal. The release is from Harvard University, where two physicists have claimed that they managed to create metallic hydrogen in their lab over eight decades after its existence was predicted. Their technique isn’t new – they simply improved on an existing method well – nor is their claim the first in history. Nonetheless, there is some doubt that metallic hydrogen is finally among us.

Metals are distinguished from other states of matter by many properties. The most apparent are what you were taught in high school: they are lustrous, ductile, sonorous, malleable, dense, etc. Isaac Silvera and Ranga Dias, two scientists at Harvard, cooled some hydrogen gas to 15 kelvin, loaded it into a small gasket of rhenium and then used two diamond-tipped screws to begin pressurising it. At 335 gigapascal (GPa) – about the detonation pressure of one of the most powerful explosives known and some three-million-times the atmospheric pressure we experience at sea level – the gas turned black, a sign that its atoms were getting tightly packed and were absorbing light. At 495 GPa, it turned lustrous – a sign of a metal.

The experimental setup. Credit: http://science.sciencemag.org/content/early/2017/01/25/science.aal1579

The experimental setup. Credit: R. Dias and I.F. Silvera

According to their paper, Silvera and Dias then measured the properties of hydrogen, rhenium and diamond using various techniques. One required a ‘high-resolution long working-distance microscope’. It allowed the duo directly image what was happening inside the gasket. They also used low-power lasers to measure the pressure, temperature and reflectivity measurements without disturbing the delicately setup system. Apart from the reflectivity, or shininess, at different wavelengths, other properties that hinted at the creation of metallic hydrogen were electron density (about one electron per atom) and physical density (15-times higher than gaseous hydrogen).

However, the reflectivity measurement has been presented as the clincher, and that is where the problem lies. There is only one measurement. Silvera and Dias did not repeat their experiment because they did not have the space, nor the resources, for it. According to other researchers, they have also not provided conclusive evidence that what they measured the shininess of was indeed hydrogen. The complaints are fair. In their experiment, Silvera and Dias coated the diamond at the end of the screw they were turning with aluminium oxide, a compound renowned for its hardness, to ensure that the hydrogen didn’t diffuse into the diamond at high pressures. And at 495 GPa, it is unknown if aluminium oxide also turns into a metal.

The pressure-load relation. Credit: http://science.sciencemag.org/content/early/2017/01/25/science.aal1579

The pressure-load relation. Credit: R. Dias and I.F. Silvera

Additionally, they were able to directly measure the pressure within the gasket depending on how much they turned the screw until about 335 GPa. On the flipside, the pressures beyond – all the way up to 495 GPa – were inferred by studying vibrations within the diamond crystal and then comparing their results to an extrapolated plot (see above). Some scientists, especially those who have been attempting to create metallic hydrogen themselves, think the extrapolation may have led to inaccuracies much before the 495 GPa mark, with the effect that turning the diamond screw may not at all have led to an increase in pressure. Together with the uncertainty about aluminium oxide’s behaviour, some speculate Silvera and Dias may have measured the reflectivity of the compound instead of any metallic hydrogen’s.

Until these doubts have been dispelled, the metallic hydrogen claim will be a difficult sell. The Harvard physicists have said they were more excited about getting their results out before conducting further tests (that they have since promised are coming), hinting at the presence of a closely fought race. After all, if the substance is indeed proven to exist, validating what Eugene Wigner and Hillard B. Huntington first hypothesised in 1935, then there would really be a wonder among us. Once the applied pressure is removed, metallic hydrogen is believed to be metastable at room temperature: able to withstand only mild shocks, dissipating at the first sign of something stronger. But if engineers find a way to reinforce it while allowing it to maintain its properties, scientists think it could be used to make high-power rocket fuels and high-temperature superconductors.