Dense metallic hydrogen has been theorized for over a century. The hydrogen acts like an electrical conductor, and it makes up the interiors of giant planets like Jupiter, Saturn, Uranus, and Neptune. While scientists knew it was there, they couldn’t explain how it changed. 

Now, researchers using a combination of artificial intelligence and quantum mechanics have been able to mimic the interactions of the hydrogen particles. They found that the hydrogen doesn’t have a sudden transition from molecular gas to metal but rather transitions smoothly and gradually as the pressure increases.

What’s fascinating to me about this story is that they tried to analyze the transition in an experimental fashion for years, using diamond anvils to compress the hydrogen atoms. Diamond, of course, is the hardest substance on Earth, but even it could not withstand the immense pressure required, and the anvils would fail. So much for a diamond is forever, y’all. Also, talk about an expensive experiment.

Then scientists tried theoretical calculations, and the computational power required to crunch the numbers with the relevant equations also turned out to be too much. None of our current supercomputers have the processing power. 

Enter in AI and quantum mechanics. The machine learning mimicked the interactions, overcoming all the limitations of trying to do the quantum mechanical calculations.

So why is the transition smooth and gradual instead of sudden as it is for most substances? To have a substance change its basic form, it has to have a critical point where those two forms – for example gas and liquid – exist simultaneously, which means on either side of the point, you have one form or the other. We’re familiar with the transition between liquid water and water vapor, and we know this happens at 100 degrees Celsius. It’s an “exposed” critical point and has a first-order phase transition.

As the press release explains: A common example of a first-order phase transition is boiling liquid water: once the liquid becomes a vapour, its appearance and behaviour completely change despite the fact that the temperature and the pressure remain the same.

However, hydrogen’s critical point is apparently hidden. Since there isn’t one specific combination of temperature and pressure that makes hydrogen undergo a transformation, it does so gradually.

More Information

University of Cambridge press release 

“Evidence for Supercritical Behaviour of High-Pressure Liquid Hydrogen,” Bingqing Cheng et al., 2020 Sep. 9, Nature