Hydrogen is the simplest and most common element in the universe. Stars and giants planets are made of it. It's also a very strange element with some puzzling properties. One particularly curious phenomenon happens to hydrogen inside these giant planets: under incredible densities, the gas turns into a metal.
Metallic hydrogen is a poorly understood phase of the element and difficult to study but a new paper published in Nature reports an important breakthrough. Researchers from the University of Cambridge, IBM Research and EPFL, used a machine learning technique combined with quantum mechanics to understand how this peculiar phase forms.
At the surface of the giant planets, hydrogen remains a molecular gas, but deep in their interiors, under extreme compression, the covalent bonds inside hydrogen molecules break and the gas becomes a metal that conducts electricity. The team believes that the transition is not sudden. Hydrogen under extreme pressure won’t suddenly change into its metallic form. The artificial intelligent (AI) algorithm employed suggests that the change from molecular to atomic hydrogen is gradual and smooth.
“The existence of metallic hydrogen was theorized a century ago, but what we haven’t known is how this process occurs, due to the difficulties in recreating the extreme pressure conditions of the interior of a giant planet in a laboratory setting, and the enormous complexities of predicting the behavior of large hydrogen systems,” lead author Dr Bingqing Cheng from Cambridge’s Cavendish Laboratory, said in a statement.
The smooth transition into the metallic phase is very important. Water boiling and turning into vapor is an example of a drastic, or first-order, change. It also has an exposed critical point. We know that at sea level pressure, water boils at 100°C (212°F).
It is commonly assumed that the transition of dense hydrogen is first-order, but by using machine learning to mimic the interaction between hydrogen atoms, they discovered this wasn't the case.
“We reached a surprising conclusion and found evidence for a continuous molecular to atomic transition in the dense hydrogen fluid, instead of a first-order one,” said Cheng.
A smooth transition suggests that a hidden critical point exists instead of an exposed one. This usually induces other unusual phenomena and could help explain what goes on inside gas giants.
The experimental creation of metallic hydrogen has been a controversial area of research where claims of discovery have been refuted. It is expected that this state can only happen at a pressure beyond 5 million atmospheres, if not more, a limiting condition for our experimental setups.
Simulations have not fared better. The quantum and statistical mechanics equation that underpins the behavior of moving hydrogen atoms under the extreme conditions of turning into metal was too much even for the most advanced supercomputer. That’s where machine learning comes in.
This approach allowed the team to cyclically refine the simulation getting a more and more accurate description of reality without going beyond computational capabilities.