Metallic hydrogen

Metallic hydrogen is a unique phase of hydrogen that acts as an electrical conductor. Under normal conditions, hydrogen is a gas made of diatomic molecules. These molecules consist of two protons with two electrons bound between them. However, when subjected to extreme pressure and temperature, hydrogen undergoes a massive change. It transitions from an insulator into a state where electrons can move freely. This phase is of immense interest to scientists studying the physics of high pressure.

The transition to a metallic state involves a fundamental change in atomic structure. As pressure increases, the discrete diatomic molecules are forced to break apart. Instead of separate pairs, a bulk phase forms. This phase consists of a solid lattice of protons. The electrons become delocalized, meaning they are no longer tied to a single pair of protons. They can flow throughout the entire structure, which allows electricity to pass through. This movement of electrons is what makes the substance a conductor.

Scientists believe this phase can exist in different forms. It can appear as a solid lattice or as a partial liquid. At pressures around 400 GPa, there may be a range of densities where hydrogen exists as a liquid metal. This could even happen at low temperatures. There is also a theoretical possibility of an elemental mesophase. This would be an intermediate state between a liquid and a solid. Some theories suggest this could even enter a supersolid state.

The concept of metallic hydrogen began with theoretical work in 1935. Physicists Eugene Wigner and Hillard Bell Huntington predicted this phase using theoretical grounds. Their initial prediction suggested a lower pressure than what is actually required. Modern calculations now point to much higher pressures, potentially around 450 GPa. Because it is so difficult to create, it is often called the "holy grail" of high-pressure physics. Many researchers have spent decades trying to replicate this state in a laboratory.

We can see the importance of this material by looking at giant planets. It is thought to exist in large quantities inside Jupiter and Saturn. The intense gravity of these planets compresses their interiors to extreme levels. This compression creates the liquid metallic hydrogen that may generate their powerful magnetic fields. This process might also occur in other exoplanets. In 1996, researchers at Lawrence Livermore National Laboratory reported a brief success. They used a light-gas gun to compress liquid hydrogen for about one microsecond. They observed the electronic energy band gap fall to almost zero.

Metallic hydrogen could also change how we travel through space. If a metastable form could be mass-produced, it might serve as a highly efficient rocket propellant. When the metallic hydrogen decompresses, it converts back into diatomic gas. This conversion releases a massive amount of energy. The theoretical specific impulse could reach up to 1700 seconds. For comparison, the most efficient current chemical propellants have a specific impulse of less than 500 seconds. However, the reaction heat exceeds 6000 K. This is too hot for most known engine materials to handle without dilution.

Beyond propulsion, the study of metallic hydrogen connects to advanced quantum physics. In 1968, Neil Ashcroft suggested that metallic hydrogen might be a superconductor. A superconductor is a material that can conduct electricity without any resistance. This hypothesis relies on a strong coupling between conduction electrons and lattice vibrations. Other scientists, like Egor Babaev, have predicted even more unusual states. He suggested the existence of superconducting superfluids or metallic superfluids. These would be novel types of quantum fluids that react uniquely to magnetic fields and rotation.

Researchers are also exploring ways to make this state easier to reach. In 2009, scientists predicted that alloying hydrogen with lithium could reduce the required pressure. They suggested that an alloy might be stable at only one quarter of the usual pressure. This idea was later supported by studies of other compounds like LaH10. These findings suggest that room-temperature superconductivity might be possible at much lower pressures. The quest to understand these high-pressure states continues through experiments with diamond anvil cells and pulsed lasers.