Everything you always wanted to know about metallic hydrogen but were afraid to ask

Abstract

The hydrogen molecule is made from the first and lightest element in the periodic table. When hydrogen gas is either compressed or cooled, it forms the simplest molecular solid. This solid exhibits many interesting and fundamental physical phenomena. It is believed that if the density of the solid is increased by compressing it to very high pressures, hydrogen will transform into the lightest known metal with very unusual and fascinating properties, such as room temperature superconductivity and/or superfluidity. In this article, we provide a critical look at the numerous claims of hydrogen metallization and the current experimental state of affairs.