HYDROGEN is the simplest and most abundant of elements. Composed of one proton and one electron, it makes up 90% of our universe (by number of atoms). On Earth, hydrogen is commonly found as a diatomic molecular gas. But on Jupiter, where interior pressure is millions of times greater than that at our planet’s surface, the hydrogen molecule is theorized to exist as a superhot liquid metal. It’s under so much pressure that its protons and electrons are extremely closely confined, close to the limits imposed by the Pauli exclusion principle. Any closer, and two electrons would be forced into the same quantum state, which is forbidden. In this dense, high-pressure mix, the electrons of hydrogen atoms and their neighbors are so close together that it’s no longer clear which electron belongs to which proton. As well, electrons, having significantly less mass than protons, have higher average velocities. Momentum is the product of mass and velocity, so protons with the same momentum as electrons would move much slower. The electrons thus are free to move independently of their protons, and this hydrogen soup becomes a metal — a conductor of electricity, and a very good one at that! This also helps justify its place at the top of the alkali metal column in the periodic table.
The theory that hydrogen turns metallic under extreme pressure was first advanced in 1935 by Eugene Wigner, who would go on to win a 1963 Nobel Prize in physics for his work in quantum mechanics. Finding experimental evidence of Wigner’s hydrogen metallization theory, however, has proven to be extremely difficult for the scientific community. While studies of the universe’s lightest material led to discovery of hydrogen’s solid and liquid phases, metallic hydrogen remained out of reach–until recently.
At Lawrence Livermore National Laboratory, in a series of shock compression experiments funded by Laboratory Directed Research and Development grants, we successfully ended a 60-year search for hard evidence of metallic hydrogen and the precise pressure at which metallization occurs at a particular temperature. The success in metallizing hydrogen would not have been achieved without the shock-wave technology built up over more than two decades to support Lawrence Livermore’s nuclear weapons program. It represents the integration of the Laboratory’s broad capabilities and expertise in gas-gun technology, shock physics, target diagnostics, hydrodynamic computational simulations, cryogenics, and hydrogen and condensed-matter physics.
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