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17. HYDROGEN GAS Hydrogen was literally the first atom that we understood in the quantum mechanical sense, and in that sense maybe hydrogen gas is the "simplest" material of all. But what a cool moment it is when you first see the atomic lines of hydrogen gas for yourself.
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18. METALLIC HYDROGEN It has been predicted since the 1930s that, at high enough pressure, hydrogen gas could be compressed into a crystalline form. This crystal should be a very high-temperature superconductor, and finding it has been a huge goal for a long time.
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In 2017 a group at Harvard published a Science paper claiming to have created metallic hydrogen by achieving a pressure of ~500 GPa (5000x the pressure at the deepest point of the ocean). One of their key pieces of evidence was an iPhone photo of something becoming shiny:
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19. GOLD is so valuable largely because it is so chemically inert. This also makes it widely used in experiments when you want something that will just act as a metal without doing anything annoying But apparently gold also has topological surface states!
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20. SILICON is literally just sand, and people who studied silicon used to get made fun of by other physicsts for "studying dirt". But now modern life revolves around semiconductors.
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The big deal about silicon (and other semiconductors) is that they have allowed and disallowed bands of energy that can carry current. By controllably injecting electrons into those bands, you can control whether something acts like a metal or insulator, and hence digital logic.
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The mechanical properties of sand piles are generally mysterious. The pile is supported by long "force chains" with fractal structure. And if you make a histogram of the force F on each sand grain, there is a mysterious power-law vanishing of the probability when F gets small.
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I was intrigued about whether there was any solid-state physics angle to why iron ended up disrupting bronze on the civilizational stage besides just better availability and the fragility of the global bronze age tin trade
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