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Title: At what pressure does solid hydrogen become a metal
Plain: Theory predicted in 1935 that squeezed hydrogen becomes a metal; experiments near 4 to 5 million atmospheres claim it, but the evidence is disputed.
Precise: At $T < 100\,\mathrm{K}$, determine the pressure at which solid hydrogen becomes metallic and whether the transition goes from insulating molecular phase III to a metallic molecular phase or to atomic hydrogen. An answer combines infrared reflectivity showing a Drude response, dc electrical transport, and structure determination above $\sim 4\,\mathrm{Mbar}$.
Settled by: Reproducible dc transport with metallic temperature dependence plus diffraction or Raman identification of the phase, at a stated calibrated pressure.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: contested
Status note: Synchrotron infrared data (2020) showed a first-order gap collapse near 4.25 Mbar at about 80 K; the 2017 reflectivity claim at about 4.95 Mbar and dc confirmation remain disputed.
Posed since: 1935