At what pressure does solid hydrogen become a metal
In plain words
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 statement
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 $\sim 4\,\mathrm{Mbar}$.
What would settle it
Reproducible dc transport with metallic temperature dependence plus diffraction or Raman identification of the phase, at a stated calibrated pressure.
Status in the literature
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.