Comments on the claimed observation of the Wigner-Huntington Transition to Metallic Hydrogen

In their recent work Dias and Silvera (Science 2017) claim to have observed the Wigner-Huntington transition of hydrogen to a metallic state (MH) at a pressure of 495 GPa at low temperatures. The evidence for this transition is based on a high electron carrier density deduced from a Drude free electron model fitted to the reflectivity of the sample. Based on our analysis of the reflectivity data we find no convincing evidence for metallic hydrogen in their published data. The pressure determination is also ambiguous - it should be ~630 GPa according to the presented Raman spectrum. For comparison, we present our own data on the observation of highly reflecting hydrogen at pressures of 350-400 GPa. The appearance of metallic reflectivity is accompanied with a finite electrical conductivity of the sample. We argue that the actual pressure in the experiment of Dias and Silvera is likely below 400 GPa. In this case the observed enhanced reflectivity would be related to the phase transformation to conductive state published in arXiv:1601.04479.