...As Silvera and Dias ramped up the pressure on the hydrogen in their DAC, they monitored the sample’s appearance as it changed from transparent to opaque to highly reflective. Then, by analyzing their data with a mathematical model of reflectivity, they concluded that the shiny appearance of the sample observed at 495 GPa corresponded to hydrogen becoming metallic.
Some researchers voiced objections to the claims, noting, for example, that the team’s method for measuring pressure may have been inaccurate. Rather than using a laser-based Raman spectroscopy technique to monitor pressure throughout the experiment, as is commonly done, the Harvard group used that method only at the highest pressure.
Silvera and Dias determined the pressure in the earlier part of the experiment from the number of turns of the “screws” that control the position of the diamonds. Silvera counters that he did so to minimize using powerful lasers that can easily damage the diamonds. And he contends that the mechanical method was properly calibrated.
Other scientists suggested that there is insufficient evidence to attribute the reflections recorded by Silvera and Dias to metallic hydrogen. Perhaps they came instead from the metal gasket or from AlH3, a material that some researchers say may form by reaction of the alumina coating with high-pressure hydrogen. Again, Silvera defends his science, arguing that the reflectivity data point to metallic hydrogen, not the other materials.
Whether the Harvard researchers actually achieved their goal remains an open question. “If they really made metallic hydrogen, that would be wonderful,” says Reinhard Boehler, a physicist at the Carnegie Institution for Science in Washington, D.C. “These experiments are very hard to reproduce,” he adds, but they must be reproduced to address the doubts...