Under pressure: Harvard researchers claim to have successfully squeezed hydrogen into a metallic state using diamond anvils, but the physics community is not convinced. The pictures in this article are all WeChat public numbers of "Nature Nature Research"

Five experts told the Nature news team that they do not yet believe the statement and still need more evidence to reach a conclusion. "I don't think this paper is convincing at all," said Paul Loubeyre, a physicist at the French Atomic Energy Commission.

Silvera and Dias said they hope to publish preliminary observations before further validating the experiment with the fragile experimental material.

Metallic Hydrogen Dreams

Producing metallic hydrogen in the laboratory has been a dream of high-pressure researchers since theoretical physicists first predicted its existence in 1935. According to theoretical predictions, when squeezed with enough pressure in a high-pressure anvil, hydrogen can obtain conductivity-a sign of the metallic state. Theorists have also suggested that metallic hydrogen may have other exotic properties, such as superconductivity (zero resistance) at room temperature.

By producing metallic hydrogen, physicists may be able to explore planetary science in the laboratory: it is theorized that the cores of gas giants such as Jupiter contain metallic hydrogen, which may be why they can maintain magnetic fields.

In recent years, physicists have squeezed trace hydrogen samples with diamond anvils at pressures higher than the pressure in the center of the earth. This kind of experiment is very delicate and prone to errors. Researchers have observed a change in the color of the material from transparent to dark during the squeezing process, which means that electrons squeezed together have the ability to absorb visible light photons. But the existence of light-reflecting, shiny metallic hydrogen has not been proven. The results reported by physicists at the Max Planck Institute for Chemistry in Germany in 2011 are also controversial. The team leader, Mikhail Eremets, said his team has yet to obtain conclusive evidence of the presence of metallic hydrogen.

Dias and Silvera claim that they can squeeze hydrogen at a higher pressure than any previous team. To achieve this, they used an anvil that can be placed in a cryostat to cool the hydrogen sample to a temperature slightly above absolute zero. They also claim to have found a way to better grind the diamond indenter, thereby eliminating the irregularities that can cause the diamond to break. They then added the pressure to 495 billion Pascals (495GPa), almost 5 million times the atmospheric pressure at sea level.

"All of a sudden, hydrogen becomes a shiny, reflective substance that could only have been a metal," Silvera said. Silvera said that when viewed through a microscope, the hydrogen sample shines and reflects light the way metallic hydrogen should.