Metallic Hydrogen: Pressure Record

The quest to turn hydrogen into a metal is often described as the most significant challenge in high-pressure physics. For nearly a century, scientists have theorized that the simplest element in the universe could conduct electricity if subjected to enough force. Recent experiments have pushed pressure boundaries to historic highs, claiming to finally observe this elusive state.

The 80-Year Scientific Chase

The concept of metallic hydrogen dates back to 1935. Physicists Eugene Wigner and Hillard Huntington published a theory suggesting that under high pressure, hydrogen molecules would break their bonds and the atoms would dissociate. This would allow electrons to flow freely, transforming the gas into a conductive metal.

Wigner and Huntington originally estimated this transition would occur at 25 gigapascals (GPa). They were drastically incorrect regarding the amount of force required. Modern experiments have shown the necessary pressure is closer to 400 to 500 GPa. To put that in perspective, 1 GPa is roughly 10,000 atmospheres of pressure. The center of the Earth sits at roughly 360 GPa. Creating metallic hydrogen requires pressures greater than those found at the Earth’s core.

Crushing Atoms with Diamond Anvils

Achieving these extreme conditions requires a device called a Diamond Anvil Cell (DAC). This device squeezes a microscopic sample of hydrogen between the tips of two polished diamonds. Diamonds are used because they are the hardest known material and are transparent to X-rays, allowing scientists to observe what happens inside the cell.

However, standard diamond anvils usually shatter before reaching the necessary pressure for metallization. To overcome this, recent breakthroughs have involved changing the shape of the diamond tips.

The Toroidal Design

In a landmark 2020 study published in Nature, French physicists Paul Loubeyre, Florent Occelli, and Paul Dumas achieved a pressure of 425 GPa. They utilized a specific “toroidal” cut on the diamond tips. This doughnut-like shape distributed the stress more effectively than flat tips, preventing the diamonds from shattering prematurely.

At 425 GPa, the team observed the hydrogen sample change properties in distinct stages:

1. Transparency: Initially, the hydrogen was transparent.
2. Opaqueness: As pressure increased, it turned black and blocked light.
3. Reflectivity: Finally, at peak pressure, the sample became reflective to infrared light. This reflectivity is a key signature of a metal.

Why Verification is Difficult

While the Loubeyre experiment provided some of the strongest evidence to date, the scientific community remains cautious. Verifying metallic hydrogen is incredibly difficult for several reasons.

First, the samples are microscopic. The hydrogen is squeezed into a space smaller than the width of a human hair. This makes it difficult to attach electrodes to measure electrical conductivity directly. Instead, scientists often rely on spectroscopic measurements using facility-sized machines like the Synchrotron SOLEIL in France. They blast the sample with extremely bright light to analyze how it absorbs and reflects energy.

Second, the experiments are destructive. The diamonds often break upon releasing the pressure, destroying the sample and making it impossible to repeat the test on the same material. In 2017, Harvard researchers Isaac Silvera and Ranga Dias claimed to reach 495 GPa and observe metallic hydrogen. However, that claim faced skepticism because the sample was lost when the diamonds shattered, preventing independent verification.

The "Holy Grail" of Superconductivity

The intense interest in metallic hydrogen comes from its potential applications. Theorists believe it could be a room-temperature superconductor. Current superconductors, used in MRI machines and maglev trains, must be cooled to extremely low temperatures (often below -200 degrees Fahrenheit) to work. This makes them expensive and difficult to maintain.

A material that conducts electricity with zero resistance at room temperature would revolutionize energy.

• Power Grids: Electrical grids could transmit power without losing energy to heat, potentially saving billions of dollars and reducing carbon emissions.
• Transportation: High-speed magnetic levitation trains could become cheaper and more widespread.
• Electronics: Computer chips could run faster without overheating.

Rocket Fuel Revolution

Beyond electricity, metallic hydrogen has massive implications for space travel. If the substance is “metastable”—meaning it stays metallic even after the pressure is released, similar to how diamonds stay solid after leaving the Earth’s crust—it could serve as the ultimate rocket fuel.

Recombining metallic hydrogen back into gas releases a massive amount of energy. It is estimated to have a specific impulse (efficiency) four times greater than the best liquid hydrogen/oxygen fuels currently used by NASA and SpaceX. This would allow rockets to carry heavier payloads to Mars or reach the outer solar system in a fraction of the current time.

Current Status of the Record

The record for high-pressure physics is constantly under siege. While the 425 GPa mark set by Loubeyre’s team stands as a verified benchmark for observing reflectivity, the race continues to prove actual superconductivity.

Other research groups, such as the Eremets group at the Max Planck Institute for Chemistry, continue to refine conductivity measurements at pressures exceeding 250 GPa. The ultimate proof will come when a lab can not only crush the hydrogen into a metal but also successfully measure zero electrical resistance and the Meissner effect (magnetic levitation) without the data being retracted or disputed.

Frequently Asked Questions

What is the pressure required for metallic hydrogen? Current research indicates hydrogen transitions to a metallic state at pressures between 400 and 500 gigapascals (GPa). This is greater than the pressure at the center of the Earth.

Why is it called the “Holy Grail” of physics? It is called the Holy Grail because it was predicted nearly 90 years ago but remains difficult to create. Additionally, it predicts room-temperature superconductivity, a property that would fundamentally change energy and technology sectors.

Has anyone definitely created metallic hydrogen? Several groups claim to have observed it, notably Paul Loubeyre’s team in 2020 at 425 GPa. However, creating a sample that survives long enough for undisputed electrical testing remains a hurdle.

Is metallic hydrogen a solid or a liquid? At the extreme pressures discussed (400+ GPa) and lower temperatures, it is expected to be a solid crystal lattice. However, at high temperatures, liquid metallic hydrogen is thought to exist in the interiors of gas giants like Jupiter and Saturn.

Will metallic hydrogen explode if pressure is released? If the material is not metastable, it will revert to gas instantly and violently. If it is metastable, it would remain a solid metal, similar to how a diamond remains stable at low pressure. Scientists are currently unsure if it will be metastable.