Hydrogen absolutely can be a solid, and under the right conditions it may even become a metal. At everyday pressures, hydrogen is the lightest gas in the universe, but squeeze it hard enough and it crystallizes into a molecular solid. Push the pressure further still, into the realm of hundreds of gigapascals (millions of atmospheres), and theory predicts the molecules will break apart and hydrogen’s single electrons will flow freely, turning the element into a shiny, electrically conductive metal. This transition, first predicted in 1935, remains one of the most stubbornly elusive goals in experimental physics, tangled in controversy, diamond failures, and pressures so extreme they challenge the limits of any apparatus humans have built.
How Hydrogen Becomes a Solid
Under ordinary conditions hydrogen exists as a diatomic gas, two atoms bonded into H₂ molecules buzzing around at room temperature. Cool it to about 14 kelvin (around −259 °C) and it freezes into a transparent molecular solid. You can also solidify it by applying high pressure at somewhat warmer temperatures. In this solid state, H₂ molecules sit on a crystal lattice but still rotate freely at lower pressures, a phase known as Phase I. As pressure climbs, that free rotation gets locked down, and hydrogen enters Phase II, where the molecules begin to adopt preferred orientations. Push past roughly 160 to 200 gigapascals and you reach Phase III, then Phase IV, each involving progressively more complex rearrangements of how the molecules orient and stack.
These phases are all still molecular: the hydrogen atoms remain paired into H₂ units. The solid is transparent or translucent, an insulator with a wide energy gap that prevents electrons from moving freely. But each successive phase transition narrows that gap, and the spectroscopic signatures shift in ways that tell researchers the electronic structure is changing. Raman spectroscopy performed up to 325 gigapascals at room temperature has revealed a cascade of subtle transitions, with discontinuities in vibrational frequencies and intensities appearing near 250, 285, and 300 gigapascals, indicating ongoing changes in bonding and electronic structure as density increases.
The 1935 Prediction That Started It All
The idea that hydrogen could become a metal dates to a landmark 1935 paper by Eugene Wigner and Hillard Bell Huntington. They calculated the energy of a body-centered cubic lattice of individual hydrogen atoms and concluded that any arrangement in which the atoms sit on a regular lattice without pairing into molecules would have metallic properties.1The Journal of Chemical Physics. On the Possibility of a Metallic Modification of Hydrogen Their estimated transition pressure was around 25 gigapascals. That number turned out to be wildly optimistic. Nearly nine decades later, experimentalists have squeezed hydrogen past 400 gigapascals without producing an unambiguous, universally accepted metallic sample. But the basic physics of the prediction has held up: compress hydrogen enough, and the molecules must eventually dissociate into individual atoms whose electrons delocalize across the lattice, creating a metal.
Why Is It So Hard to Squeeze Hydrogen?
The standard tool for generating extreme static pressures is the diamond anvil cell, a device that uses two gem-quality diamond tips pressed together to crush a tiny sample. Diamonds are the hardest known natural material, but even they fail catastrophically at the pressures needed to metallize hydrogen. Samples are microscopic, often just a few microns across, and the diamonds frequently shatter at the worst possible moment, destroying the sample before measurements can be completed.
A major step forward came with the development of toroidal diamond anvil cells, where the diamond tips are sculpted into a donut-like shape that distributes stress more evenly. This design has extended achievable pressures to around 600 gigapascals using culets (the flat tip faces) as small as 16 micrometers, and researchers have projected that even higher pressures, approaching a terapascal, could be reached with further refinements.2PubMed Central. Toroidal diamond anvil cell for detailed measurements under extreme static pressures These toroidal cells have already been used to probe hydrogen’s behavior at 80 kelvin above 400 gigapascals using synchrotron infrared absorption spectroscopy, pushing into the pressure range where metallization is expected.3PubMed. Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen
Even with better hardware, the measurements themselves are brutal. The sample is so small and confined that optical and electrical probes have to be threaded through or around the diamonds. Raman spectroscopy and infrared absorption are the workhorses, since they can be performed through the diamond windows, but electrical conductivity measurements require metallic leads that must survive the same crushing pressures. Each technique gives a partial picture, and interpreting the results requires careful cross-referencing.
The Most Controversial Claim
In 2017, Ranga Dias and Isaac Silvera at Harvard announced they had achieved the Wigner-Huntington transition. They reported that at 495 gigapascals and 5.5 kelvin, their hydrogen sample became reflective, with a reflectance as high as 0.91, consistent with a metallic surface. From the reflectance data they extracted properties matching those expected of an atomic metal.4PubMed. Observation of the Wigner-Huntington transition to metallic hydrogen The announcement made headlines worldwide.
The sample was lost almost immediately. While attempting to lower the pressure for further measurements, the diamonds failed and the hydrogen escaped. Without the sample, independent verification was impossible, and skepticism set in quickly. Critics questioned the pressure calibration, the optical measurements, and whether the reflective surface might have been alumina coating from the diamonds rather than the hydrogen itself. Dias and Silvera published a response defending their methods and noting that they had achieved the highest static pressures ever applied to hydrogen at low temperatures.5PubMed. Response to Comment on “Observation of the Wigner-Huntington transition to metallic hydrogen”
The controversy deepened years later when an independent re-examination of the raw data files released alongside the 2017 paper found troubling inconsistencies. The analysis identified nonuniform intervals in the diamond Raman spectrum consistent with artificial removal or adjustment of data points, pronounced mismatches between released infrared transmission spectra and the curves shown in the publication’s supplementary materials, and a reflectance dataset too limited for independent verification of the metallicity claim.6Matter and Radiation at Extremes. Evidence of inconsistencies between publicly released raw data and published figures for the claimed observation of the Wigner–Huntington transition to metallic hydrogen This episode sits alongside broader concerns about Dias’s research record, as some of his other high-profile claims have faced retractions and investigations. As a result, the 2017 metallic hydrogen claim is widely regarded as unconfirmed.
What Has Actually Been Observed
While the definitive atomic metallic phase remains contested, there is solid evidence that hydrogen undergoes dramatic electronic changes at very high pressures. In 2011, Mikhail Eremets and Ivan Troyan reported that above 220 gigapascals, hydrogen became opaque and electrically conductive. At 260 to 270 gigapascals, they observed a sharp increase in conductance that changed little with further pressurization up to 300 gigapascals or cooling to 30 kelvin, and the sample reflected light well.7PubMed. Conductive dense hydrogen This behavior is consistent with a metallic or semimetallic state, though some researchers have argued it could represent a band-overlap semiconductor rather than a true metal in the Wigner-Huntington sense.
Raman spectroscopy on both hydrogen and deuterium has mapped out the transitions through Phases I, III, and IV up to 325 gigapascals at room temperature, documenting the progressive changes in bonding and electronic structure.8PubMed Central. Raman measurements of phase transitions in dense solid hydrogen and deuterium to 325 GPa More recent experiments have compressed deuterium to 460 gigapascals at 80 kelvin, combining Raman and synchrotron infrared techniques to probe how hydrogen’s heavier isotope behaves along the path toward metallization.9PubMed. Compression of D_{2} to 460 GPa and Isotopic Effects in the Path to Metal Hydrogen Isotopic comparisons matter because deuterium, being heavier, has smaller quantum mechanical zero-point motion, which shifts the phase boundaries and can help distinguish between competing theoretical models.
Why Theory Keeps Getting It Wrong
One of the frustrating features of this quest is that standard computational methods have consistently predicted metallization at pressures lower than what experiments find. Density functional theory, the workhorse of computational materials science, tends to favor metallic structures at pressures where real hydrogen stubbornly remains insulating. More sophisticated approaches, like diffusion quantum Monte Carlo calculations, correct this bias and produce phase diagrams in better agreement with experiment, pushing the predicted metallization to higher pressures.10PubMed Central. Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures
Machine-learning-based potentials trained on quantum mechanical data have added another tool, allowing researchers to simulate the phase diagram across a wide range of pressures and temperatures. These simulations reproduce the known phase sequence and predict that Phase III appears above roughly 160 gigapascals, with a melting curve that peaks near 900 kelvin and 90 gigapascals before turning over and dropping at higher pressures.11Nature Communications. Understanding high pressure molecular hydrogen with a hierarchical machine-learned potential The turnover of the melting curve is itself interesting: it means that at very high pressures, solid hydrogen actually melts at lower temperatures than you might expect, creating an unusual region in the phase diagram.
The gap between theory and experiment is not just an academic headache. If we cannot accurately predict where the transition occurs, it is hard to design experiments that aim for exactly the right conditions, and it is hard to interpret ambiguous data from experiments that land in the borderland between molecular and atomic phases.
Would Metallic Hydrogen Stay Metallic?
Even if metallic hydrogen is produced, a critical question is whether it would survive once the pressure is released. Some materials, once forced into a new crystal structure, remain stuck there at ambient conditions, the way diamond (a high-pressure form of carbon) persists indefinitely at normal pressures. If metallic hydrogen were similarly metastable, it could be produced inside a diamond anvil cell and then extracted for use. Theoretical analyses suggest this is unlikely to work in a simple way. Calculations indicate the metallic state could persist down to about 10 to 20 gigapascals, but decays instantly at lower pressures.12Low Temperature Physics. On the lifetime of metastable metallic hydrogen That means metallic hydrogen would probably revert to a molecular form long before you could bring it to normal atmospheric pressure.
This is a significant disappointment for speculative applications. If metallic hydrogen were metastable at ambient conditions, it could in principle serve as an extraordinarily energy-dense fuel or as a room-temperature superconductor. The energy stored in compressing hydrogen to a metallic state is enormous, and its release upon decompression would be dramatic. But a material that instantly reverts when you let go of the pressure is not something you can put in a fuel tank.
Metallic Hydrogen Inside Giant Planets
While laboratory experiments struggle to reach the necessary conditions, nature produces metallic hydrogen effortlessly inside gas giant planets. Jupiter’s interior reaches pressures of hundreds of gigapascals and temperatures of thousands of kelvin. Under these conditions, hydrogen transitions from a molecular fluid near the surface to a metallic fluid deeper inside. This metallic hydrogen is not a solid crystal lattice as Wigner and Huntington envisioned but rather a hot, dense liquid in which electrons move freely, conducting electricity and generating Jupiter’s powerful magnetic field.
Research on hydrogen at high pressures and temperatures has indicated that the transition to a metallic state in Jupiter occurs much closer to the planet’s surface than older models assumed, and that there may be no sharp core-mantle boundary of the kind found in rocky planets.13Chemistry – A European Journal. Metallic Hydrogen at High Pressures and Temperatures in Jupiter Instead, the change from molecular to metallic hydrogen is thought to be gradual, a smooth crossover rather than a sudden phase transition. Accurate equations of state for hydrogen-helium mixtures, including the immiscibility of helium in metallic hydrogen (which causes helium to rain downward), are essential for modeling the internal structure and thermal evolution of Jupiter, Saturn, and the growing catalog of giant exoplanets.14The Astrophysical Journal Supplement Series. Equations of State, Thermodynamics, and Miscibility Curves for Jovian Planet and Giant Exoplanet Evolutionary Models
The Superhydride Workaround
Frustrated by the difficulty of metallizing pure hydrogen, researchers have found a productive shortcut: combine hydrogen with other elements to form compounds that capture some of metallic hydrogen’s predicted properties at lower pressures. These “superhydrides” are hydrogen-rich compounds where a heavier atom sits inside a cage of hydrogen atoms, and the hydrogen sublattice behaves much like metallic hydrogen would. The idea traces back to theoretical work suggesting that alloys where hydrogen is the dominant constituent should exhibit the same high-temperature superconductivity predicted for pure metallic hydrogen, but at pressures achievable with existing diamond anvil cells.15Physical Review Letters. Hydrogen Dominant Metallic Alloys: High Temperature Superconductors?
This strategy has paid off spectacularly. Hydrogen sulfide (H₃S) was found to superconduct above 200 kelvin at high pressure, and lanthanum hydride (LaH₁₀) pushed that threshold even higher. A carbonaceous sulfur hydride system then reportedly achieved superconductivity at 288 kelvin (about 15 °C) at 267 gigapascals, tantalizingly close to room temperature.16PubMed. Compressed superhydrides: the road to room temperature superconductivity Although some of these claims have also faced scrutiny and controversy, the overall trajectory is clear: hydrogen-rich compounds under pressure can exhibit remarkable superconducting properties.
Computational searches have extended into even more exotic territory. Predictions for lanthanide and actinide superhydrides with the formula MH₁₈ suggest these clathrate structures could superconduct above room temperature, with bonding and electronic properties that closely resemble those of atomic metallic hydrogen itself.17PubMed. Prediction of Above-Room-Temperature Superconductivity in Lanthanide/Actinide Extreme Superhydrides The superhydride approach cannot replace the fundamental scientific goal of understanding pure hydrogen’s behavior, but it offers a practical path toward harvesting the properties that make metallic hydrogen so appealing.
What Superconducting Hydrogen Would Mean
The deepest reason physicists care about metallic hydrogen, beyond scientific curiosity, is superconductivity. Hydrogen is the lightest element, which means its atoms vibrate at extremely high frequencies in a crystal lattice. Those high-frequency vibrations are, in simplified terms, the engine that drives conventional superconductivity, and lighter atoms drive it harder. Theoretical estimates have long suggested that metallic hydrogen could superconduct at very high temperatures, possibly even at room temperature. If that prediction is correct, and if the material could somehow be stabilized, it would represent a revolution in energy technology: lossless electrical transmission, compact and powerful magnets, and transformative advances in computing and transportation.
The catch, of course, is the twin problem of producing it and keeping it. The superhydrides described above offer a glimpse of what hydrogen-based superconductivity looks like in practice, but they still require enormous pressures. No one has yet found a way to bring those properties to conditions you could use outside a specialized laboratory. The gap between a microscopic sample at hundreds of gigapascals and a practical superconducting wire remains vast.
The Credibility Problem in High-Pressure Research
The metallic hydrogen story is inseparable from broader credibility issues that have shaken high-pressure physics in recent years. Ranga Dias, the lead author of the 2017 metallic hydrogen paper, later co-authored claims of room-temperature superconductivity in other materials that were retracted by Nature amid concerns about data integrity. The forensic re-examination of the 2017 hydrogen data, identifying mismatches between released raw data and published figures, fits into this wider pattern.6Matter and Radiation at Extremes. Evidence of inconsistencies between publicly released raw data and published figures for the claimed observation of the Wigner–Huntington transition to metallic hydrogen These controversies have not undermined the underlying physics, but they have made the community far more cautious about accepting dramatic claims without independent reproduction.
Independent reproduction is uniquely difficult in this field. Only a handful of laboratories worldwide have the equipment to reach pressures above 300 gigapascals on hydrogen, and each experiment takes months of preparation with a high probability of diamond failure. When a sample is lost before anyone else can examine it, the result sits in a gray zone: not necessarily wrong, but not confirmed either. The field’s culture is slowly adapting, with greater emphasis on raw data sharing, standardized pressure calibration, and multiple diagnostic probes used simultaneously. But the fundamental challenge remains that these experiments operate at the ragged edge of what materials can withstand, leaving very little margin for the kind of careful, reproducible measurements that would settle debates definitively.
Helium Rain and Other Oddities Inside Giant Planets
Metallic hydrogen’s role inside gas giants involves some genuinely strange physics beyond simple electrical conductivity. As hydrogen transitions to a metallic state deep inside Jupiter and Saturn, helium becomes less soluble in it. This immiscibility causes helium to separate out and form droplets that sink toward the planet’s core, a phenomenon known as helium rain. The gravitational energy released by this settling helium contributes meaningfully to the planet’s total heat output, helping explain why Saturn radiates more energy than it receives from the Sun.
Modeling this process requires accurate knowledge of how hydrogen and helium mix (or refuse to mix) across a wide range of pressures and temperatures. Updated equations of state that account for hydrogen-helium interactions and immiscibility curves have improved evolutionary models for both Jupiter and Saturn, and they extend to the diverse giant exoplanets now being discovered in large numbers.14The Astrophysical Journal Supplement Series. Equations of State, Thermodynamics, and Miscibility Curves for Jovian Planet and Giant Exoplanet Evolutionary Models Understanding metallic hydrogen is not just a laboratory curiosity; it is necessary for understanding the internal workings of the most common type of large planet in our galaxy.