Two physicists say that they have crushed hydrogen under such immense pressures that the gas became a shiny metal—a feat that physicists have been trying to accomplish for more than 80 years.

But other researchers have serious doubts about the claim, the latest in a field with a long history of failed attempts.

Ranga Dias and Isaac Silvera, both physicists at Harvard University in Cambridge, Massachusetts, first posted a report of their results on the arXiv preprint server last October, which attracted immediate criticism. A peer-reviewed version of the report was published on 26 January in Science, but sceptics say that it includes little new information.

Five experts told Nature’s news team that they do not yet believe the claim, and need more evidence. “I don’t think the paper is convincing at all,” says Paul Loubeyre, a physicist at France’s Atomic Energy Commission in Bruyères-le-Châtel.

Silvera and Dias say that they wanted to publish their first observation before making further tests on their fragile material.

Metallic dream

Producing metallic hydrogen in the laboratory has been a dream of high-pressure researchers ever since 1935, when theorists first predicted its existence. When squeezed with enough pressure inside an anvil, hydrogen should be able to conduct electricity, the hallmark of a metallic state. And theorists say that the material could have other exotic properties, such as being a superconductor—able to conduct electricity without resistance—even at room temperature.

By making metallic hydrogen, physicists might also be able to explore planetary science at their lab bench: gas-giant planets such as Jupiter are theorized to have metallic hydrogen in their cores, which would perhap explain how they can sustain a magnetic field.

In recent years, physicists have crushed tiny samples of hydrogen between diamond anvils at pressures exceeding those in the centre of Earth. The experiments are delicate and fraught with potential for error. Researchers have seen the material change from transparent to dark as it is compressed, which suggests that as electrons are crowded together, they are able to absorb photons of visible light. But no one has proven the existence of metallic, shiny hydrogen, which would reflect light. In 2011, a report by physicists at the Max Planck Institute for Chemistry in Mainz, Germany, was controversial. Mikhail Eremets, who leads that group, says his team has not yet provided conclusive evidence.

Dias and Silvera say that they were able to squeeze their hydrogen gas at greater pressures than anyone else has managed. To do so, they used an anvil that can fit inside a cryostat, enabling them to cool their hydrogen sample to just above absolute zero. They also say they have found a better way to polish the tips of their diamonds, to remove irregularities that could break the gems. They then turned a screw to crank up the pressure to 495 billion pascals (495 GPa), or almost 5 million times higher than atmospheric pressure at sea level.

“Then, suddenly, it becomes a lustry, reflective sample, which you can only believe is a metal,” Silvera says. Seen through a microscope, the sample appeared shiny, and it reflected light in the way metallic hydrogen should do, he says.

Redo the measurement

Other researchers aren't convinced. It’s far from clear that the shiny material the researchers see is actually hydrogen, says geophysicist Alexander Goncharov of the Carnegie Institution for Science in Washington DC. Goncharov has criticized the Silvera lab’s methods before. He suggests that the shiny material may be alumina (aluminium oxide), which coats the tips of the diamonds in the anvil, and may behave differently under pressure.

Loubeyre and others think that Silvera and Dias are overestimating the pressure that they reached, by relying on an imprecise calibration between turns of the screw and pressure inside the anvil. Eugene Gregoryanz, a physicist at the University of Edinburgh, UK, adds that part of the problem is that the researchers took only a single detailed measurement of their sample at the highest pressure—making it hard to see how pressure shifted during the experiment.

“If they want to be convincing, they have to redo the measurement, really measuring the evolution of pressure,” says Loubeyre. “Then they have to show that, in this pressure range, the alumina is not becoming metallic.”

But Silvera says that he just wanted to get the news out there before making confirmation tests, which, he says, could break their precious specimen. “We wanted to publish this breakthrough event on this sample,” he says. To preserve the material, he and Dias have kept it in the cryostat; the lab has only two cryostats, and the other is in use for other experiments, he says. “Now that the paper has been accepted, we’re going to do further experiments.”

Despite the scepticism, press-release materials issued by Science and by Harvard University confidently proclaim that metallic hydrogen has been made. “This is the holy grail of high-pressure physics,” Silvera says in Harvard’s press release. “It's the first-ever sample of metallic hydrogen on Earth.”

This article is reproduced with permission and was first published on January 26, 2017.</s>Image copyright HARVARD UNIVERSIT/R.DIAS/I.SILVERA Image caption Smaller than the cross-section of a hair on your head: The tiny sample went from transparent, to black, to highly reflective as the pressure was increased. Becoming shiny and reflective is a sign the material has become a solid metal, the Harvard team claims

Scientists in the US say they have at last managed to turn hydrogen into a state where it behaves like a metal.

If that is true - and it is a controversial claim - it fulfils a more than 80-year quest to produce what many have said would be a wonder material.

Theory suggests metallic hydrogen could be used to make zero-resistance electrical wiring and super-powerful rocket fuel, among many applications.

Ranga Dias and Isaac Silvera are the Harvard researchers behind the work.

They report their experiments in this week’s Science Magazine.

"It's the first time solid metallic hydrogen has ever existed on Earth," Prof Silvera told the BBC.

The pair say the quantity of metallic hydrogen created so far is tiny - a speck that is less than the cross section of a human hair. But in time, they believe, ways could be found to boost its production.

Image copyright Harvard University Image caption Ranga Dias stands in front of the experimental set-up

Their laboratory approach - as is common in this field of study - was to squeeze a cell containing a small sample of molecular hydrogen between two synthetic diamonds.

In the Science paper, Dias and Silvera say this diamond anvil cell (DAC) achieved pressures up to 495 gigapascals.

That is the equivalent of sitting under almost five million Earth atmospheres. The DAC was also chilled down to close to minus 270 Celsius.

The scepticism here is probably a good thing, in that it will drive many groups towards attempting to reproduce this experiment Marcus Knudson, Sandia National Laboratories

The intention was to pack the atoms of hydrogen so close together that they formed a crystal lattice and started to share their electrons - behaviour that is exactly that of a metal.

Dias and Silvera report that this state became obvious to them when the material in the diamond anvil cell developed the kind of lustrous surface so ubiquitous of metals.

"As we turned the pressure up, it went to a transparent molecular solid. And then as the pressure kept going up, it went black, and we think it goes black because it becomes like a semi-conductor and it can absorb light," Prof Silvera recalled on the BBC’s Science In Action programme.

"And then we turned the pressure up higher and it started shining. It was very exciting. It's got extremely high reflectance. The reflectance we measured is about 90%. It's about the reflectivity of an aluminium mirror."

Image copyright SCIENCE PHOTO LIBRARY Image caption Artwork: In a metal, the atoms pack closely together in a lattice arrangement, which enables them to share their electrons. This is why metals make such good electrical conductors

It has to be said, however, that news of the Harvard experiment has already attracted a good degree of scepticism.

Other scientists working in the same and related fields have told the BBC that the team's paper is short on the kind of data needed to make a proper assessment of its achievements.

"Complete garbage," is how Eugene Gregoryanz from Edinburgh University described the research. "Like everybody else who works with hydrogen at high pressures, I am appalled by what is being published in Science."

That there is so much scepticism is natural. If what is being claimed pans out, it would represent one of the major physics breakthroughs of recent decades.

Metallic hydrogen was first predicted more than 80 years ago, and there has been a race to create it ever since. That is because of the stunning properties that are promised.

It has been suggested for example that metallic hydrogen might be metastable; that is - once made under extreme conditions it would maintain its state even when brought back up to ambient pressures and temperatures.

And if, as some think, it is also a zero-resistance superconductor, that could lead to a revolution in the transmission and storage of electricity. For example, cables made from metallic hydrogen could feed energy across a country without the sort of electrical losses experienced in standard power grids.

The US space agency is also fascinated by the material. Already super-cold liquid hydrogen makes for a very powerful rocket propellant, but the dense metallic form of hydrogen promises to deliver really colossal levels of thrust that would enable huge payloads to be lifted off Earth.

But all this is in the realms of speculation for now. First, it must be shown that the Harvard work can be reproduced. If that's possible - and some are saying that is a very big "if" - then more sizeable quantities of metallic hydrogen need to be created.

Image copyright NASA Image caption Metallic hydrogen would give a rocket prodigious capability

"I understand that others in the DAC community have been rather sceptical (arguing that the apparent reflectivity might be coming from contaminants in the sample, the aluminium oxide coating on the diamonds, etc.). However, if they really have achieved nearly 500 gigapascals in the DAC it is not unreasonable to have observed a transition to metallic hydrogen,” commented Marcus Knudson from Sandia National Laboratories.

"The scepticism here is probably a good thing, in that it will drive many groups towards attempting to reproduce this experiment. This publication will certainly incite the field. Again, if it holds up, this is an exciting result. I think in this case time will tell," he told BBC News.

And Jeffrey McMahon from Washington State University concurred: "With respect to the tiny sample amount: Such experiments are performed in small diamond anvil cells. One challenge would be to make a larger quantity (at once); another, perhaps bigger challenge is to recover even the small sample (ie, remove it from the extreme pressures that it is under in the diamond anvil cell).

"Whether the latter is possible is an important open question."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos</s>State of hydrogen predicted to be a room temperature superconductor reportedly seen – but the methods used are being questioned

It has been over 80 years in the making but the metallic form of hydrogen has finally been created in the lab at extreme pressures. Isaac Silvera and Ranga Dias from Harvard University in the US estimate that they subjected hydrogen to 495 gigapascals (GPa), 4.95 million times the standard pressure exerted by Earth’s atmosphere. ‘It’s the first time that metallic hydrogen has existed on Earth,’ Silvera tells Chemistry World.

Yet, with this achievement long prized in the high pressure science community, Silvera’s rival experimentalists have responded with extreme scepticism. Eugene Gregoryanz, whose University of Edinburgh team took hydrogen to 380GPa in 2016, expects the study to be debunked within a year. Doubts centre on approximations used by the Harvard researchers in measuring the pressures involved.

It’s the first time that metallic hydrogen has existed on Earth Isaac Silvera, Harvard University

Modern calculations suggest metallic hydrogen could be a valuable room temperature superconductor and stable on returning to atmospheric pressure, and in its dense form also potentially useful as a rocket propellant. University of Cambridge materials scientist Chris Pickard is one of those trying to predict metallic hydrogen’s properties, and the pressure at which it might form. ‘Interestingly, it’s been settling down around the pressure that [Silvera and Dias] have found,’ Pickard says.

Crushing hydrogen so comprehensively means squeezing it between micrometre-scale points in diamond anvil cells (DACs). At such extreme conditions, however, diamonds regularly break, especially because they become brittle when hydrogen works its way among their carbon atoms. Consequently, Silvera and Dias were extremely cautious, starting by making anvils from synthetic diamonds that are less likely to have defects than natural ones.

‘We remove five or six microns of the surface and the possible defects that might be there,’ Silvera explains. ‘Then we coat the diamonds with alumina as a diffusion barrier against hydrogen. We never let the diamonds get above 80K, because at low temperature you supress diffusion. We put a lot of attention into getting good alignment. And we don’t shine intense laser beams at the sample until the end, because it’s known that lasers break diamonds.’ However, the final measure casts a shadow over what researchers can learn from the experiment, because they rely on lasers to gather information.

More illumination needed?

Silvera and Dias assert that their hydrogen sample is metallic because it became reflective, which they can see with laser beams that don’t damage the diamond. They also use lasers to measure the pressure the hydrogen and diamond experience, largely by monitoring variations in the light wavelengths the materials absorb, which indicate structural changes. However, above 335GPa that does require damaging beams, and so past that point they only risked one pressure measurement, when they had observed reflection.

The changes in reflectivity are too big to believe it’s actually hydrogen Eugene Gregoryanz, University of Edinburgh

The resulting pressure estimations in the paper are ‘strange, if not incorrect’, according to Mikhail Eremets from the Max Planck Institute for Chemistry in Mainz, Germany. Dias and Silvera find that pressure changes in direct proportion to the force applied by turning the screws that crush the anvils together. That’s not what Eremets has observed in his own high pressure hydrogen research. The Harvard scientists extrapolate this relationship to get the 495GPa measurement, he explains, whereas if they’d used the more conventional pressure–force relationship they would get 590GPa. That stretches plausibility, he underlines.

‘They might be lucky,’ Eremets says. ‘They used good diamonds, maybe it helped. But we’ve done the same for three years at least. In our experience it’s impossible to reach such high pressures.’ He’s therefore ‘very surprised’ the paper was published.

Gregoryanz’s many objections include – but go beyond – pressure measurements. He also questions whether the Harvard team even saw metallic hydrogen, and suggests they might instead be measuring a rhenium gasket used in the DAC. ‘The changes in reflectivity are too big to believe it’s actually hydrogen,’ Gregoryanz says.

Peer pressure

Paul McMillan from University College London notes Silvera’s team is ‘known internationally for careful and pioneering work’. ‘Although the researchers record the existence of a metallic state, they could not conclude on the crystalline nature of the sample,’ he adds. ‘Now the worldwide community, including extreme conditions researchers, planetary physicists and energy scientists, will look forward to discussing and debating these new data.’ Pickard echoes this. ‘As a landmark scientific result, this will have to stand up to the scrutiny of the experimental groups, but it’s very exciting,’ he says.

Silvera stands by his results but accepts that the pressure measurement may be revised. ‘In high pressure research things aren’t always well calibrated,’ Silvera says. ‘If people disagree, they should go to measure it and try to show that it’s different than what was claimed.’ Among the next steps for his own team will be measuring the high pressure hydrogen sample’s conductivity directly.

In the meantime, many have already visited Silvera’s lab to see the DAC containing a 10µm diameter hydrogen particle that they believe is unique in the planet’s history. ‘After we made it we told the staff here at Harvard,’ Silvera says. ‘We got 200 or 300 people come to see it, it went on all day.’</s>Scientists harnessed pressures greater than those found at the centre of the earth to turn hydrogen into a metal that could be an extremely efficient at conducting electricity.

The breakthrough, which created a material that has never existed before, has been hailed as the holy grail of high-pressure physics.

Metallic hydrogen is highly prized because of its potential as a room-temperature superconductor.

It could find its way into next-generation MRI scanners, replacing the huge, energy-intensive, super-cooled superconductors to make the medical imaging system more affordable and practical for hospitals around the world.

The material could also have a range of other uses from electronics and electric cars, through to the generation and storage of electricity.

The huge forces used to form the metal could also be used, in theory, as a propellant for much more powerful and efficient space rockets.

Harvard physicist Isaac Silvera said: This is the Holy Grail of high-pressure physics.

It’s the first-ever sample of metallic hydrogen on Earth, so when you’re looking at it, you’re looking at something that’s never existed before.

Details of the research, which is the first confirmation of a long-standing theory that hydrogen, normally a gas, could occur in a metallic state if exposed to extreme pressure, was published in the journal Science.

Professor Silvera and his fellow scientist fellow Ranga Dias squeezed a sample of frozen hydrogen to nearly 72 million pounds per square inch, which is greater than the pressure at the center of the Earth.

They created the immense force using synthetic diamonds mounted opposite each other in a device known as a diamond anvil cell.

However, a key question remains whether the pressurised hydrogen maintains its metallic properties at room temperature, which would make it very useful as a superconductor.

Both scientists believe it will, but this still has to be proved.</s>Researchers have been chasing the dream of metallic hydrogen for decades, ever since the material was first theorized by Princeton University scientists Eugene Wigner and Hillard Bell Huntington in 1935. If it exists, metallic hydrogen could fundamentally transform the way we used energy. Now, two scientists claim they’ve successfully created solid metallic hydrogen, but the scientific community is not yet convinced.

Metallic hydrogen is exactly what it sounds like — pure atomic hydrogen that has been put under such immense pressure that it collapses into a shiny metal. Isaac Silvera and post-doctoral fellow Ranga Dias from Harvard subjected a sample of hydrogen to 495 gigapascals of pressure, which is around 71.7 million pounds per square inch. That’s more pressure than it would experience in the center of the Earth. The pair claim the material that resulted from this process is the fabled metallic hydrogen.

To exert all that pressure, the scientists used a device known as a diamond anvil cell that compresses a sample between two diamond surfaces. However, they could not use just any diamond. The imperfections in polished synthetic diamonds could cause them to break or allow hydrogen to diffuse through the structure. The solution was to shave a five-micron layer off the diamond surface to remove imperfections and then coat it with alumina.

Silvera and Dias claim they’ve created metallic hydrogen based on their measurement of pressure and the formation of material that matches the expected properties of metallic hydrogen. They have not, however, performed any more rigorous tests on the sample. This is what most scientists who have evaluated the peer-reviewed paper (published in Science) have called for, worrying that the shiny material created in the anvil could just be alumina from the diamonds. Silvera and Dias say they do intend to do this, but they were waiting for publication first. They worry the sample could be destroyed by additional testing, so it has been preserved in a cryostat container until publication.

If the presence of metallic hydrogen is confirmed, this could change numerous industries. Metallic hydrogen is believed to be a room-temperature superconductor. That means it has zero electrical resistance, making it ideal for the construction of super-efficient batteries and electrical systems that don’t lose any energy to heat. Metallic hydrogen could also be used as a fuel source, as it retains all the energy used to compress it into a metallic state. Metallic hydrogen returning to regular molecular hydrogen would release a huge amount of energy, which could be used to propel a rocket.

There’s still more work to do in order to confirm the presence of metallic hydrogen, but this could be an amazing advancement. Even if it turns out to be real, this is just a tiny sample and all that exists on Earth. Making enough metallic hydrogen to be useful could be a completely different problem.

Now read: What are superconductors, and when will we all get maglev trains and unlimited electrical power?</s>For some time, scientists have been fascinated by the concept of metallic hydrogen. Such an element is believed to exist naturally when hydrogen is placed under extreme pressures (like in the interior of gas giants like Jupiter). But as a synthetic material, it would have endless applications, since it is believed to have superconducting properties at room temperature and the ability to retain its solidity once it has been brought back to normal pressure.

For this reason, condensed matter physicists have been attempting to create metallic hydrogen for decades. And according to a recent study published in Science Magazine, a pair of physicists from the Lyman Laboratory of Physics at Harvard University claim to have done this very thing. If true, this accomplishment could usher in a new age of super materials and high-pressure physics.

The existence of metallic hydrogen was first predicted in 1935 Princeton physicists Eugene Wigner and Hillard Bell Huntington. For years, Isaac Silvera (the Thomas D. Cabot Professor at Harvard University) and Ranga Dias, a postdoctorate fellow, have sought to create it. They claim to have succeeded, using a process which they described in their recently-published study, “Observation of the Wigner-Huntington transition to metallic hydrogen“.

Such a feat, which is tantamount to creating the heart of Jupiter between two diamonds, is unparalleled in the history of science. As Silvera described the accomplishment in a recent Harvard press release:

In the past, scientists have succeeded in creating liquid hydrogen at high temperature conditions by ramping up the pressures it was exposed to (as opposed to cryogenically cooling it). But metallic hydrogen has continued to elude experimental scientists, despite repeated (and unproven) claims in the past to have achieved synthesis. The reason for this is because such experiments are extremely temperamental.

For instance, the diamond anvil method (which Silvera and Dias used a variation of) consists of holding a sample of hydrogen in place with a thin metal gasket, then compressing it between two diamond-tipped vices. This puts the sample under extreme pressure, and a laser sensor is used to monitor for any changes. In the past, this has proved problematic since the pressure can cause the hydrogen to fill imperfections in the diamonds and crack them.

While protective coatings can ensure the diamonds don’t crack, the additional materials makes it harder to get accurate readings from laser measurements. What’s more, scientists attempting to experiment with hydrogen have found that pressures of ~400 gigapascals (GPa) or more need to be involved – which turns the hydrogen samples black, thus preventing the laser light from being able to penetrate it.

For the sake of their experiment, Professors Ranga Dias and Isaac Silvera took a different approach. For starters, they used two small pieces of polished synthetic diamond rather than natural ones. They then used a reactive ion etching process to shave their surfaces, then coated them with a thin layer of alumina to prevent hydrogen from diffusing into the crystal structure.

They also simplified the experiment by removing the need for high-intensity laser monitoring, relying on Raman spectroscopy instead. When they reached a pressure of 495 GPa (greater than that at the center of the Earth), their sample reportedly became metallic and changed from black to shiny red. This was revealed by measuring the spectrum of the sample, which showed that it had become highly reflective (which is expected for a sample of metal).

As Silvera explained, these experimental results (if verified) could lead to all kinds of possibilities:

In short, metallic hydrogen could speed the revolution in electronics already underway, thanks to the discovery of materials like graphene. Since metallic hydrogen is also believed to be a superconductor at room temperature, its synthetic production would have immense implications for high-energy research and physics – such as that being conducted by CERN.

Beyond that, it would also enable research into the interior’s of gas giants. For some time, scientists have suspected that a layer of metallic hydrogen may surround the cores of gas giants like Jupiter and Saturn. Naturally, the temperature and pressure conditions in the interiors of these planets make direct study impossible. But by being able to produce metallic hydrogen synthetically, scientists could conduct experiment to see how it behaves.

Naturally, the news of this experiment and its results is being met with skepticism. For instance, critics wonder if the pressure reading of 495 GPa was in fact accurate, since Silvera and Dias only obtained that as a final measurement and were forced to rely on estimates prior to that. Second, there are those who question if the reddish speck that resulted is in fact hydrogen, and some material that came from the gasket or diamond coating during the process.

However, Silvera and Dias are confident in their results and believe they can be replicated (which would go far to silence doubts about their results). For one, they emphasize that a comparative measurement of the reflective properties of the hydrogen dot and the surrounding gasket suggest that the hydrogen is pure. They also claim their pressure measurements were properly calibrated and verified.

In the future, they intend to obtain additional spectrographic readings from the sample to confirm that it is in fact metallic. Once that is done, they plan to test the sample to see if it is truly metastable, which will consist of them opening the vise and seeing if it remains in a solid state. Given the implications of success, there are many who would like to see their experiment borne out!

Be sure to check out this video produced by Harvard University that talks about the experiment:

By Matt Williams - 

 Matt Williams is the Curator of the Guide to Space for Universe Today, a regular contributor to HeroX, a science fiction author, and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful BC.</s>CAMBRIDGE (CBS) – Harvard University researchers are touting a scientific breakthrough as something that could lead to “revolutionary new materials.”

Science professor Isaac Silvera and his postdoctoral fellow Ranga Dias say they’ve created metallic hydrogen for the first time. The university says it’s the rarest material on Earth and it could become one of the most valuable.

“This is the Holy Grail of high-pressure physics,” Silvera said in a statement.

Metallic hydrogen was only a theory before. The researchers say they used diamonds to squeeze a small hydrogen sample with a pressure of 71.7 million pounds per square inch.

The new material could one day be used as a room temperature superconductor, which might make futuristic transportation, like levitating high-speed trains, feasible. Converting metallic hydrogen back to its molecular form would also release an enormous amount of energy that could propel rockets to make outer space exploration easier.

However, one scientific journal is casting doubt on the report. Nature says they’ve heard from five experts saying they want more evidence, questioning whether the material created is actually hydrogen.</s>Metallic hydrogen is a potential wonder substance first proposed by Eugene Wigner and Hillard Bell Huntington back in 1935, but since conditions here on Earth are not extreme enough to create it, its existence has remained theoretical — that is, until now.

Harvard scientists Isaac Silvera and Ranga Dias have created metallic hydrogen by squeezing a hydrogen sample with pressures never before produced on Earth, even greater than the pressure that exists at the center of the planet, reports Phys.org.

"This is the holy grail of high-pressure physics," said Silvera. "It's the first-ever sample of metallic hydrogen on Earth, so when you're looking at it, you're looking at something that's never existed before."



They created it using a synthetic diamond that was immaculately polished to remove even the tiniest of imperfections that might weaken it. Since diamond is one of the hardest materials in nature, researchers were able to use it to create pressures greater than 71.7 million pounds-per-square inch, thus transforming solid molecular hydrogen into atomic hydrogen, which is a metal.

This is important because as a metal, hydrogen can function as a superconductor at room temperature. Furthermore, the material is theorized to remain in its metallic state even after the pressure is removed.

"One prediction that's very important is metallic hydrogen is predicted to be meta-stable," explained Silvera. "That means if you take the pressure off, it will stay metallic, similar to the way diamonds form from graphite under intense heat and pressure, but remains a diamond when that pressure and heat is removed."



The work is described in a paper published in the journal Science.

It's impossible to understate just how important a stable, room temperature superconductor could be. It could, quite seriously, change the world as we know it. Or at least, it could usher in a new era of technological breakthroughs.

For instance, it would make magnetic levitation for high-speed trains far more feasible, revolutionizing our transportation infrastructure. Electric cars could be made immensely more efficient, and the performance of our electronic devices would be greatly enhanced.

That's just scratching the surface, though. Superconductors have zero resistance, so energy could be stored by maintaining currents in superconducting coils, to be used as needed. Furthermore, since it takes such tremendous pressure to create metallic hydrogen, when it's converted back to molecular hydrogen, all of that energy gets released. In other words, it could potentially create the most powerful rocket propellant known to man, making long-distance space travel more feasible than ever before.

"That would easily allow you to explore the outer planets," Silvera said. "We would be able to put rockets into orbit with only one stage, versus two, and could send up larger payloads, so it could be very important."

Researchers still have some work to do before these technologies can be realized, however. First and foremost, they need to test to make sure that the properties of theoretical metallic hydrogen match up with the properties of the real thing. It's still a remarkable accomplishment either way.

"It's a tremendous achievement, and even if it only exists in this diamond anvil cell at high pressure, it's a very fundamental and transformative discovery," said Silvera.</s>Scientists may not have turned hydrogen into metal, a move that was hailed as an astonishing scientific breakthrough.

Two physicists at Harvard University claimed that they had finally managed to do what scientists have been trying to do for almost a century.

That is, crushing hydrogen and turning into metal through an ‘alchemical’ process.

But the scientific community has questioned the claims of Ranga Dias and Isaac Silvera, claiming they have made a mistake when identifying the tiny amount of shiny substance they saw as hydrogen metal.

Dias and Silvera first published their work on arXiv in October, where it attracted a lot of criticism from other scientists who claimed it was based on a mistake. However, it was still published this week in the journal Science.

Five different experts have since spoken to Nature magazine, saying that they don’t believe the claim and they need more evidence.

‘I don’t think the paper is convincing at all,’ Paul Loubeyre, a physicist at the Atomic Energy Commission in Bruyeres-le-Chatel, France.

Dias and Silvera had crushed tiny amounts of hydrogen underneath diamond anvils, putting more pressure on it than is found at the Earth’s core.

What they found was a small amount of shiny material that can be viewed through a microscope. After seeing it, the two said that ‘you can only believe [it] is a metal’.

However, this ‘metal’ that they apparently produced is still stuck between the jaws of the anvil – and the two researchers say that removing it could cause it to completely disappear.

‘It is far from clear that the shiny material the researchers see is actually hydrogen,’ Alexander Goncharov, a geophysicist from the Carnegie Institution for Science in Washington DC, told Nature.

Goncharov suggested that the reflective substance may be alumina (aluminium oxide), which coats the tips of the diamonds in the anvil.

Loubeyre added that they need to redo the measurement, using a much more precise method of measuring the amount of pressure they’re placing.

Silvera, however, said he just wanted to get the information out there before he carried out confirmation tests – which, he said, could break the specimen.

Despite the doubt cast on the study’s results, press releases issued by Science and Harvard University confidently claimed that metallic hydrogen had been made.

‘This is the holy grail of high-pressure physics,’ Silvera says in the Harvard press release. ‘It’s the first ever sample of metallic hydrogen on Earth.’</s>Scientists in the US say they have at last managed to turn hydrogen into a state where it behaves like a metal.

If that is true - and it is a controversial claim - it fulfils a more than 80-year quest to produce what many have said would be a wonder material.

Theory suggests metallic hydrogen could be used to make zero-resistance electrical wiring and super-powerful rocket fuel, among many applications.

Ranga Dias and Isaac Silvera are the Harvard researchers behind the work.

They report their experiments in this week’s Science Magazine.

"It's the first time solid metallic hydrogen has ever existed on Earth," Prof Silvera told the BBC.

The pair say the quantity of metallic hydrogen created so far is tiny - an amount less than the cross section of a human hair. But in time, they believe, ways could be found to boost its production.

Their laboratory approach - as is common in this field of study - was to squeeze a cell containing a small sample of molecular hydrogen between two synthetic diamonds.

In the Science paper, Dias and Silvera say this diamond anvil cell (DAC) achieved pressures up to 495 gigapascals.

That is the equivalent of sitting under almost five million Earth atmospheres. The DAC was also chilled down to close to minus 270 Celsius.

The intention was to pack the atoms of hydrogen so close together that they formed a crystal lattice and started to share their electrons - behaviour that is exactly that of a metal.

Dias and Silvera report that this state became obvious to them when the material in the diamond anvil cell developed a lustrous surface.

"As we turned the pressure up, it went to a transparent molecular solid. And then as the pressure kept going up, it went black, and we think it goes black because it becomes like a semi-conductor and it can absorb light," he recalled on the BBC’s Science In Action programme.

"And then we turned the pressure up higher and it started shining. It was very exciting. It's got extremely high reflectance. The reflectance we measured is about 90%. It's about the reflectivity of an aluminium mirror."

It has to be said, however, that news of the Harvard experiment has already attracted a good degree of scepticism.

Other scientists working in the same and related fields have told the BBC that the team's paper is short on the kind of data needed to make a proper assessment of its achievements.

"Complete garbage," is how Eugene Gregoryanz from Edinburgh University described the research. "Like everybody else who works with hydrogen at high pressures, I am appalled by what is being published in Science."

Such resistance is natural. If what is being claimed pans out, it would represent one of the major physics breakthroughs of recent decades.

Metallic hydrogen was first predicted more than 80 years ago, and there has been a race to create it ever since. That is because of the stunning properties that are promised.

It has been suggested for example that metallic hydrogen might be metastable; that is - once made under extreme conditions it would maintain its state even when brought back up to ambient pressures and temperatures.

And if, as some think, it is also a zero-resistance superconductor, that could lead to a revolution in the transmission and storage of electricity.

The US space agency is also fascinated by the material. Already super-cold liquid hydrogen makes for a very powerful rocket propellant, but the dense metallic form of hydrogen promises to deliver really colossal levels of thrust that would enable huge payloads to be lifted off Earth.

But all this is in the realms of speculation for now. First, it must be shown that the Harvard work can be reproduced. If that's possible - and some are saying that is a very big "if" - then more sizeable quantities of metallic hydrogen need to be created.

"I understand that others in the DAC community have been rather sceptical (arguing that the apparent reflectivity might be coming from contaminants in the sample, the aluminium oxide coating on the diamonds, etc.). However, if they really have achieved nearly 500 gigapascals in the DAC it is not unreasonable to have observed a transition to metallic hydrogen,” commented Marcus Knudson from Sandia National Laboratories.

"The scepticism here is probably a good thing, in that it will drive many groups towards attempting to reproduce this experiment. This publication will certainly incite the field. Again, if it holds up, this is an exciting result. I think in this case time will tell," he told BBC News.

And Jeffrey McMahon from Washington State University concurred: "With respect to the tiny sample amount: Such experiments are performed in small diamond anvil cells. One challenge would be to make a larger quantity (at once); another, perhaps bigger challenge is to recover even the small sample (ie, remove it from the extreme pressures that it is under in the diamond anvil cell).

"Whether the latter is possible is an important open question."

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
Two Harvard scientists report, in the peer-reviewed journal Science, the first creation of metallic hydrogen in a laboratory, using a diamond anvil cell. Other researchers did not agree with this finding when it was first posted on the arXiv preprint server in October 2016, and have not yet changed their positions.