Everyone ages. Growing old is a fundamental feature of human existence.
Though we might not always be aware of aging, it looms in all of our futures. As Science News editor in chief Eva Emerson writes, “Aging happens to each of us, everywhere, all the time. It is so ever-present and slow that we tend to take little notice of it. Until we do.”
But, our scientific understanding of aging pales in comparison to its significance in our lives. While new studies reveal exciting prospects for slowing the effects of aging, its causes and extensive effects remain enigmatic. Scientists are still divided on some fundamentals of aging, and that’s why aging research raises some interesting questions. For example, how does it change the brain? How did different life histories evolve? How old is the oldest blue whale? This special report addresses those questions and more.
In Colorado’s Rocky Mountains, male and female valerian plants have responded differently to hotter, drier conditions, a new study shows. Rapidly changing ratios of the sexes could be a quick sign of climate change, the researchers say.
Valerian (Valeriana edulis) plants range from hot, scrubby lowlands to cold alpine slopes. In each patch of plants, some are male and some are female. The exact proportion of each sex varies with elevation. High on the mountain, females are much more common than males; they can make up 80 percent of some populations. Four decades ago, in patches of valerian growing in the middle of the plant’s elevation range, 33.4 percent of the plants were males. Those patches grew in the Rockies at elevations around 3,000 meters. Today, you would have to hike considerably higher to find the same proportion of male plants. Males, now 5.5 percent more common on average, are reaching higher elevations than in the past, researchers report in the July 1 Science.
“We think climate is acting almost like a filter on males and females,” says Will Petry of ETH Zurich, who led the study while at the University of California, Irvine. “The settings on this filter are controlling the sex ratio.” Those settings are sweeping up the mountainside like a rising tide at a rate of 175 meters per decade, Petry and colleagues found. Ecologists already knew that the ratio of male to female plants can vary with altitude or water availability, says ecologist Spencer Barrett of the University of Toronto, who was not involved in this study. But “the idea that a sex ratio is moving upslope — nobody’s ever done that before.”
Those moving sex ratios have kept pace with climate change since the late 1970s. Today, winter snows are melting earlier and summers are hotter, with less rain. As a result, the same amount of precipitation that would have fallen at one elevation in 1978 now falls at higher elevations instead; it has moved upslope by 133 meters per decade. Soil moisture has moved up the mountain, too, by 195 meters per decade.
The parallel shifts mean that changing sex ratios could be a marker of climate change, says population biologist Tom Miller of Rice University in Houston, a coauthor of the study. Today, movements of whole species — often up in latitude or altitude — are a hallmark of climate change. But proportions of males and females are changing “substantially faster than species are moving,” Miller says. They “might be a much more rapid fingerprint of climate change than where species are migrating to.” Petry’s team found that fingerprint while hiking around the Rocky Mountain Biological Laboratory in Crested Butte, Colo. As the scientists walked through the mountains in Chaffee and Gunnison counties, they counted flowering males and females at 31 sites in 2011, then compared their modern data with historical counts from nine of the same populations, made by coauthor Judy Soule from 1978 to 1980. When Petry saw that the percentage of males and females had changed, “we also started thinking about the consequences,” he says.
If one sex vastly outnumbers the other, populations could die out. “Imagine if it became an Amazonia situation,” says Kailen Mooney, whose lab at UC Irvine led the new study. A 100 percent female population wouldn’t be pollinated, and would disappear once the mature females died, he says.
If those female-only populations grew above a certain altitude and died out because males couldn’t reach them, then male plants would set the upper boundary for the whole species. Sex ratios “add nuance” to the way scientists think about climate-driven migration, Mooney says, because one sex could determine geographic limits for whole species.
A new book on aging starts with what sounds like a promise: “It is a common belief that aging is inevitable and universal. Nothing could be further from the truth.” From this, you might expect the final pages to offer a list of options for fending off the ravages of time. But this is less a how-to guide and more of a dive into why aging happens. The authors, theoretical biologist Josh Mitteldorf and writer Dorion Sagan, take an extensive stroll through evolutionary theory and aging research in support of an off-center view. After pointing out problems with several theories of why aging evolved, the authors present the controversial premise that aging is a programmed march toward oblivion that evolved as a form of population control. “Aging in animals enforces a common, predictable life span, helping to prevent the dominance of any one individual or one gene type. Diversity is preserved for the health of the community.” Other researchers have been skeptical of that idea.
Aging, however, is unyielding. The authors describe how certain hardships — starvation, exertion, even small amounts of poison — can paradoxically lead to life extension in lab animals. From these findings, Mitteldorf and Sagan make antiaging recommendations that start with familiar medical advice: exercise, lose weight and take a daily aspirin or ibuprofen. But then they jump to suggestions that have not yet been proven, including supplementation with “huge doses of vitamin D” and melatonin, plus metformin (a diabetes drug) and selegiline (a drug used to treat early Parkinson’s and depression). Next comes a list of herbs that could restore telomeres, the protective tips of chromosomes. The book spends much less real estate describing the research behind all of these recommendations, perhaps because the human studies haven’t been done yet.
The crystal ball section of the book is an optimistic look at very preliminary research on the benefits of lengthening telomeres, removing senescent cells from the body and regrowing the shrinking thymus, the organ that produces immune system T cells. The authors may be onto something. But none of these ideas have yet had a chance to mature.
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NASA’s Juno spacecraft has sent back its first picture of Jupiter since arriving at the planet July 4 (SN: 7/23/16, p. 14). The image, taken July 10 when the spacecraft was 4.3 million kilometers from Jupiter, shows off the planet’s clouds, its Great Red Spot (a storm a bit wider than Earth) and three of its moons (Io, Europa and Ganymede).
Juno is on the outbound leg of its first of two 53.5-day orbits of the gas giant (Juno will then settle into 14-day orbits). During orbit insertion, all of Juno’s scientific instruments were turned off while the spacecraft made its first dive through the harsh radiation belts that encircle the planet. This first image indicates that Juno is in good health and ready to study the largest planet in the solar system.
The probe is the ninth to visit Jupiter and the second to stay in orbit (SN: 6/25/2016, p. 32). For the next 20 months, Juno will investigate what lurks beneath the opaque clouds that enshroud the planet (SN: 6/25/2016, p. 16). The spacecraft won’t take its first intimate pictures of Jupiter until August 27, when it flies within 5,000 kilometers of the cloud tops.
Standoffish electrons typically keep one another at arm’s length, repelling their neighbors. But surprisingly, under certain circumstances, this repulsion can cause pairs of electrons to soften their stance toward one another and attract instead, new research shows. The effect may be the key to someday producing a new type of high-temperature superconductor, scientists report in the July 21 Nature.
Though the effect was first predicted over 50 years ago, previous attempts to coerce electrons to behave in this chummy way have failed. Like charges repel, so negatively charged electrons ordinarily rebuff one another. But now researchers have validated the counterintuitive idea that an attraction between electrons can emerge. “Somehow, you have [this] magic that out of all this repulsion you can create attraction,” says study coauthor Shahal Ilani, a physicist at the Weizmann Institute of Science in Rehovot, Israel. Ilani and colleagues produced the effect in a bare-bones system of electrons in carbon nanotubes. Operating at temperatures just above absolute zero, the system is made up of two perpendicular carbon nanotubes — hollow cylinders of carbon atoms — about 1 nanometer in diameter.
Two electrons sit at sites inside the first nanotube. Left to their own devices, those two electrons repel one another. A second nanotube, known as the “polarizer,” acts as the “glue” that allows the two electrons to attract. When the scientists brought the two nanotubes close together, says Ilani, “the electrons in the first nanotube changed their nature; they became attractive instead of repulsive.”
This flip is due to the nature of the polarizer. It contains one electron, which is located at one of two sites in the carbon nanotube — either between the first nanotube’s pair of electrons or farther away. The pair of electrons in the first nanotube repels the polarizer’s electron, kicking it from the near to the far site. And the electron’s absence leaves behind a positively charged vacancy, which attracts the pair of electrons toward it — and toward each other. It’s a “tour de force,” says Takis Kontos, a physicist at the École Normale Supérieure in Paris, who wrote a commentary on the paper in the same issue of Nature. Although the system the scientists created is very simple, he says, “the whole experiment built around it is extremely complex.” Electrons are known to attract in certain situations. In conventional superconductors, electrons pair up due to their interactions with ions in the material. This buddy system allows superconductors to conduct electricity without resistance. But such superconductors must be cooled to very low temperatures for this effect to occur.
But in 1964, physicist William Little of Stanford University theorized that electron pairs could likewise attract due to their interactions with other electrons, instead of ions. Such pairs should stay linked at higher temperatures. This realization sparked hopes that a material with these attracting electrons could be a room-temperature superconductor, which would open up a wealth of technological possibilities for efficiently transmitting and storing energy.
It’s yet to be seen whether the effect can produce a superconductor, and whether such a superconductor might work at higher temperatures — the new discovery shows only that the attraction can occur due to electrons’ repulsion. It’s “the first important step,” says Ilani. Now, scientists can start thinking of how to build “interesting new materials that are very different than what you can find in nature.”
U.S. drivers love to hit the road. The problem is doing so safely.
In 2013, 32,894 people in the United States died in motor vehicle crashes. Although down since 2000, the overall death rate — 10.3 per 100,000 people — tops 19 other high-income countries, the U.S. Centers for Disease Control and Prevention reported July 8. Belgium is a distant second with 6.5 deaths per 100,000. Researchers reviewed World Health Organization and other data on vehicle crash deaths, seat belt use and alcohol-impaired driving in 2000 and 2013. Canada had the highest percentage of fatal crashes caused by drunk drivers: 33.6 percent. New Zealand and the United States tied for second at 31 percent. But Canada and 16 other countries outperformed the United States on seat belt use — even though, in 2013, 87 percent of people in the United States reported wearing safety belts while riding in the front seat.
Spain saw the biggest drop — 75 percent — in its crash death rate. That country improved nearly all aspects of road safety, including decreasing alcohol-impaired driving and increasing seat belt use, the researchers say.
The newest and thorniest members of a diverse ant family may have extra help holding their heads high.
Found in the rainforests of Papua New Guinea, Pheidole drogon and Pheidole viserion worker ants have spines protruding from their thoraxes. For many ant species, the spiky growths are a defense against birds and other predators. But Eli Sarnat and colleagues suggest the spines might instead be a muscular support for the ants’ oversized heads, which the insects use to crush seeds. The heads “are so big that it looks like it would be difficult to walk,” says Sarnat, an entomologist at the Okinawa Institute of Science & Technology Graduate University in Japan. Micro‒CT scans of worker ants with larger heads revealed bundles of thoracic muscle fibers within spines just behind their heads. Worker ants with smaller heads did not have muscles in their spines, the researchers report online July 27 in PLOS One. More research is needed to establish the spines’ function and understand why they evolved, Sarnat says. While buff spines may support big heads, hollow spines probably keep predators at bay, the researchers suspect. Researchers named the ants after two fearsome dragons, Drogon and Viserion, in the popular book and TV series Game of Thrones.
Thirst drove one of the last populations of woolly mammoths to extinction.
A small group of holdouts on an isolated Alaskan island managed to last about 8,000 years longer than most of their mainland-dwelling brethren. But by about 5,600 years ago, the island’s lakes — the only source of freshwater — became too small to support the mammoths (Mammuthus primigenius), scientists report online the week of August 1 in the Proceedings of the National Academy of Sciences. “I don’t think I’ve ever seen something so conclusive about an extinction before,” says Love Dalén, an evolutionary geneticist at the Swedish Museum of Natural History in Stockholm who was not involved in the research. The study highlights “how sensitive small populations are and how easily they can become extinct.”
Surprisingly recent woolly mammoth bones had previously been discovered in a cave on St. Paul Island, which became isolated from the mainland roughly 14,000 years ago. Since there’s no evidence that prehistoric humans lived on St. Paul, the find provided a chance to study extinction in the absence of human influence, says Russell Graham, a paleontologist at Penn State who led the study.
The scientists extracted a core of sediment from a lake bed near the cave to see how environmental conditions had changed over the last 11,000 years. The team found remnants of ancient plants, animals and fungi in the sediment — including traces of mammoth DNA in some layers. By analyzing and dating the different sediment layers, the team could infer when and how the mammoths went extinct.
“We initially thought that vegetation change and habitat would be the major driving factor,” Graham says. Instead, his team found a wealth of evidence — including an increase in salt-tolerant algae and crustaceans 6,000 years ago — suggesting freshwater shortages as the culprit. A warmer climate after the last Ice Age ended contributed to the St. Paul mammoths’ downfall. Sea level rise shrank the mammoths’ island habitat and cut into their freshwater supplies by raising the water table and making the lake saltier over time, the team concluded. Warmer, drier conditions also caused water to evaporate more quickly from the lake surface.
The study highlights an often-overlooked vulnerability of island and coastal communities. Some islands in the South Pacific are currently experiencing similar freshwater shortages thanks to rising seas, Graham says – and Florida could be next in line. That’s particularly bad news for large island-dwelling and coastal mammals, which tend to need more water to survive than smaller species.
A small device with a heart of crystal can eavesdrop on muscles and nerves, scientists report August 3 in Neuron. Called neural dust, the device is wireless and needs no batteries, appealing attributes for scientists seeking better ways to monitor and influence the body and brain.
“It’s certainly promising,” says electrical engineer Khalil Najafi of the University of Michigan in Ann Arbor. “They have a system that operates, and operates well.”
Michel Maharbiz of the University of California, Berkeley and colleagues presented their neural dust idea in 2013. But the paper in Neuron represents the first time the system has been used in animals. Neural dust detected activity when researchers artificially stimulated rats’ sciatic nerves and muscles. Unlike other devices that rely on electromagnetic waves, neural dust is powered by ultrasound. When hit with ultrasound generated by a source outside the body, a specialized crystal begins to vibrate. This mechanical motion powers the system, allowing electrodes to pick up electrical activity. This activity can then change ultrasound signals that travel back to the source, offering a readout in a way that’s similar to a sonar measurement.
Neural dust devices may help scientists avoid some of the problems with current implants, such as a limited life span. Implantable devices can falter in the brain’s hostile environment. “It’s like throwing a piece of electronics in the ocean and wanting it to run for 20 years,” Maharbiz says. “Eventually things start to degrade and break down.” But having a simple, small device may increase the life span of such implants — although Maharbiz and colleagues don’t yet know how long their system could last.
What’s more, the brain can mount a defense against the foreign object, which can result in thick tissue surrounding the implant. Smaller systems damage the brain less. At over 2 millimeters long and just under 1 millimeter wide, a particle of the neural dust described in the paper is larger than most actual specks of dust. But the system is still shrinking. “There’s a lot of room here to just really push it, and that’s what excites us,” Maharbiz says. “You can keep getting smaller and smaller and smaller.”
Neural dust could ultimately be used to detect different sorts of data in the body, not just electrical activity, Maharbiz says. The device could be tweaked to sense temperature, pressure, oxygen or pH.
Najafi cautions that it remains to be seen whether the system will prove useful for listening to nerve cell behavior inside the brain. The system would need to include many different pieces of neural dust, and it’s not clear how effective that would be. “It’s a lot harder than the notion of dust implies,” he says.
In a few highly specialized laboratories, scientists bombard matter with the world’s most powerful electrical pulses or zap it with sophisticated lasers. Other labs squeeze heavy-duty diamonds together hard enough to crack them.
All this is in pursuit of a priceless metal. It’s not gold, silver or platinum. The scientists’ quarry is hydrogen in its most elusive of forms.
Several rival teams are striving to transform hydrogen, ordinarily a gas, into a metal. It’s a high-stakes, high-passion pursuit that sparks dreams of a coveted new material that could unlock enormous technological advances in electronics. “Everybody knows very well about the rewards you could get by doing this, so jealousy and envy [are] kind of high,” says Eugene Gregoryanz, a physicist at the University of Edinburgh who’s been hunting metallic hydrogen for more than a decade.
Metallic hydrogen in its solid form, scientists propose, could be a superconductor: a material that allows electrons to flow through it effortlessly, with no loss of energy. All known superconductors function only at extremely low temperatures, a major drawback. Theorists suspect that superconducting metallic hydrogen might work at room temperature. A room-temperature superconductor is one of the most eagerly sought goals in physics; it would offer enormous energy savings and vast improvements in the transmission and storage of energy.
Metallic hydrogen’s significance extends beyond earthly pursuits. The material could also help scientists understand our own solar system. At high temperatures, compressed hydrogen becomes a metallic liquid — a form that is thought to lurk beneath the clouds of monstrous gas planets, like Jupiter and Saturn. Sorting out the properties of hydrogen at extreme heat and high pressure could resolve certain persistent puzzles about the gas giants. Researchers have reported brief glimpses of the liquid metal form of hydrogen in the lab — although questions linger about the true nature of the material.
While no lab has yet produced solid metallic hydrogen, the combined efforts of many scientists are rapidly closing in on a more complete understanding of the element itself — as well as better insight into the complex inner workings of solids. Hydrogen, the first element in the periodic table and the most common element in the universe, ought to be easy to understand: a single proton paired with a single electron. “What could be more simple than an assembly of electrons and protons?” asks theoretical physicist Neil Ashcroft of Cornell University. But at high pressures, the physics of hydrogen rapidly becomes complex.
At room temperature and atmospheric pressure, hydrogen is a gas. But like other materials, altered conditions can transform hydrogen into a solid or a liquid. With low enough temperatures or a sufficiently forceful squeeze, hydrogen shape-shifts into a solid. Add heat while squeezing, and it becomes a liquid.
If subjected to still more extreme conditions, hydrogen can — at least theoretically — undergo another transformation, into a metal. All metals have one thing in common: They conduct electricity, due to free-flowing electrons that can go where they please within the material. Squeeze anything hard enough and it will become a metal. “Pressure does a great job of dislodging the outer electrons,” Ashcroft says. This is what scientists are aiming to do with hydrogen: create a sloshing soup of roving electrons in either a liquid or a solid.
When hydrogen is compressed, many atoms begin to interact with one another, while paired in molecules of two hydrogen atoms each. The underlying physics becomes a thorny jumble. “It is amazing; the stuff takes up incredibly complex arrangements in the solid state,” says Ashcroft, the first scientist to propose, in 1968, that metallic hydrogen could be a high-temperature superconductor.
Hydrogen’s complexity fascinates scientists. “It’s not just the metallization question that’s of interest to me,” says Russell Hemley, a chemist at the Carnegie Institution for Science in Washington, D.C., and Lawrence Livermore National Laboratory in California. Studying the intricacies of hydrogen’s behavior can help scientists refine their understanding of the physics of materials.
In 1935, when physicists Eugene Wigner and Hillard Bell Huntington of Princeton University first predicted that compressed solid hydrogen would be metallic, they thought the transition to a metal might occur at a pressure 250,000 times that of Earth’s atmosphere. That may sound like a lot, but scientists have since squeezed hydrogen to pressures more than 10 times as high — and still no solid metal.
Scientists originally expected that the transition would be a simple flip to metallic behavior. Not so, says theoretical physicist David Ceperley of the University of Illinois at Urbana-Champaign. “Nature has a lot more possibilities.” Solid hydrogen exists in multiple forms, each with a different crystal structure. As the pressure climbs, the wily hydrogen molecules shift into ever-more-complex arrangements, or phases. (For physicists, the “phase” of matter goes deeper than the simple states of solid, liquid or gas.) The number of known solid phases of hydrogen has grown steadily as higher pressures are reached, with four phases now well established. The next phase scientists find could be a metal — they hope.
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Timeline: The race to make metallic hydrogen Rival research teams are rushing to transform solid or liquid hydrogen into a metal. With each experiment, the pressure rises. They have two very different aims: a room-temperature superconductor and a window into the gas giants. If solid metallic hydrogen turns out to be a room-temperature superconductor, it would have to be crushed to work, making it impractical for many applications. But if hydrogen could hold its metallic form after the pressure is released, as some researchers have suggested, “it would be revolutionary,” says physicist Isaac Silvera, who leads the metallic hydrogen hunt at Harvard University. Such a material could be used in electrical wires to reduce loss of energy and decrease the world’s power consumption. And it might lead to efficient, magnetically levitated trains and technological advances in nuclear fusion, supercomputing and more.
While one group of would-be metallurgists is searching for solid metal, other investigators seek the scientifically intriguing liquid hydrogen metal. Their techniques differ in timescale and size. To produce liquid metal, scientists violently slam hydrogen for fractions of a second at a time, using enormous machines at national laboratories. Scientists searching for a solid metal, on the other hand, use fist-sized devices to capture hydrogen between the tips of two tiny diamonds and slowly squeeze.
Diamonds have it rough Crushing an ethereal, normally gaseous substance between two diamonds sounds nearly impossible. Such tricky experiments make for a field where researchers are in regular disagreement over their latest results. “We’re still missing high-quality, reliable data,” says physicist Alexander Goncharov. “The issue is the experiments are too challenging.”
In his office at the Geophysical Laboratory of the Carnegie Institution, Goncharov opens a desk drawer and pulls out a device called a diamond anvil cell. The cylinder of metal is small enough for Goncharov to cradle in his palm. Bits of precisely machined steel and tough tungsten carbide are held together with four screws. Through portals in the top and bottom, bright sparkles shimmer: diamonds.
Inside the capsule, two diamonds taper to tiny points a few hundredths of a millimeter wide. They pinch the material within, squashing it at over a million times atmospheric pressure. The gap between the minuscule anvils can be as small as a few thousandths of a millimeter, about the size of a human red blood cell.
Once they’ve been pressurized, diamond anvil cells will hold the pressure almost indefinitely. The prepared cells can be carried around — inspected in the laboratory, transported to specialized facilities around the world — or simply stored in a desk drawer. Goncharov regularly travels with them. (Tip from the itinerant scientist: If questions arise at airport security, “never use the word ‘diamonds.’ ”) The diamond anvil can squeeze more than hydrogen — materials from iron to sodium to argon can be crushed in the diamond vise. To identify new, potentially metallic phases of solid hydrogen within the pressurized capsules, scientists shine laser light onto the material, measuring how molecules vibrate and rotate — a technique called Raman spectroscopy (SN: 8/2/08, p. 22). If a new phase is reached, molecules shift configurations, altering how they jiggle. Certain types of changes in how the atoms wobble are a sign that the new phase is metallic. If the material conducts electricity, that’s another dead giveaway. A final telltale sign: The normally translucent hydrogen should acquire a shiny, reflective surface.
Significant hurdles exist for diamond anvil cell experiments. Diamonds, which cost upwards of $600 a pop, can crack under such intense pressures. Hydrogen can escape from the capsule, or diffuse into the diamonds, weakening them. So scientists coat their diamonds with thin layers of protective material. The teams each have their own unique recipe, Goncharov says. “Of course, everyone believes that their recipe is the best.”
Three phases of solid hydrogen have been known since the late 1980s. With the discovery of a fourth phase in 2011, “the excitement was enormous,” Gregoryanz says. In Nature Materials, Mikhail Eremets and Ivan Troyan at the Max Planck Institute for Chemistry in Mainz, Germany, reported that a new phase appeared when they squashed room-temperature hydrogen to over 2 million times atmospheric pressure. Goncharov, Gregoryanz and colleagues created the new phase and deduced its structure in Physical Review Letters in 2012. In phase IV, as it’s known, hydrogen arranges itself into thin sheets — somewhat like the single-atom-thick sheets of carbon known as graphene, the scientists wrote.
Progress doesn’t come easy. With each new paper, scientists disagree about what the results mean. When Eremets and colleagues discovered the fourth phase, they thought they also had found metallic hydrogen (SN: 12/17/11, p. 9). But that assertion was swiftly criticized, and it didn’t stand up to scrutiny.
The field has been plagued by hasty claims. “If you look at the literature for the last 30 years,” Gregoryanz says, “I think every five years there is a claim that we finally metallized hydrogen.” But the claims haven’t been borne out, leaving scientists perpetually skeptical of new results.
In a recent flurry of papers, scientists have proposed new phases — some that might be metallic or metal-like precursors to a true metal — and they are waiting to see which claims stick. Competing factions have volleyed papers back and forth, alternately disagreeing and agreeing.
A paper from Gregoryanz’s group, published in Nature in January, provided evidence for a phase that was enticingly close to a metal (SN Online: 1/6/16), at more than 3 million times atmospheric pressure. But other scientists disputed the evidence. In their own experiments, Eremets and colleagues failed to confirm the new phase. In a paper posted online at arXiv.org just days after Gregoryanz’s paper was published, Eremets’ team unveiled hints of a “likely metallic” phase, which occurred at a different temperature and pressure than Gregoryanz’s new phase.
A few months later, Silvera’s group squeezed hydrogen hard enough to make it nearly opaque, though not reflective — not quite a metal. “We think we’re just below the pressure that you need to make metallic hydrogen,” Silvera says. His findings are consistent with Eremets’ new phase, but Silvera disputes Eremets’ speculations of metallicity. “Every time they see something change they call it metallic,” Silvera says. “But they don’t really have evidence of metallic hydrogen.”
All this back and forth may seem chaotic, but it’s also a sign of a swiftly progressing field, the researchers say. “I think it’s very healthy competition,” Gregoryanz says. “When you realize that somebody is getting ahead of you, you work hard.”
The current results are “not very well consistent, so everybody can criticize each other,” Eremets says. “For me it’s very clear. We should do more experiments. That’s it.”
There are signs of progress. In 2015, Eremets and colleagues discovered a record-breaking superconductor: hydrogen sulfide, a compound of hydrogen and sulfur. When tightly compressed into solid form, hydrogen sulfide superconducts at temperatures higher than ever seen before: 203 kelvins (−70° Celsius) (SN: 8/8/15, p. 12).
Adding sulfur to the mix stabilizes and strengthens the hydrogen structure but doesn’t contribute much to its superconducting properties. Hydrogen sulfide is so similar to pure hydrogen, Eremets says, that, “in some respects we already found superconductivity in metallic hydrogen.”
Brief glimmers Giant, gassy planets are chock-full of hydrogen, and the element’s behavior under pressure could explain some of these planets’ characteristics. A sea of flowing liquid hydrogen metal may be the source of Jupiter’s magnetic field (SN: 6/25/16, p. 16). Learning more about metallic hydrogen’s behavior deep inside such planets could also help resolve a long-standing puzzle regarding Saturn: The ringed behemoth is unexpectedly bright. The physics of hydrogen’s interactions with helium inside the planet could provide the explanation.
Using a radically different set of technologies, a second band of scientists is on the hunt for such liquid metallic hydrogen. These researchers have gone big, harnessing the capabilities of new, powerful machines designed for nuclear fusion experiments at government-funded national labs. These experiments show the most convincing evidence of metallic behavior so far — but in hydrogen’s liquid, not solid, form. These enormous machines blast hydrogen for brief instants, temporarily sending pressures and temperatures skyrocketing. Such experiments reach searing temperatures, thousands of kelvins. With that kind of heat, metallic hydrogen appears at lower, more accessible pressures. Creating such conditions requires sophisticated equipment. The Z machine, located at Sandia National Laboratories in Albuquerque, generates extremely intense bursts of electrical power and strong magnetic fields; for a tiny instant, the machine can deliver about 80 terawatts (one terawatt is about the total electrical power–generating capacity in the entire United States).
A group of scientists recently used the Z machine to launch a metal plate into a sample of deuterium — an isotope of hydrogen with one proton and one neutron in its nucleus — generating high pressures that compressed the material. The pummeled deuterium showed reflective glimmers of shiny metal, the scientists reported last year in Science. “It starts out transparent, it goes opaque, and then later we see this reflectivity increase,” says Marcus Knudson of Sandia and Washington State University in Pullman.
Another group is pursuing a different tactic, using some of the most advanced lasers in the world, at the National Ignition Facility at Lawrence Livermore. Scientists there zap hydrogen to produce high pressures and temperatures. Though the conclusions of this experiment are not yet published, says physicist Gilbert Collins of Lawrence Livermore, one of the leaders of the experiment, “we have some really beautiful results.”
The first experiment to show evidence of liquid metallic hydrogen was performed at Lawrence Livermore in the 1990s. A team of physicists including William Nellis, now at Harvard, used a sophisticated gunlike apparatus to shoot projectiles into hydrogen at blisteringly fast speeds. The resulting hydrogen briefly conducted electricity (SN: 4/20/96, p. 250).
These experiments face hurdles of their own — it’s a struggle to measure temperature in such systems, so scientists calculate it rather than measuring it directly. But many researchers are still convinced by these results. Metallic hydrogen “certainly has been produced by shock techniques,” Cornell’s Ashcroft says.
Some scientists still have questions. “It’s certainly difficult to tell if something is a metal or not at such high temperature,” Collins says. Although they need high temperatures to reach the metal liquid phase, some physicists define a metal based on its behavior at a temperature of absolute zero. Current experiments hit high pressures at temperatures as close to zero as possible to produce relatively cool liquid metallic hydrogen.
Scientists who conduct palm-sized diamond anvil cell experiments refuse to be left out of the liquid-metal action. They’ve begun to use laser pulses to heat and melt hydrogen crammed into the cells. The results have stirred up new disagreements among competing groups. In April’s Physical Review B, Silvera and coauthors reported forming liquid metallic hydrogen. But under similar conditions, Goncharov and others found only semiconducting hydrogen, not a metal. They reported their results in Physical Review Letters in June.
“There’s kind of a crisis now with these different experiments,” says Illinois theorist Ceperley. “And there’s a lot of activity trying to see who’s right.” For now, scientists will continue refining their techniques until they can reach agreement.
The major players have managed to reach consensus before. Four phases of solid hydrogen are now well established, and researchers agree on certain conditions under which solid hydrogen melts.
Solid metallic hydrogen, however, has perpetually seemed just out of reach, as theoretical predictions of the pressure required to produce it have gradually shifted upward. As the goalposts have moved, physicists have reached further, achieving ever-higher pressures. Current theoretical predictions put the metal tantalizingly close — perhaps only an additional half a million times atmospheric pressure away.
The quest continues, propelled by a handful of hydrogen-obsessed scientists.
“We all love hydrogen,” Collins says. “It has the essence of being simple, so that we think we can calculate something and understand it, while at the same time it has such a devious nature that it’s perhaps the least understandable material there is.”
