Quantum physics requires imaginary numbers to explain reality

Imaginary numbers might seem like unicorns and goblins — interesting but irrelevant to reality.

But for describing matter at its roots, imaginary numbers turn out to be essential. They seem to be woven into the fabric of quantum mechanics, the math describing the realm of molecules, atoms and subatomic particles. A theory obeying the rules of quantum physics needs imaginary numbers to describe the real world, two new experiments suggest.

Imaginary numbers result from taking the square root of a negative number. They often pop up in equations as a mathematical tool to make calculations easier. But everything we can actually measure about the world is described by real numbers, the normal, nonimaginary figures we’re used to (SN: 5/8/18). That’s true in quantum physics too. Although imaginary numbers appear in the inner workings of the theory, all possible measurements generate real numbers.

Quantum theory’s prominent use of complex numbers — sums of imaginary and real numbers — was disconcerting to its founders, including physicist Erwin Schrödinger. “From the early days of quantum theory, complex numbers were treated more as a mathematical convenience than a fundamental building block,” says physicist Jingyun Fan of the Southern University of Science and Technology in Shenzhen, China.
Some physicists have attempted to build quantum theory using real numbers only, avoiding the imaginary realm with versions called “real quantum mechanics.” But without an experimental test of such theories, the question remained whether imaginary numbers were truly necessary in quantum physics, or just a useful computational tool.

A type of experiment known as a Bell test resolved a different quantum quandary, proving that quantum mechanics really requires strange quantum linkages between particles called entanglement (SN: 8/28/15). “We started thinking about whether an experiment of this sort could also refute real quantum mechanics,” says theoretical physicist Miguel Navascués of the Institute for Quantum Optics and Quantum Information Vienna. He and colleagues laid out a plan for an experiment in a paper posted online at arXiv.org in January 2021 and published December 15 in Nature.

In this plan, researchers would send pairs of entangled particles from two different sources to three different people, named according to conventional physics lingo as Alice, Bob and Charlie. Alice receives one particle, and can measure it using various settings that she chooses. Charlie does the same. Bob receives two particles and performs a special type of measurement to entangle the particles that Alice and Charlie receive. A real quantum theory, with no imaginary numbers, would predict different results than standard quantum physics, allowing the experiment to distinguish which one is correct.

Fan and colleagues performed such an experiment using photons, or particles of light, they report in a paper to be published in Physical Review Letters. By studying how Alice, Charlie and Bob’s results compare across many measurements, Fan, Navascués and colleagues show that the data could be described only by a quantum theory with complex numbers.

Another team of physicists conducted an experiment based on the same concept using a quantum computer made with superconductors, materials which conduct electricity without resistance. Those researchers, too, found that quantum physics requires complex numbers, they report in another paper to be published in Physical Review Letters. “We are curious about why complex numbers are necessary and play a fundamental role in quantum mechanics,” says quantum physicist Chao-Yang Lu of the University of Science and Technology of China in Hefei, a coauthor of the study.

But the results don’t rule out all theories that eschew imaginary numbers, notes theoretical physicist Jerry Finkelstein of Lawrence Berkeley National Laboratory in California, who was not involved with the new studies. The study eliminated certain theories based on real numbers, namely those that still follow the conventions of quantum mechanics. It’s still possible to explain the results without imaginary numbers by using a theory that breaks standard quantum rules. But those theories run into other conceptual issues, making them “ugly,” he says. But “if you’re willing to put up with the ugliness, then you can have a real quantum theory.”

Despite the caveat, other physicists agree that the quandaries raised by the new findings are compelling. “I find it intriguing when you ask questions about why is quantum mechanics the way it is,” says physicist Krister Shalm of the National Institute of Standards and Technology in Boulder, Colo. Asking whether quantum theory could be simpler or if it contains anything unnecessary, “these are very interesting and thought-provoking questions.”

Neandertals were the first hominids to turn forest into grassland 125,000 years ago

Neandertals took Stone Age landscaping to a previously unrecognized level.

Around 125,000 years ago, these close human relatives transformed a largely forested area bordering two central European lakes into a relatively open landscape, say archaeologist Wil Roebroeks of Leiden University in the Netherlands, and his colleagues. Analyses of pollen, charcoal, animal fossils and other material previously unearthed at two ancient lake basins in Germany provide the oldest known evidence of hominids reshaping their environments, the scientists report December 15 in Science Advances.

The excavated areas are located within a site called Neumark-Nord. Neandertals’ daily activities there, apparently ongoing throughout the year, had a big environmental impact, the researchers suspect. Those pursuits, which occurred over a span of about 2,000 years, included setting campfires, butchering game, collecting wood, making tools and constructing shelters, they say.

“We might be dealing with larger and less mobile groups of [Neandertals] than commonly acknowledged,” Roebroeks says, thanks in part to warming temperatures after around 150,000 years ago that cleared ice sheets from resource-rich locations such as Neumark-Nord.
His team can’t say whether Neandertals set fires to clear large tracts of land at Neumark-Nord, a practice that has been observed among some modern hunter-gatherers. The geological remnants of many small campfires may look much like those of a small number of large fires, Roebroeks says.

Finds at Neumark-Nord play into an ongoing debate about when humans began to have a dominating influence on the natural world. Some scientists regard this period as a new geological epoch, the Anthropocene (SN: 4/1/13). It’s unclear when the Anthropocene began and whether its roots extend back to the Stone Age.

Regular fire use by members of the Homo genus began around 400,000 years ago (SN: 4/2/12). Evidence of human occupations associated with increased fire setting and shifts to open habitats date to around 40,000 years ago in Australia; 45,000 years ago in highland New Guinea; and 50,000 years ago in Borneo.

Analyses of lake cores and stone-tool sites in southern-central Africa indicate that fires set by increasing numbers of humans kept the landscape open even as rainy conditions conducive to forest growth developed around 85,000 years ago. Open environments still predominate in this part of Africa, Yale University paleoanthropologist Jessica Thompson and her colleagues reported May 5 in Science Advances. “Humans and close human relatives like Neandertals have likely been [modifying] their ecosystems for a very long time,” Thompson says.

A large coal mining operation revealed ancient Neumark-Nord sediments in 1985. German scientists then excavated a large lakeside site, wrapping up that project in the mid-1990s. The same team excavated a smaller site at a lake basin located about 100 meters from the first site between 2004 and 2008.

Pollen from these sites indicates that grasses and herbs, hallmarks of an open landscape, appeared in a brief window of time around 125,000 years ago, Roebroeks and his colleagues say. Large numbers of stone artifacts — some showing signs of having been heated, possibly to make finished edges sharper — and animal bones displaying butchery marks date to the same time at Neumark-Nord, when Neandertals but not Homo sapiens inhabited Europe.
Stone tools and bone fragments displaying signs of heating, burned wood, charred seeds and dense patches of charcoal particles suggested that Neandertals had frequently set fires near the Neumark-Nord lakes.

Pollen from two other sites in the same mountainous part of Germany, where researchers previously found small numbers of stone tools suggesting a limited Neandertal presence, show that forests dominated there when Neandertals inhabited Neumark-Nord’s grasslands. That strengthens the view that Neandertals altered the Neumark-Nord landscape rather than settling there after forests had shrunk, Roebroeks says.

Archaeologist Manuel Will of Eberhard Karls University of Tübingen in Germany agrees. “Neandertal evidence from Neumark-Nord should be a wake-up call for the international scientific community to include archaeologists [studying] the Paleolithic record as part of any team trying to define and identify the beginning of the Anthropocene,” says Will, who did not participate in the new study.

The Parker Solar Probe is the first spacecraft to visit the sun’s atmosphere

For the first time, a spacecraft has made contact with the sun. During a recent flyby, NASA’s Parker Solar Probe entered the sun’s atmosphere.

“We have finally arrived,” Nicola Fox, director of NASA’s Heliophysics Science Division in Washington, D.C., said December 14 in a news briefing at the fall meeting of the American Geophysical Union. “Humanity has touched the sun.”

Parker left interplanetary space and crossed into solar territory on April 28, 2021, during one of its close encounters with the sun. While there, the probe took the first measurements of exactly where this boundary, called the Alfvén critical surface, lies. It was about 13 million kilometers above the sun’s surface, physicists reported at the meeting, held online and in New Orleans, and in Physical Review Letters on December 14.

“We knew the Alfvén critical surface had to exist,” solar physicist Justin Kasper of the University of Michigan in Ann Arbor said at the news briefing. “We just didn’t know where it was.”
Finding this crucial layer was one of Parker’s main goals when it launched in 2018 (SN: 7/5/18). The Alfvén critical surface is important because it marks where packets of plasma can separate from the sun and become part of the solar wind, the speedy stream of charged particles that constantly emanates from the sun (SN: 8/18/17). The solar wind and other, more dramatic forms of space weather can wreak havoc on Earth’s satellites and even on life (SN: 2/26/21). Scientists want to pinpoint exactly how the wind gets started to better understand how it can impact Earth.

The Alfvén critical surface also may hold the key to one of the biggest solar mysteries: why the sun’s corona, its wispy outer atmosphere, is so much hotter than the sun’s surface (SN: 8/20/17). With most heat sources, temperatures drop as you move farther away. But the sun’s corona sizzles at more than a million degrees Celsius, while the surface is only a few thousand degrees.

In 1942, physicist Hannes Alfvén proposed a solution to the mystery: A type of magnetic wave might carry energy from the solar surface and heat up the corona. It took until 2009 to directly observe such waves, in the lower corona, but they didn’t carry enough energy there to explain all the heat (SN: 3/19/09). Solar physicists have suspected that what happens as those waves climb higher and meet the Alfvén critical surface might play a role in heating the corona. But until now, scientists didn’t know where this frontier began.

With the boundary identified, “we’ll now be able to witness directly how coronal heating happens,” Kasper said.

As Parker crossed the invisible boundary, its instruments recorded a marked increase in the strength of the local magnetic field and a drop in the density of charged material. Out in the solar wind, waves of charged particles gush away from the sun. But below the Alfvén critical surface, some of those waves bend back toward the surface of the sun.
Surprisingly, Parker’s measurements showed that the Alfvén critical surface is wrinkly. “That was one of the big outstanding questions,” says solar physicist Craig DeForest of the Southwest Research Institute in Boulder, Colo., who is a member of the Parker probe team but was not part of this measurement.

“There was some debate in the community about whether the Alfvén surface would exist as a surface at all,” he says. Decades ago, scientists imagined the boundary as a smooth sphere surrounding the sun like a snow globe. More recently, some thought it would be so ragged that it wouldn’t be apparent when the spacecraft crossed it.

Neither of those images turned out to be correct. The surface is smooth enough that the moment of crossing was noticeable, Kasper said. But during the spacecraft’s close approach to the sun in April, it crossed in and out of the boundary three times. The first dip lasted about five hours, the last only half an hour.

“The surface clearly has some structure and warp to it,” Kasper said.

That structure could influence everything from the way solar eruptions leave the sun to the way the solar wind interacts with itself farther out from the sun, DeForest says. “That has consequences that we don’t know yet, but are likely to be profound,” he says. “This is very exciting. It’s terra incognita.”

Parker is still orbiting the sun and planning to make several more close approaches over the next few years, eventually getting within 6 million kilometers of the solar surface. That should bring Parker into the solar corona again and again, solar physicist Nour Raouafi of the Johns Hopkins Applied Physics Laboratory in Laurel, Md., said in the news briefing. The spacecraft may have made another journey past the Alfvén critical surface in August and will have another opportunity in January.

“The expectation is that as we fly closer and closer to the sun, we’ll keep crossing this boundary,” Raouafi said. But the boundary might not be in the same place every time. As the sun’s activity changes, the level of the Alfvén critical surface is expected to rise and fall as if the corona is breathing in and out, he said.

That’s another thing that scientists hope to observe for the first time.

Huge numbers of fish-eating jaguars prowl Brazil’s wetlands

In a tract of central Brazilian wetlands, jaguars spend their days wading through chest-deep waters searching for fish. When not hunting, the big cats playfully grapple with each other back on land. Their life is unlike any other known jaguar population’s existence in the world.

New findings reveal a degree of flexibility in diet and lifestyle previously unseen among jaguars. The discovery may provide key context on the cats’ role in food webs, helping scientists better understand the effect of environmental changes on the species, researchers report October 6 in Ecology.

Jaguars (Panthera onca), which are usually territorial loners that hunt on land, live in a wide array of habitats, ranging from North American deserts to grasslands and tropical rainforests in Central and South America. The cats are also found in the Pantanal, an immense tropical wetland — the largest of its kind in the world — that sprawls over parts of Brazil, Bolivia and Paraguay.

Ecologists Manoel dos Santos-Filho of the Universidade do Estado de Mato Grosso in Cáceres, Brazil, and Carlos Peres of the University of East Anglia in Norwich, England, knew of rumors of large numbers of jaguars sighted near Brazil’s Taiamã Ecological Station. That large ecological reserve is located in the remote, northern reaches of the Pantanal.
After relaying these anecdotes to Taal Levi, a wildlife ecologist at Oregon State University in Corvallis, the researchers started a project to better understand the jaguars’ biology and population status in the protected area.

Taiamã is seasonally flooded, with no roads or trails, so the team had to access the reserve by boat, setting up motion-activated cameras along waterways to gather data on jaguar numbers. The area’s abundance of jaguars, however, was obvious immediately.

“You set your foot out of the boat, and there’s a jaguar footprint there already,” says Charlotte Eriksson, a wildlife scientist also at Oregon State University. “There are scratches on trees. There are jaguar scats. There’s just an unbelievable presence of this apex predator wherever you go, which is something I’ve never experienced anywhere before.”

The team deployed 59 cameras, which operated from 2014 to 2018, and collected more than 1,500 videos of jaguars. The researchers also captured 13 jaguars and fitted them with GPS or radio-tracking collars to gain insight into the animals’ population density, movements and social interactions.

Based on their data, Eriksson and colleagues estimate that the Taiamã Ecological Station hosts the highest density of jaguars ever recorded: 12.4 animals per 100 square kilometers, nearly triple some of the next highest estimates elsewhere. Jaguars were also the most common mammal spotted on the cameras.

Video footage showed jaguars carrying off large fish. When the team analyzed 138 scat samples, the researchers found 46 percent had fish remains in them and 55 percent contained aquatic reptiles, such as caiman or turtles. Just 11 percent contained mammal remains.
Jaguars are well-documented in taking on challenging prey, including underwater fare (SN: 7/15/16). Eriksson and her team think that the Taiamã felines have not only the most fish-dependent diet among jaguars, but also among all big cats. There are tigers in Bangladesh that live in flooded mangrove forests and sometimes eat fish, but those cats still primarily eat land-based food, the researchers say.

The cameras and tracking collars also showed that the Taiamã jaguars were spending a lot of time near each other, sometimes traveling, fishing and playing together. This is all exceptionally odd behavior for jaguars, at least based on what scientists know about the cats elsewhere in the world.

In terms of social behavior, “what we knew of jaguars from before this study is basically that they are solitary, and they meet up to mate. And that’s about it,” Eriksson says, noting anecdotes of the cats sharing prey carcasses as rare counterexamples.
The profusion of aquatic prey in the flooded preserve — protected from human encroachment — may be responsible for the jaguars’ superlative density and their rich social lives. It’s possible there’s so much food available, Eriksson says, that there is “no real need to fight over it.”

Another idea is that aquatic prey concentrated along the river margins are accessible in only certain areas, Levi says. This may encourage jaguar territories to dissolve, since obtaining access to multiple fishing spots requires getting along with other jaguars. Other animals behave in similar ways. Brown bears, for example, congregate in great numbers to feed at salmon spawning grounds, despite the bears’ typically solitary nature, Levi says.

The abundance of jaguars and their social behavior is not surprising, given the available food resources, says Todd Fuller, a conservation biologist at the University of Massachusetts Amherst. Still, he finds the new information exciting.

Fuller, who was not involved with the research, says the study helps bring researchers’ understanding of jaguars’ ecology and conservation closer to what’s known about most other large cat species, and “that is a very good thing.”

Jaguars in the Pantanal face many threats and are declining within Brazil, Eriksson says, suffering from drought, fire and agricultural expansion. Evaluating how jaguars might respond to such changes is paramount. In 2020, half of the study area burned, so Eriksson is currently assessing the impact of the fires on the jaguars and their periodically submerged home.

She also wants to investigate how the Taiamã jaguars’ taste for fish is affecting how often the animals eat land-living prey and what strategies the cats use to catch fish.

“We think we know a lot about these charismatic, large predators,” she says, “but there are still things to learn.”

How missing data makes it harder to measure racial bias in policing

From 2012 to 2015, a team of researchers collected 2.9 million police officer patrol records in Chicago. The team’s analysis of that data, from nearly 7,000 officers, showed that Black police officers were less likely to arrest civilians than white police officers patrolling the same neighborhood (SN: 2/11/21). Officers arrested on average eight people per shift, with Black officers making 24 percent fewer arrests than white officers. But an alternate analysis, one that excluded shifts where no arrests occurred, flipped the results. That made it appear as if Black officers issued 12 percent more arrests than white officers.

Failing to account for events that don’t happen — police allowing a jaywalker to pass, opting not to make an arrest (usually for minor issues like possessing a small amount of drugs) or never firing a drawn gun — is problematic, says policing expert Dean Knox of the University of Pennsylvania. “Instead of drawing the conclusion that minority officers are engaging in less enforcement,” he says of his Chicago study, “you could mistakenly conclude that they are engaging in more enforcement.” The flip occurred because, compared with white officers, Black officers more often went out on patrols without issuing any arrests.

Nonevents of this nature are commonly excluded in policing data. Though a large body of evidence suggests that police in the United States discriminate against Black people, Knox says, many police departments only collect data on a smattering of the interactions between their officers and civilians. Cell phone videos, like those of Eric Garner in a chokehold and George Floyd struggling to breathe, tend to emerge only when encounters have spiraled out of control. That makes it difficult to measure racial bias in policing or come up with targeted solutions to reduce that bias.

How, though, can researchers studying policing account for nonevents? The laborious Chicago data collection by Knox and his team is not always feasible. And even that rigorous study, reported in Science earlier this year, still had gaps: The team had data on when police stopped, arrested or used force on civilians, but not on minor interactions that didn’t meet the department’s recording requirements.

When research teams accept these problematic datasets at face value, writes Knox in a November 4 essay in Science, they often arrive at contradictory conclusions. Disagreements in the literature allow public officials and the media to cherry-pick studies that support their viewpoint, whether arguing for or against implicit bias training to overcome unconscious stereotypes or prioritizing the recruitment of minority officers.
A long chain of events
Knox wrote the essay following the publication of a controversial, and now retracted, study that appeared in 2019 in the Proceedings of the National Academy of Sciences. “White officers are not more likely to shoot minority civilians than non-White officers,” the authors of that study wrote. They concluded that policies aimed at increasing police diversity would do little to stem racial disparities in police killings.

The study gained enormous traction, especially among conservative media outlets and politicians, Knox says. “This was one of the go-to pieces that people use to deny the existence of bias in policing.”

But the authors’ findings were mathematically baseless, says Knox, who along with Jonathan Mummolo, a policing expert at Princeton University, wrote an article debunking the study in Medium. Some 800 academics and researchers signed the piece. The team failed to consider total police encounters and then measure what fraction of those encounters resulted in deadly violence, Knox says.
But that narrow focus on fatal police shootings, a rare occurrence that typically happens at the culmination of a long chain of events, ignores all potential biases earlier in the chain, Knox says. The first potential bias in a chain of events starts with an officer’s decision to approach a civilian or let them pass. Knox acknowledges that a separate layer of research is needed to account for societal level disparities, such as the presence of more officers in Black, often impoverished, neighborhoods and longstanding discriminatory practices that reduce the quality of education and other services in such neighborhoods.

“Even if you can’t see all the things that happened before, just acknowledging they exist is imperative,” Knox says.

Consider this real-life example. On July 10, 2015, Texas state trooper Brian Encinia pulled over Sandra Bland, a Black woman, for failing to signal a lane change. The exchange grew heated and culminated with Encinia arresting Bland for failing to follow orders. Bland’s subsequent death in a county jail caused public outcry.

Focusing solely on Bland’s arrest, and not all that happened before, would provide little information on how Bland wound up in jail for such a minor offense, or how to prevent such an outcome in the future. But because Encinia’s body camera recorded the entire exchange, policing researchers, in this case interested in tone and language, could identify key steps leading up to her arrest. For instance, the researchers reported in Law and Society Review in 2017, Encinia’s language starts off polite but becomes increasingly agitated as Bland refuses to comply with his orders. His once formal commands, such as “step out of the car” become informal and unprofessional: “I’m going to yank you out of here.”

That word “yank” indicates that Encinia is losing control of the situation, says Belén Lowrey-Kinberg, a criminologist at St. Francis College in New York City. Previous research has shown that when officers pivot from formal to informal language, violence can follow.

While this is a case study of a single event, the study provides “a great example of how situations can escalate,” says criminologist Justin Nix of the University of Nebraska Omaha.

Fixing flawed data
Flawed police data does not need to be thrown out, Knox says. His team has developed an algorithm to account for gaps in the data at all points in a police-civilian interaction. The algorithm weights the various possible degrees of discrimination at each point in a chain of events — perhaps race did not factor into Encinia’s decision to pull Bland over because he could not see her face, for example, or maybe race played a large role because most drivers in that area are white. The range of values resulting from the summation of those events suggests the possible amounts of discrimination in any given scenario, Knox says.

The program operates on a very general principle, Knox says. “What are the data that you see?” and “What are the data that you don’t see?”

Thinking about the whole chain of events also points to how to collect better statistics.

Consider a study of police shootings by Nix and policing expert John Shjarback of Rowan University in Glassboro, N.J., that appeared November 10 in PLOS One. The researchers were interested in racial disparities in officers’ use of force against Black and white civilians. National databases include only shootings that result in a civilian’s death. But whether someone lives or dies after being shot hinges on several factors, such as proximity to a trauma center, location of the gunshot wound and access to first aid. So researchers sought to examine all police shootings, including those that resulted in injury but not death. To do so, they relied on records from four states — California, Colorado, Florida and Texas — that have collected this information for years.
The data revealed that some 45 percent of victims suffer nonfatal injuries. Factoring in the relative populations of Black and white civilians showed that for all four states, racial disparities in injuries were higher than racial disparities in fatalities. For example, from 2009 to 2014 in Florida, Black people were roughly three times more likely than whites to be shot and killed by police, but over five times more likely to be injured. Across all four states, and for reasons that are not entirely clear, Black victims are 7 percent less likely to die of their injuries than white victims.

National databases that only include records of civilians who die at the hands of the police underestimate officers’ use of deadly force against Black civilians, Nix says. Death “is the end of a very long sequence of events. In our paper we backed up one link in the chain.” That is, the researchers looked at all instances where officers used deadly force and not just those that resulted in death.

Knox is now working with two police departments to break down police-civilian encounters in more detail. Those departments require officers to turn on their body cameras when they believe an interaction with a civilian will rise to the level of an official interaction. (Officers have discretion at this point in the process, Knox acknowledges, so as with the Chicago study, that first link in the chain remains elusive.) Knox and his team will analyze scripts from each encounter for language and tone, such as normal voice or shouting — a quantitative version of the approach Lowrey-Kinberg used to unpack the encounter between Encinia and Bland. Computer vision techniques will parse out gestures, such as “weapon drawn.” Knox says he hopes the data will help his team get closer to reconstructing entire interactions, including identifying nonevents in any given chain.

“You don’t want just the side of the story as written by an officer,” Knox says. “You want the whole interaction.”

How massive stars in binary systems turn into carbon factories

The next time you thank your lucky stars, you might want to bless the binaries. New calculations indicate that a massive star whose outer layer gets torn off by a companion star ends up shedding a lot more carbon than if the star had been born a loner.

“That star is making about twice as much carbon as a single star would make,” says Rob Farmer, an astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany.

All life on Earth is based on carbon, the fourth most abundant element in the cosmos, after hydrogen, helium and oxygen. Like nearly every chemical element heavier than helium, carbon is formed in stars (SN: 2/12/21). For many elements, astronomers have been able to pin down the main source. For example, oxygen comes almost entirely from massive stars, most of which explode, while nitrogen is made mostly in lower-mass stars, which don’t explode. In contrast, carbon arises both in massive and lower-mass stars. Astronomers would like to know exactly which types of stars forged the lion’s share of this vital element.

Farmer and his colleagues looked specifically at massive stars, which are at least eight times heavier than the sun, and calculated how they behave with and without partners. Nuclear reactions at the core of a massive star first turn hydrogen into helium. When the core runs out of hydrogen, the star expands, and soon the core starts converting helium into carbon.

But massive stars usually have companion stars, adding a twist to the storyline: When the star expands, the companion’s gravity can tear off the larger star’s outer envelope, exposing the helium core. That allows freshly minted carbon to stream into space via a flow of particles.

“In these very massive stars, these winds are quite strong,” Farmer says. For instance, his team’s calculations indicate that the wind of a star born 40 times as massive as the sun with a close companion ejects 1.1 solar masses of carbon before dying. In comparison, a single star born with the same mass ejects just 0.2 solar masses worth of carbon, the researchers report in a paper submitted to arXiv.org October 8 and in press at the Astrophysical Journal.

If the massive star then explodes, it also can outperform a supernova from a solo massive star. That’s because, when the companion star removes the massive star’s envelope, the helium core shrinks. This contraction leaves some carbon behind, outside the core. As a result, nuclear reactions can’t convert that carbon into heavier elements such as oxygen, leaving more carbon to be cast into space by the explosion. Had the star been single, the core would have destroyed much of that carbon.

By analyzing the output from massive stars of different masses, Farmer’s team concludes that the average massive star in a binary ejects 1.4 to 2.6 times as much carbon through winds and supernova explosions as the average massive star that’s single.

Given how many massive stars are in binaries, astronomer Stan Woosley says emphasizing binary-star evolution, as the researchers have done, is helpful in pinning down the origin of a crucial element. But “I think they are making too strong a claim based on models that may be sensitive to uncertain physics,” says Woosley, of the University of California, Santa Cruz. In particular, he says, mass-loss rates for massive stars are not known well enough to assert a specific difference in carbon production between single and binary stars.

Farmer acknowledges the uncertainty, but “the overall picture is sound,” he says. “The binaries are making more [carbon].”