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      Freshwater ice can melt into scallops and spikes

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      Water’s wacky density leads to strange effects that researchers are still uncovering.

      Typically, liquids become denser the more they cool. But freshwater is densest at 4° Celsius. As it cools below that temperature, the water becomes less dense and rises. As a result, ice columns submerged in liquid water can melt into three different shapes, depending on the water’s temperature, researchers report in the Jan. 28 Physical Review Letters.  

      “Almost everything” about the findings was surprising, says mathematician Leif Ristroph of New York University.

      Ristroph and colleagues anchored ultrapure ice cylinders up to 30 centimeters long in place and submerged them in tanks of water at temperatures from 2° to 10° C.

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      The ice melted into smooth, downward-pointing spikes if placed in water lower than about 5° C. Simulations showed “a strange thing — that the cold liquid water near the ice is actually buoyant” due to being less dense than the rest of the water in the tank, Ristroph says. So that upward flow draws warmer water closer to the ice’s base, causing it to melt faster than the top.  

      The opposite occurred above about 7° C; the ice formed an upward-pointing spike. That’s because colder water near the ice is denser than the surrounding water and sinks, pulling in warmer water at the top of the ice and causing it to melt faster than the bottom, simulations showed. This matches “what your intuition would expect,” Ristroph says. 

      Between about 5° to 7° C, the ice melted into scalloped columns. “Basically, the water is confused,” Ristroph says, so it forms different layers, some of which tend to rise and others which tend to sink, depending on their density. Ultimately, the water organizes into “swirls or vortices of fluid that carve the weird ripples into the ice.”

      More work is needed to understand the complex interplay of factors that may generate these and other shapes on ice melting in nature (SN: 4/9/21). More

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      in Space & Astronomy

      Neutron star collisions probably make more gold than other cosmic smashups

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      The cosmic origins of elements heavier than iron are mysterious. One elemental birthplace came to light in 2017 when two neutron-rich dead stars collided and spewed out gold, platinum and other hefty elements (SN: 10/16/17). A few years later, a smashup of another neutron star and a black hole left scientists wondering which type of cosmic clash was the more prolific element foundry (SN: 6/29/21).

      Now, they have an answer. Collisions of two neutron stars probably take the cake, scientists report October 25 in Astrophysical Journal Letters.

      To create heavy elements after either type of collision, neutron star material must be flung into space, where a series of nuclear reactions called the r-process can transform the material (SN: 4/22/16).

      How much material escapes into space, if any, depends on various factors. For example, in collisions of a neutron star and black hole, the black hole has to be relatively small, or “there’s no hope at all,” says astrophysicist Hsin-Yu Chen of MIT. “It’s going to swallow the neutron star right away,” without ejecting anything.

      Questions remain about both types of collisions, spotted via the ripples in spacetime that they kick up. So Chen and colleagues considered a range of possibilities for the properties of neutron stars and black holes, such as the distributions of their masses and how fast they spin. The team then calculated the mass ejected by each type of collision under those varied conditions. In most scenarios, the neutron star–black hole mergers made a smaller quantity of heavy elements than the neutron star duos — in one case only about a hundredth the amount.

      Still, the ultimate element factory ranking remains up in the air. The scientists compared just these two types of collisions, not other possible sources of heavy elements such as exploding stars (SN: 7/7/21). More

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      Work on complex systems, including Earth’s climate, wins the physics Nobel Prize

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      Earth’s climate is a vastly complex system on a grand scale. On a microscopic level, so is the complicated physics of atoms and molecules found within materials. The 2021 Nobel Prize in physics knits together the work of three scientists who illuminated such intricate physical systems by harnessing basic tools of physics. 

      Half of the prize goes to climate scientists Syukuro Manabe of Princeton University and Klaus Hasselmann of the Max Planck Institute for Meteorology in Hamburg, Germany, for their work on simulations of Earth’s climate and predictions of global warming, the Royal Swedish Academy of Sciences announced October 5. The other half of the 10 million Swedish kronor (more than $1.1 million) prize goes to physicist Giorgio Parisi of Sapienza University of Rome, who worked on understanding the roiling fluctuations within disordered materials.

      All three researchers used a similar strategy of isolating a specific piece of a complex system in a model, a mathematical representation of something found in nature. By studying that model, and then integrating that understanding into more complicated descriptions, the researchers made progress on understanding otherwise perplexing systems, says physicist Brad Marston of Brown University. “There’s an art to constructing a model that is rich enough to give you interesting and perhaps surprising results, but simple enough that you can hope to understand it.”

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      The prize, normally an apolitical affair, sends a message to world leaders: “The notion of global warming is resting on solid science,” said Göran Hansson, secretary-general of the Royal Swedish Academy of Sciences, during the announcement of the prize winners. Human emissions of greenhouse gases, including carbon dioxide, have increased Earth’s average temperature by more than 1 degree Celsius since preindustrial times. That warming is affecting every region on Earth, exacerbating extreme weather events such as heat waves, wildfires and drought (SN: 8/9/21). 

      Syukuro Manabe of Princeton University (left) and Klaus Hasselmann of the Max Planck Institute for Meteorology (right) worked on early simulations of Earth’s climate, laying the foundation for today’s more detailed climate models that are used to grapple with the potential impacts of global warming.From left: Bengt Nyman/Wikimedia Commons (CC BY 2.0); Sueddeutsche Zeitung Photo/Alamy Stock Photo

      Manabe’s work laid the foundation for climate modeling, said John Wettlaufer of Yale University, a member of the Nobel Committee for Physics. “He really did construct the models from which all future climate models were built,” Wettlaufer explained during an interview after the prize announcement. “That scaffolding is essential for the improvement of predictions of climate.” 

      Manabe studied how rising carbon dioxide levels would change temperatures on Earth. A simplified climate model from a 1967 paper coauthored by Manabe simulated a single column of the atmosphere in which air masses rise and fall as they warm and cool, which revealed that doubling the amount of carbon dioxide in the atmosphere increased the temperature by over 2 degrees C. This understanding could then be integrated into more complex models that simulated the entire atmosphere or included the effects of the oceans, for example (SN: 5/30/70). 

      “I never imagined that this thing I would begin to study had such huge consequences,” Manabe said at a news conference at Princeton. “I was doing it just because of my curiosity.”

      Hasselmann studied the evolution of Earth’s climate while taking into account the variety of timescales over which different processes operate. The randomness of daily weather stands in contrast to seasonal variations and much slower processes like gradual heating of the Earth’s oceans. Hassleman’s work helped to show how the short-term jitter could be incorporated into models to understand the long-term change in climate. 

      Giorgio Parisi of Sapienza University of Rome is known for his work delving into the physics of disordered materials, such as spin glasses, in which different atoms can’t come to agreement about which direction to point their spins. Lorenza Parisi/Wikimedia Commons

      The prize is an affirmation of scientists’ understanding of climate, says Michael Moloney, CEO of the American Institute of Physics in College Park, Md. “The climate models which we depend on in order to understand the impact of the climate crisis are world-class science up there with all the other great discoveries that are recognized [by] Nobel Prizes of years past.”

      In a spin glass, illustrated here, iron atoms (red), within a lattice of copper atoms (blue), have spins (black arrows) that can’t agree on a direction to point.C. Chang

      Much like the weather patterns on Earth, the inner world of atoms within materials can be complex and disorderly. Parisi’s work took aim at understanding the processes within disordered systems such as a type of material called a spin glass (SN: 10/18/02). In spin glasses, atoms behave like small magnets, due to a quantum property called spin. But the atoms can’t agree on which direction to point their magnets, resulting in a disordered arrangement.

      That’s similar to more familiar types of glass — a material in which atoms don’t reach an orderly arrangement. Parisi came up with a mathematical description for such spin glasses. His work also touches on a variety of other complex topics, from turbulence to flocking patterns that describe the motions of animals such as starlings (SN: 7/31/14). 

      Although his work doesn’t directly focus on climate, in an interview during the Nobel announcement, Parisi commented on that half of the prize: “It’s clear that for the future generation we have to act now in a very fast way.” 

      Carolyn Gramling contributed to reporting this story. More

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      in Physics

      New ‘vortex beams’ of atoms and molecules are the first of their kind

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      Like soft serve ice cream, beams of atoms and molecules now come with a swirl.

      Scientists already knew how to dish up spiraling beams of light or electrons, known as vortex beams (SN: 1/14/11). Now, the first vortex beams of atoms and molecules are on the menu, researchers report in the Sept. 3 Science.

      Vortex beams made of light or electrons have shown promise for making special types of microscope images and for transmitting information using quantum physics (SN: 8/5/15). But vortex beams of larger particles such as atoms or molecules are so new that the possible applications aren’t yet clear, says physicist Sonja Franke-Arnold of the University of Glasgow in Scotland, who was not involved with the research. “It’s maybe too early to really know what we can do with it.”

      In quantum physics, particles are described by a wave function, a wavelike pattern that allows scientists to calculate the probability of finding a particle in a particular place (SN: 6/8/11). But vortex beams’ waves don’t slosh up and down like ripples on water. Instead, the beams’ particles have wave functions that move in a corkscrewing motion as a beam travels through space. That means the beam carries a rotational oomph known as orbital angular momentum. “This is something really very strange, very nonintuitive,” says physicist Edvardas Narevicius of the Weizmann Institute of Science in Rehovot, Israel.

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      Narevicius and colleagues created the new beams by passing helium atoms through a grid of specially shaped slit patterns, each just 600 nanometers wide. The team detected a hallmark of vortex beams: a row of doughnut-shaped rings imprinted on a detector by the atoms, in which each doughnut corresponds to a beam with a different orbital angular momentum.

      Another set of doughnuts revealed the presence of vortex beams of helium excimers, molecules created when a helium atom in an excited, or energized, state pairs up with another helium atom.

      A pattern of rings reveals the presence of vortex beams of atoms and molecules. Each doughnut shape corresponds to a beam of helium atoms with a different angular momentum. Two hard-to-see circles from helium molecules sit in between the center dot and the first two doughnuts left and right of the center.A. Luski et al/Science 2021

      A pattern of rings reveals the presence of vortex beams of atoms and molecules. Each doughnut shape corresponds to a beam of helium atoms with a different angular momentum. Two hard-to-see circles from helium molecules sit in between the center dot and the first two doughnuts left and right of the center.A. Luski et al/Science 2021

      Next, scientists might investigate what happens when vortex beams of molecules or atoms collide with light, electrons or other atoms or molecules. Such collisions are well-understood for normal particle beams, but not for those with orbital angular momentum. Similar vortex beams made with protons might also serve as a method for probing the subatomic particle’s mysterious innards (SN: 4/18/17).

      In physics, “most important things are achieved when we are revisiting known phenomena with a fresh perspective,” says physicist Ivan Madan of EPFL, the Swiss Federal Institute of Technology in Lausanne, who was not involved with the research. “And, for sure, this experiment allows us to do that.” More

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      Windbreaks, surprisingly, could help wind farms boost power output

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      Windbreaks may sound like a counterintuitive idea for boosting the performance of a wind turbine. But physicists report that low walls that block wind could actually help wind farms produce more power.

      Scientists already knew that the output of a single wind turbine could be improved with a windbreak. While windbreaks slow wind speed close to the ground, above the height of the windbreak, wind speeds actually increase as air rushes over the top. But for large wind farms, there’s a drawback. A windbreak’s wake slows the flow of air as it travels farther through the rows of turbines. That could suggest that windbreaks would be a wash for wind farms with many turbines.

      But by striking a balance between these competing effects, windbreaks placed in front of each turbine can increase power output, new computer simulations suggest. It comes down to the windbreaks’ dimensions. Squat, wide barriers are the way to go, according to a simulated wind farm with six rows of turbines. To optimize performance, windbreaks should be a tenth the height of the turbine and at least five times the width of the blades, physicists report July 30 in Physical Review Fluids. Such an arrangement could increase the total power by about 10 percent, the researchers found. That’s the equivalent of adding an additional turbine, on average, for every 10 in a wind farm.

      In the simulations, the wind always came from the same direction, suggesting the technique might be useful in locations where wind tends to blow one way, such as coastal regions. Future studies could investigate how this technique might apply in places where wind direction varies.

      In a computer simulation of a wind farm with 24 turbines, scientists found that windbreaks (red) improved the overall power output. Wakes created by the windbreaks appear in dark blue, and wakes of the turbines are light blue.L. Liu and R.J.A.M. Stevens/Physical Review Fluids 2021, Visualizations by Srinidhi N. Gadde More

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      in Space & Astronomy

      A bounty of potential gravitational wave events hints at exciting possibilities

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      A new crew of potential ripples in spacetime has just debuted — emphasis on the word “potential.”

      By loosening the criteria for what qualifies as evidence for gravitational waves, physicists identified 1,201 possible tremors. Most are probably fakes, spurious jitters in the data that can mimic the cosmic vibrations, the team reports August 2 at arXiv.org. But by allowing in more false alarms, the new tally may also include some weak but genuine signals that would otherwise be missed, potentially revealing exciting new information about the sources of gravitational waves.

      Scientists can now look for signs that may corroborate some of the uncertain detections, such as flashes of light in the sky that flared from the cosmic smashups that set off the ripples. Gravitational waves are typically spawned by collisions of dense, massive objects, such as black holes or neutron stars, the remnants of dead stars (SN: 1/21/21).

      To come up with the new census, physicists reanalyzed six months of data from the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, and Virgo gravitational wave observatories. Scientists had already identified 39 of the events as likely gravitational waves in earlier analyses.

      Eight events that hadn’t been previously identified stand a solid chance of being legitimate — with greater than a 50 percent probability of coming from an actual collision.

      The physicists analyzed the data from those eight events to see how they might have occurred. In one, two black holes may have slammed together, melding into a whopper black hole with about 180 times the mass of the sun, which would make it the biggest black hole merger seen yet (SN: 9/2/20). Another event could be a rare sighting of a black hole swallowing a neutron star (SN: 6/29/21). More

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      Black holes born with magnetic fields quickly shed them

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      Like a shaggy dog in springtime, some black holes have to shed. New computer simulations reveal how black holes might discard their magnetic fields.

      Unlike dogs with their varied fur coats, isolated black holes are mostly identical. They are characterized by only their mass, spin and electric charge. According to a rule known as the no-hair theorem, any other distinguishing characteristics, or “hair,” are quickly cast off. That includes magnetic fields.

      The rule applies to black holes in a vacuum, where magnetic fields can simply slip away. But, says astrophysicist Ashley Bransgrove of Columbia University, “what we were thinking about is what happens in a more realistic scenario.” A magnetized black hole would typically be surrounded by electrically charged matter called plasma, and scientists didn’t know how — or even if — such black holes would undergo hair loss.

      Black holes can be born with magnetic fields or gain them later, for example by swallowing a neutron star, a highly magnetic dead star (SN: 6/29/21). When Bransgrove and colleagues simulated the plasma surrounding a magnetized black hole, they found that a process called magnetic reconnection allows the magnetic field to escape the black hole. The magnetic field lines that map out the field’s direction break apart and reconnect. Loops of magnetic field form around blobs of plasma, some of which blast outward, while others fall into the black hole. That process eliminates the black hole’s magnetic field, the researchers report in the July 30 Physical Review Letters.

      Magnetic reconnection in balding black holes could spew X-rays that astronomers could detect. So scientists may one day glimpse a black hole losing its hair. More

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      Gravitational waves reveal the first known mergers of a black hole and neutron star

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      Caught in a fatal inward spiral, a neutron star met its end when a black hole swallowed it whole. Gravitational ripples from that collision spread outward through the cosmos, eventually reaching Earth. The detection of those waves marks the first reported sighting of a black hole engulfing the dense remnant of dead star. And in a surprise twist, scientists spotted a second such merger just days after the first.

      Until now, all identified sources of gravitational waves were twos of a kind: either two black holes or two neutron stars, spiraling around one another before colliding and coalescing (SN: 1/21/21). The violent cosmic collisions create waves that stretch and squeeze the fabric of spacetime, undulations that can be sussed out by sensitive detectors.

      The mismatched pairing of a black hole and neutron star was the final type of merger that scientists expected to find with current gravitational wave observatories. By pure coincidence, researchers spotted two of these events within 10 days of one another, the LIGO, Virgo and KAGRA collaborations report in the July 1 Astrophysical Journal Letters.

      Not only have unions between black holes and neutron stars not been seen before via gravitational waves, the smashups have also never been spotted at all by any other means.

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      “This is an absolute first look,” says theoretical physicist Susan Scott of the Australian National University in Canberra, a member of the LIGO collaboration.

      The result adds another tick mark to the tally of new discoveries made with gravitational waves. “That’s worth celebration,” says astrophysicist Cole Miller of the University of Maryland in College Park, who was not involved with the research. Since the first gravitational waves were detected in 2015, the observatories keep revealing new secrets. “It’s fantastic new things; it’s not just the same old, same old,” he says.

      Signs of the black hole-neutron star collisions registered in the LIGO and Virgo gravitational wave observatories in 2020, on January 5 and January 15. The first merger consisted of a black hole about 8.9 times the mass of the sun and a neutron star about 1.9 times the sun’s mass. The second merger had a 5.7 solar mass black hole and a 1.5 solar mass neutron star. Both collisions occurred more than 900 million light-years from Earth, the scientists estimate.

      To form detectable gravitational waves, the objects that coalesce must be extremely dense, with identities that can be pinned down by their masses. Anything with a mass above five solar masses could only be a black hole, scientists think. Anything less than about three solar masses must be a neutron star.

      One earlier gravitational wave detection involved a black hole merging with an object that couldn’t be identified, as its mass seemed to fall in between the cutoffs that separate black holes and neutron stars (SN: 6/23/20). Another previous merger may have resulted from a black hole melding with a neutron star, but the signal from that event wasn’t strong enough for scientists to be certain that the detection was the real deal. The two new detections clinch the case for black hole and neutron star meetups.

      One of the new events is more convincing than the other. The Jan. 5 merger was seen in just one of LIGO’s two gravitational wave detectors, and the signal has a relatively high probability of being a false alarm, Miller says. “If this were the only event, then you would not be as confident.” The Jan. 15 event, however, “seems pretty solid,” he says.

      Epic rendezvous between neutron stars and black holes happen regularly throughout the cosmos, the detections suggest. Based on the pace of detections, the researchers estimate that these events take place about once a month within 1 billion light-years of Earth.

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      In a newly reported class of cosmic smashup, a neutron star (apparent in orange in this computer simulation, after the video zooms in) and black hole (dark gray) spiral inward, producing gravitational waves (blue) in a dance that ends when the black hole swallows the neutron star.

      Scientists don’t yet know how neutron stars and black holes come to meet up. They might form together, as two stars that orbit one another until both run out of fuel and die, with one collapsing into a black hole and the other forming a neutron star. Or the two objects might have formed separately and met up in a crowded region packed with many neutron stars and black holes.

      As a black hole and neutron star spiral inward and merge, scientists expect that the black hole could rip the neutron star to shreds, producing a light show that could be observed with telescopes. But astronomers found no fireworks in the aftermath of the two newly reported encounters, nor any evidence that the black holes deformed the neutron stars.

      That could be because in both cases the black hole was significantly larger than the neutron star, suggesting that the black hole gulped down the neutron star whole in a meal worthy of Pac-Man, Scott says.

      If scientists could spot a black hole shredding a neutron star in the future, that could help researchers pin down the properties of the ultradense, neutron-rich material that makes up the dead stars (SN: 4/20/21).

      In past detections of gravitational waves, the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, based in the United States, has teamed up with Virgo, in Italy. The new observations are the first to include members of a third observatory, KAGRA, in Japan (SN: 1/18/19). But the KAGRA detector itself didn’t contribute to the results, as scientists were still preparing it to detect gravitational waves at the time. LIGO, Virgo and KAGRA are all currently offline while scientists tinker with the detectors, and will resume their communal search for cosmic collisions in 2022. More