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    Experiments hint at why bird nests are so sturdy

    To build its nest, a bird won’t go for any old twig. Somehow, birds pick and choose material that will create a cozy, sturdy nest.

    “That’s just totally mystifying to me,” says physicist Hunter King of the University of Akron in Ohio. Birds seem to have a sense for how the properties of an individual stick will translate to the characteristics of the nest. That relationship “is something we don’t know the first thing about predicting,” King says.

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    A bird’s nest is a special version of a granular material: a substance, such as sand, made up of many smaller objects (SN: 4/30/19). King and colleagues combined laboratory experiments and computer simulations to better understand the quirks of nestlike granular materials, the researchers report in a study to appear in Physical Review Letters.

    In the experiments, a piston repeatedly compressed 460 bamboo rods scattered inside a cylinder. The computer simulations let researchers analyze the points where sticks touched, which is key to understanding the material, the team says.

    The more force the piston applied to the pile, the stiffer the pile became, meaning it resisted further deformation. As the piston bore down, sticks slid against one another, and the contact points between them rearranged. That stiffened the pile by allowing additional contact points to form between sticks, which prevented them from flexing further, the simulations showed.

    Changes in the pile’s stiffness seemed to lag behind the piston’s motion, a phenomenon called hysteresis. That effect caused the pile to be stiffer when the piston pushed in than when the material bounced back as the piston retracted. Simulations suggest that the hysteresis arose because the initial friction between sticks needed to be overcome before the contact points started to rearrange.

    Beyond bird nests, this research could be applied to other materials made of disordered arrangements of long fibers, such as felt. With a better understanding of the physical qualities of such materials, engineers could use them to create new structures designed to protect not only bird eggs, but other cargo that humans consider precious. More

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    Gravitational waves gave a new black hole a high-speed ‘kick’

    This black hole really knows how to kick back.

    Scientists recently observed two black holes that united into one, and in the process got a “kick” that flung the newly formed black hole away at high speed. That black hole zoomed off at about 5 million kilometers per hour, give or take a few million, researchers report in a paper in press in Physical Review Letters. That’s blazingly quick: The speed of light is just 200 times as fast.

    Ripples in spacetime, called gravitational waves, launched the black hole on its breakneck exit. As any two paired-up black holes spiral inward and coalesce, they emit these ripples, which stretch and squeeze space. If those gravitational waves are shot off into the cosmos in one direction preferentially, the black hole will recoil in response.

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    It’s akin to a gun kicking back after shooting a bullet, says astrophysicist Vijay Varma of the Max Planck Institute for Gravitational Physics in Potsdam, Germany.

    Gravitational wave observatories LIGO and Virgo, located in the United States and Italy, detected the black holes’ spacetime ripples when they reached Earth on January 29, 2020. Those waves revealed details of how the black holes merged, hinting that a large kick was probable. As the black holes orbited one another, the plane in which they orbited rotated, or precessed, similar to how a top wobbles as it spins. Precessing black holes are expected to get bigger kicks when they merge.

    So Varma and colleagues delved deeper into the data, gauging whether the black hole got the boot. To estimate the kick velocity, the researchers compared the data with various predicted versions of black hole mergers, created based on computer simulations that solve the equations of general relativity, Einstein’s theory of gravity (SN: 2/3/21). The recoil was so large, the researchers found, that the black hole was probably ejected from its home and kicked to the cosmic curb.

    Dense groups of stars and black holes called globular clusters are one locale where black holes are thought to partner up and merge. The probability that the kicked black hole would stay within a globular cluster home is only about 0.5 percent, the team calculated. For a black hole in another type of dense environment, called a nuclear star cluster, the probability of sticking around was about 8 percent.

    The black hole’s great escape could have big implications. LIGO and Virgo detect mergers of stellar-mass black holes, which form when a star explodes in a supernova and collapses into a black hole. Scientists want to understand if black holes that partner up in crowded clusters could partner up again, going through multiple rounds of melding. If they do, that could help explain some surprisingly bulky black holes previously seen in mergers (SN: 9/2/20). But if merged black holes commonly get rocketed away from home, that would make multiple mergers less likely.

    “Kicks are very important in understanding how heavy stellar-mass black holes form,” Varma says.

    Previously, astronomers have gleaned evidence of gravitational waves giving big kicks to supermassive black holes, the much larger beasts found at the centers of galaxies (SN: 3/28/17). But that conclusion hinges on observations of light, rather than gravitational waves. “Gravitational waves, in a way, are cleaner and easier to interpret,” says astrophysicist Manuela Campanelli of the Rochester Institute of Technology in New York, who was not involved in the new study.

    LIGO and Virgo data had already revealed some evidence of black holes getting small kicks. The new study is the first to report using gravitational waves to spot a black hole on the receiving end of a large kick.

    That big kick isn’t a surprise, Campanelli says. Earlier theoretical predictions by Campanelli and colleagues suggested that such powerful kicks were possible. “It’s always exciting when someone can measure from observations what you predicted from calculations.” More

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    Levitating plastic beads mimic the physics of spinning asteroids

    Some asteroids can barely hold it together.

    Rather than solid lumps of rock, ‘rubble pile’ asteroids are loose collections of material, which can split apart as they rotate (SN: 3/16/20). To understand the inner workings of such asteroids, one team of scientists turned to levitating plastic beads. The beads clump together, forming collections that can spin and break up, physicist Melody Lim of the University of Chicago reported March 15 at a meeting of the American Physical Society in Chicago.

    It’s an elegant dance that mimics the physics of asteroid formation, which happens too slowly to observe in real-life space rocks. “These ‘tabletop asteroids’ compress phenomena that take place over kilometers [and] over hundreds of thousands of years to just centimeters and seconds in the lab,” Lim said. The results are also reported in a paper accepted in Physical Review X.

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    Lim and colleagues used sound waves to levitate the plastic beads, which arranged themselves into two-dimensional clumps. Acoustic forces attract the beads to one another, mimicking the gravitational attraction between bits of debris in space. Separate clumps then coalesced similarly to how asteroids are thought to glom onto one another to grow.

    [embedded content]
    Levitated by sound waves, plastic beads, which are about 150 micrometers across, clump together into a loosely bound 2-D conglomeration (shown at 1/50th the original speed). When spun too fast, one such structure deforms then splits apart (shown at 1/70th the original speed).

    When the experimenters gave the structures a spin using the sound waves, the clumps changed shape above a certain speed, becoming elongated. That could help scientists understand why ‘rubble pile’ asteroids, can have odd structures, such as the ‘spinning tops’ formed by asteroids Bennu and Ryugu (SN: 12/18/18).

    Eventually, the fast-spinning clumps broke apart. This observation could help explain why asteroids are typically seen to spin up to a certain rate, but not beyond: Speed demons get split up. More

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    Freshwater ice can melt into scallops and spikes

    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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    Neutron star collisions probably make more gold than other cosmic smashups

    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

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

    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

    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