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    A proposed ‘quantum compass’ for songbirds just got more plausible

    Scientists could be a step closer to understanding how some birds might exploit quantum physics to navigate.

    Researchers suspect that some songbirds use a “quantum compass” that senses the Earth’s magnetic field, helping them tell north from south during their annual migrations (SN: 4/3/18). New measurements support the idea that a protein in birds’ eyes called cryptochrome 4, or CRY4, could serve as a magnetic sensor. That protein’s magnetic sensitivity is thought to rely on quantum mechanics, the math that describes physical processes on the scale of atoms and electrons (SN: 6/27/16). If the idea is shown to be correct, it would be a step forward for biophysicists who want to understand how and when quantum principles can become important in various biological processes.

    In laboratory experiments, the type of CRY4 in retinas of European robins (Erithacus rubecula) responded to magnetic fields, researchers report in the June 24 Nature. That’s a crucial property for it to serve as a compass. “This is the first paper that actually shows that birds’ cryptochrome 4 is magnetically sensitive,” says sensory biologist Rachel Muheim of Lund University in Sweden, who was not involved with the research.

    Scientists think that the magnetic sensing abilities of CRY4 are initiated when blue light hits the protein. That light sets off a series of reactions that shuttle around an electron, resulting in two unpaired electrons in different parts of the protein. Those lone electrons behave like tiny magnets, thanks to a quantum property of the electrons called spin.

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    The two electrons’ magnets can point either parallel to one another or in opposite directions. But quantum physics dictates that the electrons do not settle on either arrangement. Rather they exist in a limbo called a quantum superposition, which describes only the probability of finding the electrons in either configuration.

    Magnetic fields change those probabilities. That, in turn, affects how likely the protein is to form an altered version instead of returning to its original state. Birds may be able to determine their orientation in a magnetic field based on how much of the altered protein is produced, although that process is not yet understood. “How does the bird perceive this? We don’t know,” says chemist Peter Hore of the University of Oxford, a coauthor of the new study.

    The idea that cryptochromes play a role in birds’ internal compasses has been around for decades, but “no one could confirm this experimentally,” says Jingjing Xu of the University of Oldenburg in Germany. So in the new study, Xu, Hore and colleagues observed what happened when the isolated proteins were hit with blue laser light. After the laser pulse, the researchers measured how much light the sample absorbed. For robin CRY4, the addition of a magnetic field changed the amount of absorbance, a sign that the magnetic field was affecting how much of the altered form of the protein was produced.

    When the researchers performed the same test on CRY4 found in nonmigratory chickens and pigeons, the magnetic field had little effect. The stronger response to the magnetic field in CRY4 from a migratory bird “could suggest that maybe there is really something special about the cryptochromes of migratory birds that use this for a compass,” says biophysicist Thorsten Ritz of the University of California, Irvine.

    But laboratory tests with chickens and pigeons have shown that those birds can sense magnetic fields, Ritz and Muheim both note. It’s not clear whether the higher sensitivity of robin CRY4 in laboratory tests is a result of evolutionary pressure for migratory birds to have a better magnetic sensor.

    One factor making interpretation of the results more difficult is that experiments on isolated proteins don’t match the conditions in birds’ eyes. For example, Xu says, scientists think the proteins may be aligned in one direction within the retina. To further illuminate the process, the researchers hope to perform future studies on actual retinas, to get a literal bird’s-eye view. More

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    Physicists used LIGO’s mirrors to approach a quantum limit

    Quantum mechanics usually applies to very small objects: atoms, electrons and the like. But physicists have now brought the equivalent of a 10-kilogram object to the edge of the quantum realm.

    Scientists with the Advanced Laser Interferometer Gravitational-Wave Observatory, or LIGO, reduced vibrations in a combination of the facility’s mirrors to nearly the lowest level allowed by quantum mechanics, they report in the June 18 Science.

    The researchers quelled differences between the jiggling of LIGO’s four 40-kilogram mirrors, putting them in near-perfect sync. When the mirrors are combined in this way, they behave effectively like a single, 10-kilogram object.

    LIGO is designed to measure gravitational waves, using laser light that bounces between sets of mirrors in the detector’s two long arms (SN: 2/11/16). But physicist Vivishek Sudhir of MIT and colleagues instead used the laser light to monitor the mirrors’ movements to extreme precision and apply electric fields to resist the motion. “It’s almost like a noise-canceling headphone,” says Sudhir. But instead of measuring nearby sounds and canceling out that noise, the technique cancels out motion.

    The researchers reduced the mirrors’ relative motions to about 10.8 phonons, or quantum units of vibration, close to the zero-phonon quantum limit.

    The study’s purpose is not to better understand gravitational waves, but to get closer to revealing secrets of quantum mechanics. Scientists are still trying to understand why large objects don’t typically follow the laws of quantum mechanics. Such objects lose their quantum properties, or decohere. Studying quantum states of more massive objects could help scientists pin down how decoherence happens.

    Previous studies have observed much smaller objects in quantum states. In 2020, physicist Markus Aspelmeyer of the University of Vienna and colleagues brought vibrations of a nanoparticle to the quantum limit (SN: 1/30/20). LIGO’s mirrors are “a fantastic system to study decoherence effects on super-massive objects in the quantum regime,” says Aspelmeyer. More

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    The teeth of ‘wandering meatloaf’ contain a rare mineral found only in rocks

    The hard, magnetic teeth of a leathery red-brown mollusk nicknamed “the wandering meatloaf” possess a rare mineral previously seen only in rocks. The mineral may help the mollusk — the giant Pacific chiton (Cryptochiton stelleri) — meld its soft flesh to the hard teeth it uses for grazing on rocky coastlines, researchers report online May 31 in Proceedings of the National Academy of Sciences.

    C. stelleri is the world’s largest chiton, reaching up to roughly 35 centimeters long. It is equipped with several dozen rows of teeth on a slender, flexible, tonguelike appendage called a radula that it uses to scrape algae off rocks. Those teeth are covered in magnetite, the hardest, stiffest known biomineral to date: It’s as much as three times as hard as human enamel and mollusk shells.

    C. stelleri uses its radula, a tonguelike structure (pictured) studded with hard magnetic teeth (dark objects), to graze on rocks. This composite image shows the radula’s stages of development, from earliest (left) to latest (right).Northwestern University

    Materials scientist Derk Joester and colleagues analyzed these teeth using high-energy X-rays from the Advanced Photon Source at Argonne National Laboratory in Lemont, Ill. They discovered that the interface between the teeth and flesh contained nanoparticles of santabarbaraite, an iron-loaded mineral never seen before in a living organism’s body.

    These nanoparticles help the underpinnings of the teeth vary in hardness and stiffness by at least a factor of two over distances of just several hundred micrometers — a few times the average width of a human hair. Such variations let these structures bridge the hard and soft parts of the mollusk’s body. Now that santabarbaraite has been found in one organism, the researchers suggest looking for it in insect cuticles and bacteria that sense magnetic fields.

    The teeth on C. stelleri’s tonguelike organ, seen in closeup in this scanning electron microscope image, help the mollusk scrape algae off of rocks.Northwestern University

    Using nanoparticles of a mineral similar to santabarbaraite, the scientists also 3-D printed strong, light materials with a range of hardness and stiffness. These composites might find use in soft robotics, including marrying soft and hard parts in bots that can squirm past obstacles that conventional robots cannot given their rigid parts, says Joester, of Northwestern University in Evanston, Ill. More

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    A newfound quasicrystal formed in the first atomic bomb test

    In an instant, the bomb obliterated everything.

    The tower it sat on and the copper wires strung around it: vaporized. The desert sand below: melted.

    In the aftermath of the first test of an atomic bomb, in July 1945, all this debris fused together, leaving the ground of the New Mexico test site coated with a glassy substance now called trinitite. High temperatures and pressures helped forge an unusual structure within one piece of trinitite, in a grain of the material just 10 micrometers across — a bit longer than a red blood cell.

    That grain contains a rare form of matter called a quasicrystal, born the moment the nuclear age began, scientists report May 17 in Proceedings of the National Academy of Sciences.

    Normal crystals are made of atoms locked in a lattice that repeats in a regular pattern. Quasicrystals have a structure that is orderly like a normal crystal but that doesn’t repeat. This means quasicrystals can have properties that are forbidden for normal crystals. First discovered in the lab in 1980s, quasicrystals also appear in nature in meteorites (SN: 12/8/16).

    Penrose tilings (one shown) are an example of a structure that is ordered but does not repeat. Quasicrystals are a three-dimensional version of this idea.Inductiveload/Wikimedia Commons

    The newly discovered quasicrystal from the New Mexico test site is the oldest one known that was made by humans.

    Trinitite takes its moniker from the nuclear test, named Trinity, in which the material was created in abundance (SN: 4/8/21). “You can still buy lots of it on eBay,” says geophysicist Terry Wallace, a coauthor of the study and emeritus director of Los Alamos National Laboratory in New Mexico.

    But, he notes, the trinitite the team studied was a rarer variety, called red trinitite. Most trinitite has a greenish tinge, but red trinitite contains copper, remnants of the wires that stretched from the ground to the bomb. Quasicrystals tend to be found in materials that have experienced a violent impact and usually involve metals. Red trinitite fit both criteria.

    But first the team had to find some.

    “I was asking around for months looking for red trinitite,” says theoretical physicist Paul Steinhardt of Princeton University. But Steinhardt, who is known for trekking to Siberia to seek out quasicrystals, wasn’t deterred (SN: 2/19/19). Eventually he and his colleagues got some from an expert in trinitite who began collaborating with the team. Then, the painstaking work started, “looking through every little microscopic speck” of the trinitite sample, says Steinhardt. Finally, the researchers extracted the tiny grain. By scattering X-rays through it, the researchers revealed that the material had a type of symmetry found only in quasicrystals.

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    The new quasicrystal, formed of silicon, copper, calcium and iron, is “brand new to science,” says mineralogist Chi Ma of Caltech, who was not involved with the study. “It’s a quite cool and exciting discovery,” he says.

    Future searches for quasicrystals could examine other materials that experienced a punishing blow, such as impact craters or fulgurites, fused structures formed when lightning strikes soil (SN: 3/16/21).

    The study shows that artifacts from the birth of the atomic age are still of scientific interest, says materials scientist Miriam Hiebert of the University of Maryland in College Park, who has analyzed materials from other pivotal moments in nuclear history (SN: 5/1/19). “Historic objects and materials are not just curiosities in collectors’ cabinets but can be of real scientific value,” she says. More

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    Morphing noodles start flat but bend into curly pasta shapes as they’re cooked

    This pasta is no limp noodle.

    When imprinted with carefully designed arrangements of grooves, flat pasta morphs as it cooks, forming tubes, spirals and other shapes traditional for the starchy sustenance. The technique could allow for pasta that takes up less space, Lining Yao and colleagues report May 5 in Science Advances.

    Pasta aficionados “are very picky about the shapes of pasta and how they pair with different sauces,” says Yao, who studies the design of smart materials at Carnegie Mellon University in Pittsburgh. But those shapes come at a cost of excess packaging and inefficient shipping: For some varieties of curly pasta, more than 60 percent of the packaging space is used to hold air, the researchers calculated.

    Yao and colleagues stamped a series of grooves onto one side of each noodle. As the pasta absorbed water during cooking, the liquid couldn’t penetrate as fully on the grooved side, causing it to swell less than the smooth side of the pasta. That asymmetric swelling bent the previously flat noodle into a curve. By changing the arrangement of the grooves, the researchers controlled the final shape. Computer simulations of swelling pasta replicated the shapes seen in the experiments.

    [embedded content]
    Flat pasta (top) with the right pattern of grooves imprinted on it curls into traditional pasta shapes when boiled. Computer simulations of the pasta (bottom) show the same behavior.

    The technique isn’t limited to pasta: Another series of experiments, performed with silicone rubber in a solvent, produced similar results. But whereas the pasta held its curved shape, the silicone rubber eventually absorbed enough solvent to flatten out again. The gluey nature of cooked pasta helps lock in the twists by fusing neighboring grooves together, the researchers determined. Removing the silicone from the solvent caused the silicone to bend in the opposite direction. This reversible bending process could be harnessed for other purposes, such as a grabber for robot hands, Yao says.

    The pasta makes particularly good camping food, Yao says. A member of her team brought it along on a recent hiking trip. The pasta slips easily into a cramped pack but cooks into a satisfying shape. More

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    Capturing the sense of touch could upgrade prosthetics and our digital lives

    On most mornings, Jeremy D. Brown eats an avocado. But first, he gives it a little squeeze. A ripe avocado will yield to that pressure, but not too much. Brown also gauges the fruit’s weight in his hand and feels the waxy skin, with its bumps and ridges.

    “I can’t imagine not having the sense of touch to be able to do something as simple as judging the ripeness of that avocado,” says Brown, a mechanical engineer who studies haptic feedback — how information is gained or transmitted through touch — at Johns Hopkins University.

    Many of us have thought about touch more than usual during the COVID-19 pandemic. Hugs and high fives rarely happen outside of the immediate household these days. A surge in online shopping has meant fewer chances to touch things before buying. And many people have skipped travel, such as visits to the beach where they might sift sand through their fingers. A lot goes into each of those actions.

    “Anytime we touch anything, our perceptual experience is the product of the activity of thousands of nerve fibers and millions of neurons in the brain,” says neuroscientist Sliman Bensmaia of the University of Chicago. The body’s natural sense of touch is remarkably complex. Nerve receptors detect cues about pressure, shape, motion, texture, temperature and more. Those cues cause patterns of neural activity, which the central nervous system interprets so we can tell if something is smooth or rough, wet or dry, moving or still.

    Scientists at the University of Chicago attached strips of different materials to a rotating drum to measure vibrations produced in the skin as a variety of textures move across a person’s fingertips.
    Matt Wood/Univ. of Chicago

    Neuroscience is at the heart of research on touch. Yet mechanical engineers like Brown and others, along with experts in math and materials science, are studying touch with an eye toward translating the science into helpful applications. Researchers hope their work will lead to new and improved technologies that mimic tactile sensations.

    As scientists and engineers learn more about how our nervous system responds to touch stimuli, they’re also studying how our skin interacts with different materials. And they’ll need ways for people to send and receive simulated touch sensations. All these efforts present challenges, but progress is happening. In the near term, people who have lost limbs might recover some sense of touch through their artificial limbs. Longer term, haptics research might add touch to online shopping, enable new forms of remote medicine and expand the world of virtual reality.

    “Anytime you’re interacting with an object, your skin deforms,” or squishes a bit.Sliman Bensmaia

    Good vibrations

    Virtual reality programs already give users a sense of what it’s like to wander through the International Space Station or trek around a natural gas well. For touch to be part of such experiences, researchers will need to reproduce the signals that trigger haptic sensations.

    Our bodies are covered in nerve endings that respond to touch, and our hands are really loaded up, especially our fingertips. Some receptors tell where parts of us are in relation to the rest of the body. Others sense pain and temperature. One goal for haptics researchers is to mimic sensations resulting from force and movement, such as pressure, sliding or rubbing.

    “Anytime you’re interacting with an object, your skin deforms,” or squishes a bit, Bensmaia explains. Press on the raised dots of a braille letter, and the dots will poke your skin. A soapy glass slipping through your fingers produces a shearing force — and possibly a crash. Rub fabric between your fingers, and the action produces vibrations.

    Four main categories of touch receptors respond to those and other mechanical stimuli. There’s some overlap among the types. And a single contact with an object can affect multiple types of receptors, Bensmaia notes.

    One type, called Pacinian corpuscles, sits deep in the skin. They are especially good at detecting vibrations created when we interact with different textures. When stimulated, the receptors produce sequences of signals that travel to the brain over a period of time. Our brains interpret the signals as a particular texture. Bensmaia compares it to the way we hear a series of notes and recognize a tune.

    “Corduroy will produce one set of vibrations. Organza will produce another set,” Bensmaia says. Each texture produces “a different set of vibrations in your skin that we can measure.” Such measurements are a first step toward trying to reproduce the feel of different textures.

    Additionally, any stimulus meant to mimic a texture sensation must be strong enough to trigger responses in the nervous system’s touch receptors. That’s where work by researchers at the University of Birmingham in England comes in. The vibrations from contact with various textures create different kinds of wave energy. Rolling-type waves called Rayleigh waves go deep enough to reach the Pacinian receptors, the team reported last October in Science Advances. Much larger versions of the same types of waves cause much of the damage from earthquakes.

    Not all touches are forceful enough to trigger a response from the Pacinian receptors. To gain more insight into which interactions will stimulate those receptors, the team looked at studies that have collected data on touches to the limbs, head or neck of dogs, dolphins, rhinos, elephants and other mammals. A pattern emerged. The group calls it a “universal scaling law” of touch for mammals.

    For the most part, a touch at the surface will trigger a response in a Pacinian receptor deep in the skin if the ratio is 5-to-2 between the length of the Rayleigh waves resulting from the touch and the depth of the receptor. At that ratio or higher, a person and most other mammals will feel the sensation, says mathematician James Andrews, lead author of the study.

    Also, the amount of skin displacement needed to cause wavelengths long enough to trigger a sensation by the Pacinian receptors will be the same across most mammal species, the group found. Different species will need more or less force to cause that displacement, however, which may depend on skin composition or other factors. Rodents did not fit the 5–2 ratio, perhaps because their paws and limbs are so small compared with the wavelengths created when they touch things, Andrews notes.

    Beyond that, the work sheds light on “what types of information you’d need to realistically capture the haptic experience — the touch experience — and send that digitally anywhere,” Andrews says. People could then feel sensations with a device or perhaps with ultrasonic waves. Someday the research might help provide a wide range of virtual reality experiences, including virtual hugs.

    Online tactile shopping

    Mechanical engineer Cynthia Hipwell of Texas A&M University in College Station moved into a new house before the pandemic. She looked at some couches online but couldn’t bring herself to buy one from a website. “I didn’t want to choose couch fabric without feeling it,” Hipwell says.

    “Ideally, in the long run, if you’re shopping on Amazon, you could feel fabric,” she says. Web pages’ computer codes would make certain areas on a screen mimic different textures, perhaps with shifts in electrical charge, vibration signals, ultrasound or other methods. Touching the screen would clue you in to whether a sweater is soft or scratchy, or if a couch’s fabric feels bumpy or smooth. Before that can happen, researchers need to understand conditions that affect our perception of how a computer screen feels.

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    Surface features at the nanometer scale (billionths of a meter) can affect how we perceive the texture of a piece of glass, Hipwell says. Likewise, we may not consciously feel any wetness as humidity in the air mixes with our skin’s oil and sweat. But tiny changes in that moisture can alter the friction our fingers encounter as they move on a screen, she says. And that friction can influence how we perceive the screen’s texture.

    Shifts in electric charge also can change the attraction between a finger and a touch screen. That attraction is called electroadhesion, and it affects our tactile experience as we touch a screen. Hipwell’s group recently developed a computer model that accounts for the effects of electroadhesion, moisture and the deformation of skin pressing against glass. The team reported on the work in March 2020 in IEEE Transactions on Haptics.

    Hipwell hopes the model can help product designers develop haptic touch screens that go beyond online shopping. A car’s computerized dashboard might have sections that change texture for each menu, she suggests. A driver could change temperature or radio settings by touch while keeping eyes on the road.

    “Ideally, in the long run, if you’re shopping on Amazon, you could feel fabric.”Cynthia Hipwell

    Wireless touch patches

    Telemedicine visits rose dramatically during the early days of the COVID-19 pandemic. But video doesn’t let doctors feel for swollen glands or press an abdomen to check for lumps. Remote medicine with a sense of touch might help during pandemics like this one — and long after for people in remote areas with few doctors.

    People in those places might eventually have remote sensing equipment in their own homes or at a pharmacy or workplace. If that becomes feasible, a robot, glove or other equipment with sensors could touch parts of a patient’s body. The information would be relayed to a device somewhere else. A doctor at that other location could then experience the sensations of touching the patient.

    Researchers are already working on materials that can translate digital information about touch into sensations people — in this case, doctors — can feel. The same materials could communicate information for virtual reality applications. One possibility is a skin patch developed by physical chemist John Rogers of Northwestern University in Evanston, Ill., and others.

    One layer of the flexible patch sticks to a person’s skin. Other layers include a stretchable circuit board and tiny actuators that create vibrations as current flows around them. Wireless signals tell the actuators to turn on or off. Energy to run the patch also comes in wirelessly. The team described the patch in Nature in 2019.

    Retired U.S. Army Sgt. Garrett Anderson shakes hands with researcher Aadeel Akhtar, CEO of Psyonic, a prosthesis developer. A wireless skin patch on Anderson’s upper arm gives him sensory feedback when grasping an object.Northwestern Univ.

    Inside the patch are circular actuators that vibrate in response to signals. The prototype device might give the sensation of touch pressure in artificial limbs, in virtual reality and telemedicine.

    Since then, Rogers’ group has reduced the patch’s thickness and weight. The patch now also provides more detailed information to a wearer. “We have scaled the systems into a modular form to allow custom sizes [and] shapes in a kind of plug-and-play scheme,” Rogers notes. So far, up to six separate patches can work at the same time on different parts of the body.

    The group also wants to make its technology work with electronics that many consumers have, such as smartphones. Toward that end, Rogers and colleagues have developed a pressure-sensitive touch screen interface for sending information to the device. The interface lets someone provide haptic sensations by moving their fingers on a smartphone or touch screen–based computer screen. A person wearing the patch then feels stroking, tapping or other touch sensations.

    Pressure points

    Additionally, Rogers’ team has developed a way to use the patch system to pick up signals from pressure on a prosthetic arm’s fingertips. Those signals can then be relayed to a patch worn by the person with the artificial limb. Other researchers also are testing ways to add tactile feedback to prostheses. European researchers reported in 2019 that adding feedback for pressure and motion helped people with an artificial leg walk with more confidence (SN: 10/12/19, p. 8). The device reduced phantom limb pain as well.

    Brown, the mechanical engineer at Johns Hopkins, hopes to help people control the force of their artificial limbs. Nondisabled people adjust their hands’ force instinctively, he notes. He often takes his young daughter’s hand when they’re in a parking lot. If she starts to pull away, he gently squeezes. But he might easily hurt her if he couldn’t sense the stiffness of her flesh and bones.

    Two types of prosthetic limbs can let people who lost an arm do certain movements again. Hands on “body-controlled” limbs open or close when the user moves other muscle groups. The movement works a cable on a harness that connects to the hand. Force on those other muscles tells the person if the hand is open or closed. Myoelectric prosthetic limbs, in contrast, are directly controlled by the muscles on the residual limb. Those muscle-controlled electronic limbs generally don’t give any feedback about touch. Compared with the body-controlled options, however, they allow a greater range of motion and can offer other advantages.

    In one study, Brown’s group tested two ways to add feedback about the force that a muscle-controlled electronic limb exerts on an object. One method used an exoskeleton that applied force around a person’s elbow. The other technique used a device strapped near the wrist. The stiffer an object is, the stronger the vibrations on someone’s wrist. Volunteers without limb loss tried using each setup to judge the stiffness of blocks.

    In a study of two different haptic feedback methods, one system applied force near the elbow. N. Thomas et al/J. NeuroEng. Rehab. 2019

    The other system tested in the study provided vibrations near the wrist. N. Thomas et al/J. NeuroEng. Rehab. 2019

    Both methods worked better than no feedback. And compared with each other, the two types of feedback “worked equally well,” Brown says. “We think that is because, in the end, what the human user is doing is creating a map.” Basically, people match up how much force corresponds to the intensity of each type of feedback. The work suggests ways to improve muscle-controlled electronic limbs, Brown and colleagues reported in 2019 in the Journal of NeuroEngineering and Rehabilitation.

    Still, people’s brains may not be able to match up all types of feedback for touch sensations. Bensmaia’s group at the University of Chicago has worked with colleagues in Sweden who built tactile sensors into bionic hands: Signals from a sensor on the thumb went to an electrode implanted around the ulnar nerve on people’s arms. Three people who had lost a hand tested the bionic hands and felt a touch when the thumb was prodded, but the touch felt as if it came from somewhere else on the hand.

    Doctors can choose which nerve an electrode will stimulate. But they don’t know in advance which bundle of fibers it will affect within the nerve, Bensmaia explains. And different bundles receive and supply sensations to different parts of the hand. Even after the people had used the prosthesis for more than a year, the mismatch didn’t improve. The brain didn’t adapt to correct the sensation. The team shared its findings last December in Cell Reports.

    Despite that, in previous studies, those same people using the bionic hands had better precision and more control over their force when grasping objects, compared with those using versions without direct stimulation of the nerve. People getting the direct nerve stimulation also reported feeling as if the hand was more a part of them.

    As with the bionic hands, advances in haptic technology probably won’t start out working perfectly. Indeed, virtual hugs and other simulated touch experiences may never be as good as the real thing. Yet haptics may help us get a feel for the future, with new ways to explore our world and stay in touch with those we love. More

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    Muon magnetism could hint at a breakdown of physics’ standard model

    A mysterious magnetic property of subatomic particles called muons hints that new fundamental particles may be lurking undiscovered.

    In a painstakingly precise experiment, muons’ gyrations within a magnetic field seem to defy predictions of the standard model of particle physics, which describes known fundamental particles and forces. The result strengthens earlier evidence that muons, the heavy kin of electrons, behave unexpectedly.

    “It’s a very big deal,” says theoretical physicist Bhupal Dev of Washington University in St. Louis. “This could be the long-awaited sign of new physics that we’ve all hoped for.”

    Muons’ misbehavior could point to the existence of new types of particles that alter muons’ magnetic properties. Muons behave like tiny magnets, each with a north and south pole. The strength of that magnet is tweaked by transient quantum particles that constantly flit into and out of existence, adjusting the muon’s magnetism by an amount known as the muon magnetic anomaly. Physicists can predict the value of the magnetic anomaly by considering the contributions of all known particles. If any fundamental particles are in hiding, their additional effects on the magnetic anomaly could give them away.

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    Muons and electrons share a family resemblance, but muons are about 200 times as massive. That makes muons more sensitive to the effects of hypothetical heavy particles. “The muon kind of hits the sweet spot,” says Aida El-Khadra of the University of Illinois at Urbana-Champaign.

    To measure the magnetic subtleties of the muon, physicists flung billions of the particles around the huge, doughnut-shaped magnet of the Muon g−2 experiment at Fermilab in Batavia, Ill. (SN: 9/19/18). Inside that magnet, the orientation of the muons’ magnetic poles wobbled, or precessed. Notably, the rate of that precession diverged slightly from the standard model expectation, physicists report April 7 in a virtual seminar, and in a paper published in Physical Review Letters.

    “This is a really complex experiment,” says Tsutomu Mibe of the KEK High Energy Accelerator Research Organization in Japan. “This is excellent work.”

    To avoid bias, the team worked under self-imposed secrecy, keeping the final number hidden from themselves as they analyzed the data. At the moment the answer was finally revealed, says physicist Meghna Bhattacharya of the University of Mississippi in Oxford, “I was having goose bumps.” The researchers found a muon magnetic anomaly of 0.00116592040, accurate to within 46 millionths of a percent. The theoretical prediction pegs the number at 0.00116591810. That discrepancy “hints toward new physics,” Bhattacharya says.

    A previous measurement of this type, from an experiment completed in 2001 at Brookhaven National Laboratory in Upton, N.Y., also seemed to disagree with theoretical predictions  (SN: 2/15/01). When the new result is combined with the earlier discrepancy, the measurement diverges from the prediction by a statistical measure of 4.2 sigma — tantalizingly close to the typical five-sigma benchmark for claiming a discovery. “We have to wait for more data from the Fermilab experiment to really be convinced that this is a real discovery, but it is becoming more and more interesting,” says theoretical physicist Carlos Wagner of the University of Chicago.

    According to quantum physics, muons are constantly emitting and absorbing particles in a frenzy that makes theoretical calculations of the magnetic anomaly extremely complex. An international team of more than 170 physicists, co-led by El-Khadra, finalized the theoretical prediction in December 2020 in Physics Reports.

    Many physicists believe that this theoretical prediction is solid, and unlikely to budge with further investigation. But some debate lingers. Using a computational technique called lattice QCD for a particularly thorny part of the calculation gives an estimate that falls closer to the experimentally measured value, physicist Zoltan Fodor and colleagues report April 7 in Nature. If Fodor and colleagues’ calculation is correct, “it could change how we see the experiment,” says Fodor, of Pennsylvania State University, perhaps making it easier to explain the experimental results with the standard model. But he notes that his team’s prediction would need to be confirmed by other calculations before being taken as seriously as the “gold standard” prediction.

    As theoretical physicists continue to refine their predictions, experimental estimates will improve too: Muon g−2 (pronounced gee-minus-two) physicists have analyzed only a fraction of their data so far. And Mibe and colleagues are planning an experiment using a different technique at J-PARC, the Japan Proton Accelerator Research Complex in Tokai, to begin in 2025.

    If the discrepancy between experiment and prediction holds up, scientists will need to find an explanation that goes beyond the standard model. Physicists already believe that the standard model can’t explain everything that’s out there: The universe seems to be pervaded by invisible dark matter, for example, that standard model particles can’t account for.

    Some physicists speculate that the explanation for the muon magnetic anomaly may be connected to known puzzles of particle physics. For example, a new particle might simultaneously explain dark matter and the Muon g−2 result. Or there may be a connection to unexpected features of certain particle decays observed in the LHCb experiment at the CERN particle physics lab near Geneva (SN: 4/20/17), recently strengthened by new results posted at arXiv.org on March 22.

    The Muon g−2 measurement will intensify such investigations, says Muon g−2 physicist Jason Crnkovic of the University of Mississippi. “This is an exciting result because it’s going to generate a lot of conversations.” More

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    Microscopic images reveal the science and beauty of face masks

    Studying fabrics at very high magnification helps determine how some face masks filter out particles better than others. And the close-ups reveal an unseen beauty of the mundane objects that have now become an essential part of life around the world.

    As scientists continue to show how effective masks can be at slowing the spread of the new coronavirus, particularly when they have a good fit and are worn correctly, some have taken microscopic approaches (SN: 2/12/21).

    “Embedded in microscale textures are clues as to why materials have various properties,” says Edward Vicenzi, a microanalysis expert at the Smithsonian’s Museum Conservation Institute in Suitland, Md. “Unraveling that evidence turns out to be a fun job.”

    Before the pandemic, Vicenzi spent his days observing meteorites, stones and other museum specimens under the microscope. But in March 2020, as the COVID-19 pandemic progressed, he and colleagues from the National Institute for Standards and Technology in Gaithersburg, Md., felt a strong desire to contribute to beating back the virus. So they started studying face-covering materials instead.

    Cotton flannel: A network of cotton fibers “hovers” above a woven surface in this view of the fabric. This chaotic arrangement gives cotton flannel fibers additional opportunities to grab particles as they flow through the fabric. E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

    Polyester-cotton blend: Disheveled natural cotton fibers (pale) contrast with nearly identical polyester fibers (blue) in this false-color image. Polyester fibers are highly organized, mostly straight and smooth, making them less effective than cotton fibers alone at trapping nanoscale particles. E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

    Rayon: Like patterns observed on rigatoni pasta, grooves run along the length of rayon fibers. Unlike cotton flannels, rayon has no apparent weblike structures formed from raised fibers, making it easier for particles to move from one side of the synthetic fabric to the other. E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

    Wool flannel: Seen in cross-section, these fibers resemble a hurricane swirl. Wool flannel can also form fiber webs that block particles, but those webs are not as effective as ones in 100-percent cotton, researchers found. E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

    N95 mask: In an N95 mask (seen in false color cross-section), a thin outer layer (top) and thick inner layer (bottom) sandwich a filtration layer (purple), which traps the smallest particles. The multilayered assemblage made of plastic is melted and blown into a weblike fabric, which makes N95s filter particles better than cloth masks, even cotton ones. E.P. Vicenzi/Smithsonian’s Museum Conservation Institute and NIST

    Using a scanning electron microscope, Vicenzi and colleagues have examined dozens of materials, including coffee filters, pillowcases, surgical masks and N95 masks. In 2020, the team found that N95 respirator masks are the most effective at providing protection from aerosols like the ones in which SARS-CoV-2, the virus that causes COVID-19, travels. And the researchers reported that synthetic fabrics, like chiffon or rayon, don’t trap as many particles as tightly woven cotton flannels.

    Microscopic textures can explain each fabric’s ability to filter out aerosols. The random nature of cotton fibers — with its wrinkled texture and complex shapes such as kinks, bends and folds — probably allows cotton to trap more nanoscale particles than other fabrics, Vicenzi says. In contrast, polyester fabrics have highly organized, mostly straight and smooth fibers, which makes them less efficient as face masks.

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    Cotton flannels also provide additional protection by absorbing moisture from breath, Vicenzi and colleagues report March 8 in ACS Applied Nano Materials.

    “Since cotton loves water, it swells up in humid environments, and that makes it harder for particles to make their way through a mask,” says Vicenzi. Polyester and nylon masks, on the other hand, “repel water from your breath, so there’s no added benefit.”

    Through his work, Vicenzi has explored the unseen world of face-covering materials. Some textiles remind him of food, such as rayon’s fibers that resemble the texture of rigatoni pasta. Others, like wool, remind him of atmospheric patterns such as the swirl of a hurricane.

    Vicenzi plans to keep observing face masks up close. And he hopes his research helps people decide how to best protect themselves and others during the COVID-19 pandemic. “It’s nice to use an effective material for a mask if you can,” he says. “However, wearing any mask compared to none at all makes the biggest difference in slowing the spread of pathogens.”

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