Most crossword puzzle fans have experienced that moment where, after a period of struggle on a particularly difficult puzzle, everything suddenly starts to come together, and they are able to fill in a bunch of squares correctly. Alexander Hartmann, a statistical physicist at the University of Oldenburg in Germany, had an intriguing insight when this happened while he was trying to solve a puzzle one day. According to his paper published in the journal Physical Review E, the crossword puzzle-solving process resembles a type of phase transition known as percolation—one that seems to be unique compared to standard percolation models.

Traditional mathematical models of percolation date back to the 1940s. Directed percolation is when the flow occurs in a specific direction, akin to how water moves through freshly ground coffee beans, flowing down in the direction of gravity. (In physical systems, percolation is one of the primary mechanisms behind the Brazil nut effect, along with convection.) Such models can also be applicable to a wide range of large networked systems: power grids, financial markets, and social networks, for example.

Individual nodes in a random network start linking together, one by one, via short-range connections, until the number of connections reaches a critical threshold (tipping point). At that point, there is a phase shift in which the largest cluster of nodes grows rapidly, giving rise to more long-range connections, resulting in uber-connectivity. The likelihood of two clusters merging is proportional to their size, and once a large cluster forms, it dominates the networked system, absorbing smaller clusters.

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The piano-mover puzzle involves trying to transport an oddly shaped load across a constricted environment with various obstructions. It's one of several variations on classic computational motion-planning problems, a key element in numerous robotics applications. But what would happen if you pitted human beings against ants in a competition to solve the piano-mover puzzle?

According to a paper published in the Proceedings of the National Academy of Sciences, humans have superior cognitive abilities and, hence, would be expected to outperform the ants. However, depriving people of verbal or nonverbal communication can level the playing field, with ants performing better in some trials. And while ants improved their cognitive performance when acting collectively as a group, the same did not hold true for humans.

Co-author Ofer Feinerman of the Weizmann Institute of Science and colleagues saw an opportunity to use the piano-mover puzzle to shed light on group decision-making, as well as the question of whether it is better to cooperate as a group or maintain individuality. "It allows us to compare problem-solving skills and performances across group sizes and down to a single individual and also enables a comparison of collective problem-solving across species," the authors wrote.

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Some version of the Hula-Hoop has been around for millennia, but the popular plastic version was introduced by Wham-O in the 1950s and quickly became a fad. Now, researchers have taken a closer look at the underlying physics of the toy, revealing that certain body types are better at keeping the spinning hoops elevated than others, according to a new paper published in the Proceedings of the National Academy of Sciences.

“We were surprised that an activity as popular, fun, and healthy as hula hooping wasn’t understood even at a basic physics level,” said co-author Leif Ristroph of New York University. “As we made progress on the research, we realized that the math and physics involved are very subtle, and the knowledge gained could be useful in inspiring engineering innovations, harvesting energy from vibrations, and improving in robotic positioners and movers used in industrial processing and manufacturing.”

Ristroph's lab frequently addresses these kinds of colorful real-world puzzles. For instance, in 2018, Ristroph and colleagues fine-tuned the recipe for the perfect bubble based on experiments with soapy thin films. In 2021, the Ristroph lab looked into the formation processes underlying so-called "stone forests" common in certain regions of China and Madagascar.

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Back in 2019, we told you about an intriguing experiment to test a famous anthropological legend about an elderly Inuit man in the 1950s who fashioned a knife out of his own frozen feces. He used it to kill and skin a dog, using its rib cage as a makeshift sled to venture off into the Arctic. Metin Eren, an archaeologist at Kent State University, fashioned rudimentary blades out of his own frozen feces to test whether they could cut through pig hide, muscle, and tendon.

Sadly for the legend, the blades failed every test, but the study was colorful enough to snag Eren an Ig Nobel Prize the following year. And it's just one of the many fascinating projects routinely undertaken in his Experimental Archaeology Laboratory, where he and his team try to reverse-engineer all manner of ancient technologies, whether they involve stone tools, ceramics, metal, butchery, textiles, and so forth.

Eren's lab is quite prolific, publishing 15 to 20 papers a year. “The only thing we’re limited by is time,” he said. Many have colorful or quirky elements and hence tend to garner media attention, but Eren emphasizes that what he does is very much serious science, not entertainment. “I think sometimes people look at experimental archaeology and think it’s no different from LARPing,” Eren told Ars. “I have nothing against LARPers, but it’s very different. It’s not playtime. It’s hardcore science. Me making a stone tool is no different than a chemist pouring chemicals into a beaker. But that act alone is not the experiment. It might be the flashiest bit, but that's not the experimental process.”

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Over the holiday weekend, all but one member of the editorial board of Elsevier's Journal of Human Evolution (JHE) resigned "with heartfelt sadness and great regret," according to Retraction Watch, which helpfully provided an online PDF of the editors' full statement. It's the 20th mass resignation from a science journal since 2023 over various points of contention, per Retraction Watch, many in response to controversial changes in the business models used by the scientific publishing industry.

"This has been an exceptionally painful decision for each of us," the board members wrote in their statement. "The editors who have stewarded the journal over the past 38 years have invested immense time and energy in making JHE the leading journal in paleoanthropological research and have remained loyal and committed to the journal and our authors long after their terms ended. The [associate editors] have been equally loyal and committed. We all care deeply about the journal, our discipline, and our academic community; however, we find we can no longer work with Elsevier in good conscience."

The editorial board cited several changes made over the last ten years that it believes are counter to the journal's longstanding editorial principles. These included eliminating support for a copy editor and a special issues editor, leaving it to the editorial board to handle those duties. When the board expressed the need for a copy editor, Elsevier's response, they said, was "to maintain that the editors should not be paying attention to language, grammar, readability, consistency, or accuracy of proper nomenclature or formatting."

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There is rarely time to write about every cool science paper that comes our way; many worthy candidates sadly fall through the cracks over the course of the year. But as 2024 comes to a close, we've gathered ten of our favorite such papers at the intersection of science and culture as a special treat, covering a broad range of topics: from reenacting Bronze Age spear combat and applying network theory to the music of Johann Sebastian Bach, to Spider-Man inspired web-slinging tech and a mathematical connection between a turbulent phase transition and your morning cup of coffee. Enjoy!

Reenacting Bronze Age spear combat

An experiment with experienced fighters who spar freely using different styles. Credit: Valerio Gentile/CC BY

The European Bronze Age saw the rise of institutionalized warfare, evidenced by the many spearheads and similar weaponry archaeologists have unearthed. But how might these artifacts be used in actual combat? Dutch researchers decided to find out by constructing replicas of Bronze Age shields and spears and using them in realistic combat scenarios. They described their findings in an October paper published in the Journal of Archaeological Science.

There have been a couple of prior experimental studies on bronze spears, but per Valerio Gentile (now at the University of Gottingen) and coauthors, practical research to date has been quite narrow in scope, focusing on throwing weapons against static shields. Coauthors C.J. van Dijk of the National Military Museum in the Netherlands and independent researcher O. Ter Mors each had more than a decade of experience teaching traditional martial arts, specializing in medieval polearms and one-handed weapons. So they were ideal candidates for testing the replica spears and shields.

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'Tis the season for many holiday traditions, including the Ugly Christmas Sweater—you know, those 1950s-style heavy knits featuring some kind of cartoonish seasonal decoration, like snowflakes, Santa Claus, or—in the case of Mark Darcy from Bridget Jones' Diary (2001)—Rudolph the Red-Nosed Reindeer. "It’s obnoxious and tacky, but also fuzzy and kind of wholesome—the fashion equivalent of a Hallmark Christmas movie (with a healthy dose of tongue-in-cheek)," as CNN's Marianna Cerini recently observed.

Fashion (or lack thereof) aside, sweaters and other knitted fabric are also fascinating to physicists and mathematicians. Case in point: a recent paper published in the journal Physical Review Letters examining the complex mechanics behind the many resting shapes a good Jersey knit can form while at rest.

Knitted fabrics are part of a class of intertwined materials—which also includes birds' nests, surgical knots, knotted shoelaces, and even the degradation of paper fibers in ancient manuscripts. Knitted fabrics are technically a type of metamaterial: an engineered material that gets its properties not from the base materials but from their designed structures. The elasticity (aka, stretchiness) of knitted fabrics is an emergent property: the whole is more than the sum of its parts. How those components (strands of yarn) are arranged at an intermediate scale (the structure) determines the macro scale properties of the resulting fabric.

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There are thousands of YouTube videos in which DIY science enthusiasts cut grapes in half—leaving just a thin bit of skin connecting them—and put the grapes in the microwave, just to marvel at the sparks and plume of ionized gas (plasma) that the grapes produce. This quirky property of grapes might help make more efficient quantum sensors, according to a new paper published in the journal Physical Review Applied.

The plasma-inducing grape effect was first observed in 1994, per the authors. As previously reported, the usual explanation for the generation of plasmas is that grapes are so small that the irradiating microwaves become highly concentrated in the grape tissue, ripping some the molecules apart to generate charged ions (adding to the electrolytes already present in the grapes). The electromagnetic field that forms causes ions to flow from one grape half to the other via the connecting skin—at least at first. That's when you get the initial sparks. Eventually, the ions start passing through the surrounding air as well, ionizing it to produce that hot plume of plasma.

But in 2019, Trent University scientists showed that explanation isn't quite right. The skin bridge isn't necessary for the effect to occur. Rather, the plasma is generated by an electromagnetic "hot spot." The grapes have the right refractive index and size to "trap" microwaves, so putting two of them close together leads to the generation of a hot spot between them. The trick also works with gooseberries, large blackberries, and quail eggs, as well as hydrogel beads—plastic beads soaked in water. ("Many microwaves were in fact harmed during the experiments," co-author Hamza Khattak admitted at the time.)

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We think of squirrels as adorably harmless creatures, admiring their bushy tails and twitchy little noses and the way they cram their cheeks with nuts or seeds to bring back to their nests for later. But the rodents turn out to be a bit more bloodthirsty than we thought. According to a new paper published in the Journal of Ethology, California ground squirrels have been caught in the act—many times over—of chasing, killing, and eating voles.

Co-author Jennifer Smith, a biologist at the University of Wisconsin, Eau Claire, described the behavior as "shocking," given the sheer number of times they watched squirrels do this. “We had never seen this behavior before," she said. "Squirrels are one of the most familiar animals to people. We see them right outside our windows; we interact with them regularly. Yet here’s this never-before-encountered-in-science behavior that sheds light on the fact that there’s so much more to learn about the natural history of the world around us.”

Squirrels mainly consume acorns, seeds, nuts, and fruits, but they have been known to supplement that diet with insects and, occasionally, by stealing eggs or young hatchlings from nests. And back in 1993, biologist J.R Callahan caused a stir by reporting that as many as 30 species of squirrel could be preying on smaller creatures: namely, fish, amphibians, reptiles, birds, and the occasional small mammal.

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The fall of the Berlin Wall in November 1989 was a seminal moment in 20th century history, paving the way for German reunification. Many segments, both large and small, were preserved for posterity—including portions covered in graffiti or murals. A team of Italian scientists used a combination of spectroscopic analysis and machine learning to study paint chips from wall fragments to learn more about the chemistry of the paints and pigments used, according to a new paper published in the Journal of the American Chemical Society.

There has been increased attention in recent years to preserving street art, which is vulnerable both to degradation over time as well as deliberate vandalism. For instance, in 2021, Italian chemists figured out how to use hydrogels to remove added graffiti from vandalized murals in Florence. (Over-painting by vandals is so chemically similar to the original painting underneath that it is difficult to selectively remove just the over-painting without damaging the original.) Unlike most classic masterpieces of the past, created with paints designed to last centuries, street art is more ephemeral in nature, using materials that lack such longevity.

In many cases, like the Berlin Wall, the painters didn't bother to document the specific materials they used, their application techniques, or other useful information that conservators could use to restore or conserve street art. Modern painting materials are also much more complex, and manufacturers typically do not report specific information on the composition of those materials.

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Archaeologists excavating a paleolithic cave site in Galilee, Israel, have found evidence that a deep-cave compound at the site may have been used for ritualistic gatherings, according to a new paper published in the Proceedings of the National Academy of Sciences (PNAS). That evidence includes the presence of a symbolically carved boulder in a prominent placement, and well as the remains of what may have been torches used to light the interior. And the acoustics would have been conducive to communal gatherings.

Dating back to the Early Upper Paleolithic period, Manot Cave was found accidentally when a bulldozer broke open its roof during construction in 2008. Archaeologists soon swooped in and recovered such artifacts as stone tools, bits of charcoal, remains of various animals, and a nearly complete human skull.

The latter proved to be especially significant, as subsequent analysis showed that the skull (dubbed Manot 1) had both Neanderthal and modern features and was estimated to be about 54,700 years old. That lent support to the hypothesis that modern humans co-existed and possibly interbred with Neanderthals during a crucial transition period in the region, further bolstered by genome sequencing.

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Physicists have been puzzling over conflicting observational results pertaining to the accelerating expansion rate of our Universe—a major discovery recognized by the 2011 Nobel Prize in Physics. New observational data from the James Webb Space Telescope (JWST) has confirmed that prior measurements of distances between nearby stars and galaxies made by the Hubble Space Telescope are not in error, according to a new paper published in The Astrophysical Journal. That means the discrepancy between observation and our current theoretical model of the Universe is more likely to be due to new physics.

As previously reported, the Hubble Constant is a measure of the Universe's expansion expressed in units of kilometers per second per megaparsec (Mpc). So, each second, every megaparsec of the Universe expands by a certain number of kilometers. Another way to think of this is in terms of a relatively stationary object a megaparsec away: Each second, it gets a number of kilometers more distant.

How many kilometers? That's the problem here. There are basically three methods scientists use to measure the Hubble Constant: looking at nearby objects to see how fast they are moving, gravitational waves produced by colliding black holes or neutron stars, and measuring tiny deviations in the afterglow of the Big Bang known as the Cosmic Microwave Background (CMB). However, the various methods have come up with different values. For instance, tracking distant supernovae produced a value of 73 km/s Mpc, while measurements of the CMB using the Planck satellite produced a value of 67 km/s Mpc.

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Epidermal electronics attached to the skin via temporary tattoos (e-tattoos) have been around for more than a decade, but they have their limitations, most notably that they don't function well on curved and/or hairy surfaces. Scientists have now developed special conductive inks that can be printed right onto a person's scalp to measure brain waves, even if they have hair. According to a new paper published in the journal Cell Biomaterials, this could one day enable mobile EEG monitoring outside a clinical setting, among other potential applications.

EEGs are a well-established, non-invasive method for recording the electrical activity of the brain, a crucial diagnostic tool for monitoring such conditions as epilepsy, sleep disorders, and brain injuries. It's also an important tool in many aspects of neuroscience research, including the ongoing development of brain-computer interfaces (BCIs). But there are issues. Subjects must wear uncomfortable caps that aren't designed to handle the variation in people's' head shapes, so a clinician must painstakingly map out the electrode positions on a given patient's head—a time-consuming process. And the gel used to apply the electrodes dries out and loses conductivity within a couple of hours, limiting how long one can make recordings.

By contrast, e-tattoos connect to skin without adhesives, are practically unnoticeable, and are typically attached via temporary tattoo, allowing electrical measurements (and other measurements, such as temperature and strain) using ultra-thin polymers with embedded circuit elements. They can measure heartbeats on the chest (ECG), muscle contractions in the leg (EMG), stress levels, and alpha waves through the forehead (EEG), for example.

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Ray spiders deploy an unusual strategy to capture prey in their webs. They essentially pull it back into a cone shape and release it when prey approaches, trapping said prey in the sticky silken threads. A few years ago, scientists noticed that they could get the spiders to release their webs just by snapping their fingers nearby, suggesting that the spiders relied at least in part on sound vibrations to know when to strike. Evidence for that hypothesis has now been confirmed in a new paper published in the Journal of Experimental Biology.

Most spider orb webs are static: the spiders weave them and fix them in place and then wait for prey to fly into the webs. That causes the silk threads to vibrate, alerting the spider that dinner is served. There are some species that actively actuate their webs, however, per the authors.

For instance, the triangle weaver spring-loads its triangular web once an insect has made contact so that the threads wrap around the prey in fractions of a second. Bolas spiders seem to detect prey in their vicinity through auditory cues, throwing a line of silk with a sticky end at passing moths to catch them. Ogre-faced spiders also seem to be able to hear potential prey, striking backward with a small silk net held in their front legs. It's a more proactive hunting strategy than merely waiting for prey to fly into a web.

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On April 15, 2019, the world watched in transfixed horror as a fire ravaged the famed Cathedral of Notre Dame in Paris, collapsing the spire and melting the lead roof. After years of painstaking restoration costing around $740 million, the cathedral reopens to the public this weekend. The December issue of National Geographic features an exclusive look inside the restored cathedral, accompanied by striking photographs by Paris-based photographer and visual artist Tomas van Houtryve.

For several hours, it seemed as if the flames would utterly destroy the 800-year-old cathedral. But after a long night of work by more than 400 Paris firefighters, the fire finally began to cool and attention began to shift to what could be salvaged and rebuilt. French President Emmanuel Macron vowed to restore Notre Dame to its former glory and set a five-year deadline. The COVID-19 pandemic caused some delays, but France nearly met that deadline regardless.

Those reconstruction efforts were helped by the fact that, a few years before the fire, scientist Andrew Tallon had used laser scanning to create precisely detailed maps of the interior and exterior of the cathedral—an invaluable aid as Paris rebuilds this landmark structure. French acousticians had also made detailed measurements of Notre Dame's "soundscape" that were instrumental in helping architects factor acoustics into their reconstruction plans. The resulting model even enabled Brian FG Katz, research director of the National Center for Scientific Research (CNRS) at Sorbonne University, to create a virtual reality version of Notre Dame with all the acoustical parameters in place.

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There's a common popular science demonstration involving "soap boats," in which liquid soap poured onto the surface of water creates a propulsive flow driven by gradients in surface tension. But it doesn't last very long since the soapy surfactants rapidly saturate the water surface, eliminating that surface tension. Using ethanol to create similar "cocktail boats" can significantly extend the effect because the alcohol evaporates rather than saturating the water.

That simple classroom demonstration could also be used to propel tiny robotic devices across liquid surfaces to carry out various environmental or industrial tasks, according to a preprint posted to the physics arXiv. The authors also exploited the so-called "Cheerios effect" as a means of self-assembly to create clusters of tiny ethanol-powered robots.

As previously reported, those who love their Cheerios for breakfast are well acquainted with how those last few tasty little "O"s tend to clump together in the bowl: either drifting to the center or to the outer edges. The "Cheerios effect is found throughout nature, such as in grains of pollen (or, alternatively, mosquito eggs or beetles) floating on top of a pond; small coins floating in a bowl of water; or fire ants clumping together to form life-saving rafts during floods. A 2005 paper in the American Journal of Physics outlined the underlying physics, identifying the culprit as a combination of buoyancy, surface tension, and the so-called "meniscus effect."

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© Jackson K. Wilt et al. 2024

Paleontologists have long puzzled over how the dinosaurs—originally relatively small and of minor importance to the broader ecosystem—evolved to become the dominant species some 30 million years later. Fossilized feces and vomit from dinosaurs might hold important clues to how and why this evolutionary milestone came about, according to a new paper published in the journal Nature.

Co-author Martin Qvarnström, an evolutionary biologist with Uppsala University in Sweden, and his collaborators studied trace fossils known as bromalites, a designation that includes coprolites as well as vomit or other fossilized matter from an organism's digestive tract. As previously reported, coprolites aren't quite the same as paleofeces, which retain a lot of organic components that can be reconstituted and analyzed for chemical properties. Coprolites are fossils, so most organic components have been replaced by mineral deposits like silicate and calcium carbonates.

For archaeologists keen on learning more about the health and diet of past populations—as well as how certain parasites evolved in the evolutionary history of the microbiome—coprolites and paleofeces can be a veritable goldmine of information. For instance, in 2021 we reported on an analysis of preserved paleo-poop revealing that ancient Iron Age miners in what is now Austria were fond of beer and blue cheese.

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Virtual reality (VR) technology has long sought to incorporate the human senses into virtual and mixed-reality environments. In addition to sight and sound, researchers have been trying to add the sensation of human touch and smell via various user interfaces, as well as taste. But the latter has proved to be quite challenging. A team of Hong Kong scientists has now developed a handheld user interface shaped like a lollipop capable of re-creating several different flavors in a virtual environment, according to a new paper published in the Proceedings of the National Academy of Sciences (PNAS).

It's well established that human taste consists of sweet, salty, sour, bitter, and umami—five basic flavors induced by chemical stimulation of the tongue and, to a lesser extent, in parts of the pharynx, larynx, and epiglottis. Recreating those sensations in VR has resulted in a handful of attempts at a flavor user interface, relying on such mechanisms as chemical, thermal, and electrical stimulation, as well as iontophoresis.

The chemical approach usually involves applying flavoring chemicals directly onto the tongue, but this requires room for bulk storage of said chemicals, and there is a long delay time that is not ideal for VR applications. Thermal variations applied directly to the tongue can stimulate taste sensations but require a complicated system incorporating a cooling subsystem and temperature sensors, among other components.

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In the wee hours of December 26, 2004, a massive 9.2 earthquake occurred in the Indian Ocean, generating an equally massive tsunami that caused unprecedented devastation to 14 countries and killed more than 230,000 people. Twenty years later, National Geographic has revisited one of the deadliest natural disasters in recorded history with a new documentary: Tsunami: Race Against Time. The four-part series offers an in-depth account of the tsunami's destructive path, told from the perspectives of those who survived, as well as the scientists, journalists, doctors, nurses, and everyday heroes who worked to save as many as possible.

Geophysicist Barry Hirshorn—now with Scripps Institution of Oceanography at the University of California, San Diego—was on duty at the Pacific Tsunami Warning Center in Hawaii that day (3 pm on Christmas Day local time). His pager went off, indicating that seismic waves had set off a seismometer in Australia, and Hirshorn rushed to the control room to locate the quake's epicenter with his colleague, Stuart Weinstein.

They initially pegged the quake at 8.5 magnitude. (It was later upgraded to 8.9 and subsequently to a whopping 9.2 to 9.3 magnitude.) But despite its strength, they initially did not think the quake would generate a tsunami, at least in the Pacific. And such events were incredibly rare in the Indian Ocean.

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