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      A new gravity sensor used atoms’ weird quantum behavior to peer underground

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      The best way to find buried treasure may be with a quantum gravity sensor.

      In these devices, free-falling atoms reveal subtle variations in Earth’s gravitational pull at different places. Those variations reflect differences in the density of material beneath the sensor — effectively letting the instrument peer underground. In a new experiment, one of these machines teased out the tiny gravitational signature of an underground tunnel, researchers report in the Feb. 24 Nature.

      “Instruments like this would find many, many applications,” says Nicola Poli, an experimental physicist at the University of Florence, who coauthored a commentary on the study in the same issue of Nature.

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      Poli imagines using quantum gravity sensors to monitor groundwater or magma beneath volcanoes, or to help archaeologists uncover hidden tombs or other artifacts without having to dig them up (SN: 11/2/17). These devices could also help farmers check soil quality or help engineers inspect potential construction sites for unstable ground.

      “There are many tools to measure gravity,” says Xuejian Wu, an atomic physicist at Rutgers University in Newark, N.J., who wasn’t involved in the study. Some devices measure how far gravity pulls down a mass hanging from a spring. Other tools use lasers to clock how fast an object tumbles down a vacuum chamber. But free-falling atoms, like those in quantum gravity sensors, are the most pristine, reliable test masses out there, Wu says. As a result, quantum sensors promise to be more accurate and stable in the long run than other gravity probes.

      Inside a quantum gravity sensor, a cloud of supercooled atoms is dropped down a chute. A pulse of light then splits each of the falling atoms into a superposition state — a quantum limbo where each atom exists in two places at once (SN: 11/7/19). Due to their slightly different positions in Earth’s gravitational field, the two versions of each atom feel a different downward tug as they fall. Another light pulse then recombines the split atoms.

      Thanks to the atoms’ wave-particle duality — a strange rule of quantum physics that says atoms can act like waves — the reunited atoms interfere with each other (SN: 1/13/22). That is, as the atom waves overlap, their crests and troughs can reinforce or cancel each other out, creating an interference pattern. That pattern reflects the slightly different downward pulls that the split versions of each atom felt as they fell — revealing the gravity field at the atom cloud’s location.

      Extremely precise measurements made by such atom-based devices have helped test Einstein’s theory of gravity (SN: 10/28/20) and measure fundamental constants, such as Newton’s gravitational constant (SN: 4/12/18). But atom-based gravity sensors are highly sensitive to vibrations from seismic activity, traffic and other sources.

      “Even very, very small vibrations create enough noise that you have to measure for a long time” at any location to weed out background tremors, says Michael Holynski, a physicist at the University of Birmingham in England. That has made quantum gravity sensing impractical for many uses outside the lab.  

      Holynski’s team solved that problem by building a gravity sensor with not one but two falling clouds of rubidium atoms. With one cloud suspended a meter above the other, the instrument could gauge the strength of gravity at two different heights in a single location. Comparing those measurements allowed the researchers to cancel out the effects of background noise.

      Holynski and colleagues tested whether their sensor — a 2-meter-tall chute on wheels tethered to a rolling cart of equipment — could detect an underground passageway on the University of Birmingham campus. The 2-by-2-meter concrete tunnel lay beneath a road between two multistory buildings. The quantum sensor measured the local gravitational field every 0.5 meters along an 8.5-meter line that crossed over the tunnel. Those readouts matched the predictions of a computer simulation, which had estimated the gravitational signal of the tunnel based on its structure and other factors that could influence the local gravitational field, such as nearby buildings.

      Based on the machine’s sensitivity in this experiment, it could probably provide a reliable gravity measurement at each location in less than two minutes, the researchers estimate. That’s about one-tenth the time needed for other types of gravity sensors.

      The team has since built a downsized version of the gravity sensor used in the tunnel-detecting experiment. The new machine weighs about 15 kilograms, compared with the 300-kilogram beast used for the tunnel test. Other upgrades could also boost the gravity sensor’s speed.

      In the future, engineer Nicole Metje envisions building a quantum gravity sensor that could be pushed from place to place like a lawn mower. But portability isn’t the only challenge for making these tools more user-friendly, says Metje, a coauthor on the study who is also at the University of Birmingham. “At the moment, we still need someone with a physics degree to operate the sensor.”

      So hopeful beachcombers may be waiting a long time to trade in their metal detectors for quantum gravity sensors. More

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      Core memory weavers and Navajo women made the Apollo missions possible

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      The historic Apollo moon missions are often associated with high-visibility test flights, dazzling launches and spectacular feats of engineering. But intricate, challenging handiwork — comparable to weaving — was just as essential to putting men on the moon. Beyond Neil Armstrong, Buzz Aldrin and a handful of other names that we remember were hundreds of thousands of men and women who contributed to Apollo over a decade. Among them: the Navajo women who assembled state-of-the-art integrated circuits for the Apollo Guidance Computer and the women employees of Raytheon who wove the computer’s core memory.

      In 1962, when President John F. Kennedy declared that putting Americans on the moon should be the top priority for NASA, computers were large mainframes; they occupied entire rooms. And so one of the most daunting yet crucial challenges was developing a highly stable, reliable and portable computer to control and navigate the spacecraft.

      NASA chose to use cutting-edge integrated circuits in the Apollo Guidance Computer. These commercial circuits had been introduced only recently. Also known as microchips, they were revolutionizing electronics and computing, contributing to the gradual miniaturization of computers from mainframes to today’s smartphones. NASA sourced the circuits from the original Silicon Valley start-up, Fairchild Semiconductor. Fairchild was also leading the way in the practice known as outsourcing; the company opened a factory in Hong Kong in the early 1960s, which by 1966 employed 5,000 people, compared with Fairchild’s 3,000 California employees.

      At the same time, Fairchild sought low-cost labor within the United States. Lured by tax incentives and the promise of a labor force with almost no other employment options, Fairchild opened a plant in Shiprock, N.M., within the Navajo reservation, in 1965. The Fairchild factory operated until 1975 and employed more than 1,000 individuals at its peak, most of them Navajo women manufacturing integrated circuits.

      It was challenging work. Electrical components had to be placed on tiny chips made of a semiconductor such as silicon and connected by wires in precise locations, creating complex and varying patterns of lines and geometric shapes. The Navajo women’s work “was performed using a microscope and required painstaking attention to detail, excellent eyesight, high standards of quality and intense focus,” writes digital media scholar Lisa Nakamura.

      A brochure commemorating the dedication of Fairchild Semiconductor’s plant in Shiprock, N.M., included this Fairchild 9040 integrated circuit.Courtesy of the Computer History Museum

      In a brochure commemorating the dedication of the Shiprock plant, Fairchild directly compared the assembly of integrated circuits with what the company portrayed as the traditional, feminine, Indigenous craft of rug-weaving. The Shiprock brochure juxtaposed a photo of a microchip with one of a geometric-patterned rug, and another of a woman weaving such a rug. That portrayal, Nakamura argues, reinforced racial and gender stereotypes. The work was dismissed as “women’s work,” depriving the Navajo women of appropriate recognition and commensurate compensation.  Journalists and Fairchild employees also “depict[ed] electronics manufacture as a high-tech version of blanket weaving performed by willing and skillful Indigenous women,” Nakamura notes, yet “the women who performed this labor did so for the same reason that women have performed factory labor for centuries — to survive.”

      Far from the Shiprock desert, outside of Boston, women employees at Raytheon assembled the Apollo Guidance Computer’s core memory with a process that in this case directly mimicked weaving. Again, the moon missions demanded a stable and compact way of storing Apollo’s computing instructions. Core memory used metal wires threaded through tiny doughnut-shaped ferrite rings, or “cores,” to represent 1s and 0s. All of this core memory was woven by hand, with women sitting on opposite sides of a panel passing a wire-threaded needle back and forth to create a particular pattern. (In some cases, a woman worked alone, passing the needle through the panel to herself.)

      Women employees of Raytheon assembled core memory for the Apollo Guidance Computer by threading metal wires through rings. (This unnamed woman was described as a “space age needleworker” in a Raytheon press kit.)Courtesy of the collection of David Meerman Scott, Raytheon public relations

      Apollo engineers referred to this process of building memory as the “LOL,” or “Little Old Ladies,” method. Yet this work was so mission critical that it was tested and inspected multiple times. Mary Lou Rogers, who worked on Apollo, recalled, “[Each component] had to be looked at by three of four people before it was stamped off. We had a group of inspectors come in for the federal government to check our work all the time.”

      The core memory was also known as rope memory, and those who supervised its development were “rope mothers.” We know a great deal about one rope mother — Margaret Hamilton. She has been recognized with the Presidential Medal of Freedom, among other awards, and is now remembered as the woman who oversaw most of the Apollo software. But her efforts were unrecognized by many at the time. Hamilton recalled, “At the beginning, nobody thought software was that big a deal. But then they began to realize how much they were relying on it…. Astronauts‘ lives were at stake. Our software needed to be ultrareliable and it needed to be able to detect an error and recover from it at any time during the mission. And it all had to fit on the hardware.” Yet, little is known about the thousands of others who performed this mission-critical work of weaving integrated circuits and core memory.

      Margaret Hamilton is known for overseeing the development of the Apollo software. Draper Laboratory, restored by Adam Cuerden/Wikimedia Commons

      At the time, Fairchild’s representation of the Navajo women’s work as a feminine craft differentiated it from the high-status and masculine work of engineering. As Nakamura has written, the work “came to be understood as affective labor, or a ‘labor of love.’” Similarly, the work performed at Raytheon was described by Eldon Hall, who led the Apollo Guidance Computer’s hardware design, as “tender loving care.” Journalists and even a Raytheon manager presented this work as requiring no thinking and no skill.

      Recently, the communications scholar Samantha Shorey, engineer Daniela Rosner, technologist Brock Craft and quilt artist Helen Remick firmly overturned the notion that weaving core memory was a “no-brainer” with their Making Core Memory project. In nine workshops, they invited participants to weave core memory “patches” using metal matrices, beads and conductive threads, showcasing the deep focus and meticulous attention to detail required. The patches were then assembled in an electronic quilt that played aloud accounts from 1960s Apollo engineers and Raytheon managers. The Making Core Memory collaboration challenged the dichotomy of masculine, high-status, well-paid science and engineering cognitive labor versus feminine, low-status, low-paid, manual labor.

      A 1975 NASA report that summarized the Apollo missions spoke glowingly of the Apollo computing systems — but mentioned none of the Navajo or Raytheon women. “The performance of the computer was flawless,” the report declared. “Perhaps the most significant accomplishment during Apollo pertaining to guidance, navigation, and control was the demonstration of the versatility and adaptability of the computer software.”

      That computer, and that software, relied on the skilled, technical, embodied expertise and labor of thousands of women, including women of color. They were indubitably women of science, and their untold stories call us to reconsider who does science, and what counts as scientific expertise.  More

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      ‘From Data to Quanta’ defends Niels Bohr’s view of quantum mechanics

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      From Data to QuantaSlobodan PerovićUniv. of Chicago, $45

      Ever since Max Planck introduced the idea of the quantum to the world, physicists have argued about whether reality is more like sand or water.

      Planck’s famous 1900 discovery that energy is grainy — at least when absorbed or emitted — moved him to label those smallest bits of energy grains “quanta.” But he believed that once emitted, as in light from a fire, those grains merged into smooth, continuous waves, just as water seems a smooth liquid to human perception. Einstein, on the other hand, insisted that light quanta traveled through space on their own, behaving like particles later called photons.

      By the mid-1920s, both the wave and particle views of light had gained experimental support, with the additional paradox that electrons — supposedly particles — could sometimes disguise themselves as waves.

      Into this arena of controversy stepped the famed Danish physicist Niels Bohr, the pioneer of exploring the architecture of the atom. Bohr announced that resolving the wave-particle paradox required a new view of reality, in which both notions shared a role in explaining experimental phenomena. In experiments designed to observe waves, waves you would find, whether electrons or light. In experiments designed to detect particles, you’d see particles. But in no experiment could you demonstrate both at once. Bohr called this viewpoint the principle of complementarity, and it successfully guided the pursuit of quantum mechanics during the following decades.

      More recently, as philosopher Slobodan Perović recounts in From Data to Quanta, Bohr’s success has been questioned by some physicists and philosophers and even popular science writers (SN: 1/19/19, p. 26). Complementarity has been derided as an incoherent application of vague philosophy expressed in incomprehensible language. But as Perović’s investigations reveal, such criticisms are rarely rooted in any deep understanding of Bohr’s methods. Rather than Bohr’s philosophy contaminating his science, Perović argues, it is his opponents’ philosophical prejudices that have led to misstatements, misunderstandings and misrepresentations of Bohr’s physics. And Bohr can’t be understood by attempting to understand his philosophy, Perović asserts, because philosophy did not guide him — experiments did.

      In fact, Bohr’s drive to understand the wave-particle paradox was fueled by a deep devotion to comprehending the experimental evidence in its totality. It was the same approach the younger Bohr took when developing his model of the atom in 1913 (SN: 7/13/13, p. 20). Various experiments suggested properties of the atom that seemed irreconcilable. But Bohr forged those experimental clues into a “master hypothesis” that produced a thoroughly novel understanding of the atom and its structure.

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      Perović describes how Bohr’s process began with lower-level hypotheses stemming from features directly given by experiment. Spectral lines — different specific colors of light emitted by atoms — led to basic hypotheses that some vibratory process, of an atom itself or its constituents, produced electromagnetic radiation exhibiting precise patterns. Intermediate hypotheses about the structure of the atom did not explain such lines, though. And then Ernest Rutherford, on the basis of experiments in his lab, inferred that an atom was mostly empty space. It contained a dense, tiny central nucleus encompassing most of the mass, while lightweight electrons orbited at a distance. But that hypothesis didn’t mesh with the precise patterns of spectral lines. And such an atom would be unstable, persisting for less than a millisecond. From all these disparate experiment-based hypotheses, Bohr applied Planck’s quantum idea to construct a master hypothesis. He reconciled the spectral lines and Rutherford’s nuclear atom with a new atomic model, in which electrons maintained stability of the atom but jumped from one orbit to another, emitting specific patterns of spectral lines in the process.

      As Perović demonstrates, Bohr followed a similar course in arriving at complementarity. While numerous experiments showed that light was a wave, by the early 1920s other experiments established that X-rays, highly energetic light, collided with electrons just as though both were particles (momentum and energy were conserved in the collisions just as the particle view required). Bohr’s master hypothesis, complementarity, seemed the only way forward.

      Throughout the book, Perović relates how Bohr has been misinterpreted, his views misleadingly conflated with those of others (like John von Neumann and Werner Heisenberg), and his philosophy incorrectly portrayed as antirealist — suggesting that only observations brought reality into existence. Bohr never said any such thing, and in fact cautioned against using language so loosely.

      Perović’s account offers a thorough survey of other historical investigations into Bohr’s work and draws liberally from Bohr’s own writings. It’s a nuanced and insightful presentation of the interplay of experiment and theory in the scientific process. This book is not easy reading, though. It’s not the place to seek clear explanations of quantum physics and Bohr’s interpretation of it. Perović opts for scholarly thoroughness and careful reasoning with a propensity for long sentences. But then again, Bohr’s writings were no breeze, either. In fact, a major complaint against Bohr has been expressed by authors who say his writings are very difficult to understand. It’s unfortunate that so many seem to think that because they can’t understand Bohr, he must have been wrong. Perović’s book provides a useful antidote to that attitude.

      Buy From Data to Quanta from Bookshop.org. Science News is a Bookshop.org affiliate and will earn a commission on purchases made from links in this article. More

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      A diamondlike structure gives some starfish skeletons their strength

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      Some starfish made of a brittle material fortify themselves with architectural antics.

      Beneath a starfish’s skin lies a skeleton made of pebbly growths, called ossicles, which mostly consist of the mineral calcite. Calcite is usually fragile, and even more so when it is porous. But the hole-riddled ossicles of the knobby starfish (Protoreaster nodosus) are strengthened through an unexpected internal arrangement, researchers report in the Feb. 11 Science.

      “When we first saw the structure, we were really amazed,” says Ling Li, a materials scientist at Virginia Tech in Blacksburg. It looks like it’s been 3-D printed, he says.

      Li and colleagues used an electron microscope to zoom in on ossicles from several dozen dead knobby starfish. At a scale of 50 micrometers, about half the width of a human hair, the seemingly featureless body of each ossicle gives way to a meshlike pattern that mirrors how carbon atoms are arranged in a diamond.

      Zooming in on the bumpy growths called ossicles (seen in this electron microscope image) that make up a knobby starfish’s skeleton reveals a meshlike structure similar to the arrangement of carbon atoms in diamond. This arrangement strengthens the ossicles, which are mostly made of calcite, a relatively weak mineral.Ling Li/Virginia Tech

      But the diamondlike lattice alone doesn’t fully explain how the ossicles stay strong.

      Within that lattice, the atoms that make up the calcite have their own pattern, which resembles a series of stacked hexagons. That pattern affects the strength of the calcite too. In general, a mineral’s strength isn’t uniform in all directions. So pushing on calcite in some directions is more likely to break it than force from other directions. In the ossicles, the atomic pattern and the diamondlike lattice align in a way that compensates for calcite’s intrinsic weakness.

      It’s a mystery how the animals make the diamondlike lattice. Li’s team is studying live knobby starfish, surveying the chemistry of how ossicles form. Understanding how the starfish build their ossicles may provide insights for creating stronger porous materials, including some ceramics.

      We can learn a lot from a creature like a starfish that we may think is primitive, Li says.

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      The quantum ‘boomerang’ effect has been seen for the first time

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      Some quantum particles gotta get right back to where they started from.

      Physicists have confirmed a theoretically predicted phenomenon called the quantum boomerang effect. An experiment reveals that, after being given a nudge, particles in certain materials return to their starting points, on average, researchers report in a paper accepted in Physical Review X.

      Particles can boomerang if they’re in a material that has lots of disorder. Instead of a pristine material made up of orderly arranged atoms, the material must have many defects, such as atoms that are missing or misaligned, or other types of atoms sprinkled throughout.

      In 1958, physicist Philip Anderson realized that with enough disorder, electrons in a material become localized: They get stuck in place, unable to travel very far from where they started. The pinned-down electrons prevent the material from conducting electricity, thereby turning what might otherwise be a metal into an insulator. That localization is also necessary for the boomerang effect.

      To picture the boomerang in action, physicist David Weld of the University of California, Santa Barbara imagines shrinking himself down and slipping inside a disordered material. If he tries to fling away an electron, he says, “it will not only turn around and come straight back to me, it’ll come right back to me and stop.” (Actually, he says, in this sense the electron is “more like a dog than a boomerang.” The boomerang will keep going past you if you don’t catch it, but a well-trained dog will sit by your side.)

      Weld and colleagues demonstrated this effect using ultracold lithium atoms as stand-ins for the electrons. Instead of looking for atoms returning to their original position, the team studied the analogous situation for momentum, because that was relatively straightforward to create in the lab. The atoms were initially stationary, but after being given kicks from lasers to give them momenta, the atoms returned, on average, to their original standstill states, making a momentum boomerang.

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      The team also determined what’s needed to break the boomerang. To work, the boomerang effect requires time-reversal symmetry, meaning that the particles should behave the same when time runs forward as they would on rewind. By changing the timing of the first kick from the lasers so that the kicking pattern was off-kilter, the researchers broke time-reversal symmetry, and the boomerang effect disappeared, as predicted.

      “I was so happy,” says Patrizia Vignolo, a coauthor of the study. “It was perfect agreement” with their theoretical calculations, says Vignolo, a theoretical physicist at Université Côte d’Azur based in Valbonne, France.

      Even though Anderson made his discovery about localized particles more than 60 years ago, the quantum boomerang effect is a recent newcomer to physics. “Nobody thought about it, apparently, probably because it’s very counterintuitive,” says physicist Dominique Delande of CNRS and Kastler Brossel Laboratory in Paris, who predicted the effect with colleagues in 2019.

      The weird effect is the result of quantum physics. Quantum particles act like waves, with ripples that can add and subtract in complicated ways (SN: 5/3/19). Those waves combine to enhance the trajectory that returns a particle to its origin and cancel out paths that go off in other directions. “This is a pure quantum effect,” Delande says, “so it has no equivalent in classical physics.” More

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      Quantum particles can feel the influence of gravitational fields they never touch

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      If you’re superstitious, a black cat in your path is bad luck, even if you keep your distance. Likewise, in quantum physics, particles can feel the influence of magnetic fields that they never come into direct contact with. Now scientists have shown that this eerie quantum effect holds not just for magnetic fields, but for gravity too — and it’s no superstition.

      Usually, to feel the influence of a magnetic field, a particle would have to pass through it. But in 1959, physicists Yakir Aharonov and David Bohm predicted that, in a specific scenario, the conventional wisdom would fail. A magnetic field contained within a cylindrical region can affect particles — electrons, in their example — that never enter the cylinder. In this scenario, the electrons don’t have well-defined locations, but are in “superpositions,” quantum states described by the odds of a particle materializing in two different places. Each fractured particle simultaneously takes two different paths around the magnetic cylinder. Despite never touching the electrons, and hence exerting no force on them, the magnetic field shifts the pattern of where particles are found at the end of this journey, as various experiments have confirmed (SN: 3/1/86).

      In the new experiment, the same uncanny physics is at play for gravitational fields, physicists report in the Jan. 14 Science. “Every time I look at this experiment, I’m like, ‘It’s amazing that nature is that way,’” says physicist Mark Kasevich of Stanford University.

      Kasevich and colleagues launched rubidium atoms inside a 10-meter-tall vacuum chamber, hit them with lasers to put them in quantum superpositions tracing two different paths, and watched how the atoms fell. Notably, the particles weren’t in a gravitational field–free zone. Instead, the experiment was designed so that the researchers could filter out the effects of gravitational forces, laying bare the eerie Aharonov-Bohm influence.

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      The study not only reveals a famed physics effect in a new context, but also showcases the potential to study subtle effects in gravitational systems. For example, researchers aim to use this type of technique to better measure Newton’s gravitational constant, G, which reveals the strength of gravity, and is currently known less precisely than other fundamental constants of nature (SN: 8/29/18).

      A phenomenon called interference is key to this experiment. In quantum physics, atoms and other particles behave like waves that can add and subtract, just as two swells merging in the ocean make a larger wave. At the end of the atoms’ flight, the scientists recombined the atoms’ two paths so their waves would interfere, then measured where the atoms arrived. The arrival locations are highly sensitive to tweaks that alter where the peaks and troughs of the waves land, known as phase shifts.

      At the top of the vacuum chamber, the researchers placed a hunk of tungsten with a mass of 1.25 kilograms. To isolate the Aharonov-Bohm effect, the scientists performed the same experiment with and without this mass, and for two different sets of launched atoms, one which flew close to the mass, and the other lower. Each of those two sets of atoms were split into superpositions, with one path traveling closer to the mass than the other, separated by about 25 centimeters. Other sets of atoms, with superpositions split across smaller distances, rounded out the crew. Comparing how the various sets of atoms interfered, both with and without the tungsten mass, teased out a phase shift that was not due to the gravitational force. Instead, that tweak was from time dilation, a feature of Einstein’s theory of gravity, general relativity, which causes time to pass more slowly close to a massive object.

      The two theories that underlie this experiment, general relativity and quantum mechanics, don’t work well together. Scientists don’t know how to combine them to describe reality. So, for physicists, says Guglielmo Tino of the University of Florence, who was not involved with the new study, “probing gravity with a quantum sensor, I think it’s really one of … the most important challenges at the moment.” More