Counting on Grains of Sand

Civil engineers have developed a new method that measures the way forces move through granular materials—one that could improve our understanding of everything from how soils bear the weight of buildings to what stresses are at work deep below the surface of the earth.

Granular materials—conglomerations of solid particles larger than a micrometer, such as gravel or coffee grounds—are everywhere in the world around us. The snow on a mountain top is a granular material, as is the grain stored in a silo. Granular materials exhibit distinct and sometimes unusual properties. For example, if you shake an aggregate composed of particles spanning a wide range of sizes, the larger particles rise to the top; this counterintuitive behavior is known as granular size separation, although it is sometimes referred to as the "Brazil nut effect" because large Brazil nuts tend to rise to the top of a packet of mixed nuts.

Because of their ubiquity, granular materials are of significant interest to scientists and engineers. And an important goal of granular material science is to be able to measure how forces move through such materials, says Caltech's Jose Andrade, professor of civil and mechanical engineering in the Division of Engineering and Applied Sciences. "It is the ultimate frontier in granular materials," he says. "Understanding the way they move and carry forces will help us reconstruct their collective behavior."

Andrade and his colleagues used a combination of computed tomography (CT) scanning and X-ray diffraction to measure the deformation of individual grains, in effect turning each particle into a force gauge that shows the direction and intensity of force.

Imagine two identical houses: one sitting on bedrock and the other on a sandy soil. Both are subject to gravity's downward pull. The mass of the house on bedrock, because it pushes on a solid object, generates a force that is easy to model. However, the force generated by the house on sand, although identical, is much more difficult to model because it is dispersed across many millions of sand grains, each with a different shape and orientation and moving with respect to one another and in different directions.

In this example, the house on sand exerts force on the grains that it touches, which exert force on the grains around them, which in turn exert force on the grains below them, and so on. The force is not transmitted straight down, but rather radiates out in asymmetric patterns determined by each grain's shape and interaction with its immediate neighbors.

Prior research measuring forces in granular materials often used grains made of a special material that shines when placed under stress. By observing which grains were shining and which were not, researchers could track the propagation of stress through the material as a whole. Andrade and his colleagues, however, wanted to be able to track the propagation of stress through any granular material. Their new technique is based on the fact that, when under stress, the shape of an individual grain will change slightly, much the way a foam ball deforms to varying degrees based on how hard you squeeze it.

The team used CT scanning to reveal how particles are shaped and oriented and X-ray diffraction to show—at an atomic level—how those particles deform under pressure. This information was then used to calculate how much force each individual grain is under and to quantify the transmission of force through granular materials.

More than just a proof-of-concept, the test revealed a surprising characteristic of granular materials: the more external pressure is placed on them, the more homogenous the substance acts, regardless of how heterogenous the grains may actually be. As the team slowly increased the amount of force on their test particles, they noted that the dispersion of that force grew more equitable throughout the entire material.

The observation led to an unforeseen connection with the social sciences.

Social scientists use a statistical expression called the Gini coefficient to measure income and wealth inequality in societies. For example, a Gini coefficient of 0 signifies a society in which all of the wealth is concentrated in the hands of a single individual, while a coefficient of 1 signifies a society in which everyone has an equal share.

Andrade and his colleagues found that this same coefficient can be used to model the dispersion of force through granular materials. "To draw a correlation, the granular 'societies' in our samples become more equitable as external pressure increases," Andrade says. "We can call this observation granular solidarity."

Being able to quantify granular solidarity means that engineers will know how much external pressure a granular material needs to be under before it behaves like a single, unbroken material—as opposed than a collection of tiny grains. The finding could simplify future engineering calculations about granular materials.

The work is described in a paper, titled "Quantifying Interparticle Forces and Heterogeneity in 3D Granular Materials," published online on July 18 in the journal Physical Review Letters. Coauthors on the paper include Ryan Hurley, formerly of Caltech but now with the Lawrence Livermore National Laboratory; S. A. Hall of Lund University in Sweden; and J. Wright of the European Synchrotron Radiation Facility in France. The research was funded by the U.S. Air Force Office of Scientific Research, the U.S. Defense Threat Reduction Agency, Lawrence Livermore National Laboratory, and the European Commission.

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The Utility of Instability

A team of researchers from Caltech and Harvard has designed and created mechanical chains made of soft matter that can transmit signals across long distances. Because they are flexible, the circuits could be used in machines such as soft robots or lightweight aircraft constructed from pliable, nonmetallic materials.

Unlike hard materials, which transit signals readily, soft materials tend to absorb energy as it passes through them. An analogy is hitting a firm punching bag versus a soft one: with the firm bag, the energy of your punch moves through the bag and sends it swinging, but the soft bag deforms your fist like a lump of dough and therefore will swing less.

To overcome that response, Caltech's Dennis Kochmann, Chiara Daraio, and their colleagues created an unstable, "nonlinear" system. Their findings have appeared in three papers published over the past few months.

"Engineers tend to shy away from instability. Instead, we take advantage of it," says Kochmann, assistant professor of aerospace in the Division of Engineering and Applied Sciences, and one of the lead researchers on the project.

Stable, or "linear," systems are attractive to engineers because they are easy to model and predict. Take, for example, a spring: If you push on a spring, it will respond by pushing back with a force that is linearly proportional to how much force you apply. The response of a nonlinear system to that same push, by comparison, is not proportional, and can include sudden changes in the direction or amplitude of the responsive force.

The nonlinear systems that Kochmann and his colleagues designed rely on bistable elements, or elements that can be stable in two distinct states. The bistable elements that the team developed consist of arches of an elastic material, each a few millimeters in size. The elements can be in either a convex or a concave position—and are stable in either configuration. However, if you push on the element in its convex position, it responds by pushing back against the direction of force until it snaps into a concave position, accompanied by a sudden release of energy in the opposite direction.

"It's an elastic response, and then a snap-through," explains Daraio, professor of aeronautics and applied physics.

Collaborating with Katia Bertoldi, Jennifer Lewis, and Jordan Raney of Harvard University, Kochmann, Daraio, and Caltech graduate student Neel Nadkarni designed chains of the bistable elements, connected to one another by springs. When one link "pops" from the concave to the convex state, its spring tugs at the link that is next downstream in the chain, popping it to a convex position as well. The signal travels unidirectionally down the chain. The energy released by the popping balances out any energy absorbed by the soft material, allowing the process to continue down the chain across long distances and at constant speed.

"We were inspired by the physics of phase transformations. A phase transformation is a process where a switching occurs between two stable states of a system. It is governed by strongly nonlinear mathematical equations that are not very well understood," Nadkarni says.

A proof-of-concept version of the design constructed from 3-D printed elements is described in a paper published August 8, 2016 in the Proceedings of the National Academy of Sciences. This paper was the third in the series of publications outlining the new concept for transmitting signals. It outlined how the design can be used to build mechanical AND and OR logic gates such as those used in computer processors. Logic gates are the building blocks of circuits, allowing signals to be processed.

"These systems could be used as actuators to control robotic limbs, while passively performing simple logic decisions," Daraio says. Actuators use the transfer of energy to perform mechanical work, and in this case, the transfer of energy would occur via a mechanical rather than an electrical system.

The first paper in the series was published in March in the journal Physical Review B, and it described Kochmann's theoretical, mathematical framework for the system.  The second paper was published in Physical Review Letters in June, and it describes Daraio's first experimental model for the system.

While springs can be employed between the bistable elements, the team also demonstrated in the Physical Review Letters paper how magnets could be used to connect the elements—potentially allowing the chain to be reset to its original position with a reversal of polarity.

"Though there are many applications, the fundamental principles that we explore are most exciting to me," Kochmann says. "These nonlinear systems show very similar behavior to materials at the atomic scale but these are difficult to access experimentally or computationally. Now we have built a simple macroscale analogue that mimics how they behave."

The PNAS paper is titled "Stable propogation of mechanical signals in soft media using stored elastic energy." The authors are Nadkarni, Daraio, and Kochmann of Caltech and Jordan Raney, Jennifer Lewis, and Katia Bertoldi of Harvard University. The work was funded by the National Science Foundation.

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Researchers from Caltech and Harvard have designed and created mechanical chains made of soft matter that can transmit signals across long distances.

Chorus of Black Holes Radiates X-Rays

Supermassive black holes do not give off any of their own light, hence the word "black" in their name. However, many black holes pull in, or accrete, surrounding material, and emit powerful bursts of X-rays. Collectively, these active black holes throughout the sky can be thought of a cosmic choir, singing in the language of X-rays. Their "song" is what astronomers call the cosmic X-ray background.

To date, NASA's Chandra mission has managed to pinpoint many of the individual black holes contributing to the X-ray background, but the ones that let out high-energy X-rays—those with the highest-pitched "voices"—have remained elusive.

New data from NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has, for the first time, begun to pinpoint large numbers of the black holes sending out the high-energy X-rays. More technically, NuSTAR has made significant progress in resolving the high-energy X-ray background.

"We've gone from resolving just 2 percent of the high-energy X-ray background to 35 percent," says Fiona Harrison, Benjamin M. Rosen Professor of Physics and Astronomy at Caltech, the principal investigator of NuSTAR, and lead author of a new study describing the findings in an upcoming issue of The Astrophysical Journal. "We can see the most obscured black holes, hidden in thick gas and dust."

The results will ultimately help astronomers understand how the growth patterns of supermassive black holes change over time—a key factor in the development of black holes and the galaxies that host them. For instance, the supermassive black hole at the center of our Milky Way galaxy is dormant now, but at some point in the past, it would have siphoned gas and bulked up in size.

As black holes grow, their intense gravity pulls matter toward them. The matter heats up to extremely high temperatures and particles get boosted to close to the speed of light. Together, these processes make the black hole surroundings glow with X-rays. A supermassive black hole with an ample supply of fuel, or gas, will give off more high-energy X-rays.

NuSTAR is the first telescope capable of focusing these high-energy X-rays into sharp pictures.

"Before NuSTAR, the X-ray background in high-energies was just one blur with no resolved sources," says Harrison. "To untangle what's going on, you have to pinpoint and count up the individual sources of the X-rays."

"We knew this cosmic choir had a strong high-pitched component, but we still don't know if it comes from a lot of smaller, quiet singers, or a few with loud voices," says coauthor Daniel Stern, the project scientist for NuSTAR at JPL. "Now, thanks to NuSTAR, we're gaining a better understanding of the black holes and starting to address these questions."

High-energy X-rays can reveal what lies around the most obscured supermassive black holes, which are otherwise hard to see. In the same way that medical X-rays can travel through your skin to reveal pictures of bones, NuSTAR can see through the gas and dust around black holes, to get a deeper view of what is going on inside.

With NuSTAR's more complete picture of supermassive black hole populations, astronomers can begin to puzzle together how these objects evolve and change over time. When did they start and stop growing? What is the distribution of the gas and dust that both feed and hide the black holes?

The team expects that over time, NuSTAR will be able to resolve more of the high-energy X-ray background—and better decipher the X-ray song of the universe's black holes.

The Astrophysical Journal study, titled "The NuSTAR Extragalactic Surveys: The Number Counts of Active Galactic Nuclei and the Resolved Fraction of the Cosmic X-Ray Background," is funded by NASA. Other Caltech authors include: Mislav Baloković, Murray Brightman, Karl Forster, Brian Grefenstette, Kristin Madsen, Peter Mao, Hiromasa Miyasaka, and Vikram Rana.

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Improving Computer Graphics with Quantum Mechanics

Caltech applied scientists have developed a new way to simulate large-scale motion numerically using the mathematics that govern the universe at the quantum level.

The new technique, presented at the International Conference and Exhibition on Computer Graphics & Interactive Techniques (SIGGRAPH), held in Anaheim, California, from July 24-28, allows computers to more accurately simulate vorticity, the spinning motion of a flowing fluid.

A smoke ring, which seems to turn itself inside out endlessly as it floats along, is a complex demonstration of vorticity, and is incredibly difficult to simulate accurately, says Peter Schröder, Shaler Arthur Hanisch Professor of Computer Science and Applied and Computational Mathematics in the Division of Engineering and Applied Science.

"Since we are computer graphics folks, we are interested in methods that capture the visual variety and drama of fluids well," says Schröder. "What's unique about our method is that we took a page from the quantum mechanics's 'playbook.'"

The Schrödinger equation, the basic description of quantum mechanical behavior, can be used to describe the motion of superfluids, which are fluids supercooled to temperatures near absolute zero that behave as though they are without viscosity. Viscosity is a fluid's resistance to deformation.  

"Caltech's Richard Feynman was one of the first to recognize that superfluids are governed by so-called vortex filaments, which are basically long strings of pure vorticity," Schröder says. "While we are not interested in quantum mechanics, we realized that the Schrödinger equation—with some tweaks—can also approximate fluids at the macroscopic level, from smoke gently rising from a flame to the concentrated vorticity of a tornadic storm."

When asked why the Schrödinger equation, usually reserved for effects at the atomic level, does so well for fluids at the macroscopic level, Schröder says, "The Schrödinger equation, as we use it, is a close relative of the non-linear Schrödinger equation which is used for the description of superfluids. Their vorticity behavior is in many ways very similar to the behavior we can also observe in the macroscopic world."

Schröder hopes his work will have an impact on computer-generated graphics, and may also be used to model real-world phenomena, such as the curling motion of a hurricane.

Schröder's paper, entitled "Schrödinger's Smoke," was presented on July 26. His coauthors include Albert Chern, a graduate student at Caltech; Felix Knöppel and Ulrich Pinkall of Technische Universität Berlin; and Steffen Weißmann of Google. This research was supported by the German Research Foundation, the Office of Naval Research, and the German Academic Exchange Service.

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Team of Proteins Works Together to Turn on T Cells

The fates of various cells in our bodies—whether they become skin or another type of tissue—are controlled by genetic switches. In a new study, Caltech scientists investigate the switch for T cells, which are immune cells produced in the thymus that destroy virus-infected cells and cancers. The researchers wanted to know how cells make the choice to become T cells.

"We already know which genetic switch directs cells to commit to becoming T cells, but we wanted to figure out what enables that switch to be turned on," says Hao Yuan Kueh, a postdoctoral scholar at Caltech and lead author of a Nature Immunology report about the work, published on July 4.

The study found that a group of four proteins, specifically DNA-binding proteins known as transcription factors, work in a multi-tiered fashion to control the T-cell genetic switch in a series of steps. This was a surprise because transcription factors are widely assumed to work in a simultaneous, all-at-once fashion when collaborating to regulate genes.

The results may ultimately allow doctors to boost a person's T-cell population. This has potential applications in fighting various diseases, including AIDS, which infects mature T cells.

"In the past, combinatorial gene regulation was thought to involve all the transcription factors being required at the same time," says Kueh, who works in the lab of  Ellen Rothenberg, Caltech's Albert Billings Ruddock Professor of Biology. "This was particularly true in the case of the genetic switch for T-cell commitment, where it was thought that a quorum of the factors working simultaneously was needed to ensure that the gene would only be expressed in the right cell type."

The authors report that a key to their finding was the ability to image live cells in real-time. They genetically engineered mouse cells so that a gene called Bcl11b—the key switch for T cells—would express a fluorescent protein in addition to its own Bcl11b protein. This caused the mouse cells to glow when the Bcl11b gene was turn on. By monitoring how different transcription factors, or proteins, affected the activation of this genetic switch in individual cells, the researchers were able to isolate the distinct roles of the proteins.

The results showed that four proteins work together in three distinct steps to flip the switch for T cells. Kueh says to think of the process as a team of people working together to get a light turned on. He says first two proteins in the chain (TCF1 and GATA3) open a door where the main light switch is housed, while the next protein (Notch) essentially switches the light on. A fourth protein (Runx1) controls the amplitude of the signal, like sliding a light dimmer.

"We identify the contributions of four regulators of Bcl11b, which are all needed for its activation but carry out surprisingly different functions in enabling the gene to be turned on," says Rothenberg. "It's interesting—the gene still needs the full quorum of transcription factors, but we now find that it also needs them to work in the right order. This makes the gene respond not only to the cell's current state, but also to the cell's recent developmental history."

Team member Kenneth Ng, a visiting student from California Polytechnic State University, says he was surprised by how much detail they could learn about gene regulation using live imaging of cells.

"I had read about this process in textbooks, but here in this study we could pinpoint what the proteins are really doing," he says.

The next step in the research is to get a closer look at precisely how the T cell genetic switch itself works. Kueh says he wants to "unscrew the panels" of the switch and understand what is physically going on in the chromosomal material around the Bcl11b gene.

The Nature Immunology paper, titled, "Asynchronous combinatorial action of four regulatory factors activates Bcl11b for T cell commitment," includes seven additional Caltech coauthors: Mary Yui, Shirley Pease, Jingli Zhang, Sagar Damle, George Freedman, Sharmayne Siu, and Michael Elowitz; as well as a collaborator at the Fred Hutchinson Cancer Research Center, Irwin Bernstein. The work at Caltech was funded by a CRI/Irvington Postdoctoral Fellowship, the National Institutes of Health, the California Institute for Regenerative Medicine, the Al Sherman Foundation, and the Louis A. Garfinkle Memorial Laboratory Fund.

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Community Seismic Network Detected Air Pulse From Refinery Explosion

Tight network of low-cost detectors improve resolution of seismic data gathering and could offer city inspectors crucial information on building damage after a quake

On February 18, 2015, an explosion rattled the ExxonMobil refinery in Torrance, causing ground shaking equivalent to that of a magnitude-2.0 earthquake and blasting out an air pressure wave similar to a sonic boom.

Traveling at 343 meters per second—about the speed of sound—the air pressure wave reached a 52-story high-rise in downtown Los Angeles 66 seconds after the blast.

The building's occupants probably did not notice a thing; the building shifted at most three-hundredths of a millimeter in response. But the building's seismometers—one is installed on every floor, as well as on the basement levels—noted and recorded the motion of each individual floor.

Those sensors are part of the Community Seismic Network (CSN), a project launched at Caltech in 2011 to seed the Los Angeles area with relatively inexpensive seismometers aimed at providing a high level of detail of how an earthquake shakes the Southern California region, as well as how individual buildings respond. That level of detail has the potential to provide critical and immediate information about whether the building is structurally compromised in the wake of an earthquake, says Caltech's Monica Kohler, research assistant professor in the Division of Engineering and Applied Science.

For example, if building inspectors know that inter-story drift—the displacement of each floor relative to the floors immediately below and above it—has exceeded certain limits based on the building's size and construction, then it is a safe bet that the building has suffered damage in a quake. Alternately, if inspectors know that a building has experienced shaking well within its tolerances, it could potentially be reoccupied sooner—helping an earthquake-struck city to more quickly get back to normal.

"We want first responders, structural engineers, and facilities engineers to be able to make decisions based on what the data say," says Kohler, the lead author of a paper detailing the high-rise's response that recently appeared in the journal Earthquake Spectra.

The keys to the CSN's success are affordability and ease of installation of its seismic detectors. Standard, high-quality seismic detectors can cost tens of thousands of dollars and need special vaults to house and protect them that can easily double the price. By contrast, the CSN detectors use $40 accelerometers and other off-the-shelf hardware, cost roughly $300 to build, and require minimal training to install. Approximately 700 of the devices have been installed so far, mostly in Los Angeles.

However, the CSN sensors are roughly 250 times less sensitive than their more expensive counterparts, which is why the ability to successfully detect and quantify the downtown building's response to the ExxonMobil explosion was such an important proof-of-concept.

"It's a validation of our approach," says CSN's project manager, Richard Guy.

Sonic booms have been noted by seismic networks dozens of times before, beginning in the 1980s with the first detections of seismic shaking caused by space-shuttle reentries. The sonic booms, found Hiroo Kanamori and colleagues at Caltech and the United States Geological Survey, rattled buildings that, in turn, shook the ground around them.

"Seismologists try to understand what is happening in the earth and how that affects buildings by looking at everything we see on seismograms," says Kanamori, Caltech's John E. and Hazel S. Smits Professor of Geophysics, Emeritus, and coauthor of the Earthquake Spectra paper. "In most cases, signals come from the interior of the earth, but nothing prevents us from studying signals from the air. Though rare, the signals from the air provide a new dimension in the field of seismology."

The earlier sonic boom detections were made using single-channel devices, which typically record motion in one direction only. While this information is useful for understanding ground shaking, a three-dimensional record of the floor-by-floor motion of a building can reveal how much a building is rocking, swaying, and shifting; two or more sensors installed per floor can show the twisting of the structure.

"The more sensors you have in a small area, the more detail you're going to see. If there are things happening on a small scale, you'll never see it until you have sensors deployed on that scale," Kohler says.

Kohler and her colleagues found that the air pressure wave from the explosion had about the same impact on the high-rise as an 8 mile-per-hour gust of wind. A pressure wave about 100 times larger would have been required to have broken windows in the building; a wave 1,000 times larger would have been necessary to cause significant damage to the building.

The ExxonMobil blast was not the first shaking recorded by the building's seismometers. A number of earthquakes—including a magnitude-4.2 quake on January 4, 2015, with an epicenter in Castaic Lake, about 40 miles northwest of downtown Los Angeles—also were registered by the seismic detectors on nearly every floor of the building. But the refinery explosion-induced shaking was an important test of the sensitivity of the instruments, and of the ability of researchers to separate earthquake signals from other sources of shaking.

Other authors of the Earthquake Spectra paper, "Downtown Los Angeles 52-Story High-Rise and Free-Field Response to an Oil Refinery Explosion," include Caltech's Anthony Massari, Thomas Heaton, Egill Hauksson, Robert Clayton, Julian Bunn, and K. M. Chandy. Funding for the CSN came from the Gordon and Betty Moore Foundation, the Terrestrial Hazard Observation and Reporting Center at Caltech, and the Divisions of Geological and Planetary Sciences and Engineering and Applied Science at Caltech. 

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Community Seismic Network sensors detected motion of each floor of a building in response to the 2015 ExxonMobil refinery explosion.

NASA Rover's Sand-Dune Studies Yield Surprise

Some of the wind-sculpted sand ripples on Mars are a type not seen on Earth, and their relationship to the thin Martian atmosphere today provides new clues about the history of Mars' atmosphere.

The determination that these mid-size ripples are a distinct type resulted from observations by NASA's Curiosity Mars rover. Six months ago, Curiosity made the first up-close study of active sand dunes anywhere other than Earth, at the "Bagnold Dunes" on the northwestern flank of Mars' Mount Sharp.

"Earth and Mars both have big sand dunes and small sand ripples, but on Mars, there's something in-between that we don't have on Earth," said Mathieu Lapotre, a graduate student at Caltech, Pasadena, California, and science-team collaborator for NASA's Curiosity Mars rover mission. He is the lead author of a report about these mid-size ripples published in the July 1 issue of the journal Science. 

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Scientists Transform Lower-Body Cells into Facial Cartilage

Caltech scientists have converted cells of the lower-body region into facial tissue that makes cartilage, in new experiments using bird embryos. The researchers discovered a "gene circuit," composed of just three genes, that can alter the fate of cells destined for the lower bodies of birds, turning them instead into cells that produce cartilage and bones in the head.

The results, published in the June 24 issue of the journal Science, could eventually lead to therapies for conditions where facial bone or cartilage is lost. For example, cartilage destroyed in the nose due to cancer is particularly hard to replace. Understanding the genetic pathways that lead to the development of facial cartilage may help in future stem-cell therapies, where a patient's own skin cells could be transformed and used to repair the nose.

"When facial cartilage and bone is lost, from cancer or an accident, it has been difficult to replace," says Marianne Bronner, the Albert Billings Ruddock Professor of Biology at Caltech, and senior author of the Science report. "Our long term hope is that uncovering this gene circuit may be useful in reprogramming a patient's own stem cells to make facial cartilage."

The bones below our necks, referred to by scientists as the "long" bones, originate from a different source of tissue than the bones in our head. As embryos, we are born with a type of early tissue called the neural crest that forms along the entire body, from the head to the end of the spinal cord. Those neural crest cells which originate in the head, called cranial neural crest, differentiate into the cartilage and bone of our faces, including the jaws and skull. In contrast, the so-called trunk neural crest cells, forming below the neck, do not make cartilage or bone but instead turn into nerve cells and pigment cells elsewhere in our bodies. Bronner and her colleagues want to understand what genes regulate the development of cranial neural crest cells and enable them to make cartilage and bones in the head.

To this end, they divided the trunk and cranial neural crest cells of bird embryos into separate groups, and looked for differences in gene activity. Fifteen genes were initially identified as being turned on in only the cranial cells. The researchers chose six of these genes for further study. All six code for transcription factors—molecules that bind to DNA to turn on and off the expression of other genes. After studying how these factors interact with each other, the scientists focused on three, called Sox8, Tfap2b and Ets, that are part of the cranial neural crest circuit.

These three genes were then inserted into the bodies of developing bird embryos, in particular the trunk neural crest, using a technique called electroporation. In this method, electric current is applied to cells to open up pores through which molecules such as DNA may pass. Next, the researchers transplanted the altered trunk cells to the cranial region of the embryos. Five days later, the trunk cells were doing something entirely new: producing cartilage.

"Normally, these trunk cells will not make cartilage," says Bronner. "Introducing just three genes into these cells reprogrammed them to acquire the ability to do so."

Bronner said that she hopes other researchers will use this information for experiments in cell culture. By adding the new-found gene circuit, perhaps with other known factors, to skin cells in a petri dish it may be possible to turn them into cartilage-producing cells—a key next step in creating future therapies for facial bone and cartilage loss.

The first author of the Science paper, titled, "Reprogramming of avian neural crest axial identity and cell fate," is Marcos Simoes-Costa of Caltech. The research is funded by the National Institutes of Health and the Pew Fellows Program in Biomedical Sciences.

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Gravitational Waves Detected from Second Pair of Colliding Black Holes

The LIGO Scientific Collaboration and the Virgo collaboration identify a second gravitational wave event in the data from Advanced LIGO detectors

On December 26, 2015 at 03:38:53 UTC, scientists observed gravitational waves—ripples in the fabric of spacetime—for the second time.

The gravitational waves were detected by both of the twin Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors, located in Livingston, Louisiana, and Hanford, Washington, USA.

The LIGO Observatories are funded by the National Science Foundation (NSF), and were conceived, built, and are operated by Caltech and MIT. The discovery, accepted for publication in the journal Physical Review Letters, was made by the LIGO Scientific Collaboration (which includes the GEO Collaboration and the Australian Consortium for Interferometric Gravitational Astronomy) and the Virgo Collaboration using data from the two LIGO detectors.

Gravitational waves carry information about their origins and about the nature of gravity that cannot otherwise be obtained, and physicists have concluded that these gravitational waves were produced during the final moments of the merger of two black holes—14 and 8 times the mass of the sun—to produce a single, more massive spinning black hole that is 21 times the mass of the sun.

"It is very significant that these black holes were much less massive than those observed in the first detection," says Gabriela Gonzalez, LIGO Scientific Collaboration (LSC) spokesperson and professor of physics and astronomy at Louisiana State University. "Because of their lighter masses compared to the first detection, they spent more time—about one second—in the sensitive band of the detectors. It is a promising start to mapping the populations of black holes in our universe."

During the merger, which occurred approximately 1.4 billion years ago, a quantity of energy roughly equivalent to the mass of the sun was converted into gravitational waves. The detected signal comes from the last 27 orbits of the black holes before their merger. Based on the arrival time of the signals—with the Livingston detector measuring the waves 1.1 milliseconds before the Hanford detector—the position of the source in the sky can be roughly determined.

"In the near future, Virgo, the European interferometer, will join a growing network of gravitational wave detectors, which work together with ground-based telescopes that follow-up on the signals," notes Fulvio Ricci, the Virgo Collaboration spokesperson, a physicist at Istituto Nazionale di Fisica Nucleare (INFN) and professor at Sapienza University of Rome. "The three interferometers together will permit a far better localization in the sky of the signals."

The first detection of gravitational waves, announced on February 11, 2016, confirmed a major prediction of Albert Einstein's 1915 general theory of relativity, and marked the beginning of the new field of gravitational-wave astronomy.

The second discovery "has truly put the 'O' for Observatory in LIGO," says Caltech's Albert Lazzarini, deputy director of the LIGO Laboratory. "With detections of two strong events in the four months of our first observing run, we can begin to make predictions about how often we might be hearing gravitational waves in the future. LIGO is bringing us a new way to observe some of the darkest yet most energetic events in our universe."

"We are starting to get a glimpse of the kind of new astrophysical information that can only come from gravitational wave detectors," says MIT's David Shoemaker, who led the Advanced LIGO detector construction program.

Both discoveries were made possible by the enhanced capabilities of Advanced LIGO, a major upgrade that increases the sensitivity of the instruments compared to the first generation LIGO detectors, enabling a large increase in the volume of the universe probed.

"With the advent of Advanced LIGO, we anticipated researchers would eventually succeed at detecting unexpected phenomena, but these two detections thus far have surpassed our expectations," says NSF Director France A. Córdova. "NSF's 40-year investment in this foundational research is already yielding new information about the nature of the dark universe."

Advanced LIGO's next data-taking run will begin this fall. By then, further improvements in detector sensitivity are expected to allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe. The Virgo detector is expected to join in the latter half of the upcoming observing run.

LIGO research is carried out by the LIGO Scientific Collaboration (LSC), a group of more than 1,000 scientists from universities around the United States and in 14 other countries. More than 90 universities and research institutes in the LSC develop detector technology and analyze data; approximately 250 students are strong contributing members of the collaboration. The LSC detector network includes the LIGO interferometers and the GEO600 detector.

Virgo research is carried out by the Virgo Collaboration, consisting of more than 250 physicists and engineers belonging to 19 different European research groups: 6 from Centre National de la Recherche Scientifique (CNRS) in France; 8 from the Istituto Nazionale di Fisica Nucleare (INFN) in Italy; 2 in The Netherlands with Nikhef; the MTA Wigner RCP in Hungary; the POLGRAW group in Poland and the European Gravitational Observatory (EGO), the laboratory hosting the Virgo detector near Pisa in Italy.

The NSF provides most of the financial support for Advanced LIGO. Funding organizations in Germany (Max Planck Society), the U.K. (Science and Technology Facilities Council, STFC) and Australia (Australian Research Council) also have made significant commitments to the project.

Several of the key technologies that made Advanced LIGO so much more sensitive have been developed and tested by the German UK GEO collaboration. Significant computer resources have been contributed by the AEI Hannover Atlas Cluster, the LIGO Laboratory, Syracuse University, the ARCCA cluster at Cardiff University, the University of Wisconsin-Milwaukee, and the Open Science Grid. Several universities designed, built, and tested key components and techniques for Advanced LIGO: The Australian National University, the University of Adelaide, the University of Western Australia, the University of Florida, Stanford University, Columbia University in the City of New York, and Louisiana State University. The GEO team includes scientists at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI), Leibniz Universität Hannover, along with partners at the University of Glasgow, Cardiff University, the University of Birmingham, other universities in the United Kingdom and Germany, and the University of the Balearic Islands in Spain.

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Gravitational Waves Detected a Second Time
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Live Webcast: LIGO, Virgo Scientists to Discuss Continued Search for Gravitational Waves

The latest research in the effort to detect gravitational waves will be discussed in a press briefing at the 228th meeting of the American Astronomical Society in San Diego, California. The public can view the briefing during the live webcast, scheduled to begin at 10:15 am Pacific Daylight Time on Wednesday, June 15, 2016. The panelists for the briefing are Caltech's David Reitze, executive director of LIGO; Gabriela González, LIGO Scientific Collaboration spokesperson, from Louisiana State University; and Fulvio Ricci, Virgo spokesperson, from the University of Rome Sapienza and the Istituto Nazionale di Fisica Nucleare in Rome.

The first detection of gravitational waves, announced on February 11, 2016, confirmed a major prediction of Albert Einstein's 1915 general theory of relativity, and marked the beginning of the new field of gravitational-wave astronomy.

LIGO, a system of two identical detectors located in Livingston, Louisiana, and Hanford, Washington, was constructed to detect the tiny vibrations from passing gravitational waves, was conceived and built by Caltech and MIT with funding from the National Science Foundation and contributions from other U.S. and international partners.

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Live Webcast: Ligo, Virgo Scientists to Discuss Continued Search for Gravitational Waves
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