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  • richardmitnick 10:25 am on April 8, 2015 Permalink | Reply
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    From physicsworld.com: “Mysterious baryon resonance is a subatomic molecule, say physicists” 

    physicsworld
    physicsworld.com

    Apr 7, 2015
    Hamish Johnston

    1
    Does Λ(1405) comprise an anti-kaon and a nucleon?

    Physicists in Australia have produced further evidence that an excited state of the lambda baryon is a “subatomic molecule” – a meson and a nucleon that are bound together. While the physicists are not the first to suggest this exotic structure, they have done new computer simulations and calculations that they say “strongly suggest” that the lambda baryon can exist in this exotic configuration.

    The lambda baryon (Λ) has no electrical charge and comprises three quarks (up, down and strange). Its discovery in 1950 by physicists at the University of Melbourne played an important role in the development of the quark model of matter and ultimately quantum chromodynamics (QCD), which is the theory of the strong interaction that binds quarks together in baryons and mesons.

    Λ is a composite particle, and therefore it exists in a number of different energy states, much like an atom. Λ is the lowest-energy state and Λ(1405), which was discovered in 1961, is the lowest-lying excited state or resonance. As physicists developed the quark model in the 1960s, it became apparent that there was something not quite right about Λ(1405). In particular, the energy difference between Λ and Λ(1405) is much lower than expected, if Λ(1405) is assumed to be a “single particle” containing just three quarks.

    Growing evidence

    In the 1960s the Australian physicist Richard Dalitz and colleagues suggested that that Λ(1405) could comprise an anti-kaon meson bound to a nucleon (proton or neutron). This can occur in two ways: a negatively charged anti-kaon bound to a proton, or a neutral anti-kaon bound to a neutron. Working out the structure of Λ(1405) – or any baryon resonance for that matter – is extremely difficult because of the nonlinear nature of the strong interaction. However, over the past two decades theoretical support for molecular Λ(1405) has grown, with calculations done by several groups of physicists backing up the idea.

    Now, Ross Young and colleagues at the University of Adelaide and the Australian National University have used lattice QCD to gain further insights into the nature of Λ(1405). The team used a lattice QCD simulation that was first developed by the Japan-based PACS-CS collaboration. The most important result of the team’s calculation is that the strange quark appears to make no contribution to the magnetic moment of Λ(1405). This is expected if the strange quark is confined within an anti-kaon with zero spin and is consistent with a molecular model of Λ(1405).

    Energy levels

    The team also analysed the energy levels calculated by lattice QCD and concluded that the Λ(1405) resonance is dominated by the anti-kaon nucleon molecule with a much smaller contribution from the single-particle three-quark state (up, down, strange).

    José Antonio Oller of the University of Murcia in Spain calls the calculation of the strange quark’s magnetic contribution a “remarkable result”. However, he points out that while this zero magnetic contribution is a necessary condition for molecular Λ(1405), it is not sufficient to confirm the molecular nature of the resonance. He added that further calculations of the properties of Λ(1405) using other techniques are needed before the issue can be settled.

    The calculations are described in Physical Review Letters.

    See the full article here.

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    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 1:22 pm on April 4, 2015 Permalink | Reply
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    From FNAL: “Physics in a Nutshell Observe neutral particles with this one weird trick” 

    FNAL Home

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    April 2, 2015
    Jim Pivarski

    1
    A shower produces dozens of particles that could be observed individually (inset figure) or collectively in a calorimeter (bottom).

    The previous Physics in a Nutshell introduced tracking, a technique that allows physicists to see the trajectories of individual particles. The biggest limitation of tracking is that only charged particles ionize the medium that forms clouds, bubbles, discharges or digital signals. Neutral particles are invisible to any form of tracking.

    Calorimetry, which now complements tracking in most particle physics experiments, takes advantage of a curious effect that was first observed in cloud chambers in the 1930s. Occasionally, a single high-energy particle seemed to split into dozens of low-energy particles. These inexplicable events were called “bursts,” “explosions” or “die Stöße.” Physicists initially thought they could only be explained by a radical revision of the prevailing quantum theory.

    As it turns out, these events are due to two well-understood processes, iterated ad nauseam. Electrons and positrons recoil from atoms of matter to produce photons, and photons in matter split to form electron-positron pairs. Each of these steps doubles the total number of particles, turning a single high-energy particle into many low-energy particles.

    This cascading process is now known as a shower. The cycle of charged particles creating neutral particles and neutral particles creating charged particles can be started by either type, making it sensitive to any particle that interacts with matter, including neutral ones. Although the shower process is messy, the final particle energies should add up to the original particle’s energy, providing a way to measure the energy of the initial particle — by destroying it.

    Modern calorimeters initiate the shower using a heavy material and then measure the energy using ordinary light sensors. To accurately measure the energy of the final photons, this heavy material should also be transparent. Crystals are a common choice, as are lead-infused glass, liquid argon and liquid xenon.

    Not all calorimeters are man-made. Neutrinos produce electrons in water or ice, which cascade into showers of electrons, positrons and photons. The IceCube experiment uses a cubic kilometer of Antarctic ice to observe PeV neutrinos — a hundred times more energetic than the LHC’s beams.

    ICECUBE neutrino detector
    IceCube neutrino detector interior
    IceCube

    Cosmic rays form showers in the Earth’s atmosphere, producing about 4 watts of ultraviolet light and billions of particles. The Pierre Auger Observatory uses sky-facing cameras and 3,000 square kilometers of ground-based detectors to capture both and has measured particles that are a million times more energetic than the LHC’s beams.
    Pierre Auger Observatory
    Pierre Auger Observatory

    See the full article here.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
    collaborate at Fermilab on experiments at the frontiers of discovery.

     
  • richardmitnick 1:22 pm on March 31, 2015 Permalink | Reply
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    From Quanta: “Dark Energy Tested on a Tabletop” 

    Quanta Magazine
    Quanta Magazine

    March 31, 2015
    Maggie McKee

    1
    A vacuum chamber with a marble-size sphere at its center was used to test the nature of dark energy. Courtesy of Holger Müller

    Dark energy has topped cosmologists’ “most wanted” list since 1998, when astronomers noticed that the expansion of the universe is speeding up rather than slowing down. The entity responsible — whatever it is — must be incredibly powerful, constituting nearly 70 percent of the universe. Figuring out the identity of this dark energy is “arguably the most important problem in physics,” said Clare Burrage of the University of Nottingham in the United Kingdom.

    Now a team of physicists has directly tested one option for dark energy using not powerful telescopes or satellites, but a vacuum chamber fashioned on a tabletop.

    The most straightforward explanation for dark energy is that it is the energy inherent in the vacuum of space itself. In this model, every teaspoonful of space brims with the same amount of dark energy, a value known as the cosmological constant [Λ]. But there’s a major flaw in this simple solution. Physicists’ best calculation of this energy, which is thought to be due to the constant appearance and disappearance of “virtual” quantum particles, overshoots the actual observed value by a factor of 10120.

    So perhaps instead of — or in addition to — the cosmological constant, there may be extra quantum fields, called scalar fields, that have a given strength at each point in space, just as a measurable temperature exists everywhere.

    “We know there’s no explanation for the cosmological-constant problem within general relativity and the Standard Model of particle physics,” said Burrage. “Pretty much anytime you want to go beyond that, the new physics you try and introduce gives you new scalar fields.”

    2
    Illustration of spacetime curvature.

    4
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom).

    Scalar fields could produce dark energy in the fields’ lowest-energy, or vacuum, state, just as the cosmological constant would. But many proposed scalar fields interact with matter, which raises its own problem. If a scalar field interacts with ordinary matter — like the stuff that makes up Earth and the sun — its presence should already have been observed in our own solar system as an extra, unexplained force, and none has been seen. “If your theory of dark energy tells you these extra scalar fields are around, you have to explain why we haven’t seen them,” Burrage said.

    One solution is that, like a chameleon, the field changes depending on the surrounding environment. Such a field would produce a negligible effect near high-density matter, like Earth, slipping by unnoticed in the presence of the stronger, familiar force of gravity. But in the emptiness of space between galaxies, it would produce a long-range pull. (Unfortunately, this pull would still be imperceptible to astronomers, since it would disappear around large objects whose movements they could track.)

    2
    Dark energy has the same value everywhere in a “cosmological constant” model. If dark energy is described by a “chameleon” field instead, it would have only minor effects around massive objects such as Earth. Olena Shmahalo/Quanta Magazine. Earth via NASA/Deglr6328.

    Chameleon models are not especially well motivated from the standpoint of fundamental physics, admits Burrage, who began studying them in graduate school, but since dark energy presents such a profound mystery, physicists are willing to consider just about anything.

    Last August, Burrage and her colleagues posted a paper on the scientific preprint site arxiv.org suggesting a way to lay a trap for these cagey cosmic chameleons. They envisioned a vacuum chamber about the size of a bowling ball with a marble-size sphere at its center. The chameleon field, assuming it was there, would be minimized near the walls of the chamber and immediately around the central sphere. It would have a higher value in the empty space between them. That means that an atom — whose own mass is too small to kill off the chameleon field — placed inside the vacuum chamber would feel a different force from the field depending on its position in the chamber.

    Pulses of laser light could be used to track the atom’s movement in the chamber at three different times. If the tracking revealed an unexplained acceleration, it could be due to the force of a chameleon field. “You use the light beam as a ruler, and you just watch the atoms moving across the ruler,” said Ed Hinds, the head of the Center for Cold Matter at Imperial College London and the lead experimentalist on the team proposing the test.

    After devising the chameleon trap, Hinds and his team set out to build it; he expects to get the first results in a few months. But other physicists led by Holger Müller at the University of California, Berkeley, already had a similar setup in their lab, so they got a head start on the tests and reported their first results in a paper posted to arxiv.org on Feb. 13 and submitted to a prominent peer-reviewed journal. (Müller declined to comment for this article, as the journal’s policies forbid him from speaking directly to the media until shortly before the paper is published.)

    Using cesium atoms as the test particles, Müller’s team found that the atoms’ movement did not change depending on their distance from the sphere. That ruled out most chameleon models that could account for dark energy, Müller reported at a talk at Harvard University on Feb. 23.

    The result came out “exactly as I predicted, so it’s a little bit galling that it wasn’t in my lab,” Hinds said. “But I must say they’ve done a very fine job.” Hinds believes that the test can be made 1,000 times more sensitive, allowing him to probe energies close to the scale where quantum mechanics becomes important for gravity. But he is closemouthed about how he plans to get there. “I need to have some way to come back at the Berkeley people,” he joked.

    3
    The Berkeley team that ruled out most chameleon models. From right to left: Paul Hamilton, Matt Jaffe, Holger Müller, Philipp Haslinger. Enar de Dios Rodriguez, courtesy of Holger Müller

    Lam Hui, a theoretical astrophysicist at Columbia University, said such experiments are interesting, but not for their ability to shed light on dark energy. That is because cosmic acceleration, according to chameleon models, would be caused not by any camouflaging behavior on the part of the field but simply by the value of its lowest-energy state. Instead, the experiments are “testing the chameleon mechanism,” he said — the general idea that the universe could harbor undetected scalar fields that interact with matter.

    Mikhail Lukin, a physicist at Harvard who attended Müller’s talk there, said the method holds a lot of promise. Such high-precision instruments should “really push the frontier of our understanding of the universe,” he said, but he added that “the big thing would be to really observe something” rather than rule models out.

    To date, cosmological observations have had an edge in this regard, said Ronald Walsworth, another Harvard physicist at the talk. “They’ve actually seen effects that we can’t explain,” he said, referring to the observations that revealed dark energy.

    Still, some of those who trade in cosmic observations are impressed with the new study. “That was a very neat idea,” said Valeria Pettorino of the University of Heidelberg in Germany. “It’s quite different from other kinds of tests we are used to for dark energy.” She led a team that recently compared the predictions of various models of dark energy with observations from the Planck satellite and other telescopes. The combined data from all sources revealed the faintest hint of a deviation from the simplest dark-energy model based on the cosmological constant.

    If chameleon models are one day ruled out completely, “then that is great,” said Amanda Weltman of the University of Cape Town in South Africa, who co-developed the first such models more than a decade ago. “It is exciting to be able to propose a theory that can be tested and ruled out in a reasonable time frame.”

    See the full article here.

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    Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

     
  • richardmitnick 1:11 pm on March 27, 2015 Permalink | Reply
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    From BNL: “Physicists Solve Low-Temperature Magnetic Mystery” 

    Brookhaven Lab

    March 27, 2015
    Chelsea Whyte, (631) 344-8671 or Peter Genzer, (631) 344-3174

    1
    Ignace Jarrige shown with the sample used in the experiment.

    Researchers have made an experimental breakthrough in explaining a rare property of an exotic magnetic material, potentially opening a path to a host of new technologies. From information storage to magnetic refrigeration, many of tomorrow’s most promising innovations rely on sophisticated magnetic materials, and this discovery opens the door to harnessing the physics that governs those materials.

    The work, led by Brookhaven National Laboratory physicist Ignace Jarrige, and University of Connecticut professor Jason Hancock, together with collaborators from Japan and Argonne National Laboratory, marks a major advance in the search for practical materials that will enable several types of next-generation technology. A paper describing the team’s results was published this week in the journal Physical Review Letters.

    The work is related to the Kondo Effect, a physical phenomenon that explains how magnetic impurities affect the electrical resistance of materials. The researchers were looking at a material called ytterbium-indium-copper-four (usually written using its chemical formula: YbInCu4).

    YbInCu4 has long been known to undergo a unique transition as a result of changing temperature. Below a certain temperature, the material’s magnetism disappears, while above that temperature, it is strongly magnetic. This transition, which has puzzled physicists for decades, has recently revealed its secret. “We detected a gap in the electronic spectrum, similar to that found in semiconductors like silicon, whose energy shift at the transition causes the Kondo Effect to strengthen sharply,” said Jarrige

    2
    From Left to Right: Jason Hancock, Diego Casa, and Jung-ho Kim, shown with one of the instruments used in the experiment.

    Electronic energy gaps define how electrons move (or don’t move) within the material, and are the critical component in understanding the electrical and magnetic properties of materials. “Our discovery goes to show that tailored semiconductor gaps can be used as a convenient knob to finely control the Kondo Effect and hence magnetism in technological materials,” said Jarrige.

    To uncover the energy gap, the team used a process called Resonant Inelastic X-Ray Scattering (RIXS), a new experimental technique that is made possible by an intense X-ray beam produced at a synchrotron operated by the Department of Energy and located at Argonne National Laboratory outside of Chicago. By placing materials in the focused X-ray beam and sensitively measuring and analyzing how the X-rays are scattered, the team was able to uncover elusive properties such as the energy gap and connect them to the enigmatic magnetic behavior.

    The new physics identified through this work suggest a roadmap to the development of materials with strong “magnetocaloric” properties, the tendency of a material to change temperature in the presence of a magnetic field. “The Kondo Effect in YbInCu4 turns on at a very low temperature of 42 Kelvin (-384F),” said Hancock, “but we now understand why it happens, which suggests that it could happen in other materials near room temperature.” If that material is discovered, according to Hancock, it would revolutionize cooling technology.

    3
    During the RIXS experiment, an X-ray beam is used to excite electrons inside the sample. The X-ray loses energy during the process and then is scattered out of the sample. A fine analysis of the scattered X-rays yields insight into the mechanism that controls the strength of the Kondo Effect.

    Household use of air conditioners in the US accounts for over $11 billion in energy costs and releases 100 million tons of carbon dioxide annually. Use of the magnetocaloric effect for magnetic refrigeration as an alternative to the mechanical fans and pumps in widespread use today could significantly reduce those numbers.

    In addition to its potential applications to technology, the work has advanced the state of the art in research. “The RIXS technique we have developed can be applied in other areas of basic energy science,” said Hancock, noting that the development is very timely, and that it may be useful in the search for “topological Kondo insulators,” materials which have been predicted in theory, but have yet to be discovered.

    See the full article here.

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    BNL Campus

    One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. The Laboratory’s almost 3,000 scientists, engineers, and support staff are joined each year by more than 5,000 visiting researchers from around the world.Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit, applied science and technology organization.
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  • richardmitnick 12:09 pm on March 16, 2015 Permalink | Reply
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    From MIT: “Quantum sensor’s advantages survive entanglement breakdown” 


    MIT News

    March 9, 2015
    Larry Hardesty | MIT News Office

    1
    In the researchers’ new system, a returning beam of light is mixed with a locally stored beam, and the correlation of their phase, or period of oscillation, helps remove noise caused by interactions with the environment. Illustration: Jose-Luis Olivares/MIT

    Preserving the fragile quantum property known as entanglement isn’t necessary to reap benefits.

    The extraordinary promise of quantum information processing — solving problems that classical computers can’t, perfectly secure communication — depends on a phenomenon called “entanglement,” in which the physical states of different quantum particles become interrelated. But entanglement is very fragile, and the difficulty of preserving it is a major obstacle to developing practical quantum information systems.

    In a series of papers since 2008, members of the Optical and Quantum Communications Group at MIT’s Research Laboratory of Electronics have argued that optical systems that use entangled light can outperform classical optical systems — even when the entanglement breaks down.

    Two years ago, they showed that systems that begin with entangled light could offer much more efficient means of securing optical communications. And now, in a paper appearing in Physical Review Letters, they demonstrate that entanglement can also improve the performance of optical sensors, even when it doesn’t survive light’s interaction with the environment.

    “That is something that has been missing in the understanding that a lot of people have in this field,” says senior research scientist Franco Wong, one of the paper’s co-authors and, together with Jeffrey Shapiro, the Julius A. Stratton Professor of Electrical Engineering, co-director of the Optical and Quantum Communications Group. “They feel that if unavoidable loss and noise make the light being measured look completely classical, then there’s no benefit to starting out with something quantum. Because how can it help? And what this experiment shows is that yes, it can still help.”

    Phased in

    Entanglement means that the physical state of one particle constrains the possible states of another. Electrons, for instance, have a property called spin, which describes their magnetic orientation. If two electrons are orbiting an atom’s nucleus at the same distance, they must have opposite spins. This spin entanglement can persist even if the electrons leave the atom’s orbit, but interactions with the environment break it down quickly.

    In the MIT researchers’ system, two beams of light are entangled, and one of them is stored locally — racing through an optical fiber — while the other is projected into the environment. When light from the projected beam — the “probe” — is reflected back, it carries information about the objects it has encountered. But this light is also corrupted by the environmental influences that engineers call “noise.” Recombining it with the locally stored beam helps suppress the noise, recovering the information.

    The local beam is useful for noise suppression because its phase is correlated with that of the probe. If you think of light as a wave, with regular crests and troughs, two beams are in phase if their crests and troughs coincide. If the crests of one are aligned with the troughs of the other, their phases are anti-correlated.

    But light can also be thought of as consisting of particles, or photons. And at the particle level, phase is a murkier concept.

    “Classically, you can prepare beams that are completely opposite in phase, but this is only a valid concept on average,” says Zheshen Zhang, a postdoc in the Optical and Quantum Communications Group and first author on the new paper. “On average, they’re opposite in phase, but quantum mechanics does not allow you to precisely measure the phase of each individual photon.”

    Improving the odds

    Instead, quantum mechanics interprets phase statistically. Given particular measurements of two photons, from two separate beams of light, there’s some probability that the phases of the beams are correlated. The more photons you measure, the greater your certainty that the beams are either correlated or not. With entangled beams, that certainty increases much more rapidly than it does with classical beams.

    When a probe beam interacts with the environment, the noise it accumulates also increases the uncertainty of the ensuing phase measurements. But that’s as true of classical beams as it is of entangled beams. Because entangled beams start out with stronger correlations, even when noise causes them to fall back within classical limits, they still fare better than classical beams do under the same circumstances.

    “Going out to the target and reflecting and then coming back from the target attenuates the correlation between the probe and the reference beam by the same factor, regardless of whether you started out at the quantum limit or started out at the classical limit,” Shapiro says. “If you started with the quantum case that’s so many times bigger than the classical case, that relative advantage stays the same, even as both beams become classical due to the loss and the noise.”

    In experiments that compared optical systems that used entangled light and classical light, the researchers found that the entangled-light systems increased the signal-to-noise ratio — a measure of how much information can be recaptured from the reflected probe — by 20 percent. That accorded very well with their theoretical predictions.

    But the theory also predicts that improvements in the quality of the optical equipment used in the experiment could double or perhaps even quadruple the signal-to-noise ratio. Since detection error declines exponentially with the signal-to-noise ratio, that could translate to a million-fold increase in sensitivity.

    “This is a breakthrough,” says Stefano Pirandola, an associate professor of computer science at the University of York in England. “One of the main technical challenges was the experimental realization of a practical receiver for quantum illumination. Shapiro and Wong experimentally implemented a quantum receiver, which is not optimal but is still able to prove the quantum illumination advantage. In particular, they were able to overcome the major problem associated with the loss in the optical storage of the idler beam.”

    “This research can potentially lead to the development of a quantum LIDAR which is able to spot almost-invisible objects in a very noisy background,” he adds. “The working mechanism of quantum illumination could in fact be exploited at short-distances as well, for instance to develop non-invasive techniques of quantum sensing with potential applications in biomedicine.”

    See the full article here.

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  • richardmitnick 5:04 am on March 4, 2015 Permalink | Reply
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    From AAAS: “Physicists gear up to catch a gravitational wave” 

    AAAS

    AAAS

    3 March 2015
    Adrian Cho

    1
    The twin 4-kilometer arms of LIGO Livingston embrace a working forest, where logging generates vibrations that the instrument must damp out.

    This patch of woodland just north of Livingston, Louisiana, population 1893, isn’t the first place you’d go looking for a breakthrough in physics. Standing on a small overpass that crosses an odd arching tunnel, Joseph Giaime, a physicist at Louisiana State University (LSU), 55 kilometers west in Baton Rouge, gestures toward an expanse of spindly loblolly pine, parts of it freshly reduced to stumps and mud. “It’s a working forest,” he says, “so they come in here to harvest the logs.” On a quiet late fall morning, it seems like only a logger or perhaps a hunter would ever come here.

    Yet it is here that physicists may fulfill perhaps the most spectacular prediction of Albert Einstein’s theory of gravity, or general relativity. The tunnel runs east to west for 4 kilometers and meets a similar one running north to south in a nearby warehouselike building. The structures house the Laser Interferometer Gravitational-Wave Observatory (LIGO), an ultrasensitive instrument that may soon detect ripples in space and time set off when neutron stars or black holes merge.

    Einstein himself predicted the existence of such gravitational waves nearly a century ago. But only now is the quest to detect them coming to a culmination. The device in Livingston and its twin in Hanford, Washington, ran from 2002 to 2010 and saw nothing. But those Initial LIGO instruments aimed only to prove that the experiment was technologically feasible, physicists say. Now, they’re finishing a $205 million rebuild of the detectors, known as Advanced LIGO, which should make them 10 times more sensitive and, they say, virtually ensure a detection. “It’s as close to a guarantee as one gets in life,” says Peter Saulson, a physicist at Syracuse University in New York, who works on LIGO.

    Detecting those ripples would open a new window on the cosmos. But it won’t come easy. Each tunnel contains a pair of mirrors that form an “optical cavity,” within which infrared light bounces back and forth. To look for the stretching of space, physicists will compare the cavities’ lengths. But they’ll have to sense that motion through the din of other vibrations. Glancing at the pavement on the overpass, Giaime says that the ground constantly jiggles by about a millionth of a meter, shaken by seismic waves, the rumble of nearby trains, and other things. LIGO physicists have to shield the mirrors from such vibrations so that they can see the cavities stretch or shorten by distances 10 trillion times smaller—just a billionth the width of an atom.

    IN 1915, Einstein explained that gravity arises when mass and energy warp space and time, or spacetime. A year later, he predicted that massive objects undergoing the right kind of oscillating motion should emit ripples in spacetime—gravitational waves that zip along at light speed.

    For decades that prediction remained controversial, in part because the mathematics of general relativity is so complicated. Einstein himself at first made a technical error, says Rainer Weiss, a physicist at the Massachusetts Institute of Technology (MIT) in Cambridge. “Einstein had it right,” he says, “but then he [messed] up.” Some theorists argued that the waves were a mathematical artifact and shouldn’t actually exist. In 1936, Einstein himself briefly took that mistaken position.

    2
    Rainer Weiss of the Massachusetts Institute of Technology laid out the basic plan for LIGO 43 years ago. © MATT WEBER

    Even if the waves were real, detecting them seemed impossible, Weiss says. At a time when scientists knew nothing of the cosmos’s gravitational powerhouses—neutron stars and black holes—the only obvious source of waves was a pair of stars orbiting each other. Calculations showed that they would produce a signal too faint to be detected.

    By the 1950s, theorists were speculating about neutron stars and black holes, and they finally agreed that the waves should exist. In 1969, Joseph Weber, a physicist at the University of Maryland, College Park, even claimed to have discovered them. His setup included two massive aluminum cylinders 1.5 meters long and 0.6 meters wide, one of them in Illinois. A gravitational wave would stretch a bar and cause it to vibrate like a tuning fork, and electrical sensors would then detect the stretching. Weber saw signs of waves pinging the bars together. But other experimenters couldn’t reproduce Weber’s published results, and theorists argued that his claimed signals were implausibly strong.

    Still, Weber’s efforts triggered the development of LIGO. In 1969, Weiss, a laser expert, had been assigned to teach general relativity. “I knew bugger all about it,” he says. In particular, he couldn’t understand Weber’s method. So he devised his own optical method, identifying the relevant sources of noise. “I worked it out for myself, and I gave it to the students as a homework problem,” he says.

    Weiss’s idea, which he published in 1972 in an internal MIT publication, was slow to catch on. “It was obvious to me that this was pie in the sky and it would never work,” recalls Kip Thorne, a theorist at the California Institute of Technology (Caltech) in Pasadena, California. Thorne recorded his skepticism in Gravitation, the massive textbook that he co-wrote and published in 1973. “I had an exercise that said ‘Show that this technology will never work to detect gravitational waves,’ ” Thorne says.

    But by 1978 Thorne had warmed to the idea, and he persuaded Caltech to put up $2 million to build a 40-meter prototype interferometer. “It wasn’t a hard sell at all,” Thorne says, “which was a contrast to the situation at MIT.” Weiss says that Thorne played a vital role in winning support for a full-scale detector from the National Science Foundation in 1990. Construction in Livingston and Hanford finally began in 1994.

    Now, many physicists say Advanced LIGO is all but a sure winner. On a bright Monday morning in December, researchers at Livingston are embarking on a 10-day stint that will mark their first attempt to run as if making observations. LIGO Livingston has the feel of an outpost. Roughly 30 physicists, engineers, technicians, and operators gather in the large room that serves as the facility’s foyer, auditorium, and—with a table-tennis table in one corner—rec room. “Engineering run 6 began 8 minutes ago,” announces Janeen Romie, an engineer from Caltech. It seems odd that so few people can run such a big rig.

    But in principle, LIGO is simple. Within the interferometer’s sewer pipe–like vacuum chamber, at the elbow of the device, a laser beam shines on a beam splitter, which sends half the light down each of the interferometer’s arms. Within each arm, the light builds up as it bounces between the mirrors at either end. Some of the light leaks through the mirrors at the near ends of the arms and shines back on the beam splitter. If the two arms are exactly the same length, the merging waves will overlap and interfere with each other in a way that directs the light back toward the laser.

    The ultimate motion sensor

    3
    In a LIGO interferometer, light waves leaking out of the two storage arms ordinarily interfere to send light back to the laser. By stretching the two arms by different amounts, a gravitational wave would alter the interference and send light toward a photodetector. G. GRULLÓN/SCIENCE

    But if the lengths are slightly different, then the recombining waves will be out of sync and light will emerge from the beam splitter perpendicular to the original beam. From that “dark port” output, physicists can measure any difference in the arms’ lengths to an iota of the light’s wavelength. Because a gravitational wave sweeping across the apparatus would generally stretch one arm more than the other, it would cause light to warble out of the dark port at the frequency at which the wave ripples. That light would be the signal of the gravitational wave.

    In practice, LIGO is a monumental challenge in sifting an infinitesimal signal from a mountain of vibrational noise. Sources of gravitational waves should “sing” at frequencies ranging from 10 to 1000 cycles per second, or hertz. But at frequencies of hundreds or thousands of hertz the individual photons in the laser beam produce noise as they jostle the mirrors. To smooth out such noise, researchers crank up the amount of light and deploy massive mirrors. At frequencies of tens of hertz and lower, seismic vibrations dominate, so researchers dangle the mirrors from elaborate suspension systems and actively counteract that motion. Still, a large earthquake anywhere in the world or even the surf pounding the distant coast can knock the interferometer off line.

    To boost the Hanford and Livingston detectors’ sensitivity 10-fold, to a ten-billionth of a nanometer, physicists have completely rebuilt the devices. Each of the original 22-kilogram mirrors hung like a pendulum from a single steel fiber; the new 40-kilogram mirrors hang on silica fibers at the end of a four-pendulum chain. Instead of LIGO’s original 10 kilowatts of light power, researchers aim to circulate 750 kilowatts. They will collect 100,000 channels of data to monitor the interferometer. Comparing the new and old LIGO is “like comparing a car to a wheel,” says Frederick Raab, a Caltech physicist who leads the Hanford site.

    The new Livingston machine has already doubled Initial LIGO’s sensitivity. “In 6 months they’ve made equivalent progress to what Initial LIGO made in 3 or 4 years,” says Raab, who adds that the Hanford site is about 6 months behind. But Valery Frolov, a Caltech physicist in charge of commissioning the Livingston detector, cautions that machine isn’t running anywhere close to specs. The seismic isolation was supposed to be better, he says, and researchers haven’t been able to keep the interferometer “locked” and running for long periods. As for reaching design sensitivity, “I don’t know whether it will take 1 year or whether it will take 5 years like Initial LIGO did,” he warns.

    Still, LIGO researchers plan to make a first observing run this year and hope to reach design sensitivity next year. “We will have detections that we will be able to stand up and defend, if not in 2016, then in 2017 or 2018,” says Gabriela González, a physicist at LSU and spokesperson for the more than 900-member LIGO Science Collaboration.

    That forecast is based on the statistics of the stars. LIGO’s prime target is the waves generated by a pair of neutron stars—the cores of exploded stars that weigh more than the sun but measure tens of kilometers across—whirling into each other in a death spiral lasting several minutes. Initial LIGO could sense such a pair up to 50 million light-years way. Given the rarity of neutron-star pairs, that search volume was too small to guarantee seeing one. Advanced LIGO should see 10 times as far and probe 1000 times as much space, enough to contain about 10 sources per year, González says. However, Clifford Will, a theorist at the University of Florida in Gainesville, notes that the number of sources is the most uncertain part of the experiment. “If it’s less than one per year, that’s not going to be too good,” he says.

    Enlarging the search

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    Compared with Initial LIGO, Advanced LIGO will be able to detect gravitational wave sources up to 10 times as far away, probing 1000 times as much space. Such a volume will likely yield multiple sources. ADAPTED FROM NSF BY G. GRULLÓN/SCIENCE

    The hunt will be global. As well as combining data from the two LIGO detectors, researchers will share data with their peers working on the VIRGO detector, an interferometer with 3-kilometer arms near Pisa, Italy, that is undergoing upgrades, and on GEO600, one with 600-meter arms near Hannover, Germany.

    VIRGO interferometer EGO
    VIRGO interferometer EGO Campus
    VIRGO

    GEO600
    GEO600

    By comparing data, collaborators can better sift signals from noise and can pinpoint sources on the sky. Japanese researchers are also building a detector, and LIGO leaders hope to add a third detector, in India.

    FOR THEORISTS—if not for the rest of the world—seeing gravitational waves for the first time will be something of an anti-climax. “We are so confident that gravitational waves exist that we don’t actually need to see one,” says Marc Kamionkowski, a theorist at Johns Hopkins University in Baltimore, Maryland. That’s because in 1974 American astrophysicists Russell Hulse and Joseph Taylor Jr. found indirect but convincing evidence of the waves. They spotted two pulsars—neutron stars that emit radio signals with clockwork regularity—orbiting each other. From the timing of the radio pulses, Hulse and Taylor could monitor the pulsars’ orbit. They found it is decaying at exactly the rate expected if the pulsars were radiating energy in the form of gravitational waves.

    LIGO’s real payoff will come in opening a new frontier in astronomy, says Robert Wald, a gravitational theorist at the University of Chicago in Illinois. “It’s kind of like after being able to see for a while, being able to hear, too,” Wald says. For example, if a black hole tears apart a neutron star, then details of the gravitational waves may reveal the properties of matter in neutron stars.

    All told, detecting gravitational waves would merit science’s highest accolade, physicists say. “As soon as they detect a gravitational wave, it’s a Nobel Prize,” Kamionkowski predicts. “It’s such an extraordinary experimental accomplishment.” But the prize can be shared by at most three people, so the question is who should get it.

    Weiss is a shoo-in, many say, but he demurs. “I don’t want to deny that there was some innovation [in my work], but it didn’t come out of the blue,” he says. “The lone crazy man working in a box, that just doesn’t hold true.” In 1962 two Russian physicists published a paper on detecting gravitational waves with an interferometer, as Weiss says he learned long after his 1972 work. In the 1970s, Robert Forward of the Hughes Aircraft Company in Malibu, California, ran a small interferometer. Key design elements of LIGO came from Ronald Drever, project director at Caltech from 1979 to 1987, who, Thorne says, “has to be recognized as one of the fathers of the LIGO idea.”

    But to make that prize-winning discovery, physicists must get Advanced LIGO up and running. At 8 a.m. on Tuesday morning, LIGO operator Gary Traylor comes off the night shift. “Last night was a total washout,” he says in his soft Southern accent, swiveling in a chair in the brightly lit control room. “There’s a low pressure area moving over the Atlantic that’s causing 20-foot waves to crash into the coast,” Traylor says, and that distant drumming overwhelmed the detector. So in the small hours, LIGO did sense waves. But not the ones everybody is hoping to see.

    See the full article here.

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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  • richardmitnick 11:48 am on March 3, 2015 Permalink | Reply
    Tags: , , , Physics   

    From FNAL: “Detecting something with nothing” 

    FNAL Home


    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Tuesday, March 3, 2015
    Lauren Biron

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    From left: Jason Bono (Rice University), Dan Ambrose (University of Minnesota) and Richie Bonventre (Lawrence Berkeley National Laboratory) work on the Mu2e straw chamber tracker unit at Lab 3. Photo: Reidar Hahn

    Researchers are one step closer to finding new physics with the completion of a harp-shaped prototype detector element for the Mu2e experiment.

    FNAL Mu2e experiment
    Mu2e

    Mu2e will look for the conversion of a muon to only an electron (with no other particles emitted) — something predicted but never before seen. This experiment will help scientists better understand how these heavy cousins of the electron decay. A successful sighting would bring us nearer to a unifying theory of the four forces of nature.

    The experiment will be 10,000 times as sensitive as other experiments looking for this conversion, and a crucial part is the detector that will track the whizzing electrons. Researchers want to find one whose sole signature is its energy of 105 MeV, indicating that it is the product of the elusive muon decay.

    In order to measure the electron, scientists track the helical path it takes through the detector. But there’s a catch. Every interaction with detector material skews the path of the electron slightly, disturbing the measurement. The challenge for Mu2e designers is thus to make a detector with as little material as possible, says Mu2e scientist Vadim Rusu.

    “You want to detect the electron with nothing — and this is as close to nothing as we can get,” he said.

    So how to detect the invisible using as little as possible? That’s where the Mu2e tracker design comes in. Panels made of thin straws of metalized Mylar, each only 15 microns thick, will sit inside a cylindrical magnet. Rusu says that these are the thinnest straws that people have ever used in a particle physics experiment.

    These straws, filled with a combination of argon and carbon dioxide gas and threaded with a thin wire, will wait in vacuum for the electrons. Circuit boards placed on both ends of the straws will gather the electrical signal produced when electrons hit the gas inside the straw. Scientists will measure the arrival times at each end of the wire to help accurately plot the electron’s overall trajectory.

    “This is another tricky thing that very few have attempted in the past,” Rusu said.

    The group working on the Mu2e tracker electronics have also created the tiny, low-power circuit boards that will sit at the end of each straw. With limited space to run cooling lines, necessary features that whisk away heat that would otherwise sit in the vacuum, the electronics needed to be as cool and small as possible.

    “We actually spent a lot of time designing very low-power electronics,” Rusu said.

    This first prototype, which researchers began putting together in October, gives scientists a chance to work out kinks, improve design and assembly procedures, and develop the necessary components.

    One lesson already learned? Machining curved metal with elongated holes that can properly hold the straws is difficult and expensive. The solution? Using 3-D printing to make a high-tech, transparent plastic version instead.

    Researchers also came up with a system to properly stretch the straws into place. While running a current through the straw, they use a magnet to pluck the straw — just like strumming a guitar string — and measure the vibration. This lets them set the proper tension that will keep the straw straight throughout the lifetime of the experiment.

    Although the first prototype of the tracker is complete, scientists are already hard at work on a second version (using the 3D-printed plastic), which should be ready in June or July. The prototype will then be tested for leaks and to see if the electronics pick up and transmit signals properly.

    A recent review of Mu2e went well, and Rusu expects work on the tracker construction to begin in 2016.

    See the full article here.

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    Fermilab Campus

    Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics.

     
  • richardmitnick 8:39 am on March 3, 2015 Permalink | Reply
    Tags: , Physics,   

    From AAAS: “A step closer to explaining high-temperature superconductivity?” 

    AAAS

    AAAS

    27 February 2015
    Adrian Cho

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    In the new experiment, scientists glimpsed a pattern of up- and down-spinning atoms, which mimics the up-and-down pattern of magnetism seen in high-temperature superconductors. R. A. Hart et al., Nature (2015)

    For years some physicists have been hoping to crack the mystery of high-temperature superconductivity—the ability of some complex materials to carry electricity without resistance at temperatures high above absolute zero—by simulating crystals with patterns of laser light and individual atoms. Now, a team has taken—almost—the next-to-last step in such “optical lattice” simulation by reproducing the pattern of magnetism seen in high-temperature superconductors from which the resistance-free flow of electricity emerges.

    “It’s a very big improvement over previous results,” says Tilman Esslinger, an experimentalist at the Swiss Federal Institute of Technology in Zurich, who was not involved in the work. “It’s very exciting to see steady progress.”

    An optical lattice simulation is essentially a crystal made of light. A real crystal contains a repeating 3D pattern of ions, and electrons flow from ion to ion. In the simulation, spots of laser light replace the ions, and ultracold atoms moving among spots replace the electrons. Physicists can adjust the pattern of spots, how strongly the spots attract the atoms, and how strongly the atoms repel one another. That makes the experiments ideal for probing physics such as high-temperature superconductivity, in which materials such as mercury barium calcium copper oxide carry electricity without resistance at temperatures up to 138 K, far higher above absolute zero than ordinary superconductors such as niobium can.

    Just how the copper-and-oxygen, or cuprate, superconductors work remains unclear. The materials contain planes of copper and oxygen ions with the coppers arranged in a square pattern. Repelling one another, the electrons get stuck in a one-to-a-copper traffic jam called a Mott insulator state. They also spin like tops, and at low temperatures neighboring electrons spin in opposite directions, creating an up-down-up-down pattern of magnetism called antiferromagnetism. Superconductivity sets in when impurities soak up a few electrons and ease the traffic jam. The remaining electrons then pair to glide freely along the planes.

    Theorists do not yet agree how that pairing occurs. Some think that wavelike ripples in the antiferromagnetic pattern act as a glue to attract one electron to the other. Others argue that the pairing arises, paradoxically, from the repulsion among the electrons alone. Theorists can write down a mathematical model of electrons on a checkerboard plane, known as the Fermi-Hubbard model, but it is so hard to “solve” that nobody has been able to show whether it produces superconductivity.

    Experimentalists hope to reproduce the Fermi-Hubbard model in laser light and cold atoms to see if it yields superconductivity. In 2002, Immanuel Bloch, a physicist at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, and colleagues realized a Mott insulator state in an optical lattice. Six years later, Esslinger and colleagues achieved the Mott state with atoms with the right amount of spin to mimic electrons. Now, Randall Hulet, a physicist at Rice University in Houston, Texas, and colleagues have nearly achieved the next-to-last step along the way: antiferromagnetism.

    Hulet and colleagues trapped between 100,000 and 250,000 lithium-6 atoms in laser light. They then ramped up the optical lattice and ramped it back down to put them in order. Shining laser light of a specific wavelength on the atoms, they observed evidence of an emerging up-down-up-down spin pattern. The laser light was redirected, or diffracted, at a particular angle by the rows of atoms—just as x-rays diffract off the ions in a real crystal. Crucially, the light probed the spin of the atoms: The light wave flipped if it bounced off an atom spinning one way but not the other. Without that flipping, the diffraction wouldn’t have occurred, so observation confirms the emergence of the up-down-up-down pattern, Hulet says.

    Hulet’s team solved a problem that has plagued other efforts. Usually, turning the optical lattice on heats the atoms. To avoid that, the researchers added another laser that slightly repelled the atoms, so that the most energetic ones were just barely held by the trap. Then, as the atoms heated, the most energetic ones “evaporated” like steam from hot soup to keep the other ones cool, the researchers report online this week in Nature. They didn’t quite reach a full stable antiferromagnetic pattern: The temperature was 40% too high. But the technique might get there and further, Hulet says. “We don’t have a good sense of what the limit of this method is,” he says. “We could get a factor of two lower, we could get a factor of 10 lower.”

    “It is indeed very promising,” says Tin-Lun “Jason” Ho, a theorist at Ohio State University, Columbus. Reducing the temperature by a factor of two or three might be enough to reach the superconducting state, he says. However, MPQ’s Bloch cautions that it may take still other techniques to get that cold. “There are several cooling techniques that people are developing and interesting experiments coming up,” he says.

    Physicists are also exploring other systems and problems with optical lattices. The approach is still gaining steam, Hulet says: “It’s an exciting time.”

    See the full article here.

    The American Association for the Advancement of Science is an international non-profit organization dedicated to advancing science for the benefit of all people.

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  • richardmitnick 7:54 pm on March 2, 2015 Permalink | Reply
    Tags: , , , Physics,   

    From EPFL: “The first ever photograph of light as both a particle and wave” 

    EPFL bloc

    Ecole Polytechnique Federale Lausanne

    1

    March 2, 2015

    Light behaves both as a particle and as a wave. Since the days of [Albert] Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.

    Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

    When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this “photoelectric” effect by proposing that light – thought to only be a wave – is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

    A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle.

    The experiment is set up like this: A pulse of laser light is fired at a tiny metallic nanowire. The laser adds energy to the charged particles in the nanowire, causing them to vibrate. Light travels along this tiny wire in two possible directions, like cars on a highway. When waves traveling in opposite directions meet each other they form a new wave that looks like it is standing in place. Here, this standing wave becomes the source of light for the experiment, radiating around the nanowire.

    This is where the experiment’s trick comes in: The scientists shot a stream of electrons close to the nanowire, using them to image the standing wave of light. As the electrons interacted with the confined light on the nanowire, they either sped up or slowed down. Using the ultrafast microscope to image the position where this change in speed occurred, Carbone’s team could now visualize the standing wave, which acts as a fingerprint of the wave-nature of light.

    While this phenomenon shows the wave-like nature of light, it simultaneously demonstrated its particle aspect as well. As the electrons pass close to the standing wave of light, they “hit” the light’s particles, the photons. As mentioned above, this affects their speed, making them move faster or slower. This change in speed appears as an exchange of energy “packets” (quanta) between electrons and photons. The very occurrence of these energy packets shows that the light on the nanowire behaves as a particle.

    “This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” says Fabrizio Carbone. In addition, the importance of this pioneering work can extend beyond fundamental science and to future technologies. As Carbone explains: “Being able to image and control quantum phenomena at the nanometer scale like this opens up a new route towards quantum computing.”

    This work represents a collaboration between the Laboratory for Ultrafast Microscopy and Electron Scattering of EPFL, the Department of Physics of Trinity College (US) and the Physical and Life Sciences Directorate of the Lawrence Livermore National Laboratory. The imaging was carried out EPFL’s ultrafast energy-filtered transmission electron microscope – one of the two in the world.

    See the full article here.

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    EPFL is Europe’s most cosmopolitan technical university. It receives students, professors and staff from over 120 nationalities. With both a Swiss and international calling, it is therefore guided by a constant wish to open up; its missions of teaching, research and partnership impact various circles: universities and engineering schools, developing and emerging countries, secondary schools and gymnasiums, industry and economy, political circles and the general public.

     
  • richardmitnick 1:48 pm on March 1, 2015 Permalink | Reply
    Tags: , , , Physics   

    From Perimeter: “Pioneering Women of Physics” 

    Perimeter Institute
    Perimeter Institute

    February 25, 2015

    For more information, contact:
    Lisa Lambert
    Manager, External Relations and Public Affairs
    llambert@perimeterinstitute.ca
    (519) 569-7600 x5051

    They discovered pulsars, found the first evidence of dark matter, pioneered mathematics, radioactivity, nuclear fission, elasticity, and computer programming, and have even stopped light.
    Perimeter celebrates women who made pioneering contributions to physics, often overcoming tremendous challenges to do so.

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    See the full article here.

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    About Perimeter

    Perimeter Institute is a leading centre for scientific research, training and educational outreach in foundational theoretical physics. Founded in 1999 in Waterloo, Ontario, Canada, its mission is to advance our understanding of the universe at the most fundamental level, stimulating the breakthroughs that could transform our future. Perimeter also trains the next generation of physicists through innovative programs, and shares the excitement and wonder of science with students, teachers and the general public.

     
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