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  • richardmitnick 9:18 am on October 8, 2015 Permalink | Reply
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    From FNAL: “Physics in a Nutshell – Is the universe getting bigger or am I getting smaller?” 

    FNAL II photo

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

    Oct. 8, 2015
    Jim Pivarski

    Temp 1
    Alice knows she’s getting bigger only because the room isn’t.

    It is a well-established fact that the universe is expanding. It grows without center, like an inflating raisin cake, but an infinite raisin cake filling all of space in all directions. The raisins are the galaxies.

    A problem I’ve had with this explanation is that if everything were to double in size — galaxies, houses, you and me, rulers — then we’d never notice. I might be a towering giant, but if the room is equally huge, I wouldn’t know. We can only see relative differences in sizes.

    When scientists say the universe is expanding, they don’t mean that its occupants are expanding along with it. The raisins do not grow with the cake. Imagine cake batter so full of raisins that they’re pressed against each other when you first put the cake in the oven, but by the time it’s done, there’s only one raisin per mouthful. This would be a better analogy, but it raises another question: How do we know the raisins aren’t shrinking?

    Putting the question another way, what if the distances between galaxies are fixed, but everything except those distances are getting smaller? Or somewhere in between — the universe grows a little while we shrink a little. For that matter, where should we put the boundary line between the scales that grow relative to the scales that shrink?

    Fundamentally, the expansion of the universe is described by one ratio that relates lengths in space with durations in time, sometimes called the cosmic scale factor. As time passes, this ratio changes: the scale of space increases with each second. But since this ratio, length divided by time, is a speed, suppose we think of space as fixed and all speeds slowing down.

    What would happen if every object, from particles to planets, suddenly slowed down? Planets would fall in closer to the sun because they would have less angular momentum. Similarly, electrons would get closer to the nuclei of atoms. Molecular bonds would shorten. Every system bound by a force would shrink, but the distances between unconnected systems would stay the same.

    Alternatively, what would happen if particle speeds were left alone but everything expanded uniformly, like a plate of marshmallows in the microwave? Again, electron and planetary orbits would then shrink to their natural sizes, like marshmallows taken out of the microwave, but the gaps between them wouldn’t.

    Regardless of how we interpret the underlying theory, we have the same picture: Distances between bound systems increase relative to the sizes of those systems. But that shouldn’t be a surprise, since we’re talking about the same physics theory in two different ways. It’s all a matter of perspective.

    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 3:25 pm on October 6, 2015 Permalink | Reply
    Tags: , , , Physics   

    From DESY: “Physicists shrink particle accelerator” 


    No Writer Credit

    Terahertz accelerator modules easily fit into two fingers. Credit: DESY/Heiner Müller-Elsner

    An interdisciplinary team of researchers has built the first prototype of a miniature particle accelerator that uses terahertz radiation instead of radio frequency structures. A single accelerator module is no more than 1.5 centimetres long and one millimetre thick. The terahertz technology holds the promise of miniaturising the entire set-up by at least a factor of 100, as the scientists surrounding DESY’s Franz Kärtner from the Center for Free-Electron Laser Science (CFEL) point out. They are presenting their prototype, that was set up in Kärtner’s lab at the Massachusetts Institute of Technology (MIT) in the U.S., in the journal Nature Communications. The authors see numerous applications for terahertz accelerators, in materials science, medicine and particle physics, as well as in building X-ray lasers. CFEL is a cooperation between DESY, the University of Hamburg and the Max Planck Society.

    In the electromagnetic spectrum, terahertz radiation lies between infrared radiation and microwaves. Particle accelerators usually rely on electromagnetic radiation from the radio frequency range; DESY’s particle accelerator PETRA III, for example, uses a frequency of around 500 megahertz.

    DESI Petra III interior

    The wavelength of the terahertz radiation used in this experiment is around one thousand times shorter. “The advantage is that everything else can be a thousand times smaller too,” explains Kärtner, who is also a professor at the University of Hamburg and at MIT, as well as being a member of the Hamburg Centre for Ultrafast Imaging (CUI), one of Germany’s Clusters of Excellence.

    For their prototype the scientists used a special microstructured accelerator module, specifically tailored to be used with terahertz radiation. The physicists fired fast electrons into the miniature accelerator module using a type of electron gun provided by the group of CFEL Professor Dwayne Miller, Director at the Max Planck Institute for the Structure and Dynamics of Matter and also a member of CUI. The electrons were then further accelerated by the terahertz radiation fed into the module. This first prototype of a terahertz accelerator was able to increase the energy of the particles by seven kiloelectronvolts (keV).

    “This is not a particularly large acceleration, but the experiment demonstrates that the principle does work in practice,” explains co-author Arya Fallahi of CFEL, who did the theoretical calculations. “The theory indicates that we should be able to achieve an accelerating gradient of up to one gigavolt per metre.” This is more than ten times what can be achieved with the best conventional accelerator modules available today. Plasma accelerator technology, which is also at an experimental stage right now, promises to produce even higher accelerations, however it also requires significantly more powerful lasers than those needed for terahertz accelerators.

    The physicists underline that terahertz technology is of great interest both with regard to future linear accelerators for use in particle physics, and as a means of building compact X-ray lasers and electron sources for use in materials research, as well as medical applications using X-rays and electron radiation. “The rapid advances we are seeing in terahertz generation with optical methods will enable the future development of terahertz accelerators for these applications,” says first author Emilio Nanni of MIT. Over the coming years, the CFEL team in Hamburg plans to build a compact, experimental free-electron X-ray laser (XFEL) on a laboratory scale using terahertz technology. This project is supported by a Synergy Grant of the European Research Council.

    So-called free-electron lasers (FELs) generate flashes of laser light by sending high-speed electrons from a particle accelerator down an undulating path, whereby these emit light every time they are deflected. This is the same principle that will be used by the X-ray laser European XFEL, which is currently being built by an international consortium, reaching from the DESY Campus in Hamburg to the neighbouring town of Schenefeld, in Schleswig-Holstein. The entire facility will be more than three kilometres long and will be the best and most modern of its kind after completion.

    The experimental XFEL using terahertz technology is expected to be less than a metre long. “We expect this sort of device to produce much shorter X-ray pulses lasting less than a femtosecond”, says Kärtner. Because the pulses are so short, they reach a comparable peak brightness to those produced by larger facilities, even if there is significant less light in each pulse. “With these very short pulses we are hoping to gain new insights into extremely rapid chemical processes, such as those involved in photosynthesis.”

    Developing a detailed understanding of photosynthesis would open up the possibility of implementing this efficient process artificially and thus tapping into increasingly efficient solar energy conversion and new pathways for CO2 reduction. Beyond this, researchers are interested in numerous other chemical reactions. As Kärtner points out, “photosynthesis is just one example of many possible catalytic processes we would like to investigate.” The compact XFEL can be potentially also used to seed pulses in large scale facilities to enhance the quality of their pulses. Also, certain medical imaging techniques could benefit from the enhanced characteristics of the novel X-ray source.

    „Terahertz-driven linear electron acceleration“; Emilio A. Nanni, Wenqian R. Huang, Kyung-Han Hong, Koustuban Ravi, Arya Fallahi, Gustavo Moriena, R. J. Dwayne Miller & Franz X. Kärtner; Nature Communications, 2015; DOI: 10.1038/NCOMMS9486

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    DESY is one of the world’s leading accelerator centres. Researchers use the large-scale facilities at DESY to explore the microcosm in all its variety – from the interactions of tiny elementary particles and the behaviour of new types of nanomaterials to biomolecular processes that are essential to life. The accelerators and detectors that DESY develops and builds are unique research tools. The facilities generate the world’s most intense X-ray light, accelerate particles to record energies and open completely new windows onto the universe. 
That makes DESY not only a magnet for more than 3000 guest researchers from over 40 countries every year, but also a coveted partner for national and international cooperations. Committed young researchers find an exciting interdisciplinary setting at DESY. The research centre offers specialized training for a large number of professions. DESY cooperates with industry and business to promote new technologies that will benefit society and encourage innovations. This also benefits the metropolitan regions of the two DESY locations, Hamburg and Zeuthen near Berlin.

  • richardmitnick 9:43 am on October 2, 2015 Permalink | Reply
    Tags: , , Heisenberg's Uncertainty Principle, Physics,   

    From AAAS: “Physicists observe weird quantum fluctuations of empty space—maybe” 



    1 October 2015
    Adrian Cho

    The setup in which a long “pump” light pulse (red) changes the polarization of a short “probe” light pulse (green) also serves to measure the effect of vacuum fluctuations—just by turning off the pump beam. ADAPTED FROM C. RIEK ET AL., SCIENCE (2015)

    Empty space is anything but, according to quantum mechanics: Instead, it roils with quantum particles flitting in and out of existence. Now, a team of physicists claims it has measured those fluctuations directly, without disturbing or amplifying them. However, others say it’s unclear exactly what the new experiment measures—which may be fitting for a phenomenon that originates in quantum mechanics’ famous uncertainty principle.

    “There are many experiments that have observed indirect effects of vacuum fluctuations,” says Diego Dalvit, a theorist at Los Alamos National Laboratory in New Mexico who was not involved in the current work. “If this [new experiment] is correct, it would be the first direct observation of the field [of fluctuations] itself.”

    Thanks to the [Heisenberg’s] uncertainty principle, the vacuum buzzes with particle-antiparticle pairs popping in and out of existence. They include, among many others, electron-positron pairs and pairs of photons, which are their own antiparticles. Ordinarily, those “virtual” particles cannot be directly captured. But like some spooky Greek chorus, they exert subtle influences on the “real” world.

    For example, the virtual photons flitting in and out of existence produce a randomly fluctuating electric field. In 1947, physicists found that the field shifts the energy levels of an electron inside a hydrogen atom and hence the spectrum of radiation the atom emits. A year later, Dutch theorist Hendrik Casimir predicted that the field would also exert a subtle force on two closely spaced metal plates, squeezing them together. That’s because the electric field must vanish on the plates’ surfaces, so only certain wavelike ripples of the electric field can fit between the plates. In contrast, more ripples can push on the plates from the outside, exerting a net force. The Casimir effect was observed in 1997.

    But now, Claudius Riek, Alfred Leitenstorfer, and colleagues at the University of Konstanz in Germany say they have directly observed those electric field fluctuations by charting their influence on a light wave. The experiment riffs on a technique they developed to study a longer light pulse with a much shorter one by shooting them simultaneously through a crystal (see diagram). The longer “pump” pulse is polarized horizontally, meaning that the electric field in it oscillates sideways. The shorter “probe” pulse starts out polarized vertically. However, the properties of the crystal depend on the electric field in it, so the pump beam causes the polarization of the probe beam to change and emerge from the crystal tracing an elliptical pattern. By adjusting the timing of the pulses, researchers can use the polarization effect to map out the wiggles in the electric field in the pump wave.

    But vacuum fluctuations themselves will affect the crystal and hence the polarization of the probe pulse, Leitensdorfer says. So to measure the fluctuations of the vacuum field, “we only put in the probe pulse, nothing else.” On average the polarization of the lone probe pulse remained vertical. But over many repeated trials, it varied slightly, and that noise was the sign of the vacuum fluctuations, the team says.

    Spotting the effect is no mean feat, as the polarization also varies because of random variation in the number of photons in each pulse, or “shot noise.” To tease the two apart, the physicists vary the duration and width of the pulse, but not the number of photons in it. The shot noise should stay constant, whereas the noise from quantum fluctuations should shrink as the pulses become bigger. The researchers saw a change of a few percent in the noise, an effect they attribute to vacuum fluctuations.

    Some physicists question what the new experiment actually measures, however. The researchers assume that fluctuating optical properties of the crystal reflect the vacuum fluctuations, says Steve Lamoreaux, a physicist at Yale University and one of the first to observe the Casimir effect. But the variations in the crystal’s optical properties could have some other source, such as thermal fluctuations, he says. “The material properties will fluctuate on their own,” he says, so “how does one attribute these fluctuations to the vacuum alone?”

    Moreover, Leitenstorfer’s group is not the first to directly probe such fluctuations. In 2011, Christopher Wilson, a physicist now at the University of Waterloo in Canada, and colleagues reported in Nature that they had pumped up vacuum fluctuations and turned them into real photons. In principle, that can be done by accelerating a mirror back and forth at near light speed. Wilson used a more practical analog: a system in in which the effective length of a small superconducting cavity could be changed electronically. Leitenstorfer notes that the new experiment differs from Wilson’s in that it does not require amplifying the fluctuations. Wilson responds, “While I agree that that’s a difference, I don’t think that it’s fundamental.”

    Leitenstorfer contends that the new work makes a qualitative advance over previous efforts. “We clearly have gone a significant step further in comparison to anybody else by directly measuring the electric field amplitude of the vacuum as it fluctuates in space and time,” he says. Others seem less certain about that.

    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 3:40 pm on September 24, 2015 Permalink | Reply
    Tags: , , Physics,   

    From SLAC: “Mysterious Neutrinos Take the Stage at SLAC” 

    SLAC Lab

    Of all known fundamental particles, neutrinos may be the most mysterious: Although they are highly abundant in the universe and were discovered experimentally in 1956, researchers still have a lot left to learn about them. To find out more about the elusive particles and their potential links to cosmic evolution, invisible dark matter and matter’s dominance over antimatter in the universe, the Department of Energy’s SLAC National Accelerator Laboratory is taking on key roles in four neutrino experiments: EXO, DUNE, MicroBooNE and ICARUS.

    Neutrinos were also the central theme of the 43rd annual SLAC Summer Institute for particle physics and astrophysics. The tradition-rich educational event, held Aug. 10-21, attracted more than 150 scientists from around the globe and featured lectures by some of the world’s leading neutrino experts.

    “Neutrinos are a hot research topic and have become a major focus of U.S. high-energy physics,” said SLAC theorist Thomas Rizzo, one of the summer school’s organizers. “There are many things we want to know about neutrinos. For instance, what are the masses of the known neutrinos? Are there other types of neutrinos that we don’t know about? Do neutrinos and antineutrinos behave differently? Or are neutrinos their own antiparticles?”

    Francis Halzen, principal investigator of the international IceCube Neutrino Observatory at the South Pole and neutrino specialist at the University of Wisconsin, gave the summer institute’s opening lecture.

    ICECUBE neutrino detector
    IceCube neutrino detector interior

    He said neutrinos have a high potential for scientific discovery – and they are also never boring. As a matter of fact, the history of neutrino research has seen a few surprising twists and turns.

    Neutrinos were the central theme of the 43rd annual SLAC Summer Institute for particle physics and astrophysics, which featured lectures by some of the world’s leading neutrino experts. (SLAC National Accelerator Laboratory)

    Elusive and Mysterious Neutrinos

    Neutrinos are one of the most common fundamental particles in the universe. They are abundantly produced in supernova explosions, star-powering nuclear fusion and other nuclear processes, resulting in trillions of neutrinos passing through us every minute. Yet, they are very difficult to study because they rarely interact with their surroundings and easily evade detection. This explains why it took researchers nearly 30 years to catch a first glimpse of neutrinos, although their existence had been first postulated in 1930 to explain an apparent violation of the conservation of energy in the radioactive decay of unstable atomic nuclei known as beta decay.

    A few years after the initial discovery in 1956, researchers were caught by surprise when more than one type of neutrino showed up in their experiments. By the turn of the millennium, they had identified three different types, or flavors, each associated with another fundamental charged partner particle: the electron, muon and tau.

    For the longest time, neutrinos were thought to be massless. But in 1998, scientists discovered that neutrinos frequently change from one flavor into another – a process called neutrino oscillation that can only occur if neutrinos do, in fact, have mass. Although the exact masses remain unknown, researchers estimate neutrinos to be two million times lighter than the next heavier particle, the electron, and this large mass difference is one of the great puzzles of neutrino physics.

    Neutrinos are abundantly produced in nuclear processes in the universe, for instance inside the sun. This image shows the sun in “neutrino light” as seen by the Super-Kamiokande neutrino detector in Japan. (Kamioka Observatory, ICRR, University of Tokyo)

    Super-Kamiokande experiment Japan
    Super-Kamiokande neutrino detector

    EXO: The Origin of the Neutrino Mass

    The origin of neutrino masses could be different from the origin of the masses of other particles. This could explain why neutrinos are incredibly light. One sign that this is true would be if they were their own antiparticles. This is only possible for neutrinos, since they carry no electric charge. The Enriched Xenon Observatory (EXO) is searching for a theorized rare nuclear process – neutrinoless double beta decay – that would prove that neutrinos and antineutrinos are identical.

    EXO experiment
    Part of the EXO-200 underground detector used to search for a hypothesized radioactive decay that could reveal how neutrinos acquire their incredibly small mass. (EXO Collaboration)

    Located almost half a mile underground at the Waste Isolation Pilot Plant in New Mexico, protected from cosmic radiation, the sensitive EXO experiment uses 200 kilograms of enriched liquid xenon that could potentially undergo the sought-after decay. If it exists, it would be so rare that it would take billions of times longer than the age of the universe for half of the radioactive xenon nuclei to decay. Only the large number of xenon atoms in the experiment allows researchers to search for such a long-lived decay.

    “Neutrinoless double beta decay would not only tell us that neutrinos must be their own antiparticles,” said SLAC particle physicist and EXO team member Martin Breidenbach. “From the measured decay rate, we could also determine the effective neutrino mass.”

    SLAC co-led the construction of the experiment’s 200-kilogram version (EXO-200), which also serves as a test bed for a more sensitive future ton-scale version (nEXO) that would give researchers a much better chance of seeing neutrinoless double beta decay.

    DUNE: Trio of Neutrino Masses and Matter-Antimatter Imbalance

    SLAC researchers are also taking part in another neutrino experiment – the Deep Underground Neutrino Experiment (DUNE), which will be constructed by a new international collaboration hosted at the Long-Baseline Neutrino Facility (LBNF) as the centerpiece of the particle physics program in the U.S.

    As part of LBNF, neutrinos and antineutrinos will be sent 800 miles through the Earth from Fermi National Accelerator Laboratory in Illinois to the DUNE detector in South Dakota – an “eye” for neutrinos that will eventually consist of four 10,000-ton modules of liquid argon. Scientists will then track how the particles morph from one neutrino flavor into another along the way.

    By comparing the oscillations of antineutrinos with those of neutrinos, DUNE researchers will be able to determine if the matter-antimatter siblings behave differently. If they do, the difference could potentially help explain why our universe is made of matter rather than antimatter.

    “Since each neutrino flavor interacts differently with the material in the Earth, the experiment will also tell us which of the three neutrino types is the lightest and which is the heaviest,” said researcher Mark Convery, who heads SLAC’s LBNF/DUNE group.

    DUNE’s liquid argon detector may also make other experiments possible. It could be used, for instance, to catch a glimpse of neutrino bursts from supernova explosions, which could tell us more about the physics of collapsing stars. Scientists at the joint SLAC/Stanford University Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) are particularly interested in this research opportunity.

    The future DUNE experiment will send neutrinos and antineutrinos 800 miles through the Earth to determine the relative masses of the three known neutrino types and study whether neutrinos and antineutrinos behave differently. (Fermi National Accelerator Laboratory)

    MicroBooNE and ICARUS: Search for Unknown Neutrinos

    However, DUNE will not be ready until the mid-2020s. In the meantime, Convery and his team are also engaging in the current MicroBooNE and future ICARUS experiments at Fermilab. These are so-called short-baseline experiments with detectors just hundreds of yards away from the neutrino source, rather than hundreds of miles away.

    FNAL MicroBooNE
    MicroBooNE detector


    “MicroBooNE and ICARUS will help us prepare for DUNE, but they also have the potential to discover completely new physics,” Convery said. “They’ll follow up on previous short-baseline studies that observed anomalies in neutrino oscillations.”

    Researchers believe that these anomalies could hint at the existence of a fourth, “sterile” neutrino. This hypothetical particle could potentially be linked to dark matter, the invisible substance that is five times more prevalent in the universe than regular matter.

    MicroBooNE’s 170-ton liquid argon detector began collecting data in August 2015, while ICARUS, which is three-and-a-half times heavier, is being upgraded at the European particle physics laboratory CERN. Both experiments will eventually become part of a three-detector short-baseline neutrino program at Fermilab, scheduled to launch in 2018 and designed to clarify whether previous hints at sterile neutrinos are correct or not.

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    SLAC Campus
    SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

  • richardmitnick 1:25 pm on September 22, 2015 Permalink | Reply
    Tags: , Physics, , Weyl fermions   

    From Princeton: “After 85-year search, massless particle with promise for next-generation electronics discovered” 

    Princeton University
    Princeton University

    July 16, 2015
    Morgan Kelly

    An international team led by Princeton University scientists has discovered an elusive massless particle theorized 85 years ago. The particle could give rise to faster and more efficient electronics because of its unusual ability to behave as matter and antimatter inside a crystal, according to new research.

    The researchers report in the journal Science July 16 the first observation of Weyl fermions, which, if applied to next-generation electronics, could allow for a nearly free and efficient flow of electricity in electronics, and thus greater power, especially for computers, the researchers suggest.

    Proposed by the mathematician and physicist Hermann Weyl in 1929, Weyl fermions have been long sought by scientists because they have been regarded as possible building blocks of other subatomic particles, and are even more basic than the ubiquitous, negative-charge carrying electron (when electrons are moving inside a crystal). Their basic nature means that Weyl fermions could provide a much more stable and efficient transport of particles than electrons, which are the principle particle behind modern electronics. Unlike electrons, Weyl fermions are massless and possess a high degree of mobility; the particle’s spin is both in the same direction as its motion — which is known as being right-handed — and in the opposite direction in which it moves, or left-handed.

    “The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we’re just not capable of imagining now,” said corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team.

    An international team led by Princeton University scientists has discovered Weyl fermions, elusive massless particles theorized 85 years ago that could give rise to faster and more efficient electronics because of their unusual ability to behave as matter and antimatter inside a crystal. The team included numerous researchers from Princeton’s Department of Physics, including (from left to right) graduate students Ilya Belopolski and Daniel Sanchez; Guang Bian, a postdoctoral research associate; corresponding author M. Zahid Hasan, a Princeton professor of physics who led the research team; and associate research scholar Hao Zheng. (Photo by Danielle Alio, Office of Communications)

    The researchers’ find differs from the other particle discoveries in that the Weyl fermion can be reproduced and potentially applied, Hasan said. Typically, particles such as the famous Higgs boson are detected in the fleeting aftermath of particle collisions, he said. The Weyl fermion, however, was discovered inside a synthetic metallic crystal called tantalum arsenide that the Princeton researchers designed in collaboration with researchers at the Collaborative Innovation Center of Quantum Matter in Beijing and at National Taiwan University.

    The Weyl fermion possesses two characteristics that could make its discovery a boon for future electronics, including the development of the highly prized field of efficient quantum computing, Hasan explained.

    For a physicist, the Weyl fermions are most notable for behaving like a composite of monopole– and antimonopole-like particles when inside a crystal, Hasan said. This means that Weyl particles that have opposite magnetic-like charges can nonetheless move independently of one another with a high degree of mobility.

    A detector image (top) signals the existence of Weyl fermions. The plus and minus signs note whether the particle’s spin is in the same direction as its motion — which is known as being right-handed — or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. A schematic (bottom) shows how Weyl fermions also can behave like monopole and antimonopole particles when inside a crystal, meaning that they have opposite magnetic-like charges can nonetheless move independently of one another, which also allows for a high degree of mobility. (Image by Su-Yang Xu and M. Zahid Hasan, Princeton Department of Physics)

    The researchers also found that Weyl fermions can be used to create massless electrons that move very quickly with no backscattering, wherein electrons are lost when they collide with an obstruction. In electronics, backscattering hinders efficiency and generates heat. Weyl electrons simply move through and around roadblocks, Hasan said.

    “It’s like they have their own GPS and steer themselves without scattering,” Hasan said. “They will move and move only in one direction since they are either right-handed or left-handed and never come to an end because they just tunnel through. These are very fast electrons that behave like unidirectional light beams and can be used for new types of quantum computing.”

    Prior to the Science paper, Hasan and his co-authors published a report in the journal Nature Communications in June that theorized that Weyl fermions could exist in a tantalum arsenide crystal. Guided by that paper, the researchers used the Princeton Institute for the Science and Technology of Materials (PRISM) and Laboratory for Topological Quantum Matter and Spectroscopy in Princeton’s Jadwin Hall to research and simulate dozens of crystal structures before seizing upon the asymmetrical tantalum arsenide crystal, which has a differently shaped top and bottom.

    The crystals were then loaded into a two-story device known as a scanning tunneling spectromicroscope that is cooled to near absolute zero and suspended from the ceiling to prevent even atom-sized vibrations. The spectromicroscope determined if the crystal matched the theoretical specifications for hosting a Weyl fermion. “It told us if the crystal was the house of the particle,” Hasan said.

    The Princeton team took the crystals passing the spectromicroscope test to the Lawrence Berkeley National Laboratory in California to be tested with high-energy accelerator-based photon beams. Once fired through the crystal, the beams’ shape, size and direction indicated the presence of the long-elusive Weyl fermion.

    First author Su-Yang Xu, a postdoctoral research associate in Princeton’s Department of Physics, said that the work was unique for encompassing theory and experimentalism.

    “The nature of this research and how it emerged is really different and more exciting than most of other work we have done before,” Xu said. “Usually, theorists tell us that some compound might show some new or interesting properties, then we as experimentalists grow that sample and perform experiments to test the prediction. In this case, we came up with the theoretical prediction ourselves and then performed the experiments. This makes the final success even more exciting and satisfying than before.”

    In pursuing the elusive particle, the researchers had to pull from a number of disciplines, as well as just have faith in their quest and scientific instincts, Xu said.

    “Solving this problem involved physics theory, chemistry, material science and, most importantly, intuition,” he said. “This work really shows why research is so fascinating, because it involved both rational, logical thinking, and also sparks and inspiration.”

    Weyl, who worked at the Institute for Advanced Study, suggested his fermion as an alternative to the theory of relativity proposed by his colleague Albert Einstein. Although that application never panned out, the characteristics of his theoretical particle intrigued physicists for nearly a century, Hasan said. Actually observing the particle was a trying process — one ambitious experiment proposed colliding high-energy neutrinos to test if the Weyl fermion was produced in the aftermath, he said.

    Hasan (pictured) and his research group researched and simulated dozens of crystal structures before finding the one suitable for holding Weyl fermions. Once fashioned, the crystals were loaded into this two-story device known as a scanning tunneling spectromicroscope to ensure that they matched theoretical specifications. Located in the Laboratory for Topological Quantum Matter and Spectroscopy in Princeton’s Jadwin Hall, the spectromicroscope is cooled to near absolute zero and suspended from the ceiling to prevent even atom-sized vibrations. (Photo by Danielle Alio, Office of Communications)

    The hunt for the Weyl fermion began in the earliest days of quantum theory when physicists first realized that their equations implied the existence of antimatter counterparts to commonly known particles such as electrons, Hasan said.

    “People figured that although Weyl’s theory was not applicable to relativity or neutrinos, it is the most basic form of fermion and had all other kinds of weird and beautiful properties that could be useful,” he said.

    “After more than 80 years, we found that this fermion was already there, waiting. It is the most basic building block of all electrons,” he said. “It is exciting that we could finally make it come out following Weyl’s 1929 theoretical recipe.”

    Ashvin Vishwanath, a professor of physics at the University of California-Berkeley who was not involved in the study, commented: “Professor Hasan’s experiments report the observation of both the unusual properties in the bulk of the crystal as well as the exotic surface states that were theoretically predicted. While it is early to say what practical implications this discovery might have, it is worth noting that Weyl materials are direct 3-D electronic analogs of graphene, which is being seriously studied for potential applications.”

    The team included numerous researchers from Princeton’s Department of Physics, including graduate students Ilya Belopolski, Nasser Alidoust and Daniel Sanchez; Guang Bian, a postdoctoral research associate; associate research scholar Hao Zheng; and Madhab Neupane, a Princeton postdoctoral research associate now at the Los Alamos National Laboratory; and Class of 2015 undergraduate Pavel Shibayev.

    Other co-authors were Chenglong Zhang, Zhujun Yuan and Shuang Jia from Peking University; Raman Sankar and Fangcheng Chou from National Taiwan University; Guoqing Chang, Chi-Cheng Lee, Shin-Ming Huang, BaoKai Wang and Hsin Lin from the National University of Singapore; Jie Ma from Oak Ridge National Laboratory; and Arun Bansil from Northeastern University. Wang is also affiliated with Northeastern University, and Jia is affiliated with the Collaborative Innovation Center of Quantum Matter in Beijing.

    The paper, Discovery of Weyl fermions and topological Fermi arcs, was published online by Science on July 16. The work was supported by the Gordon and Betty Moore Foundations Emergent Phenomena in Quantum Systems (EPiQS) Initiative (grant no. GBMF4547); the Singapore National Research Foundation (grant no. NRF-NRFF2013-03); the National Basic Research Program of China(grant nos. 2013CB921901 and 2014CB239302); the U.S. Department of Energy (grant no. DE-FG-02-05ER462000); and the Taiwan Ministry of Science and Technology (project no. 102-2119-M- 002-004).

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  • richardmitnick 12:14 pm on September 22, 2015 Permalink | Reply
    Tags: , Physics, Proton Decay (?),   

    From Symmetry: “Do protons decay?” 



    September 22, 2015
    Matthew R. Francis

    The stuff of daily existence is made of atoms, and all those atoms are made of the same three things: electrons, protons and neutrons.

    Protons and neutrons are very similar particles in most respects. They’re made of the same quarks, which are even smaller particles, and they have almost exactly the same mass.

    Yet neutrons appear to be different from protons in an important way: They aren’t stable. A neutron outside of an atomic nucleus decays in a matter of minutes into other particles.

    What about protons?

    A free proton is a pretty common sight in the cosmos. Much of the ordinary matter (as opposed to dark matter) in galaxies and beyond comes in the form of hydrogen plasma, a hot gas made of unattached protons and electrons. If protons were as unstable as neutrons, that plasma would eventually vanish.

    But that isn’t happening. Protons—whether inside atoms or drifting free in space—appear to be remarkably stable. We’ve never seen one decay.

    However, nothing essential in physics forbids a proton from decaying. In fact, a stable proton would be exceptional in the world of particle physics, and several theories demand that protons decay.

    If protons are not immortal, what happens to them when they die, and what does that mean for the stability of atoms?

    Following the rules

    Fundamental physics relies on conservation laws: certain quantities that are preserved, such as energy, momentum and electric charge. The conservation of energy—combined with the famous equation E=mc2—means that lower-mass particles can’t change into higher-mass ones without an infusion of energy. Combining conservation of energy with conservation of electric charge tells us that electrons are probably stable forever: No lower-mass particle with a negative electric charge exists, to the best of our knowledge.

    Protons aren’t constrained the same way: They are more massive than a number of other particles, and the fact that they are made of quarks allows for several possible ways for them to die.

    For comparison, a neutron decays into a proton, an electron and a neutrino. Both energy and electric charge are preserved in the decay: A neutron is a wee bit heftier than a proton and electron combined, and the positively-charged proton balances out the negatively-charged electron to make sure the total electric charge is zero both before and after the decay. (The neutrino—or technically an antineutrino, the antimatter version—is necessary to balance other things, but that’s a story for another day.)

    Because atoms are stable and we’ve never seen a proton die, perhaps protons are intrinsically stable. However, as Kaladi Babu of Oklahoma State University points out, there’s no “proton conservation law” like charge conservation to preserve a proton.

    “You ask this question: What if the proton decays?” he says. “Does it violate any fundamental principle of physics? And the answer is no.”

    No GUTs, no glory

    So if there’s no rule against proton decay, is there a reason scientists expect to see it? Yes. Proton decay is the strongest testable prediction of several grand unified theories, or GUTs.

    GUTs unify three of the four fundamental forces of nature: electromagnetism, the weak force and the strong force. (Gravity isn’t included because we don’t have a quantum theory for it yet.)

    The first GUT, proposed in the 1970s, failed. Among other things, it predicted a proton lifetime short enough that experiments should have seen decays when they didn’t. However, the idea of grand unification was still valuable enough that particle physicists kept looking for it. (You might say they had a GUT feeling. Or you might not.)

    “The idea of grand unification is really beautiful and explains many things that seem like bizarre coincidences,” says theorist Jonathan Feng, a physicist at the University of California, Irvine.

    Feng is particularly interested in a GUT that involves Supersymmetry, a brand of particle physics that potentially could explain a wide variety of phenomena, including the invisible dark matter that binds galaxies together.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    Supersymmetric GUTs predict some new interactions that, as a pleasant side effect, result in a longer lifetime for protons, yet still leave proton decay within the realm of experimental detection. Because of the differences between supersymmetric and non-supersymmetric GUTs, Feng says the proton decay rate could be the first real sign of Supersymmetry in the lab.

    However, Supersymmetry is not necessary for GUTs. Babu is fond of a GUT that shares many of the advantages of the supersymmetric versions. This GUT’s technical name is SO(10), named because its mathematical structure involves rotations in 10 imaginary dimensions. The theory includes important features absent from the Standard Model such as neutrino masses, and might explain why there is more matter than antimatter in the cosmos. Naturally, it predicts proton decay.

    The search for proton decay

    Much rests on the existence of proton decay, and yet we’ve never seen a proton die. The reason may simply be that protons rarely decay, a hypothesis borne out by both experiment and theory. Experiments say the proton lifetime has to be greater than about 1034 years: That’s a 1 followed by 34 zeroes.

    For reference, the universe is only 13.8 billion years old, which is roughly a 1 followed by 10 zeros. Protons on average will outlast every star, galaxy and planet, even the ones not yet born.

    The key phrase in that last sentence is “on average.” As Feng says, it’s not like “every single proton will last for 1034 years and then at 1034 years they all boom! poof! in a puff of smoke, they all disappear.”

    Because of quantum physics, the time any given proton decays is random, so a tiny fraction will decay long before that 1034-year lifetime. So, “what you need to do is to get a whole bunch of protons together,” he says. Increasing the number of protons increases the chance that one of them will decay while you’re watching.

    The second essential step is to isolate the experiment from particles that could mimic proton decay, so any realistic proton decay experiment must be located deep underground to isolate it from random particle passers-by. That’s the strategy pursued by the currently operating Super-Kamiokande experiment in Japan, which consists of a huge tank with 50,000 tons of water in a mine.

    Super-Kamiokande experiment Japan
    Super-Kamiokande experiment

    The upcoming Deep Underground Neutrino Experiment, to be located in a former gold mine in South Dakota, will consist of 40,000 tons of liquid argon.

    FNAL Dune & LBNF
    DUNE, managed by FNAL

    Because the two experiments are based on different types of atoms, they are sensitive to different ways protons might decay, which will reveal which GUT is correct … if any of the current models is right. Both Super-Kamiokande and DUNE are neutrino experiments first, Feng says, “but we’re just as interested in the proton decay possibilities of these experiments as in the neutrino aspects.”

    After all, proton decay follows from profound concepts of how the cosmos fundamentally operates. If protons do decay, it’s so rare that human bodies would be unaffected, but not our understanding. The impact of that knowledge would be immense, and worth a tiny bit of instability.

    See the full article here .

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 4:00 pm on September 19, 2015 Permalink | Reply
    Tags: , , Optics and Biology, Physics   

    From NSF: “Year of Light: The brilliance of mixing physics with biology” 

    National Science Foundation

    September 18, 2015
    No Writer Credit

    Photo credit: Matthew Comstock

    If you thought fluorescence was just meant for eye-shocking crayons, paints, t-shirts and shoelaces, think again. When physics and biology come together to better understand molecules like DNA, using a mixture of techniques known as fluorescence microscopy and optical traps allows researchers to see and learn so much more.

    A good deal of biological research today now intertwines physics to better comprehend molecules and their dynamic processes. In modern medicine, for example, this approach enables us to better recognize molecular interactions in living systems, such as the actual mechanisms of cellular components and how they move and interact within a cell or on an even smaller level, parsing how parts – of DNA or other molecules so small they refer to them in piconewtons – ambulate in sickness and health, thereby strengthening our ability to combat critical diseases in the long-term, and simultaneously improving our economy, especially in the field of commercial pharmaceuticals.

    To advance research in this field, two physics professors, Taekjip Ha and Yann Chemla at the University of Illinois at Urbana-Champaign and the NSF-funded Center for Physics of Living Cells, have coupled two unique biophysical techniques – optical traps and fluorescence microscopy – to examine the binding processes that underlie DNA strands. Optical traps, which are also referred to as optical tweezers, use a highly focused laser beam to provide an attractive or repulsive force to physically hold and move microscopic objects that are susceptible to this kind of control. The fluorescence microscope is based on the phenomenon that certain materials emit energy detectable as visible light when irradiated with the light of a specific wavelength. The material can be naturally fluorescent or be treated to make it so. The light then in this kind of microscope “excites” the material, allowing researchers to view molecules, for example, in an active state and observe mechanisms they wouldn’t in a static environment.

    Together, this combined technique paves the foundation for others to clearly visualize protein motion and conformational changes, thereby greatly enhancing our ability to measure how molecules interact with one another. The image displayed above offers a small visual sample of the advanced capabilities their new technique offers the field. DNA (blue double helix) is stretched out between two beads (gray spheres) held by optical traps (red cones) with a bound protein glowing with fluorescence excited by a “confocal” laser (green cones).

    See the full article here .

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    The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 “to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense…we are the funding source for approximately 24 percent of all federally supported basic research conducted by America’s colleges and universities. In many fields such as mathematics, computer science and the social sciences, NSF is the major source of federal backing.


  • richardmitnick 4:43 pm on September 10, 2015 Permalink | Reply
    Tags: , , , Physics,   

    From BNL: “Tiny Drops of Early Universe ‘Perfect’ Fluid” 

    Brookhaven Lab

    August 31, 2015
    Karen McNulty Walsh, (631) 344-8350 or Peter Genzer, (631) 344-3174

    The upper panel of this image, created based on calculations by Brookhaven Lab nuclear theorist Bjoern Schenke, represents initial hot spots created by collisions of one, two, and three-particle ions with heavy nuclei. The lower panel shows the geometrical patterns of particle flow that would be expected if the small-particle collisions are creating tiny hot spots of quark-gluon plasma. No image credit.

    The Relativistic Heavy Ion Collider (RHIC), a particle collider for nuclear physics research at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, smashes large nuclei together at close to the speed of light to recreate the primordial soup of fundamental particles that existed in the very early universe.

    BNL RHIC Campus

    Experiments at RHIC—a DOE Office of Science User Facility that attracts more than 1,000 collaborators from around the world—have shown that this primordial soup, known as quark-gluon plasma (QGP), flows like a nearly friction free “perfect” liquid. New RHIC data just accepted for publication in the journal Physical Review Letters now confirm earlier suspicions that collisions of much smaller particles can also create droplets of this free-flowing primordial soup, albeit on a much smaller scale, when they collide with the large nuclei.

    “These tiny droplets of quark-gluon plasma were at first an intriguing surprise,” said Berndt Mueller, Associate Laboratory Director for Nuclear and Particle Physics at Brookhaven. “Physicists initially thought that only the nuclei of large atoms such as gold would have enough matter and energy to set free the quark and gluon building blocks that make up protons and neutrons. But the flow patterns detected by RHIC’s PHENIX collaboration in collisions of helium-3 nuclei with gold ions now confirm that these smaller particles are creating tiny samples of perfect liquid QGP.”

    These results build on earlier findings from collisions of two-particle ions known as deuterons with gold ions at RHIC, as well as proton-lead and proton-proton collisions at Europe’s Large Hadron Collider (LHC).

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    They also set the stage for the current run colliding protons with gold at RHIC.

    “The idea that collisions of small particles with larger nuclei might create minute droplets of primordial quark-gluon plasma has guided a series of experiments to test this idea and alternative explanations, and stimulated a rich debate about the implications of these findings,” said University of Colorado physicist Jamie Nagle, a co-spokesperson of the PHENIX collaboration at RHIC. “These experiments are revealing the key elements required for creating quark-gluon plasma and could also offer insight into the initial state characteristics of the colliding particles.”

    Geometrical flow patterns

    RHIC’s PHENIX detector

    The discovery of the “perfect” liquid at RHIC, announced definitively in 2005, was largely based on observations of particles flowing in an elliptical pattern from the matter created in RHIC’s most energetic gold-gold collisions. This flow was a clear sign that particles emerging from the collisions were behaving in a correlated, or collective, way that contrasted dramatically with the uniformly expanding gas the scientists had expected. Additional experiments confirmed that this liquid is indeed composed of visible matter’s most fundamental building blocks, quarks and gluons, no longer confined within individual protons and neutrons, and that the flow occurs with minimal resistance—making it a nearly “perfect” liquid QGP.

    “Experiments colliding smaller particles with the heavy ions were originally designed as control experiments because they weren’t supposed to create the QGP,” Nagle said. “But observations at the LHC of very energetic proton-proton collisions and later experiments there colliding protons with lead revealed hints that particles streaming from those tiny collisions were also behaving collectively and flowing. It looked a lot like some of the perfect liquid signatures originally discovered in gold-gold collisions at RHIC, and later in lead-lead collisions at the LHC.”

    When RHIC physicists checked data from the RHIC run of 2008, when deuterons (a nucleus made of one proton and one neutron) were smashed into gold ions, they saw a similar pattern.

    “Since the deuteron is two particles, it creates two separate impacts on the nucleus—two hot spots that appear to merge and form an elongated drop of QGP,” Nagle said.

    Definitive tests

    Those observations triggered the idea of testing for flow patterns in a range of more tightly controlled experiments, which is only possible at RHIC, where physicists can collide a wide variety of ions to control the shape of the droplets of matter created. With additional deuteron-gold collisions already in hand, the RHIC scientists set out to collide three-particle helium-3 nuclei (each made of two protons and one neutron) with gold—and later, single protons with gold.

    “The PHENIX detector can pick up particles coming out of collisions very far forward and backward from the collision point. This large angle coverage allows us to measure the flow in these small collision systems,” said Shengli Huang, a PHENIX collaborator from Vanderbilt University who carried out the analysis. “PHENIX also has a trigger detector that picks up and records the most violent collisions—the ones in which the flow pattern is most apparent,” he said.

    The analysis of those events, as described in the new paper, reveals that the helium-gold collisions exhibit a triangular pattern of flow that the scientists say is consistent with the creation of three tiny droplets of QGP. They also say the data indicate that these small particle collisions could be producing the extreme temperatures required to free quarks and gluons—albeit at a much smaller, more localized scale than in the relatively big domains of QGP created in collisions of two heavy ions.

    “This is a pretty definitive measurement,” Nagle said. “The paper has a plot of elliptical and triangular flow that pretty much matches the hydrodynamic flow calculations we’d expect for QGP. We are really engineering different shapes of the QGP to manipulate it and see how it behaves.”

    There are other key signatures of QGP formation, such as the stopping of energetic particle jets, which have not been detected in the tiny droplets. And other theoretical explanations suggest the flow patterns resulting from some of the small particle-nucleus collisions could emerge from the interactions of gluons within the colliding particles, rather than from the formation of QGP.

    “At this time, the only theoretical framework that reproduces the patterns we’re observing in deuteron-gold and helium-3-gold collisions is fluid dynamics,” said Bjoern Schenke, a nuclear theorist at Brookhaven Lab. “It remains to be seen if alternative models can describe these patterns as well.”

    If other models also turn out to be compatible with the helium-3-gold data, physicists will want to explore whether these models make predictions that differ from those of the hydrodynamic flow model, and for which types of collisions.

    “The good news is that RHIC, with its unrivaled versatility, will likely be able to study any system that can discriminate between different models,” Mueller said.

    Research at RHIC is funded primarily by the DOE Office of Science.

    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.

  • richardmitnick 4:01 pm on September 9, 2015 Permalink | Reply
    Tags: , , , , Physics   

    From MIT: “How to spawn an ‘exceptional ring'” 

    MIT News

    September 9, 2015
    David L. Chandler

    A schematic drawing of how a ring of exceptional points (shown in white) can be spawned from a Dirac point (a dot), and thus change the dispersion from the normal, widely known conical shape into an exotic lantern-like shape. Courtesy of the researchers

    A schematic picture showing the conical dispersion of a Dirac cone being deformed into a new hour-glass-like shape due to radiation. Courtesy of the researchers

    The Dirac cone, named after British physicist Paul Dirac, started as a concept in particle and high-energy physics and has recently became important in research in condensed matter physics and material science. It has since been found to describe aspects of graphene, a two dimensional form of carbon, suggesting the possibility of applications across various fields.

    Graphene is an atomic-scale honeycomb lattice made of carbon atoms.

    Now physicists at MIT have found another unusual phenomenon produced by the Dirac cone: It can spawn a phenomenon described as a “ring of exceptional points.” This connects two fields of research in physics and may have applications in building powerful lasers, precise optical sensors, and other devices.

    The results are published this week in the journal Nature by MIT postdoc Bo Zhen, Yale University postdoc Chia Wei Hsu, MIT physics professors Marin Soljačić and John Joannopoulos, and five others.

    This work represents “the first experimental demonstration of a ring of exceptional points,” Zhen says, and is the first study that relates research in exceptional points with the physical concepts of parity-time symmetry and Dirac cones.

    Individual exceptional points are a peculiar phenomenon unique to an unusual class of physical systems that can lead to counterintuitive phenomena. For example, around these points, opaque materials may seem more transparent, and light may be transmitted only in one direction. However, the practical usefulness of these properties is limited by absorption loss introduced in the materials.

    The MIT team used a nanoengineered material called a photonic crystal to produce the exceptional ring. This new ring of exceptional points is different from those studied by other groups, making it potentially more practical, the researchers say.

    “Instead of absorption loss, we adopt a different loss mechanism — radiation loss — which does not affect the device performance,” Zhen says. “In fact, radiation loss is useful and is necessary in devices like lasers.”

    This phenomenon could enable creation of new kinds of optical systems with novel features, the MIT team says.

    “One important possible application of this work is in creating a more powerful laser system than existing technologies allow,” Soljačić says. To build a more powerful laser requires a bigger lasing area, but that introduces more unwanted “modes” for light, which compete for power, limiting the final output.

    “Photonic crystal surface-emitting lasers are a very promising candidate for the next generation of high-quality, high-power compact laser systems,” Soljačić says, “and we estimate we can improve the output power limit of such lasers by a factor of at least 10.”

    “Our system could also be used for high-precision detectors for biological or chemical materials, because of its extreme sensitivity,” Hsu says. This improved sensitivity is due to another exotic property of the exceptional points: Their response to perturbations is not linear to the perturbation strength.

    Normally, Hsu says, it becomes very difficult to detect a substance when its concentration is low. When the concentration of the target substance is reduced by a million times, the overall signal also decreases by a million times, which can make it too small to detect.

    “But at an exceptional point, it’s not linear anymore,” Hsu says, “and the signal goes down by only 1,000 times, providing a much bigger response that can now be detected.”

    Demetrios Christodoulides, a professor of optics and photonics at the University of Central Florida who was not involved in this work, says, “This represents the first observation of an exceptional ring in a 2-D crystal associated with a two-dimensional band. The MIT work opens up a number of opportunities … in particular, around exceptional points where systems are known on many occasions to behave in a peculiar fashion.”

    The research team also included Yuichi Igarashi of NEC Corp. in Japan and MIT research scientist Ling Lu, postdoc Ido Kaminer, Harvard University graduate student Adi Pick, and Song-Liang Chua at DSO National Laboratory in Singapore. The work was supported, in part, by the Army Research Office through MIT’s Institute for Soldier Nanotechnologies, the National Science Foundation, and the Department of Energy.

    See the full article here .

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  • richardmitnick 8:41 am on September 7, 2015 Permalink | Reply
    Tags: , Physics,   

    From Weizmann: “Stick and Slip” 

    Weizmann Institute of Science logo

    Weizmann Institute of Science

    No Writer Credit

    What do the sounds of a creaky old hinge and a cello have in common? Both rely on the same kind of friction: two surfaces that alternately stick and slide against one another. This physical phenomenon is called stick-slip and, in the case of the creaky hinge, it is often mitigated by the application of a lubricant between the surfaces. It has long been accepted that such a thin layer of lubrication between sliding surfaces alternates along with the cycles of sticking and slipping; it starts as a solid, turns to liquid in the slipping phase and then back to a solid when the surfaces stick once again. But a recent paper in the Proceedings of the National Academy of Sciences (PNAS) suggests this model is incorrect. The findings arose out of research by the group of Prof. Jacob Klein of the Weizmann Institute’s Materials and Interfaces Department, with the collaboration of Prof. Arie Yeredor of Tel Aviv University.

    The sound of a violin arises from a type of friction known as “stick-slip”

    The group – including research student Irit Rosenhek-Goldian and associate staff scientist Dr. Nir Kampf, both members of Klein’s lab – tested this assumption in an ideal system: two perfectly smooth surfaces separated by a thin layer of lubricating material. The material in question contains molecules organized in layers, totaling around four to five nanometers in thickness. The idea was that as the surfaces stuck and then slipped, the lubricant molecules would also stick – as a solid – and then liquefy and slip over one another fluidly.

    The shift from solid to liquid should entail another change: “When a material is solid, it is generally denser than its liquid form,” says Klein. “Thus, when the lubricating material turns liquid, it should physically expand by around ten percent; that is, the thin lubricant layer should expand by around 0.5 nanometers.” But where there should have been a bit more space between the surfaces in the slip stage, there was none. The measurements, which were accurate down to 0.1 nanometers, revealed no change at all in the volume of the lubricant as it went from stick to slip and back again. The conclusion: The lubricant does not change from solid to liquid after all.

    Since friction and wear account for billions of dollars of loss annually, a better understanding of the basic science underlying lubrication may lead to significant improvements in both biomedical and industrial applications.

    Prof. Jacob Klein’s research is supported by the European Research Council. Prof. Klein is the incumbent of the Hermann Mark Professorial Chair of Polymer Physics.

    See the full article here .

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    Weizmann Institute Campus

    The Weizmann Institute of Science is one of the world’s leading multidisciplinary research institutions. Hundreds of scientists, laboratory technicians and research students working on its lushly landscaped campus embark daily on fascinating journeys into the unknown, seeking to improve our understanding of nature and our place within it.

    Guiding these scientists is the spirit of inquiry so characteristic of the human race. It is this spirit that propelled humans upward along the evolutionary ladder, helping them reach their utmost heights. It prompted humankind to pursue agriculture, learn to build lodgings, invent writing, harness electricity to power emerging technologies, observe distant galaxies, design drugs to combat various diseases, develop new materials and decipher the genetic code embedded in all the plants and animals on Earth.

    The quest to maintain this increasing momentum compels Weizmann Institute scientists to seek out places that have not yet been reached by the human mind. What awaits us in these places? No one has the answer to this question. But one thing is certain – the journey fired by curiosity will lead onward to a better future.

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