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FİZİKTE YENİ GELİŞMELER
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	The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 854 January 23, 2008 www.aip.org/pnu by Phillip F. Schewe and Jason S. Bardi A NEW CALCULATION EXPLAINS THE MECHANISM BEHIND CARBON DATING in terms of the way the mass of mesons changes as they travel through an atomic nucleus. Mesons (particles such as pions, containing a quark and an antiquark) are thought to mediate the nuclear force between two nuclei. Radiocarbon dating began in 1949 when Willard Libby said that the amount of carbon-14 (the radioactive cousin of carbon-12) left in an object (such as a fossil tree) could provide an estimate of how old the object was. The thinking was that the organism, while it was alive, would constantly ingest enough of the rare C-14 to replace those nuclei that were decaying into -14 (the other products being an electron and a neutrino). But as soon as the organism died, the ratio of C-14/C-12 would begin to drop exponentially since the C-14 was no longer being replaced. Measuring the ratio in terms of radioactive half-lifes would provide a good estimate of the fossil. This method has been used by archeologists ever since to measure the age of things, at least those things that had been alive. A big questions presented itself: if the radioactive half-life of C-11 is 20 minutes, and that of O-14 is 1 minute, and that of O-15 is 2 minutes, and that of N-13 is 10 minutes, why is the life-time of C-14 some 3 billion minutes (5730 years)? This is what Jeremy Holt and his colleagues at Stony Brook, TRIUMF (the accelerator facility in Vancouver), and the University of Idaho have set out to determine. Holt says that the anomalously long C-14 half-life has been a mystery to theorists for half a century. An earlier theory, called Brown-Rho scaling (named for Gerry Brown and Mannque Rho, advanced in 1991), suggested that the masses of most mesons decrease uniformly when (insofar as they carry the nuclear force operating inside nuclei) they travel through dense nuclear material (see figure at http://www.aip.org/png/2008/294.htm ). Holt (jeholt@tonic.physics.sunysb.edu, 631-632-9843) and his fellow authors bring things up to date by accounting, with fair accuracy, for the observed long C-14 lifetime. (Holt et al., Physical Review Letters, upcoming article; designated as an Editors Suggested article) GRAPHENE SPEED RECORD.* Andre Geim and his colleagues at the University of Manchester have observed the highest electron mobility for an electron in any electronic material. In this case the electrons were moving through graphene, single-atom-thick sheets of carbon, with an electron mobility of 200,000 cm^2/volt-second. Graphene was only discovered a few years ago (by Geim: see http://www.aip.org/pnu/2006/split/769-2.html ). A true two-dimensional material is striking enough, but even more unusual was the observed ease with which electrons moved in graphene. Electrons moving through any crystal lattice are constantly interacting with the atoms in that lattice, especially if there are irregularities present. This causes the electrons to slow. Their effective mass will be different for each type of crystal. In graphene, the effective mass of electrons is zero. Still another way of quantitatively describing an electrons journey through the alleyways of a crystal is in terms of its mobility,* in units of square centimeters per volt/sec. The charge-carrier mobility is perhaps the most important figure of merit for an electronic material, so researchers have sought a larger mobility. To take some examples: the mobility in silicon is 1500, while in GaAs it is 8500. Thats w hy the circuitry in cell phones is based on GaAs. For InSb, the mobility is even higher: 80,000. Geims new mobility record of 200,000 wont cause the electronics industry to ditch Si or GaAs any time soon. The problems with early graphene circuits right now, says Geim, are, first, that graphene cant yet be made into uniform high-quality wafers; and second that prototype graphene transistor switching (going from Off to On) is too slow. However, Geim predicts that over the short run (3-5 years) graphene might emerge as a basis for chemical sensors and for generators of terahertz-range light-a frequency span (and not yet achieved in any practical way) where human bodies are transparent-making possible security or medical scanning machines. (Morozov et al., Physical Review Letters, upcoming article) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 853 January 11, 2008 www.aip.org/pnu by Phillip F. Schewe and Jason S. Bardi UNPRECEDENTED SPECTROSCOPY USING THE BEST EVER RULER FOR LIGHT. Physicists at NIST-Boulder have carried out a powerful new spectroscopic study of a sample of gas using optical frequency combs. The NIST work, which might well change the way spectroscopy is done, is remarkable in that it provides the full spectrum of the gas over a broad spectral region and with frequency accuracy that can reach 1 Hz (for spectral frequencies of the order of 2 x 10^14 Hz). The NIST spectroscopic feat is equivalent to simultaneously sending 155,000 individual single frequency lasers through the sample and measuring the resulting amplitude and phase shift on each individual laser. Moreover, the spectrum is measured rapidly, using a device with no moving mechanical parts. The invention of the optical frequency comb method was a great step forward in laser science. John Hall (NIST) and Ted Haensch (Max Planck) the Nobel prize in 2005 for their pioneering work in this area. (For a tutorial on frequency combs, see http://www.nist.gov/public_affairs/newsfromnist_frequency_combs.htm) In the comb process, a pulsed laser emits light not merely at a single frequency, but at a series of frequencies. A frequency spectrum of this composite laser output looks like a comb, with light occurring at regularly spaced frequencies, covering the infrared part of the light spectrum. In many ways the frequency comb is an ideal tool for spectroscopy. Its light covers enormous amounts of the optical spectrum and the frequency of each individual comb line can be known to 1-Hz precision. When you pass a frequency comb through a gas cell a given comb line will, like any laser beam, be absorbed when it is resonant with any of the many quantum energy levels of the gas. The challenge with frequency combs is to figure out which of the more than one-hundred thousand comb lines experience absorption and which do not. To solve this problem NIST researchers take the comb used for spectroscopy and mix it with a second carefully crafted frequency-comb. This ensemble of light pulses results in a beat-frequency pulse which can be measured with conventional electronics. From this beat-frequency pulse the absorption and phase shift experienced by each individual comb line can be separately observed. This work represents by far the largest number of frequency comb teeth that have been individually observed. The present NIST experiment interrogates the effect of the absorption from the gas on 155,000 comb lines, spanning a wavelength range of 125 nm. The NIST precision of 1 Hz for spectral lines is to be compared with tens of MHz precision characterizing other spectroscopic techniques. NIST researchers believe that this new work might change the way people perform spectroscopy. (Coddington{ian@nist.gov, 303-497-4889}, Swann, Newbury, Physical Review Letters, 11 January 2008; PRL editors designate this as a Suggested Article) ACOUSTIC CLOAKING. Computer simulations and the use of wave scattering theory have demonstrated that, contrary to earlier predictions, it should be possible to produce a 3-dimensional material shell which is invisible to sound waves, analogous to optical cloaking, the process in which light waves are guided around an object and then refocused on the far side and in the same direction (with no reflected light to betray position) so as to make the object seem invisible. Full optical cloaking has not been achieved yet, but researchers expect to be able to do it. Can the same thing be done with sound waves? In principle there is no reason why it couldnt be done. The leader of a group of scientists examining this issue, Steven Cummer at Duke University, says that many of the principles that pertain to the channeling of light waves around an object also apply to sound waves. To be sure, there are differences. Sound waves oscillate in the direction of their motion while the electric and magnetic fields composing light waves oscillate perpendicularly to the wave motion. In the optical case, cloaking will require a material (actually a meta-material) tailored, highly anisotropic (varying widely according to the direction through the material) index of refraction. In practice, the index of refraction for electromagnetic waves depends on the permittivity, a measure of the material's response to an applied electric field, and permeability, its response to an applied magnetic field (for an account of the demonstration of negative-index materials, see http://www.aip.org/pnu/2000/split/pnu476-1.htm). The acoustic equivalent of these two parameters are the mass density and the bulk modulus (the springiness) of the background fluid (usually air or water) in which the object sits. Cummer (919-660-5256, cummer@ee.duke.edu) says that in the short run acoustic cloaking might be more practical than optical cloaking. A limitation of electromagnetic cloaking, he says, is that it requires portions of the wave to move faster than the speed of light (in full accordance with special relativity); this can be done for very limited frequency ranges but not for wider ranges, limiting the applicability of optical cloaking. This limitation does not apply to sound waves moving through matter. Furthermore, the acoustic properties of most materials means that sound waves might not be absorbed as readily in acoustic cloaking as light waves are absorbed in optical cloaking (in which case the cloaking would be something less than perfect). Applications of acoustic cloaking come easily to mind: hiding submarines from sonar, for example. Another potential practical application might be in architecture, where acoustic considerations (reducing noise) might not have to be sacrificed in the interest of structural integrity. Among Cummers collaborators are David Smith of Duke (one of the early pioneers in the field of negative-index materials) and John Pendry of Imperial College (the early theorist of negative-index studies). (Cummer et al., Physical Review Letters, 11 January 2008; considered an editors Suggested article in PRL) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 852 January 3, 2008 www.aip.org/pnu by Phillip F. Schewe and Jason S. Bardi AMOEBAS ANTICIPATE CLIMATE CHANGE* A new experiment shows that amoebas will slow their motion in synch with periodic adverse changes in their environment, and will, as if in anticipation, even slow down when the adverse condition is not delivered. A team of scientists from Hokkaido University and the ATR Wave Engineering Laboratories in Japan cultured the single-celled slime mold Physarum polycephalum (a member of the amoeba clan) in a bed of oat flakes on agar. Every ten minutes the air was made slightly cooler and drier, which had the effect of slowing the movement of the amoebas down a narrow lane. Then more favorable air would be restored and the motion continued as before. After several cycles, the amoebas slowed even when the adverse conditions did not materialize. Later still, when the organisms have been tricked into anticipating impending climate change several times, they refrain from slowing without an actual change in conditions. One of the researchers, Toshiyuki Nakagaki from Hokkaido (nakagaki@es.hokudai.ac.jp), cautions that amoebas do not have a brain and that this is not example of classic Pavlovian conditioned response behavior. Nevertheless, it might represent more evidence for a primitive sensitivity or intelligence based on the dynamic behavior of the tubular structures deployed by the amoeba. (Saigusa et al., Physical Review Letters, 11 January 2008; journalists can obtain the article from www.aip.org/physnews/select) SHATTERING VIRUSES. A new study is trying to establish the intrinsic vibration modes of capsids---the protein shells of virus particles that package its genetic material---with a view toward rupturing them and thereby killing the pathogenic virus. If the capsid resonant frequencies could be determined, then possibly light or sound waves might be used to shatter the capsids the way the opera singer Enrico Caruso supposedly shattered wine glasses by sustaining a note at exactly the resonant frequency of the glass. This approach to attacking viruses is alternative to treating them with chemicals, which is not always effective; furthermore, the chemicals can do damage to healthy cells, or the viruses can mutate and defeat chemical defenses. Hence the importance of attempting to undo viruses with mechanical means. Eric Dykeman and Otto Sankey, physicists at Arizona State University, are modeling capsid vibrations at the atomic level for comparisons with experiments being performed by K.T. Tsen at ASU in which picosecond laser pulses are scattered from capsids. The capsids, which are mostly made of complex protein assemblies, will typically absorb some of the laser light, a process which causes them to vibrate. The rest of the laser beam, its energy somewhat depleted, will be downshifted in frequency. This allows observers to deduce the resonant frequency of the capsids. By staging the short laser pulse in different ways, a whole catalog of capsid resonant frequencies can be made. Sankey (otto.sankey@asu.edu, 480-965-4334) says that the simulations performed so far suggest that resonant frequencies for their chosen virus, the satellite tobacco necrosis virus (see vibration movie at http://www.aip.org/png/2008/292.htm) are in the vicinity of 60 to 90 GHz. (Dykeman and Sankey, Physical Review Letters, upcoming article; journalists can obtain the text from www.aip.org/physnews/select. This article is designated by the PRL editors as one of their Suggestions. To learn what this m eans, see the PRL editorial in the 5 January 2007 issue) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 851 December 21, 2007 www.aip.org/pnu by Phillip F. Schewe and Jason S. Bardi A PERSISTENT FLOW OF BOSE-CONDENSED ATOMS IN A TOROIDAL TRAP, the first time this has been achieved, offers physicists a better chance to study the kinship between Bose-Einstein condensates (BEC) and superfluids. Both involve the establishment of an ensemble in which many atoms join together in a single quantum entity. But theyre not quite the same thing. In a bath of liquid helium at low temperatures, for example, nearly 100% of the atoms are in a superfluid state but only about 10% are in a BEC state (in a BEC millions of atoms have become, in a sense, a single atom). But physicists generally believe that most or all of a BEC is superfluid. Scientists have been able to stir up quantized vortices in BEC samples, one indication that BECs are superfluid. But until now researchers had not been able to get BECs to move around a track in a persistent flow, another sign of superfluidity. The new experiment, performed by Nobel laureate William Philips and his colleagues at NIST-Gaithersburg and the Joint Quantum Institute of NIST and the University of Maryland, chilled sodium atoms in a toroidal trap, set them into motion with laser light, and observed a flow for as long as 10 seconds, when the condensate started to come undone because the delicate magnetic and optical trapping parameters tuned to contain the atoms had drifted from their ideal settings. One of the scientists on the project, Kristian Helmerson (kristian@nist.gov), says that neutral atoms flowing in a toroidal vessel could be fashioned into the atom analog of a superconducting quantum interference device (or a SQUID, for short, which is used as a sensitive detector of magnetism); this BEC device, sensing not magnetism but slight changes in direction, could serve as a sensitive gyroscope, possibly for navigation purposes (Ryu et al., Physical Review Letters, upcoming article) THE SIZE OF THE HELIUM-8 NUCLEUS* has been measured. To be more precise, the charge radius of this heaviest of helium isotopes (containing two protons and six neutrons) has been measured for the first time. The charge radius tells you how widely the proton charge is spread out in space. The new work, conducted by a Argonne-Chicago-GANIL-Windsor (Canada)-Los Alamos collaboration, arrives at a value of 1.93 fm (1 fermi equals 10^-15 m). For comparison, the charge radius of the He-6 isotope, is 2.068 fm; that is, the lighter isotope actually has a larger charge radius, the result of the binding effect of the strong nuclear force. He-8 is very rare, hard to make, and represents the most neutron-rich material known on Earth. Still heavier helium groupings, such as He-10, are not really bound entities-they can only be considered as resonances. For the new experiment, He-8 was produced by bombarding a carbon target with 1-GeV beam of C-13 ions. The charge radius of the respective isotopes-He-4, He-6, and He-8-is determined by comparing the subtle shifting of the atomic spectra from the three different species of helium atom. The spectroscopy measurements involve only the electromagnetic force between the electrons and the nucleus in these atoms, and not the strong nuclear force that holds each nucleus together. However, once the charge distribution is determined, it can be used to infer things about the binding force operating in the nucleus. The current thinking on the distribution of protons and neutrons (illustrated in the figure at http://www.aip.org/png/2007/291.htm) suggests that the He-4 nucleus, composed of two protons and two neutrons ( a unit usually referred to as an alpha particle), forms the default nucleus, while in He-6 the extra two neutrons are thought to orbit the core as a sort of halo. In this model, the alpha core wobbles a bit around the joint center of mass with the halo neutron pair. In He-8, the halo consists of two two-neutron pairings. This actually allows the core to wobble a bit less than in the case of He-6, allowing the charge radius of He-8 to be a bit less. One of the researchers, Peter Mueller (630-252-7276, pmeuller@anl.gov), says that the current nuclear theory did an excellent job of predicting the charge radius for He-8, giving confidence to those who model heavier nuclei. (Mueller et al., Physical Review Letters, 21 December 2007; lab website, at http://www.phy.anl.gov/mep/atta/) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 850 December 13, 2007 www.aip.org/pnu by Phillip F. Schewe and Jason S. Bardi TEN TOP PHYSICS STORIES FOR 2007, in chronological order during the year: light, slowed in one Bose Einstein condensate (BEC), is passed on to another BEC (http://www.aip.org/pnu/2007/split/812-1.html); electron tunneling in real time can be observed with the use of attosecond pulses (http://www.aip.org/pnu/2007/split/818-2.html); laser cooling of coin-sized object, at least in one dimension (http://www.aip.org/pnu/2007/split/818-1.html); the best test ever of Newtons second law, using a tabletop torsion pendulum (http://www.aip.org/pnu/2007/split/819-1.html); first Gravity Probe B first results, the measurement of the geodetic effect---the warping of spacetime in the vicinity of and caused by Earth-to a precision of 1%, with better precision yet to come (http://www.aip.org/pnu/2007/split/820-2.html); the MiniBooNE experiment at Fermilab solves a neutrino mystery, apparently dismissing the possibility of a fourth species of neutrino (http://www.aip.org/pnu/2007/split/820-1.html); the Tevatron, in its quest to observe the Higgs boson, updated the top quark mass and observed several new types of collision events, such as those in which only a single top quark is made, and those in which a W and Z boson or two Z bosons are made simultaneously (http://www.aip.org/pnu/2007/split/821-1.html); the shortest light pulse, a 130-attosecond burst of extreme ultraviolet light (http://www.aip.org/pnu/2007/split/823-1.html); based on data recorded at the Auger Observatory, astronomers conclude that the highest energy cosmic rays come from active galactic nuclei (http://www.aip.org/pnu/2007/split/846-1.html); and the observation of Cooper pairs in insulators (http://www.aip.org/pnu/2007/split/849-1.html). HIGH-INTENSITY PHOTOELECTRIC EFFECT.* Physicists at the Free-electron LASer in Hamburg (FLASH) have performed a photoelectric-effect experiment at an extreme-ultraviolet wavelength, 13 nm, and ultra-high photon intensities. In the process, they removed electrons from xenon atoms, sometimes 21 of them. The photoelectric effect---in which ultraviolet or extreme-ultraviolet light impinging on a metal surface kicks electrons out---was used by Albert Einstein to argue in favor of the idea of light existing in quantized form, what we now call photons. The explanation, winning Einstein the Nobel Prize in 1921, is a landmark in early quantum theory since it suggested that the light at a fixed wavelength consists of photons with fixed (quantized) energy. In the Hamburg experiment, radiation from the free electron laser (FEL) is brought to a focus (3 microns wide and about 350 microns long) within a cell containing xenon gas. The irradiance of the laser beam, the amount of power per unit area, was 10^16 W/cm^2, a record for extreme-ultraviolet light. The light ejects electrons from the xenon, and the resultant ions are detected. In this case, charged ions with as many as 21 electrons removed were observed. This was the first time that as many as 21 electrons were removed during a photoelectric experiment, and the surprising results are not well explained by the quantization of light and photons as the light particles. (Sorokin et al., Physical Review Letters, 23 November 2007; contact Mathias Richter of the Physikalische-Technische Bundesanstalt (PTB) at mathias.richter@ptb.de, www-hasylab.desy.de) VOYAGER 2 REACHES THE HELIOSPHERE. Like its sister craft, Voyager 1 several years ago, Voyager 2 has now flown far enough out into the solar system to enc ounter the heliosphere, where the wind of solar particles meets the interstellar medium. We already know that the surface of this boundary zone is irregular in shape because of earlier measurements by Voyager 1 (http://www.aip.org/pnu/2006/split/778-1.html. Voyager 1 is currently about 9.8 billion miles from Earth and traveling out at a speed of 38,000 miles per hour. Voyager 2 is about 7.8 billion miles away and traveling at about 35,000 miles per hour. Voyager 1 might be faster, further, and earlier, but Voyager 2's plasma measuring instrument is functioning, unlike Voyager 1's. Voyager 2 confirms that the boundary layer is irregular and has found that the temperature just beyond the boundary is some ten times cooler than expected. (Results reported at this weeks meeting of the American Geophysical Union in San Francisco.) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 849 December 5, 2007 by Phillip F. Schewe and Jason S. Bardi www.aip.org/pnu COOPER PAIRS IN INSULATORS.* Cooper pairs are the extraordinary link-up of like-charged electrons through the subtle flexings of a crystal. ; They act as the backbone of the superconducting phenomenon, but have also now been observed in a material that is not only non-superconducting but actually an insulator. An experiment at Brown University measures electrical resistance in a Swiss-cheese-like plank of bismuth atoms made by spritzing a cloud of atoms onto a substrate with 27-nm-wide holes spaced 100 nm apart. Bismuth films made this way are superconducting if the sample is many atom-layers thick but is insulating if the film is only a few atoms thick, owing to subtle effects which arise from the restrictive geometry. The superconducting and insulating states are easily distinguished; as the temperature is lowered below the transition temperature (2 K) the resistance goes to zero for bismuth-as-superconductor, whereas for the insulating bismuth the resistance becomes extremely high. Cooper pairs are certainly present in the superconducting sample; they team up to form a non-resistive supercurrent. But how do the researchers know that pairs are present in the insulator too? Because of an additional test. By seeing what happens to resistance as an external magnetic field is increased. The resistance should vary periodically, with a period proportional to the charge of the electrical objects in question. From the periodicity, proportional in this case to two times the charge of the electron, the Brown physicists could deduce that they were seeing doubly-charged objects moving through the sample. In other words, Cooper pairs are present in the insulator. This is true only at the lowest temperatures. One of the researchers, James Valles (james_valles_jr@brown.edu), says that there have been previous hints of Cooper pairs in some films related to superconductors, but that in those cases the evidence for pairs in the insulating state was ambiguous, and not as direct as the observation recorded in the Brown lab. He asserts that the realization of a boson insulator (in which the charge carriers are electron pairs) will help to further explore the odd kinship between insulators and superconductors. (Stewart et al., Science 23 November 2007) A MOON LIKE OURS IS RARELY FORMED. Interpretations of recent infrared observations might be changing our view of the Moon. About 4.5 billion years ago, our Earth was utterly shattered-the victim of a giant impact with an object the size of Mars. The collision that was powerful enough to vaporize rock and throw a massive plume of Earths mantle into space was not all bad, though. The impactor soon merged with the Earth giving it a fast spin, while chunks of Earth's mantle settled into a disk around our planet. Within a year or so, a large moon was formed out of this debris. The left-over rocks continued to circle around the sun over the next million years, occasionally colliding and creating a flow of dust, until it was all cleaned up by gravity and solar radiation. Many scientists are interested in knowing how common such impacts are in other young solar systems because the heavy tidal mixing driven by the moons gravity may have played an important role in making conditions favorable for the origins of life on Earth. Recently Nadya Gorlova of the University of Florida and her colleagues at the Steward observatory in Tucson, Arizona and the European Southern Observatory in Santiago, Chile reported in The Astrophysical Journal that they may not be very common at all. Using the cryogenically-cooled infrared orbiting Spitzer Space Telescope, Gorlova and her colleagues surveyed the 30-million-year old star cluster NGC 2547. They selected this cluster because of its age. The planetary building process usually ends by approximately 50 million years, making the odds of a giant impact unlikely to occur outside this window. The other advantage of NGC 2547 is that it is old enough for the material left out from the original cloud of which solar systems formed to dissipate (this takes about 3-10 million years). By focusing on radiation at a wavelength of about 8 microns, they could detect the heat they would expect from dust at a distance of about one astronomical unit (1 AU) from a solar-type star. The NGC 2547 cluster was previously surveyed spectroscopically, so they could cross-check to make sure that the emission they detected was not due to gas (which would be evident by spectral emission lines). Out of about 400 stars in the NGC 2547 cluster, they found only one that showed evidence of dust from a massive impact. From this they conclude that collisions like the one that gave rise to our moon dont happen in every system. This means that moons like ours may be rare. (The Astrophysical Journal, 20 November 2007) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 848 November 27, 2007 by Phillip F. Schewe and Jason Bardi www.aip.org/pnu BETTER DETECTION OF THYROID CANCER* should be attainable through a new technique being developed at the Mayo Clinic. Ultrasound is currently the most sensitive tool for detecting thyroid nodules and the most cost-effective imaging method for evaluating the thyroid gland. However, the overwhelming majority of nodules discovered by ultrasound (as high as 95 percent) are benign. Often the ultrasound and other imaging results are ambiguous and cannot differentiate between malignant and benign thyroid nodules. The only way to definitively rule out a cancer diagnosis is through fine needle aspiration and biopsy. More than half these biopsies prove benign. While that may be reassuring to the people who undergo the biopsies, it would be better if they could receive that reassurance without having an expensive, invasive, and (as it turned out) unnecessary procedure. Azra Alizad of Mayo Clinic College of Medicine has developed a novel non-invasive imaging technique called vibro-acoustography (VA) for identifying thyroid nodules in excised human thyroids imbedded in tissue gel. In this method, ultrasound is used to vibrate tissue at low frequencies, and the resulting vibrations can be detected by a sensitive microphone. Harder tissues normally produce a significantly different acoustic field than softer tissues, and detecting the difference may reveal a more definitive diagnosis. Malignant lesions are stiffer than benign lesions; therefore it is reasonable to expect that VA will be a better tool for detection and differentiation of thyroid nodules than the conventional ultrasound imaging. While the technique is not yet tested for actually detecting thyroid cancers in clinical trials, vibro-acoustography is currently undergoing clinical evaluation for detecting breast cancer lesions in people. If successful, this inexpensive and non-invasive imaging tool would represent a major advance in our ability to provide care for people with potential cancer. Alizad presents his new results this week at the meeting of the Acoustical Society of America (ASA) in New Orleans. (Paper 3pBB3, http://www.acoustics.org/press/) TISSUE STIFFNESS AS A MEASURE OF A HEALTH. Matthew Urban (Urban.Matthew@mayo.edu) and his colleagues at the Mayo Clinic College of Medicine are designing ways to measure the stiffness of tissues as a non-invasive diagnostic tool. Monitoring a tissues material properties may not be as obvious a gauge of its health as looking at its biological or chemical properties, but changes to these properties can be a good indicator of disease. Areas of stiffness in a tissue, for instance, are often a good warning sign of cancer---the basic premise behind breast self-examination. Likewise when cancerous tumors form on the liver or another one of the bodys organs, they are often stiffer than the surrounding tissues because there are more blood vessels to support the tumors. The problem is, how can you measure stiffness in tissues deep within the body? There is no such thing as a liver self-exam. At this weeks ASA meeting, Urban reports on his latest experiments, in which he and his colleagues used focused ultrasound waves to deliver tiny vibrations to a steel sphere encased in gelatin, a model of a tissue with a stiff lesion. They were able to measure the frequency response of the sphere to acoustical waves of multiple frequencies, which can then be used to determine the stiffness of the tissue-mimicking material. The method also provides new ways to non-invasively cause vibration for assessment of tissue stiffness without the presence of the steel sphere. Moreover, they were able to deliver the energy to the sphere without heating the surrounding gelatin. This is one of the challenges of using highly focused ultrasound, because acoustical energy can be absorbed by nearby tissues in the form of heat. (Talk 3pBB1, meeting website: http://www.acoustics.org/press/) RECREATING THE WORLD INSIDE YOUR HEAD.* The first use of individualized virtual-reality sounds in a functional MRI (fMRI) environment to reproduce a naturalistic acoustic experience for studying brain function might provide a better explanation of the cocktail party effect-the process by which we try to make sense of a conversation at a crowded party even as several other potentially distracting conversations proceed at the same time. New brain scans using fMRI are helping researchers to understand how the brain segregates objects in space when a person hears, but not necessarily sees, multiple sources of sound. At Kourosh Saberi's (saberi@uci.edu) lab at the University of California, Irvine, human subjects are exposed to several sounds. Sometimes the sounds come from different locations near the subject, while sometimes several sounds come from a single location. When looking at fMRI scans showing areas of enhanced blood flow, which provides 2-mm-resolution maps of brain activity, the U.C. Irvine scientists report two main results. First, no specific brain region accounts exclusively for identifying auditory motion, in contrast to the visual cortex which does have specific motion-sensing regions. And second, spatial auditory information seems to be processed in a neural region, called the Planum Temporale, in a way that can facilitate the segregation of multiple sound sources. (ASA meeting talk 2aPP8, http://www.acoustics.org/press/) The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 847 November 20, 2007 by Phillip F. Schewe www.aip.org/pnu EL NINO WEATHER IN A WARMER WORLD will cause a decrease in the number of frost days in the southwestern states, an increase in precipitation intensity in southeastern states, and an increase in heat-wave intensity in the southern tier of states, according to a new study. The study looks at the weather impact of El Nino events on weather extremes in North American if, as is often predicted, global warming raises temperatures by a degree or two in coming decades. El Nino is the name for a huge ocean-atmosphere interaction and transfer of energy across the tropical Pacific Ocean between South America and Asia. El Nino events occur irregularly in intervals of between two and seven years and can have a large impact on weather in places around and beyond the Pacific basin. Gerald Meehl (meehl@ncar.ucar.edu) and his colleagues at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado have attempted to model what happens when El Nino events occur in a hypothetical warmer world, especially for weather patterns in the US. The model, first of all, does a pretty good job of simulating weather extremes (such as number of frost days-days when the temperature goes below freezing---and intense precipitation) in the world as it is now. Furthermore, the same model has been used to demonstrate that the temperature increase over the US in recent years has been mostly due to human-related forcings over and above any natural fluctuations in effect. Giving the model a new slightly higher base temperature, a number of specific changes in weather extremes (during El Nino events) in the US emerge, such as those shifts in extremes mentioned above. (Meehl et al., Geophysical Review Letters, current issue.) EGYPTIAN PYRAMIDS, DINOSAUR EXTINCTION, THE JFK ASSASSINATION: all were studied by Berkeley physicist Luis Alvarez. Alvarez won a Nobel Prize for his discovery of new particles using a bubble chamber, but some of his fame comes from his work applying physics principles and methods outside the normal physics-research world. In the November issue of the American Journal of Physics, Charles Wohl of the Lawrence Berkeley National Lab (cgwohl@lbl.gov) looks at three notable examples of Alvarezs extracurricular effort. (1) To search for possible hidden chambers in the Chephren pyramid in Cairo-one of the three great pyramids built in the third millennium BCE-Alvarez designed an experiment in which cosmic rays would strike a detector set up inside a known chamber beneath the pyramid. Observing the penetrating muons from cosmic-ray showers, this detector would discern any intervening empty spaces in the overlying pyramid structure. The upshot: no hidden chambers. (2) In scrutinizing the so called Zapruder film,* a short filmed sequence that caught the assassination in progress, experts had been puzzled by the backwards jerk of President Kennedys head after one of the bullet impacts. Some took this to be evidence for another assassin shooting from in front of the presidents car. Alvarez and some of his colleagues performed impromptu experiments at a shooting range, and also considered the conservation of momentum and the forward-moving matter from the wound. From this they concluded that the movie sequence was consistent with a shot coming from the rear. (3) Most famous of all was Alvarezs hypothesis, made in collaboration with his son Walter Alvarez, that a thin but conspicuous layer of the otherwise rare element iridium in numerous places aroun the world, all at a geological stratum corresponding to the era just around the boundary between the Cretaceous and Tertiary periods (the KT boundary), signified a large asteroid impact at that time. This impact, it was further thought, cast enough dust into the air from a long enough time as to kill off many living things, including a large portion of dinosaurs. _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 846 November 12, 2007 by Phillip F. Schewe www.aip.org/pnu THE HIGHEST-ENERGY COSMIC RAYS* probably come from the cores of active galactic nuclei (AGN), where supermassive black holes are thought to supply vast energy for flinging the rays across the cosmos. This is the conclusion reached by scientists who operate the Pierre Auger Observatory in Argentina. This gigantic array of detectors spread across 3000 sq. km of terrain, looks for one thing: cosmic ray showers. These arise when extremely energetic particles strike our atmosphere, spawning a gush of secondary particles. Many of the rays come from inside our own Milky Way, especially from our sun, but many others come from far away. Of most interest are the highest-energy showers, with energies above 10^19 electron volts, far higher than any particle energy that can be produced in terrestrial accelerators. The origin of such potent physical artifacts offers physicists a tool for studying the most violent events in the universe. To arrive at Earth most cosmic rays will have crossed a great deal of intergalactic space, where magnetic fields can deflect them from their starting trajectories. But for the highest-energy rays, the magnetic fields cant exert as much influence, and consequently the starting point for the cosmic rays can be traced with some confidence. This allowed the Auger scientists to assert that the premier cosmic rays were not coming uniformly from all directions but rather preferentially from galaxies with active cores, where the engine for particle acceleration was probably black holes of enormous size. The very largest of cosmic ray showers, those with an energy higher than 57 EeV (1EeV equals 10^18 eV), correlated pretty well with known AGNs. (Science, 9 November 2007) BREATHING EXERCISES FOR ENZYMES. A new model of proteins seeks to explain how enzymes extract energy form their vicinity and put it to use in regulating cell chemistry. Enzymes are huge protein molecules that play a crucial role in catalyzing chemical reactions among other molecules or atoms by lowering the energy barrier that would otherwise keep the reaction from happening. Enzymes can therefore be considered as energy-processing chemical-reaction-facilitating machines. They are usually large, typically containing thousands of heavy (non-hydrogen) atoms, but of these only a few dozen atoms actually participate in the catalytic process. Addressing this important issue, a team of scientists at the Ecole Normale Superieure (Lyon, France) and the Ecole Polytechnique Federale de Lausanne (Switzerland) have concentrated on modeling the behavior of the stiff parts of the enzyme since they believe that some of the energy used in carrying out the catalytic task is stored not just as chemical energy (in the form of adenosine triphosphate, or ATP, the all-purpose food of cells) but also as mechanical energy in the form of a waggling or breathing motion in the stiffer parts of the enzyme. Extending this research to proteins in general, Yves-Henri Sanejouand (yves-henri.sanejouand@ens-lyon.fr, 33-04-72-72-8870) says that he and his colleagues would like to scrutinize in more detail the nonlinear process by which some proteins catch and store thermal energy from their environment and also how chemical energy can be turned into mechanical energy, such as in muscle contraction. (Juanico et al., Physical Review Letters, upcoming article;) DIGITAL DROPLET SORTING. A new microfluidic lab-on-a-chip setup forms tiny droplets, passes them through a pair of electrodes which can perform an identification of the droplets, passes them through a second pair which gives them a charge, and then through a third pair which sorts the drops according to their properties. Basically the charge imparted to the droplet is proportional to the droplet size, and the charge is gauged by the effect it has when passing through the first set of capacitor electrodes. Scientists at the Hong Kong University of Science and Technology form a supply of drops moving in a microchannel by having the fluid of interest (in one channel) merge with a running rivulet of oil (silicon or sunflower oil) in a second channel (see the figure at http://www.aip.org/png/2007/290.htm). By regulating the flow rate of the fluid and the oil, droplets of many sizes and rates can be formed. The Hong Kong Scientists currently can look at droplets smaller than a pico-liter (10^-12 liter) in size with a capacitive sensitivity of a pico-Farad (10^-12 F). The detection rate right now is about 10,000 drops per second, which is already pretty high. According to one of the researchers, Weijia Wen (phwen@ust.hk), this capacitance-based detection rate is better than that can be accomplished with optical means (such as with a CCD camera), and the capacitance method is intrinsically cheaper than the optical equivalent. In the Hong Kong approach the detection and the sorting are both performed electrostatically: sorting happens when an electric field sends the higher-charged drops into one channel, and the lesser-charged drops into another channel. In this way nano- or micro-particles can be sorted digitally. The goal is to furnish a useful digitally-controlled bio-chemical chip for performing various experiments with nano-liter volumes of reactants or biological samples. (Niu et al., Biomicrofluidics, Oct-Dec 2007; lab website at www.phys.ust.hk/phwen) The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 845 November 2, 2007 by Phillip F. Schewe www.aip.org/pnu MARTIAN DUNES TAKE THEIR TIME: they need 1000 years to travel a few meters. This is the conclusion of a new study that tries to simulate the observed structures of dunes on the red planet and to determine whether present conditions could have been responsible. On Earth, a sand dune is shaped by wind and water. On Mars, there doesnt seem to be any surface water movement (at least not any that would shape dunes), and as for wind, there isnt much of that either. With an atmosphere only 1/100 the density of Earths the wind speed on Mars would have to be considerable to move sand around. Eric Parteli of the Universitaet Stuttgart in Germany and his colleague Hans Herrmann of the Universidade Federal do Ceara in Brazil calculate that on Mars (where the gravity is only 1/3 the Earth strength) a dune at a height of 1 meter would require a wind velocity of 35 m/s (roughly 75 mph) to be moved appreciably. This speed occurs only a few times a decade, hence the glacial pace of dunes on Mars. Their most surprising finding, Parteli said, comes from their study of bimodal sand dunes, those that bear evidence of being shaped by winds from two perpendicular directions. They deduce a wind oscillation period on Mars of 50,000 years (the time it takes for winds to shift around by 90 degrees), roughly the same as the period for the precession of Marss axis. (Physical Review E, October 2007; parteli@icp.uni-stuttgart.de; more information at http://www.icp.uni-stuttgart.de/~hans/dunes.html) GRANULAR LIQUIDS WITH ZERO SURFACE TENSION. New experiments with spherical glass beads show that liquid behavior can arise simply from rapid collisions among a sufficiently dense stream of particles. The experiment was undertaken by Xiang Cheng, Heinrich Jaeger and Sidney Nagel and their colleagues at the University of Chicago, experts on discovering novel effects with granular materials (see http://www.aip.org/pnu/2005/split/725-3.html and http://www.aip.org/pnu/2005/split/759-2.html). If one or two beads are dropped from above on a horizontal surface, they will bounce back in the direction from which they came. If, however, many beads are dropped all at once---constituting a dense granular stream hitting a target---then something else happens: the grains deflect out laterally in the form of a very thin, symmetrical sheet or cone as if they were a liquid. Indeed, the experiments using granular matter quantitatively reproduce results obtained with streams of water. However, with beads, the liquid is one in the limit of vanishing surface tension. (To ensure there was no cohesiveness between the beads, which range in size between 50 microns and millimeter, they were baked in a vacuum oven beforehand, evaporating any lurking moisture.) During the short interval the beads inside the stream collide with each other in front of the target, liquid-like conditions are established whose observable consequence are the thin sheets. This novel, zero-surface-tension liquid state, the experimenters believe, might be of interest to physicists at the Relativistic Heavy Ion Collider (RHIC), where heavy nuclei colliding at high energies (see http://www.aip.org/pnu/2005/split/728-1.html) form a plasma of quarks and gluons that also resembles a liquid. Intriguingly, the collision pattern produced by the completely classical, macroscopic granular liquid can match that produced by the quark-gluon plasma. (Cheng et al., Physical Review Letters, 2 November 2007) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 844 October 25, 2007 by Phillip F. Schewe www.aip.org/pnu SOLID-STATE ATTOSECOND MEASUREMENTS. Physicists at the Max Planck Institute fur Quantum Optics in Munich have previously looked at the behavior of electrons in atoms in the gas phase over a timescale of a hundred attoseconds (1 as=10^-18 second) or so. Now the scientists, led by Ferenc Krausz, have, in collaboration with their colleagues from Bielefeld, Hamburg, Vienna and San Sebastian, made a measurement of electron motion in a solid-state environment over a comparable timescale. The specific measurement-observing the difference in arrival times of electrons flying out of an atom struck by laser light-represents the sharpest time-resolution ever achieved in a condensed-matter experiment. To bring about this feat, a near infrared (NIR) laser pulse consisting of only a few well-chosen cycles is sent through a column of neon, producing a number of secondary beams of shorter wavelength. One of these beams, at extreme ultraviolet (XUV) wavelengths, appears in very truncated bursts lasting only 300 as. Next, the XUV pulse is directed at a tungsten target where atoms lying close to surface can be ionized. Actually the ultraviolet light tends to liberate an out-lying (delocalized) electron from the atom as well as an inner-lying (localized) electron. These two electrons can proceed through the crystal and toward a detector where, depending on the time of their arrival, they can be told apart. This identification process is enhanced in a clever way. Traveling co-linearly with the XUV pulse (and coherently linked to it) is part of the original NIR laser beam. The NIR intensity was carefully chosen so that it would not do the work of ionization (that task being assigned to the XUV light) but would be strong enough to accelerate the ionized electrons as they sprang out of the sample surface. The arrival of the NIR pulse with its well-controlled electric field was staged so that the first of the two electrons to appear (the faster-moving outer electron) would receive a boost in speed from the electric field of the NIR radiation, while the second electron (the slower inner-shell electron) received less of a boost. In other words the NIR light acted like a atomic-sized accelerator, speeding up the electrons, but in differing amounts. This accentuated the difference in the arrival times of the two electrons, making it easier to tell them apart. The net result was an ability to measure the time delay of the two electrons coming across the top few layers of the solid-state sample. The measured interval, 110 attoseconds with an accuracy of 70 attoseconds, constitutes the unprecedented attosecond measurement. One of the researchers, Adrian Cavalieri (adrian.cavalieri@mpq.mpg.de) says that monitoring electron motions in a crystal with this level of precision is the first step in developing a much faster style of electronics, maybe even at a petahertz (10^15 Hz) rate. First comes measurement at 100-attosecond levels, later comes control of electron activity. (Cavalieri et al., Nature 25 October 2007; http://www.attoworld.de/) NUCLEAR DRIPLINE DROOPS. Several new heavy isotopes have been discovered, at least one of which pushes beyond the neutron dripline. Driplines are the outer edges defining the zone of observed or expected bound nuclei on a map whose horizontal axis is the number of neutrons in a nucleus (denoted by the letter N) and whose vertical axis corresponds to the number of protons (Z). Unlike the Coulomb force which holds atoms togethe r, and where electron behavior and the expected chemical properties of that element can be predicted pretty accurately, with nuclei its different. The nuclear force holding neutrons and protons together (even as the like-charged protons repel each other electrostatically) is so strong that no theory (not even the so called nuclear shell model, fashioned in analogy to the atomic model) can confidently predict whether a particular combination of neutrons and protons will form a bound nucleus. Instead experimenters must help theorists by going out and finding or making each nuclide in the lab. In an experiment conducted recently at the National Superconducting Cyclotron Lab (NSCL) at Michigan State University, a beam of calcium ions was smashed into a tungsten target. A myriad of different nuclides emerged and streamed into a sensitive detector for identification. Two newly found nuclides-Mg-40 and Al-43-came as no surprise. But another, Al-42, was more unusual since it violated the provisional prohibition against nuclei of this size having an odd number of protons and neutrons. The new nuclides are not stable since they decay within a few milliseconds. But this is pretty long by nuclear standards. Why study such fleeting nuclei? Even though they might not exist naturally, the new nuclides still might play a role inside stars or novas where heavy elements, including those that make up our planet and our bodies, are created. Thomas Baumann (baumann@nscl.msu.edu) suggests that even heavier aluminum-isotopes might exist, and that it is worth exploring any possible islands of stability, not just those at the very edge of the periodic table. (Baumann et al., Nature 25 October 2007; http://www.nscl.msu.edu/magnesium40) The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 843 October 18, 2007 by Phillip F. Schewe www.aip.org/pnu RELATIVISTIC THERMODYNAMICS.* Einsteins special theory of relativity has formulas, called Lorentz transformations, that convert time or distance intervals from a resting frame of reference to a frame zooming by at nearly the speed of light. But how about temperature? That is, if a speeding observer, carrying her thermometer with her, tries to measure the temperature of a gas in a stationary bottle, what temperature will she measure? A new look at this contentious subject suggests that the temperature will be the same as that measured in the rest frame. In other words, moving bodies will not appear hotter or colder. Youd think that such an issue would have been settled decades ago, but this is not the case. Einstein and Planck thought, at one time, that the speeding thermometer would measure a lower temperature, while others thought the temperature would be higher. One problem is how to define or measure a gas temperature in the first place. James Clerk Maxwell in 1866 enunciated his famous formula predicting that the distribution of gas particle velocities would look like a Gaussian-shaped curve. But how would this curve appear to be for someone flying past? What would the equivalent average gas temperature be to this other observer? Jorn Dunkel and his colleagues at the Universitat Augsburg (Germany) and the Universidad de Sevilla (Spain) could not exactly make direct measurements (no one has figured out how to maintain a contained gas at relativistic speeds in a terrestrial lab), but they performed extensive simulations of the matter. Dunkel (joern.dunkel@physik.uni-augsburg.de ) says that some astrophysical systems might eventually offer a chance to experimentally judge the issue. In general the effort to marry thermodynamics with special relativity is still at an early stage. It is not exactly known how several thermodynamic parameters change at high speeds. Absolute zero, Dunkel says, will always be absolute zero, even for quickly-moving observers. But producing proper Lorentz transformations for other quantities such as entropy will be trickier to do. (Cubero et al., Physical Review Letters, 26 October 2007; text available to journalists at www.aip.org/physnews/select) NUCLEAR SYRUP. A new measurement of how long it takes certain nuclei to fission into large fragments suggests that the liquid-drop model of the nucleus should be replaced with a nuclear syrupmodel. Fission is the most dramatic form of radioactivity, when a nucleus loses not merely a small fragment-such as an electron, gamma ray, or an alpha particle-but actually splits in half. The fission of many nuclei has been studied through the years, most famously uranium-235. As early as 1939 Niels Bohr and John Wheeler tried to model the nature of fission by saying that the nucleus is like a drop of water in which the tendency of the drop to fly apart is checked by the force of surface tension; something like this, they said, kept a nucleus intact until such time as the rapid oscillations of an unstable nucleus became so large that the surface tension normally keeping the nucleus together was overcome. Sometimes as a prelude to fission, the nucleus relieves some of its instability and effectively reduces its internal nuclear temperature by flinging out neutrons or gamma rays. In fact, the lifetime for fission has been indirectly measured by observing those cast-off neutrons. The results suggest that the old liquid-drop model was off by a factor of ten or so in predicting lifetimes. Some scientists have begun to think that an additional stickiness in the nuclear substance is at work, which slows up the fission process. An experiment at Oak Ridge National Laboratory has probed this proposition by creating several fissionable nuclei artificially with heavy-ion beams bombarding a tungsten target; the projectile and target nuclei temporarily fuse together, travel a short distance through the tungsten crystal, and then fission. The spacing of the atoms in the crystal is used as a reference to measure the recoil of the composite nucleus before fission. According to team member Jens Andersen of the University of Aarhus in Denmark (jua@phys.au.dk, 45-8942-3713), the Oak Ridge experiment suggests that the fission lifetimes are even longer (an additional factor of ten to one hundred) than those derived with the more indirect neutron-emission method. This could imply that the nuclear shape does not oscillate as rapidly as a water droplet would but instead deforms very slowly like a drop of syrup. (Andersen et al., Physical Review Letters, 19 October 2007; journalists can obtain the text from www.aip.org/physnews/select) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 842 October 9, 2007 by Phillip F. Schewe www.aip.org/pnu THE 2007 NOBEL PRIZE IN PHYSICS WILL BE AWARDED TO Albert Fert (Université Paris-Sud, Orsay, France) and Peter Grünberg (Forschungszentrum Jülich, Germany) for the discovery of giant magnetoresistance, or GMR for short. GMR is the process whereby a tiny magnetic field, such as that of an oriented domain on the surface of a computer hard drive can, when the proper read head is brought nearby, trigger a large change in electrical resistance, thus reading the data vested in the magnetic orientation. This is the heart of modern hard drive technology and makes possible the immense hard-drive data storage industry. Fert and Grünberg pioneered the making of stacks consisting of alternating thin layers of magnetic and non-magnetic atoms needed to produce the GMR effect. GMR is a prominent example of how quantum effects (a large electrical response to a tiny magnetic input) come about through confinement (the atomic layers being so thin.); that is, atoms interact differently with each other when they are confined to a tiny volume or a thin plane. All these magnetic interactions involve the spin of an electron. Spin is a quantum attribute that shouldnt be associated too closely in the mind with the electron literally spinning (in the way that a top spins). Still more innovative technology can be expected through quantum effects depending on electrons spin. Most of the electronics industry is based on manipulating the charges of electrons moving through circuits. But the electrons* spins might also be exploited to gain new control over data storage and processing. Spintronics is the general name for this budding branch of electronics. (Nobel Prize website: http://nobelprize.org/nobel_prizes/physics/laureates/2007/info.html) NEW THEORY EXPLAINS HOW CELLULAR COMPASSES WORK. Scientists from the Politecnico di Torino in Italy and the Landau Institute of Theoretical Physics in Russia have derived a theory to describe how eukaryotic cells (such as those found in all higher organisms) respond to chemical signals in their environments. Considering that coordinated sensing of and movement toward chemical signals is a vital processes in embryology (how cells know where to go in fashioning the organism), inflammation, and immune response, directional maneuvering at the cellular level is quite important. Here's what happens. First, receptors in the membranes of the cells become activated by the presence of trace amounts of chemicals---even down to the nano-molar level or about one molecule in a cubic micron---in the cells' vicinity. Not only do the receptors sense the presence of the attractants but, through the differential activation of 10,000 or more receptors distributed along the body of the cell, the direction of the source of the attractant can be located to within a few degrees. Ability to train upon a 5% chemical gradient allows the cell to know where it should be going, whether to find food, antigens, or to take up its place in a larger multi-cellular structure. Second, a cascade of polymerization steps now ensues within a few minutes. Consequently the cell develops head and tail structures, the better to make possible travel along the chemical gradient (chemotaxis). In nature, cells have also been known to plan their travel by exploiting thermal gradients (thermotaxis) and electrical gradients (galvanotaxis). According to Andrea Gamba (andrea.gamba@polito.it) and coauthors the new results consist of being able now to demonstrate in a mech anistic way how the cell's directional sensing and response comes about through a kind of self-organized phase transition; when the chemical gradient exceeds a certain threshold level the dynamic of growth of clusters of signaling molecules on the cell surface fine-tunes to sense the slight unbalance in activated receptors and provides a fast polarization in the direction of the gradient, thus providing a compass bearing which is able to initiate the modification in the cellular structure. The scientists argue that the physical amount of space along the body of large eukaryotic cells needed for making such an astute directional assessment might explain why bacteria (with much smaller bodies) do not have a spatial system of directional sensing. (Gamba et al., Physical Review Letters, 12 October 2007) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 841 October 2, 2007 by Phillip F. Schewe www.aip.org/pnu THE VACUUM STRIKES BACK. Modern physics has shown that the vacuum, previously thought of as a state of total nothingness, is really a seething background of virtual particles springing in and out of existence until they can seize enough energy to materialize as real particles. In high energy collisions at accelerator labs, some of the original beam energy can be consumed by ripping particle-antiparticle pairs out of the vacuum. Sometimes this process is the very reason for doing the experiment, but sometimes it is only a detriment. For example, in the Large Hadron Collider (LHC), under construction at the CERN lab in Geneva, a major source of beam losses (particles exiting from the usable beam) for heavy-ion collisions is expected to be a class of event in which the counter-moving ions pass each other and dont interact except to spawn a pair of particles---an electron and positron---one of which (the positron) goes off to oblivion while the other (the electron) latches onto one of the ions. This ion, bearing an extra electric charge, will now behave slightly differently as it races through the chain of powerful magnets that normally steer the particles around the accelerator. Going a certain distance, the modified ion will leave its fellows and smash into the beam pipe carrying the beams, thus heating up the pipe and surrounding magnet coils. Fearing these future beam losses, accelerator physicists have sought to observe this effect at an existing machine, the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven Lab on Long Island. And they found what they were looking for, a tiny splash of energy amounting to about .0002 watts, or about what a firefly puts out. The RHIC beam for these tests consisted of copper ions each carrying 6.3 TeV of energy (about 100 GeV per nucleon). According to CERN scientist John Jowett (john.jowett@cern.ch, 41-22-7676-643) this troublesome class of events, referred to as bound-free-pair production (or BFPP, the bound referring to the electron and the free to the positron), will be much more formidable at LHC than at RHIC. First of all, the pair production scales as the atomic number of the nucleus (or the charge of the nucleus, denoted by the letter Z) raised to the seventh power. The LHC heavy-ion collisions will use beams composed of lead ions. The more highly charged nucleus and the larger energies (574 TeV per lead nucleus) mean the BFPP process should be some 100,000 times more prominent than in the test at RHIC. This would amount to about 25 watts, the equivalent of a reading lamp. That doesn't sound like much but, when deposited in the ultra-cold (1.9 K) magnets of the LHC, it could bring them to the brink of "quenching" out of their superconducting state, interrupting the operation of the huge machine. (Bruce et al., Physical Review Letters, 5 October 2007; journalists can obtain the text from www.aip.org/physnews/select; other background material at arxiv.org/abs/0706.3356v2), http://cern.ch/AccelConf/e04/PAPERS/MOPLT020.PDF, Vol. I, Chapter 21 of the LHC Design Report, available at http://ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html ) GAMMA RAYS FROM THUNDERCLOUDS* have been observed by ground-based detectors, providing new insights into mechanisms for accelerating electrons to high energies, as high as 10 MeV, in the atmosphere. Ground observations of thundercloud gammas has been made before as part of monitoring regular nuclear plant operations. The new measurements, ho wever, represent the first time that such gamma studies were made with detailed scientific objectives in mind, including determinations of particle species, arrival direction, and energy spectrum. On the night of 6 January 2007 two powerful low-pressure air masses collided over the Sea of Japan. A nearby array of gamma detectors provided information on the energy and the timing of the gammas, which are the highest-category of electromagnetic radiation. The array is operated by the University of Tokyo and the Cosmic Radiation Laboratory of RIKEN in Japan. The gamma production, the researchers believe, works like this: an energetic seed electron, perhaps liberated from an atom by an intruding cosmic ray, ionizes many air molecules, which in turn are accelerated by the high electric fields present in the thunderclouds. This flock of fast electrons can then emit gamma radiation (bremsstrahlung, or braking radiation) as they are slowed by surrounding air. The gamma production actually occurs before the eventual lightning strike, says Teruaki Enoto of the University of Tokyo (enoto@amalthea.phys.s.u-tokyo.ac.jp, 81-3-5841-4173), and the reason for this is not entirely known. Previous thundercloud-related gammas were studied by satellite and only measured very brief bursts, with durations of msec. By contrast, the Tokyo-RIKEN work indicates bursting behavior that could last for minutes, testifying to the quasi-static nature of the acceleration mechanism at work in the clouds. The electric fields in the clouds might be as high as 10 million volts. (Tsuchiya et al., Physical Review Letters, upcoming article; text can be obtained from www.aip.org/physnews/select ) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 840 September 26, 2007 by Phillip F. Schewe www.aip.org/pnu CONTROLLING CARDIAC CHAOS-a gentler approach. Physics can save lives: a new type of defibrillation aims to reduce the voltage needed to shock out-of-control hearts back to a normal beating pattern. Ordinarily the beating heart is an orderly process (called systole) in which the heart muscle cells contract cooperatively to insure that blood is pumped about once every second. If, however, some portions of cardiac tissue are electrically triggered in a non-coordinated way, the overall activity of the heart can become chaotic. An irregular systole (fibrillation) in the atrial chambers of the heart can be tolerated for some time, but fibrillation of the ventricles can kill a person within a few minutes. The most extreme remedy for ventricular fibrillation (VF) is the application of a huge electrical shock (administered by paddles applied to the chest). Conventional defibrillators applied to the outside of the body can deliver a voltage difference of up to 5000 volts and a current of 20 amps. The shock delivered by implanted defibrillators is much less, but can still result in trauma. The goal of the shock is to overwhelm the electrical environment of the entire heart---disrupting electrical waves even in the parts of the heart beating normally---hoping a global coordinated rhythm will resume. (One could compare this to brute-force method of chemotherapy, in which toxic chemical meant to kill cancer cells will also kill many healthy cells, resulting in unpleasant side effects.) To see how the general assault on fibrillation can be modified, consider that the threatening arrhythmias take the form of rotating waves (spirals) of electrical excitation passing across the volume of the heart. These spirals are enhanced (and dangerously pinned in position) by the presence of scars (dead tissue) on the heart caused at the scene of previous attacks and even by other heterogeneities present in healthy hearts such as blood vessels, connective tissues, and oriented bundles of cardiac muscle fibers. Alain Pumir and Valentin Krinsky and their colleagues at the University of Nice, France and at the Centre National de la Recherche Scientifique (CNRS) Nonlinear Institute try to undo threatening vortices not by jolting the whole heart but by aiming their countermeasures at the vortices exclusively. This permits a much smaller voltage to be used, and hence less trauma to the patient and less damage to the heart itself. One of their earlier efforts in this direction (Physical Review Letters, 30 July 2004) allowed a rotating vortex in the heart to be removed using an input electrical energy lower by a factor of 20. Later the approach was confirmed to be effective using rabbit hearts. Now Pumir and Krinsky (33-6-6844-1415, alain.pumir@unice.fr , valentin.krinski@inln.cnrs.fr) have designed an even better scheme, one that would counteract a chaotic cardiac crisis consisting of many vortices. In addition, this approach permits the energy to be reduced by a factor of a hundred or a thousand from present levels. A sophisticated implant device, programmed to mitigate potential fibrillation with the new shock method, would be almost unnoticeable to the patient. Teams led by R. Gilmour (Cornell) and E.Bodenschatz (Max Planck Institute, Goettingen eberhard.bodenschatz@ds.mpg.de ) are currently testing the method. An estimated 250,000 people have implanted defibrillators, so the scope for medical benefits are enormous. (Pumir et al., Physical Review Letters, upcoming article) THERMAL LOGIC GATES. Information processing in the world's computers is mostly carried out in compact electronic devices, which use the flow of electrons both to carry and control information. There are, however, other potential information carriers, such as photons, which are parcels of light. Indeed a major industry, photonics, has developed around the sending of messages encoded in pulsed light. Heat pulses, or phonons, rippling through a crystal might also become a major carrier, says Baowen Li of the National University of Singapore (phylibw@nus.edu.sg). Li, with his colleague Lei Wang, have now shown how circuitry could use heat---energy already present in abundance in electronic devices---to carry and process information. They suggest that thermal transistors (also proposed by Li's group in Applied Physics Letters, 3 April 2006) could be combined into all the type of logic gates---such as OR, AND, NOT, etc.-used in conventional processors and that therefore a thermal computer, one that manipulates heat on the microscopic level, should be possible. Given the fact that a solid state thermal rectifier has been demonstrated experimentally in nanotubes by a group at UC Berkeley (Chang et al., Science, 17 November 2006) only a few years after the theoretical proposal of "thermal diode," the heat analog of an electrical diode which would oblige heat to flow preferentially in one direction (Li et al, Physical Review Letters, 29 October 2004). Li is confident that thermal devices can be successfully realized in the foreseeable future. (Wang and Li, Physical Review Letters, upcoming article) _{ANA SAYFA FİZİK FORUM} The American Institute of Physics Bulletin of Physics News The American Institute of Physics Bulletin of Physics News Number 839 September 17, 2007 by Phillip F. Schewe www.aip.org/pnu RADIO-COOLED MACROSCOPIC OBJECT. Lasers have long been used to cool atoms in traps. By using light slightly mistuned with the atoms own internal quantum energy levels, the light can progressively slow the atoms almost to a halt. The same principles can be applied to larger objects made of trillions of atoms, such as a thin silicon cantilever. Although light cooling of a cantilever-specifically the cantilevers oscillatory motions---has been achieved before, scientists at the NIST lab in Boulder, Colorado are the first to do this using very radio-frequency circuitry. In the NIST experiment, a micron-sized cantilever is chilled from room temperature down to 45 K in a process called capacitive cooling, in which the cantilever, pelted with radio waves, slows down (vibrates less) by transferring energy to the surrounding radio frequency resonant circuit. One of the NIST scientists, Kenton Brown (krbrown@boulder.nist.gov, 303-497-4364) says that the potential advantage here is that the cooling of the cantilever can be accomplished with standard radio-frequency technology instead of with precision optical elements or lasers, making it easier to put the whole setup on a chip and to immerse the chip in a cryogenic environment. Why chill the cantilever (think of a tiny up-and-down vibrating diving board) in the first place? Because a cold enough cantilever could demonstrate quantum behavior in a macroscopic object. Besides the fundamental interest in such a feat, it might pave the way to very sensitive detectors. (Brown et al., Physical Review Letters, upcoming article; journalists can obtain the text at www.aip.org/physnews/select; lab website for NIST Time and Frequency Division, http://tf.nist.gov/ion/index.htm) AN ULTRAFAST, ULTRALARGE CHANGE IN REFLECTIVITY can be brought about with femtosecond lasers. In a recent experiment short laser pulses, falling on an organic salt target, momentarily changed the material from an insulator (a bad reflector of light) to a semi-metal (good reflector of light). The change in reflectivity this large---more than 100%-has never been achieved before in a photonic material; photo-induced changes are usually more like a few percent. The laser pulse required doesnt even have to be particularly intense to cause the change. Thus gigantic photo-response work began as a Tokyo-Kyoto collaboration but now includes also LBL and Oxford. The new advance is that the change in reflectivity can be brought about in tens of femtoseconds rather than 150 ns. The new results are being reported this week at the Frontiers in Optics meeting in San Jose by Jiro Itatani, who has a joint appointment at LBL (jitatani@lbl.gov) and the Japan Science and Technology Agency. He says that dramatic reflectivity changes will be useful in bringing about direct ultrafast optical-to-optical switching. (Meeting website: http://www.osa.org/meetings/annual/default.aspx) EXPLAINING A PLASMON VERSION OF YOUNGS EXPERIMENT.** When light strikes a metallic array of subwavelength apertures surface plasmons may be created. An electromagnetic phenomenon like light itself, the plasmons propagate in the plane of the metal but with a wavelength smaller, sometimes appreciably smaller, than the illuminating light. Just as light can couple to surface plasmons, these plasmons propagating between apertures can also be reconstituted as light. The overall effect is that large light can pass through tiny holes. If now the number of openings is limited t o two, then one has the makings of a plasmonic version of the famous Young's experiment, the early nineteenth-century experiment in which light falling on two slits in a baffle produced an interference pattern---revealing the wave nature of light. A number of experiments have now been performed on exactly this version of Young's experiment. At the Frontiers in Optics meeting C.H. Gan of the University of North Carolina (Charlotte) reports on some new theoretical predictions relating to the coherence properties of light transmitted through the slits. His detailed simulations, done with collaborators G. Gbur of UNC Charlotte and T.D. Visser of the Free University of Amsterdam, show how surface plasmons traveling between the apertures result in a correlation of the light fields emitted from the apertures. Gan (chgan@uncc.edu) shows how this effect can be tuned (such as by varying the size or spacing of the slits) to achieve varying degrees of spatial coherence (that is, the amount by which the waves are in step) of the emergent reconstituted light waves. This tunability in turn has the potential to be exploited in new forms of coherence-relating imaging, such as 'variable coherence scattering microscopy. _{ANA SAYFA FİZİK FORUM}