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Aktuelle Meldungen bei MM-Physik 02 März 2005 © email: Krahmer |
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Aktuelle Meldungen bei MM-Physik 02 März 2005 © email: Krahmer |
"The American Institute of Physics Bulletin of Physics News" AIP Auswahl 1/2002 by Phillip F. Schewe and Ben Stein, and James Riordon
| April 9, 2002 DIMMING SUPERNOVAS WITHOUT COSMIC ACCELERATION. Several years ago two different studies of distant supernovas seemed to suggest that the expansion of the universe was not slowing but actually accelerating (go to www.aip.org/physnews/update and see Update 361). One implication of this would be the existence of some kind of anti- gravity or "dark energy" responsible for counteracting the mutual gravitational attractiveness thought to be operating among all the galaxies. But could there be another explanation for the observed dimness of distant supernovas? Scientists from Los Alamos and Stanford say yes, there is. John Terning (terning@particle.lanl.gov, 505-665-0437), Csaba Csaki, and Nemanja say that the dimness might arise when photons from the supernovas turn into axions on their way to Earth. Axions are hypothetical particles which are thought to account for some of the asymmetries between left-handed and right-handed things in the universe. The occasional transformation of a photon into an axion and back again would be analogous to the oscillation of one neutrino species into another and back again; in the oscillation process at least one of the species must have some mass. The axions would probably have a very low mass, something like 10^- 16 eV. Terning says that the axion hypothesis nicely recreates the observed supernova luminosity actually observed. A direct search for axions is underway at the CERN Axion Solar Telescope (CAST), http://axnd02.cern.ch/CAST/ ). (Csaki et al., Physical Review Letters, 22 April 2002; text at www.aip.org/physnews/select ; see also http://t8web.lanl.gov/people/terning/axion.html) April
1, 2002 |
| March 22, 2002 A MOLECULAR BEC has been achieved by Carl Wieman and his colleagues at the University of Colorado. To be precise, what Wieman reported at this week's APS March Meeting in Indianapolis was the observation of a quantum superposition of diatomic molecules and disassociated atoms in a trap. Having long used Rb-87 in his BEC experiments, Wieman has as of late been studying Rb-85 which, although it is harder to condense, possesses just the right fine-grained set of quantum energy levels (hyperfine levels) so that the application of a magnetic field can alter the interaction force among the atoms in the trap, even as they reside in the single quantum state which is the hallmark of Bose Einstein condensates. By adjusting the magnetic field to be very close to the point where the interatomic force goes from attractive to repulsive, a "Feshbach resonance" occurs and some of the atoms form molecules. The atoms and molecules are thought to be coherent (share a single quantum state) at least locally, and maybe over longer distances too. In this process the condensate appears first to implode and then rebound somewhat like a supernova, even to the extent of sending out jets of particles and leaving behind a remnant. The physics behind this "Bosenova" behavior is still a mystery. Wolfgang Ketterle of MIT, like Wieman a winner of the 2001 Nobel Prize in physics for BEC discoveries, spoke at the same APS session and reported findings in three areas. (1) He has used a sodium-23 BEC to help cool a gas of lithium-6. Li-6 is a fermionic atom (one with a half-integral net spin). The Pauli- exclusion principle forbids such atoms from falling into the single state available to bosonic atoms such as Na-23, but the Li-6 atoms can, if cooled low enough, occupy all the lowest energy quantum states possible. This has now been done in the MIT experiment, the first time such a "degenerate Fermi sea" has co-existed with a large BEC. One wants to see how such a fermi gas behaves at nK temperatures and whether the atoms can be coaxed (by manipulating the interaction between them) into forming Cooper pairs, becoming thereby a superfluid. (2) Ketterle reported the propagation of a condensate in a magnetic waveguide. First, his group made a large (2 million atoms) BEC in the usual way (in a magnet trap), then loaded it into a magnetic trap by 40 cm, and finally loaded it into a microtrap on a printed circuit board. The micro-journey around the chip was partly smooth and partly bumpy, especially when the cigar-shaped BEC came toward a Y divide. (Such a beam splitter would be a useful step toward making an interferometer for atom waves.) At the divide the condensate wiggled itself into a snake shape. Close to the chip surface, the condensate broke up into several detached segments. Future atom chips will need better control of surface roughness. (3) Another goal is the generation of pair correlated atoms. Ironically, the atoms in a condensate all share a single quantum state but are not otherwise entangled. The MIT researchers have created two BEC blobs (let us call them 1 and 2) together with another small "seed" condensate (blob 3). The elastic collision of these blobs produced a fourth blob in a process called four-wave mixing (for an earlier version of this experiment, at NIST, see Update 422). In effect the atoms in blobs 1 and 2 help to amplify blob 3 (a gain of 20, in this case). For each atom added to blob 3 one atom is put into blob 4. This created two pair-correlated atomic beams. In some future experiment this pair correlation might be verified directly if one could detect single atoms in the two condensates, which are moving off in opposite directions. Right now it is difficult to spot single neutral atoms in a BEC. Single-atom detection is likely in helium BECs since the atoms, deliberately put into an excited state in order to confine and cool them in the first place, are easily ionized, making it far easier to detect them. Chris Westbrook, a member of Alain Aspect's team at Orsay, summarized recent helium work and described a scheme for producing helium molecules within a BEC. This, he said, might allow an atom-wave equivalent to the current process of down- conversion, by which UV photons can be converted, in a special crystal, into a pair of lower-energy but entangled photons (if one photon has a horizontal polarization, the other must have a vertical polarization; see Update 519). A beam of related atoms could, analogously, be sundered into beams of pair-correlated atoms. Finally, Jakob Reichel (Max Planck/Univ Munich), a member of one of Ted Hansch's groups, said at the APS meeting that he and his colleagues were aiming to achieve single-atom detection in rubidium condensates. Furthermore, he hoped that the single atoms, maneuvered into resonant cavities, might be able to carry out quantum computing chores. Wed, 13 Mar
2002 |
| March 5, 2002 HAS NUCLEAR FUSION BEEN OBSERVED IN A BUBBLE TANK? A team of scientists has claimed evidence for deuterium- deuterium fusion in a tabletop apparatus at Oak Ridge National Lab (Taleyarkhan et al., Science, 8 March 2002), but other scientists (including a separate group at Oak Ridge) are raising serious concerns about the validity of the result. In their experiment, Taleyarkhan et al. (a collaboration of scientists from Oak Ridge, Rensselaer Polytechnic Institute and the Russian Academy of Sciences) utilize sonoluminescence (SL), itself a well- studied and highly regarded area of research (see, for example, Updates 34, 299, 307, 327, and 355), in which powerful sound waves sent into a liquid tank trigger the creation of single or multiple bubbles which then collapse and release short flashes of light. Sonoluminescence, literally the conversion of sound into light, is a remarkable process in that sound itself is not a densely packed form of energy. Even the sound in the most powerful car stereo has a much lower energy density than the light in a penlight laser beam. In an SL experiment, however, the energy from the sound wave gets focused into a very small region, namely a collapsing bubble. This highly concentrated energy heats the gas inside the bubble to incandescent temperatures resulting in the release of light. The conversion of sound energy into light energy represents an energy concentration of over a trillion. Researchers have long speculated whether the conditions inside the collapsing bubbles could be made to approach the high temperatures and densities necessary to trigger energy-producing nuclear fusion reactions such as those that occur inside the sun. This is a great matter of debate, as some details of the bubble collapse and light emission are still incompletely understood. With this incomplete knowledge, researchers cannot discount the possibility that the conditions can be tweaked to generate nuclear fusion, modest as these fusion reactions are likely to be. However, according to leading sonoluminescence theorist William Moss of Lawrence Livermore National Laboratory, "We are all pretty sure that normal SL conditions are nowhere near fusion temperatures--typical SL temperatures don't exceed 11,000 degrees Kelvin or so, at least from theoretical estimates"---as opposed to the millions of degrees that nuclear fusion would typically require. In the newly reported experiment, many details are similar to a traditional SL setup: researchers aimed 19.3-kHz sound waves at a glass flask containing deuterated acetone. But here's the novel part of the experiment: a pulsed neutron generator injected 14.3 MeV neutrons into the flask, in sync with the sound waves. The researchers claim that the neutrons trigger the creation of extremely small bubbles which then grow to relatively large sizes and then collapse to generate pulses of light. In conjunction with the light pulses, the researchers report the detection of significant amounts of tritium and evidence for neutrons with an energy of 2.5 MeV. Such neutrons would be produced in the fusion of deuterium atoms in the glass flask. They repeated the experiment with normal acetone (lacking deuterium) and did not detect the tritium or neutrons. However, another group at Oak Ridge, consisting of D. Shapira and M.J. Saltmarsh, attempted to reproduce the experiment, except for the fact that they used a larger neutron/gamma-ray detector and what they report to be a more sophisticated data acquisition system ( http://www.ornl.gov/slsite/SLan5av2.pdf ). They found a 1% increase in the neutron/gamma ray signal when the experiment was set up to trigger cavitation (formation of bubbles), as opposed to when the sound wave was turned off. However, they did not find the 10-fold increase that they expected if the reported tritium levels occurred as a result of deuterium-deuterium fusion. And they found nothing when they looked for neutrons or gamma rays being emitted in coincidence with the light pulses. Outside researchers who have studied the Science paper have expressed very significant concerns about its validity. According to Moss, the key measurement is the 2.5 MeV neutron peak. "If measured neutrons are thermonuclear in origin, then there must be a peak at 2.5MeV, and measuring and reporting that peak constitutes a minimum requirement to support the claim of thermonuclear origin," he says. "Tritium production (claimed in the paper) is not sufficient evidence, since it is difficult to determine the source." Moss rejects the conclusions of the paper based on the "lack of a properly resolved neutron peak." He says, "Extraordinary claims require unambiguous data, which they did not provide. This doesn't mean that thermonuclear neutrons from a sonoluminescence source are impossible, only that they didn't show data to support the claim." Seth Putterman, a leading sonoluminescence experimentalist at UCLA, points out that the researchers claim a 1000-to-1 production of output neutrons to input neutrons that hit the acoustically sensitive region of the resonator. It should be possible, he says, to turn this data into a huge signal and a clearly detectable neutron spectrum, but this is not presented in the paper. He also points out that no other paper on sonoluminescence has ever detected a single neutron as a result of the SL process. The authors of the Science paper have invited other researchers to attempt to reproduce the experiments. They say that they have reanalyzed the Shapira and Saltmarsh data and find that these data are actually compatible with sonofusion and provide an independent confirmation of their controversial claim ( http://www.rpi.edu/~laheyr/SciencePaper.pdf ). However, according to Putterman and Moss, the experiment by Taleyarkhan et al. does nothing to resolve the question of whether acoustic cavitation can generate nuclear fusion reactions. "The actual scientific experiment appears to be flawed," Putterman says. "If confirmed, however," adds sonoluminescence pioneer Lawrence Crum of the University of Washington, "it would be a remarkable result, demonstrating that mechanical systems could induce nuclear reactions." However, Crum also adds, "I am very skeptical that their results will ever be duplicated." "This is an interesting, high-risk direction of research that should go on," Putterman says. "These results may be so premature and so flawed, however, that it may taint future attempts in the field." February 27, 2002 USING CLOCKS IN SPACE TO SEARCH FOR NEW PHYSICS. Einstein's theory of relativity holds several things sacred. One is the idea that if you rotate a particle or object, or boost it up to a high velocity, the laws of physics affecting the object should stay the same. This is called Lorentz invariance. But in some "extensions" of the standard model of particle physics, interactions of particles with certain hypothetical universal fields (very roughly analogous to the way in which Higgs bosons are supposed to make some particles massive) might lead to subtle violations of Lorentz invariance. In a new paper Alan Kostelecky of Indiana University and his colleagues show how this can happen, and how such a violation could be detected in clock- comparison experiments now being readied for the International Space Station (ISS). In general an atomic clock works by shooting microwaves into a sample of cooled cesium atoms and reading out the microwave- absorption frequency which corresponds to a specific quantum transition for electrons in the cesium atoms. The microwave frequency setting is used to define the "second." If one can cool the atoms to lower temperatures (thus reducing the blurring caused by their movement) or observe them for longer periods, the precision of the whole readout process (and the standardization of the second) would improve. The world's best clock, NIST F-1, currently has an uncertainty of one part in 10^15. It achieves this by chilling Cs atoms in a trap and then gently boosting them upwards. Where they reach the top of their trajectory (subject always to the attraction of gravity) and are at their slowest is where they are subjected to the microwave bath. A related apparatus mounted on the ISS could gain in precision because the atoms would never fall (at least not relative to the atom trap setup) and could be sampled for longer periods. The goal is to have several such "space clocks" in orbit within a few years (see, for example, www.boulder.nist.gov/timefreq/cesium/parcs.htm). According to Kostelecky (kostelec@indiana.edu, 812-855-1485) certain Lorentz-violation effects, expected to show up as a tiny shifting of an atom's energy level, would be more readily accessible in space thanks to the speeds, rotation rates, and clock orientations available on space platforms (see animations at http://physics.indiana.edu/~kostelec/mov.html ). With sensitivities in space comparable to those in Earth-based experiments, the expected tests of Lorentz-violating effects would be measured with uncertainties at the level of parts in 10^27. (Bluhm et al., Physical Review Letters, 4 March 2002) |
| February 20, 2002 COLD ANTIHYDROGEN ATOMS might have been made, for the first time, in an experiment at the CERN lab, where positrons and antiprotons are brought together in a bottle made of electric and magnetic fields. Nature allows the existence of antiparticles but hasn't seen fit to make a lot of them. Modest amounts of antiprotons show up in cosmic ray showers, and positrons (antielectrons) are forged in certain high-energy regions of the sky such as galactic nuclei. But if larger forms of anti-matter like anti- atoms, anti-stars, and anti-galaxies were plentiful in the visible part of the universe then we would see the catastrophic gamma ray glare from places where matter brushes up against antimatter. Such radiation has not been seen and scientists must make their own anti-atoms artificially. Making antihydrogen is difficult, however, because positrons and antiprotons, even when they can be marshaled and brought near each other, are usually going past each other too quickly for neutral atoms to form. A few years ago a dozen or so hot antihydrogen atoms were made on the fly amid violent scattering interactions at CERN and Fermilab (Updates 253, 297). These did not dally long enough to be studied, but instead expired quickly when they crashed into detectors that established the antihydrogen's brief existence. At CERN several experiments are devoted to making cold anti- atoms in a controlled environment amenable to detailed studies. The main goal here is to determine whether the laws of physics (gravity, quantum mechanics, relativity, etc.) apply to anti-atoms the same as they do to regular atoms. At this week's meeting of the American Association for the Advancement of Science (AAAS) in Boston, Gerald Gabrielse of Harvard, spokesperson for the Antihydrogen Trap collaboration (ATRAP: website at hussle.harvard.edu/~atrap), reported new results. In his experiment 6-MeV antiprotons (themselves made by smashing a beam of protons into a target) are slowed by a factor of 10 billion (to an equivalent temperature of 4 K), partly by mixing them with cold electrons, and then collected in a trap. Positrons from the decay of sodium-22 nuclei are cooled and collected at the other end of the device. Eventually about 300,000 positrons are electrically nudged into the vicinity of about 50,000 antiprotons. Gabrielse believes that what sits in his trap isn't entirely a neutral plasma consisting of coincident positron and antiproton clouds, and that cold antihydrogen atoms might have formed. More diagnostic equipment being installed now may settle the issue in the coming months. A larger version of the ATRAP apparatus, which might be in operation as early as this fall, should allow the researchers to introduce some lasers for the purpose of studying the spectroscopy of prospective anti-hydrogen atoms in the trap. COMPUTER AIDED DIAGNOSIS OF BREAST CANCER. Ultrasound is one of the primary tools available for diagnosing breast lesions, but the final word on the malignancy of a particular lesion generally requires a biopsy or other invasive technique. Recently, some researchers have begun developing methods to classify lesions based on their appearance in ultrasound scans. Now a group at the University of Chicago (Maryellen Giger, 773- 702-6778, m-giger@uchicago.edu) is taking such schemes one step further by attempting to automate lesion classification with computer-aided diagnosis (CAD). Clues to a lesion's malignancy lie in its shape, texture, and the sharpness of its boarder, as well as its response to the acoustic signals emitted by ultrasound machines. None of these features alone characterize a given lesion, therefore both humans and computers must consider multiple characteristics to make a diagnosis. To test their CAD method, the researchers studied 400 breast lesion cases that were each documented with one to six ultrasound images. The method correctly identified 95% of the malignant lesions and 60% of the benign lesions. In a study presented at the 2001 meeting of the Radiological Society of North America, the researcher group compared their CAD method to human diagnostic skills. Although imperfect, the computer's performance was marginally better than radiologists who studied the same cases, and only slightly worse than a comparison group of expert mammographers. In the study, both community radiographers (who routinely interpret breast images, but are not experts) and mammography experts improved their accuracy when they were given access to the CAD data. In fact, radiologists aided by the CAD system performed as well as the expert mammographers performed without the aid. Although some lesions stymie human experts and CAD systems alike when they rely solely on ultrasound data, advances in computer-aided diagnosis promise to help reduce the need for biopsies and other procedures that are frequently both expensive and traumatic. (K. Horsch, M. L. Giger, L. A. Venta, C. J. Vyborny, Medical Physics, February 2002) |
| January 16, 2002 QUANTUM GRAVITATIONAL STATES have been observed for the first time. An experiment with ultracold neutrons shows that their vertical motion in Earth's gravitational field come in discrete sizes. Quantum properties such as the quantization of energies, wavelike dynamics including interference, and an irreducible uncertainty in the simultaneous measurement of position and momentum usually emerge only at the atomic level or under special circumstances (e.g., low temperatures) wherein a particle is trapped in a potential well by a controlling force. Observing such properties in phenomena governed by the electromagnetic or the weak and strong nuclear forces is common enough, but the strength of gravity, many orders of magnitude weaker than the other forces, has not previously been strong enough to enforce the kind of confinement needed to make quantum reality manifest. Such an effect has now been seen. Physicists at the Institute Laue-Langevin reactor in Grenoble, France employ a beam of ultracold neutrons. Moving at a pace of 8 m/sec (compared to 300 m/sec for an oxygen molecule at room temperature), the neutrons are sent on a gently parabolic trajectory through a baffle and onto a horizontal plate. Because the neutrons bounce at such a grazing angle, the plate is essentially a mirror for the neutrons, which are reflected back upwards until gravity saps their ascent; then the neutrons start falling again, eventually to be captured by a detector. In effect the neutrons are caught in a vertical potential well: gravity pulls down, while atoms in the surface of the mirror push up. The researchers report seeing a minimum (quantum) energy of 1.4 picoelectron volts (1.4 x 10^-12 eV), which corresponds to a vertical velocity of 1.7 cm/sec. A comparison of this energy level to the minimum energy for an electron trapped inside a hydrogen atom, -13.6 eV, demonstrates why this kind of detection has not been made before. The experiment provides also preliminary evidence for higher quantized motion states as well. In the horizontal direction there is no confinement and therefore no quantum effect. (By the way, neutron-interferometry experiments, in which neutron waves are split apart, moved around separate paths, and then brought back together in order to produce an interference pattern, have been influenced by gravity, but these neutron waves were not quantum states owing to the gravitational field. By contrast, the Laue-Langevin experiment is the first to observe quantum states of matter (neutrons) in Earth's gravitational field.) The next step is to use a more intense beam and an enclosure mirrored on all sides (the energy resolution improves the longer the neutrons spend in the device). An energy resolution as sharp as 10^-18 eV is expected, which would allow one to test such basic propositions as the equivalence principle, according to which the neutron's gravitational mass (as measured by its free fall in gravity) is the same as its inertial mass (as prescribed by Newton's second law, F=ma, where F is a generic force and a the acceleration imparted). (Nesvizhevsky et al., Nature, 17 Jan 2002.) LOOKING AT EXTRASOLAR PLANETS BY DIRECT OBSERVATION will be possible soon, says UC Berkeley astronomer Ray Jayawardhana. Because a star is so much brighter than any planet (viewed from outside our solar system, Jupiter would be only one billionth as bright as the sun), the presence of extrasolar worlds around distant stars has so far been inferred only indirectly, by the slight distortion imparted to the star's spectrum. But with new adaptive optics technology---which, with computer- controlled flexing of secondary mirrored surfaces, can partly undo the fuzzy distortions of incoming light introduced by atmospheric air currents overhead attached to the largest optical telescopes, such as the 8.1-m-diameter Gemini North and the 10-m Keck telescopes, the prospect of gaining the needed clarity for seeing planets has improved greatly. At last week's meeting of the American Astronomical Society in Washington, DC, Jayawardhana reported an example of the new, sharper viewing: a picture taken with Gemini showing not yet a planet exactly but a planet in the making near the star MBM12, some 900 light years away. This protoplanetary disk ( http://www.noao.edu/outreach/press/pr02/pr0201.html ) is the first such disk imaged for a four-star system and the first edge-on disk discovered with the help of adaptive optics. Furthermore, this star is still quite young and the disk itself only an estimated 2 million years along on its planet-building mission. It is young star systems like this that offer hope of seeing planets directly since the star-to- planet brightness ratio might be only as little as 100,000. With the higher angular resolution available (80 milli-arcseconds for the case of this disk, which lies at a distance of only 150 AU from the star) from adaptive optics coupled with large ground-based telescopes Jayawardhana believes planets, and not just disks, can be spotted in the next few years. Indeed he referred to some planetary candidates already glimpsed but not yet subjected to the full battery of tests needed for planetary designation such as observing the planet candidate co-move with its star and recording a spectrum consonant with planets (methane, water, etc.). |
| December 21, 2001 A TINY MICROPHONE DIAPHRAGM BASED ON FLY EARS has been built by researchers (Ronald Miles, Binghamton University, 607-777-4038, miles@binghamton.edu), offering such possibilities as compact hearing aids that respond only to sound in front of the wearer. The diaphragm is the part of a microphone that vibrates in response to incoming sound waves; other components then convert the diaphragm's vibrations into electrical signals which can then be amplified or recorded. The researchers based their novel diaphragm on Ormia orchracea, a small parasitic fly that uses sound to track down its cricket host even in complete darkness. The fly can detect changes as small as two degrees in the direction of an incoming sound, as good as humans. This is remarkable since the fly's ears are just a couple hundred microns apart. Mammals, on the other hand, rely on the fact that their ears are well separated from one another, so that sound can arrive at each ear at sufficiently different times and with sufficiently different intensities. What's even more remarkable about the fly is that its hearing organs, a pair of rectangle-shaped membranes, are connected to each other. Specifically, they are "torsionally coupled" so that a sound wave that lands on one membrane can deflect the other membrane. The connection between the membranes enables them to vibrate in several different ways so that the fly can obtain both the average pressure of an incoming sound and its pressure gradient, the change in sound pressure as you move from one ear to the other. This provides lots of information with which to determine the direction of the sound. The researchers built a silicon nitride prototype microphone diaphragm that closely reproduces the characteristics of the fly ears. While the researchers face challenges in mass-producing such a design, they hope that its unconventional approach to localizing sound will inspire lots of applications. (Paper 2aEA1 at Acoustical Society of America meeting in Ft. Lauderdale, 3-7 Dec 2001.) A QUANTUM COMPUTER HAS FACTORED THE NUMBER 15. This may sound like a trivial achievement, but it is actually a considerable physics milestone. It represents the most complex calculation yet performed in quantum computing, which offers a radically different means of information processing through the use of quantum mechanics. Even more noteworthy, it is the first experimental demonstration of Shor's algorithm, a quantum- computer program which can potentially factor large numbers in a fraction of the time needed for the world's currently fastest supercomputers. Such large numbers are used as the basis of encryption codes; the codes are broken by finding the prime- number factors of the large numbers. IBM-Almaden and Stanford University researchers (Isaac Chuang, now at MIT, ichuang@cba.mit.edu) built a quantum computer whose working substance was a liquid consisting of a billion billion molecules. The molecules were specially designed to contain 7 nuclear "spins"--5 from fluorine nuclei and 2 from carbon-13 nuclei. Analogous to a bar magnet which could point north or south, each spin could represent the binary digits "0" or "1" (or both 0 and 1 at the same time through the subtleties of quantum mechanics) and could be controlled by magnetic fields and radio waves (i.e., nuclear magnetic resonance techniques). By manipulating the 7 qubits, the computer could take advantage of quantum computing's unique parallel processing capabilities to determine that the factors of 15 were 3 and 5. Enormous challenges must be surmounted to build larger-scale quantum computers which could factor very large numbers, and this is an early step forward. (Vandersypen et al., Nature, 20/27 December 2001; also see IBM-Almaden news release at http://www.research.ibm.com/resources/news/20011219_quantum. shtml ) |
