The American Institute of Physics 3 страница

HIGH PROTON POLARIZATION, up to 32%, has been achieved at liquid-nitrogen temperatures (77 K) and with modest 0.3-Tesla magnetic fields in a experiment at Kyoto University in Japan. Among a proton’s attributes is the orientation of its intrinsic spin; this directionality can come into play when the proton interacts with the spins of other particles or with a radio frequency field. For comparison, proton polarization levels in MRI medical imaging is a paltry .0003% (still good enough for spotting tumors) using room temperature and magnetic fields typically of 1 Tesla (10,000 gauss). Targets for particle physics using accelerators can achieve polarizations of up to about 70% but even higher fields (2 or 5 T) are needed as well as low liquid-helium temperatures (typically 0.3 K). In the Kyoto experiment, the electrons in pentacene (an aromatic organic molecule chain) are polarized optically with a laser beam. Next, microwaves force the polarization to be transferred to protons in the molecules. The researchers suspect that their approach will find applications in particle physics (where targets polarized in smaller fields and warmer temperatures would permit the detection of slower charged particles amid high intensity beams) and in chemistry / biology (where the new method provides higher sensitivity than the existing NMR). Polarized protons would be portable in a small box for more than 3 hours at almost zero magnetic field. The new polarization method should also benefit MRI imaging (where high polarization can improve spatial resolution of pictures), the task of transferring spin to normally-hard-to-detect C-13 atoms, and NMR-based quantum computing (wherein information storage and processing are vested in spins). The Kyoto physicists, through various improvements, hope to extend their method to room temperatures. (Iinuma et al., Physical Review Letters, 3 January 2000; Select Article; figure at www.aip.org/physnews/graphics)

COSMIC RAYS OBSERVED BY GRAVITY-WAVE DETECTOR. The NAUTILUS detector at the Frascati Laboratory in Italy consists of a 2300-kg aluminum cylinder cooled to a temperature of 0.1 K. The plan is that a passing gravitational wave (broadcast, say, by the collision of two neutron stars) would excite a noticeable vibration in the cylinder. NAUTILUS has not yet recorded any gravitational waves, but scientists have now witnessed the cylinder vibrated by energetic particle showers initiated when cosmic rays strike the atmosphere. The signal generated by the rays is believable because conventional cosmic-ray detectors surrounding the bar also lit up when they were struck by the particles. In effect the detector is able to discern a mechanical vibration as small as 10ˆ-18 meters, corresponding to an energy deposit as small as 10ˆ-6 eV.

NEUTRONS HAVE BEEN CAPTURED AND STORED IN A MAGNETIC TRAP, a development which should lead to a better estimation of the neutron’s lifetime and in turn a better understanding of the weak nuclear force. Neutral atoms have been confined in magnetic traps before (even uncharged atoms can have a magnetic moment which can be influenced by a strong magnetic field), but neutrons are more difficult to deal with in the same way since their intrinsic magnetic moment is so much weaker. Now a collaboration of scientists from Harvard, NIST, Los Alamos National Laboratory, and the Hahn-Meitner Institute (Berlin) has succeeded in trapping neutrons in a magnetic bottle, thereby restricting neutron movement in all three dimensions (a decade ago, neutrons were magnetically trapped in a storage ring, but this confined neutron motion in only two dimensions). To bring about 3D trapping, a beam of already cold (11 K) neutrons from a reactor was directed into a trapping vessel surrounded by magnetic coils and filled with liquid helium at a temperature of less than 250 mK. The helium acts as a coolant, slowing the neutrons, and as a scintillator for recording the subsequent decay of neutrons into a proton, positron, and anti-neutrino.

The neutron lifetime measured in this experiment was 750 seconds, with an uncertainty of +300 and -200 seconds. The researchers hope to push their method to an accuracy of a part in 10ˆ5, which would exceed the accuracy of the currently accepted best value for the neutron lifetime, 886.7 (+/-1.9) seconds. (P.R. Huffman et al., Nature, 6 January 2000.)

TWO-ELECTRON PRISON BREAK. New experiments studying the cooperation among electrons undergoing ionization show that electrons do not act alone when intense light liberates two of them at once from helium and other rare-gas atoms. When an intense light pulse removes more than one electron from an atom, it’s simplest to assume that electrons respond to the light independently of their brethren and leave one by one.

However, this “independent electron model” fails by many orders of magnitude in predicting double-ionization rates of atoms. Using the COLTRIMS “momentum microscope” for atoms and molecules, two multi-institutional experiments in Germany at the University of Marburg and the Max Born Institute in Berlin have measured the complete 3D momentum values for singly and multiply ionized helium and neon. If the electrons had acted independently, and left one by one in two successive steps, then the momentum data for double ionization would look like single ionization occurring twice. But the data show otherwise, leaving only the possibility of coordinated behavior. Going further, the authors of the neon observations suggest that their data support a cooperative-behavior scenario known as “rescattering”: the laser pulse’s oscillating electric field first removes one electron, then pushes electron and ion back together, and finally the electron knocks out one of its comrades. These experiments can begin to test the extensive theoretical models of strongly interacting electrons in intense light fields.

SCANNING GATE MICROSCOPY. Scanned probe microscope not only provide images of surface atoms, they also allow one to move atoms and to study the spectroscopy (the quantum energy levels) of those surface atoms (or molecules or metallic clusters). Concerning the latter, physicists at the Delft University of Technology (in the Netherlands) can better assay the energy levels of target particles at a surface positioning a second probe right next to the main probe in a standard scanning tunneling microscope (STM) setup, giving it a tong-like appearance. The second probe acts much like a gate in a transistor: by shifting energy levels of the target particle, it allows or disallows the passage of the tunneling current. In the reported experiment, the so-called Coulomb blockade (the difficulty of yet another electron to join many other electrons already on a tiny electrode) for single-electron tunneling in a 20 nm gold cluster was controlled using the gate electrode.

THE X-RAY BACKGROUND, the glow of x rays seen in all directions in space, has now largely been resolved into emissions from discrete sources by the Chandra X-Ray Telescope, ending the notion that the x rays come from distant hot gas. Previously only about 20-30% of the x-ray background had been ascribed to point sources (by such telescopes as ASCA). Chandra was launched in July 1999 and put in an elliptical orbit.

With its high angular resolution and acute sensitivity, it could tell apart x-ray objects (many of them thought to be accretion disks around black holes) that before had been blurred into a continuous x-ray curtain. (Of course, now that the background has been resolved into points it ceases to be a background.) Richard Mushotzky of Goddard Space Flight Center reported these Chandra results at last week’s meeting in Atlanta of the American Astronomical Society (AAS). Resolving the x-ray background was not all. Mushotzky added that the Chandra survey had revealed the existence of two categories of energetic galaxies that had been imaged only poorly or not at all by optical telescopes. He referred to one category as “veiled galactic nuclei,” objects (with a redshift of about 1) bright in x-rays but obscured by dust at optical wavelengths. The other category was “ultrafaint galaxies.” One interpretation of these galaxies is that optical emission is suppressed owing to absorption over what could be a very long pathway to Earth. Mushotzky speculated that such high redshift (z-greater than 5) galaxies could be the most distant, and hence earliest, objects ever identified. The XMM x-ray telescope, just launched, should provide complementary information in the form of high-precision spectra (from which redshifts are derived) of the distant objects.

OTHER CHANDRA RESULTS at the meeting included the mapping of a thousand x-ray stars in the Orion Nebula portion of our galaxy 1500 light years away, making this the highest density of x-ray sources yet recorded.

Gordon Garmire of Penn State spoke about this finding as well as about the effort to find x-ray counterparts for objects cataloged in the Hubble Deep Field image made with visible light; some tentative matches were made. Meanwhile, Frederick Baganoff of MIT reported that Chandra’s inspection of the center of the Milky Way revealed what might be the first recorded x-ray signal from the vicinity of the massive (2 million solar mass) black hole residing at or near the radio-bright object called Sagittarius A*. In X rays, this object proved to be fainter than expected by a factor of 5. The supermassive black hole at the heart of our sister spiral galaxy, Andromeda, also is much cooler than expected. According to Stephan Murray from Harvard-Smithsonian, the measured temperature was only a few million K, compared to temperatures of tens of millions for much more modest x-ray stars in the same galaxy. None of this fits with theories of supermassive black holes. Finally, Claude Canizares of MIT summarized Chandra observations of supernova remnant EO102-72, located in the Small Magellanic Cloud. EO102-72 is the leftover from an explosion 1000 years ago of a huge star of 15-20 solar masses. A diffraction grating on the telescope was used to spread out incoming x-rays into a spectrum, which could be scanned for the presence of specific elements in the stellar debris. Canizares estimated that as much as 10 solar masses’ worth of oxygen was present in the wreckage of the older star, enough to furnish thousands of solar systems like ours with the breathable element needed for much of life on Earth.

SOLITARY, WANDERING BALCK HOLES, unheralded by any bright accretion disk or rapidly orbiting stars or gas, have been detected through the process of gravitational microlensing. The Massive Compact Halo Object (MACHO) collaboration regularly views millions of stars in the direction of the dense bulge of our galaxy hoping to observe, every now and then, stars brightening courtesy of the lensing caused by the passage of some nonluminous object (hovering in the galaxy’s halo) between us and the star. The brightening can last as short as two days or as long as 1000. Longer duration suggest either large or very slow lensing objects.

David Bennett of Notre Dame reported at the ASS meeting on two such long-duration events in which the mass of the lens was calculated to be roughly 6 solar masses, too heavy to be a neutron star and more likely to be a black hole. Bennett speculates that the lone-wolf black holes form from supernova collapse and might be as common as neutron stars in the galaxy.

OPTICAL BLACK HOLES, objects that attract and trap specific colors of light, can be made in earthly laboratories; two researchers have shown theoretically, offering possibilities for lab-based analogs of general relativity and potentially even quantum gravity phenomena. According to researchers at the Royal Institute of Technology in Sweden and at the University of St Andrews in Scotland, the trick is to create a vortex of fluid that whirls at velocities comparable to the speed of light inside the fluid. Such a feat is now possible, with the advent of techniques for slowing down light to just a few meters per second through such substances as a Bose-Einstein condensate or a rubidium gas (Phys. Rev. Focus, 29 June 1999). If a sufficiently fast-spinning vortex of these or similar materials could be created, light inside the fluid could lose maneuverability and become trapped in the vortex. Since light in an optical black hole would behave analogously to matter in a real black hole, these light-trapping whirlpools would permit laboratory study of Hawking radiation, the hypothetical emissions from evaporating black holes; this radiation, which consists of particles made near the hole’s boundary, is next-to-impossible to observe directly since it is obscured by the cosmic microwave background. In addition, the researchers speculate that studying quanta of light interacting with the quantum-mechanical matter waves in BECs could even help establish “a testable prototype model of quantum gravity.” In the meantime, physicists are also pursuing the idea of creating “acoustical black holes” (dumb holes), regions that capture and trap sound waves. (Leonhardt and Piwincki, Physical Review Letters, 31 January 2000; Physical Review A, December 1999; Select Articles)

“THE FORMATION AND EVOLUTION OF GALAXIES are intimately connected to the presence of a central massive black hole,” asserts Douglas Richstone of the University of Michigan. Richstone was at the recent meeting of the American Astronomical Society in Atlanta to report the new identification of supermassive black holes at the cores of three nearby elliptical galaxies, adding to an already substantial association between galaxies possessing centralized, high-density spheroidal clumps or bulges of stars and nearby heavy black holes (star concentration correlating closely with black hole mass). Richstone pointed to the growing consensus that these massive black holes are the remnants of quasars (a notion underscored at the meeting by the report given by Andrew Wilson of the University of Maryland – of many “dying quasars” in nearby galaxies, objects whose radio spectra resemble a quieter version of quasar spectra) and to the historical fact that the age of quasar formation occurred before the time when most stars were forming in galaxies (to judge from high redshift observations). Richstone concluded that “Radiation and high-energy particles released by the formation and growth of black holes are the dominant sources of heat and kinetic energy for star-forming gas in protogalaxies.”

SNOW SCREENING ON WATER. With its ability to create muffled winter landscapes, snow is usually associated with quiet. When the white stuff falls on a body of water, one would expect it to be just as silent, since it doesn’t make much of an impact. But as researchers have discovered, it unexpectedly creates high-pitched screeching sounds that can sometimes disrupt underwater sonar experiments. Investigating these sounds, which last for roughly a ten-thousandth of a second, Larry Crum of the University of Washington and his colleagues implicate air bubbles as the source of snowflake noise. According to their explanation, the snowflake’s presence on a water surface creates capillary action (the attraction between a liquid and solid surface), causing water to rush upwards. The upward flow of water either generates air bubbles on its own, or unleashes air bubbles in the snowflake as it melts. The bubbles oscillate as they reach equilibrium with their environment, creating sound waves of up to 200 kilohertz – out of the range of human hearing (which stops at 20 kHz) but potentially audible to dolphins. Researchers have been known to shut down sonar surveys of salmon population during snowfall because of these sounds. (Select Article, Journal of the Acoustical Society of America, October 1999; see also New Scientist, 25 December 1999.)

A NEW FORM OF NUCLEAR MATTER has been detected at the CERN lab in Geneva. Results from seven different experiments, conducted at CERN over a span of several years, were announced at a series of seminars today. In the experiments a high energy beam of lead ions (160 GeV/nucleon, times 208 nucleons, for a total energy of about 33TeV) smashes into fixed targets of lead or gold atoms. The center-of-mass energy of these collisions, the true energy available for producing new matter, is about 3.5 TeV.

From the debris that flies out of the smashups, the CERN scientists estimate that the “temperature” of the ensuing nuclear fireball might have been as high as 240 MeV (under these extreme conditions energy units are substituted for degrees Kelvin), well above the temperature where new nuclear effects are expected to occur. In the CERN collisions the effective, momentary, nuclear matter density was calculated to be 20 times normal nuclear density. It is not quite certain whether the novel nuclear state is some kind of denser arrangement of known nuclear matter or a manifestation of the much-sought quark-gluon plasma (QGP), in which quarks, and the gluons which normally bind the quarks into clumps of two quarks (mesons) or three quarks (baryons), spill together in a seething soup analogous to the condition of ionized atoms in a plasma. Such nuclear plasma might have existed in the very early universe only microseconds after the big bang. Evidence for the transition form a hadron phase (baryons and mesons) into a QGP phase was expected to consist of (1) an enhanced production of strange mesons, (2) a decrease in the production of heavy psi-mesons (each consisting of a charm and anticharm quarks), and (3) an increase in the creation of energetic photons and lepton-antilepton pairs. Just this sort of (indirect) evidence (at least of types 1 and 2) has now turned up in the CERN data. (CERN press release, www.cern.ch) To demonstrate the existence of QGP more directly, one would like the plasma state to last longer, and one should observe the sorts of particle jets and gamma rays that come with still higher –energy fireballs. That energy (about 40 TeV, center-of-mass) will be available in the next few months at the Relativistic Heavy Ion Collider undergoing final preparations at Brookhaven.

D-WAVE SQUID. The working fluid of superconductors consists of pairs of electrons (or pairs of the holes left behind in a crystal when an electron moves somewhere else). These Cooper pairs form a coherent state with specific symmetry properties. For example, in most low temperature superconductors, the pairs are fairly isotropic; if you imagine one electron at the origin of some coordinate system, the likelihood of finding a second electron is pretty much the same in all directions. Thus, the Cooper pair is essentially spherical and the pair is said to possess “s-wave” symmetry. In high-temperature superconductors, the symmetry is thought to resemble a four-leave clover, referred to as a “d-wave”. A fundamental consequence of the d-wave symmetry is a phase-change of pi between neighboring lobes of the clover in the quantum wave function describing the Cooper pair. All of this can be important in the design of superconducting quantum interference devices, SQUIDs, which consist of a superconducting loop interrupted in two places by thin insulating junctions, through which the Cooper pairs must tunnel. SQUIDs are highly sensitive to applied magnetic fields and are used in a variety of magnetometer applications (in biology, geology, new materials research, etc.). Furthermore, SQUIDs form the building blocks of superconducting electronics. A group at Augsburg University in Germany has developed a SQUID that exploits the special nature of the d-wave symmetry of the high-Tc superconductors. Using specially prepared tetracrystalline crystals as substances, they devised and built a SQUID in which the symmetry properties give rise to a pi phase-change over one of the two junctions. For this reason, the Augsburg researchers call their device a pi-SQUID. The pi-SQUID is a realization of the recently proposed complementary Josephson electronics and its operation provides strong evidence for the d-wave symmetry in the high-Tc superconductors. Such devices present a novel approach for the fabrication of quantum computers. (Schulz et al., Applied Physics Letters, 7 Feb; Select Article.)

A MOLECULA BOSE EINSTEIN CONDENSATE (BEC) has been made at the University of Texas, where physicists first create a condensate of rubidium atoms in a trap. Then diatomic molecules (dimmers) are formed by getting two nearby Rb atoms to first absorb a photon together and then to emit a second photon. This photo-association process leaves the Rb2 dimer essentially at reast, with an equivalent temperature of about 100 nK; “…perhaps the coldest molecules in the universe,” says Paul Julienne of NIST (Science News, 12 Feb2000). The stillness makes possible high precision spectroscopy of the molecules, which constitute about 1% of the condensate. The dimmers hold together typically less than one millisecond. (Wynar et al., Science, 11 Feb 2000.)

QUANTUM MIRAGE. The scanning tunneling microscope (STM) allows one both to push individual atoms around on a surface and to image them. Especially intriguing are images of “quantum corrals,” circular or elliptical arrangements on a surface inside of which the waves corresponding to electrons near the substrate surface can be revealed. The latest entry in the gallery of fine pictures comes from IBM, where physicists placed 36 cobalt atoms in an elliptical “Stonehenge” pattern on a copper surface. An extra magnetic cobalt atom was placed at one of the two foci of the ellipse, causing visible interactions with the surface electron waves. But the waves seem also to be interacting with a phantom cobalt atom at the other focus, an atom that is not really there.

FIRST SPACECRAFT IN ORBIT AROUND AN ASTEROID. The Near Earth Asteroid Rendevous (NEAR) spacecraft has arrived at, and gone into orbit around, asteroid Eros, which was at a distance of 160 million miles from Earth when the rendezvous occurred. The asteroid, whose gravity is about one thousandth that of Earth, might represent a chunk of matter not much altered from the time the solar system was formed 4 billion years ago, and so it is of great interest to planetary scientists.

(NASA press conference, 17 Feb; http://near.jhuapl.edu/iod/20000215/index.html)

ATTOSECOND LIGHT PULSES. A curtailed wave pulse can be represented mathematically as the weighted sum of a number of wavetrains of various wavelengths. In this way, scientists at the Foundation for Research and Technology-Hellas (FORTH) in Crete have created light pulses less than a femtoseocnd (10ˆ-15 seconds) in duration (Papadogiannis et al., Physical Review Letters, 22 November 1999). First they split a beam of light (wavelength of 800 nm) into two parts; each of these, when sent through an argon vapor, produces sets of higher-harmonic wavetrains (at wavelengths equal to several fractions of the original 800 nm) which add together in a synchronized way to form the ultrashort wave pulse with a duration estimated to be less than 100 attoseconds. Before this, the record short pulse was 4.5 fs in duration. (Physics World Feb 2000.)

UNEMPLOYMENT LEVELS WERE ONLY 2 PERCENT FOR U.S. PHYSICS PHDS receiving their degrees in 1997 and seeking employment in the winter after their graduation, dropping from a recent high of 6% for the class of 1993, according to a new report from the American Institute of Physics. However, most Ph.D.s in permanent positions stated that they were working in an area that was not primarily physics, although this does not mean that their jobs involved little or no physics. Perhaps surprisingly in the post-Cold War era, bachelor recipients from the class of 1998 appear to be, if anything, exceedingly hopeful about their long-range career goals: for example, 61% said they intended to become a college or university professor, but this is far higher then the percentage historically attaining this goal. Most employed master’s degree recipients from the class of 1997 (62%) work in industry, with three-fourths viewing their job as being related to physics. After many years of decreasing steadily, the number of students earning physics bachelor’s degrees has stabilized at least for the time being, with a total of 3,821 granted in the 1997-98 academic year. (Report available at www.aip.org/statistics, the AIP Education and Employment Statistical Division.)

ULTRAVIOLET LASER AT DESY. A free electron laser (FEL) built at the DESY lab in Hamburg by the international TESLA collaboration has achieved a beam of radiation with a wavelength of only 93 nm. FELs normally operate in the following way : a beam of energetic electrons passes through a series of S curves (an undulator) where they are made to radiate light which is stored inside a mirrored cavity. The photons, reflecting back and forth in the cavity, help to stimulate the electrons to radiate even more, thus amplifying the higher-energy light beam. The resultant light is tunable and coherent. At wavelengths below about 150 nm, however, mirrors are not effective and light accumulation cannot occur. Scientists of the TESLA collaborations have now succeeded at DESY in carrying out a scheme suggested 20 years ago: give up the accumulation of light in an optical cavity and let the radiation amplify itself in a single pass as the electrons travel through a very long undulator section, thereby increasingly interacting with the radiation. The product is essentially coherent synchrotron radiation.

The TESLA collaboration consists of 38 institutes from 9 countries. Major hardware contributions came from DESY, Italy, France and the USA (US institutes: ANL, Cornell, Fermilab, UCLA). The work with the UV laser is part of an effort to produce an x-ray laser with 6-nm light (by the year 2003). And beam-optics lessons learned might in turn contribute to a more ambitious plan to develop a next-generation linear 500-GeV electron linear collider with integrated x-ray lasers called TESLA. (Joerg Rossbach, [email protected]; www.desy.de/pr-infor/News; figure at www.aip.org/physnews/graphics)

SNOWBALLS SURVIVE IN HELLISH CONDITIONS. Many of the unique and unusual properties of liquid water at ambient conditions are due to the ability of water molecules to form hydrogen bonds, which in turn causes the oxygen atoms to be arranged in a three-dimensional diamond-like network. However, under extreme pressures the properties of water can change drastically. For example, although water ice normally melts at 0 C at ambient conditions, at a pressure of 10 Giga-pascals (10,000 atm) water remains “frozen” up to 320 C! New computer simulations carried out at the Lawrence Livermore National Laboratory (Eric Schwgler, 925-424-3098, [email protected]) have explored what happens to the microscopic structure of the compressed liquid, in a region of the phase diagram where experimentally determined structural data do not exist. These simulations indicate that when the liquid is squeezed up to a pressure of 10 GPa, the hydrogen bonds and oxygen network are substantially altered. At this high pressure, each water molecule is close packed and surrounded by 12.9 molecules, as opposed to 4.5 neighbors for ambient conditions.

(E.Schwegler, G.Galli, F.Gygi, Phys. Rev.Lett., 13 March 2000; figure at www.aip.org/physnews/graphics. Select Article.)

MAXIMALLY RANDOM JAMMING. Packing particles into a container has been important since antiquity, when basketfuls of grain were traded or collected as taxation. Packing applies not just to grains of wheat of course, but also to ball bearings, living cells, a variety of granular media, and the placement of atoms and molecules in solids and liquids. Hence packing has become a science, and the maximum fraction of space that can be filled with spheres is a conjectured 74%. This is for an ordered “face-centered cubic” array that looks like a stack of cannonballs or oranges. (Kepler came very close to the 74% figure four centuries ago.) The mathematics for estimating the maximum filling fraction for an array of disordered, or randomly packed, balls is much more slippery. Salvatore Torquato and his colleagues at Princeton consider that the whole problem of random close packing (RCP) is ill posed and have proposed in its place a new concept which they call maximally random jamming, a precisely defined condition in which spheres are deployed in the most disordered way. Computer simulations show that eh packing fraction for the maximally jammed state is about 64%.

Torquato ([email protected], 609-258-3341) believes that he new model will help to study randomness in many-body systems in general.

(Torquato, Truskett, Debenedetti, Physical Review Letters, 6 March; see figure at www.aip.org/physnews/graphics. Select Article.)

DARK MATTER UPDATE. At the dark matter detection meeting in Marina del Rey, California last week (Update 437) a group from Gran Sasso, Italy reported detecting evidence for dark matter particles. The Cryogenic Dark Matter Search collaboration (10 US institutions), using a different detection scheme, reported finding no evidence for such particles, and asserted that their results were incompatible with the Gran Sasso finding. (Stanford press release, 2/24. See preprint at http://arXiv.org/abs/astro-ph/?0002471.)