PHYSICS NEWS

The American Institute of Physics Bulletin of Physics News

DIGITAL ENTROPY. How much information does it take to control something? By combining thermodynamics with information theory, MIT researchers have determined the minimum amount of information one needs to bring an unruly object under control, providing quantitative answers to such subjects as taming chaos. From the perspective of thermodynamics, controlling an object means reducing its disorder, or entropy. Lowering the disorder of a hot gas, for example, decreases the number of possible microscopic arrangements in the gas. This in turn removes some of the uncertainty from the gas's detailed properties. According to information theory, this reduced uncertainty is tantamount to increased information about the gas. Applying this "digital entropy" perspective to the notion of control, the researchers found that controlling an object becomes possible when one acquires enough information about it (and then applies this information to the object) to keep the uncertainties in its properties at manageable levels. Chaotic systems are particularly hard to control because they constantly manifest new amounts of uncertainty in their properties. Perhaps there is no better everyday example of chaos than steering a car: a tiny change in steering can quickly be amplified into a huge change in course. For example, if a blindfolded driver initially knows that her car is within two feet from a curb, tiny fluctuations in steering can make this uncertainty 4 feet after one second, 8 feet after two seconds, and so on. Only if the driver receives second-by-second instructions for adjusting the steering to keep the uncertainty down to the two-feet level does she have any hope of controlling it. If the driver makes such steering adjustments only half as frequently, her car will go out of control (crash into the curb) but it will take exactly twice the amount of time than if no adjustments were made.

THE MOST PROTON-RICH NUCLEUS, nickel-48, has been produced for the first time at the GANIL accelerator in France, where beams of nickel-58 atoms are smashed into a target. (Nickel is conspicuous for the range of its isotope varieties: Ni-78, in contrast to Ni-48, is one of the most neutron-rich of nuclei.) Ni-48 has been of special interest to physicists since it is a "doubly magic" nucleus. A nucleus is exalted as being "magic" if the neutrons or protons exactly fill up one of those shells (analogous to the electron shells in atom) that nature decrees as the model for stability. It was not easy making the Ni-48. Producing just four Ni-48 nuclei required more than 10^17 incoming Ni-58 atoms. The likelihood for creating Ni-48 in this collision process is expressed as a "cross section" of only 50 "femtobarns," the smallest cross section ever measured in nuclear physics. Nevertheless, the apparent lifetime of the Ni-48 nuclei, about half a microsecond, gives the researchers hope that they can look for signs of a never-before-seen form of radioactivity, di-proton decay. That is, with a larger sample, the GANIL scientists believe they might observe one of the Ni-48 nuclei spitting out a two-proton parcel.

GUIDING NEUTRAL ATOMS AROUND CURVES can be performed with tiny current-carrying wires which deflect the atoms through a lithographically patterned "atom waveguide.' Physicists at the University of Colorado and from NIST-Boulder send laser-cooled (42 micro-Kelvin) atoms into a 10-cm guide where they undergo three curves (with a 15-cm radius of curvature). Three million atoms per second can be sent through the course; at the far end, the atoms are ionized and then counted. A possible use for the new waveguide, part of a growing toolbox of atom optics components, will be in atom interferometry and other forms of high-precision metrology. The researchers hope to send atoms (or should we say atom?) from a Bose-Einstein condensate into the waveguide.

SANDSTONE TORTUOSITY. In conventional nuclear magnetic resonance (NMR) imaging, a liquid is the working substance. For example, the hydrogen nuclei in watery living tissue are weakly oriented by a powerful magnet, and then these nuclei signal their positions by emitting radio waves. By contrast, gas-phase NMR imaging has been difficult because of the low density of gases, which yields only a weak NMR signal. Recently, however, practical NMR imaging has been realized for noble-gas atoms by strongly orienting the nuclei (with polarized laser light) outside the sample and then injecting them into, say, the lungs, where they rapidly diffuse into the deepest of alleyways, providing data that can't be collected in any other way. In a new extension of gas-phase NMR to the study of porous materials such as oil-bearing sandstone and carbonate rocks, the aim right now is not so much to provide images (the rapid diffusion of the gas atoms limits the spatial resolution, as one would expect for a moving target, to about one millimeter) as it is to characterize internal topology. Ronald Walsworth and his colleagues at the Harvard-Smithsonian Center for Astrophysics and Schlumberger-Doll inject xenon atoms into various porous rock samples filled with countless pores and connections, which affect the rate of gas diffusion and flow in the porous solid. They determine such things as the pore surface-area-to-volume ratio and a property called 'tortuosity,' which is an indication of how the structure of the porous medium restricts the flow of gases or liquids through the material. In this sense, tortuosity is to fluid flow what the structure of a wire (cross-section, length, etc.) is to the flow of electricity. Noble gases may be handier to use than liquids in NMR studies of rocks and other porous materials since the gas can flow further and faster through the pores without losing its orientation.

WAVY MICROSTRUCTURES, induced to grow in a polymer surface by a stressful puckering process, might be useful as a diffraction grating or as a part of various microelectromechanical systems (MEMS). George Whitesides, Ned Bowden, and their colleagues at Harvard begin by heating a film of the elastic polymer material PDMS (polydimethylsiloxane) attached to a glass slide. The top coating of the film expands when heated, after which it is exposed to an oxygen plasma, which makes a silica-like crust. When the whole sample is cooled, the silica layer relieves the stress by puckering. The waves are locally ordered but will be rather disorderly on a global level unless an extra organizational rule can be imposed, in this case in the form of a bas-relief pattern on the PDMS surface. The resulting wavy structures can be made with wavelengths as small as half a micron. This might facilitate a variety of uses, such as being part of a detection system for microfluidic devices, as stamps for microcontact printing, as masks for photolithography, or as surfaces on which cells can be grown and oriented.

NEPTUNE DIAMONDS. The crushing conditions inside Neptune and Uranus are recreated at UC Berkeley, where a tiny sample of methane is squeezed in a diamond anvil press up to pressures of 30-50 GPa (more than 10 million atm) and heated with laser light to temperatures to 3000 K. Scattered x rays and infrared light indicate that some of the methane is being converted into 10-micron-sized diamonds and certain polymers at pressures much below what had been expected. This result might lead to some re-assessment of planetary interiors since a widespread dissociation of methane would release considerable energy, affecting the dynamics and evolution of the planet in a big way.

THE 1999 NOBEL PRIZE FOR PHYSICS goes to Gerardus 't Hooft of the University of Utrecht and Martinus Veltman, formerly of the University of Michigan and now retired, for their work toward deriving a unified framework for all the physical forces. Their efforts, part of a tradition going back to the nineteenth century, centers around the search for underlying similarities or symmetries among disparate phenomena, and the formulation of these relations in a complex but elegant mathematical language. A past example would be James Clerk Maxwell's demonstration that electricity and magnetism are two aspects of a single electro-magnetic force. Naturally, this unification enterprise has met with various obstacles along the way. In this century, quantum mechanics was combined with special relativity, resulting in quantum field theory. This theory successfully explained many phenomena, such as how particles could be created or annihilated or how unstable particles decay, but it also seemed to predict, nonsensically, that the likelihood for certain interactions could be infinitely large. Richard Feynman, along with Julian Schwinger and Sin-Itiro Tomonaga, tamed these infinities by redefining the mass and charge of the electron in a process called renormalization. Their theory, quantum electrodynamics (QED), is the most precise theory known, and it serves as a prototype for other gauge theories (theories which show how forces arise from underlying symmetries), such as the electroweak theory, which assimilates the electromagnetic and weak nuclear forces into a single model. However, the electroweak model too was vulnerable to infinities and physicists were worried that the theory would be useless. Then 't Hooft and Veltman overcame the difficulty (and the anxiety) through a renormalization comparable to Feynman's. To draw out the distinctiveness of Veltman's and Hooft's work further, one can say that they succeeded in renormalizing a non-Abelian gauge theory, whereas Feynman had renormalized an Abelian gauge theory (quantum electrodynamics). What does this mean? A mathematical function (such as the quantum field representing a particle's whereabouts) is invariant under a transformation (such as a shift in the phase of the field) if it remains the same after the transformation. One can consider the effect of two such transformations, A and B. An Abelian theory is one in which the effect of applying A and then B is the same as applying B first and then A. A non-Abelian theory is one in which the order for applying A and B does make a difference. Getting the non-Abelian electroweak model to work was a formidable theoretical problem. An essential ingredient in this scheme was the existence of another particle, the Higgs boson (named for Peter Higgs), whose role (in a behind-the-scenes capacity) is to confer mass upon many of the known particles. For example, interactions between the Higgs boson and the various force-carrying particles result in the W and Z bosons (carriers of the weak force) being massive (with masses of 80 and 91 GeV, respectively) but the photon (carrier of the electromagnetic force) remaining massless. With Veltman's and 't Hooft's theoretical machinery in hand, physicists could more reliably estimate the masses of the W and Z, as well as produce at least a crude guide as to the likely mass of the top quark. (Mass estimates for exotic particles are of billion-dollar importance if Congress, say, is trying to decide whether or not to build an accelerator designed to discover that particle.) Happily, the W, Z, and top quark were subsequently created and detected in high-energy collision experiments, and the Higgs boson is now itself an important quarry at places like Fermilab's Tevatron and CERN's Large Hadron Collider, under construction in Geneva.

THE 1999 NOBEL PRIZE IN CHEMISTRY goes to Ahmed H. Zewail of Caltech, for developing a technique that enables scientists to watch the extremely rapid middle stages of a chemical reaction. Relying on ultra-fast laser pulses, "femtosecond spectroscopy" can provide snapshots far faster than any camera--it can capture the motions of atoms within molecules in the time scale of femtoseconds (10^-15 s). An atom in a molecule typically performs a single vibration in just 10-100 femtoseconds, so this technique is fast enough to discern each and every step of any known chemical reaction. Shining pairs of femtosecond laser pulses on molecules (the first to initiate a reaction and the second to probe it) and studying what type of light they absorb yields information on the atoms' positions within the molecules at every step of a chemical reaction. With this technique, Zewail and his colleagues first studied (in the late 1980s) a 200-femtosecond disintegration of iodocyanide (ICN-->I+CN), observing the precise moment at which a chemical bond between iodine and carbon was about to break. Since then, femtochemistry has revealed a completely new class of intermediate chemical compounds that exist less than a trillionth of a second between the beginning and end of a reaction. It has also provided a way for controlling the courses of chemical reaction and developing desirable new materials for electronics. It has provided insights on the dissolving of liquids, corrosion, and catalysis on surfaces; and the molecular-level details of how chlorophyll molecules can efficiently convert sunlight into useable energy for plants during the process of photosynthesis.

EXTRA INVISIBLE DIMENSIONS are for particle physicists what they are for Star Trek captains: a device for covering a lot of ground quickly and explaining anomalous behavior. In physics the importation of extra dimensions into the standard theory helps to make peace between quantum mechanics and general relativity, but it doesn't explain the great disparity (the "hierarchy problem") between the temperature at which the weak and electromagnetic forces fuse together (10^2 GeV, expressed in energy units) and the temperature at which gravity joins up with the other forces (10^18 GeV), a temperature so hot, or an energy so high, that such conditions have not prevailed since a tiny moment after the big bang. Some theories contend that we are not aware of the extra dimensions because they extend only a very short distance, far smaller than the size of an atom. Yet, another way of playing with spacetime is to introduce a new dimension essentially infinite in extent but one in which gravitons, the carriers of gravity, would largely be locked up in localized regions, at least in the extra dimension. This exciting new idea, advanced by Lisa Randall of Princeton and Raman Sundrum, now at Stanford, has the effect of fusing gravity with the other known forces at the more reasonable energy of 10^3 GeV (rather than at 10^18 GeV), thus solving the hierarchy problem. One testable implication of the new hypothesis would be the existence of exotic new particles, which could be detectable at energies to be available in a few years at the Large Hadron Collider (LHC) under construction in Geneva.

WAVE PROPERTIES OF BUCKYBALLS have been observed in an experiment at the University of Vienna. Physical objects from quarks to planets have wavelike attributes. The quantum nature of a bowling bowl, unfortunately, is not manifest since its equivalent quantum (or de Broglie) wavelength is so tiny that interference effects (for example, the left part of the ball negating the right part of the ball) cannot be detected in a practical experiment. However, the wave properties of some composite entities, such as atoms and even small molecules, have previously been demonstrated. Now Anton Zeilinger at the University of Vienna has been able to perform the same feat for fullerenes, the largest objects (by a factor of ten) for which wavelike behavior has been seen. The researchers send a beam of the soccerball-shaped C-60 molecules (with velocities of around 200 m/sec) through a system of baffles and a grating (with slits 50 nm wide, 100 nm apart), which yields a striking interference pattern characteristic of quantum behavior. Ironically, the pattern indicating wave behavior is built up from an ensemble of individual sightings, each of which depends upon a buckyball's particle-like ability to make itself felt in an electrode. The interference is not negated thereby since it is not known by which path the C-60 came to be at the electrode.

STRIPED SUPERCONDUCTIVITY. In high-temperature ceramic superconductors, currents flow mostly in the plane. But if special dopants (such as neodymium) are added to La-Sr-Cu-O materials, the supercurrents seem to be further restricted to narrow lanes or stripes. In these materials, rows of charges are separated by insulating antiferromagnetic regions (in which neighboring atomic spins oppose each other), so they are referred to as charge-ordered or spin-ordered materials. Since the stripes occur preferentially at lower temperatures, physicists are not sure whether the stripes help or hurt superconductivity. Two new experiments (in which the superconductivity is turned off, the better to study underlying electronic properties) add some fresh perspective. A University of Tokyo team (Noda et al.) uses a strong magnetic field to produce a Hall effect, in which electrons should be pushed sideways by the field. A resistance to this effect is taken as evidence for a "self-organized" one-dimensional charge flow. Meanwhile a Stanford-LBL-Tokyo team (Zhou et al.) shoots UV photons into their samples and observes the ejected electrons that come flying out. The telltale photo-electron pattern maps back to charge flows in the sample that must have been organized into stripes.

GRAVITY'S GRAVITY. A new experiment at the University of Washington seeks to determine whether the gravitational binding energy of an object generates gravity of its own. As formulated by Albert Einstein, the Equivalence Principle (EP) states that if we stand in a closed room we cannot tell whether the weight we feel is the result of gravity pulling down or the force of a rocket carrying us forward through otherwise empty space. All of this gets complicated in some theories of gravity, which predict that the EP will be violated to a small degree since in addition to the usual gravity, carried from place to place by spin-two particles called gravitons, there should exist another, fainter kind of gravity carried by spin-zero particles (sometimes called dilatons). For this reason, and because recent observations of supernovas suggest that some repulsive gravitational effects might be at work in the cosmos, scientists want to explore the possibility of EP violations. Three decades of lunar laser ranging (bouncing light off reflectors placed on the Moon) show that the Moon and the Earth fall toward the Sun with the same acceleration to within half a part in a trillion (10^12). What the Washington physicists have done is focus attention on the subject of gravitational binding energy, or self-energy, and whether it too obeys the EP. To illustrate the concept of binding energy, consider that the mass of an alpha particle is actually about 28 MeV less than the sum of its constituents. This energy (about 7.6 parts in a thousand of the alpha mass) represents the energy (vested in the strong nuclear force) needed to hold two protons and two neutrons together inside the alpha. Gravity being very much weaker than the strong nuclear force, the gravitational binding energy, the self-energy of gravity attraction, is almost infinitesimal. For example, self-energy effectively reduces the mass energy of the Earth by a factor of only about 4.6 parts in 10^10. Is this tiny "mass" also subject to the EP? Supplementing existing lunar laser ranging results with new data from special test masses mounted on a sensitive torsion balance to take into account the different compositions of the Earth and Moon, the Washington physicists show that gravitational self-energy does obey the equivalence principle at the level of at least one part in a thousand. Thus, gravitational self-energy does indeed generate its own gravity.

VACUUM TUBES ATTEMPT A COMEBACK. Vacuum tubes were the backbone of the electronics industry until the 1960s, when their large size, excessive power dissipation, and lack of integration allowed solid-state technology to win out. Now forests of 100-nm sized nanotriodes might bring vacuum designs back, at least for niche applications. Researchers at the University of Cambridge have made an anode-gate-cathode device in which the cathode consists of multiple nanopillars, which can be crowded together in a dense formation. This will eventually enable nanotriode densities of 10^9 per cm^2 (including interconnects) to be reached, comparable with the best packing densities for metal-oxide-semiconductor (MOS) transistors, the electronics industry workhorse. Shooting electrons through vacuum rather than a semiconductor not only makes switching fast (the ballistic electrons always travel without scattering), but gives nanotriodes a few advantages over MOS technology: the nanotriodes are radiation resistant, operate well at high and low temperatures, and, because they are vertically-oriented, will permit integration in the third dimension, allowing even greater packing densities. Electrons (or, more accurately, the electron waves) issuing from the nanopillars are coherent and highly focused, and might be useful for doing holography or nanolithography. Remaining problems with this vacuum design include a relatively high operating voltage (10 V) for large scale integration applications and the reproducibility and longevity of the nanotriodes.

ORIGIN OF RADIO JETS NEAR A BLACK HOLE. Black holes don't just sit there spiderlike swallowing stars. They also fling out vast plumes of light-emitting material; these collimated streams can stretch for hundreds of thousands of light years. One of the closest of these conspicuous jets is to be found at the heart of galaxy M87, about 50 million light years away from Earth. Presumably, the jet originates at an accretion disk surrounding a supermassive black hole. Previously radio mapping of this spot in the sky did not possess sufficient resolving power to see precisely where the jet begins. But now, by pooling the extended radiowave gathering power of the Very Long Baseline Array (VLBA), the Very Large Array (VLA), and telescopes in Italy Sweden, Finland, Germany, and Spain, astronomers have nailed down the jet origin to within tenths of a light year of the black hole's location. The resulting image shows that the jet's initial opening angle is 60 degrees, the widest ever seen for a jet, although the jet becomes much more focused (6 degrees) further downstream. (Junor et al., Nature, 28 Oct.)

GOLD CHAINS ARE PRIZED not only as jewelry but also for their atomic properties. By plunging a scanning microscope probe into a gold surface and then retracting the tip a string of several (perhaps as many as seven) gold atoms can be produced. The binding strength between atoms in the chain is at least about half that between atoms in bulk gold and so the chain is somewhat stable. Transmission electron microscope (TEM) pictures of the chains seem to indicate that the atoms are much as 4 to 5 angstroms apart, but other measurements, such as conductance tests, imply the gap was more like 3 angstroms or less. So what are the gold atoms doing? This puzzle is addressed by a group of scientists from several Spanish labs (plus a contingent at the University of Illinois contact Daniel Sanchez-Portal, daniel@roma.physics.uiuc.edu) whose computer simulations suggest that the atoms lie not on a straight line but on a zig-zag (spaced about 2.5 angstroms apart) and that, furthermore, the chain should be spinning around its long axis. The TEM pictures would then be explained as capturing only a misleadingly averaged position for the gold atoms. Knowledge of where the gold atoms are and what they are doing is important to those hoping to develop circuitry-using nanowires. (Sanchez-Portal et al., Physical Review Letters, 8 November 1999; Select Article.)

MACH CONES: SHOCK WAVES IN DUSTY PLASMAS. Plasmas-collections of charged particles such as ions and electrons-usually behave as a gaslike substance, with particles dancing around each other with little deflection. But under the right conditions, physicists can make plasmas act like liquids and solids, in which particles sit almost stationary, interacting almost exclusively with their nearest neighbors. This is especially true when plasmas are mixed with dust, as is the case in interstellar space. In laboratory experiments at the University of Iowa, the "dusty plasmas" are micron-sized spheres loaded up with approximately 10,000 electrons apiece. When illuminated by an intense sheet of light, the researchers can see the microscopic structure and movements of these particles in a way that is not possible with conventional atomic matter. For this reason, plasmas can serve as a model system for investigating condensed matter physics. By firing a particle at the dusty plasma at supersonic speeds, the researchers produced a Mach cone, similar to the V-shaped shock wave produced by a supersonic airplane. Mach cones are well known in gases (airplanes, for example), but almost unknown in solids. One of the only other known examples is in seismology: a sound wave traveling down the surface of a liquid-filled borehole moves faster than the sound speed in the surrounding rock, causing a Mach cone to be produced in the rock. (D. Samsonov et al, Phys. Rev. Letters, 1 November 1999; also see paper H12.02 in the upcoming American Physical Society Division of Plasma Physics meeting-http://www.aps.org/meet/DPP99/baps/ ; also Select Article.)

ULTRASOUND IMAGING WITHOUT PHYSICAL CONTACT between device and patient has been achieved, providing a potential solution to an unmet medical need-determining the depth and severity of serious burns in a convenient, accurate, and pain-free fashion. At the present time, physicians usually diagnose burns by inspecting them visually; however, such visual observation cannot provide direct information on whether there is damage to underlying blood vessels, a condition that requires surgery. Technologies such as conventional ultrasound or MRI are too slow, either time-consuming, or cumbersome. In addition, they are painful for the patient if they require direct contact with the burn area. This is certainly the case with conventional ultrasound, which requires direct contact with the body, or must at least be connected to the body via water. That's because generating ultrasound in a device and sending it through air causes a large proportion of the sound to bounce right back into the device. This results from a great mismatch between air and the device in the values of their "impedance," the product of the density of the substance and the velocity of sound through it. By more closely matching the impedance values between the device and air, a significantly greater proportion of sound can be transmitted to the body, and reflected back, to obtain enough of a signal for an image. In a non-contact ultrasound device described at last week's meeting of the Acoustical Society of America in Columbus, Joie Jones of UC-Irvine and his colleagues pass the sound wave through a multilayered material, with each succeeding layer having an impedance value closer to that of air. The transmission is improved to the point that the researchers could image burns by holding their device about two inches away from the skin, in about a minute or so. Having tested this device on over 100 patients, the researchers plan to move to larger clinical studies and develop a device that can take images in real time.

THE OXYGEN RED PHASE gets its name from the fact that this form of solid oxygen comprised of oxygen-4 molecules is deeply red in color, and gets more red at higher pressures. The red phase has now been studied in detail by physicists in Italy and their results suggest that at pressures above 10 GPa two O2 molecules combine into an O4 molecule. The pressure is necessary for altering (by brute force) the chemical bonds at work inside this molecular solid. By recording the vibrational properties of oxygen solids at pressures up to 63 GPa, Roberto Bini and his colleagues at the European Laboratory for Nonlinear Spectroscopy in Florence have concluded that the process whereby O2 molecules form into O4 units could be a kind of prelude to oxygen's transformation into longer chains (polymers) and then into a metal (superconducting oxygen was reported by Shimizu et al., in Nature, 25 June 1998). (Gorelli et al., Physical Review Letters, 15 November.)

IO SODIUM JET. Astronomers have previously known of a sodium cloud, which precedes the moon Io in its orbit around Jupiter. The cloud is believed to arise from slow escape of sodium from Io. Now the Galileo spacecraft is providing details of another sodium feature at Io, more of a fast-escaping spray or jet, thought to come about when Io plows through Jupiter's potent magnetic field, a process which induces mega-amp currents through Io's atmosphere (see schematic at www.aip.org/physnews/graphics). New pictures, reported by scientists at the University of Colorado and Boston University, localize the source of the sodium to a region smaller than Io's diameter, suggesting that Io's atmosphere might not be global; that is, the atmosphere might be patchy and not extend all the way to the poles. (Geophysical Research Letters, 15 November.)

LASER LIGHT IN, 50-MEV PROTONS OUT. At next week's meeting of the American Physical Society Division of Plasma Physics in Seattle, three groups will independently announce their ability to generate powerful, intense streams of ions by shining ultrashort laser pulses on tiny spots of solid material. Potentially, this approach offers an alternative to bulky, expensive ion accelerators for producing high-velocity ions useful for cancer therapy and electronics manufacturing. Using a single pulse of light from Livermore's Petawatt laser, the most powerful in the world, researchers at that laboratory have reported generating 30 trillion protons with energies up to 50 MeV, from a tiny spot approximately 400 microns in size. Using a tabletop terawatt laser one-thousandth the power of he Petawatt, University of Michigan researchers produce 10 billion protons with about a tenth the energy of those reported at Livermore. In addition, the Michigan team has announced that they can produce a confined beam of ions pointing roughly in the direction of the laser beam. Employing the VULCAN laser at the Rutherford Appleton Laboratory, researchers there, generated lead ions with energies up to 420 MeV (and protons up to 17 MeV). The mechanism behind each demonstration is similar. A single laser pulse strikes a thin target, ejecting electrons, which form a cloud of negative charge around the back of the target. The cloud pulls positively charged ions from the back of this target and rapidly accelerates the ions to high energies. All of this occurs over a very short distance-almost 1 MeV/micron for protons in the Livermore case, which is an order of magnitude higher than conventional ion accelerators.

20,000 LEAGUES UNDER THE FERMI SEA. Recently Stanford and UC Santa Barbara physicists used two alternating-current voltage sources to skew the quantum states in a tiny semiconducting quantum dot in such a way as to produce (without any net applied bias) a nonzero current through the dot. This was an experimental realization of a "Thouless pump" (named for David Thouless), which pumps electrons much as an Archimedean screw pump lifts water (Switkes et al., Science, 19 March 1999; see also the commentary in the same issue by Altshuler and Glazman). Now, Mathias Wagner (Hitachi Cambridge Laboratory, 011-44-1223-44-2911, wagner@phy.cam.ac.uk) and Fernando Sols (Universidad Aut-noma de Madrid) predict that a similar principle will also apply to electrons far beneath the Fermi-sea surface. The Fermi surface or Fermi level represents (in an abstract space in which all electrons are described by their momentum vectors) the highest energy an electron may possess-at zero temperature-in the conduction band of a metal or semiconductor material. Conduction electrons, those that stray from their home atoms, are usually drawn from electrons very near the Fermi surface. Electrons with lesser energies, and occupying rungs further down on an energy-level diagram, are said to reside in the "Fermi sea" and normally do not effectively contribute to the current. Wagner and Sols suggest that with high enough ac power, the resulting pump current might actually consist mostly of electrons from far beneath the Fermi-sea surface. These subsea currents would be largely immune from temperature effects (just as submarines are less vulnerable to surface storms), a very useful property in the electronics world. (Wagner and Sols, Physical Review Letters, 22 November 1999)

THE SHADOW OF A PLANET slipping across the face of a distant star has been detected, for the first time, by veteran extrasolar-planet stalkers Geoffrey Marcy of UC Berkeley and Paul Butler of the Carnegie Institution, working with Greg Henry of Tennessee State University. Prior indirect "sightings" of extrasolar planets consisted of small feints in the apparent position of the stars caused by the suspected gravity pull of an orbiting planet. Astronomers have felt that from among the growing sample of such planets (up to 25 as of now) a few (whose orbits would be viewed at Earth edge-on) might be detected directly as they pass in front of the star. One such candidate was HD 209458. Prediction of a planetary transit for the night of November 7 proved accurate and a 1.7% dimming in the star's light was seen. (Announcement made in an International Astronomical Union circular.)

MICROFLUIDICS CAN BE DRIVEN BY HEAT rather than by electric fields. Microfluidics is to the mixing of fluids (including studies of blood, DNA, etc.) what integrated circuits are to the processing of electrical signals: transactions occur quickly, controllably, in a very small space. But instead of excavating small channels in a substrate and propelling tiny fluid volumes around the nano-sized system of aqueducts customary in microfluidics, Princeton professor Sandra M. Troian and Dawn Kataoka, now at Sandia Laboratories (CA), have moved tiny liquid rivulets around a silicon wafer using temperature gradients. The capillary movement of the micro-fluids can be programmed because (1) the liquid surface tension varies with temperature and even a gradient of 3 or 4 K will cause a fluid to seek out a cold region, and (2) a lithographically applied pattern of chemical modifications on the substrate (the equivalent of an invisible scent marker or a chemical levee) further constrains the droplet rivercourses. Thus streams of hydrophilic and hydrophobic molecules, zooming across the substrate along neighboring lanes, can be shunted together at some desired meeting point. The advantages of thermo-capillary action over electronic-driven fluidics are that the use of high electric fields and the precision carving of channels are not necessary; everything happens on a plane, making easier the task of building micro-electromechanical (MEMS) "labs-on-a-chip.' Troian will report on her research at the APS division of fluid dynamics meeting in New Orleans, November 21-23: http://www.nd.edu/~apsnd/)

HYDROGEN STORAGE IN NANOTUBES. Hydrogen is a potent fuel: combined with oxygen it can power spacecraft to the Moon. Storing such a dangerous substance, however, is difficult. Physicists at MIT have now succeeded in canning hydrogen inside carbon nanotubes. Actually, hydrogen sausage has been encased in a carbon skin before, but the MIT efforts are the first to achieve reliably such a high hydrogen uptake (one hydrogen for every two carbons) at room temperature. And like a jack-in-the-box, the hydrogen came shooting out of the tubes (80% of them anyway) when the packing pressure was relaxed. (Liu et al., Science, 5 November 1999.)

THE ONLINE JOURNAL PUBLISHING SERVICE (OJPS) constitutes a shopping mall for the physics journals published by the American Institute of Physics (AIP), many of its member societies, and other scientific and engineering societies. From this site (http://ojps.aip.org/) one can handily visit the homepage for such journals as Physical Review, Applied Physics Letters, Optics Letters, and Chaos. Nonsubscribers can view tables of contents and look at all the abstracts, including those from some issues not yet published. (You can even search the full SPIN database of abstracts if you have a subscription to at least one of the OJPS journals.) In general the full texts are available only to subscribers, although a few prominent articles are supplied to science writers via a separate website called Physics News Select Articles.

UNDERSEA VOLCANO. Like astronomers who team up to view supernova eruptions at a variety of wavelengths, geophysicists have been able to mount an in-depth study of the eruption in January 1998 of the Axial Volcano, lying 1500 m underwater about 200 miles off the Oregon-Washington coast. Axial, which is a large volcanic edifice lying along a rift zone in the Northeast Pacific where new ocean floor is being created, is one of the few places on the worldwide 60,000-km mid-ocean ridge system (Iceland and the Azores are other examples) where volcanic activity can be monitored in real time. In this case, the coverage consisted of Navy hydrophone arrays (listening for quakes rather than subs), surface ships, moored sensors, and instruments placed on the very summit of the caldera in anticipation of an eruption. The 1998 event is chronicled in a variety of ways in a series of articles in the December 1 and 15 issues of Geophysical Research Letters. For example, C.G. Fox reports (via on-the-spot seafloor measurements) a 3-meter drop in the caldera floor; Baker et al. provide the first incite observation of the water temperature change above an erupting rift zone (constituting the "largest vent field heat flux yet measured"); Embley et al estimate that up to 76 million cubic meters of lava were produced, modest by land volcano standards, but the largest outpouring in 20 years of monitoring along the Juan de Fuca Ridge. (Robert Embley, Pacific Marine Environmental Laboratory)

SWIRLED SPHERE MAGIC NUMBERS. Physicists love to detect patterns in nature, whether in the crystalline structures of atoms in solids, or the groupings into "shells" of electrons inside atoms or protons and neutrons within nuclei. Even in a system as simple as a bunch of spheres swirled around in a dish patterns can emerge. Scientists at the Max Planck Institute in Dortmund, Germany, and the University of Chile have determined that for certain "magic" numbers of spheres, such as 19, 21, or 30, the spheres congregate into solid-like shell structures with stable rings. The swirled balls are a form of granular material. Studies of agitated grains had uncovered stable structures before (such as "oscillons") but not any that had depended on the number of particles present. The researchers noticed that when they increased the size of the dish a puzzling transition between stable and disordered states would occur intermittently. (Kotter et al., Physical Review E, December 1999; Select Article.)

THE TOP PHYSICISTS IN HISTORY are, according to a poll of scientists conducted by Physics World magazine, 1. Albert Einstein, 2. Isaac Newton, 3. James Clerk Maxwell, 4. Niels Bohr, 5. Werner Heisenberg, 6. Galileo Galilei, 7. Richard Feynman, 8. Paul Dirac, 9. Erwin Schrodinger, and 10. Ernest Rutherford. Other highlights of Physics World's millennium canvas: the most important physics discoveries are Einstein's relativity theories, Newton's mechanics, and quantum mechanics. Most physicists polled (70%) said that if they had to do it all over again, they would choose to study physics once more. Most do not believe that progress in constructing unified field theories spells the end of physics. Ten great unsolved problems in physics: quantum gravity, understanding the nucleus, fusion energy, climate change, turbulence, glassy materials, high-temperature superconductivity, solar magnetism, complexity, and consciousness. (December issue of Physics World, published by the Institute of Physics, the British professional organization of physicists celebrating its 125th anniversary this year.)

MEASUREMENTS OF THE COSMIC MICROWAVE BACKGROUND (CMB) provide new evidence that the expansion of the universe is accelerating. One of the greatest issues in cosmology is whether the current expansion will continue, reverse, or proceed at a diminishing rate. Supernova observations two years ago suggested that not only would the expansion not reverse but that it was in fact getting faster. The new CMB mappings, carried out with telescopes on mountains and on balloons, reveal that the temperature of the microwave background varies in clumps with an angular size of about one degree on the sky, a result indicative of an overall "flat" geometry for the universe (New York Times, 26 November 1999). Another way of saying this is that the observed energy density of the universe is apparently equal to the critical density value of about 10^-29 gm/cm^3. But the amount of known matter (luminous and dark) is insufficient for producing a flat geometry, so additional energy, probably hiding in the universal vacuum, is needed. This energy, according to many theorists, would exert an effect equivalent to a repulsive form of gravity, thus working against the mutual gravitational attraction of galaxies. Much of the new work is available only in preprint form. For example, papers for one of the experiments, the "Boomerang" collaboration, which measures the CMB with a balloon-mounted detector, can be found on the Los Alamos server.

COOPERATIVE EVAPORATION, a process whereby droplets on a substrate do not evaporate independently but in a coordinated fashion, has been observed for the first time by physicists at the University of Konstanz. The researchers begin by laying down a periodic array of diethylene glycol drops 0.75 microns in radius and spaced by 2.5 microns. (Condensing the droplets out of a supersaturated vapor onto a patterned grid of adsorption sites imposed on the surface with microcontact-printing was itself something of a feat). The Konstanz scientists found that some rows of droplets evaporated faster than other rows, leading to a sort of "superstructure." In other words, some drops would survive at the expense of the preferential evaporation of other drops in a methodical way. Previously scientists have considered how gas sensors comprised of liquid droplet arrays could be designed. The droplet size in such sensors can be made sensitive to environmental conditions by selective uptake of certain molecules. When monitoring the average droplet size by light scattering techniques, the concentration of the molecules can be determined. But for this to work the cooperative evaporation effect will have to be taken into effect. (Schafle et al., Physical Review Letters, 20 December 1999; Select Article.)

ATOM TRAP TRACE ANALYSIS, the search for tiny isotope fractions among atoms using a magneto-optic trap, may soon be preferable to accelerator mass spectrometry (in which atoms are heated, accelerated, and sent through a strong magnet, which sorts the atoms by mass) for certain radio-dating purposes. To demonstrate this idea, physicists at Argonne have detected traces of krypton-85 (with an abundance of only 10^-11) and krypton-81 (abundance of 10^-13) in an atom trap with an efficiency of 1 part in 10^7; accelerator mass spectrometry, which requires an accelerator, currently has a counting efficiency of a part in 10^5. Keeping track of Kr-85 atoms is important since they are produced chiefly in nuclear-fuel reprocessing plants, and (arising mostly since the 1950s) are used as a tracer of air and ocean currents. Kr-81, in contrast, is made in cosmic-ray showers in the upper atmosphere and (with a half life 40 times longer than C-14's) is preferable to carbon-dating for calibrating the antiquity of million-year-old samples of ice and ground water. (Chen et al., Science, 5 November 1999.)

NATURALLY OCCURRING RADIATION LEVELS ARE MUCH LOWER TODAY on Earth than when life first appeared, a new analysis has shown, suggesting that all living organisms--which have mutation-repair mechanisms very similar to those first developed by primordial life forms were once equipped to handle larger doses of background nuclear radiation than modern life forms. Presently, humans receive a dose of about 360 millirems per year of radiation from natural sources, plus typically about 63 mrem/yr from anthropogenic sources. Perhaps surprisingly, a major source (about 40 mrem/yr) of naturally occurring radiation is inside our bodies--in the form of potassium, a nutrient essential for many things such as generating signals between cells. All natural sources of potassium contain some radioactive potassium-40 (K-40). But life first began about 4 billion years ago--about 3 K-40 half-lives ago-meaning that the radiation dose from potassium today is about one-eighth of what it was 4 billion years ago. Geologic sources of radiation (about 28 mrem/yr) include uranium, thorium, and potassium present in rocks and minerals in the earth's crust. Studying published data of 1100 rocks, and assuming that the continental crust had formed early (a scenario favored by the rock record), the researchers estimated that radiation from these sources is now about one-half of what it was 4 billion years ago, because many of these radioisotopes decayed in the intervening time. Not considered in the present study were cosmic sources (about 27 mrem/yr) and radon (typically about 200 mrem/yr); the authors are making these the subject of ongoing research. (Karam and Leslie, Health Physics, December 1999.)

MAXWELL'S DEMON MADE OF SAND. The second law of thermodynamics states that within a closed system heat cannot flow unassisted from a cold to a warm place. To ponder this issue, James Clerk Maxwell, one of the pioneers of statistical mechanics, posed this thought experiment: could not a clever microscopic creature, poised at a pinhole in a baffle dividing an insulated box into two equal chambers, sort molecules in such a way that the hotter (faster) molecules would be directed into one chamber while cooler (slower) molecules would be directed into the other. "Maxwell's demon," as the sorter came to be known, itself requires energy to operate, and so the segregation of hot from cold cannot really happen as advertised. And yet in an experiment conducted at the University of Essen in Germany in which agitated sand in a two-chamber vessel (the halves being connected by a hole) "hot," quickly moving sand migrated to one side while cool sand spontaneously condensed and congregated on the other side (see sketch at www.aip.org/physnews/graphics). Jens Eggers explains that, no, the second law is not violated in this case since although moving sand can be considered as a gas, individual grains can absorb heat and dissipate heat (that is, individual grains can gain temperature), unlike the ideal gas molecules described by Maxwell, whose "temperature" is a measurement of gas motion. Thus when sand grains start to congregate in one chamber (the segregation begins as an act of spontaneous symmetry breaking) more and more grains will partake of a growing ordered state consisting of grains falling to the bottom of the container (where the grains are denser there are more collisions and hence faster cooling, leading to more congregation, etc.), while the unaffiliated grains will tend to be on the other side, still in "gaseous" form. (Eggers, Physical Review Letters, 20 December; Select Articles.)

COMPETING ARROWS OF TIME. Lawrence S. Schulman of Clarkson University has found that time might actually flow backwards in certain regions of space. This time reversal has nothing to do with quantum fluctuations or the spacetime-warping effects of a black hole. It's just ordinary matter obeying the ordinary and mostly time-symmetric laws of physics. The difference lies in its statistics. If the laws of physics have no preferred direction then why do we never see a shattered wineglass jump back up on the table and reassemble itself? The "arrow of time" concept enshrines this domestic disaster in the form of a law, the second law of thermodynamics. The arrow describes the tendency for macroscopic systems consisting of many particles (the falling wineglass) to evolve in time in such a way that disorder grows and information decreases. This tendency is statistical and does not prevail at the microscopic level, where a movie of two atoms colliding would seem credible if run in the forward or reverse direction. The wineglass, however, consists of zillions of atoms. The reason we never see the glass re-assemble and lift itself (courtesy of the warmth of the original breakage returning from the floor and air) back onto the table is that this highly specialized (and, as we would say, unlikely) scenario is but one of a myriad of possible configurations, in most of which the glass shards stay on the floor. This statistical explanation leads to two puzzles. First, why does this arrow point the way it does? Why not the other way? And second, why should it point at all? On the first question, Schulman subscribes to the view that the "thermodynamic" arrow of time is a consequence of the "cosmological" arrow reflected in the one-way expansion of the universe, a theory advanced some years ago by Thomas Gold of Cornell. As to the second question, that's exactly where Schulman's new results have their impact. The prevailing view holds that if opposite-arrow systems came into even the mildest of contact, the order in at least one of them would be destroyed. This is because from the perspective of one observer the coordination needed to reassemble the other's wineglass would be so fantastic that even a single photon could disrupt it. Not so, says Schulman who, in his computer modeling of the universe, specifies not one boundary condition in time (the big bang) but two, the other being a supposed "big crunch" when the universe would contract (or so it would seem to us; from the perspective of that arrow, the universe would be expanding). In his model, the two arrows of time (one growing out of either end of the "timeline"; see the figure at www.aip.org/physnews/graphics) can be mildly in contact and nevertheless each have its wineglass break and its rain fall appropriately. Observers associated with either arrow might even watch the other grow young-from a distance. Some relatively-isolated relics of matter subject to the opposite arrow might be found in our vicinity. By its own clock such a region would be very old and no longer luminous, although gravitationally it would not be anomalous, exactly the hallmark of dark matter. Or we might see an opposite-arrow black hole giving matter back to an accretion disk, which in turn would feed it back to a companion star, which would seem (to us) to be coming into existence. Schulman concedes that recent observations may rule out a final crunch in our actual universe but argues that there is still a lot we don't understand about our thermodynamic arrow, and that a competing time arrow might arise from another, as yet unknown, cause. (Physical Review Letters, 27 Dec.)

STARLIGHT REFLECTED FROM AN EXTRASOLAR PLANET has been reported by University of St. Andrews astronomers. Roughly, 30 planets have been detected around nearby stars through an indirect method, which monitors fluctuations in the stars' positions. More recently, the shadow of an extrasolar planet was observed to transit across the face of its star. Now light has been detected which apparently comes to us directly from a planet circling the star tau Bootis, some 50 light years away. The main difficulty was of course discerning the reflected light while blocking out the glare of the star itself. The planet seems to be blue-green in color, is twice the size of Jupiter, and 8 times as massive. (Cameron et al., Nature, 16 December 1999.)

THE SOLAR WIND DISAPPEARED for a day back on May 10/11, allowing Earth's magnetosphere to balloon out to the orbit of the Moon. Ironically, the greatly lowered solar wind flux of particles and solar magnetic field allowed high-energy electrons from the sun's corona to penetrate directly to our upper atmosphere unadulterate, where the electrons' characteristic x-ray emissions were observed by satellites over the North Pole for the first time. Such a "polar rain" had been predicted years before. Normally the coronal electrons (with energies of tens of keV, corresponding to temperatures of millions of degrees) lose much of their energy through scatterings with other particles on their ride from sun to Earth and in the topsy-turvy trajectories experienced at our magnetosphere. At last week's meeting of the American Geophysical Union in San Francisco, these results were reported by a number of speakers, including David Chenette of Lockheed, Jack Scudder of the University of Iowa, and Keith Ogilvie of NASA Goddard.

SPONGELIKE STRUCTURES NEAR THE SUN'S SURFACE, newly observed by the TRACE satellite (at extreme ultraviolet wavelengths) and the SOHO satellite (in x rays), lie between the 10,000-K chromosphere and the corona at a temperature of several million K. These filamentary structures (dubbed "solar moss" by Lockheed scientists reporting at the AGU meeting) are typically 6000-12,000 miles in size and about 1000-1500 miles above the photosphere; occur at various places around the sun's surface, usually near the footprint of huge coronal loops. The moss blobs seem to be stable for hours but can also change brightness over periods as short as 30 seconds do. Thomas Berger of Lockheed said that the new structures might provide information on how the corona gets so hot, an issue that remains one of the great unsolved mysteries of solar physics.

THE RAREST NATURALLY OCCURRING ISOTOPE, tantalum-180, is rare because it is bypassed in the two processes that produced most of the heavy elements we dig out of the ground here on Earth: the so called s process (slow neutron capture in stars) and the r process (rapid neutron capture in supernova explosions). What little Ta-180 that is produced (in stars or in reactors) is quite robust; its halflife is more than 10^15 years. Ta-180 is also unique in being the only naturally occurring isomer; it is essentially a nucleus in a perpetual excited state. A group of German physicists, essentially working with the world's supply of this priceless substance, about 7 milligrams, try to jar the tantalum nuclei out of their customary states by shooting them with gamma photons, thus re-creating stellar conditions. They observed that depending on the temperature the Ta-180 halflife varied over a range of more than 10^17! This rules out the nucleosynthesis of Ta-180 within the "canonical" s process; however, in a more realistic version of the theory, the tantalum can survive if it rapidly mixes with cooler layers of the star. (Belic et al., Physical Review Letters, 20 December 1999. Select Article.)

SUPERCONDUCTING BALLS, a new phenomenon, have been observed by physicists at Southern Illinois University. Rongjia ao (618-536-2117, rtao@physics.siu.edu) and his colleagues began by wanting to observe the motion of micron-sized copper oxide (e.g., Br-Sr-Ca-Cu-O) superconducting particles (suspended in liquid nitrogen) in an electric field running between two electrodes. Metal particles in this situation would bounce between the two electrodes or tend to line up; after all, an electric field helps to define a preferred direction in space. The superconducting particles ignored this hint and, to the researchers' great surprise, formed themselves into a ball. The ball, about .25 mm across and containing over a million particles, formed quickly and was quite sturdy, surviving constant collisions with the electrodes (see figure at www.aip.org/physnews/graphics). What binds the ball together against the dictates of the rectilinear field? Tao and his collaborator, Princeton theorist Philip Anderson, have concluded that the effect is an artifact of superconductivity (the same particles, above their superconducting transition temperature, do not ball up but instead queue into lines), perhaps something to do with the way in which the surface energy of the particle ensemble is reduced by self-assembly into a ball. This unprecedented new surface energy is related to the acquired surface charges on the particles and the reactions among the layers of the balls. Granular properties of the particles might also play a role in the process and in the ball's internal structure, but this is difficult to gauge since the inter-particle interactions (frictional dissipation being the hallmark of granular materials) are mitigated by the liquid nitrogen needed in the experiment to neutralize gravity. A way around this is to do the experiment in the microgravity of space. The basic scientific novelty of this new phenomenon is paramount, but Tao is also turning his attention to possible applications in the area of superconducting thin films and unusual forms of wetting. (Select Tao et al., Physical Review Letters, 27 Dec.)

TWO-DIMENSIONAL COLLOIDAL CRYSTALS SEEMINGLY DEFY COULOMB'S LAW as they form, experiments have shown. A colloidal crystal is a regular arrangement of tiny particles suspended in a liquid. Three-dimensional examples have long been known. Now free-floating 2D "crystallites" of colloidal particles, lashed together by bilayer membranes similar to those surrounding living cells, have been created, offering intriguing possibilities for using them as templates for artificial biomaterials and industrial catalysts. University of Pennsylvania researchers (Laurence Ramos, now at Universite de Montpellier, France, ramos@gdpc.univ-montp2.fr) created the system by adding negatively charged latex beads to a suspension of positively charged soaplike (surfactant) membranes in water. As expected, initially the beads avidly stuck to the membranes. To the researchers' surprise, though, in many cases the beads formed rafts floating on the membrane. Outside the raft the membrane actually repelled additional beads, even though they were highly oppositely charged. The researchers argued that the source of this paradoxical behavior lay in the migration of negative ions trapped on the side of the membrane opposite to the beads. With time the fluid rafts solidify into rigid, flat crystallites, near-perfect 2D crystalline structures some tens of microns on a side. (Ramos et al., Science, 17 December 1999; and Aranda-Espinoza et al., 16 June.)

AMPLIFYING AN ATOM WAVE while maintaining its original phase has been demonstrated for the first time, bringing about an atom laser that is the closest equivalent yet to an optical laser. The first atom lasers were passive devices: researchers simply prepared a Bose-Einstein condensate of atoms, and then extracted some of the BEC atoms to form a beam. In the latest round of demonstrations, two research groups (one at MIT and one at the University of Tokyo) have independently demonstrated an atom laser that amplifies its initial beam, in a way that's remarkably similar to how optical lasers augment an initial light wave. Unlike light, however, atoms cannot be created from the vacuum, so researchers must rely on a pre-existing supply of atoms to serve as the initial beam to be amplified. In the MIT demonstration, researchers shine a pair of laser pulses on sodium BEC. First, some of the BEC atoms absorb a photon from a high-frequency beam and emit a photon towards a lower-frequency beam. These atoms recoil in the same direction, forming a weak atom wave. Then the lower-frequency beam is shut off, and some of the other BEC atoms absorb light from an intensified pulse coming from the high-frequency laser. The presence of the initial atom wave stimulates these atoms to emit a photon in the direction of the lower-frequency beam. This resulted in a phase-coherent amplified beam about 4 times as strong as the initial atom wave. The Tokyo group demonstrated similar results with a rubidium-87 BEC. In both demonstrations, the amplification is limited by the size of the BEC, which is depleted in the process. However, an atom-wave amplifier promises improvements in such applications as atom-wave gyroscopes and lithography. (Inouye et al., Nature, 9 December 1999; Kozuma et al., Science, 17 December.)

HIGH PROTON POLARIZATION, up to 32%, has been achieved at liquid-nitrogen temperatures (77 K) and with modest 0.3-Tesla magnetic fields in an 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 10^-18 meters, corresponding to an energy deposit as small as 10^-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 10^5, 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 microscopes 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 by 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 "ultra-faint 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 Stephen 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 E0102-72, located in the Small Magellanic Cloud. E0102-72 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 BLACK 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 durations suggest either large or very slow lensing objects. David Bennett of Notre Dame reported at the AAS 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 Piwnicki, 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 SCREECHING 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 33 TeV) 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 from 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, or 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 substrates, 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 MOLECULAR 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 (dimers) 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 rest, with an equivalent temperature of about 100 nK; "...perhaps the coldest molecules in the universe," says Paul Julienne of NIST (Science News, 12 Feb 2000). The stillness makes possible high precision spectroscopy of the molecules, which constitute about 1% of the condensate. The dimers 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 rendevous 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 femtosecond (10^-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 than 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 Statistics 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 collaboration 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, rossbach@desy.de; www.desy.de/pr-info/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 Schwegler, 925-424-3098, schwegler@llnl.gov) 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 the packing fraction for the maximally jammed state is about 64%. Torquato (torquato@matter.princeton.edu, 609-258-3341) believes that the 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 473) a group form 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.)

HIGHLY OPTIMIZED TOLERANCE. Many natural and man-made systems exhibit power-law statistics. That is, when you plot the likelihood of an event (e.g., sizes of forest fires, power outages, and web file transfers, or losses due to hurricanes, floods, earthquakes, and man-made disasters) as a function of size the resulting graph will fall off proportionally to the size of the event raised to some exponent. Interactions or phenomena at many size scales (from very small to very large) contribute to the overall state of these systems. One theory, which tries to explain all this, is "self organized criticality." Jean Carlson of UC Santa Barbara (carlson@physics.ucsb.edu) and John Doyle of Caltech (doyle@cds.caltech.edu) now propose another theory, called highly optimized tolerance (HOT), which they believe does a better job of accounting for the tendency in interconnected systems to gain a measure of robustness against uncertainties in one area by becoming more sensitive elsewhere. As with energy conservation or the inexorable increase in entropy, efforts to violate the robustness principle will fail. Especially in biological evolution or in engineering, this means that a system might obtain robustness against common and designed-for uncertainties and yet be hypersensitive to design flaws or rare events. For example, organisms and ecosystems exhibit remarkable robustness to large variations in temperature, moisture, nutrients, and predation, but can be catastrophically sensitive to tiny perturbations, such as a genetic mutation, an exotic species, or a novel virus. Engineers deliberately design systems to be robust to common uncertainties. Cost and performance tradeoffs force an acceptance of some hypersensitivity to (one hopes) rare perturbations. In evolved or designed systems, this tradeoff leads to the "robust, yet fragile" characteristic of complexity, one feature of which is power laws. Doyle and Carlson have been exploring the application of their theory to a number of biological and engineering problems with the help of experts in those fields. (Physical Review Letters, 13 March 2000; Select Article; a longer version appears in Physical Review E, August 1999.)

16 CONSECUTIVE MONTHS OF RECORD HIGH TEMPERATURES (global mean temperature; during 1997 and 1998) suggest to Thomas Karl (director of the National Climate Data Center, NCDC) that a human-induced global warming trend, and not merely the kind of natural temperature fluctuations one expects to see in the climate record, is under way. According to Karl, the last quarter-century of data is characterized by a temperature gradient of two degrees Celsius per century. The 16-month streak is unmatched in temperature records dating back to the nineteenth century. (Thomas Karl in Geophysical Research Letters, 1 March 2000; tkarl@ncdc.noaa.gov, 828-271-4476.)

ANAMORPHIC IMAGES are those in which the painted image of an object has been distorted in such a way that the object becomes recognizable only by viewing it at an oblique angle or in some curved reflecting surface. Anyone who has visited the National Gallery in London might have seen Hans Holbein's painting "The Ambassadors," in which an odd shape at the bottom of the canvas is seen to be a skull when viewed almost edge-on. Anamorphic images were something of a rage in the Renaissance, and Leonardo and Durer tried the technique as part of their studies of perspective. An eighteenth century innovation was to create anamorphs of paintings by famous artists. A seventeenth century book by Jean-Francois Niceron worked out the geometrical algorithms for producing anamorphic art (the planar and conical cases are pretty easy but cylinders are quite challenging), but this mathematical connection was lost through the centuries. Now, scientists at Guelph University (Ontario, Canada) have re-derived the transform equations needed for producing anamorphs. (Hunt, Nickel, Gigault, American Journal of Physics, March 2000; Select Articles; James Hunt, phyjlh@physics.uoguelph.ca, 519-824-4120, x3993; Bernie Nickel, bgn@physics.uoguelph.ca; images at http://physics.uoguelph.ca/)

ATMOSPHERIC INFRASOUND. Humans hear sounds in the 20-20,000 Hz frequency range. Ultrasound, waves above 20,000 Hz, is used for things such as medical imaging and can be sensed by animals such as bats and dogs. But how about infrasound, sound at frequencies less than 20 Hz? Humans can sometimes feel (even if they can't quite hear) infrasound in the form audio systems with the bass turned way up. The monitoring of infrasound may contribute practical benefits in a number of areas: determining the location and nature of avalanches, tornadoes, and meteor strikes, volcanoes, and nuclear weapons tests. (Physics Today, March 2000.)