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 gass 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 10ˆ17 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.
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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 cant 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 Astrophyics and Schlumberger-Doll inject xenon atoms into various 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, ect.) 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.
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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 wide-spread 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 Gerardust 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 Maxwells 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 Feynmans. To draw out the distinctiveness of Veltmans and Hoofts 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 particles 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.
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With Veltmans and t Hoofts 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 Fermilabs Tevatron and CERNs 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 (10ˆ-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 doesnt explain the great disparity (the hierarchy problem) between the temperature at which the weak and electromagnetic forces fuse together (10ˆ2 GeV, expressed in energy units) and the temperature at which gravity joins up with the other forces (10ˆ18 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.
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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 103 GeV (rather than at 1018 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 5 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 buckyballs 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 he 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.
GRAVITYS 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 (1012). 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 1010. 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 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.
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VACUUM TUES 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 109 per cmˆ2 (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 dont 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 holes location. The resulting image shows that the jets 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, [email protected]) 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 10,00 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 travelling 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. Thats 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 weeks 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 bounds at work inside this molecular solid. By recording the vibrational properties of oxygen solids at pre0 00ssures up to 63 GPa, Roberto Bini and his colleagues at the European Laboratory for Non-linear Spectroscopy in Florence have concluded that the process whereby 02 molecules form into 04 units could be a kind of prelude to oxygens 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 Jupiters potent magnetic field, a process which induces mega-amp currents through Ios 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 Ios diameter, suggesting that Ios 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 weeks 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 Livermores 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.
