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9. November 2001 © Schulphysik">email: Schulphysik

"The American Institute of Physics Bulletin of Physics News" 
AIP Auswahl Oktober 2001
by Phillip F. Schewe and Ben Stein, and James Riordon 








Wed, 26 Sep 2001
ENTANGLEMENT OF MACROSCOPIC OBJECTS, a pair of gas clouds containing a trillion atoms each, has been achieved by a research team in Denmark (Eugene Polzik, University of Aarhus, 011-45-89423745, polzik@ifa.au.dk), constituting by far the largest material objects entangled on demand and paving the way for quantum teleportation between macroscopic objects. The accomplishment, published in this week's issue of Nature (Julsgaard et al., 27 September 2001), was announced in preliminary form this June at the first International Conference on Quantum Information, sponsored in part by the Optical Society of America and the American Physical Society. One of the most profound features of quantum mechanics, entanglement is a special interrelationship between objects in which measuring one object instantly influences the other, even if the two are completely isolated from one another. No previous entanglement with atoms has involved more than four particles. Furthermore, atoms have only been entangled at close proximity, either as ions spaced microns apart in a tiny trap (Update 475), or atoms flying over a short range through narrowly spaced cavities (Hagley et al., Phys. Rev. Lett., 7 July 1997). In the present experiment, researchers sent a light beam through two cesium gas samples, each held in a special paraffin-coated cell. The beam changed each sample's "collective spin," which describes, in a sense, the net direction in which all of the atoms' tiny magnets add up. First, the researchers measured the sum of the two collective spins without knowing the individual collective spin of each sample. A subsequent measurement, nearly a millisecond later, showed that the sum remained the same. This demonstrated that the two gas samples maintained their special interrelationship and were entangled. Although the two samples were just millimeters apart, they could in principle be separated, and thereby entangled, at much longer distances. Entanglement of such large objects enables "bulk" properties, like collective spin, to be "teleported," or transferred, from one gas cloud to another.

Wed, 31 Oct 2001
WHAT IS INTELLIGENCE? This may seem to be more of a question for psychologists than physicists. But two researchers (Joseph Wakeling, jwakeling@webdrake.net, now at the University of Fribourg, Switzerland, and Per Bak, Imperial College, 011-44-20- 7594-8528, p.bak@ic.ac.uk) argue that intelligence is not an abstract concept, but must be considered as a physical phenomenon. Any definition of intelligence, they say, cannot ignore a living being's environment, including its very own body. In their view, an organism is only intelligent relative to how well it solves the problems that its surroundings throw at it. This runs counter to many historical ideas, including the concept that the mind is separate from the body, or that it is possible to build a desktop computer that thinks like a human without having the same physical environment or body. To explore the idea of intelligence, the researchers ran computer simulations of artificial neural networks called "minibrains." In the simulations, 251 minibrains each attempted to pick the less popular of two choices, 0 and 1, analogous to 251 motorists all trying to pick the less congested road. This "Minority Game" would be repeated over many successive rounds. Each minibrain consisted of three layers of "neurons": "input neurons," which dictated how many past rounds it could remember, leading to an intermediary layer, which then led into an "output" layer that determined what choice was made. If the minibrain ending up making an incorrect choice, it would reduce the strength of the connections between neurons supplying the "wrong answer." The researchers were in for a surprise when they endowed all of the minibrains with equal abilities, which would be analogous to a bunch of motorists with the same amount of decision-making skill. In this situation, no minibrains correctly guessed the minority choice with even a 50 percent success rate, which is what you'd get by making the choice with a random flip of a coin. Even an E. coli bacterium, which searches for glucose by moving in random directions in its environment, is seemingly more intelligent than this. Only when the researchers introduced a "rogue" minibrain with more intermediate neurons to analyze the past rounds did it attain more than a 50 percent success rate. Their simulations suggest that intelligence often hinges on how much one can make use of the data in its physical environment. (Wakeling and Bak, Physical Review E, November 2001)

 

  7 Nov 2001
PYROELECTRIC ACCELERATOR. In a pyroelectric crystal held below a critical temperature (the Curie temperature) heating or cooling causes distortions in the lattice of atoms which in turn creates strong electric fields at the surface of the crystal. James Brownridge of the State University of New York at Binghamton (jdbjdb@binghamton.edu) and Stephen Shafroth of the University of North Carolina ( 919-962-3015, shafroth@physics.unc.edu) have used these electric fields to create stable, self-focused electron beams with energies as high as 170 keV. The energy conversion is not especially efficient: inputting watts of heating energy produces only microwatts of output electron beam energy, but this might not be important. Pyroelectric crystals (such as those made of LiNbO3) are widely used as detectors of infrared and THz radiation, but the discovery by Brownridge that they can also be used to produce energetic electron beams if heated or cooled in dilute gas atmospheres means that they can be used to produce x-ray fluorescence for elemental analysis of complex materials, such as tree leaves, rocks, air filters, blood samples, etc. Portable economical x-ray fluorescence is now a real possibility.
(Applied Physics Letters, 12 Nov; http://www.binghamton.edu/physics/brownridge.html )

 




























Wed, 26 Sep 2001
THE BLACK HOLE OF GENEVA. Black holes are known as the omnivorous destroyers of stars. In reality black holes not only take but give. Near their event horizons, where space is so drastically warped, black holes spawn particle-antiparticle pairs out of sheer vacuum. In some cases one of the pair escapes beyond the horizon while its counterpart is pulled back into the hole. Thus black holes can shed energy in the form of this "Hawking radiation." Physicists hope to bring this whole process down to earth by manufacturing tiny black holes amid the stupendous smashups of protons at the Large Hadron Collider (LHC) being built at CERN. Until recently theorists thought gravity was so weak compared to the other forces that it, and gravitationally bound objects like black holes, could be studied on an equal footing with the other forces like the strong nuclear force only at energies of 10^19 GeV. In the past few years, though, some models featuring extra spatial dimensions hint that the unification of the forces, including gravity, might set in at much more modest energies, even in the TeV realm of the LHC. Thus one can contemplate forming a TeV- mass black hole even as one can imagine creating new particles in that mass range. But what would a black hole look like? Savas Dimopoulous of Stanford (650-723-4231) and Greg Landsberg of Brown University (landsberg@hep.brown.edu, 401-863-1464) have drawn a picture in which proton-proton collisions could create black holes with a cross section (likelihood of creation) only about a factor of ten less than for producing top quarks and at a rate of up to one per second (see figure at http://www.aip.org/mgr/png ). A black hole produced in this way would quickly decay, not in the usual particle way but in a furious burst of Hawking radiation. A particularly striking signature of the black hole would involve an electron, muon, and photon in the final state of debris particles. Properties of Hawking radiation could tell physicists about the shape of extra spatial dimensions. A possibility of recreating the early moments of the universe in the lab would further unite particle physics and cosmology (Physical Review Letters, 15 October 2001; text at http://www.aip.org/physnews/select)

Wed, 3 Oct 2001
NEW MODEL OF INTERGALACTIC MAGNETIC FIELDS. On Earth strong magnetic fields, powered by currents moving through wires, steer energetic particles around an accelerator. At the Sun magnetic fields, powered by immense subsurface currents, spring upwards to facilitate the warming of the Sun's corona and, further downstream, to buffet the Earth and sometimes disrupt our terrestrial telecommunications. But where do the fields in the intergalactic medium (IGM) come from, and what role do they play in the life of the cosmos? Such fields have been observed to reside even in parts of space relatively devoid of galaxies. Earlier theories of IGM fields, such as the ideas that the fields may be partly primordial in nature (present at the Creation) or that they grew as a result of shock waves occurring at the boundary between massive colliding gas clouds, must now be amended to include the substantial contribution of galactic black holes. Philipp Kronberg and Quentin Dufton at the University of Toronto (kronberg@physics.utoronto.ca, 416-978-4971) and Hui Li and Stirling Colgate at Los Alamos believe that fully half of the energy content of those massive radio-emitting lobes (up to 10^60 ergs) exists in the form of magnetic energy thrown out of hundred- million-solar-mass black holes. This represents about 10% of their total gravitational energy (about 10^61 ergs). This latter energy, summed over many galaxies, appears to be the largest available energy reservoir in the mature universe for magnetizing intergalactic space. They also suggest that the fields don't stop there but continue on to fill up large volumes of space, even those rural areas between galaxy clusters. These expelled magnetic fields should exert a substantial influence on galaxy formation. The dynamo process whereby black holes would crank out so much energy and such strong fields remains one of the greatest problems in astrophysics. (Astrophysical Journal, 10 October 2001; Los Alamos preprint astro-ph/0106281)


Tue, 23 Oct 2001
LOOP QUANTUM GRAVITY (LQG), rival of string theory in the quest to unite quantum mechanics with general relativity, does not suffer from certain mathematical "infinities" (corresponding to ephemeral, but numerous, alternatives in the way that interactions take place in spacetime), a new study shows. This clears up some doubts as to the theory's usefulness. What is LQG, and why has it been so difficult to quantize gravity? To address this question, return to classical (pre-1900) physics, a regime in which space was fixed. Then the relativity and quantum revolutions changed everything utterly. With the advent of general relativity, space was combined with time in an integrated, but deformable, spacetime. Meanwhile, in quantum mechanics spacetime remains fixed but matter becomes fuzzy; the whereabouts of particles can only be expressed in terms of probability clouds. In a theory that would combine quantum and gravity features, spacetime would then have to be both deformable and fuzzy, and this has been difficult to do. In string theory, the merger is accomplished by imagining that matter ultimately consists of tiny strings. In loop theory, the merger is attempted by imagining that space itself consists of moveable tiny loops. Carlo Rovelli (Center for Theoretical Physics, Marseilles, rovelli@cpt.univ-mrs.fr, 33-0491-269644; also Univ Pittsburgh) argues that loop theory does not have to import the extra commodities (additional dimensions and particles) needed by string theory and that it offers, in principle, more testable predictions, such as the idea of quantized surface areas (that is, regions of space would come in discrete chunks and there would be a minimum possible size scale) and the notion that quantized spacetime might manifest itself as a minute difference in the speed of light for different colors. The new version of loop gravity studied by Rovelli and his colleagues pictures spacetime as being foamy: points in space sometimes grow into bubbles. The bubbles are not "in" space but constitute space itself. The infinities pondered in the present paper represent not difficulties posed by the reality of particles within particles (a necessary complexity dealt with in Richard Feynman's quantum electrodynamics theory) but rather, analogously, to those potentially corresponding to interactions occurring on spacetime loops within loops. (Crane et al., Physical Review Letters, 29 October 2001)

 















Tue, 23 Oct 2001
IMPLANTABLE BioMEMS. Microelectromechanical systems (MEMS), tiny devices crafted using microchip technology, have appeared in a number of settings; examples include micron-sized motors, gears, pumps, and detectors. One would also like to use MEMS in implantable medical applications, but bio-compatibility has been a problem. To address this obstacle Tejal Desai at the University of Illinois-Chicago (tdesai@uic.edu, 312-413-8723) has developed a capsule containing insulin-secreting cells. The capsule is covered with pores as small as 7 nm which allow the release of insulin while blocking the entrance of antibodies thrown up the immune system to counteract the transplanted cells. Desai, who has tested her capsules on mice and rats, will report her new results with nanopore capsules (including also compartmented 100-micron chips for drug delivery) at the AVS Science and Technology Society meeting in San Francisco, Oct 29-Nov 2
http://www.avssymposium.org/Overview.asp
Desai's abstract at : http://www.avssymposium.org/paper.asp?abstractID=145
her university website: http://www.uic.edu/depts/bioe/faculty/tejal_desai/CML%20lab/res_lab.htm


7 Nov 2001
SOUND WAVES MAKE FILTERS FINER. Generally, filters that remove particulates from fluids are limited by their pore sizes. That is, a filter with millimeter-sized pores isn't likely to catch many micron-sized particles. On the other hand, a filter with tiny pores can trap small particles at the expense of inhibiting fluid flow. Donald Feke (Case Western Reserve University, dlf4@po.cwru.edu, 216-368-2750), however, has found a way to reduce the effective pore size in highly porous media without significantly hindering fluid flow. By applying a low power acoustic signal to a filter, Feke can trap particles as much as a hundred times smaller than the nominal filter pore size. An acoustically aided filter provides relatively little resistance to the fluid that passes through it, and yet collects particles as efficiently as a much finer filter. And once the filter has done its job, the trapped particles can be released at the flip of a switch that cuts off the acoustical signal (figures at http://www.aip.org/mgr/png ). The trapping arises because acoustic signals traveling through a porous material create patterns of standing waves that focus particulate matter toward certain positions on the walls of the pores. Rather than wending their way through the filter, particles headed for the focal points line up to form intricate, stable filaments. In other locations, groups of particles collect in regions of stability within the pores, where they orbit for as long as the signal persists. In addition to novel filter designs, Feke proposes that acoustic manipulation may lead to efficient material sorting technologies or methods that aid in assembling microscopic structures. Feke presented his work at the 73rd annual Society of Rheology meeting in Bethesda, Maryland. http://www.rheology.org/sor01a/abstract.asp?PaperID=157

 

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