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16. Mai 2001 © email: Krahmer


The American Institute of Physics Bulletin of Physics News Number 536 April 27, 2001 
The American Institute of Physics Bulletin of Physics News Number 537 May 2, 2001
by Phillip F. Schewe, Ben Stein, and James Riordon

ULTRASONIC BANDGAP MATERIALS are to sound waves what semiconductors are to electrons and photonic bandgap materials to light waves: they allow some energies (or frequencies) and not others. The hope is to fabricate the acoustic equivalent of various electronic or optical elements, such as mirrors, lenses, even switches and "transistors" in some future acoustic integrated circuit. The trouble is that, as with the optical counterpart, it has been difficult to achieve full exclusion of certain acoustic frequency bands in "phononic" materials. Pressing ahead anyway, a group of physicists in Spain have produced an ultrasonic wedge which, even without perfect acoustic bandgap performance, can split a beam of sound waves or steer the sound through an angle of 90 degrees. At the Instituto de Fisica Aplicada in Madrid (contact Jose Aragon, joseluis@iec.csic.es, 34-915-618-806 x 251) researchers create a material consisting of mercury cylinders inserted into a slab of aluminum (see figure ). The researchers noticed that in refracting through their device the sound waves did not conform to Snell's law, the classical equation governing the propagation of waves from one medium into another, a phenomenon (probably related to the interaction between the waves and the compound crystalline environment of the wedge) which might be applicable to the case of light waves. (Torres et al., Physical Review Letters, 7 May 2001)

THE LIMITS OF SUPERLUMINAL PROPAGATION. Last year, L.J. Wang and his colleagues at the NEC Institute reported that a composite wave pulse traveled with little distortion through a medium at a group velocity faster than c, without violating Einstein's theory of relativity, or the notion that cause precedes effect. (Update 495) Sent into a chamber of specially prepared cesium atoms, the light pulse exited the chamber before the peak of the input pulse entered it. This can happen because the early part of the pulse, made of many component waves, contains all of the information in the wave. Once inside the chamber, the pulse is rearranged such that the peak reappears at a position a little farther ahead in the chamber. This causes the composite pulse to emerge from the chamber earlier than if it had been traveling through the chamber at the speed of c. Potential applications involve the possibility of shuttling along light waves faster in applications such as telecommunications and computers. How to define and analyze the speed of signal transfer in that setup is a subject of a new paper by the same researchers, along with two other physicists: Peter Milonni of Los Alamos and Raymond Chiao of UC Berkeley (chiao@physics.berkeley.edu). They consider the effect that quantum noise, due in part to random spontaneous emission by the medium, has on the reliability with which a signal can be measured. The more one tries to push along the signal in the medium, the greater the number of noise-producing, signal- obscuring spontaneous emissions that occur, and any attempt to boost the signal's intensity to make it more detectable introduces delays such that the signal velocity always ends up to be less than c. Therefore, the signal velocity is defined operationally as an optical signal-to-noise ratio. In summary, the researchers extended the special relativity speed limit of c for sharp wavefronts (which act like "on-off" signals), to that of a more realistic smoothly varying signal, based on a speed limit set by quantum fluctuations. (A Kuzmich et al., Phys. Rev. Lett., 30 April 2001.)

  MAJOR NEW COSMIC MICROWAVE BACKGROUND (CMB) measurements uphold the idea of an early "inflationary" era during which the observable universe expanded with superluminal speed and tiny quantum fluctuations in the density of matter were amplified into much larger structures. These structures are imprinted in the CMB as faint variations in the temperature across the microwave sky. The CMB, the curtain of photons set free when the expanding universe became cool enough to permit the existence of neutral atoms, is the earliest, largest, and furthest observable thing in all of science. The best way to extract cosmological information from the CMB is to plot the observed microwave power as a function of the angular size of regions contributing to the CMB. The inflation model predicts that this spectrum should feature a number of peaks. The first peak, at an angular size of about 1 degree (about twice the angular size of the Moon), corresponds to the largest blobs of matter in the primordial plasma at the time of the CMB (about 400,000 years after the big bang). Subsequent peaks should correspond to blobs that had come together under the action of gravity but had then rebounded outward because of radiation pressure, and later still had condensed for a second or third time, etc. A year ago the Boomerang collaboration, which used a balloon-based detector floating over Antarctica, provided a detailed map (Update 481) of the first peak which, besides falling at the 1-degree size predicted by inflation, also determined that the overall curvature of the universe was zero. But Boomerang, and another detector group, Maxima, saw scant evidence of any other peaks, and this puzzled astronomers. All this changed earlier in the week at the American Physical Society (APS) meeting in Washington, DC, where the Degree Angular Scale Interferometer (DASI) collaboration, which parks its microwave detector on the roof of NSF's South Pole station, presented solid evidence for a second and third peak. The DASI results (John Carlstrom, University of Chicago, 773-834-0269) were largely in concert with Boomerang's presentation at the meeting (Barth Netterfield, Univ Toronto, 416-946-5465); Boomerang used a new type of analysis and reported 14 times more data than last year. The microwave spectra for the two groups were similar
(see figures at http://www- news.uchicago.edu/releases/01/dasi/index-embargoed.shtml; oder http://www.physics.ucsb.edu/~boomerang/press_images) as were the values of various cosmological parameters. For example, the position of the first peak yields the total energy of the universe (a parameter, denoted by the letter omega, expressed as a fraction of the critical density needed for halting the cosmological expansion). Boomerang and DASI found values of 1.03 and 1.04, respectively, with about a 6% uncertainty. Comparing the height of the first and second peaks, one can calculate the expected percentage of all energy in the universe that exists in the form of ordinary matter (baryons). This turns out to be about 5% for both groups, a fact that agrees well with predictions made by the independent "big bang nucleosynthesis" theory. It is harder to nail down other cosmological parameters, such as the percentage of energy in the form of dark matter or dark energy (energy lurking in the vacuum and responsible for the newly discovered net acceleration in the cosmological expansion). The new CMB measurements suggest values of about 30% and 65%, respectively, again in keeping with recent expectations. New Maxima results (Shaul Hanany, Univ Minnesota, 612-626-8929) presented at the meeting did not have nearly the statistical weight of the other two groups, but were generally consistent; the three- way agreement brought a great round of applause from the audience of astronomers eager to unravel the mysteries of the early universe. Noted cosmologist Michael Turner (Univ Chicago, 773-702- 7974) observed that last year's discovery of the first microwave peak constituted the first great vindication for the Inflation model and that this new discovery of secondary peaks was the second great vindication. The third type of evidence, Turner said, would be the detection of gravity waves from before the time of the CMB. (Recently posted preprints on the Los Alamos server (http://xxx.lanl.gov/) include the following: Maxima astro- ph/014459; Boomerang astro-ph/0104460; and DASI astro- ph/0104488, 89, and 90.)

UNEXPECTED PHYSICS CONDITIONS IN RHIC COLLISIONS. At the APS meeting, speakers from all four detector groups (BRAHMS, PHENIX, PHOBOS, STAR) at the Relativistic Heavy Ion Collider (RHIC) agreed that it is too early to declare a sighting of the coveted quark-gluon plasma (QGP), a primordial soup of free-ranging quarks and gluons. But presenters said they found preliminary signs of tantalizing QGP "prerequisites." Studying the products of RHIC's collisions between near-light-speed-velocity gold-ion beams, all four detector groups measured a more equal ratio of antiprotons to protons (roughly a 3:2 ratio, according to BRAHMS measurements) than ever before seen in nuclear collisions. This is the closest reproduction yet of the matter-antimatter balance thought to prevail at the time of the Big Bang than previously achieved in the laboratory, said PHOBOS member Russell Betts (U. Illinois at Chicago). In fact, the abundance of protons and antiprotons in the collision products was surprising, raising possibilities of a new production mechanism for proton-antiproton pairs or a suppression in the production of lighter particles, said PHENIX's Sam Aronson (Brookhaven). Studying the highest-momentum products moving transversely to the direction of the ion beams, the groups found hints of "jet quenching," the idea that the particles lose significant energy while traveling through the collision fireball. Such a large energy loss does not occur in ordinary nuclear matter. In addition, the STAR collaboration observed that the collision fireball expanded violently, at supersonic speeds. Voicing a minority view, STAR member John Cramer (U. Washington) speculated that such a violently exploding fireball may mean that RHIC is operating at energies higher than those required for creating a QGP (expected by some to expand more gently). However, all agree that the picture will become clearer in RHIC's next experimental run, slated to begin later this month, in which the groups expect to gather 10-100 times more data from the accelerator, which will be able to run, for the first time, at its maximum energy of 100 GeV/nucleon.







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