SETI Physics News Update.434


Larry Klaes (lklaes@bbn.com)
Mon, 21 Jun 1999 13:01:51 -0400


>Date: Fri, 18 Jun 1999 11:07:45 -0400 (EDT) >From: AIP listserver <physnews@aip.org> >To: physnews-mailing@aip.org >Subject: update.434 > > >PHYSICS NEWS UPDATE >The American Institute of Physics Bulletin of Physics News >Number 434 June 18, 1999 by Phillip F. Schewe and Ben Stein > >MEASURING THE FREQUENCY OF LIGHT TO NEW >LEVELS OF PRECISION is now possible, opening a new chapter >in metrology which may lead to greatly improved determinations >of fundamental constants and one way of making powerful optical >versions of atomic clocks. Even the most advanced electronic >equipment cannot directly measure electromagnetic frequencies >higher than roughly 100 GHz (in the microwave range, where >frequencies can be counted in terms of the number of oscillations >induced in an electrical circuit). Now, researchers at the Max >Planck Institute for Quantum Optics (Thomas Udem, 011-49-89- >32905-257, Thomas.Udem@mpq.mpg.de) have shown that a >femtosecond laser pulse can be used as a "ruler" for precisely >determining the frequencies of visible light (which goes up to >roughly a million GHz). A femtosecond pulse does not contain a >single frequency; rather, its spectrum consists of many frequency >peaks which give the appearance of a comb with the tips pointing >upwards. The researchers have now shown that the very regular >spacing of these peaks can potentially be used to measure >differences of at least 20 THz between two electromagnetic waves >with a precision as high as 3 parts in 10^17 (Udem et al., Optics >Letters, 1 July 1999). For comparison, the best atomic clocks >today, based on measuring radio-frequency atomic transitions, >have accuracies of 2 parts in 10^15. Locking the wave of interest >to the low-frequency end of the femtosecond comb and locking a >reference wave to the high-frequency end can determine the >frequency difference between the two waves and ultimately allow >one to reconstruct the frequency of the visible-light wave. Using >femtosecond lasers, the researchers have already measured the >frequency of visible light emitted by a cesium atom undergoing a >specific transition (specifically, its "D1 line") to a precision of 120 >parts per billion, almost 1000 times more precise than previous >measurements of that light. (Udem et al, Phys. Rev. Lett., 3 May >1999). The D1 frequency can be plugged into a formula for >precisely calculating the fine structure constant, which dictates the >strength of the electromagnetic force. > >HOW DO COMPLEX ORGANISMS FORM? A Darwinian >mechanism of natural selection plus random mutation is not quite >enough to explain the complex features of life on earth. For >example, it does not predict or anticipate the fact that an ecosystem >or a global community has a hierarchical structure, with >interactions that take place at several size scales. For example, >people communicate with each other in an organization; and >organizations communicate with each other in a larger community. >Barbara Drossel of the University of Manchester in England (011- >44-161-275-4201, barbara.drossel@man.ac.uk) has introduced a >simple mathematical model for describing how originally >independent units may develop into a complex organism with a >hierarchical structure. In her model hierarchy comes about because >of the increase of a quantity she calls "productivity" (similar to >"fitness" in biology and "utility" in economics). Individual units >communicate with each other to increase productivity which leads, >at the very least, to larger groups. Drossel's model incorporates >the additional idea that the size of a group is restricted by the >limited capacity of individuals to communicate and to travel. >Therefore, she introduces a "communication cost" per partner and >per unit distance to the partner. This encourages the formation of >groups and ultimately the formation of supergroups and groups of >supergroups which interact with each other. (Drossel, Physical >Review Letters, 21 June 1999.) >



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