____________________________________________________________________ He is able who thinks he is able. Buddha The Armadillo Group ,::////;::-. James Choate Austin, Tx /:'///// ``::>/|/ ravage@ssz.com www.ssz.com .', |||| `/( e\ 512-451-7087 -====~~mm-'`-```-mm --'- -------------------------------------------------------------------- ---------- Forwarded message ---------- Date: Wed, 22 Nov 2000 14:19:13 -0500 (EST) From: AIP listserver <physnews@aip.org> To: physnews-mailing@aip.org Subject: update.513 PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 513 November 22, 2000 by Phillip F. Schewe and Ben Stein THE INTERNET IS SURPRISINGLY ROBUST, and it remains connected on a global scale even if a randomly chosen 99% of its connection points break down. However, it is relatively fragile if its most highly connected points are selectively knocked out. These are the conclusions of researchers applying physics principles and precise mathematical models to the study of the worldwide computer network. The Internet consists of computer networks (most commonly, "local area networks") connected by various devices, known as routers and hubs. For simplicity's sake, researchers consider each connection point as a generic "node." Previous work suggests the fraction of Internet nodes having k connections is proportional to k^-a, for some number a. This is a "scale-free power law distribution," which occurs commonly in nature and appears in the frequency of earthquakes and the size distributions of clouds and mountains. Unlike an exponential distribution, a scale-free power law distribution decays very slowly, meaning in this case that there is a large proportion of computers that still have a significant amount of connections. Recent computer simulations of scale-free networks have shown that the Internet is resilient for this reason (Albert et al., Nature, 27 July; Albert-Laszlo Barabasi, Notre Dame, 219-631-5767, alb@nd.edu; see also The Industrial Physicist, December 2000). The latest work now puts this conclusion on a firm mathematical footing. Two independent groups (Reuven Cohen, Bar Ilan University, Israel, 011-972-8-9370131, cohenr@shosji.ph.biu.ac.il; Duncan Callaway, Cornell, 607-255-9174; dc52@cornell.edu) apply percolation theory, developed by geophysicists interested in estimating how much oil they could extract from reservoirs in a porous medium. Percolation theory deals with systems containing points ("sites") and connections between them, and it analyzes the behavior of the system when one removes some of the sites or connections. Combined with the insights from the scale-free distribution, the powerful percolation-based approach may help Internet architects to maximize resistance against Internet attacks, by controlling the distribution of nodes having certain numbers of connections. (Cohen et al, Phys. Rev. Lett, 20 Nov (Select Articles); Callaway et al., Phys. Rev. Lett., upcoming.) COSMIC RAYS AND CLOUD COVER. Galactic cosmic rays (GCRs) play an important role in controlling global cloud cover on Earth, according to recent studies by researchers at the Danish Space Research Institute in Copenhagen (Nigel D. Marsh, 011-45- 35325740). GCRs, consisting principally of energetic protons emitted from stars within our galaxy, are a primary source of the atmospheric ionization which affects cloud formation. Because cloud cover has an impact on both the reflection of solar radiation and the retention of heat in the atmosphere, correlation between GCRs and low level clouds suggests a link between global climate changes and cosmic ray flux (see figure at http://www.aip.org/physnews/graphics). The discovery reveals a convoluted connection between solar variability and climate change. Fluctuations in the sun's radiative output are generally dismissed as too small to account directly for global warming and other climate variations. Periods of intense solar activity, however, lead to powerful solar winds which shield the atmosphere from cloud-forming GCRs, potentially modulating the global climate. (N. D. Marsh; H. Svensmark, Physical Review Letters, 4 December.) Researchers at the University of Leeds (UK), on the other hand, have observed a direct and rapid connection between atmospheric chemistry and ultraviolet light from the sun (Dwayne E. Heard, 44-113-233-6471, dwayneh@chem.leeds.ac.uk). During the 97% eclipse of the sun over Ascot, England, local ozone concentrations fell to 60% of typical daytime levels, and quickly returned to normal after the event. The study demonstrates the dynamic connection between sunlight and the photochemistry of atmospheric gasses which may contribute to global warming, smog formation, and acid rain. (J. P. Abram; et al, Geophysical Research Letters, 1 November.) PHYSICS NEWS UPDATE: subscribe or unsubscribe by sending a message to listserv@aip.org and specify either "add physnews" or "delete physnews". Our website is at http://www.aip.org/physnews/update.