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Date: Wed, 22 Nov 2000 14:19:13 -0500 (EST)
From: AIP listserver
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.)
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