Physics News Update, Number 474

Number 474, March 10, 2000 by Phillip F. Schewe and Ben Stein

HIGHLY OPTIMIZED TOLERANCE. Many natural and man-made systems exhibit power-law statistics. That is, when you plot the likelihood of an event (e.g., sizes of forest fires, power outages, and web file transfers, or losses due to hurricanes, floods, earthquakes, and man-made disasters) as a function of size the resulting graph will fall off proportionally to the size of the event raised to some exponent. Interactions or phenomena at many size scales (from very small to very large) contribute to the overall state of these systems.

One theory which tries to explain all this is "self organized criticality." Jean Carlson of UC Santa Barbara (carlson@physics.ucsb.edu) and John Doyle of Caltech (doyle@cds.caltech.edu) now propose another theory, called highly optimized tolerance (HOT), which they believe does a better job of accounting for the tendency in interconnected systems to gain a measure of robustness against uncertainties in one area by becoming more sensitive elsewhere. As with energy conservation or the inexorable increase in entropy, efforts to violate the robustness principle will fail. Especially in biological evolution or in engineering, this means that a system might obtain robustness against common and designed-for uncertainties and yet be hypersensitive to design flaws or rare events.

For example, organisms and ecosystems exhibit remarkable robustness to large variations in temperature, moisture, nutrients, and predation, but can be catastrophically sensitive to tiny perturbations, such as a genetic mutation, an exotic species, or a novel virus. Engineers deliberately design systems to be robust to common uncertainties. Cost and performance tradeoffs force an acceptance of some hypersensitivity to (one hopes) rare perturbations.

In evolved or designed systems, this tradeoff leads to the "robust, yet fragile" characteristic of complexity, one feature of which is power laws. Doyle and Carlson have been exploring the application of their theory to a number of biological and engineering problems with the help of experts in those fields. (Carlson, Doyle, Physical Review Letters, 13 March 2000; Select Article; a longer version appears in Physical Review E, August 1999.)

16 CONSECUTIVE MONTHS OF RECORD HIGH TEMPERATURES (global mean temperature; during 1997 and 1998) suggest to Thomas Karl (director of the National Climate Data Center, NCDC) that a human-induced global warming trend, and not merely the kind of natural temperature fluctuations one expects to see in the climate record, is under way. According to Karl, the last quarter-century of data are characterized by a temperature gradient of two degrees Celsius per century. The 16-month streak is unmatched in temperature records dating back to the nineteenth century. (Thomas Karl in Geophysical Research Letters, 1 March 2000; tkarl@ncdc.noaa.gov, 828-271-4476.)

ANAMORPHIC IMAGES are those in which the painted image of an object has been distorted in such a way that the object becomes recognizable only by viewing it at an oblique angle or in some curved reflecting surface. Anyone who has visited the National Gallery in London might have seen Hans Holbein's painting "The Ambassadors," in which an odd shape at the bottom of the canvas is seen to be a skull when viewed almost edge-on. Anamorphic images were something of a rage in the Renaissance, and Leonardo and Durer tried the technique as part of their studies of perspective. An eighteenth century innovation was to create anamorphs of paintings by famous artists. A seventeenth century book by Jean-Francois Niceron worked out the geometrical algorithms for producing anamorphic art (the planar and conical cases are pretty easy but cylinders are quite challenging), but this mathematical connection was lost through the centuries. Now, scientists at Guelph University (Ontario, Canada) have re-derived the transform equations needed for producing anamorphs. (Hunt, Nickel, Gigault, American Journal of Physics, March 2000; Select Articles; James Hunt, phyjlh@physics.uoguelph.ca, 519-824-4120, x3993; Bernie Nickel, bgn@physics.uoguelph.ca; images at http://physics.uoguelph.ca/)

ATMOSPHERIC INFRASOUND. Humans hear sounds in the 20-20,000 Hz frequency range. Ultrasound, waves above 20,000 Hz, is used for things such as medical imaging and can be sensed by animals such as bats and dogs. But how about infrasound, sound at frequencies less than 20 Hz? Humans can sometimes feel (even if they can't quite hear) infrasound in the form audio systems with the bass turned way up. The monitoring of infrasound may contribute practical benefits in a number of areas: determining the location and nature of avalanches, tornadoes, meteor strikes, volcanoes, and nuclear weapons tests. (Physics Today, March 2000.)