November 18, 2009
Spotting Evidence Of Directed Percolation
Convincing experimental evidence finally found for directed percolation
A team of physicists has, for the first time, seen convincing experimental evidence for directed percolation, a phenomenon that turns up in computer models of the ways diseases spread through a population or how water soaks through loose soil. Their observation strengthens the case for directed percolation's relevance to real systems, and lends new vigor to long-standing theories about how it works. Their experiment is reported in Physical Review E and highlighted with a Viewpoint in the November 16 issue of Physics.
Now a team of physicists from the University of Tokyo, in Japan, and CEA-Saclay, in France, have seen directed percolation in a layer of liquid crystals about a hundredth of a millimeter thick sandwiched between two glass plates connected to electrodes. When they increased the voltage above a threshold, they saw gray spots appearing. A spot could disappear spontaneously but also cause spots to pop up around it, similar to the way a virus can die in one individual after infecting people nearby. The team showed that the system exhibited many of the mathematical hallmarks of directed percolation"”convincing evidence that the long-theorized phenomenon occurs in real systems.
Image Caption: This is an illustration of directed percolation in 1+1 dimensions: Activity percolates through open bonds (red lines), activating nearest neighbors and giving rise to a cluster of activity. Credit: Illustration: Alan Stonebraker
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