Multiply scattered elastic and acoustic waves can reveal a wealth of useful information on the dynamics of complex media. Because the waves sample the medium many times (due to multiple scattering), they are very sensitive to small changes in the physical system. Here we see a mug of beer with Mount Merapi in the background. In the beer mug, an ultrasonic transducer produces a short acoustic pulse, which is scattered many times by the walls of the mug and the beer bubbles. The scattered signal is picked up by a miniature hydrophone, and by observing how the scattered wave changes with time, we can learn about the concentration and speed of the bubbles. Geological changes can be monitored in a similar way. At Mount Merapi, an active volcano on the island of Java in Indonesia, seismic waves are excited by an air gun and detected many kilometers away, after being multiply scattered by heterogeneities in the Earth. These examples illustrate the diversity of applications using multiply scattered waves that are being developed to monitor temporal changes with exquisite sensitivity over a huge range of length and time scales.

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William Kurt Hildebrand
Winnipeg, Manitoba
Second Prize ( Open Category )

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