Undergraduates
Why does the space shuttle returning to Earth cause two separate sonic booms?
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Professor Richard Weaver received an A.B. degree in physics from Washington University in St Louis in 1971 and a Ph.D in astrophysics from Cornell University in 1977. He came to Illinois in 1981 after a research associateship in theoretical elastic wave propagation and ultrasonics at Cornell. He was elected a fellow of the Acoustical Society of America in 1996 and received the Hetenyi Award from the Society for Experimental Mechanics in 2004. He is associate editor of the Journal of the Acoustical Society of America. Formerly a professor in the Department of Theoretical and Applied Mechanics at Illinois, he joined the Department of Physics in 2006. Description of Current ResearchUltrasonic Analog for a Random Laser (with colleagues Oleg Lobkis, UIUC, and Alexey Yamilov, U Missouri) — We report measurements on ultrasonic systems analogous to random lasers. An auto-oscillating, and spontaneously emitting, piezoelectric device is found to emit more energetically when stimulated by an incident wave field. The emission is at the same frequency as the stimulating field, and with a phase relation corresponding to super radiance. Over a wide range of parameters we observe narrow single emission lines, sensitivity to linear cavity properties, complex multi-mode emissions, and line narrowing. Line widths are more narrow than we can measure. Theory suggests they have Schawlow–Townes widths as low as 10-9 Hz. Systems of several such oscillators are observed to self-organize into a coherent state with power emission that rises faster than the first power of the number of oscillators. Statistical Elastodynamics of Large Structures and Quantum Chaos (with Oleg Lobkis, UIUC, and Thomas Seligman, Centro de Ciencias Fisicas, UNAM) — Numerical simulations, analytic theory and laboratory measurements are used to study the statistics of linear waves in complex systems. Particular attention is paid to wave energy density (or probability for quantum waves), and its mean flow and fluctuations. We seek methods to predict mean flow and fluctuations over long times, based on information in ray optics or direct numerical simulations over short times. Seismic Noise Correlations (with M Campillo, B van Tiggelen and E Larose, U Joseph Fourier, and O Lobkis UIUC and X Song UIUC Geology) — Recent attention to diffuse fields in seismology, inspired in part by laboratory experiments done at UIUC, is leading to new methods for probing the interior of the earth. We observe and exploit mesoscopic residual correlations in nominally incoherent multiply scattered elastic wave fields, on the moon, in the seismic coda, in local geophone noise, and in long period world-wide background seismicity. Determination of Thin-Film Interfacial Properties by Laser Generated Stress Waves (with N Sottos UIUC MatSE) — We investigate, both theoretically and experimentally, the generation of high-amplitude compression waves due to the sudden deposition of heat from a YAG laser pulse in a thin metallic film between two solids. The resulting pulse, with a duration of 10 nsec and a strain amplitude of the order of 1%, is measured using laser interferometry. Particular issues of concern include the effects of nonlinearity in the wave propagation and the corresponding development of shocks, and mode conversion at oblique interfaces with consequent generation of high-amplitude shear waves. Each of these effects is critical in ultimate application to the testing, by high-speed stress loading, of thin-film coatings. News About Our ResearchIn a recent publication (Weaver JASA 2013) I have extended our previous exact understanding of Greens function retrieval for fully diffuse equipartitioned fields [ eg. Weaver and Lobkis JASA and PRL 2001 ] to the case, far more important in practice, of incompletely diffuse fields, as long as they can still be comprehended by means of their specific intensities as evolving according to radiative transfer. This puts practical retrieval on a much firmer theoretical foundation and in particular permits us to interpret amplitudes of correlation functions. My papers on doing just that [ Weaver Comptes Rendus Geoscience 2011, GJI 2013) have shown, for well controlled numerical experiments, that we can retrieve attenuations and amplitudes with stunning precision. That comparable precision still escapes the practical seismologist is a mystery yet to be unraveled. Doing so will require continued vigorous communication between theorists (i.e. myself) and seismologists (esp X Song of Geology and D Yang of LANL, but more generally the entire community) I look forward to this with some trepidation and some excitement. In a collaboration with an experimentalist colleague at OSU ( two of his students visited me for a while last year) , I provided the theory side of a piece of work (Zhao et el JASA 2013) doing pump-probe studies of tiny single crystals and the subsequent extraction of elastic properties. The basic linear thermo-elastic equations that govern these experiments have long been understood. Strikingly though, the full anisotropic case has resisted theoretical solution until now. I was pleased to develop a clever reciprocity argument that substantially simplifies the analysis.For more informationHonors and awards
Semesters Ranked Excellent Teacher by Students
Selected Publications
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