ucla |A spectacular example is provided by sonoluminescence which is the phenomenon where by sound is channelled into light. In this effect a diffuse uniformly applied sound wave propagating through water can be observed to spontaneously focus its energy by over a factor of one trillion to generate a very short flash of ultraviolet light. A similar effect can be observed in the flow of water through a converging pipe. At flows which achieve velocity variations of about a meter/second bubbles form in the constriction and then emit picosecond bursts of ultraviolet light as they collapse downstream [flow cavitation].
Another example of energy focusing relates to the everday experience of generating a spark upon touching a door-knob after rubbing one's shoes on a carpet. In the laboratory a controlled realization of frictional-electricity is realized by moving glass relative to mercury. A motion of only a millimeter per second leads to the repetitive acceleration of electrons to at least 1% of the speed of light. Furthermore these electrons are emitted in bursts which are again measured in picoseconds.
Turbulence is another well known example of energy focusing. Here the phenomenon is referred to as intermittency. When a fluid is sufficiently agitated so that the effects of nonlinear dynamics overwhelm the damping effects of viscosity the motion becomes turbulent. The turbulence is not uniform being characterized by regions of unexpectedly violent and quiescent motion.
We also believe that the commonplace effect of friction is an example of the concentration of energy density, or stress. Here a pressure that is uniformly applied to a macroscopic body focuses down to tiny junctions where it reaches levels of one million atmospheres.
None of the above problems has been explained nor is there a generic understanding of the tendency of nature to form structures and focus energy off-equilibrium. In some instances models with many input effects have been generated that can parametrize a portion of the existing data, but these models become quite weak when challenged to make a prediction.
Finally it must be emphasized that these unsolved problems in physics are fundamental. Since no-one has yet succeeded to derive fluid mechanics from the first principles of quantum mechanics [or Newton's Laws] these emergent theories are, as my thesis adviser George E. Uhlenbeck was fond of emphasizing, just as fundamental as the reductionist's so-called first principles of physics.
