Sonoluminescence is a phenomenon wherein bubbles in fluid emit photons when abruptly compressed by the presence of certain strong sound waves. Sonoluminescent bubbles, driven by ultrasonic fields at discrete frequencies, oscillate with this unexpected side effect: as bubbles are “squeezed” with enough force and in the right way, they emit one short burst of broad-spectrum light for each period of the sustained sound wave (Kondic, 1995). Astounding! Where does the light come from?
Sonoluminescence was first observed in 1934, at the University of Cologne by Frenzel and Schultes. They applied an ultrasound transducer to a vat of darkroom chemicals hoping to expedite photographic development. Instead, they found minute dots on the developed plates, and through the haze of their deep initial confusion gradually understood that bubbles in the fluid were releasing light in the presence of the ultrasonic waves (Frenzel, 1934). It was nearly impossible for decades to scrutinize these sonoluminescent effects, due to complex environmental requirements and the vast number of extremely short-lived bubbles. Today, this initial discovery is known as multi-bubble sonoluminescence, or MBSL (Young, 2005).
Felipe Gaitan and Lawrence Crumin established a major development in 1989 by demonstrating stable single-bubble sonoluminescence. Single bubble sonoluminescence (SBSL) is “the natural emission of brief pulses of broadband light from a micron-wide gas bubble levitated in water by a steady external sound field.” The bubble expands and contracts in phase with the oscillating pressure field (Gaitan, 1999). In SBSL, a single bubble, ensnared in an acoustic standing wave, emits one pulse of light per compression. This more controlled technique allows further systematic study of the phenomenon by isolating the complex occurrence into one steady, predictable bubble.
The temperature inside sonoluminescing bubbles is hot enough to melt steel. Recent experiments conducted by the University of Illinois at Urbana-Champaign report temperatures around 20,000 Kelvin (Young, 2005). Supplementary research on single bubble sonoluminescence focuses on dynamic bubble motion and detailed light spectrum analysis in the 200 to 700 nm range, using various gas blends to produce the sonoluminescent bubbles. Experiments concentrating shock waves on the bubbles suggest temperatures of up to 1, 000, 000, 000 K, while other estimates range peak temperatures only from 10, 000 to 1, 000, 000 K (Young, 2005). Perhaps this broad array of figures bears witness to either the infancy of sonoluminescence research or the inadequacy of current instrumentation. Certainly there is much to be discovered.
Surely such technological magic can only be conjured by humans tinkering with powerful electronic gadgets. Contrarily, Nature herself provides living examples of this odd phenomenon: Snapping shrimp produce short bursts of light from collapsing bubbles generated as they quickly close their distinctive claws. The light produced is of low intensity and invisible to the naked eye--simply a byproduct of the shockwave the shrimp use to disorient and capture prey. However, it represents the first known example of an animal generating light by sonoluminescence, and was wryly called “shrimpoluminescence” upon its discovery (Lohse, 2001). It has consequently been established that the Mantis shrimp’s club-like forelimbs can also strike so quickly and with such force as to provoke sonoluminescent bubbles upon impact (Patek 2005).
The mechanism for sonoluminescent light emission is still not well understood. There is general agreement that the forceful collapse of a micron size bubble to its hard-core (atoms packed as tight as possible) limit is at the root of the light emission process (Young, 2005).
One exotic theory of sonoluminescence, which has received wide-ranging attention, is the Casimir energy theory suggested by Nobel laureate physicist Julian Schwinger (Schwinger, 1994) and more thoroughly considered in a paper by Claudia Eberlein of the University of Sussex. Eberlein’s paper proposes that sonoluminescent light is produced by the vacuum within the bubble in a process akin to Hawking radiation, the radiation generated along the fringes of black holes. “Quantum theory holds that vacuums contain virtual particles, and the fast moving interface between water and air changes virtual photons to real photons (Eberlein, 1996).” If true, sonoluminescence may be “the first observable example of quantum vacuum radiation (Eberlein, 1996).” An argument has countered that sonoluminescence releases too much energy too quickly to be consistent with the vacuum energy explanation (Milton, 2000). Others argue, “The vacuum energy explanation might yet prove to be correct (Liberati, 2000).”
As interest accelerates and technology improves, answers will arrive and sonoluminescence will gradually be better understood. Until that bright day, plenty of literature waits to keep us occupied. I plan to order an inexpensive kit and use household parts to construct a sonoluminescence station in my garage. It will hopefully keep me out of trouble. I anticipate publishing my results in one of several obscure journals I have had the distinct pleasure of perusing in pursuit of this complex, beguiling, yet multifariously rewarding topic. Beyond doubt, the future of sonoluminescence is as peculiar as it is dazzling.
Works Cited:
Eberlein, C. Theory of quantum radiation observed as sonoluminescence. Physics Review.1996
Frenzel H., Schultes, H. Luminescenz im ultraschallbeschickten wasser. Z. Phys. Chem. 1934
Gaitan, F. Experimental observations of bubble response and light intensity near the threshold for single bubble sonoluminescence in an air-water system. Physics Review. 1999
Kondic, L., Gersten J., Yuan, C. Theoretical studies of sonoluminescence radiation: radiative transfer and parametric dependence. Physical Review. 1995
Liberati, S., Belgiorno, F., Visser, M. Comment on ‘Dimensional and dynamical aspects of the Casimir effect: understanding the reality and significance of vacuum energy’. High Energy Physics and Theory. 2000
Lohse, D., Schmitz, B., Versluis, M. Snapping shrimp make flashing bubbles. Nature. 2001
Milton, K. A. Dimensional and dynamical aspects of the Casimir effect: understanding the reality and significance of vacuum energy. High Energy Physics and Theory. 2000
Patek, S. N., Caldwell, R. L. Extreme impact and cavitation forces of a biological hammer: strike forces of the Peacock mantis shrimp. Journal of Experimental Biology. 2005
Schwinger, J. Talk at the Fourth International Conference on Cold Fusion. ICCF4. 1994
Young, F. R. Sonoluminescence. CRC Press. 2005
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