Effects of Sonoluminescence
Project: Designing Science Fiction Scenarios

Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound.

Sonoluminescence may or may not occur whenever a sound wave of sufficient intensity induces a gaseous cavity within a liquid to quickly collapse. This cavity may take the form of a pre-existing bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained.

Some facts about sonoluminescence:

* The light flashes from the bubbles are extremely short—between 35 and a few hundred picoseconds long, with peak intensities of the order of 1-10 mW.
* The bubbles are very small when they emit the light—about 1 micrometre in diameter depending on the ambient fluid (e.g. water) and the gas content of the bubble (e.g. atmospheric air).
* Single-bubble sonoluminescence pulses can have very stable periods and positions. In fact, the frequency of light flashes can be more stable than the rated frequency stability of the oscillator making the sound waves driving them. However, the stability analysis of the bubble show that the bubble itself undergoes significant geometric instabilities, due to, for example, the Bjerknes forces and Rayleigh-Taylor instabilities.
* The addition of a small amount of noble gas (such as helium, argon, or xenon) to the gas in the bubble increases the intensity of the emitted light.

The wavelength of emitted light is very short; the spectrum can reach into the ultraviolet. Light of shorter wavelengths has higher energy, and the measured spectrum of emitted light seems to indicate a temperature in the bubble of at least 20,000 kelvins, up to a possible temperature in excess of one megakelvin. The veracity of these estimates is hindered by the fact that water, for example, absorbs nearly all wavelengths below 200 nm. This has led to differing estimates on the temperatures in the bubble, since they are extrapolated from the emission spectra taken during collapse, or estimated using a modified Rayleigh-Plesset equation (see below). Some estimates put the inside of the bubble at one gigakelvin [1]. These estimates are based on models which cannot be verified at present, and may include too many unsupported assumptions.

Temperatures this high make the study of sonoluminescence especially interesting for the possibility that it might produce a method for achieving thermonuclear fusion. If the bubble is hot enough, and the pressure in it is high enough, fusion reactions like those that occur in the Sun and other stars could be produced within these tiny bubbles. This possibility is sometimes referred to as bubble fusion.

On January 27, 2006, researchers at Rensselaer Polytechnic Institute claimed to have produced fusion reactions by sonoluminescence, without an external neutron source, according to a paper published in Physical Review Letters [2] [3]. To date, these results have not been reproduced by other members of the scientific community.

Recent experiments (2002, 2005) of R. P. Taleyarkhan, et.al., using deuterated acetone, show measurements of tritium and neutron output consistent with fusion, but these measurements have not been reproduced outside of the Taleyarkhan lab and remain controversial. Brian Naranjo of the University of California, Los Angeles, has recently completed an analysis of the Taleyarkhan results claiming that Taleyarkhan had most likely misinterpreted the radioactive decay of standard lab materials for the byproducts of nuclear fusion.

Writing in Nature, chemists David J. Flannigan and Kenneth S. Suslick study argon bubbles in sulfuric acid and show that ionized oxygen mbox{O}_2^+, sulfur monoxide, and atomic argon populating high-energy excited states are present implying that the bubble has a hot plasma core. They point out that the ionization and excitation energy of dioxygenyl cation is 18 electronvolts, and thus cannot be formed thermally; they suggested it was produced by high-energy electron impact from the hot opaque plasma at the center of the bubble (Nature 434, 52 - 55 (03 March 2005); doi:10.1038/nature03361).

An unusually exotic theory of sonoluminescence, which has received much popular attention, yet is considered to have a marginal effect on the mechanism of SBSL by the scientific community at large, is the Casimir energy theory proposed by Claudia Eberlein, a physicist at the University of Sussex. In 1996, it was suggested that the light in sonoluminescence is generated by the vacuum around the bubble in a process similar to Hawking radiation, the radiation generated by the edges of black holes. Quantum theory holds that a vacuum is filled with virtual particles, and the rapidly moving interface between water and air converts virtual photons into real photons. This is related to the Unruh effect or the Casimir effect. If true, sonoluminescence may be the first observable example of quantum vacuum radiation. It is, however, argued that the mechanism leading to the above effects do not occur on the proper time scales to describe the observed spectrum of SBSL, which is thought to likely obey a classical cavitation collapse; and thus the Casimir model has been largely relegated to the position of an ancillary remnant of the field at large.

Mon, Jun 4, 2007  Permanent link

Sent to project: Designing Science Fiction Scenarios