As the intensity of a standing sound wave is increased the pulsations
of a bubble of gas trapped at a velocity node attain sufficient amplit
ude so as to emit picosecond hashes of light with a broadband spectrum
that increases into the ultraviolet. The acoustic resonator can be tu
ned so that the flashes of light occur with a clocklike regularity: on
e flash for each cycle of sound with a jitter in the time between flas
hes that is also measured in picoseconds. This phenomenon (sonolumines
cence or ''SL'') is remarkable because it is the only means of generat
ing picosecond flashes of light that does not use a laser and the inpu
t acoustic energy density must be concentrated by twelve orders of mag
nitude in order to produce light. Light scattering measurements indica
te that the bubble wall is collapsing at more than 4 times the ambient
speed of sound in the gas just prior to the light emitting moment whe
n the gas has been compressed to a density determined by its van der W
aals, hard core. Experiments indicate that the collapse is remarkably
spherical, water is the best fluid for SL, some noble gas is essential
for stable SL, and that the light intensity increases as the ambient
temperature is lowered. In the extremely stable experimental configura
tion consisting of an air bubble in water, measurements indicate that
the bubble chooses an ambient radius that is not explained by mass dif
fusion. Experiments have not yet been able to map out the complete spe
ctrum because above 6 eV it is obscured by the cutoff imposed by water
, and furthermore experiments have only determined an upper bound on t
he hash widths. In addition to the above puzzles, the theory for the l
ight emitting mechanism is still open. The scenario of a supersonic bu
bble collapse launching an imploding shock wave which ionizes the bubb
le contents so as to cause it to emit Bremsstrahlung radiation is the
best candidate theory but it has not been shown how to extract from it
the richness of this phenomenon. Most exciting is the issue of whethe
r SL is a classical effect or whether Planck's constant should be invo
ked to explain how energy which enters a medium at the macroscopic sca
le holds together and focuses so as to be emitted at the microscopic s
cale.