Number 267, April 23, 1996 by Phillip F. Schewe and Ben Stein
A NEW THEORY OF SONOLUMINESCENCE . Sound energy, in the form of a beam
of ultrasonic waves, can be partly converted into light energy by aiming
the sound at an air bubble in a sample of water. The sound causes the bubble
to collapse and to emit sharp (less than 12 picosecond) light pulses. The
light's spectrum implies that the source of the radiation is similar to
a black-body object at a temperature of tens of thousands of kelvins. Theorists
have tried to explain sonoluminescence by saying, for example, that the
radiation comes from a plasma formed by the collapse of the bubble. But
mostly the mechanism behind the production of the pulses remains a mystery.
Now Claudia Eberlein of Cambridge University (cce20@phy.cam.ac.uk, 44-1223-337-458)
offers a more daring explanation. She believes the light is being emitted
by the vacuum surrounding the bubble. Modern quantum theory holds that
unseeable virtual photons abound in the vacuum. The behavior of these "zero-point
fluctuations" is influenced by the properties of the surrounding medium.
The rapidly moving air-water interface (where two media different indices
of refraction come together) may facilitate the conversion of virtual photons
into real photons. In fact, Eberlein says, sonoluminescence may represent
the first observable manifestation of quantum vacuum radiation. This scenario
can be compared to the "Unruh effect," a hypothetical phenomenon
in which photons are emitted by a mirror accelerating through a vacuum.
"Hawking radiation," the hypothetical emission of particles from
black holes, is yet another example of energy seemingly coming out of nowhere;
at the black hole's Schwarzschild radius (inside of which, light cannot
escape), space is so warped that energy from the black hole can be converted
into particle-antiparticle pairs; one particle falls back into the hole
while its partner escapes. Eberlein asserts that researchers can put her
theory to an experimental test and compare the results to other models
of sonoluminescence. (Claudia Eberlein, Physical Review Letters, 13 May
1996.)
COLLECTIVE SYNCHRONIZATION CAN ACT LIKE A PHASE TRANSITION. Several
systems with large populations exhibit a coordinated ensemble behavior.
For example, fireflies can flash in unison, crickets can chirp together,
and pacemaker cells in the heart work together. Theorists, such as Yoshiki
Kuramoto of the University of Kyoto (Japan), have supposed for some time
that the onset of synchronization would be equivalent to a phase transition,
like the transformation of water into ice, and have worked out formulas
to deal with the problem. (Science News, 13 April 1996.) Now, physicists
at Georgia Tech and Cornell have performed experimental studies of large
series arrays of nonidentical Josephson junctions which bear out the models.
As the input current was varied, the array showed two phase transitions,
one in which the voltage oscillations of the junctions became partially
synchronized, and a second in which the frequency coupling became complete.
Normally, the junctions, which are all slightly different, would oscillate
at their own natural frequencies. The researchers plan next to test two-dimensional
arrays. (Kurt Wiesenfeld et al., Physical Review Letters, 15 January 1996.)
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