Number 268, May 1, 1996 by Phillip F. Schewe and Ben Stein
CHAOTIC QUANTUM PINBALL. If struck correctly, a ball on a rectangular
billiard table will follow a reproducible trajectory. Changing the table
from a rectangular to a stadium shape causes the trajectory to become essentially
unpredictable; the ball's motion is chaotic. A quantum mechanical analog
of this has been devised in the form of electrons confined in a quantum
well, a semiconductor arrangement consisting of a thin layer (22 nm) of
GaAs sandwiched between layers of AlGaAs. The quantum well, part of a tunnel
diode, acts as a box inside of which electrons knock around like billiard
balls. If in addition the electrons are exposed to a high magnetic field
(up to 37 T) tilted relative to the walls of the box, the electrons' motions
become chaotic. Physicists from a Nottingham-Tokyo collaboration (L. Eaves,
44-115-951-5165) sample the current from the junction as the magnetic field
and the voltage across the well are varied. In this way the researchers
can observe the onset of chaos and can map out the wavefunction (the probability)
for the electron to be at certain positions. Despite the unstable nature
of the electron's orbit, the maps did exhibit concentrations (enhancements
in the probability amplitude) in certain parts of the well. These "scars"
in the electron wavefunction had been predicted but never before seen in
a quantum system (although scarred states had been observed for microwaves
confined in an irregular cavity). Besides serving as a laboratory for studying
quantum chaos, the tunnel diode is a workable electronic device, and it
might be possible to exploit the current fluctuations which come about
because of the scarring effect. (P.B. Wilkinson et al., Nature 18 April
1996; the group has also published in T.M. Fromhold et al., Physical Review
Letters, 7 August 1995.)
SATURN'S RINGS ARE SEEN EDGE-ON at Earth only twice every 30 years.
This is a good time to view the Saturn system since the glare of the rings
is so much less. For the recent 1995 ring-plane crossings, the Hubble Space
Telescope made extensive observations. Some findings: the ring system overall
is about 1.2 to 1.5 km thick; the F ring is inclined relative to the A
ring; the rings are covered by a tenuous sheath of OH molecules; the tiny
inner satellite Prometheus was some 19 degrees of longitude away from its
estimated position; the E ring flares (at a distance of 7.5 Saturn radii)
to a thickness of about 15,000 km. (Several articles, Science, 26 April
1995.)
THE FIRST BINARY-STAR SYSTEMS BEAMING X RAYS AT SUB-MILLISECOND rates
have been observed by the Rossi X-Ray Timing Experiment, an orbiting telescope
launched in December 1995. Speaking at this week's meeting of the American
Astronomical Society in San Diego, RXTE scientists reported that a binary
system (Sco X-1) in the constellation Scorpius was emitting x rays on and
off 1130 times per second. Another binary system (4U 1728-34) in the constellation
Sagittarius was emitting x ray bursts at a rate of up to 1100 per second.
In each case the x rays are believed to arise when material from a normal
star falls onto a companion neutron star. For Sco X-1, x rays beamed at
a slower rate are also seen. The researchers are puzzled as to why the
slower pulses and the faster 1130 pulses/sec emissions are modulating over
time in the same way. (AAS press release.)
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