Friday, May 26, 2006

Physics NewsUpdate - May 26, 2006. The Misshapen Solar System and Counting Terahertz Photons

The American Institute of Physics Bulletin of Physics News
Number 778 May 26, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi

THE MISSHAPEN SOLAR SYSTEM. Having traveled far beyond the planets
in their 28.5-year journey, the two Voyager spacecraft are providing
new information on the heliosphere, the teardrop-shaped bubble that
separates the solar system from interstellar space. At this week's
Joint Assembly Meeting in Baltimore of the American Geophysical
Union (AGU) and several other geophysics-related societies, Ed Stone
of Caltech reported that the heliosphere is deformed, according to
Voyager observations, with the teardrop's rounded edge bulging at
the top (the northern hemisphere of the solar system) and squashed
at the bottom (the southern hemisphere). (See pictures and movies at
) As Rob Decker of Johns Hopkins University Applied Physics
Laboratory explained, the asymmetry is due to a magnetic field from
interstellar space pushing on the southern hemisphere. The field is
about 1/100,000 the strength of Earth's field but its effects can be
felt for billions of miles, since it is acting over a large area on
the very dilute gas at the solar system's edge.
The interstellar field even squashes an important spherical zone
inside the heliosphere, called the termination shock. Analogous to
the circle that forms when water splatters on a sink, the
termination shock represents the boundary at which the rapidly
traveling solar wind (the stream of charged gas from the sun) slows
down abruptly and piles up. Voyager 2's measurements indicate that
the southern part of the termination sphere might be a billion miles
closer to the sun than the northern part. Moreover, forces from the
solar wind cause the termination shock to breathe in and out roughly
every dozen years. Voyager 1 has already ventured beyond the
termination shock, to the heliosheath, the region where solar wind
and interstellar gas mix. So in a way, the end of the solar system
is not clearly defined. Stone guesses it could be another 10 years
(3-4 billion miles) before the two spacecraft pass through the
heliopause (the very outermost boundary of the heliosphere) and
enter purely interstellar space. The spacecraft have about another
15 years of power left in them. (Session SH02 at meeting; see

COUNTING TERAHERTZ PHOTONS. Scientists at the University of Tokyo
and the Japan Science and Technology Corporation have been able to
detect single photons in the terahertz region of the electromagnetic
spectrum for the first time. Previously, such photons, with
energies around 4 milli-electron-volts, could not be seen singly.
THz radiation, essentially in the far-infrared, is a potentially
important telecommunications carrier. Not only detection but
microscopy at ultra-low THz light levels can be performed. By
scanning a quantum-dot probe (highly sensitive to THz light) across
the face of a sample, the sample can be imaged with a
spatial resolution of 50 microns (the radiation itself has a
wavelength of 132 microns). This is even more remarkable when you
consider that the power emitted from the surface being imaged is at
the level of 10^-19 watts (0.1 attowatt). Currently photon-counting
microscopy glimpses a few electrons at a time oscillating at THz
frequencies in semiconductor devices at high magnetic fields.
According to Kenji Ikushima (, the
extraordinarily high-sensitivity of the photon counting approach
will soon facilitate the study of a molecule shaking, rattling and
rolling at THz rates, photon-counting microscopy in this spectral
range will facility the study of a few molecules at a time
oscillating at THz frequencies in semiconductor devices at high
magnetic fields. (Ikushima et al., Applied Physics Letters, 10
April 2006; www.dbs.c.u-tokyo.acjp/~komiyama )

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