Number 276, June 21, 1996 by Phillip F. Schewe and Ben Stein
POLARIZED LIGHT AS A BLACK HOLE SIGNATURE . How can astronomers be sure
that black holes exist? The motions of stars and gas near presumed black
holes provide provisional evidence, but additional assurances are desirable.
Paul Wiita and his colleagues at Georgia State suggest that the polarization
(the preferential orientation) of x rays coming from some celestial objects
such as x-ray binaries and active galactic nuclei can be used to demonstrate
the presence of a black hole. These x rays are thought to arise when material
pulled from nearby stars toward black holes piles up on (and heats up)
the accretion disk hovering closely about the hole. According to Wiita
(wiita@chara.gsu.edu), light coming from the inner part of the disk will
not only be more energetic than light from further out on the disk, but
will show greater changes in polarization as well. Furthermore, the degree
of the polarization should be enhanced in a characteristic way by the lensing
action of the black hole's huge gravitational field. It will, however,
be difficult to test this hypothesis in the near future since the apparatus
for measuring polarization was recently dropped from plans for the orbiting
AXAF x-ray telescope, to be launched in 1998. (Gang Bao, Paul Wiita, and
Petr Hadrava, Physical Review Letters, 1 July 1996.)
IO MAY GENERATE A MAGNETIC FIELD OF ITS OWN . The Galileo spacecraft
recently measured the magnetic field in the vicinity of Jupiter's moon
Io and found the field strength to be approximately 38% lower than the
1860 nanotesla expected if only the field originating at Jupiter itself
were present. Researchers have previously speculated that additional fields
may be generated near Io by the presence of accelerating ions in the moon's
neighborhood. But at the May meeting of the American Geophysical Union
in Baltimore, Margaret Kivelson of UCLA suggested that even the most charitable
estimates on the numbers of ions encountered by Galileo during the measurements
could not account for this sharp dip in the magnetic field. The most likely
way to explain the results, Kivelson said, would be if Io's core (known
to be heavy and currently believed to consist of iron or an iron-iron sulfide
mixture) generates a magnetic field, perhaps through the sloshing of molten
fluid in the core; this is essentially what happens inside Earth and Mercury.
If this hypothesis holds up to more detailed analyses of Galileo's ion
flux measurements, Io would be the first moon known to produce its own
magnetic field. (Upcoming article in Physics Today, July 1996).
DIGITAL VERSATILE DISCS (DVDs) will appear in consumer products in early
1997. The same size as conventional compact discs (CDs), DVDs will hold
about 14 times more data because of a combination of innovations. For example,
both types of disc encode digital data as pits on thin plastic platters,
but for DVDs the pits are smaller (.4 microns versus .83 microns for CDs),
the tracks of pits are closer together (.7 versus 1.6 microns), and the
laser light used to read data has a wavelength of 635-650 nm rather than
the 780 nm used for CDs; all of these factors allow data to be crowded
in more densely. Furthermore, the DVD consists of two layers, each of which
can hold data. With a capacity of 4.7 Gbytes, the DVD will be able to provide
a variety of multimedia products, even movies, at least in compressed form.
And all of this is without resorting to blue-light lasers (still under
development), whose shorter wavelengths would permit even more data to
be compressed into a given area. (Scientific American, July 1996.)
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