Number 559, October 3, 2001
by Phil Schewe, James Riordon, and Ben Stein
New Model of Intergalactic Magnetic Fields
On Earth, strong magnetic fields, powered by currents moving through
wires, steer energetic particles around an accelerator. At the Sun,
magnetic fields, powered by immense subsurface currents, spring upwards
to facilitate the warming of the Sun's corona and, further downstream,
to buffet the Earth and sometimes disrupt our terrestrial telecommunications.
But where do the fields in the intergalactic medium (IGM) come from,
and what role do they play in the life of the cosmos? Such fields have
been observed to reside even in parts of space relatively devoid of
galaxies.
Earlier theories of IGM fields, such as the ideas that the fields may
be partly primordial in nature (present at the creation) or that they
grew as a result of shock waves occurring at the boundary between massive
colliding gas clouds, must now be amended to include the substantial
contribution of galactic black holes.
Philipp Kronberg and Quentin Dufton at the University of Toronto (kronberg@physics.utoronto.ca,
416-978-4971) and Hui Li and Stirling Colgate at Los Alamos believe
that fully half of the energy content of those massive radio-emitting
lobes (up to 1060 ergs) exists in the form of magnetic energy
thrown out of hundred-million-solar-mass black holes. This represents
about 10% of their total gravitational energy (about 1061
ergs).
This latter energy, summed over many galaxies, appears to be the largest
available energy reservoir in the mature universe for magnetizing intergalactic
space. They also suggest that the fields don't stop there but continue
on to fill up large volumes of space, even those rural areas between
galaxy clusters. These expelled magnetic fields should exert a substantial
influence on galaxy formation. The dynamo process whereby black holes
would crank out so much energy and such strong fields remains one of
the greatest problems in astrophysics. (Astrophysical
Journal, 10 October 2001; Los
Alamos preprint.)
Ultraviolet Prompts Bacterial Trek
Evidence of the progressive depletion of ozone in our upper atmosphere
has prompted several studies of the effects of increasing ultraviolet
(UV) radiation on biological systems. In general, such studies focus
on repair mechanisms at the cellular level.
A research group at Clark University in Worcester, Massachusetts has
now looked at larger scale behavior in a bacterial colony exposed to
elevated UV levels. The researchers (A. Kudrolli, akudrolli@clarku.edu,
508-793-7752; L. Tsimring, ltsimring@ucsd.edu, 858-534-0816) were surprised
to find that colonies of microscopic bacteria migrated to form macroscopic,
ring-shaped distributions in response to UV exposure (see figure).
The study began with evenly distributed colonies of the common soil
bacteria Bacillus subtilis grown on nutrient-rich media. The
bacteria launched their migration toward the edges of the colonies after
the UV light was turned on. Initially, there seemed little benefit to
the patterns because the UV intensity was uniform across the colony.
Indeed, when the radiation was switched off the bacteria returned to
the evacuated area inside the ring, confirming that the growing media
was still hospitable.
What benefit could the bacteria gain through their travels? The researchers
propose that the added stress due to UV light causes the bacteria to
become more sensitive to slightly increased levels of waste products
built up in the central portion of the colony, and that the bacteria
near the colony edges emit chemical attractants that lure their kin
to pristine media regions.
The swarming migration of bacteria under UV light is a remarkable example
of a complex reaction to stress in a biological system. It is particularly
important in the light of continuing threats to the ozone layer that
protects bacteria and humans alike from harmful radiation. (A.
M. Delprato, A. Samadani, A. Kudrolli, and L.S. Tsimring, Physical
Review Letters, 8 October 2001.)
BEC on a Chip
First it was neutral atoms guided along a wire (Update
416). Then it was a beam of atoms steered over the surface of a
microchip (Update
486). Now the latest feat of atom optics, performed by a group at
the Max Planck Institute in Munich, is the creation of a Bose-Einstein
condensate (BEC) of rubidium atoms in a microscopic magnetic trap built
into a lithographically patterned chip. Not only was this the fastest-formed
BEC (it took only 700 ms to form, faster even than the all-optical BEC
method reporter earlier this year-see Update
545) but the condensate can be maneuvered around the microchip a
few microns above the surface (see Update
516); in fact the condensate was moved a distance of 1.6 mm. This
capability opens up the possibility of numerous atomtronic applications
in interferometry, quantum computing, navigation, lithography, holography,
and entanglement experiments. (Hansel et al., Nature,
4 October 2001.
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