ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.

May 12, 2000 -- Vol. 4 Number 19

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Geological time inspires awe, and there are no
certainties about the future, but perhaps it is
almost certain that someday they will collect
our skulls and call us Early Man.
-- Anonymous

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Contents of This Issue:

1. Medical Biology:
A New Method for Delivery of Therapeutic Engineered Proteins
------------------------------------------------------------
A major problem in the use of therapeutic engineered proteins is
to control local release kinetics and local concentrations.
Researchers now report a system for direct pharmacological
control of protein secretion by mammalian biological cells, the
system developed to allow rapid and pulsatile delivery of
therapeutic engineered proteins.

2. Microbiology:
A Proposed Habitat for Cold-Loving Bacteria in Antarctic Ice
------------------------------------------------------------
Microbes have been found in ice cores drilled in Antarctica at
depths down to approximately 3600 meters and close to the surface
of the huge subglacial freshwater Lake Vostok. Interconnected
liquid interstices along the junctions of ice crystals in ice may
provide energy and nutrients for cold-loving (psychrophilic)
bacteria.

3. Zoology:
Honeybee Navigation
-------------------
One theory of honeybee navigation is that flight distance is
estimated in terms of energy consumption. Another theory is that
the primary cue is the integral over time of the image motion
that is experienced by the bee en route. A new study suggests
that honeybees use cues based primarily on image motion to
monitor flight distances of hundreds of meters in natural outdoor
environments. (Includes related background material.)

4. Medical Biology:
On the Early Origins of Autism
------------------------------
Since autism was first identified in 1943, great strides have
been made in describing its symptoms, but its biological basis
has remained elusive. Studies of twins confirm that autism has a
heritable component but suggest that environmental influences
play a role as well. (Includes related background material.)

5. Astrophysics:
An Extremely Cool White Dwarf Star
----------------------------------
White dwarf stars are extremely dense stars about the size of
Earth but with a mass approximately that of the Sun. Researchers
now report the accidental discovery of what is apparently the
coolest white dwarf yet identified, the star at a temperature of
3500 degrees kelvin. (Includes related background material.)

6. Computer Science:
Room-Temperature Quantum Cellular Automata
------------------------------------------
Quantum cellular automata using relatively large quantum dots
(approximately 100 nanometers) that work at room temperature have
been developed by using magnetic metals in the construction of
the dots. (Includes related background material.)

In Focus: On Computers in Astronomy

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1. MEDICAL BIOLOGY:
A NEW METHOD FOR DELIVERY OF THERAPEUTIC ENGINEERED PROTEINS
In mammalian cells, secreted proteins regulate various processes
ranging from rapid metabolic changes to growth and
differentiation. Since dysfunctions in these processes are
associated with many diseases, engineered proteins are an
important class of potential therapeutic agents. However, broad
use of engineered proteins as drugs is hampered by the need for
direct injection, since oral administration through the
gastrointestinal tract results in the chemical breakdown of the
administered protein by digestive processes. Major difficulties
with direct injection are that it is often impossible to maintain
administered protein concentrations within a therapeutic range,
and appropriate delivery kinetics usually cannot be provided.
Gene therapy is a promising alternative to direct injection for
delivering therapeutic engineered proteins, and sustained
therapeutic concentrations of several proteins have been obtained
after stable introduction of their genes into somatic cells.
However, in most cases, safe and effective gene therapy will
require that genes be equipped with a regulatory system that
results in natural concentrations of therapeutic proteins and
natural kinetics of therapeutic protein expression.
... ... V.M. Rivera et al (12 authors at 3 installations, US CH)
now report a system for direct pharmacological control of protein
secretion, the system developed to allow rapid and pulsatile
delivery of therapeutic proteins. In this system, the protein was
engineered so that it was accumulated as aggregates in the
*endoplasmic reticulum of cells. Secretion was then stimulated by
a synthetic small-molecule drug that induces protein
disaggregation. Rapid and transient secretion of growth hormone
and insulin was achieved in vitro and in vivo (mouse). A
regulated pulse of insulin secretion resulted in a transient
correction of serum glucose concentration in a mouse model of
*hyperglycemia. The authors suggest this approach may make gene
therapy a viable method for delivery of therapeutic polypeptides
whose efficacy requires rapid and regulated secretion by cells.
-----------
V.M. Rivera et al: Regulation of protein secretion through
controlled aggregation in the endoplasmic reticulum.
(Science 4 Feb 00 287:826)
QY: Victor M. Rivera [vrivera@ariad.com]
-----------
Text Notes:
... ... *endoplasmic reticulum: The term "endoplasmic reticulum"
refers to a complex system of flattened sacs in all biological
cells that have a nucleus (eukaryotes). The endoplasmic reticulum
is the site of many important syntheses, including the production
of new surface membrane and the intracellular transport of
various biochemical entities.
... ... *hyperglycemia: An abnormally high concentration of
glucose in circulating blood. This is seen especially in patients
with diabetes mellitus, a disease caused by an absolute or
relative deficiency of insulin.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm

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2. MICROBIOLOGY:
A PROPOSED HABITAT FOR COLD-LOVING BACTERIA IN ANTARCTIC ICE
There is no apparent physiological standard temperature for all
known living systems. Most known living systems exist in the
"moderate" temperature range 20 to 40 degrees centigrade, but
there are also organisms that thrive at relatively extreme
temperatures above and below that range. Organisms that prefer
warmer temperatures are called "thermophilic", and organisms that
prefer colder temperatures are called "psychrophilic" (from the
ancient Greek "psychros" for cold). In recent decades, there has
been considerable interest in psychrophilic organisms as a result
of some apparent evidence of biological systems in Greenland and
Antarctic ice cores.
... ... P. Buford Price (University of California Berkeley)
presents a discussion of psychrophiles and a proposal for a
possible habitat for bacterial organisms in ice. The author makes
the following points:
     1) Microbes are amazingly hardy: a) Viable specimens of a
spore-forming bacillus and of an extremely halophilic bacterium
have been found in an inclusion in a 250-million-year-old salt
crystal. b) A bacterial spore has been revived, cultured, and
identified from 40-million-year-old amber. c) Microbial life has
been found at depths down to several kilometers in the crust of
the Earth, and viable bacterial populations have been found in
sediments at Pacific Ocean sites at depths greater than 500
meters.
     2) Bacteria can grow and reproduce at temperatures equal to
or less than 0 degrees centigrade in high-altitude cloud
droplets. Microbial communities survive on wind-deposited
sediment particles within liquid-water inclusions in permanently
ice-covered Antarctic lakes. The author suggests that before
Antarctica developed its permanent ice cap approximately 14
million years ago, microbes may have existed in its continental
crust, and their descendants may now live in subglacial rock
crevices, lakes, and sediments and perhaps even in the glacial
ice itself. The author points out that far less is known about
psychrophilic bacteria than about thermophiles, and it is not
even certain yet whether life on Earth originated in a hot or
cold environment. The author suggests it thus seems worthwhile to
devise a search for live microorganisms in ancient polar ice, and
in this connection consideration of possible bacterial habitats
in such regions is of interest.
     3) The author points out that microbes have been found in
ice cores drilled at *Vostok Station in Antarctica at depths down
to approximately 3600 meters, close to the surface of the huge
subglacial apparently freshwater Lake Vostok. In these studies,
two types of ice have been found: a) The upper 3500 meters
comprises glacial ice containing traces of nutrients of *aeolian
origin, including sulfuric acid, nitric acid, methanesulfonic
acid, formic acid, sea salts, and mineral grains. b) Ice below
approximately 3500 meters comprises refrozen water from Lake
Vostok, the refrozen water (an apparent 100 meter layer) accreted
to the bottom of the glacial ice. Nutrients in the accretion ice
include salts and dissolved organic carbon. At the present time,
no one has yet observed living motile organisms in situ in
unmelted glacial freshwater ice.
     4) The author proposes a microbial habitat consisting of
interconnected liquid interstices ("veins") along the junctions
of ice crystals in ice, veins in which psychrophilic bacteria can
move and obtain energy and carbon from ions in solution. In the
accretion ice, with an age of the order of magnitude of 10^(4)
years and a temperature a few degrees below freezing, there is
some evidence that the carbon and energy sources in the veins can
maintain significant numbers of cells per cubic centimeter that
are metabolizing but not multiplying. In the 400,000 year old
colder glacial ice, at least 1 cell per cubic centimeter in acid
veins can be maintained. The author suggests that with
fluorescence microscopy tuned to detect the metabolite *reduced-
nicotinamide-adenine dinucleotide (NADH) in live organisms,
mobile bacteria could be detected by direct scanning of the veins
in ice samples.
-----------
P. Buford Price: A habitat for psychrophiles in deep Antarctic
ice.
(Nature 1 Feb 00 97:1247)
QY: P. Buford Price [bprice@uclink4.berkeley.edu]
-----------
Text Notes:
... ... *Vostok Station in Antarctica: Beneath the continental
ice sheet in Antarctica lie dozens of freshwater lakes whose
existence is known only by means of satellite observations. Most
of these lakes are small, but Lake Vostok may cover an area
similar to Lake Ontario, with a maximum length of 200 kilometers
and an average depth of 125 meters. Although the existence of the
lake was discovered in 1974, its true size was found only during
a later recharting involving radiowave echoes. The temperature in
the 4-kilometer-thick ice sheet covering Lake Vostok may be as
low as -50 degrees centigrade. At the present time, none of the
deep drillings carried out in the Antarctic has opened access to
a freshwater lake. The 1996 Vostok drilling program halted in the
ice sheet 150 meters above the surface of Lake Vostok.
... ... *aeolian origin: (eolian origin) Refers to deposits due
to the transporting action of wind.
... ... *reduced-nicotinamide-adenine dinucleotide (NADH): This
metabolite marks the existence of living cells by fluorescing at
approximately 440 nanometers. The author suggests that with
10^(3) TO 10^(6) NADH molecules per living cell, the signal from
a single cell would be strong. In a dead cell, NADH is oxidized
to NAD, which does not fluoresce.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm

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3. ZOOLOGY:
HONEYBEE NAVIGATION
There is considerable evidence that honeybees navigate accurately
and repeatedly to a food source, as well as communicate to their
nestmates by means of various "dances" the distance and direction
in which to fly to reach the source. However, the cues by which
been gauge the distance to the goal have been controversial. One
theory is that flight distance is estimated in terms of energy
consumption. Another theory is that the primary cue is the
integral over time of the image motion that is experienced by the
bee en route.
... ... M.V. Srivivasan et al (4 authors at 2 installations, AU
DE) now report a series of tests of the two theories by recording
dances of bees that have been trained to fly into a short narrow
tunnel to collect a food reward. The authors report the
following:
     1) The experimental bees were Apis mellifera ligustica
Spinola. In one series of experiments, individually marked bees
were trained to forage at a feeder carrying sugar solution placed
in a wooden tunnel 6.4 meters long, 11 centimeters wide, and 20
centimeters high. The tunnel was positioned outdoors near the
maintained hive. The far end of the tunnel was closed, and bees
could enter and leave the tunnel only at the near end. The top of
the tunnel was covered with black insect-screen cloth, which
permitted observation of the bees and provided the bees with a
view of the sky.
     2) In one experiment, the tunnel was positioned 35 meters
from the hive. It is known that bees of this species perform
"round dances" when a food source is within 50 meters of the
hive, and perform "waggle dances" when a food source is more than
50 meters from the hive. The possibility that image motion is the
cue for estimating flight distance was tested as follows: The
walls and floor of the tunnel were lined with axially oriented
stripes parallel to the direction of flight. This tunnel provided
only negligible image motion cues, since the stripes were
parallel to the direction of flight. Bees returning from this
tunnel produced predominantly round dances.
     3) The same tunnel was then set up with the walls and floor
lined with material showing a random visual pattern, and the
tunnel moved so that the entrance was only 6 meters from the hive
exit, with the tunnel pointed toward the hive and the feeder
placed 6 meters inside the tunnel. Bees returning from this
tunnel, performed mainly waggle dances, even though the feeder
was now only 12 meters (traveled distance) from the hive. The
authors suggest these experiments indicate that the distance
flown is inferred by the bees on a visual basis, the primary cue
being the extent of image motion experienced by the eyes of the
bee.
     4) The authors conclude: "Our study suggests that honeybees
use cues based primarily on image motion to monitor flight
distances of hundreds of meters in natural outdoor environments.
In the future it should be possible to study mechanisms of
navigation and path integration conveniently, and under
controlled conditions, by training bees to fly through short
tunnels arranged in various configurations."
-----------
M.V. Srinivasan et al: Honeybee navigation: Nature and
calibration of the "odometer".
(Science 4 Feb 00 287:851)
QY: Mandyan V. Srinivasan [M.Srinivasan@anu.edu.au]
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON HONEYBEE SOCIAL BEHAVIOR, GENES, AND THE ENVIRONMENT
The so-called social insects live in societies that rival human
societies in complexity and internal cohesion. Honey bees, for
example, apparently always follow 3 rules: a) they live in
colonies with overlapping generations; b) they care cooperatively
for offspring other than their own; and, c) they maintain a
reproductive division of labor. ... ... In a review of research
(much of it from the author's own laboratory) concerning the
genetic and environmental factors responsible for honey bee
behavior, Gene E. Robinson (University of Illinois Urbana-
Champaign, US) makes the following points: 1) Genes do not play
an exclusive role in regulating behavior: biologists have long
realized that behavior is influenced by genes, the environment,
and interactions between the two. 2) Genes never act alone. They
must operate in an environment where they code for proteins that
participate in many systems in an organism, with these systems in
turn influencing the expression of genes. Consequently,
biologists must take a broad approach in assessing the impact of
any gene. 3) The research group of the author uses the Western
honey bee, Apis mellifera. Honey bees pass through different life
stages as they age, and their behavioral responses to
environmental and social stimuli change in predictable ways.
Although worker bees go through a consistent path of behavioral
development, this path is not rigidly determined. Bees can
accelerate, retard, or even reverse their behavioral development
in response to changing environmental and colony conditions. 4)
Experimental evidence indicates that juvenile hormone, one of
the most important hormones influencing insect development, helps
time the pace of behavioral maturation in honey bees. The rate of
endocrine-mediated behavioral development is influenced by
inhibitory social interactions. Older bees inhibit the behavioral
development of younger bees: the rate of behavioral development
is negatively correlated with the proportion of older bees in a
colony. Inhibitory social interactions that influence the rate of
behavioral development involve chemical communication between
colony members. 5) Evidence from the laboratory of the author in
1993 indicated the so-called mushroom bodies in the bee brain are
involved in the behavioral changes occurring during maturation,
the volume of the bodies increasing, and the volume increase
associated with an increase in synapses with neurons from brain
regions devoted to sensory input. The author suggests this was
the first report of brain plasticity in an invertebrate. 6) The
author suggests that, in general, two-way interactions between
the nervous system and the genome contribute fundamentally to the
control of social behavior. Information about social conditions
that is acquired by the nervous system is likely to induce
changes in genomic function that in turn produce adaptive
modifications of the structure and function of the nervous
system. 7) The author proposes a new research initiative called
"sociogenomics", defined as a "wide-ranging approach to identify
genes that influence social behavior, determining the influence
of these genes on underlying neural and endocrine mechanisms, and
exploring the effects of the environment -- particularly the
social environment --  on gene action."
QY: Gene E. Robinson, Dept. of Entomology, Univ. of Illinois
Urbana-Champaign 217-333-3090.
(American Scientist Sep/Oct 1998 86:456)
(Science-Week 11 Sep 98)
-------------------
Related Background:
AN UNUSUAL IDEA CONCERNING HONEYBEES AND QUARKS
In mathematics, a manifold is a topological space which is
locally euclidean. A flag manifold is a type of 6-fold symmetry
manifold named after the apparent resemblance of its topology to
that of a flag. Quarks, the proposed elementary entities that
constitute protons in the atomic nucleus, evidently can be
described by 6-fold flag manifold mathematics. That is the first
set of ideas in this little elaboration. The second set of ideas
concerns insects, specifically honeybees. It has been known for
some time that honeybees engage in forms of communication,
especially communication concerning the location of food sources.
When a lone honeybee, which has only a few hundred neurons in its
nervous system, has discovered food, it returns to the hive and
executes what can be described as a geometric dance whose para-
meters convey information concerning the direction and distance
of the newly found food source. This is one of those observations
astounding to physicists and commonplace to biologists, which
puts the shoe on the other foot, so to speak, if one considers
how biologists usually react to the idea of quantum
entanglements. In any case, mathematician Barbara Shipman
(University of Rochester, US) who studies flag manifolds, has now
apparently demonstrated that the honeybee dance can be described
by a single parameter associated with a flag manifold. She has
also made the rather startling postulate that the reason why this
is so is that honeybees are somehow sensitive to the properties
or local histories of quarks, the idea based primarily on the
similarity of the mathematical descriptions rather than on any
proposed physical linkage. No matter the proposed relationship to
the physics of quarks, if the honeybee dance can indeed now be
concisely described by a particular mathematical formalism, that
is a solid advance.
QY: B. Shipman, Univ. Rochester, (716) 275-4411
(Discover Nov 97) (Science-Week 17 Oct 97)
For more information: http://scienceweek.com/swfr.htm

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4. MEDICAL BIOLOGY:
ON THE EARLY ORIGINS OF AUTISM
Autism is a behaviorally defined syndrome of unknown etiology
associated with poor social interaction, disordered language, and
atypical responses to people, objects, and events. The syndrome
is classically manifested by severe disturbances in cognition,
language, and behavior that appear before the age of 30 months.
In some cases, there is an apparent hyperarousal state. Autistic
children often exhibit ritualized body movements, repeated
touching and sniffing of objects, ritualistic ordering, checking,
and collecting, and insistence on precisely following routines.
The ratio of male to female cases ranges from 2:1 to 4:1, and
studies of monozygotic and dizygotic twins indicate an important
role for genetic factors. There is presently a controversy over
whether movement disorders play a central role in the phenomenon
of autism and even whether such movement disorders exist in
autism at all.
... ... Patricia M. Rodier (University of Rochester, US) presents
a review of current research on the embryonic origins of autism,
the author making the following points:
     1) Autism is apparently associated with anatomical changes
in the brainstem, the region immediately above the spinal cord.
The brainstem of an individual with autism is shorter than a
normal brainstem and lack certain structures. "It is almost as
though a band of tissue were missing." The author suggests that
such changes could occur only in early gestation.
     2) Since autism was first identified in 1943, great strides
have been made in describing symptoms, but the biological basis
of autism has remained elusive. Studies of twins confirm that
autism has a heritable component but suggest that environmental
influences play a role as well. For example, if genetic factors
alone were involved, identical twins (monozygotic twins), who
share the same genes, should have a 100 percent chance of sharing
the same diagnosis. Instead, when one twin has autism, the second
twin has only a 60 percent chance of being diagnosed with the
same disorder. That second twin also has an 86 percent chance of
having at least some of the symptoms of autism. These results of
twin studies indicate that other factors must modify the genetic
predisposition to the disorder.
     3) Several environmental risk factors for autism are known:
In utero exposure to German measles (rubella), or to birth
defect-causing substances such as ethanol and valproic acid,
increases the chances that autism will develop. People with
certain genetic diseases, such as phenylketonuria and tuberous
sclerosis, also have a greater chance of developing autism. None
of these factors, however, occurs frequently enough to be
responsible for many cases of autism.
     4) The author points out that even a minimal understanding
of the genetic basis of autism would be of great value. But
devising a genetic test for autism -- similar to the current
tests for cystic fibrosis, sickle cell anemia, and other
disorders -- would be a much more difficult task. "Because so
many genes appear to be involved in the disorder, one cannot
accurately predict the odds of having a child with autism by
simply testing for one or two variant genes in the parents."
-----------
Patricia M. Rodier: The early origins of autism.
(Scientific American February 2000)
QY: Patricia M. Rodier, Univ. of Rochester 617-273-4539.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEURODEVELOPMENTAL DAMAGE IN AUTISM: AN INFECTION-BASED MODEL
     Exposure of the fetus and newborn infant around the time of
birth ("perinatal exposure") to infectious agents and toxins has
been linked to the pathogenesis of certain human neuropsychiatric
disorders, particularly *autism. But the mechanisms by which
environmental factors such as infectious agents and toxins
interact with developing immune and neural systems to create
neurodevelopmental disturbances are only poorly understood.
     Autism disorders occur as frequently as 1 in 500 children, a
rate that may be increasing in some geographical regions. A
neurodevelopmental hypothesis for autism is supported by various
approaches: brain imaging, anatomic and *cytoarchitectonic
evidence, and epidemiologic evidence. The view among many
researchers is that autism has likely perinatal origins; a
linkage to microbial or immune factors; an association with
dysfunction of the *hippocampus, *amygdala, and *cerebellum; and
a connection with disturbances of certain *neurotransmitters
(e.g., *dopamine and *serotonin). Of significance are the various
neurobehavioral dysfunctions characteristic of autism: motor,
postural, and sensory deficits; *hypotonia; *stereotypies; poor
eye contact; mental retardation, and so on.
     "Borna disease" is a central nervous disease of horses and
certain other vertebrate species. The disease is caused by Borna
disease virus, an RNA virus similar to *rhabdoviruses and
*paramyxoviruses, but with a number of unique features. The virus
has a high affinity for nerve cells, and there is data to
associate the virus with neuropsychiatric disorders in humans:
Borna disease virus *antibodies have been detected in
approximately one-third of patients with certain mental
illnesses, including depression, schizophrenia, and obsessive-
compulsive disorder. In addition, Borna disease virus RNA and
*antigen have been detected in peripheral blood *monocytes and in
autopsy brain samples of psychiatric patients. Nevertheless, it
remains to be established whether Borna disease virus is
etiologically involved in the pathophysiology of certain human
mental disorders.
... ... M. Hornig et al (4 authors at 3 installations, US AT) now
present an animal model for investigating disorders of central
nervous system development, the model based on neonatal rat
infection with Borna disease virus. The authors report infection
by inoculation of neonate rats with the virus results in abnormal
righting reflexes, hyperactivity, inhibition of open-field
exploration, and stereotypic behaviors. Neuronal architecture is
markedly disrupted in the hippocampus and cerebellum, with
reduction in the numbers of certain types of nerve cells (granule
cells and Purkinje cells). Neurons are apparently lost
predominantly by *apoptosis, and a variety of *inflammatory
changes in the brain occur. The authors suggest that the
resemblance of these functional and neuropathologic abnormalities
to human neurodevelopmental disorders indicates the utility of
this model for defining cellular, biochemical, histologic, and
functional outcomes of interactions of environmental influences
with the developing central nervous system. In particular, the
authors suggest that the disturbances of central nervous system
architecture produced by Borna disease virus infection in rats
parallel the structural and behavioral abnormalities observed in
human autism.
-----------
M. Hornig et al: An infection-based model of neurodevelopmental
damage.
(Proc. Natl. Acad. Sci. US 12 Oct 99 96:12012)
QY: Ian Lipkin [ilipkin@uci.edu]
-----------
Text Notes:
... ... *autism: See main report.
... ... *cytoarchitectonic: In the central nervous system,
particularly in the brain, nerve cells arrange themselves during
development in consistent patterns, e.g., the layers of the
cerebral cortex. The same patterns are found in all individuals
without evident neural dysfunctions, and the presence of various
patterns of nerve cell arrangements has come to be called
"architecture". In this context, the term "cytoarchitectonic"
refers to various patterns of neuron arrangement in distinct
central nervous system locations: cytoarchitectonic areas are
distinct regions of the cerebral cortex identified by differences
in cell size, packing density, and laminar arrangement.
... ... *hippocampus: A brain cortex structure in the medial part
of the temporal lobe. In humans, among other functions, the
hippocampus is apparently involved in short-term memory. Analysis
of the neurological correlates of learning behavior in the rat
indicates that the hippocampus is also involved in memory in that
species.
... ... *amygdala: a cluster of nerve cell bodies (the cluster
called a "nucleus") in the temporal lobe of the brain, the
cluster with major involvements in autonomic, emotional, and
sexual behavior.
... ... *cerebellum: A large neural structure at the base of the
brain involved in motor coordination, posture, and balance.
... ... *neurotransmitters: Neurotransmitters are chemical
substances released at the terminals of nerve axons in response
to the propagation of an impulse to the end of that axon. The
neurotransmitter substance diffuses into the synapse, the
junction between the presynaptic nerve ending and the
postsynaptic neuron, and at the membrane of the postsynaptic
neuron the transmitter substance interacts with a receptor.
Depending on the type of receptor, the result may be an
excitatory or an inhibitory effect on the postsynaptic nerve
cell.
... ... *dopamine: A neurotransmitter substance of critical
importance in certain areas of the brain involved in movement
control.
... ... *serotonin: A neurotransmitter substance involved in
nearly everything occurring in the brain, including psychological
states such as anxiety and depression, and dysfunctions producing
migraine and epilepsy.
... ... *hypotonia: In this context, a loss of the tension of
relaxed muscle (loss of muscle tone.)
... ... *stereotypies: In this context, a "stereotypy" is a
persistent repetition of gestures or movements that do not appear
to be goal-directed.
... ... *rhabdoviruses: Rhabdoviruses are rod- or bullet-shaped
single-stranded RNA viruses 75 x 180 nanometers, each particle
surrounded by a membranous envelope with protruding spikes 10
nanometers long. The rabies virus is an example of a rhabdovirus.
... ... *paramyxoviruses: These viruses include the most
important agents of respiratory infections of infants and young
children, as well as the causative agents of mumps and measles.
In general, paramyxoviruses are 150 to 300 nanometers in
diameter, the viral genome a linear single-stranded RNA molecule
of 16 to 20 kilobases.
... ... *antibodies: In general, an antibody is a protein
molecule produced by the immune system of vertebrate organisms,
the molecule designed to specifically interact with a particular
chemical entity called an antigen, the antigen usually a
particular surface component of a foreign organism.
... ... *antigen: See previous note.
... ... *monocytes: The monocytes are the largest of the
leukocytes (white blood cells).
... ... *apoptosis: In general, programmed cell death produced by
control mechanisms designed to destroy defective cells.
... ... *inflammatory changes: In general, an "inflammatory
change" is a response of tissues to irritation or injury. The
response involves a dynamic complex of cellular and chemical
reactions that occur in the affected blood vessels and adjacent
tissues.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/swfr.htm]
-------------------
Related Background:
INFANT AUTISM: USE OF MOVEMENT ANALYSIS IN DIAGNOSIS
... ... P. Teitelelbaum et al (5 authors at University of Florida
Gainesville, US) now report that a study of 17 autistic children
showed disturbances of movement that could be detected at the age
of 4 to 6 months and sometimes even at birth. The authors used a
standard movement analysis system (the Eshkol-Wachman system) in
combination with still-frame videodisc analysis to study videos
obtained from parents of children who had been diagnosed as
autistic by conventional methods, the diagnosis usually occurring
at about age 3 years. The videos showed the behaviors of the
children when they were infants, long before they had been
diagnosed as autistic. The authors report that movement disorders
varied from child to child, with disturbances revealed in the
shape of the mouth and in some or all of the milestones of
development, including lying, righting, sitting, crawling, and
walking. The authors suggest their findings support the view that
movement disturbances play an intrinsic part in the phenomenon of
autism, that movement disturbances in autistic children are
present at birth, and that such movement disturbances can be used
to diagnose the presence of autism in the first few months of
life. The authors further suggest these results indicate the need
for the development of methods of therapy to be applied from the
first few months of life in autistic children.
-----------
P. Teitelbaum et al: Movement analysis in infancy may be useful
for early diagnosis of autism. (Proc. Natl. Acad. Sci. US 10 Nov
98 95:13982) QY: Philip Teitelbaum 
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 15Jan99 
[For more information: http://scienceweek.com/swfr.htm]
-------------------
Related Background:
MOLECULAR BIOLOGISTS VS. CLINICIANS IN ACCESS CONFLICT
We have had a number of reports in recent issues of this
publication concerning the resistance of corporate and university
entities to releasing scientific data, such resistance apparently
based on the desire for financial gain. Lest anyone think the
corporate and university institutional entities are alone in this
sort of behavior, there is growing evidence of a similar attitude
among individual investigators. The current situation is
apparently as follows: For many decades now, a large number of
clinical studies (in the US these are usually funded by the
National Institutes of Health) have involved the routine
collection of blood samples from families both nuclear and
extended in connection with research on the familial bases of a
number of common diseases. These blood samples of course contain
DNA, and molecular biologists now have the techniques that
usually allow them to rapidly identify genes and markers
associated with various diseases. So they would like samples of
these blood materials in order to do the molecular genetic
investigations. But the clinical investigators who spent so much
time and effort tracking down families and gathering the material
over years and even decades are loath to release it to the
molecular biologists, calling the molecular biologists scientific
"gunslingers" who will take the fruit of their labors and win
Nobel Prizes with it. The various funding agencies are in an
apparent quandary, undecided about the duration of time they
should allow clinical researchers to maintain exclusive control
of blood samples. Should it be one year or 10 years? For those
whose instinctive answer is that exclusive control should last
about 10 minutes, the funding agencies argue that without the
incentive of exclusive control of materials, individual
investigators will have no incentive to do the research in the
first place. Which of course is also the argument that is made to
justify proprietary rights over scientific data (including the
patenting of genes) already granted to corporate and university
entities. So now we have a problem whereby molecular biologists
can probably rapidly identify the genetic etiology of a number of
complex multi-gene familial based diseases (for example, autism
and schizophrenia), but they cannot get the material because the
people who gathered the DNA from the familial populations, who
are clinicians and not molecular biologists, are reluctant to
sacrifice their prospects for important discoveries that will
bring them fame and fortune.
(Science 24 Oct 97) (Science-Week 14 Nov 97)
For more information: http://scienceweek.com/swfr.htm

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5. ASTROPHYSICS:
AN EXTREMELY COOL WHITE DWARF STAR
One of the great achievements of astrophysics during the second
half of the 20th century was a detailed theoretical approach to
the birth and death of stars. The sequence of events during the
death of a star, when its fuel is exhausted, involves a radical
alteration of the internal structure of the star, including a
blow-off of many of its outer layers. According to the current
view of stellar evolution, the remnant mass after the outer-
region blow-off determines the ultimate fate of the star. If the
remnant mass is greater than 1.44 solar masses (the Chandrasekhar
limit for a star with no hydrogen content; the limit varies
slightly, depending on star composition and star rotation), the
star, which in any case can no longer support itself against the
inward pull of its own gravity, collapses into either a *neutron
star or a black hole. If, however, the remnant mass is less than
the Chandrasekhar limit, the star also undergoes gravitational
collapse, but it collapses into a so-called "white dwarf star", a
relatively cool and extremely dense star about the size of Earth
but with a mass approximately that of the Sun.
... ... S.T. Hodgkin et al (6 authors at 5 installations, UK US
ES) now report the accidental discovery of what is apparently the
coolest white dwarf yet identified, the authors making the
following points:
     1) The authors point out that white dwarf stars cool
gradually over billions of years, and it has been suggested they
make up much of the so-called "dark matter" in the halo of our
Galaxy. Although extremely cool white dwarfs have proved
difficult to detect because of both their low luminosity and
their expected similarity in color to other classes of dwarf
stars, recent models have suggested that white dwarfs may be much
more blue in color than previously supposed. The authors suggest
this may indicate that earlier searches for Galactic halo white
dwarfs may have been looking for the wrong kinds of objects.
     2) The authors report an infrared spectrum of an extremely
cool white dwarf, the spectrum consistent with the new models of
white dwarf color. The authors determined the temperature of this
star to be 3500 +- 200 degrees kelvin, "making it the coolest
known white dwarf." The authors suggest the kinematics of this
star indicate that it is in the halo of our Galaxy, and the
density of such objects, as implied by the serendipitous
discovery of this star, is "consistent with white dwarfs
dominating the dark matter in the halo."
-----------
S.T. Hodgkin et al: Infrared spectrum of an extremely cool white
dwarf star.
(Nature 6 Jan 00 403:57)
QY: S.T. Hodgkin [sth@ast.cam.ac.uk]
-----------
Text Notes:
... ... *neutron star or a black hole: The important
consideration here is the competition between two forces: a) the
enormous inward pull of gravity produced by the mass of the star;
and b) the repulsive forces between the constituents of atoms. In
an ordinary star, the radiation pressure produced by the burning
of the core of the star is enough to counteract the inward
gravitational force and keep the star from collapsing; in a star
that has exhausted its fuel, such radiation pressure is severely
reduced and the star begins a gravitational collapse due to its
enormous mass. But how far will the collapse proceed? If,
following its terminal stages, the remnant mass of a star is
between 1.4 and 2 to 3 solar masses, the star will collapse into
a neutron star, a body with a radius of 10 to 15 kilometers, with
a core so dense that its component protons and electrons have
merged into neutrons. The average density of a neutron star is
10^(15) grams per cubic centimeter, and the weight of an object
on the surface of a neutron star would be 10^(11) its weight on
the surface of the Earth. Neutron stars apparently have an outer
shell of iron, but it is iron like no Earth iron, an iron of 4
orders of magnitude greater density. On the other hand, if the
terminal stages of star death leave a remnant star mass greater
than 3 solar-masses, the ultimate gravitational collapse will
produce a black hole, a relativistic singularity. A black hole is
a localized region of space from which neither matter nor
radiation can escape. The "trapping" occurs because the requisite
escape velocity, which can be calculated from the relevant
equations, exceeds the velocity of light and is therefore
unattainable. 
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
WHITE DWARF STARS AND GRAVITATIONAL MICROLENSING
Gravitational lensing is the bending of light and other radiation
by a massive gravitational entity such as a star, a black hole, a
galaxy, or a cluster of galaxies. The effect is predicted by
Einstein's theory of relativity and was first detected during a
total solar eclipse by Eddington in 1919. Large-scale
gravitational lensing causes multiple images of an object, the
type and arrangement of the images determined by the specifics of
the lensing entity. Gravitational "microlensing" is a small-scale
lensing effect, the gravitational field of the lensing object not
strong enough to form distinct images of the background source,
but instead causing an apparent brightening of the source. Stars
are expected to vary in brightness in a characteristic manner if
low-mass stars pass in front of them, and this effect has been
detected in the Large Magellanic Cloud and in the central bulge
of our Galaxy. The Magellanic Clouds are two comparatively small
and irregular galaxies close to our own galaxy, visible by the
naked-eye but observable only in the southern hemisphere, and
named after Ferdinand Magellan, who first recorded them in 1519.
The Large Magellanic Cloud has a diameter of approximately 10,000
*parsecs, and lies at a distance of approximately 50,000 parsecs
from Earth. The analysis of gravitational microlensing events of
stars in the Large Magellanic Cloud indicates the masses of the
lensing objects are in the range 0.3 to 0.8 solar-masses, which
suggests they might be old *white-dwarf stars. ... ... In an
analysis of white dwarf stars as possible sources of
gravitational microlensing events in the Large Magellanic Cloud,
Brad Hansen points out that 1) if white dwarfs exist in abundance
in the *halo of our Galaxy, this would apparently have profound
implications for our understanding of the early generations of
stars in the Universe; 2) previous attempts to theoretically
constrain the contribution of white dwarfs to microlensing
indicated they can account for only a small fraction of the
events, but these estimates relied on models of white dwarf
cooling that may be inadequate for the oldest white dwarfs. The
author presents cooling models proposed as appropriate for very
old white dwarfs, and using these models, the author reports that
the widely held idea that old white dwarfs are red applies only
to those with a helium atmosphere. Old white dwarfs with hydrogen
atmospheres, which could be a considerable fraction of the total
population of old white dwarfs, will appear blue, with colors
similar to those of the faint blue sources in the *Hubble Deep
Field. The author suggests that observational searches for the
population of microlensing objects should therefore look for
faint blue objects, rather than for faint red objects.
-----------
Brad M.S. Hansen (University of Toronto, CA): Old and blue white-
dwarf stars as a detectable source of microlensing events.
(Nature 27 Aug 98 394:860) 
QY: Brad M.S. Hansen 
-----------
Text Notes:
... ... *parsecs: 1 parsec equals 3.262 light-years, or 30.86 x
10^(12) kilometers.
... ... *white-dwarf stars: These are extremely dense and compact
stars that have undergone gravitational collapse after nuclear
fusion burn-out in their centers. They are the final stage in the
evolution of low-mass stars after such stars have lost their
outer layers. The white dwarfs are about the size of Earth, but
with a mass about that of the Sun.
... ... *halo: A galactic halo, such as that associated with our
own Galaxy, is a spheroidal distribution of old stars and
globular clusters of old stars surrounding the galaxy. In the
case of our own Galaxy, the galactic halo has a radius of
approximately 50,000 light years.
... ... *Hubble Deep Field: The orbiting Hubble Space Telescope
has the ability to discern images of galaxies too faint to be
picked up by planet-based and other orbital telescopes. After the
Hubble Space Telescope was repaired in 1993, one of the projects
begun with this instrument was the Hubble Deep Field Project,
which involves long-exposure detection of radiation received from
a relatively small area of the sky, the long-exposure providing
information concerning "deep" objects, i.e., objects extremely
distant from the instrument (in this case, beyond our local
Universe).
-------------------
Summary & Notes by SCIENCE-WEEK  18Sep98
-------------------
Related Background:
STUDY OF A PULSATING WHITE DWARF STAR
White dwarfs are the final phase in the evolution of low mass
stars. The stars from which they originate can be up to 8 solar
masses, but they lose 90% of their mass during star death, and
the remaining mass contracts until they have diameters only 1%
that of the sun. Most dead stars are white dwarfs, approximately
the size of the Earth, with masses approximately the mass of the
sun. This of course means an enormous density, 0.1 to 100 tons
per cubic centimeter, resulting from a terminal star the size and
mass of the sun collapsing to Earth-size. In a white dwarf, the
ordinary atomic structure has broken down completely, with
electrons and nuclei compacted tightly together by the
gravitational force. The luminosity of white dwarfs is low, and
they gradually cool to become cold and dark objects. Many white
dwarfs have been identified, the first of them the dark companion
of the star Sirius. It is estimated that in our galaxy alone the
number of white dwarf stars is of the order of 10^(10). Don
Winget et al (University of Texas Austin, US; Universidade
Federal do Rio do Sul, BR) report that a study of the white dwarf
star BPM 37093 indicates this particular pulsating white dwarf is
massive enough (about twice the white dwarf average) to have a
crystalline interior despite its apparent relatively high surface
temperature of over 11,000 degrees Celsius. But more evidence is
needed to sort out effects that could mimic indicators of
crystallization, and observations next year by the Hubble Space
Telescope of this star are planned.
QY: D. Winget, Univ. Texas Austin (512) 471-3350
(Astrophys. J. 1 Oct 97) (Science-Week 31 Oct 97)
For more information: http://scienceweek.com/swfr.htm

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6. COMPUTER SCIENCE:
ROOM-TEMPERATURE QUANTUM CELLULAR AUTOMATA
     The consensus view among computer scientists and
microelectronics engineers is that in the near future the rapid
growth of information processing capability will require radical
innovations in technology because of inherent limitations in
current microchip (integrated circuit) architecture and dynamics.
This has caused attention to focus on various possibilities for
the use of quantum mechanical effects in computing. One such
possibility involves the "single-electron transistor". A
single-electron transistor is a transistor of extremely small
dimensions isolated from its leads by potential barriers narrow
enough to permit electron tunneling, with a minute electron
source that is essentially a droplet of electrons. A
single-electron transistor switches on and off with the addition
of each electron, in contrast with the ordinary transistor which
sustains a switched-on state given a flow of added electrons.
This quantized behavior of the single-electron transistor is due
to its dimensions, the electron droplet essentially behaving as
an artificial atom or "quantum dot". As realized in the
laboratory, quantum dots are small electrically conducting
regions, typically less than 1 micron in diameter, that contain
from one to a few thousand electrons. Because of the small
volume, the electron energies within the dot are quantized, and
the behavior of the quantum dot is intermediate between that of
an atom and that of a classical macroscopic object.
     In general, in this context, the term "cellular automata"
refers to theoretical computer processing units consisting of a
large number of cells, each cell containing a relatively small
number of responding entities (devices), with each cell
communicating only with neighboring cells. The main advantage of
cellular automata systems is that they eliminate the requirement
for long interconnections between devices, a requirement that is
one of the ultimate limitations of the speed of the conventional
computer microchip. What is of interest here is that quantum dots
can be configured as quantum cellular automata, and such
configurations have been shown to have the ability to perform
logic operations. Until now, however, these devices have worked
only at very low temperatures (millikelvins) -- unless the dots
are made extremely small (less than 2 nanometers in diameter).
... ... R.P. Cowburn and M.E. Welland (University of Cambridge,
UK) now report experimental development of quantum cellular
automata using relatively large quantum dots (approximately 100
nanometers) that work at room temperature, provided one uses
magnetic metals in the construction of the dots. The authors make
the following points:
     1) In the system developed by the authors, each quantum
cellular automata network consisted of a single elongated input
quantum dot followed by a chain of 69 circular dots. Each dot was
110 nanometers in diameter and placed so that the edge of each
dot was 25 nanometers from the edge of each neighboring dot
(i.e., pitch = 135 nanometers). The dots were 10 nanometers thick
and made from a common magnetic alloy (Supermalloy), which is
Ni(sub80)Fe(sub14)Mo(sub5)X, where X is other metals, all of the
dots on a single-crystal silicon substrate. The dots were
fabricated by high-resolution *electron-beam lithography.
     2) The authors point out that in electronic quantum cellular
automata, the term "quantum" is used because the system involves
*quantum mechanical tunneling of charge between dots to change a
logic state; classical electrostatics are involved thereafter in
the propagation of the change in logic state. The quantum
mechanical interactions in magnetic quantum cellular automata
networks are *exchange interactions between *spins within a
single dot in order to form a single giant classical spin. A
logic 1 is signaled when the magnetization vector of the dot
points to the right, for example, and a logic 0 when it points to
the left. The magnetic field emanating from such a magnetic
particle can be extremely large, with the result that one
magnetic dot is strongly influenced by the magnetic field
emanating from its nearest neighbor. These classical
magnetostatic interactions are then involved in the propagation
of information along the chain of dots. A further feature of
magnetostatic interactions is that they force the magnetization
to point along the length of the chain. The system is thus
intrinsically binary, with only right- and left-pointing
magnetization states being stable. An applied oscillating
magnetic field feeds energy into the system and serves as a
clock.
     3) The authors state: "These networks offer a several
thousandfold increase in integration density and a hundredfold
reduction in power dissipation over current microelectronic
technology."
-----------
R.P. Cowburn and M.E. Welland: Room temperature magnetic quantum
cellular automata.
(Science 25 Feb 00 287:1466)
-----------
Text Notes:
... ... *electron-beam lithography: In this context, lithography
is a technique used for integrated circuit fabrication, the
technique in general involving a silicon chip coated uniformly
with a radiation-sensitive film ("resist"), and an exposing
radiation source (e.g., ultraviolet light or an electron beam)
illuminating selected areas of the surface through an intervening
master template (mask) to obtain a particular pattern of resist-
coated surface after unexposed resist is washed away. Non-resist
coated portions of the silicon surface are then etched away by
acid. In electron-beam lithography, the radiation-sensitive film
used in microchip fabrication is placed in the vacuum chamber of
a scanning-beam electron microscope and exposed by an electron
beam under digital computer control. In the present report, the
quantum dots were fabricated by high-resolution electron-beam
lithography in a polymethylmethacrylate resist followed by
metalization and ultrasonically-assisted lift-off in acetone. The
report contains photographs of linear arrays of identical quantum
dots 110 nanometers in diameter fabricated by this method.
... ... *quantum mechanical tunneling: "Tunneling" is a quantum
mechanical phenomenon involving an effective penetration of an
energy barrier resulting from the width of the barrier being less
than the wavelength of the particle.
... ... *exchange interactions: In quantum mechanics, an
"exchange interaction" is an interaction represented by an
exchange of space or spin coordinates or both --  an interaction
that can be viewed as an effective exchange of particles.
... ... *spins: In quantum mechanics, electrons, protons, and
neutrons have an intrinsic angular momentum known as "spin", and
a *magnetic moment parallel or antiparallel to that angular
momentum. When electrons are combined together to form an atom or
ion, there is a resultant angular momentum which is a combination
of the intrinsic spin of the electrons and the angular momentum
due to their motion about the nucleus, and this is the "spin" of
the atom or ion. Atoms or ions with non-zero spin are magnetic
atoms or ions.
... ... *magnetic moment: (magnetic dipole moment) The intrinsic
spins of the electrons in an atom, together with the motion of
the electrons around the nucleus, give rise to a magnetic field
around the atom, and the magnitude of this field is related to
the magnetic dipole moment of the atom or ion.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12May00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MATERIALS SCIENCE: QUANTUM DOTS
It is now possible to create extremely small crystals which
contain less than 1000 atoms, each crystal measuring a few
millionths of a millimeter across and thus in the nanoscale
domain. Certain of these nano-crystals, those of cadmium
selenide, for example, have peculiar attributes: crystals of
exactly the same composition but of different size exhibit quite
different properties, with the large nano-crystals of cadmium
selenide red in color, smaller crystals orange, and the smallest
(containing barely 100 atoms) yellow in color. The differences in
properties are due to quantum mechanical effects. These extremely
small atomic arrays are called "quantum dots", and there is a
current consensus that if quantum dots could be integrated onto a
chip, their unique electrical properties could be harnessed to
perform a function similar to a conventional transistor, while
requiring only a small fraction of the space. In consequence, the
creation of an appropriate regular array of quantum dots would
allow a computer processor many times more powerful than any
current supercomputer to be constructed on single chip.
... ... F. Remacle and R.D. Levine (2 installations, BE IL)
present a theoretical discussion of assemblies of metallic
quantum dots with each dot considered as an "atom". The dots are
taken as being packed close enough to be interacting. The authors
suggest that the key point is that such dots are essentially
"designer" atoms, since their electronic properties can be
controlled via the synthetic method used to prepare the dots. Of
direct significance are the size of the dot and the nature of the
ligands used to prevent coalescence of the dots. The energy
required to remove or add an electron to the dot is determined by
the size of the dot. The ligands control how closely the dots can
be packed and hence the strength of the coupling between adjacent
dots. An important parameter is the energy cost of adding an
electron to a dot: because of the large size of the dots, the
Coulomb repulsion of the added electron is low. Unlike most
ordinary atoms, quantum dots have a high capacity for
accommodating an additional electron.
-----------
F. Remacle and R.D. Levine: Architecture with designer atoms:
Simple theoretical considerations.
(Proc. Natl. Acad. Sci. US 18 Jan 00 97:553)
QY: R.D. Levine [rafi@fh.huji.ac.il]
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 21Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MATERIALS SCIENCE: THE SEARCH FOR LOW-K DIELECTRICS
In general, a "transistor" is a semiconductor device in which it
is possible to control voltage or current in such a way as to
achieve gain or switching action. An "integrated circuit" is a
miniature electronic circuit produced within a single crystal of
a semiconductor such as silicon. Such devices, usually called
"chips", range from simple logic circuits to large-scale circuits
containing approximately 10^(6) components (transistors,
resistors, capacitors), and these devices are widely used in
memory circuits, microcomputers, pocket calculators, etc.,
because of their low cost and high speed.
... ... Robert D. Miller (IBM Almaden Research Center, US)
presents a review of current research in chip design, the author
making the following points:
     1) Within the next few years, high-performance chips
containing as many as 5 x 10^(8) transistors on a single chip
will be produced. These advanced chips may contain up to 10^(4)
meters of on-chip wiring. Such increased component and wiring
densities, however, cannot be achieved with currently used
materials. Although an intensive search is now underway for
materials that can replace silicon dioxide [SiO(sub2)] as the
insulator in these future devices, a clear candidate material has
yet to be identified.
     2) In a typical microchip, layers of copper-interconnect
wiring are separated by a dielectric insulator, traditionally
silicon dioxide. Both the resistance of the metal and the
capacitance of the insulator increase markedly as the wiring
dimensions and distances between chip components (pitch)
decrease, with resulting crosstalk, capacitative coupling between
metal-interconnect lines, and consequent increased signal delays.
Traditional aluminum-copper wiring can be replaced by pure
copper, which has a lower resistance, and then the performance
gain is limited primarily by interlayer and intralayer
capacitance, which in turn is dictated primarily by the
*dielectric constant of the insulator. Thus, there is now a
intensive search underway for new dielectric insulators with
lower dielectric constants than silicon dioxide. (The dielectric
constant (k) of silicon dioxide is in the range 3.9 to 4.2.)
     3) Any replacement low-dielectric constant material must
meet current requirements for integrated circuits: thermal
stability in excess of 400 degrees centigrade, good mechanical
properties, low ion content, breakdown fields in excess of 2
million volts per centimeter, low water uptake, lithographic
processability, low thermal expansion coefficients and film
stresses, good adhesion to a variety of substrates, and low
reactivity with conductor metals at elevated temperatures.
     4) Although the drive toward increased device densities and
improved performance in semiconductor devices makes the switch
from silicon dioxide on-chip insulators to low-dielectric
constant materials inevitable, no clear winner has yet emerged
among materials with dielectric constants less than 3.0. The
author concludes: "The switch to low-k on-chip insulators
continues to be a formidable challenge to chemists, physicists,
materials scientists, and integration engineers."
-----------
Robert D. Miller: In search of low-k dielectrics.
(Science 15 Oct 99 286:421)
QY: Robert D. Miller [rdmiller@almaden.ibm.com]
-----------
Text Notes:
... ... *dielectric constant: In general, a "dielectric" is a
substance that can sustain an electric field and act as an
insulator, e.g., a nonconductor of electric charge in which an
applied electric field causes a displacement of charge but not a
flow of charge. In physics, "permittivity" is the ratio of the
electric displacement in a dielectric medium to the applied
electric field strength, and "relative permittivity" refers to
the ratio of the permittivity of a medium to the permittivity of
*free space. Relative permittivities vary from 1 (for free space)
to over 4000 for certain ferromagnetic materials, but for most
materials, relative permittivities are less than 10. In general,
in physics, the term "dielectric constant" has been replaced by
the term "relative permittivity", but in the context of electric
circuits, the term dielectric constant is still used, and in this
context the term "dielectric constant" of a material is perhaps
best defined as the ratio of the capacitance of a capacitor
constructed of the material to the capacitance the capacitor
would possess if the material were replaced by free space.
... ... *free space: In physics, the term "free space" refers to
a region in which there is no matter and no electromagnetic or
gravitational fields, the region having a temperature of absolute
zero, unit refractive index, and the speed of light at its
maximum value.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 31Dec99
[For more information: http://scienceweek.com/swfr.htm]

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IN FOCUS: ON COMPUTERS IN ASTRONOMY
"What we fail to see with our eyes, or detectors, we can
occasionally see with our minds, aided by computer graphics.
Computers now play a key role in the search for dark matter.
Historically, astronomers have focused on observations; now the
field has evolved into an experimental science. Today's
astronomical experimenters sit neither at lab benches nor at
telescopes but at computer terminals. They scrutinize cosmic
simulations in which tens of thousands of points, representing
stars, gas and dark matter, interact gravitationally over a
galaxy's lifetime. A cosmologist can tweak a simulation by
adjusting the parameters of dark matter and then watch what
happens as virtual galaxies evolve in isolation or in a more
realistic, crowded universe. Computer models can thus predict
galactic behavior. For instance, when two galaxies suffer a close
encounter, violently merging or passing briefly in the night,
they sometimes spin off long tidal tails. Yet from the models, we
now know these tails appear only when the dark matter of each
galaxy's halo is three to ten times greater than its luminous
matter. Heavier halos produce stubbier tails. This realization
through modeling has helped observational astronomers to
interpret what they see and to understand more about the dark
matter they cannot see. For the first time in the history of
cosmology, computer simulations actually guide observations.
-----------
Vera Rubin: "Dark Matter in the Cosmos"
in D.R. Danielson: _The Book of the Cosmos_
(Helix Books, Cambridge MA (US) 2000, p.585)
[Astronomer Vera Rubin is in the Department of Terrestrial
Magnetism, Carnegie Institution, US. She received the US National
Medal of Science in 1993)
For more information: http://scienceweek.com/swfr.htm

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