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.

December 17, 1999 -- Vol. 3 Number 51

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There is no doubt that great revolutions of human scientific
thought will occur in the next century, and in the century after
that, and in thousands of centuries afterward. So which of our
current pet scientific dogmas will be among the first washed away
by new facts and sudden clarities?
-- Anonymous

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The following reports from back issues have been posted at the
ScienceWeek website. Links are on the main page (URL:
http://www.scienceweek.com) in the panel to the right of the
current issue contents (you many need to scroll down on the web
page). Main page links to these reports will be up for 1 week.
After that the reports go into the archive, where they can be
accessed via the SW website search engine.

Earth Science: The Layered Earth
Particle Physics: Nucleon Spin
Earth Science: On the Possibility of Rapid Climate Change
Materials Science: A New Approach to Materials Design
Developmental Biology: Notch Signaling
Neurobiology: Potassium Channel Gating
Science Policy: Basic Energy Research
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Contents of This Issue:

1. Neurodevelopmental Damage in Autism: An Infection-Based Model
2. Neurobiology: On Mechanisms of Olfaction
3. High Resolution Detection of ATP on Surfaces of Living Cells
4. On Wave Phenomena in Physics
5. On Gravitational Radiation and General Relativity
6. Precision Measurements of Bright Rings Around Sunspots

In Focus: On the Biophysics of Warmth and Cold Detection

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1. 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: 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.
... ... *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/search/search.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/search/search.htm]


2. NEUROBIOLOGY: ON MECHANISMS OF OLFACTION
In vertebrates, two major sensory systems are dedicated to the
detection of chemicals in the environment: olfaction and taste.
The olfactory system detects airborne molecules; the taste system
detects ingested, water-soluble molecules. In humans, olfaction
provides information about chemicals from food, one's self, other
people, and about a variety of animals, plants, and other aspects
of the environment. Olfactory information can influence feeding
behavior, social interactions, and reproduction. Taste
(gustation) provides information about the quality, quantity, and
safety of ingested food. Three articles in a recent symposium are
devoted to various aspects of the biology of olfaction:
... ... Peter Mombaerts (The Rockefeller University, US) presents
a review of the role of specific proteins as odorant and
chemosensory receptors, the author making the following points:
     1) The olfactory systems of various species solve the
problem of general molecular recognition in widely differing
ways. Despite this variety, the molecular receptors are
invariably *G protein-coupled 7-transmembrane proteins, and these
receptor proteins are encoded by the largest gene families known
to exist in a given animal genome. Such receptor gene families
have been identified in vertebrates and two invertebrate species
(the *nematode Caenorhabditis elegans and the fruit fly
*Drosophila melanogaster).
     2) The complexity of the odorant receptor repertoire is
estimated in mouse and rat at 1000 genes, or 1 percent of the
genome, surpassing that of the *immunoglobulin and *T cell genes
combined.
     3) In rodents, two distinct 7-transmembrane gene families
may encode the chemosensory receptors of the *vomeronasal organ,
which is specialized in the detection of pheromones. Remarkably,
these receptor families have practically no sequence homology
among them.
     4) Experiments indicate that vertebrate odorant receptors
may fulfill a dual role, also serving as address molecules that
guide axons of olfactory sensory neurons to their precise target
in the brain.
... ... K. Mori et al (3 authors at 2 installations, JP) present
a review of the coding and processing of odor molecule
information in the olfactory bulb, the authors making the
following points:
     1) Olfactory sensory neurons detect a large variety of odor
molecules and send information through their axons to the
olfactory bulb, the first site for the processing of olfactory
information in the brain. The axonal connection is precisely
organized so that signals from 1000 different types of odorant
receptors are sorted out in 1800 specific clusters (glomeruli) in
the mouse olfactory bulb.
     2) Individual glomeruli modules presumably represent a
single type of receptor and are thus tuned to specific molecular
features of odorants.
     3) Local neuronal circuits in the olfactory bulb mediate
*lateral inhibition among glomerular modules to sharpen the
tuning specificity of output neurons. These circuits also mediate
synchronized oscillatory discharges among specific combinations
of output neurons and may contribute to the integration of
signals from distinct odorant receptors in the *olfactory cortex
of the brain.
... ... Gilles Laurent (California Institute of Technology, US)
presents a review of systems aspects of early olfactory coding,
the author making the following points:
     1) Studying a neural code requires asking specific questions
such as the following: What information do the signals carry?
What formats are used? Why are such formats used? Although
superficially unambiguous, such questions involve hidden
difficulties and biases. The author suggests that whereas Shannon
and Weaver (1963) developed information theory to quantify
communication through noisy channels, neuroscientists have found
that brains do more than just convey information about the world.
Sensory circuits apparently evolved to detect selective patterns
relevant for survival, but these circuits also create qualities
that do not exist outside of the brain. Hence, brain codes can be
studied from many different perspectives.
     2) To understand coding, the format and information-carrying
features of signals transported from a source to a receiver must
be examined. Although the approach is clear when applied to
traditional communication channels, it is fuzzier when applied to
brain circuits. Neurons propagate signals via transmembrane
voltage changes -- in most cases, *action potentials. As far as
we know, all information carried by one neuron is conveyed by
some aspect or aspects of its "spike" (action potential)
discharge. The study of neural coding thus requires an estimate
of the discharge of the participating neuron, and in this regard
no technique is perfect.
     3) Given a defined source and receiver, what forms could
codes take? Because the relevant signals are spikes produced by
individual neurons over time, any neural code is spatiotemporal.
     4) Whereas the visual and auditory systems process energy
signals whose propagation in the external environment is
predictable, olfaction must deal with turbulent flow of the
medium (turbulent flow of air).
     5) The author concludes: "The study of olfactory coding sits
at the intersection of several established and evolving areas of
modern neuroscience... Traditional concepts transferred literally
from the study of other senses may not always be appropriate for
olfactory codes. The time seems ripe for combining theories that
emphasize global dynamics with experimental approaches that
provide cellular and spike time resolution, as well as behavior."
-----------
Peter Mombaerts: Seven-transmembrane proteins as odorant and
chemosensory receptors.
(Science 22 Oct 99 286:707)
QY: pm@rockvax.rockefeller.edu]
-----------
K. Mori et al: The olfactory bulb: Coding and processing of odor
molecule information.
(Science 22 Oct 99 286:711)
QY: Kensaku Mori [moriken@postman.riken.go.jp]
-----------
Gilles Laurent: A systems perspective on early olfactory coding.
(Science 22 Oct 99 286:723)
QY: Gilles Laurent [laurent@its.caltech.edu]
-----------
Text Notes:
... ... *G protein-coupled 7-transmembrane proteins: G-proteins
are a family of signal-coupling proteins that act as
intermediaries between activated cell receptors and effectors,
for example, the transduction of chemical messenger (hormone)
signals from the cell surface to the cell interior. The G-protein
is apparently embedded in the cell membrane with parts exposed on
the outside surface and inside surface. So-called "seven-
transmembrane proteins" (seven-transmembrane-domain proteins) are
a type of receptor protein containing, in a single polypeptide
chain, 7 hydrophobic domains that traverse the cell membrane
lipid bilayer.
... ... *nematode Caenorhabditis elegans: Nematodes are an
abundant and ubiquitous phylum of unsegmented roundworms.
Caenorhabditis elegans is a small (1 mm) nematode worm. It is
transparent, hermaphroditic, free-living, and found in soil. It
has a relatively small genome (approximately 3000 genes), and
only a few types of cells in its body. It has a 16-hr
embryogenesis that can be achieved in a petri dish, and is thus
highly suitable for the study of developmental and behavioral
genetics.
... ... *Drosophila melanogaster: A major advantage of this
experimental system is the presence of giant chromosomes in the
insect's salivary glands. (In cells with chromosomes, the
chromosomes are the physical structure into which DNA is
organized and on which genes are carried.) Drosophila also has a
short reproductive cycle (approximately 10 days), and it produces
100 to 400 progeny per mating.
... ... *immunoglobulin: The immunoglobulins are a large
glycoprotein category that includes antibodies as a subset. 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.
... ... *T cell: (lymphocyte) Lymphocytes are a type of leukocyte
(white blood cell) responsible for the immune response. There are
two classes of lymphocytes: 1) B-cells, which, when presented
with a foreign chemical entity (antigen), change into antibody
producing plasma cells; and, 2) T-cells, which interact directly
with foreign invaders such as bacteria and viruses. Certain types
of T-cells are also involved in B-cell production of antibodies.
The essential point of the text is that since the immune system
of vertebrates is so essential for survival of the organism, the
large number of genes devoted to receptor proteins is an
indication of the importance of sensory receptors in evolution.
... ... *vomeronasal organ: The vomer is a flat bone of
trapezoidal shape forming the inferior and posterior portion of
the nasal septum, and term "vomeronasal" refers to the vomer and
the nasal bone.
... ... *lateral inhibition: In general, a type of neuronal
inhibition in which activity of a neuron inhibits activities of
other neurons in the same locus. The simplest example is an axon
collateral involved in a negative feedback loop to one or more
nearby nerve cells.
... ... *olfactory cortex of the brain: The cerebral cortex is a
thin surface layering of nerve cells of the brain, the region
only several millimeters thick but covering all of the brain
surface. This is the part of the central nervous system most
intimately involved with the so-called "higher faculties",
although the cortex operates in concert with other parts of the
brain. The structure is primitive in lower mammals, and is found
progressively more pronounced and with greater surface area in
primates and man.
... ... *action potentials: (nerve impulses) In general,
transient pulses (e.g., 1 millisecond) of reversed membrane
potential propagated over the long extensions of neurons (axons),
in some cases over relatively large distances (e.g., 1 meter
between spinal motorneurons and peripheral muscle cells). The
physical characteristics of the action potentials in the nervous
systems of diverse animal forms are often quite similar.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/search/search.htm]


3. HIGH RESOLUTION DETECTION OF ATP ON SURFACES OF LIVING CELLS
     ATP (adenosine triphosphate) is the most important chemical
energy source in living cells, intimately involved in various
cell functions and cell metabolism, and an entity in numerous
cyclic chemical pathways involved in the synthesis of cell
components. Most of the research and analysis of ATP function has
involved intracellular ATP, but there has been speculation
concerning the role of extracellular ATP in biological processes.
The potential regulatory effects of extracellular ATP on
*platelet aggregation, *vascular tone, muscle contraction, the
generation of pain signals in nerve fibers, and *ion channel
dynamics have been the foci of recent studies. One difficult
problem associated with this research is the accurate detection
and localization of extracellular ATP on the surfaces of living
cells.
     An atomic force microscope is a type of microscope in which
a small probe, consisting of a tiny chip of diamond, is held on a
spring-loaded cantilever in contact with the surface of a sample.
The probe is moved slowly across the surface, and the tracking
force between the tip and the surface is monitored. The probe is
raised and lowered so as to keep this force constant, and a
profile of the surface is thus produced. Since the instrument can
be used with electrically nonconducting samples, it is useful for
biological specimens, and this technique has been used to assay
the morphology of living cells and the dynamic changes that occur
after manipulations in the physiological environment surrounding
the cell. Recent studies have also demonstrated that atomic force
microscopy can be used as a viable tool to measure elasticity,
enzyme activity, or (after chemical treatment of the microscope
tips) the presence of chemically distinct functional groups.
     Myosin is the major structural protein of muscle cells, a
large protein with a molecular weight of approximately 5 x 10^(5)
daltons. There are also several types of myosin found in non-
muscle cells, where they are active in cell movements, and at the
present time more than a dozen different structural classes of
myosin are recognized. In general, all myosins are enzymes
exhibiting ATPase activity, i.e., they have an ATP binding site
and catalyze ATP hydrolysis.
... ... S.W. Schneider et al (6 authors at 3 installations, US
DE) now report the use of atomic force microscopy, in combination
with myosin-coated cantilevers, to detect ATP concentrations in
solution and on the surfaces of living cells. Functionally active
tips were used to scan the surface of *bronchial epithelial cells
in culture and to demonstrate a basal surface ATP concentration.
The authors suggest their results represent an innovation in the
application of atomic force microscopy to biological
preparations, and that the development of ATP-detecting tips is
an example of modifying the scanning probe tip to measure
simultaneously high-resolution topography and a biologically
important molecule in the surface microenvironment of living
cells. The authors further suggest that successful implementation
of this biosensor "opens possibilities in the use of the atomic
force microscope for a continuous live update of surface
topography, as well as a direct measurement of the
microenvironment along the surface of the cell membrane in living
cells under physiological conditions."
-----------
S.W. Schneider et al: Continuous detection of extracellular ATP
on living cells by using atomic force microscopy.
(Proc. Natl. Acad. Sci. US 12 Oct 99 96:12180)
QY: John P. Geibel [john.geibel@yale.edu]
-----------
Text Notes:
... ... *platelet: In this context, a "platelet" is an
irregularly shaped disk-like cytoplasmic fragment of a large bone
marrow cell (megakaryocyte), the fragment shed from the marrow
and subsequently found in peripheral blood where it functions in
clotting.
... ... *vascular tone: In this context, "tone" refers to the
resting tension of muscle, and "vascular tone" refers to the tone
of the muscles of blood vessels.
... ... *ion channel dynamics: Ion channels are protein channels
in cell membranes that allow ions to pass from extracellular
solution to intracellular solution and vice versa. Most ion
channels are selective, allowing only certain ions to pass, and
an individual cell has ion channels with various ion
selectivities.
... ... *bronchial epithelial cells in culture: In animals,
"epithelial cells" compose the cell layers that form the
interface between a tissue and the external environment, for
example, the cells of the skin, the lining of the intestinal
tract, and the lung airway passages. "Bronchial" epithelial cells
are epithelial cells of the two main subdivisions of the trachea
(bronchi) of the lung. In general, the term "cultured cells"
refers to a population of cells maintained in vitro, the cells
proliferating in a medium of controlled composition. In many
experiments, the cultured cells are subpopulations of "immortal"
cell lines widely used in numerous laboratories.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/search/search.htm]


4. ON WAVE PHENOMENA IN PHYSICS
The idea of "wave" phenomena, the characterization of certain
phenomena as waves, must rank as one of the most important
concepts in both classical and modern physics. In general, a
"wave" is a time-varying quantity that is also a function of
position, a disturbance either continuous or transient,
travelling through a medium as a result of certain properties of
the medium, the resulting displacements of the medium returning
to zero when the disturbance has passed. The chief parameters of
a wave are its speed of propagation, its frequency, its
wavelength, and its amplitude.
... ... J.A. Scales and R. Sneider (2 installations, US NL)
present an essay on waves in physics, the authors making the
following points:
     1) The authors note that when scientists (including
physicists) are asked to define a wave, the answers are often
ambiguous. Students may state that a wave is a solution to the
wave equation; professionals may make some ambiguous statement
about propagation velocity; mathematicians tend to give a formal
characterization based on the hyperbolic character of certain
differential equations. The authors suggest that the term "wave"
be defined as an "organized propagating imbalance", with the
caveat: "Just don't ask us to define 'organized'."
     2) The authors state that at the simplest levels, the
ubiquity of (classical) waves can be attributed to the desire of
nature for stable equilibria. Whatever the forces that connect
bits of matter together (e.g., electromagnetic or gravitational),
for small perturbations about a stable equilibrium point, the
forces are approximately linear. A linear restoring force implies
harmonic oscillations, and coupled systems of oscillators support
both propagating and standing disturbances. Linearity also
implies superposition, so that periodic solutions can be added
together to obtain finite wave "packets". Thus, for small
perturbations about an equilibrium state in coupled or decoupled
(extended) systems, waves are the natural consequence of the
stability of simple harmonic motion.
     3) Wave propagation is in many situations described by a
linear differential equation. In reality, nonlinearity is of
great importance, and this nonlinearity may destroy the waves.
When this happens, organized wave motion changes into turbulent
motion, and in this process it is impossible to state exactly at
which point the wave ceases to be a wave.
     4) Heat is the manifestation of microscopic motion.
Computing the classical resonant frequencies of atoms or
molecules in a lattice gives numbers of the order of 10^(13) Hz,
i.e., in the infrared part of the electromagnetic spectrum, so
that when molecules vibrate they produce heat. These lattice
vibrations are called "phonons", and they have both wave-like and
particle-like character. Lattice vibrations are responsible for
the transport of heat in a lattice, and we know that heat is a
diffusive phenomenon. However, if the lattice is cooled to near
absolute zero, the mean free scattering path of the phonons
becomes comparable to the macroscopic size of the sample, and
when this happens, lattice vibrations no longer behave
diffusively but are actually wave-like. By controlling the
temperature of a sample, one can control the extent to which heat
is ballistic (wave-like) or diffusive.
     5) Waves have a central role in quantum mechanics, according
to which theory everything has a wave character. Einstein (1879-
1955) used the relation E = hf (energy equals Planck's constant
times frequency) to connect the wave frequency of light with the
energy of discrete light quanta (photons). De Broglie (1875-1960)
extended this to electrons and other entities of matter. For
classical waves, dissipation generally damps the wave motion, and
ultimately everything appears to come to rest. Quantum mechanics
demonstrates that matter waves do not exhibit dissipation: even
the ground state of a harmonic oscillator is in harmonic motion.
Matter waves never come to rest.
-----------
J.A. Scales and R. Sneider: What is a wave?
(Nature 21 Oct 99 401:739)
QY: John A. Scales [jscales@mines.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
NEW LINEARIZED WAVE MODEL SIMPLIFIES SOLITON ANALYSIS
A soliton is a solitary localized wave propagating with little or
no change in form in special circumstances in a nonlinear
dispersive medium. It is one of those special mathematical
circumstances that have relevance because there are many
instances in all branches of science where such solitary
propagating waves apparently occur. The mathematical physics of
solitons, however, is extremely complicated, involving nonlinear
differential wave equations with usually intractable solutions.
The result is that until now there has been little physical
understanding of solitons despite the possibility for
mathematical understanding inherent in the relevant equations.
This week Allan W. Snyder and D. John Mitchell (Australian
National University, Canberra AU) presented a heuristic
mathematical model whose basis is the degeneration of the soliton
nonlinear wave equation to a common linear wave equation for a
special set of parameters and parametric relationships. The
result is that for the first time the physical behavior of the
soliton is intuitively apparent, albeit for these special
circumstances. This is a typical mode of breaking through new
conceptual ground in theoretical physics, and physicists are
excited by this development. It will no doubt spur a vigorous
round of experimental and theoretical investigations that may
well bear fruit in new electro-optic technology.
(Science 6 Jun 97) (Science-Week 12 Jun 97)
[For more information: http://scienceweek.com/search/search.htm]


5. ON GRAVITATIONAL RADIATION AND GENERAL RELATIVITY
First proposed in 1915 by Albert Einstein (1879-1955), the
General Theory of Relativity describes the effects of the
gravitational fields of matter on space and time. One of the
major conclusions of the theory is that gravitational fields
alter the geometry of space and time, causing this geometry to
become curved. According to the theory, when gravity is weak and
the resultant space-time curvature small, the motion of matter is
close to that predicted by Newton's laws.
... ... Clifford M. Will (Washington University St. Louis, US)
presents a review of current attempts to improve the validation
of general relativity theory, the author making the following
points:
     1) Although the empirical support for general relativity
theory is strong, it is still not as solid as the support for
*Maxwell's theory of electromagnetic phenomena, and only in the
last 35 years or so have precise tests of general relativity been
feasible. Furthermore, general relativity has not been tested
fundamentally either in its radiative regime (i.e., with respect
to *gravitational waves) or in the regime of strong gravitational
fields such as those associated with *black holes or *neutron
stars. Most tests, such as those carried out within our Solar
System, check the theory only in its weak-field, slow-motion,
nonradiative limit. One famous exception, analysis of the *Hulse-
Taylor binary pulsar, does provide an important verification of
the lowest-order radiative predictions of general relativity and
is sensitive to some strong-field aspects. But more fundamental
tests of gravitational radiation and its properties are yet to be
accomplished. In addition, alternative theories to general
relativity still exist -- theories in agreement with all
observations to date. Thus, gravitational wave tests remain of
interest to the extent that such tests can further constrain the
theoretical possibilities.
     2) There are 3 aspects of gravitational radiation that can
be subjected to testing:
... ... a) The *polarization content of the gravitational waves:
general relativity predicts only 2 polarization states, while
other theories predict as many as 6.
... ... b) The speed of the gravitational waves: general
relativity predicts a speed the same as that of light, while
other theories may predict different speeds.
... ... c) The back influence of the emitted radiation on the
evolution of the source.
     2) At the heart of gravitational theory is the Einstein
equivalence principle, which essentially modernizes Newton's
postulate of the equivalence of gravitational and inertial mass.
The equivalence principal states: a) bodies fall with the same
acceleration regardless of their internal structure or
composition (the so-called "weak equivalence principle"); and b) 
the outcome of any local nongravitational experiment is both
independent of the velocity of the free-falling reference frame
in which it is performed (called "local Lorentz invariance") and
independent of where and when in the Universe it is performed
("local position invariance").
     3) The Einstein equivalence principle implies that
gravitation must be described by a theory in which matter
responds only to the geometry of space-time. Such theories are
called "metric theories". General relativity is a metric theory
of gravity, but so are many others, including the *Brans-Dicke
"scalar-tensor" theory. "*String theory", a leading contender for
a unified theory of particle interactions and for a quantum
theory of gravity, does not strictly satisfy the metric theory
definition: in string theory, matter can respond weakly to
gravitation-like fields, in addition to responding to geometry.
Consequently, testing the Einstein equivalence principle is a way
to search for new physics beyond standard metric gravity.
     4) Concerning the testing of general relativity in strong
fields, the in-spiraling and merger of two compact astronomical
objects, or the core collapse in a supernova, involve the physics
of space-time curvature in the limit of strong and highly dynamic
fields, as well as the formation and evolution of *black hole
event horizons. Although this physics is so complex that
quantitatively precise tests of general relativity are not likely
to be realized, making qualitative tests may be possible, and
signals from strong-field events may tell us much about the
validity of general relativity. But finding firm predictions for
the gravitational waves to compare the observations against
requires solving Einstein's equations in a regime in which only
large-scale numerical computation has a hope of yielding reliable
results. This challenging task has been taken up by many
"numerical relativity" groups around the world. The discovery and
study, by means of gravitational waves, of the formation of a
black hole would provide a strong test of relativistic gravity.
-----------
Clifford M. Will: Gravitational radiation and the validity of
general relativity.
(Physics Today October 1999)
QY: Clifford M. Will [cmw@wuphys.wustl.edu]
-----------
Text Notes:
... ... *Maxwell's theory of electromagnetic phenomena: James
Clerk Maxwell (1831-1879) essentially brought together
electricity, magnetism, and light in one set of equations. The
equations are a set of differential equations describing the
space and time dependence of the electromagnetic field, the
equations forming the basis of classical electrodynamics.
... ... *gravitational waves: In general, this is a wave-like
perturbation in a gravitational field, produced when a mass is
accelerated or otherwise disturbed. According to general
relativity theory, such waves travel through space-time at the
speed of light, with an amplitude proportional to the rate of
acceleration of the body producing the waves. The strongest
sources are the sources with the strongest gravitational fields,
although the gravitational waves produced are comparatively weak.
... ... *black holes: 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. Another view of a black hole
is that it is a mass that has collapsed to such a small volume
that its gravity prevents the escape of all radiation.
... ... *neutron stars: 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.
... ... *Hulse-Taylor binary pulsar:  A pulsar is a pulsing
source of stellar radiation believed to originate with a *neutron
star. They were originally discovered at radio wavelengths, but
there are optical, gamma-ray, and x-ray pulsars, and some of the
gamma-ray pulsars are extremely powerful gamma-ray emitters. A
"binary pulsar" is a pulsar in orbit with another star, the
existence of the companion revealed by a cyclic change in the
pulse period as the two stars orbit each other. The first known
binary pulsar, denoted as PSR 1913+16, was discovered by Russell
Hulse and Joseph Taylor in 1974. This binary pulsar consists of a
pulsar that pulses 17 times a second in a highly eccentric orbit
around a second neutron star from which pulses are not observed.
The orbital period of each star is shortening, apparently due to
the loss of energy resulting from gravitational radiation.
... ... *polarization: In general, the term "polarization" refers
to the vibration of waves in a preferred plane or planes, or to
the process of confining the vibrations to certain planes. In
this context, the waves are gravitational waves. Ordinarily, the
term refers to light waves.
... ... *Brans-Dicke "scalar-tensor" theory: This is an
alternative to Einstein's general relativity theory. The Brans-
Dicke theory attempts to incorporate *Mach's principle, and among
other things predicts a time-dependent gravitational constant.
The theory was formulated by C.H. Brans and R.H. Dicke.
... ... *Mach's principle: Ernst Mach (1838-1916) proposed that
the inertial mass of a particle is determined by the
gravitational effect of all the other matter in the Universe,
with the concept of "mass" meaningless in an empty universe.
... ... *String theory: In particle physics, string theory is a
theory of elementary particles based on the idea that the
fundamental entities are not point-like particles but finite
lines (strings), or closed loops formed by strings, the strings
one-dimensional curves with zero thickness and lengths (or loop
diameters) of the order of the Planck length of 10^(-35) meters. 
... ... *black hole event horizons: The boundary of a black hole
is called the "event horizon", because any event within the
boundary is invisible outside, the invisibility resulting from
the fact that no radiation can escape to be detected.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ASTROPHYSICS: PERSPECTIVES IN GENERAL RELATIVITY
... In recent years, the small deviations from Newton's laws
predicted by general relativity theory have been indirectly
confirmed by studies of *millisecond radiopulsars [*Note #1].
However, no direct measurements have ever been performed to
measure the gross deviations from Newtonian mechanics predicted
in strongly-curved space-time. Thus, these strong-field
predictions of general relativity theory have not yet been
tested.
... ... Michiel van der Klis (University of Amsterdam, NL)
presents a review of current research on general relativistic
effects produced by massive astronomical objects, the author
making the following points:
     1) The strongest space-time curvatures that are accessible
to observations are those near collapsed massive stars. These
objects concentrate within a few kilometers the mass of a star
one to several times the mass of the Sun (1 solar-mass = 2 x
10^(30) kg). Such objects (e.g., black holes and neutron stars)
have the strongest gravity, and hence curve space-time the
tightest of all known objects in the Universe. Some of these
objects are so massive that they can only be black holes [*Note
#2].
     2) X-ray binaries, discovered in the early 1960s, are double
star systems in which a compact object (a neutron star or a black
hole) orbits a normal star and pulls a flow of *plasma out of the
normal star's atmosphere onto itself. Such binary systems have
long been considered promising natural laboratories for studying
orbital motion in strongly curved space-time, and since the
launch of the *Rossi Explorer 3 in 1996, these systems have
finally begun to make good on their promise.
     3) The process of "accretion", the flow of matter from the
normal star onto the neutron star or into the black hole,
produces a differentially rotating disk around the compact
object: the closer material is to the compact object, the faster
the material rotates around the object. Thus, the plasma orbits
at ever-increasing speed as it spirals down toward the center. In
the strong gravity region near the compact object, speeds
approach that of light and orbital radii of the plasma are only a
few kilometers, so that the orbit of a particle around the center
takes less than a millisecond to complete.
     4) The Rossi x-ray satellite has made it possible to detect
such extremely tight orbits. The fastest signals, seen in neutron
stars, have periods as short as 0.75 milliseconds, corresponding
to orbital motion at a 12-kilometer radius, which is deep within
the strong-gravity region and close to the marginal orbital
radius within which no stable orbits are possible. Such phenomena
have now been seen in approximately 20 neutron stars. Three black
holes have also shown periods as short as 3 milliseconds, which,
given their higher masses, implies that the signals come from a
similar depth in the strong-gravity region around a black hole.
     5) The author suggests that the success of the Rossi
satellite has demonstrated that when probing dynamics in strongly
curved space-time, there is no substitute for size. It is the
huge effective area of 0.7 square meters of the main x-ray
instrument onboard the satellite that gave Rossi the sensitivity
to make the first direct measurements of orbital motion near
collapsed stars.
-----------
Michiel van der Klis: The buzz of general relativity.
(Science 3 Sep 99 285:1499)
QY: Michiel van der Klis [michiel@astro.uva.nl]
-----------
Text Notes:
... ... *millisecond pulsars: (see Note #2 below)
... ... *Note #1: The essential basis of general relativity
theory is the so-called "equivalence principle", which identifies
the mass involved in the gravitational force equation with the
inertial mass in the equation that relates any force to the
product of inertial mass and acceleration. (In general
relativity, the equivalence principle states that the observable
local effects of a gravitational field are indistinguishable from
those arising from acceleration of the frame of reference.)
... ... *Note #2: ... 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. Theory predicts that a
neutron star should rotate very rapidly, be extremely hot, and
have an intense magnetic field. Pulsars, sources of pulsed radio
energy, are evidently spinning neutron stars which emit beams of
radiation from their magnetic poles. A few pulsars have been
found in binary systems, and the empirical estimated masses of
the pulsars are consistent with the masses predicted by neutron
star models. Pulsars were originally discovered at radio
wavelengths, but they have also been detected at optical and
gamma-ray wavelengths. They can be powerful gamma-ray emitters
(gamma-ray pulsars), and there is also a class of x-ray pulsars.
The periods of pulsars range from approximately 1.5 milliseconds
to 4 seconds and can typically be measured to accuracies of one
part in 10^(10). Pulsars with periods shorter than approximately
0.01 seconds constitute the distinct class of millisecond
pulsars. Most pulsars are single stars, but binary pulsars are
known, about half of which are millisecond pulsars. The
millisecond-pulsar neutron star is believed to be rotating
hundreds of times per second, and a large number of millisecond
binary pulsars have been discovered in globular clusters. The
origin and evolution of pulsars has not been clear, but it has
been thought that millisecond radio pulsars, which are often
found in binary systems, start as ordinary pulsars, then lose
most of their magnetic field and "spin up" to millisecond periods
by the accretion of matter (and transfer of angular momentum)
from a companion star in an x-ray binary system.
... ... *plasma: In this context, a "plasma" is a state of matter
consisting of ions and electrons moving freely. Stars, for
example, consist of plasma. Because a plasma is highly ionized,
it can be affected by external electrical and magnetic fields,
and the charged particles of the plasma interact electrically and
magnetically.
... ... *Rossi Explorer 3: The Rossi X-Ray Timing Explorer (RXTE)
is a NASA satellite launched in December 1995. It carries the
largest array of proportional counters ever flown, nearly a meter
square. The purpose is to study the variability of the brightest
x-ray sources at time-scales down to 10 microseconds. The
satellite is named after the astronomer Bruno Benedetto Rossi
(1905-1993).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 5Nov99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
IN FOCUS: ON GENERAL RELATIVITY
"During the two decades 1960-80, the subject of general
relativity experienced a rebirth. Despite its enormous influence
on scientific thought in its early years, by the late 1950s
general relativity had become a sterile, formalistic subject, cut
off from the mainstream of physics. It was thought to have very
little observational contact, outside of cosmology and a few
tests. It was believed to be an extremely difficult subject to
learn and comprehend. It was also viewed as a field that was full
of ambiguities and unanswerable questions... One of the
outgrowths of the renaissance of general relativity that occurred
between 1960 and 1980 has been a change in attitude about the
importance and use of the theory. Its importance as a fundamental
theory of the nature of space-time and gravitation has not been
diminished in the least: if anything it has been enhanced by the
flowering of research in the subject that has taken place. Its
importance as a foundation for other theories of physics has been
strengthened by current searches for unified and grand unified
quantum theories of nature that incorporate gravity along with
other interactions. But the real change in attitude about general
relativity has been its use as a tool in the real world. [In
astrophysics, for example] the general relativistic bending of
light in gravitational lenses can help astrophysicists probe the
structure of galaxies. General relativistic effects in the binary
pulsar gave a high-precision determination of the mass of the
pulsar. Had the result been very different from 1.4 solar masses
it could have affected our understanding of supernovae in close
binary systems. Neutron-star mass limits from general relativity
are important in the observational search for black holes.
Finally, gravitational radiation may one day provide a completely
new tool for exploring and examining the universe. Relativity
even plays a role in everyday life. For example, the
gravitational redshift effect on clocks _must_ be taken into
account in satellite-based navigation systems, such as the US
Global Positioning System, in order to achieve the required
positional accuracy of a few meters or time transfer accuracy of
a few nanoseconds."
-----------
Clifford Will: The Renaissance of General Relativity.
in: Paul Davies (ed.): _The New Physics_
(Cambridge University Press, Cambridge UK 1989, p.7,33)
[For more information: http://scienceweek.com/search/search.htm]


6. PRECISION MEASUREMENTS OF BRIGHT RINGS AROUND SUNSPOTS
A "sunspot" is a dark area of the solar surface. The center of
the spot, called the "umbra", is darker than the outer border,
which is called the "penumbra". The average sunspot is
approximately twice the diameter of the Earth and may last for
several weeks. Sunspots tend to form in pairs or groups, and a
large group may contain up to 100 spots and may last as long as 2
months. Sunspots appear dark because they are cooler than the
photosphere (the visible surface of the Sun or a star). The
temperature at the center of a typical sunspot is approximately
4240 degrees kelvin, while the solar photosphere is at
approximately 6000 degrees kelvin. Temperatures of the order of
4000 degrees kelvin, however, are significant: a sunspot emits
enough radiation so that a single sunspot on its own in the
absence of the remainder of the Sun would glow a brilliant
orange-red and would be brighter that the full Moon. Analysis of
the *Zeeman effect in sunspots indicates that the magnetic field
in a typical sunspot is approximately 1000 times stronger than
the average magnetic field of the Sun, and one theory is that
this powerful localized magnetic field inhibits gas motion below
the photosphere, with the result that rising gas cannot deliver
its heat to the surface. Thus, the area cools and a sunspot is
the result. Infrared observations of sunspots have suggested that
the heat that does not emerge through the sunspot is deflected
and produces a slight increase in the temperature of the
photosphere around the sunspot, but so far these measurements
have not been precise and the slight increase has not been
confirmed. The other major theory of sunspots proposes that the
removal of energy from the sunspot location is the result of
enhanced hydromagnetic wave radiation associated with so-called
"*plage fields". Of the two theories, the first theory is
currently favored.
... ... M.P. Rast et al (6 authors at 2 installations, US) now
report high-photometric-precision observations of bright rings
around 8 sunspots. The authors report the rings are approximately
10 degrees kelvin warmer than the surrounding photosphere and
extend at least one sunspot radius out from the penumbra.
Approximately 10 percent of the radiative energy missing from the
sunspots is apparently emitted through these bright rings. The
authors conclude: "Thus, isolated sunspots are seen to be
commonly surrounded by a ring of enhanced radiation, the origin
of which is probably not bright vertical magnetic elements (plage
field), but the re-emergence of heat blocked by magnetic
inhibition of convective transport in the spot itself."
-----------
M.P. Rast et al: Bright rings around sunspots.
(Nature 14 Oct 99 401:678)
QY: M.P. Rast [mprast@ucar.edu]
-----------
Text Notes:
... ... *Zeeman effect: (Zeeman splitting) The splitting of a
spectral line due to a magnetic field. Named after Peter Zeeman
(1865-1943). The effect is widely used for the determination of
magnetic fields in astronomical objects, especially concerning
the Sun and sunspots. In general, the Zeeman effect occurs when
atoms emit or absorb radiation in the presence of a magnetic
field: the field modifies the energy configuration of the atom
with the result that a spectral line is split into 2, 3, or more
closely spaced components. The spacing of the components is a
measure of the magnetic field strength.
... ... *plage fields: A "plage" is a brighter, hotter patch in
the *chromosphere of the Sun, and a region of particularly strong
magnetic field.
... ... *chromosphere: The region of the Sun's atmosphere
directly above its photosphere. Visible only immediately before
or after a total solar eclipse.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON THE SOLAR CORONA
The surface of the Sun and the regions immediately exterior to it
constitute a domain in which various forces are played out on an
immense scale. The bright surface layer of the Sun is called the
"photosphere", a region a few hundred kilometers thick at a
temperature that ranges from 5770 degrees kelvin at its innermost
part to 4400 degrees kelvin at its outermost part, the Sun's
temperature minimum. Above this is the "chromosphere",
approximately 9000 kilometers thick, where the temperature ranges
from the minimum at the photosphere-chromosphere interface to
approximately 20,000 degrees kelvin. And above the chromosphere
is the "corona", the Sun's faint outer atmosphere, where the
temperature is 2 million degrees kelvin or more. The corona is a
low-density hot gas that glows with a pale white color, and one
of the major problems of solar astrophysics is to explain the
extreme temperature increase from the surface of the Sun to its
corona region. ... ... B. Haisch and J. Schmitt (2 installations,
US DE) present a review of recent research concerning the solar
corona, the authors making the following points:
     1) Solar physics has traditionally focused on short-term
changes in the atmosphere of the Sun. In contrast, stellar
studies have for decades concentrated on static properties in
order to understand the evolution of stars on long time scales.
Now a change in research perspective has occurred as space-based
instruments have made possible observations of stars previously
precluded by the Earth's atmosphere. Astronomers have now shifted
much of their attention from stellar interiors and surfaces to
activity occurring in the outer atmospheres of stars, and these
observations have contributed to our understanding of the outer
atmosphere of the Sun.
     2) The corona of the Sun has been observed throughout human
history during total eclipses, which make it possible to see
sunlight scattered by coronal electrons. By the 1930s,
spectroscopic observations of the visible-light corona had
precisely measured a number of prominent spectroscopic emission
lines, but the source of these lines was unknown.
     3) In the 1940s, it became clear that highly ionized iron
and calcium atoms were responsible for the puzzling spectral
emission lines. The highly ionized state of these elements
indicated that the tenuous outer region of the Sun was somehow
being heated to temperatures of more than 1 million degrees
kelvin. The problem then became to explain the temperature
gradient rising from approximately 5000 degrees kelvin at the
light-emitting surface -- the photosphere -- to the million-
degree temperature of the corona.
     4) The breakthrough occurred in 1968, when the first high-
resolution x-ray pictures of the Sun became available from an x-
ray telescope at an altitude of 250 kilometers aboard a rocket.
This telescope provided x-ray pictures with unprecedented
resolution, and the corona that appeared in these pictures
consisted of giant loops filled with x-ray emitting hot ionized
gas (plasma). These new images provided the first hint that the
outer atmosphere of the Sun is structured by magnetic fields.
Shortly afterward, time-lapse x-ray photographs from *Skylab
indicated these coronal structures constantly change. Both the
inhomogeneity of the solar corona and its variability forced
theoreticians away from the reigning hypothesis, in which sound
waves generated by turbulent gas flows below the surface of the
Sun propagated into the corona and heated it. Theory indicated
the collective action of these sound waves would be uniform and
unchanging, which was opposite to what was observed. Astronomers
then began to realize that shifting and changing magnetic fields
must be heating the corona. From recent studies of the
atmospheres and rotations of stars other than Sun, it is now
clear that the rotation of the Sun powers the dynamic magnetic
fields which are responsible for the extraordinary heating of the
corona region.
-----------
B. Haisch and J. Schmitt: The solar-stellar connection.
(Sky & Telescope October 1999)
QY: Jurgen Schmitt, University of Hamburg, DE.
-----------
Text Notes:
... ... *Skylab: A NASA space station launched in 1973. The
manned station contained 6 telescopes for observing the Sun's
chromosphere and corona at x-ray, ultraviolet, and visible
wavelengths. Three crews of 3 astronauts each spent a total of
171 days aboard the space station. Skylab was then abandoned, and
the empty station eventually reentered Earth's atmosphere in
1979, with fragments falling in Western Australia.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 17Sep99
-------------------
Related Background:
HELIOSEISMOLOGY: PROBING THE INTERIOR OF THE SUN
The Sun, a *main-sequence star 1.4 million kilometers in
diameter, is composed predominantly of hydrogen and helium
(approximately 70 percent hydrogen by mass, 28 percent helium by
mass, and 2 percent heavy elements by mass) and it generates its
energy via nuclear fusion processes, particularly via the
*proton-proton chain reaction. As a result, the Sun is losing
mass at a rate of approximately 4 million metric tons per second.
The generation of energy occurs in the "central core", which has
a temperature of approximately 15 million degrees kelvin, is
approximately 400,000 kilometers in diameter, and contains
approximately 60 percent of the mass of the Sun in 2 percent of
its volume. Outside the core is the "radiative zone", an envelope
of unevolved material through which energy from the core is
diffusively transported by successive absorption and emission of
radiation in collisions between atomic particles. It has been
estimated that it may take as long as 10 to 20 million years for
the energy generated in the core to reach the surface. The
radiative zone extends to within 200,000 kilometers of the
surface. In the surface layer (the "convective zone"), where the
temperature is only 1 million degrees kelvin, convection is the
most important mode of energy transport. The science of
helioseismology is the study of the solar interior using
observations of solar surface manifestations of resonant sound
waves (pressure modes; p-modes) traveling in the solar interior.
In other words, helioseismology is the study of the solar
interior structure by using the oscillations of its surface.
Since p-mode frequencies are *Doppler-shifted by motions in the
line of sight, they can also be used to study the internal
dynamics of the Sun, such as internal rotation and convection.
... ... P. Demarque and D.B. Guenther (2 installations, US CA)
present a review of current research in helioseismology, the
authors making the following points:
     1) In 1962, Leighton et al discovered patches of the surface
of the Sun moving up and down with a velocity of the order of 15
centimeters per second, with periods of approximately 5 minutes.
Called the "5-minute oscillation", the motions were originally
believed to be local in character and somehow related to
turbulent convection in the solar atmosphere. In 1970, Ulrich
suggested that the phenomenon is global and that the observed
oscillations are the manifestation at the solar surface of
resonant sound waves (pressure modes or "p-modes") traveling in
the solar interior.
     2) Stellar oscillation theory, the main theoretical
framework for helioseismology, also predicts the existence of
buoyancy driven modes (gravity modes, or "g-modes") that have
been observed in other astrophysical contexts, but it is not
clear at present whether g-modes are excited in the Sun. G-modes
are expected to be exponentially damped in convective regions, so
their amplitudes at the top of the solar convective envelope are
expected to be much smaller than in the radiative core. In
contrast to p-modes, which have maximum amplitudes in the outer
parts of the Sun, g-modes exhibit their largest amplitudes in the
solar core. If observable, g-modes would be sensitive probes of
the solar core, where p-modes are least sensitive.
     3) There are many facets of helioseismology, and the field
has contributed to the study of stellar evolution and to
astrophysics and physics in general. The interpretation of a
wealth of ground-based data, most recently provided by the Global
Oscillation Network Group (GONG) project, a network of observing
stations distributed around the globe to observe the Sun
continuously, and the Solar and Heliospheric Observatory (SOHO)
space mission, have led to many advances. The most important
accomplishments of helioseismology include the following:
... ... a) The testing of the physical assumptions of stellar
evolution theory.
... ... b) The determination of the depth of the solar convection
zone.
... ... c) The reconstruction of the internal rotation profile in
the outer half of the solar radius.
... ... d) The detailed probing of the *superadiabatic transition
layer near the solar surface.
... ... e) The realization of the important role played by the
diffusion of helium in the interior of the Sun and the seismic
determination of the helium abundance in the convection zone.
... ... f) The determination of the age of the Sun by seismic
means.
... ... g) The setting of a strong constraint on *varying-
gravitational-constant cosmologies.
... ... h) The demonstration that the *solar neutrino discrepancy
is likely to reveal fundamental new knowledge about neutrinos and
their interaction with matter.
-----------
P. Demarque and D.B. Guenther: Helioseismology: Probing the
interior of a star.
(Proc. Natl. Acad. Sci. US 11 May 99 96:5356)
QY: P. Demarque, Yale University, 203-432-4771.
-----------
Text Notes:
... ... *main-sequence star: The Hertzsprung-Russell diagram is
a plot of stellar absolute magnitude against spectral type, and
is one of the most useful diagrammatic aids in astrophysics. The
Main Sequence is a region on the Hertzsprung-Russell diagram
where most stars, including our own Sun, are situated. The course
of a star's evolution can be traced as a particular path in the
H-R diagram, with the paths of various types of stars showing
significant differences.
... ... *proton-proton chain reaction: A chain of nuclear
reactions inside a star that converts hydrogen to helium, with
the associated release of energy. In the reaction, 4 hydrogen
nuclei (protons) fuse to form one nucleus of helium, with the
production of a number of intermediate nuclei such as deuterium
and isotopes of lithium, beryllium, and boron. The proton-proton
reaction is the most important stellar reaction at temperatures
below 18 million degrees kelvin, and thus operates chiefly in
stars of less than 2 solar masses.
... ... *Doppler-shifted: In general, the term "Doppler shift"
refers to the change in wavelength of electromagnetic radiation
as a result of relative movement between the source and the
observer.
... ... *superadiabatic transition layer: An adiabatic process is
any thermodynamic process, reversible or irreversible, that takes
place in a system without exchange of heat with the surroundings
of the system. All real processes are nonadiabatic in the sense
that some heat exchange always occurs. But close approximation to
an adiabatic ideal can be realized in practice. In the context of
this report, the "superadiabatic transition layer" is the
transition between deep convection, where the temperature
gradient is nearly adiabatic, and the shallow outer layers of the
Sun, where radiative losses dominate.
... ... *varying-gravitational-constant cosmologies: In general,
this term refers to cosmological theories dependent on a time-
varying universal gravitational constant. Recent
helioseismological data have provided a strong limit on the
variation of the universal gravitational constant during the
lifetime of the Sun, and this limit is stronger by almost one
order of magnitude than previous constraints.
... ... *solar neutrino discrepancy: Neutrinos are fundamental
particles with zero charge, possibly zero mass, and an angular
momentum factor (spin) of 1/2. Various processes produce
neutrinos: stellar nuclear reactions, reactions occurring during
supernova explosions, cosmic ray collisions with matter, etc.
Measurements of solar neutrinos have produced a mystery: the
neutrino density measured by detectors is approximately one-third
that expected from theoretical calculations of solar neutrino
emission. Two kinds of solutions have been proposed to resolve
this mystery, one solution involving revisions to the theory of
stellar structure, and the other solution involving revisions to
nuclear particle theory.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 30Jul99
-------------------
Related Background:
ANALYSIS OF SUB-SURFACE MATERIAL FLOW IN THE SUN
... Helioseismology is the analysis of wave motions of the Sun's
surface to determine the structure of the Sun's interior, and
helioseismic tomography is simply such an analysis in terms of
sections of the Sun's sphere. In general, the idea is that
systematic and quantitative analysis of surface wave phenomena
can reveal the sub-surface conditions that produce these
phenomena. Giles et al (4 authors at 3 installations, US) report
the detection, using helioseismic tomography, of the
longitude-averaged sub-surface flow in the outer 4 percent of the
Sun. The sub-surface flow is apparently constant in this depth
range, and its speed is similar to that seen on the surface. (In
this context, "speed" is the scalar quantity -- distance per unit
time.) The authors suggest the observations demonstrate that
surface flow penetrates deeply and is likely to be an important
factor in solar dynamics.
QY: P. M. Giles [pgiles@solar.stanford.edu]
(Nature 6 Nov 97) (Science-Week 28 Nov 97)
[For more information: http://scienceweek.com/search/search.htm]


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IN FOCUS: ON THE BIOPHYSICS OF WARMTH AND COLD DETECTION
"The sensations of warmth and cold are known to follow from the
excitation of separate warm and cold cutaneous thermoreceptors as
measured by the firing rates of the afferent nerves that form the
receptors. For each modality, the firing rates depend statically
upon temperature, T, and dynamically on the rate of temperature
change, dT/dt. with positive coefficients for warm receptors and
negative coefficients for cold receptors. Hence, in the
homeotherm skin, the firing rate of the afferent nerves that
serve the warm-receptors increases with increasing temperature,
while the firing rate of the nerves that serve the cold sensors
is reduced. The systems adapt to long-term temperature changes
and an increase in external calcium concentration results in a
feeling of warmth... The density of receptors in humans varies
according to the location, and there are usually more cold
sensors than warm sensors. Where there are, typically, from one
to five cold points per square centimeter on the skin of the
hand, there seems to be less than one warm point per square
centimeter. Elsewhere, such as on the face, the densities are
larger, of the order of 10 cold spots per square centimeter and,
perhaps, 2 warm spots per square centimeter. These warm and cold
spots are generally considered to be associated with single
receptors. The body of data shows that the detailed character of
the warm-cold response varies from species to species and varies
over the body area of specific species... Humans show
sensitivities to temperature changes in the skin of approximately
0.02 degrees centigrade near the neutral point of approximately
38 degrees centigrade. There is both temporal summation and area
integration; to a good approximation, a warm-cold response is
proportional to the time integral of the temperature signal up to
a period of 3 seconds and to a spatial summation up to areas
which vary over the body but are of the magnitude of 25 square
centimeters on the forearm or back."
-----------
Robert K. Adair: "A model of the detection of warmth and cold by
cutaneous sensors through effects on voltage-gated membrane
channels."
(Proc. Natl. Acad. Sci. US 12 Oct 99 96:11825)
-----------
[The author is in the Department of Physics, Yale University, US]


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