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 29, 2000 -- Vol. 4 Number 52

The Editors extend their best wishes to all for
the Holiday Season.

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When a distinguished but elderly scientist states
that something is possible, he is almost certainly
right. When he states that something is impossible,
he is very probably wrong.
-- Arther C. Clarke 

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Section 1
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Contents of this Issue (Full reports in Section 2):

1. SCIENCE IN HISTORY: SOFT TISSUE PALEOPATHOLOGY
The branch of science called "paleopathology" has two aspects:
Research in this area seeks a) to arrive at a broader
understanding of human disease by correlating the pathological
findings of mummified human tissue with known historical and
cultural trends; and b) to provide insights for historical
research, particularly in connection with the history of specific
populations. It is known that in addition to the ancient
Egyptians, many historical populations practiced mummification,
including the people living along the Torres Strait between Papua
New Guinea and Australia, and including the Incas of South
America. Mummification does not preserve all soft tissues, and
paleopathological analysis of mummies has its constraints, but
the science has provided information unobtainable by other means.
(J. Amer. Med. Assoc. 22/29 Nov 00 284:2571)

2. NEUROBIOLOGY:
ON REGENERATING THE DAMAGED CENTRAL NERVOUS SYSTEM
For most of the 20th century it was believed that in the
mammalian central nervous system, including in humans, the nerve
fibers of the brain and spinal cord were incapable of
regeneration sufficient to restore function. It is now understood
that the regenerative capacity of the central nervous system is
not intrinsic to central nervous system nerve cells, but depends
on the circumstances of damage and the immediate environment of
the nerve cells. Regeneration can occur in the damaged central
nervous system, and this new understanding has caused
considerable excitement in the neurobiological and medical
communities. (Nature 26 Oct 00 407:963)

3. CELL BIOLOGY: ON CELL BIOLOGY AND SOCIETY
The rapid advances in cell biology of the past several decades
have been exhilarating and the promise for the future is
extraordinary. Cell biology has grown to encompass many other
disciplines and has benefitted from the synergy of different
approaches. Cell biologists are just beginning to see impact from
the fields of genomics and computational biology, and they can
look forward to increasing impact on medicine. Cell biology is
poised to make even more rapid progress, and it is incumbent on
cell biologists to communicate and share the excitement and
promise with society at large. (The Scientist 11 Dec 2000)

4. COSMOLOGY: ON QUINTESSENCE
Perhaps the central question in cosmology concerns the future:
Will the Universe continue to expand indefinitely, or will the
expansion eventually slow and stop and be replaced by a
contraction? At present, attempts to answer this fundamental
question involve two considerations: a) the total mass of the
Universe, which will determine whether gravity can slow the
expansion and produce a contraction; and b) the possible presence
of special energy fields that may also influence the rate of
expansion. In 1997, R.R. Caldwell, R. Dave, and P.J. Steinhardt
introduced the term "quintessence" to refer to a dynamical
quantum field, not unlike an electrical or magnetic field, that
gravitationally repels. (Scientific American January 2001)

5. CONDENSED MATTER PHYSICS: A NEW APPLICATION OF CLASSIC THEORY
TO HIGH-TEMPERATURE FERROMAGNETISM
A new theoretical model, which introduces small-system
thermodynamics into a mean-field formalism, provides theoretical
results in close agreement with data from several different
ferromagnets. The new model also produces realistic results on
the temperature dependence of the local magnetic order above the
Curie temperature, thereby creating a robust formalism that
appears to solve a very old problem. The success of the model
demonstrates that nanothermodynamics -- of clear importance for
dealing with small systems in many areas of nanoscience -- is
also crucial for describing internal fluctuations in bulk
materials.  (Nature 16 Nov 00 408:299)

6. MATERIALS SCIENCE: A MAGNETIC AND ELECTRICAL CONDUCTOR HYBRID
Whereas conducting magnets, such as nickel or iron, are common
among metals and alloys, a new study is the first reported
example involving molecular materials. Because the electronic
peculiarities of individual molecules are different from those of
bulk metals, a conducting molecular magnet is likely to have
unexpected properties. The new work involves the synthesis of
single crystals formed by infinite sheets of a magnetic
coordination polymer (a bimetallic oxalato complex) interleaved
with layers of an organic conducting cation, and demonstrates
that this molecule-based compound displays both ferromagnetism
and metallic conductivity. (Nature 23 Nov 00 408:421)

7. IN FOCUS: ON THE BOUNDARIES OF ANIMAL LIFE

8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE QUESTION OF THE DANGERS OF SCIENCE

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Section 2
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1. SCIENCE IN HISTORY: SOFT TISSUE PALEOPATHOLOGY
     The branch of science called "paleopathology" has two
aspects: Research in this area seeks a) to arrive at a broader
understanding of human disease by correlating the pathological
findings of mummified human tissue with known historical and
cultural trends; and b) to provide insights for historical
research, particularly in connection with the history of specific
populations. It is known that in addition to the ancient
Egyptians, many historical populations practiced mummification,
including the people living along the Torres Strait between Papua
New Guinea and Australia, and including the Incas of South
America. Mummification does not preserve all soft tissues, and
paleopathological analysis of mummies has its constraints, but
the science has provided information unobtainable by other means.
... ... Arthur C. Aufderheide (University of Minnesota Duluth,
US) presents a review of current progress in soft tissue
paleopathology, the author making the following points:
     1) The term "mummification" refers to the preservation of
soft tissue to resist the usual enzyme-mediated process of
postmortem decay. Such preservation can be brought about by
environmental effects without human intervention (spontaneous or
natural mummification) or as a deliberate human effort
(anthropogenic or artificial mummification). Spontaneous
mummification occurs most frequently by rapid desiccation of the
body in hyperarid deserts or equivalent microclimates.
Anthropogenic mummification is commonly effected by reducing the
bacterial burden of the corpse by evisceration and the promotion
of tissue dehydration via exposure to heat. In ancient Egypt, the
body was desiccated by packing it in a mixture of carbonate,
bicarbonate, and chloride salts of sodium ( the mixture called
"natron"); then the body was painted with a waterproofing agent
such as tree resin to prevent rehydration.
     2) It is known that postmortem *pleural adhesions reflect an
episode of pneumonia from which the individual recovered, while
the lung of a fatal case of pneumonia is easily identifiable by
its increased thickness and abundance of electrolyte crystals and
powdery protein deposit from the dried pleural effusion covering
it. Tabulation of such observations permits calculation of the
case fatality rate of pneumonia in ancient populations. For
example, infants of an early hunting population of the Chilean
coast (Atacama Desert) were found to have a pneumonia case
fatality rate similar to that of the US before antibiotics were
available, while children of their sedentary agricultural
successors had a much higher rate, probably due to increased
smoke exposure and more crowded living conditions.
     2) The author points out that *radioimmunoassay techniques
have identified cocaine in mummy hair in archeological
populations from northern Chile that are as much as 5000 years
old. The results have defined the antiquity, geography, and
demography of the ancient Andean practice of chewing cocaine-
containing leaves of the coca plant and have demonstrated the
transplacental and mammary transmission of cocaine in fetuses and
nurslings.
     3) Concerning DNA analysis, in a sample of a *hilar lymph
node in a 1000-year-old Peruvian mummy, W.L. Salo et al (1994)
identified a segment of DNA unique to the tuberculosis-causing
Mycobacterium group of pathogens, thus proving the presence of
tuberculosis in the New World 500 years before the arrival of
Columbus. Although current paleopathologic methods of DNA
analysis still have certain technical problems, the methods
promise enormous expansions of soft-tissue paleopathology
research.
     4) Energy-dispersive analysis is a technique in which x-rays
are focused on crystals, the reflected wavelengths providing a
signature unique to each element. Combined with morphological
examination, this technique has led to the identification of
*silicate pneumoconiosis as an occupational disease of ancient
farmers.
-----------
Arthur C. Aufderheide: Progress in soft-tissue paleopathology.
(J. Amer. Med. Assoc. 22/29 Nov 00 284:2571)
QY: Arthur C. Aufderheide: aaufderh@d.umn.edu
-----------
Text Notes:
... ... *pleural: The term "pleura" refers to the membranes
enveloping the lung and lining the internal surface of the
thoracic cavity.
... ... *radioimmunoassay: In general, any method for detecting
or quantitating antigens or antibodies using radiolabeled
reactants. Minute quantities of enzymes, hormones, or other
substances can be assayed. In general, an antigen is any
substance or moiety that produces an immune response, and an
antibody is a protein molecule produced by the immune system of
vertebrate organisms, the molecule designed to specifically
interact with a particular antigen.
... ... *hilar lymph node: The lymphatic system is a complex
network for the distribution of lymph fluid (which is similar to
blood plasma -- blood without red cells). Lymph is collected by
drainage from the tissues throughout the body, flows in the
lymphatic vessels through the lymph nodes, and is eventually
added to the venous blood circulation. Lymph consists of a clear
liquid portion, varying numbers of white blood cells (chiefly
lymphocytes), and a few red blood cells. The lymph nodes are
small bodies located throughout the lymph system and varying in
diameter from 0.1 to 2.5 centimeters. In general, a "hilum" is a
part of an organ where nerves and blood vessels enter and leave.
... ... *silicate pneumoconiosis: The term "pneumoconioses"
refers to a group of lung diseases caused by inhalation of
inorganic dusts. Coal-workers' pneumoconiosis (Black Lung
disease), silicosis, and asbestosis are the best known varieties
of pneumoconiosis.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm

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2. NEUROBIOLOGY:
ON REGENERATING THE DAMAGED CENTRAL NERVOUS SYSTEM
     The ability to regenerate at least certain parts of the
organism is found in all living systems, including plants and
animals, unicellular and multicellular. With higher organisms,
however, for example with mammals, the process of regeneration
involves many constraints. Of great concern in clinical medicine
are injuries to the nervous system, injuries which are often
permanently debilitating because of poor or absent regeneration
of neural tissue. Important advances have recently been made in
our understanding of nervous system injury and regeneration, and
there are now indications that significant breakthroughs will
occur in the near future.
     What happens when a nerve cell is injured? Consider the case
when the *axon of the nerve cell is severed. When a *peripheral
nerve fiber is cut, certain events follow in different parts of
the neuron. The distal segment of the nerve fiber, the part on
the far end of the cut, undergoes degeneration, which begins
slowly, requiring days to be completed, and involves the separate
parts of the nerve fiber differently. The axon gradually breaks
up and the segments are digested and absorbed. If there is a
*myelin sheath, it is gradually transformed into a chain of lipid
droplets, the larger of which may in the early stages contain
degenerating fragments of the axon. The fragments of the axon
disappear in a few days; parts of the degenerating myelin sheath,
in the form of droplets, may persist for six months or more. When
a nerve fiber is cut, the parts of the neuron from the break
toward the cell body (the proximal parts) also show
characteristic changes. The cell body undergoes evident changes
in *endoplasmic reticulum and *ribosomes (chromatic changes in
Nissl substance). This changes reaches its peak in 7 to 15 days,
after which there may be recovery, or complete degeneration if
there is too much damage. If the cell body completely
degenerates, the nerve fiber between the cell body and the cut
undergoes degeneration (Wallerian degeneration) just as the
distal segment does. But if the cell body survives, only a small
amount of destruction of the proximal segment occurs, and that
near the cut. Since this is a peripheral nerve, what happen then
is that from each axon near its cut end a number of small sprouts
grow out in all directions. Some of the sprouts grow in the
direction of the former distal axon segment and grow into the
connective tissue matrix that has formed scar tissue. The
haphazard arrangement of connective tissue fibers influences the
amoeboid growing tips of the nerve sprouts. Not all of the fibers
get across the scar, but a few do, and even fewer manage to
regain the original neural pathway.
     The above is a description of a mammalian peripheral nerve
degeneration and regeneration, the process first described at the
beginning of the 20th century. For most of the 20th century,
there was a clear dogma in neurobiology: It was believed that in
the mammalian central nervous system, including in humans, the
nerve fibers of the brain and spinal cord were incapable of
regeneration sufficient to restore function. A most important
corollary of this dogma was that this incapability of sufficient
regeneration (or any regeneration at all) was intrinsic to
central nervous system nerve cells. In 1980, that corollary dogma
was overturned, and it is now understood that the regenerative
capacity of the central nervous system is not intrinsic to
central nervous system nerve cells, but depends on the
circumstances of damage and the immediate environment of the
nerve cells. Regeneration can occur in the damaged central
nervous system, and this new understanding has caused
considerable excitement in the neurobiological and medical
communities.
... ... P.J. Horner and F.H. Gage (Salk Institute, US) present an
extensive review of regeneration in the damaged central nervous
system, the authors making the following points:
     1) The authors point out that in contrast to fish, amphibia,
and the mammalian peripheral nerves and developing central
nerves, adult central mammalian neurons do not regrow functional
axons after damage. This inability of adult central nervous
system neurons to regrow after injury cannot be entirely
attributed to intrinsic differences between adult central nervous
system neurons and all other neurons, since it has been known
since the early years of the 20th century that adult central
nervous system neurons could regrow in a permissive environment.
In 1980, P.M Richardson et al replicated the early studies with
new methods that definitely confirmed that adult central nervous
system neurons have regenerative capabilities. This finding
revealed that the failure of central nervous system neurons to
regenerate was not an intrinsic deficit of the neuron, but rather
a characteristic feature of the damaged environment that either
did not support or prevented regeneration. In the past 20 years,
progress has been made in identifying the elements that are
responsible for the differences between the adult central nervous
system and peripheral nervous system environments, and in the
past few years the molecular and cellular bases of regenerative
compared with non-regenerative responses are beginning to be
revealed.
     2) The authors suggest that regeneration strategies
developed from these new discoveries will be applicable to many
central nervous system disorders. Spinal cord injury could be the
most approachable, owing to the well-defined loss of cells and
axons and the relative lack of consequent chronic pathology.
Genetic disorders that result in aberrant axonal pathfinding or
neuronal cell loss may also be amenable to regeneration.
Degenerative diseases where a defined cell type is lost (e.g.,
Parkinson's disease, Alzheimer's disease, amyotrophic lateral
sclerosis) are also good targets, but may be more challenging
because of the potential for continued cell loss or axonal
degeneration. Finally, regeneration strategies may also be
applied to less well-defined disorders where diffuse cell and
axonal loss can occur, such as cerebrovascular disease, tumor,
and infection of the central nervous system.
     3) Concerning recent work, an increasing number of studies
have demonstrated that an adult cut axon in the central nervous
system can be induced to regrow by either increasing the
permissive cues or decreasing the non-permissive cues of the
existing environment. Furthermore, a growing list of reports
indicate that one strategy or another can induce some level of
functional recovery following damage. The authors (Horner and
Gage), however, point out that it is not sufficient to
demonstrate axon elongation and behavioral improvement after
injury to conclude that authentic functional regeneration is
responsible for the outcome. There are many mechanisms that may
account for observed functional recovery that do not require
regeneration, and these non-regenerative mechanisms are common in
most experimental models of traumatic injury and need to be
excluded before invoking functional regeneration as the cause of
repair and recovery. The reason for sorting out the authentic
mechanisms of functional recovery is that without understanding
the underlying basis of regeneration, little progress can be made
beyond the phenomenological observation of recovery from injury.
     4) The authors conclude: "Despite the progress in the last
century of research on regeneration... *Cajal's [1928] flowery
decree, as translated by Raoul May, still resonates: 'Once the
development was ended, the founts of growth and regeneration of
the axons and *dendrites dried up irrevocably. In the adult
centers the nerve paths are something fixed, ended, and
immutable. Everything may die, nothing may be regenerated. It is
for the science of the future to change, if possible, this harsh
decree.' The decree is lifted; the solution remains elusive."
-----------
P.J. Horner and F.H. Gage: Regenerating the damaged central
nervous system.
(Nature 26 Oct 00 407:963)
QY: Fred H. Gage: gage@salk.edu
-----------
Text Notes:
... ... *axon: In general, nerve cells have a single long
extension (the "axon") that propagates the electrical output (the
action potential) of the cell. In some types of nerve cells,
axons are extensively branched into a multitude of fine fibers
that make contact (synapses) with other nerve cells.
... ... *peripheral nerve fiber: In mammals, neural tissue
comprising the brain and spinal cord is called the "central
nervous system", while neural tissue outside the brain and spinal
cord is called the "peripheral nervous system". The dichotomy is
more than formal, since anatomical, functional, and in this
context regeneration differences are significant.
... ... *myelin sheath: High signal propagation velocities in
motor and sensory neurons in vertebrates are achieved by
association of the nerve fiber with an enfolding "myelin sheath".
The myelin sheath consists of concentric layers of electrically
insulating lipid material (myelin), but the sheath is
periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. 
... ... *endoplasmic reticulum: The term "endoplasmic reticulum"
refers to a complex system of intracellular flattened sacs, and
it is the site of many important syntheses, including the
production of new surface membrane and the intracellular
transport of various biochemical entities.
... ... *ribosomes: A ribosome (not to be confused with riboZYME)
is a small particle, a complex of various ribonucleic acid
component subunits and proteins that functions as the site of
protein synthesis.
... ... *Cajal: Santiago Ramon y Cajal (1852-1934), one of the
founders of microscopic neuroanatomy, was awarded the 1906 Nobel
Prize in Physiology or Medicine for establishing the neuron as
the fundamental unit of the nervous system.
... ... *dendrites: The general input extensions of nerve cells
are called "dendrites", and they may be extensively branched. In
general, dendrites are considered to receive input and axons to
propagate output, but the electrical architecture of most neurons
is complicated, and in many types of nerve cells activation of
the axon produces electrical activity that not only propagates
down the axon but also propagates backward through the cell body
and dendrites.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
MEDICAL BIOLOGY:
PROSPECTS FOR NEURAL STEM CELL REPAIR OF INJURED SPINAL CORD
What has happened in vertebrate evolution is that the brain has
evolved from a mere head cluster of nerve cells (a head ganglion)
of the spinal array of ganglia (the spinal cord) to a burgeoned
structure that dominates the spinal cord almost completely.
In terms of both function and anatomy, the human spinal cord can
thus be viewed as a "service" extension of the commanding brain,
the two together constituting the "central nervous system", and
like in the brain, traumatic injury to the spinal cord is usually
irreversible: brain and spinal nerve cells and nerve fibers
usually do not regenerate when damaged. Since many nerve cells
and nerve fibers in the spinal cord are essential to the control
of various voluntary and involuntary muscles of the body below
the head, traumatic injury to the spinal cord can be devastating
in its consequences. An acceleration of research into possible
mechanisms of neuronal regeneration has occurred during the past
several decades, and there is now some hope for applications of
this research to the treatment and repair of spinal cord
injuries.
... ... S.S.W. Han and I. Fischer (Hahnemann University School of
Medicine, US) present a review of current research in this field,
the authors making the following points:
      1) Recent observations that several regions of the
mammalian central nervous system do continue to produce neurons
throughout life suggests there are prospects for repairing an
injured spinal cord. Researchers have developed efficient
methods for culturing the neural *stem cells of rodents,
genetically modifying these cells to produce therapeutic genes,
and then transplanting these cells into animal models of brain
diseases. These same gene therapy and grafting techniques are
being explored as possible methods for restoring function
following traumatic spinal cord injury.
      2) In the developing embryo, *epithelial cells of the
*neural tube generate a variety of precursor cells that migrate
and *differentiate into neurons, *astrocytes, and
*oligodendrocytes. Central nervous system stem cells have now
been discovered in the human central nervous system and appear
to behave similarly to their rodent counterparts, and these stem
cells could potentially be used to promote the generation of new
nerve cells (neurogenesis) following injury and disease.
      3) Transplantation studies have demonstrated that neural
stem cells have the capacity to differentiate in response to the
environment into which they are reintroduced and to integrate
appropriately with the host tissue. Neural stem cells can be
isolated from different areas and propagated for long periods in
culture without losing their ability for varied differentiations
(their "multipotentiality"). When transplanted back into the
central nervous system, these stem cells have the capacity to
migrate, to integrate with the host tissue, and to respond to
local cues for differentiation.
      4) The authors conclude: "Transplantation of neural stem
cells and precursor cells together with gene therapy offers
great promise for spinal cord repair. Specific research goals
include improving neuronal survival, promoting functional
recovery through *axonal regeneration, compensating for
*demyelination, and replacing lost cells. Many issues will need
to be resolved before stem cells can be considered for use in
human subjects, but continued basic research on the properties
of these cells and development of appropriate animal models of
repair will pave the way for successful clinical applications."
-----------
S.S.W. Han and I. Fischer: Neural stem cells and gene therapy:
Prospects for repairing the injured spinal cord.
(J. Amer. Med. Assoc. 3 May 20 283:2300)
QY: S.S.W. Han, MCP Hahnemann University School of
Medicine, Philadelphia, PA US.
-----------
Text Notes:
... ... *stem cells: In general, a stem cell is any precursor
cell, a form prior to cell differentiation. E.g., stem cells in
bone marrow that give rise to blood cells.
... ... *epithelial cells: 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.
... ... *neural tube: The term "neural tube" refers to the early
embryonic structure (an actual hollow tube of cells formed by the
infolding and closing of a long sheet of cells) that subsequently
gives rise to the entire brain and spinal cord.
... ... *differentiate: In this context, the term
"differentiation" refers to developmental cell specialization
(morphology and biochemistry) resulting from activation of
specific parts of the cell genome. E.g., the differentiation of a
stem cell into a nerve cell.
... ... *astrocytes: (astroglial cell) Neuroglia are non-neuronal
cellular elements of the central and peripheral nervous systems,
and astroglia (astrocytes) are a type of neuroglia. In general,
neuroglia are thought to have important metabolic functions.
... ... *oligodendrocytes: (oligodendroglia) Glial cells
characterized by sheet-like processes that are wrapped around
individual neuron axons to form the myelin sheath of nerve
fibers in the central nervous system. (The myelin sheath of a
nerve fiber is effectively a periodically interrupted insulation
which increases the propagation velocity of nerve impulses. See
note on "demyelination" below.)
... ... *axonal regeneration: In general, nerve cells have a
single long extension (the "axon") that propagates the electrical
output (the action potential) of the cell. In some types of nerve
cells, axons are extensively branched into a multitude of fine
fibers that make contact (synapses) with other nerve cells.
... ... *demyelination: (demyelinization) A number of
neurodegenerative diseases involve progressive demyelination of
various myelinated nerve fibers. High signal propagation
velocities in motor and sensory neurons in vertebrates are
achieved by association of the nerve fiber with an enfolding
sheath called myelin. The myelin sheath consists of concentric
layers of electrically insulating lipid material, but the sheath
is periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. 
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 2Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEUROBIOLOGY: FUNCTIONAL REGENERATION OF SENSORY AXONS IN ADULT
SPINAL CORD
      In vertebrates, the spinal cord is continuous with the
brain, and the two together constitute what is called the
"central nervous system". In addition to other functional
involvements, the spinal cord, and the nerves extending from and
leading into the spinal cord ("spinal nerves"), comprise neuronal
circuits that among other things mediate a number of fast
responses to environmental changes. For example, if you
inadvertently pick up a hot object, the grasping muscles in your
hand may relax and the object drop even before the sensation of
extreme heat or pain reaches your brain and your conscious
perception. This is an example of a "spinal cord reflex", a fast
automatic response to certain types of stimuli, the response
requiring only nerve fibers and nerve cells in the spinal nerves
and spinal cord. In addition to processing such reflexes, the
spinal cord also is the site for integration of nerve impulses
that originate locally in the spinal cord or that arrive from the
periphery and brain. Of great importance is that the spinal cord
is the "highway" traveled by sensory nerve impulses carrying
sensory information to the brain, and by motor nerve impulses
originating in the brain and destined for voluntary muscles via
the spinal nerves. In humans, there are 31 pairs of spinal nerves
arranged with bilateral symmetry to serve the two sides of the
body.
      Sensory input to the spinal cord (and to nerve cells in the
spinal cord) occurs via sensory neurons with a special
morphology. Ordinary neurons have a cell body with short (often
arborized) extensions (dendrites) to receive input, and a long
extension (axon) to propagate output away from the cell body to
either another neuron or to a muscle cell. But most sensory
neurons conveying input to the spinal cord are quite different:
such neurons have a long input extension, as much as 1 meter long
in humans, that propagates nerve impulses at high speed _toward_
the cell body, and a short or long (depending on the specific
type of sensory nerve cell) output extension into the spinal cord
from the sensory neuron cell body located just outside the spinal
cord.
      Spinal nerves are "mixed nerves", containing both input
(afferent) nerve fibers and output (efferent) nerve fibers. In
humans and other higher vertebrates, the anatomy is such that
near the spinal cord, just before joining it, each spinal nerve
bifurcates into a "dorsal root" and a "ventral root" (in humans,
posterior root and anterior root, respectively). The ventral root
contains output nerve fibers to "effector cells" (in muscles,
glands, etc.), while the dorsal root contains input nerve fibers
propagating peripheral sensory information to the central nervous
system. Each dorsal root, as seen in gross morphology, has a
bulge which contains the numerous cell bodies of the sensory
nerve fibers, and each of these bulges is called a "dorsal root
ganglion".
      When the human spinal cord is injured by physical trauma
(as in an automobile accident), one of common consequences is a
traction-caused ripping of the spinal nerves (spinal nerve roots)
out of the spinal cord at a particular location in the spinal
cord axis ("spinal root avulsion"). Root avulsion usually
produces complete paralysis of those regions of the body
controlled by those particular spinal nerves, with loss of local
motor control and loss of local sensation. Natural repair of
severed connections between the spinal cord and spinal nerves
does not occur in humans, but in the past decade there has been
much progress in understanding the mechanisms of nerve fiber
regeneration, and there is now some hope of defining
interventions that may possibly provoke regeneration in cases of
human spinal nerve avulsion.
... ... M.S. Ramer et al (3 authors at 2 installations, UK) now
report evidence of functional regeneration of sensory axons in
adult mammalian spinal cord. The authors point out that the
arrest of dorsal root axonal regeneration at the transition zone
between the peripheral and central nervous system (e.g., between
the spinal cord and the spinal nerves) has been repeatedly
described since the early 20th century. The authors report their
work indicates that with *neurotrophic support to damaged sensory
neuron axons, this regenerative barrier is surmountable. In adult
rats with experimentally injured dorsal roots, *intrathecal
treatment with *nerve growth factor, *neurotrophin-3, and
*glial-cell-line-derived neurotrophic factor, resulted in
selective regrowth of damaged axons across the dorsal root entry
zone and into the spinal cord, where neurons that ordinarily
receive sensory input (dorsal horn neurons) were found to be
synaptically driven by peripheral nerve stimulation in treated
animals, demonstrating functional reconnection. In behavioral
studies, rats treated with nerve growth factor and glial-
cell-line derived neurotrophic factor recovered sensitivity to
noxious heat and pressure. The authors report that the observed
effects of neurotrophic factors corresponded to their known
actions on distinct subpopulations of sensory neurons. The
authors suggest that neurotrophic factor intervention may serve
as a viable treatment in promoting recovery from root avulsion
injuries. The authors further suggest that apart from dorsal root
injuries, once the nature of traumatic injuries in general in the
human central nervous are better understood, neurotrophic
treatment may have vast therapeutic potential for such tissue
damage.
-----------
M.S. Ramer et al: Functional regeneration of sensory axons into
the adult spinal cord.
(Nature 20 Jan 00 403:312)
QY: Matt S. Ramer [matt.ramer@kcl.ac.uk]
-----------
Text Notes:
... ... *neurotrophic treatment: (treatment with neurotrophins)
In general, neurotrophins are chemical entities apparently
essential for the viability of nerve cells. These substances are
polypeptides of 200 to 300 amino acids, and a number of different
neurotrophins have been identified.
... ... *intrathecal treatment: In general, treatment involving
injection into a local area surrounding the spinal cord:
injection beneath one or more of the protective sheaths that
cover the spinal cord.
... ... *nerve growth factor: A type of neurotrophin. The various
neurotrophins can be differentiated on the basis of tissue
specificities. Nerve growth factor has apparent specificity for
dorsal root ganglion cells.
... ...*neurotrophin-3: A specific type of neurotrophin: 257
amino acids, molecular weight 29.32 kilodaltons.
... ... *glial-cell-line-derived neurotrophic factor: Glial cells
are cells of the central and peripheral nervous system that
metabolically support neurons. Such cells also produce the
multiple membrane layers called myelin and enfold nerve cell
axons with it. The glial cells are found everywhere in the brain
and spinal cord, and one result of a localized injury to the
central nervous system is a local proliferation of glial cells
to form a scar matrix. In this context, the term
"glial-cell-line" refers to a line of laboratory cultured glial
cells.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
REGENERATION OF A GERMINAL LAYER IN THE ADULT MAMMALIAN BRAIN
Until recently, one of the dogmas of neurobiology was that the
adult mammalian brain is incapable of regeneration: after injury,
neurons in the central nervous system do not spontaneously
reestablish their connections. During the past decade, however,
progress has been made in identifying various factors and types
of cells that can promote a degree of *axonal regeneration, and
an unanticipated form of *neuroplasticity in the adult mammalian
brain has been demonstrated  -- the continued production of new
neurons in certain brain regions. For example, proliferating
cells apparently persist throughout adult life along the length
of the lateral wall of the internal brain fluid space known as
the "lateral *ventricles". This germinal region, called the
"subventricular zone", generates new neurons destined for the
part of the brain receiving olfactory sensory information
(olfactory bulb). The olfactory bulb is a major mammalian brain
structure, considerably reduced in size in man, but still of
physiological importance [*Note #1]. The subventricular zone is
organized as an extensive network of chains of migrating cells
destined to become neurons (neuroblasts). The newly generated
neuroblasts migrate through the subventricular zone to join a
migrating stream of precursor neurons that leads to the olfactory
bulb, and in the olfactory bulb, the new neurons differentiate
into various types of nerve cells
... ... F. Doetsch et al (3 authors at 2 installations, US ES)
now report that after treatment of the adult mouse brain with an
antimitotic agent (cytosine-beta-D-arabinofuranoside) (i.e., an
agent that stops cell mitosis), subventricular zone neuroblasts
are eliminated, but the subventricular zone network then rapidly
regenerates. In 2 days, precursor cells reappear, followed at 4.5
days by migrating neuroblasts. By 10 days, the subventricular
zone network is fully regenerated, and the orientation and
organization of chains of migrating neuroblasts resemble that of
normal mice. The authors suggest this regeneration reveals an
unexpected plasticity in the adult central nervous system and
should provide a model system to study the early stages of
neurogenesis in the adult brain.
-----------
F. Doetsch et al: Regeneration of a germinal layer in the adult
mammalian brain.
(Proc. Natl. Acad. Sci. US 28 Sep 99 96:11619)
QY: Arturo Alavarz-Buylla [alvarez@rockvax.rockefeller.edu]
-----------
Text Notes:
... ... *axonal regeneration: In general, nerve cells have a
single long extension (the "axon") that propagates the electrical
output (the action potential) of the cell. In some types of nerve
cells, axons are extensively branched into a multitude of fine
fibers that make contact (synapses) with other nerve cells. 
... ... *neuroplasticity: In neurobiology, the term "plasticity"
is the name given to the capacity of neural tissue to adjust to
change. One variant of this concerns the dependence of the
"wiring" of the nervous system on its input. Another variant
concerns the degree to which one region can under certain
conditions assume the function of another region. A 3rd meaning,
used in this report, refers to the ability of the nervous system
to repair itself after damage, i.e., to regenerate both cells and
connections between cells.
... ... *ventricles: The ventricles are spaces in the vertebrate
brain that comprise the remnants of the embryonic neural tube.
These spaces are filled with cerebrospinal fluid (CSF), a clear
colorless fluid that flows continuously in the brain and spinal
cord, the fluid containing proteins, glucose, and various
electrolytes.
... ... **Note #1: The olfactory sensory tissue system (olfactory
epithelium) comprises approximately 10 square centimeters in a
70-kilogram human and 20 square centimeters in a 3-kilogram cat.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 3Dec99
[For more information: http://scienceweek.com/search/search.htm]
-------------------
Related Background:
ON NEW NERVE CELLS IN THE ADULT HUMAN BRAIN
During most of this century, one of the dogmas in neurobiology
was that in the adult human brain new connections between neurons
may arise, but never new neurons. The dogma, in other words, was
that in the adult human brain new nerve cells are not produced,
and the neurons present at birth are the neurons present in the
adult, albeit a maximum number of nerve cells at birth, since a)
the number of neurons in the healthy adult human brain apparently
decreases with age beginning at about age 35; and b) various
neurodegenerative diseases can markedly reduce the population of
neurons in either specific regions of the brain or globally
nearly everywhere in the brain. In recent years, this dogma, the
idea that new nerve cells are not produced in the adult human
brain, has effectively crumbled for at least one specific and
important brain locus called the "hippocampus", which is a region
of the cerebral cortex in the *medial part of the temporal lobe.
In humans, among other functions, the hippocampus is apparently
involved in short-term memory, and analysis of the neurological
correlates of learning behavior in animals indicates that the
hippocampus is also involved in memory in other species.
... ... G. Kempermann and F.H. Gage (2 installations, DE US)
present a review of past and current research in adult
neurogenesis in humans, the authors making the following points:
     1) In 1965, Altman and Das reported neurogenesis in the
hippocampus of rats, in a subregion of the hippocampus called the
"dentate gyrus". But this data was not viewed as evidence of
significant neurogenesis in adult mammals, primarily because the
methods available then could not accurately estimate the number
of new neurons nor demonstrate definitively that the new cells
were indeed nerve cells. In addition, the concept of *stem cells
in the brain had not yet been introduced, and the belief was that
for new neurons to appear, the only source would be replication
(i.e., mitosis) of adult neurons. There was also no evidence that
neurogenesis occurred in non-human primates, and so the relevance
of the rat data for the human brain seemed remote.
     2) In the mid 1980s, Nottebohm discovered that neurogenesis
occurred in adult canaries in brain centers responsible for song
learning, and that the process accelerated during the seasons in
which the adult birds acquired their songs. Nottebohm and his co-
workers then demonstrated that neurogenesis also occurred in the
hippocampus of adult chickadees, particularly during seasons when
the birds had to keep track of dispersed food storage sites.
     3) In 1997, Gould and McEwan reported that some neurogenesis
occurs in the hippocampus of the primate-like tree shrew, and in
1998, these authors found the same phenomenon in marmoset
monkeys, which are classified as actual primates.
     4) Because of research difficulties, demonstration of
neurogenesis in the adult human brain had to await special
techniques. In 1998, Peter S. Eriksson reported the use of
bromodeoxyruridine as a marker for neurogenesis and the first
evidence for neurogenesis in the hippocampus of adult humans. The
use of this marker depended on its already established use as a
tumor marker in cancer patients. Bromodeoxyuridine is a marker
that becomes integrated only into the DNA of cells preparing to
divide, and the marker was in use with terminally ill patients
with cancer of the tongue or larynx. Eriksson obtained consent
from a number of patients to investigate their brains after
death, and when 5 patients died, all 5 brains displayed new
neurons in the dentate gyrus subregion of the hippocampus. At the
same time as this study was reported, other research groups
reported nerve cell production in the hippocampus of adult rhesus
monkeys, which are primates closer to humans than marmoset
monkeys.
     5) In their review, the authors refer to their own work,
noting that beginning in 1997, they have demonstrated that adult
mice given enriched living conditions generate substantial
increases in dentate gyrus hippocampal neurons over that found in
genetically identical control animals.
     6) The authors suggest that studies of neurogenesis in the
adult human brain, while difficult, may lead to better treatments
for a variety of neurological diseases. The authors conclude:
"The expected benefits of unlocking the brain's regenerative
potential justify all the effort that will be required."
-----------
G. Kempermann and F.H. Gage: New nerve cells in the adult brain.
(Scientific American May 1999)
QY: Gerd Kempermann, University of Regensburg, DE.
-----------
Text Notes: 
... ... *medial part of the temporal lobe: The temporal lobes are
roughly the lower sides of the brain, above the ears and behind
the temporal bones of the skull, but when the human brain is
viewed from the side, as it usually is in common gross
depictions, the large and functionally important ventral and
infolded parts of the temporal lobes are not visible. In general,
the larger anatomical regions of the human brain are best
visualized as highly corrugated lobular structures extensively
folded and densely packed to fit inside the volume-limiting
protective skull. Isolated verbal descriptions of the
architecture are of limited use: anatomical graphics are the best
sources for visualization of gross brain structures.
... ... *stem cells: In general, the term "stem" cells
refers to undifferentiated cells that upon differentiation can
give rise to various specialized cell lines such as blood cells,
skin cells, nerve cells, etc. Adult bone marrow, for example,
contains stem cells that are the precursors of the various
specialized types of blood cells.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 18Jun99
-------------------
Related Background:
REGENERATION OF AXONS IN CENTRAL NERVOUS SYSTEM WHITE MATTER
When examining the gross anatomy of the mammalian brain and
spinal cord, a striking feature is the presence of large regions
with an opalescent ivory color. The color is due to myelin, the
substance that sheaths many nerve fibers in the central nervous
system. In the vertebrate central nervous system, the axons of
nerve cells involved in physiological functions that require
rapid signaling (for example, the neural control of voluntary
muscle) are wrapped in myelin with a special consequence. The
myelin sheath consists of concentric layers of electrically
insulating lipid material, but the sheath is periodically
interrupted, and at the points where the sheath is interrupted so
is the electrical insulation interrupted. The result, predictable
from the classical physics of electrical transmission lines and
the electrical parameters of nerve fibers, is that the
propagation of an electrical pulse along such nerve fibers occurs
at a velocity much higher than that found in unmyelinated fibers.
Glial cells are cells of the central and peripheral nervous
system that metabolically support neurons and produce the
multiple membrane layers called myelin and enfold nerve cell
axons with it. The glial cells are found everywhere in the brain
and spinal cord, and one result of a localized injury to the
central nervous system is a local proliferation of glial cells to
form a scar matrix. Concerning brain and spinal cord injury, it
has always been a canon of neurobiology that adult central
nervous system neurons cannot regenerate after injury to re-
establish the connections to other cells necessary for proper
functioning. Davies et al (6 authors at 2 installations, US UK),
using microtransplantation techniques, now report that adult
central nervous system white matter can support long-distance
regeneration of adult axons provided the reactive glial
extracellular matrix at the site of the lesion can be bypassed.
The authors suggest this is the first time this glial barrier to
axon regeneration has been noted.
QY: Jerry Silver [jsx10@po.cwru.edu]
(Nature 18/25 Dec 97) (Science-Week 9 Jan 98)
-------------------
Related Background:
REGENERATION OF MOTOR NEURONS: IDENTIFICATION OF A MITOGEN
Motor neurons are nerve cells that transmit nerve signals from
the brain or spinal cord to muscle or gland tissue, and sensory
neurons are nerve cells that carry signals from various parts of
the body to the brain or spinal cord. High signal propagation
velocities in motor and sensory neurons in vertebrates are
achieved by association of the nerve fiber with an enfolding
sheath called myelin. The myelin sheath consists of concentric
layers of electrically insulating lipid material, but the sheath
is periodically interrupted, and at the points where the sheath
is interrupted so is the electrical insulation interrupted. The
result, predictable from the classical physics of electrical
transmission lines and the electrical parameters of nerve fibers,
is that the propagation of an electrical pulse along such nerve
fibers occurs at a velocity much higher than that found in
unmyelinated fibers. Glial cells are the cells of the central and
peripheral nervous system that produce the multiple membrane
layers called myelin and enfold nerve cell axons with it, and
Schwann cells are a particular type of glial cell. A mitogen is
any compound that stimulates mitotic cell division. Livesey et al
(6 authors at 3 installations, UK CA) report the identification
of an extracellular signaling molecule, previously described as
the pancreatic secreted protein Reg-2, that is expressed solely
in regenerating and developing rat motor and sensory neurons,
with Reg-2 a potent Schwann cell mitogen in vitro. In vivo, Reg-2
is apparently transported along regrowing axons, and inhibition
of Reg-2 significantly retards the regeneration of axons
containing the protein. The authors suggest that Reg-2 is an
essential component of the neuron-glia interactions underlying
development and regeneration of mammalian motor neurons.
QY: Frederick J. Livesey [rlivesey@mail.tcd.ie]
(Nature 11 Dec 97) (Science-Week 2 Jan 98)
[For more information: http://scienceweek.com/search/search.htm]

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

3. CELL BIOLOGY: ON CELL BIOLOGY AND SOCIETY
In general, the term "cell biology" refers to a branch of biology
that focuses on the structure and dynamics of biological cells as
intact systems. But the fact is that during the past five
decades, as biochemistry and molecular biology and biophysics and
genetics have attacked (and in many cases solved) significant
problems in cell biology, the field of cell biology can no longer
be sharply separated from these other fields by either
methodology or content, and these categorizations are becoming
more a question of emphasis. At the advanced undergraduate
teaching level, for example, it is now common to find textbooks
carrying the title "Cell and Molecular Biology", the material
presented for a single course without much attempt to distinguish
between the two disciplines. Nevertheless, there are a number of
important problems in classical cell biology that have not yet
been solved, and the people who work on these problems identify
themselves as "cell biologists" rather than "molecular
biologists", and they have membership in cell biology societies
and hold positions in cell biology departments. Current academic
categorizations differ from those that existed 100 years ago, and
certainly 100 years from now the extant academic categorizations
will differ from those existing today. All the sciences have such
categorization histories; meanwhile, the natural world remains
indifferent to the finesses of the academy.
... ... Richard Hynes (Massachusetts Institute of Technology,
US), Director of the MIT Center for Cancer Research and also
current president of the American Society for Cell Biology,
presents an essay on the future of cell biology, the author
making the following points concerning cell biology and society:
     1) The author suggests that the enormous promise of
applications of cell biology to the treatment of human disease
exemplifies the impact that the science of cell biology will have
in coming years. Knowledge about cells and the power to
manipulate cells brings with it a responsibility to think about
the potential effects on society. Our increasing ability to
intervene in the initiation, amelioration, and prolongation of
life raises issues of ethics and public policy. There is already
considerable debate about the merits and demerits of stem cells,
fetal tissue research, cloning, and genetically modified foods,
and such debate can only increase as potential applications
proliferate. To many scientists, the benefits seem obvious, but
much of the public debate suffers from a chronic lack of adequate
information.
     2) The author points out that the science of cell biology
has moved rapidly and appears bewilderingly complex to many
outside the field. The power to manipulate cells, genes, and
fundamental life processes seems frightening to some, and cell
biologists have a real responsibility to communicate their
science, to explain what they do and how it can benefit humankind
and society and why cell biology is worthy of enthusiastic
support, particularly moral support.
     3) The author points out that the rapid advances of the past
several decades have been exhilarating and the promise for the
future is extraordinary. Cell biology has grown to encompass many
other disciplines and has benefitted from the synergy of
different approaches. Cell biologists are just beginning to see
impact from the fields of genomics and computational biology, and
they can look forward to increasing impact on medicine. Cell
biology is poised to make even more rapid progress, and it is
incumbent on cell biologists to communicate and share the
excitement and promise with society at large.
-----------
Richard Hynes: Whither cell biology?
(The Scientist 11 Dec 2000)
QY: Richard Hynes: Mass. Inst. of Technology 617-253-1000
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

4. COSMOLOGY: ON QUINTESSENCE
     The conceptual turmoil in current cosmology, a turmoil
caused by significant apparent paradoxes and the lack of an
acceptable broad theory of the evolution of the Universe that
both explains observations and solves the paradoxes, portends
important breakthroughs in our understanding of the Cosmos.
Dramatic new insights may come soon or may require decades, but
meanwhile the intellectual ferment continues without abatement.
     Perhaps the central question in cosmology concerns the
future: Will the Universe continue to expand indefinitely, or
will the expansion eventually slow and stop and be replaced by a
contraction? At present, attempts to answer this fundamental
question involve two considerations: a) the total mass of the
Universe, which will determine whether gravity can slow the
expansion and produce a contraction; and b) the possible presence
of special energy fields that may also influence the rate of
expansion.
     In this context, the term "dark matter" refers to material
whose presence can be inferred from its effects on the motions of
stars and galaxies, but which cannot be seen directly because it
emits little or no radiation. It is believed that as much as 90
percent of the mass in the Universe may exist as some form or
dark matter, although the proposed percentage of dark matter
varies widely with different cosmological models.
     In cosmology, what is called the "cosmological constant"
refers to a mathematical term introduced by Einstein into the
equations of general relativity, the purpose to obtain a solution
of the equations corresponding to a "static Universe" (i.e., flat
Universe). The term describes a pressure (if positive) or a
tension (if negative) which can cause the Universe to expand or
contract even in the absence of any matter (i.e., a "vacuum
energy"). When the expansion of the Universe was discovered,
Einstein apparently began to regard the introduction of this term
as a mistake, and he described the cosmological constant as the
"greatest mistake of my life". But the term has reappeared as the
proposed source of apparent accelerated cosmic expansion. 
     Contrary to expectations, current evidence indicates that
the Universe is expanding at approximately twice the velocity
required to overcome the gravitational pull of all the matter the
Universe contains. The implication of this is that in the past
the greater density of mass in the Universe gravitationally
slowed the expansion, while in the future the expansion rate will
be close to constant or perhaps increasing under the influence of
a new type of matter that some call "quintessence". Quintessence
began theoretically as Einstein's cosmological constant, but in
current theory it is more a time-variant parameter than a
constant. It has negative gravitational mass: its gravity pushes
things apart, and it is thus the origin of a repulsive force.
... ... J.P. Ostriker and P.J. Steinhardt (Princeton University,
US) present a review of current work on quintessence, the authors
making the following points:
     1) The authors point out that during the past 5 years,
observations have convinced cosmologists that the chemical
elements and the dark matter combined amount to less than half
the content of the Universe. The bulk is a ubiquitous "dark
energy" with a strange and remarkable feature: its gravity does
not attract, it repels. "Whereas gravity pulls the chemical
elements and dark matter into stars and galaxies, it pushes the
dark energy into a nearly uniform haze that permeates space. The
Universe is a battleground between the two tendencies, and
repulsive gravity is winning."
     2) Where does the dark energy come from? The best-known
possibility is that the energy is inherent in the fabric of space
(vacuum energy as represented by Einstein's cosmological
constant). Many cosmologists, however, are now leaning towards a
different idea -- quintessence. In 1997, R.R. Caldwell, R. Dave,
and P.J. Steinhardt introduced the term to refer to a dynamical
quantum field, not unlike an electrical or magnetic field, that
gravitationally repels.
     3) Vacuum energy is completely inert, maintaining the same
density for all time. Consequently, to explain the amount of dark
energy present today, the value of the cosmological constant
would have to be fine-tuned at the creation of the Universe,
which is conceptually unsatisfactory and reinforces the criticism
of the cosmological constant as a fudge factor. In contrast,
quintessence interacts with matter and evolves with time, so it
might naturally adjust itself to reach the value observed today.
     4) In general, according to the current conception, the main
ingredient of the Universe is dark energy, which consists of
either vacuum energy or quintessence. The other ingredients are
dark matter composed of exotic elementary particles, ordinary
matter (both nonluminous and visible) and a trace amount of
radiation.
     5) In general, the Universe expands at different rates
depending on which form of energy predominates. Matter causes the
expansion to decelerate, whereas vacuum energy causes it
accelerate. Quintessence is intermediate: quintessence forces an
accelerating expansion, but one less rapid than that produced by
vacuum energy.
     6) Supernova data may be a way to decide between
quintessence and vacuum energy: for supernova at a given distance
(given redshift), cosmic expansion results in a dimming of
apparent luminosity; measurements can thus distinguish an
acceleration of expansion due to a vacuum energy model from an
acceleration of expansion due to a quintessence model. Existing
telescopes cannot resolve the two cases, but the proposed
Supernova Acceleration Probe should be able to accomplish this.
-----------
J.P. Ostriker and P.J. Steinhardt: The quintessential Universe.
(Scientific American January 2001)
QY: J.P. Ostriker: Princeton University 609-258-3000
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON QUINTESSENCE AND THE EVOLUTION OF THE COSMOLOGICAL CONSTANT
... P.J.E. Peebles (Princeton University, US) presents a short
review of current ideas concerning the cosmological constant, the
author making the following points: 1) Contrary to expectations,
the evidence is that the Universe is expanding at approximately
twice the velocity required to overcome the gravitational pull of
all the matter the Universe contains. The implication of this is
that in the past the greater density of mass in the Universe
gravitationally slowed the expansion, while in the future the
expansion rate will be close to constant or perhaps increasing
under the influence of a new type of matter that some call
"quintessence". 2) Quintessence began as Einstein's cosmological
constant, Lambda. It has negative gravitational mass: its gravity
pushes things apart. 3) Particle physicists later adopted
Einstein's Lambda as a good model for the gravitational effect of
the *active vacuum of quantum physics, although the idea is at
odds with the small value of Lambda indicated by cosmology. 4)
Theoretical cosmologists have noted that as the Universe expands
and cools, Lambda tends to decrease. As the Universe cools,
*symmetries among forces are broken, particles acquire masses,
and these processes tend to release an analogue of *latent heat.
The vacuum energy density accordingly decreases, and with it the
value of Lambda. Perhaps an enormous Lambda drove an early rapid
expansion that smoothed the primeval chaos to make the near
uniform Universe we see today, with a decrease in Lambda over
time to its current value. This is the cosmological *inflation
concept. 5) The author suggests that the recent great advances in
detectors, telescopes, and observatories on the ground and in
space have given us a rough picture of what happened as our
Universe evolved from a dense, hot, and perhaps quite simple
early state to its present complexity. Observations in progress
are filling in the details, and that in turn is driving intense
debate on how the behavior of our Universe can be understood
within fundamental physics.
----------
P.J.E. Peebles: Evolution of the cosmological constant.
(Nature 4 Mar 99 398:25)
QY: P.J.E. Peebles: pjep@pupgg.princeton.edu
-----------
Text Notes:
... ... *active vacuum of quantum physics: This refers to the
idea that the vacuum state in quantum mechanics has a zero-point
energy (minimum energy) which gives rise to vacuum fluctuations,
so the vacuum state does not mean a state of nothing, but is
instead an active state.
... ... *symmetries among forces are broken: If a theory or
process does not change when certain operations are performed on
it, the theory or process is said to possess a symmetry with
respect to those operations. For example, a circle remains
unchanged under rotation or reflection, and a circle therefore
has rotational and reflection symmetry. The term "symmetry
breaking" refers to the deviation from exact symmetry exhibited
by many physical systems, and in general, symmetry breaking
encompasses both "explicit" symmetry breaking and "spontaneous"
symmetry breaking. Explicit symmetry breaking is a phenomenon in
which a system is not quite, but almost, the same for two
configurations related by exact symmetry. Spontaneous symmetry
breaking refers to a situation in which the solution of a set of
physical equations fails to exhibit a symmetry possessed by the
equations themselves.
... ... *latent heat: In general, this is the quantity of heat
absorbed or released when a substance changes its physical phase
(e.g., solid to liquid) at constant temperature.
... ... *inflation concept: The inflationary model, first
proposed by Alan Guth in 1980, proposes that quantum
fluctuations in the time period 10^(-35) to 10^(-32) seconds
after time zero were quickly amplified into large density
variations during the "inflationary" 10^(50) expansion of the
universe in that time frame.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 30Apr99
-------------------
Related Background:
INFLATION IN A LOW-DENSITY UNIVERSE
There is an apparent consensus among cosmologists that recent
observational evidence is not consistent with the current
"*inflation theory" of the early evolution of the Universe, and
that to keep this theory relevant requires either the postulate
of an exotic form of energy or the addition of "a layer of
complexity" to inflation theory. ... ... M.A. Bucher and D.N.
Spergel (2 installations, UK US) present a review of the second
option, the authors making the following points: 1) Despite its
success, the standard *Big Bang theory cannot answer several
important questions: Why is the density and temperature of the
present Universe so uniform? Why did the early Universe have any
density variations at all? Why is the rate of cosmic expansion
just enough to counteract the collective gravity of all the
matter in the Universe? 2) The failure of the standard Big Bang
theory to answer these questions provoked, in the 1980s, the
formulation of the theory of inflation by Guth, Sato, Linde,
Albrecht, Steinhardt and others. 3) Inflation theory predicts a
flat (i.e., Euclidean) and uniform Universe, with an observed
value of the *Omega parameter either exactly 1 or so close to 1
that the deviation is not detectable. The implication of an Omega
value of 1 is that the cosmic gravitational energy exactly equals
the cosmic kinetic energy (i.e., the energy contained in the
motion of matter as space expands). The problem is that a wide
variety of recent astronomical observations involving galaxy
clusters and distant supernovae suggest that gravity is too weak
to combat cosmic expansion, that the density of matter must be
much less than predicted, and that the value of the Omega
parameter is equal to approximately 0.3. 3) The authors propose
there are 3 ways to interpret this result: a) Standard inflation
theory is completely wrong. Or b) Standard inflation theory is
correct: the Universe is flat, but an additional new form of
energy exists, and this is responsible for what appears to be an
accelerating expansion. Or c) Standard inflation theory is
partially correct, and its assumption of the inevitability of a
flat Universe needs to be revised. 4) The focus of the authors is
on the 3rd option. They review a revision of standard inflation
theory, the revision involving the introduction of a "*false-
vacuum decay" preceding the standard inflation, this false-vacuum
decay producing nonuniform "bubbles" of expansion [*Note #1]. The
new conception is called "open inflationary theory". 5) The
authors state that at the current levels of precision,
observations cannot distinguish between the predictions of the 2
theories of inflation. The authors suggest the "moment of truth"
will come with the planned deployment late next year of the
*Microwave Anisotropy Probe, and the launch in 2007 of its
European counterpart, *Planck. These satellites will perform
observations similar to those of the *Cosmic Microwave Background
Explorer (COBE) nearly a decade ago, but at a much higher
resolution. The authors suggest these new satellites will be able
to resolve which of the 3 theoretical options is correct: a) an
abandonment of any inflation theory; b) standard inflation theory
with a new form of energy; c) open inflation theory.
-----------
M.A. Bucher and D.N. Spergel: Inflation in a low-density
Universe.
(Scientific American January 1998)
QY: Martin A. Bucher, University of Cambridge, UK.
-----------
Editor's note: In addition to the background material below,
further material on this subject was presented last week in the
SW issue of 5 Feb 99. Also, there are a number of relevant SW
Focus Reports available at [http://scienceweek.com/swfr.htm].
-----------
Text Notes:
... ... *inflation theory: The inflationary model, first proposed
by Alan Guth in 1980, proposes that quantum fluctuations in the
time period 10^(-35) to 10^(-32) were quickly amplified into
large density variations during the "inflationary" 10^(50)
expansion of the universe in that time frame.
... ... *Big Bang theory: The Big Bang theory is the general
cosmological model that proposes that all matter and radiation in
the universe originated in an explosion at a finite time in the
past.
... ... *Omega parameter: Another approach to the Omega parameter
is to define it as the ratio of the density of matter (or energy)
in the Universe to the theoretical density required for flatness.
An Omega with a value of greater than 1 implies a closed
Universe; a value less than 1 implies an open Universe; a value
equal to 1 implies a flat Universe.
... ... *false-vacuum decay: A "false vacuum" is a peculiar state
of matter which has never been observed but whose properties are
unambiguously predicted by *quantum field theory. Essentially,
the idea of a false vacuum refers to a miniature energy minimum
above the true minimum, a saddle "trap". The most peculiar
property of the false vacuum is probably its pressure, which is
both large and negative. The term "false-vacuum decay" refers to
a breaking out of the trap, in this case via *quantum mechanical
tunneling through the miniature energy barrier, and then a fall
to the true zero-point (minimum vacuum energy). The application
of the idea of false vacuum to the inflation model was already
well underway in the late 1980s by Guth and others.
... ... *quantum field theory: Quantum field theory is the
mathematical fusion of quantum mechanics with special relativity
theory.
... ... *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.
... ... *Note #1: It should be noted that this idea was already
described in the late 1980s by Alan Guth and others.
... ... *Microwave Anisotropy Probe: Information on this
satellite project can be found at URL [http://map.gsfc.nasa.gov].
... ... *Planck: Information on this satellite project can be
found at [http://astro.esctec.esa.nl/SA-general/Projects/Planck].
... ... *Cosmic Microwave Background Explorer (COBE): A NASA
orbiting satellite launched in 1989 and dedicated to the study of
the *cosmic microwave background radiation. The most important
results were the discoveries of irregularities in the cosmic
background radiation on the level of one part in 10^(5), and the
confirmation that the spectrum of the cosmic background radiation
is that of a black body with a temperature of 2.73 degrees
kelvin.
... ... *cosmic microwave background radiation: The cosmic
microwave background is black-body radiation (the emission
radiation of a perfect absorber of radiation) at a present
temperature of 2.73 degrees Kelvin, and has an almost equal
intensity in all directions in space. The deviations from
isotropic intensity, however, are of extreme importance in
theoretical cosmology. The cosmic background radiation is
predicted by the Big Bang theory and is considered one of the
most important pieces of evidence for it.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 12Feb99
-------------------
Related Background:
ASTROPHYSICS: ON ACCELERATING COSMIC EXPANSION
The idea of cosmic expansion derives from the observation that
radiation from distant galaxies is *redshifted, and the consensus
is that the distance between clusters of galaxies is continuously
increasing, with all galaxies beyond the Local Group apparently
receding from us [*Note #1]. In other words, the Universe as a
whole is expanding, a phenomenon discovered by Edwin Hubble in
1929 but previously suggested by several theoretical cosmologists
(e.g., A. Friedmann [1922], G. Lemaitre [1927]). This expansion
is the observational basis of the *Big Bang theory. Essential to
the study of cosmic expansion is the accurate measurement of
intergalactic distances, and such measurement is dependent on the
use of "standard candles", astronomical objects whose intrinsic
brightness is known and whose distance can therefore be
calculated from apparent brightness. ... ... C.J. Hogan et al (3
authors at 3 installations, US CL) present a review of current
research on the temporal history of cosmic expansion, with
emphasis on recent work concerning the use of *type 1a supernovae
as standard candles. The authors make the following points: 1)
Until recently, the intrinsic brightness of all standard candles
used in observations has been found to be too variable, changing
with the evolution of the object or showing too much diversity
from one object to the other. However, during the past decade,
astrophysicists have been able to precisely determine the
intrinsic brightness of one kind of astronomical object, the type
1a supernova, and these objects have become the best calibrated
standard candles known to astronomers. Currently, observations of
type 1a supernovae are challenging decades of conventional ideas
concerning cosmic expansion. 2) Locating distant supernovae
involves taking images of the same part of the sky a few weeks
apart and searching for changes that might be exploding stars.
Because the digital light detectors can precisely count the
number of photons in each picture element, one makes a simple
subtraction of the first image from the second and looks for
significant differences from zero. With present equipment,
thousands of galaxies are checked in each image pair... After
supernovae candidates are located, the *Keck telescopes in
Hawaii, the largest optical instruments in the world, are pointed
at the objects, and critical observations establish whether or
not the objects discovered are in fact type 1a supernovae. The
observations are then used to gauge the intrinsic brightness of
the objects more exactly and to determine their redshifts... Two
teams have now studied a total of approximately 40 high redshift
supernovae, objects that erupted between 4 and 7 billion years
ago, when the universe was between one-half and two-thirds of its
present age. The results have been surprising: the supernovae are
fainter than expected. The difference is slight, the distant
supernovae on average only 25 percent dimmer than forecast, but
this result is enough to call long-standing cosmological theories
into question. The conclusion from the observations is that the
cosmic expansion is slowing less quickly than previously thought.
3) If the Universe is made of normal matter, gravity must
steadily slow the cosmic expansion. A reduced slowing, as
indicated by the supernovae measurements, implies that the
overall density of matter in the Universe is low... However, the
big surprise is that the observed supernovae are fainter than
predicted even for a nearly empty universe. Taken at face value,
the observations appear to require that expansion is actually
accelerating with time. This is consistent with the "vacuum
energy" embodied in Einstein's equations as the so-called
"*cosmological constant". Unlike ordinary forms of mass and
energy, the vacuum energy adds gravity that is repulsive and can
drive the Universe apart at ever increasing speeds. The authors
conclude: "Evidence for a strange form of energy imparting a
repulsive gravitational force is the most interesting result we
could have hoped for, yet it is so astonishing that we and others
remain suitably skeptical."
-----------
C.J. Hogan et al: Surveying space-time with supernovae.
(Scientific American January 1999)
QY: Craig J. Hogan, Univ. of Washington Seattle 206-543-8992.
-----------
Text Notes:
... ... *redshifted: Redshift (symbol: z) is a lengthening of the
wavelengths of electromagnetic radiation from a source caused
either by the movement of the source (Doppler effect) or by the
expansion of the universe (cosmological redshift). Redshift is
defined as the change in wavelength of a particular spectral line
divided by the unshifted wavelength of that line. Large redshifts
imply large radial velocities (which imply large distances,
according to current cosmological theory), but at redshifts
greater than about 0.2 there is a relativistic divergence from a
linear relation. A redshift of 4.0 corresponds to an object
receding with a radial velocity 92% that of the velocity of
light. The largest astrophysical redshifts so far observed are of
the order of z = 4.9.
... ... *Note #1: In the expansion model, it is the space between
widely separated objects that is expanding. Neighboring objects,
such as close pairs of galaxies, do not move apart because their
mutual gravitational attraction exceeds the effect of the
cosmological expansion. However, the distance between two widely
separated galaxies, or clusters of galaxies, will increase as the
Universe expands.
... ... *Big Bang theory: The Big Bang theory is the general
cosmological model that proposes that all matter and radiation in
the universe originated in an explosion at a finite time in the
past.
... ... *type 1a supernovae: Type 1a supernovae are believed to
be *white dwarf stars that have accreted enough matter from
another star to be pushed over a mass threshold and into a
thermonuclear explosion.
... ... *white dwarf star: White dwarf stars are extremely dense
and compact stars that have undergone gravitational collapse.
They are the final stage in the evolution of low-mass stars after
they have lost their outer layers. White dwarf stars are about
the size of Earth, but with a mass about that of the Sun.
... ... *Keck telescopes: The Keck telescopes are a pair of twin
telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
each with 10 meter mirrors, the pair constructed 1992-1996. The
installation is managed by the University of California (US) and
the California Institute of Technology (US).
... ... *cosmological constant: A mathematical term introduced by
Einstein into the equations of general relativity, the purpose to
obtain a solution of the equations corresponding to a "static
universe". The term describes a pressure (if positive) or a
tension (if negative) which can cause the Universe to expand or
contract even in the absence of any matter ("vacuum energy").
When the expansion of the Universe was discovered, Einstein
apparently began to regard the introduction of this term as a
mistake, and he described the cosmological constant as the
"greatest mistake of my life". But the term has reappeared as the
proposed source of apparent accelerated cosmic expansion.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 22Jan99
-------------------
Related Background:
COSMOLOGY: THE END OF THE OLD MODEL UNIVERSE
Cosmologists are apparently expecting the near-future necessity
for profound conceptual alterations in their field. Peter Coles
(University of London, UK) presents a short review of the current
situation and makes the following points: 1) Observations only
recently made possible by improvements in astronomical
instrumentation have put theoretical models of the Universe under
intense pressure. The standard ideas of the 1980s about the shape
and history of the Universe have now been abandoned -- and
cosmologists are now taking seriously the possibility that the
Universe is pervaded by some sort of "vacuum energy" whose origin
is not at all understood. 2) The weakness of the Big Bang model
is that the numerical values of certain essential parameters in
the model (the Hubble constant, the density parameter, and, in
some versions, the cosmological constant) are not predicted by
theory, and thus the parameters must be inferred from
observations. 3) The Big Bang model does not deserve to be called
a "theory" unless and until it can explain how nonuniformities of
galaxies and clusters of galaxies came into being and evolved. 4)
The Cold Dark Matter model of structure formation, first proposed
in the 1980s, is in serious difficulty because the consequent
significant gravitational brake on expansion is not evident, and
in fact expansion may be accelerating. Current observations
coupled with current dynamical arguments all suggest a global
density of matter in the Universe less than the value required to
make the Universe recollapse. 5) The existence of a cosmological
constant (or vacuum energy) of the required size necessary to
make the basic cosmological models work is not at all explained
by current theories of the fundamental interactions of matter. 6)
There is every reason to be confident that the important issues
will soon be resolved, because a data explosion is about to
engulf cosmology, a new generation of galaxy surveys. The Sloan
Digital Sky Survey, for example, will encompass more than a
million galaxies. The cosmological community is bracing itself
for the arrival of these enormous new data sets and the new
insights they will surely bring. 7) It is possible that none of
the available models will fit all the new data. Coles concludes:
"For many of us, that is the most exciting possibility of all, as
we would have to move to stranger theories, perhaps not even
based on General Relativity."
QY: Peter Coles: p.coles@qmw.ac.uk
(Nature 25 Jun 98 393:741) (Science-Week 17 Jul 98)
For more information: http://scienceweek.com/swfr.htm

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5. CONDENSED MATTER PHYSICS: A NEW APPLICATION OF CLASSIC THEORY
TO HIGH-TEMPERATURE FERROMAGNETISM
     In theoretical physics, in general, the term "field" refers
to a mathematical representation of the spatiotemporal
distribution of a force or forces experienced by a test object at
each point in a region, the field essentially serving as the
mediating entity between any objects in that field. The field may
result from an application from outside the system, or from
components of the system, or both.
     In general, in this context, the term "mean field" refers to
the average field produced at a point in space in a system of
distributed field sources, and a "mean-field theory" is any
theory based on a mean field approach. The essential idea of a
mean field approach is that by considering only the average field
at any point the field equations that describe the system often
become tractable.
     The major experimental observations concerning magnetism
were clearly delineated by Pierre Curie (1859-1906) in a 100-page
paper in 1895, a paper which reported his work on the effects of
temperature on magnetic materials, the subject of his doctoral
dissertation. In this monumental paper, Curie clarified the
knowledge of his time and indicated directions for future
research. Already in 1895, it had long been recognized that
substances with magnetic properties can be divided into 3
categories -- diamagnetic, paramagnetic, or ferromagnetic --
according to their behavior in magnetic fields. According to the
current view: a) In "diamagnetism", the magnetization of a
substance is in the opposite direction to that of the applied
field. Diamagnetism results from changes induced in the orbits of
electrons in the atoms of a substance by the applied field. All
substances are to some extent diamagnetic, but it is a weak form
of magnetism and it may be masked by stronger forms of magnetism.
b) In "paramagnetism", the atoms or molecules of the substance
are capable of being aligned in the direction of the applied
field. Paramagnetism occurs in all atoms or molecules with
unpaired electrons. c) In ferromagnetic substances, within a
certain temperature range, there are intrinsic net atomic
magnetic moments, which line up in such a way that magnetization
persists after the removal of the applied magnetic field. Below a
certain temperature (Curie point; Curie temperature), an
increasing magnetic field applied to a ferromagnetic substance
will cause increasing magnetization to a high value. Above the
Curie point, ferromagnetic materials become paramagnetic.
     In this context, the term "molecular field" refers to the
molecular field theory of Pierre Weiss (1865-1940), a theory
first proposed in 1907 to explain the behavior of ferromagnetic
materials. A Weiss molecular field is an effective magnetic field
which acts on atomic magnetic moments within a domain, the field
tending to align the moments, and the field in turn generated by
these magnetic moments. The Weiss molecular field theory is a
mean-field theory.
     In 1928, Werner Heisenberg (1901-1976) demonstrated that
from the perspective of quantum mechanics, the cause of
ferromagnetism lies in the *quantum-mechanical exchange
interaction between electrons, the interaction imposed by the
Pauli exclusion principle. This was really an explanation rather
than a contradiction of the Weiss mean-field theory, an
amplification in terms of quantum considerations. Many
physicists, however, felt uncomfortable with the new quantum-
mechanical approach to magnetic materials: the classical theory
gave a more immediate feeling of understanding, with a closer
relationship to directly observable phenomena. In addition, the
mathematical difficulties of quantum mechanics as applied to such
systems resulted in many problems remaining unsolved. For these
reasons, the classical approach to the magnetic properties of
materials persisted for another generation.
... ... Tom Giebultowicz (Oregan State University, US) presents a
commentary on recent theoretical work in ferromagnetism (R.V.
Chamberlin, Nature 408:337 2000), Giebultowicz making the
following points:
     1) Giebultowicz points out that with the development of
modern experimental tools such as nuclear magnetic resonance
(NMR) spectroscopy and neutron scattering, the deficiencies of
the classic Weiss approach to ferromagnetism became more
apparent. In the 1960s and 1970s, powerful new theories emerged
that were better at describing the critical behavior of
ferromagnets near the Curie temperature (especially
renormalization group theory [see related background material
below]). In those days, many experts believed that the usefulness
of the mean-field model had been exhausted and that, like the
Bohr model of the hydrogen atom, it should be consigned to the
history books. But this did not happen. None of the newer models
offer the same versatility as the mean-field theory: each model
focuses on effects occurring at different temperatures. Moreover,
the underlying formalisms of the various new theories are
incompatible, so they cannot be united into one comprehensive
picture. The mean-field model is still the only one that
describes the qualitative behavior of ferromagnets over the
entire temperature range from extremely low temperatures to
extremely high temperatures. For that reason, mean-field theory
offers a common platform for researchers working on different
types of interacting systems.
     2) The new model of R.V. Chamberlin, which introduces small-
system (finite-size) thermodynamics (nanothermodynamics) into a
mean-field formalism, provides theoretical results that agree
very well with data from several different ferromagnets. The new
model also produces realistic results on the temperature
dependence of the local magnetic order above the Curie
temperature, thereby creating a robust formalism that appears to
solve a very old problem. The success of the model demonstrates
that nanothermodynamics -- of clear importance for dealing with
small systems in many areas of nanoscience -- is also crucial for
describing internal fluctuations in bulk materials. The author
(Giebultowicz) concludes: "At last we have a unified picture of
the paramagnetic behavior of ferromagnetic materials."
-----------
Tom Giebultowicz: Breathing life into an old model.
(Nature 16 Nov 00 408:299)
QY: Tom Giebultowicz: tgiebult@physics.orst.edu
-----------
Text Notes:
... ... *quantum-mechanical exchange interaction: In general, an
"exchange interaction" is an interaction resulting from the
continued exchange of particles in a manner that bonds their
hosts together (e.g., covalent bonding). Within each domain in
ferromagnetic materials, the individual atomic magnetic moments
are spontaneously aligned by exchange forces. In this context,
the "Pauli exclusion principal" is the rule that no two electrons
in a system can possess the same set of quantum numbers.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CONDENSED-MATTER PHYSICS: ON THE ISING MODEL
     In theoretical physics, one approach that has proved to be
of great general utility is to begin with an attempt to identify
and understand the simplest model exhibiting the same essential
features as the physical problem in question. In condensed-matter
physics, such a model is the so-called "Ising model", an approach
that has been applied to ferromagnetism, and also to a number of
other systems. In general, the Ising model consists of an array
of entities in one, two, or three dimensions, with each entity
capable of being in one of two possible states, with each entity
interacting only with its nearest neighbors, with a condition
that when two neighboring entities are in the same state the
total energy of the pair is reduced compared to when the same two
neighboring entities are in opposite states. These are the
elements of the model, with other conditions imposed depending on
how the model is used. Various versions of the model have been of
great utility in studies of cooperative phenomena in condensed-
matter systems, and the model itself has an interesting human
story attached to it.
     A "ferromagnet" is a material (e.g., iron) in which there
may be a permanent magnetic moment (magnetic dipole moment), and
in which the *spins of the atoms are aligned parallel to each
other. Concerning permanent magnetic moments: In general,
according to theory, 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. The term "Curie point" refers to the
temperature above which ferromagnetic materials lose their
ferromagnetism. The Curie point is thus the critical point for a
phase transition.
... ... Brian Hayes (American Scientist, US) presents an essay on
the Ising model and its application to ferromagnetism, the author
making the following points:
     1) The Ising model was invented in 1920 by Wilhelm Lenz, who
proposed it as a simplified version of a ferromagnet (Physik. Z.
1920 21:613). In 1925, a student of Lenz, Ernst Ising, chose the
model as the subject of his doctoral dissertation at the
University of Hamburg (DE), and the model has subsequently borne
Ising's name.
     2) Lenz and Ising formulated the original model in terms of
"spins", although the concept of rotation is never used. In the
original model, a spin is merely one of two states, characterized
by an arrow pointing either up or down but in no other direction.
The spins are arranged in a grid or lattice pattern. Spins at
neighboring sites prefer to point the same way: the energy is
lower when adjacent spins are parallel, and the energy is higher
when adjacent spins are antiparallel. Except for these nearest
neighbor preferences, the spins do not interact at all. Thermal
fluctuations tend to randomize the spins. Finally, an external
magnetic field may impose a bias on the spin directions.
     3) Hayes points out that the Ising model is indeed a crude
picture of a ferromagnet: a) the Ising spins correspond to
spinning electrons in iron atoms; b) the lattice represents the
crystal structure; c) the nearest neighbor interaction mimics the
overlap of quantum mechanical wave functions in adjacent iron
atoms. The one element in the model that has no obvious
counterpart in real systems is the requirement that spins take on
only two possible orientations.
     4) Ising's doctoral dissertation examined whether the
1-dimensional version of the model exhibited a Curie point. The
results were negative: the 1-dimensional Ising model exhibits no
phase transition at any temperature above absolute zero. Ising
apparently believed this negative result would hold in higher
dimensions as well, but in this conjecture he was wrong.
     5) Ising's published results (Z. fur Physik 1925 31:253)
were essentially ignored until 1936, when Rudolf Peierls (1907-
1995) showed that a 2-dimensional Ising model might exhibit a
temperature-dependent phase transition . An exact calculation of
such a system, a mathematical tour de force, was made by Lars
Onsager (1903-1976) in 1944. Exact calculations for 3-dimensional
Ising models have remained intractable, but approximations and
computer simulations involving the model have proved extremely
useful, and the value of the model has grown rather than
diminished through the years. An important approximation method
is known as "the renormalization group": the simplest version of
this algorithm gathers sets of spins into blocks, replaces each
block with a single new spin, and finally adjusts the couplings
between spins to compensate for the coarsening of the lattice.
     6) Concerning Ernst Ising, there is no record of Ising ever
publishing anything else in physics. After receiving his
doctorate, Ising taught physics in German public high schools,
but as a Jew he was dismissed from his teaching post when Hitler
came to power in 1933. Ising then taught at a Jewish boarding
school in Potsdam (DE), until that school was destroyed in the
Kristallnacht pogrom of 1938. Ising and his wife fled Germany,
but they escaped only as far as Luxembourg before the war
overtook them. They managed to survive the occupation, and they
finally reached the US in 1947. Ising taught physics and
mathematics in Minot, North Dakota (US), and then taught for
almost 30 years more at Bradley University in Peoria, Illinois
(US). In 1998, Ernst Ising died at the age of 98.
-----------
Brian Hayes: The world in a spin.
(American Scientist Sep/Oct 2000 88:384)
QY: Brian Hayes bhayes@amsci.org
-----------
Text Notes:
... ... *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.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 15Sep00
For more information: http://scienceweek.com/swfr.htm

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6. MATERIALS SCIENCE: A MAGNETIC AND ELECTRICAL CONDUCTOR HYBRID
     Most researchers agree that the physical limits of
conventional silicon-based electronics will soon be reached. One
possible path is so-called "molecular electronics". The use of
individual molecules as functional electronic devices was first
proposed by Aviram and Ratner in 1974, and since then molecular
electronics has attracted much interest, particularly because it
could lead to conceptually new miniaturization strategies in the
electronics and computer industries. In general, current
molecular electronics is an approach which uses assemblies of
individual molecules to mimic larger conventional structures such
as switches or semiconductors, with the goal of full control over
the composition, size, and function of the molecules, and thus of
their behavior. During the past several decades, some of the
basic building blocks needed for molecular electronics have been
created, including so-called "single-molecule magnets".
... ... F. Palacio and J.S. Miller (2 installations, ES US)
present a commentary on a report of a new hybrid conducting-
magnetic material (E. Coronado et al: Nature 408:447 2000), the
authors (Palacio and Miller) making the following points:
     1) The authors point out that whereas conducting magnets,
such as nickel or iron, are common among metals and alloys, the
work of Coronado et al is the first reported example involving
molecular materials. Because the electronic peculiarities of
individual molecules are different from those of bulk metals, a
conducting molecular magnet is likely to have unexpected
properties. Coronado et al report the synthesis of single
crystals formed by infinite sheets of a magnetic coordination
polymer (a bimetallic oxalato complex) interleaved with layers of
the organic conducting cation
bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF), and demonstrate
that this molecule-based compound displays both ferromagnetism
and metallic conductivity.
     2) Palacio and Miller point out that materials whose
properties are based on their component molecules are more
versatile than those whose properties derive from their component
atoms. Thus the bulk properties of molecules -- whether optical,
magnetic, or electrical -- can be controlled using conventional
syntheses, such as those used in the pharmaceutical industry.
This means that molecular electronic devices are tunable and can
more readily be tailored to respond to the changing demands of
technology. By combining an organic conductor and a magnetic
complex, Coronado et al have now introduced the possibility of
materials with multiple functions.
     3) Palacio and Miller point out that the ability of small
organic molecules to exhibit metal-like electrical conductivity
was first reported in 1965, demonstrating that metal-like
conductivity as well as metal-like optical properties could be
seen in soluble organic materials. This work led to the
development of electrically conducing polymers -- essentially
plastic conductors -- for which A. MacDiarmid, A. Heeger, and H.
Shirakawa received the 2000 Nobel Prize in Chemistry.
     4) Molecular-based ferromagnetism (see Report #5, this issue
of SW) was first reported in 1972 for an iron chloride
coordination compound. Then, in 1986, ferromagnetism was
discovered in an organic-based material. These materials become
ferromagnetic at extremely low temperatures (below 5 degrees
kelvin) and are soluble in conventional organic solvents. In
1992, Okawa et al reported a layered bimetallic compound that
behaved as a two-dimensional ferromagnet.
     5) In conventional metallic ferromagnets, the mobile
electrons play a crucial role in both the magnetic interactions
and in the electrical conductivity. But in the new system of
Coronado et al, the conducting electrons in the organic layer do
not appear to interact with the magnetic moments of the
ferromagnetic layer. Palacio and Miller suggest that this unique
feature, which is only possible because of the molecular nature
of the system, may yield unforseen physical behavior.
-----------
F. Palacio and J.S. Miller: A dual-action material.
(Nature 23 Nov 00 408:421)
QY: Joel S. Miller: jsmiller@chemistry.utah.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 29Dec00
For more information: http://scienceweek.com/swfr.htm

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7. IN FOCUS: ON THE BOUNDARIES OF ANIMAL LIFE
"Where does the boundary of an organism lie? For a cell biologist
looking at a squirrel or a laboratory mouse, the answer would
probably be the skin. For a physiologist, the answer would
probably be the fur -- even though nonliving mammalian fur does
play a practical role in thermoregulation. If the animal were an
orb-weaving spider, however, the answer is less straightforward.
Spiders build webs to catch their prey, and without a web many
spiders can neither catch prey nor feed. The web is protein
produced and arranged by, but clearly distinct from, the spider.
Without it, many spiders are not viable entities. For that
matter, the squirrel and laboratory mouse both build nests using
materials wholly external to their bodies. Such nests play a
vital role in these animals' thermoregulation and energy balance.
In the ecological and physiological senses, where is the boundary
of these organisms? The classical work on this topic is Karl von
Frisch's... _Animal Architecture_ [1974]. Richard Dawkins [1982]
argues that animal constructs, and indeed all aspects of
behavior, must be considered part of an organism's phenotype and
thus part of the organism. J. Scott Turner [2000] poses a more
limited but more radical thesis: that these external constructs
in many cases represent the physical manifestations of an
'external physiology' whereby organisms adapt their environment 
to themselves... Richard Lewontin [2000] also argues that
organism modify their environment as much as the environment
modifies them, but his perspective is strictly evolutionary, not
physiological."
-----------
Michael LaBarbera: Fuzzing the boundary of animal life.
(Science 15 Sep 00 289:1882)
-------------------
SCIENCE-WEEK http://scienceweek.com 29Dec00

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8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE QUESTION OF THE DANGERS OF SCIENCE
Lewis Wolpert (University College London, UK), in a "commentary"
article, considers the classic question whether science is
dangerous, the author making the following points:
     1) The idea that knowledge is dangerous is deeply embedded
in our culture. Indeed, Western literature is filled with images
of scientists meddling with nature with disastrous results.
Scientists are portrayed as a soulless group, unconcerned with
ethical issues.
     2) The social obligations that scientists have, as distinct
from those responsibilities they share with all citizens, come
from scientists having access to specialized knowledge of how the
world works, knowledge that is not easily accessible to others.
The obligation of scientists is to make public any social
implications of their work and its technological applications,
and to give some assessment of the reliability of their work. In
most areas of science, it matters little whether a particular
theory is right or wrong, but in some areas, such as human and
plant genetics, it matters a great deal.
     3) The most clear case of immorality in scientific research
was the eugenics movement. The scientific assumptions behind this
movement were crucial: that most human attributes (desirable and
undesirable) are inherited. The scientists concerned completely
failed to give an assessment of the reliability of their ideas or
sufficiently to consider the implications of their ideas. On the
contrary, and even more blameworthy, their conclusions seem to
have been driven by what they saw as desirable social
implications. In contrast, the Allied scientists who built the
atomic bomb behaved morally, and fulfilled their social
obligations by informing their governments about the implications
of atomic theory. The decision to build the bomb was taken by
politicians, not scientists.
     4) The very term "genetic engineering" conjures up the image
of Frankenstein and his monster. The media are aware of this and
often report what can be regarded as genetic pornography --
reports dressed up to titillate and frighten. Newspapers print
sensational and unjustifiable headlines such as the "Frankenstein
foods" idiocy surrounding genetically modified organisms in the
UK.
     5) Bioethics is a growth industry that purports to address
questions concerning the dangers to society posed by biological
science. But one should regard this field with caution, as
bioethicists have a vested interest in finding difficulties.
     6) Are there areas of research that are so socially
sensitive that they should be avoided, even proscribed? Once one
begins to censor the acquisition of objective knowledge, one is
on the most slippery edge of all. Scientists cannot easily
predict the social and technological implications of research, as
is demonstrated by numerous examples in the history of science
and technology.
     7) The author concludes: "National and international
councils that can assess the ethical issues relating to the
applications of science and promote public debate are no doubt
valuable. But one wonders what such a committee would have said
if the public had been offered a convenient form of transport,
but at the cost, in the United Kingdom alone, of more than 3000
lives per year, a quarter of a million injured and the untold
damage of pollution. Where are the car-ethicists?"
-----------
Lewis Wolpert: Is science dangerous?
(Nature 25 Mar 99 398:281)
QY: Lewis Wolpert: l.wolpert@ucl.ac.uk
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 25Jun99
For more information: http://scienceweek.com/swfr.htm


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