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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.
June 9, 2000 -- Vol. 4 Number 23
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There is no place for dogma in science. The scientist
is free to ask any question, to doubt any assertion,
to seek any evidence, to correct any error.
-- J. Robert Oppenheimer (1904-1967)
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1. Biochemistry:
On the Future of Biochemistry
-----------------------------
A biochemist argues that life and the living cell elude
biochemistry, which rarely attains the exactness of chemistry and
physics, and that if it is to keep its promise, biochemistry must
undergo a scientific revolution.
2. Neurobiology:
Optical Probing of Neuronal Circuits
------------------------------------
Researchers report a method using calcium imaging that allows
rapid identification of neuronal connections, and which may be
applied to reconstruct and assay circuits in the mammalian
central nervous system.
3. Medical Biology:
Misregulation of Mitosis and Human Aging
----------------------------------------
An analysis of aging correlates in human genetic profiles
suggests that an altered expression profile of genes involved in
mitosis occurs with age, and that these changes result in
increased rates of somatic mutation, leading to numerical and
structural chromosome aberrations and mutations that produce the
phenotype of aging. (Includes related background material.)
4. Astrophysics:
On the Near Disappearance of the Solar Wind
-------------------------------------------
On 11 May 1999, the particle density of the solar wind
unexpectedly decreased to a remarkably low value, and a special
meeting of astrophysicists was devoted to the phenomenon last
December.
5. Medical Physics:
On Localized Radionuclide Therapy
---------------------------------
The essential idea of localized radionuclide therapy is to
deliver high radiation doses to a tumor but minimal doses to the
surrounding healthy tissue. Radiation sources are implanted
either directly in the tumor or close to it, an advantage over
external beam radiation therapy.
6. Cosmology:
On Cosmic Strings
-----------------
If 10^(9) neutron stars were squeezed into the size of an
electron, the resulting matter-energy density would still hardly
attain the matter-energy density of cosmic strings.
(Includes related background material.)
Focus Report: On Phantom Problems in Science
Erratum: Report #4, 2 June 00
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1. BIOCHEMISTRY:
ON THE FUTURE OF BIOCHEMISTRY
The gist of the usual trumpeting in biochemistry and molecular
biology is that great things have been done in the past, and
great things are being done now, and more great achievements will
be made in the near future. Although much of the trumpeting is
perhaps lifted from research grant proposals (where trumpet-
blowing is often a prerequisite for adequate funding of
research), the fact is the past four or five decades in
biochemistry and molecular biology have indeed been of tremendous
significance for these sciences and for biology and medicine in
general, and important developments will apparently occur in the
near future. On the other hand, there is still much to be done,
biological systems (and most human diseases) are still as complex
as ever, and a sobering focus on realities is always useful.
... ... Hans V. Westerhoff (University of Amsterdam, NL) presents
a commentary on the future of biochemistry, the author making the
following points:
1) The static structures and catalytic mechanisms of
biological macromolecules are becoming increasingly understood,
and the macromolecules that participate in metabolic and signal-
transduction pathways are being identified. However, life and the
living cell elude biochemistry, which rarely attains the
exactness of chemistry and physics. If it is to keep its promise,
biochemistry must undergo a scientific revolution.
2) The author suggests that since the intracellular milieu
differs greatly from that in the test-tube, biochemistry must
enter the living cell and aim at understanding novel functions
arising from nonlinear macromolecular interactions. Living
systems of even minor sophistication require more than 1000
different molecular activities, each activity depending on the
collaboration of hundreds of amino acids. The author suggests
that biochemistry should become a quantitative science,
implementing numerical tools and quantitative experimentation and
concepts.
3) The author suggests that we are far from understanding
living systems in terms of chemistry and physics, and that the
reason for this is that many of the "secrets of life" are not in
the individual macromolecules or their functional domains but in
the interactions between them, "and this is not where our focus
has been." We are also far from understanding living systems
because those interactions are nonlinear: the combined effect of
two interactions does not equal the sum of the effects of the
individual interactions, and the combined effect depends strongly
on the conditions. The author suggests that when studying
processes with an aim to understand the role of these processes
in the living cell, one should either do the experiments in the
living cell, or otherwise account for the interactions. The
author states: "We are far from understanding life because of a
scarcity of methods to perform biochemistry and biophysics inside
the living cell. And we are far from understanding life because
most of us dislike the mathematics required to analyze the
complex interactions."
-----------
Hans V. Westerhoff: Live perspectives of biochemistry.
(The Biochemist April 2000)
QY: Hans V. Westerhoff, Dept. of Mathematical Biochemistry,
University of Amsterdam, NL.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
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2. NEUROBIOLOGY:
OPTICAL PROBING OF NEURONAL CIRCUITS
Since the functioning of any nervous system depends on the
interconnections of nerve cells, one of the important objectives
of neurobiological research is the characterization of neuronal
circuitry. But in nerve cell systems containing thousands or
millions of nerve cells and their interconnections, this is no
easy task, and in most nervous systems, the detailed circuits are
as yet unknown, even when the nature of the computation carried
out by particular circuits is evident.
A direct approach to deciphering neuronal circuits is their
reconstruction with electron microscopy, but this has been
achieved only for the nervous system of the *nematode worm
Caenorhabditis elegans, which nervous system consists of 302
neurons exhibiting a stereotyped connectivity from animal to
animal. For most organisms, electron microscopic reconstructions
of entire circuits are impractical because of the high number of
neurons present and the laboriousness of serial reconstruction
from thin slices of tissue.
Another approach to identify circuits involves intracellular
recordings from connected cells. and this has been done
extensively in invertebrates. In vertebrate preparations, dual
recordings of randomly chosen neurons in *brain slices have been
combined with anatomical reconstruction to identify *synaptic
contacts. But this approach suffers from the problem that the
probability that randomly chosen neurons are connected is low.
This, and the large number of neuronal types, makes testing of
possible connections and sequential examination of circuits
impractical.
... ... Z.A. Peterlin et al (5 authors at Columbia University,
US) now report a method using calcium imaging that allows rapid
identification of neuronal connections. The method consists of
stimulating one neuron (the "trigger neuron") while imaging a
population of nerve cells to detect which other neurons
("follower neurons") are activated by the trigger. By using a
bulk-loaded fluorescent calcium indicator in slices of mouse
visual cortex, the authors report they have demonstrated that
neurons that display *somatic calcium transients time-locked to
the spikes of a trigger neuron are directly connected to the
trigger neuron. The bulk-loaded fluorescent calcium indicator was
fura-2 acetoxymethyl ester, a previously established calcium
indicator in nerve cells. The authors suggest this technique
could be applied to reconstruct and assay circuits in the
mammalian central nervous system.
-----------
Z.A. Peterlin: Optical probing of neuronal circuits with calcium
indicators.
(Proc. Natl. Acad. Sci. US 28 Mar 00 97:3619)
QY: Rafael Yuste [rmy5@columbia.edu]
-----------
Text Notes:
... ... *nematode worm: An abundant and ubiquitous phylum of
unsegmented roundworms.
... ... *brain slices: "Brain slices" are exactly that, the brain
removed from the animal and a thin slice of a particular region
prepared in an appropriate solution for electrophysiological
recording of nerve cell activity.
... ... *synaptic contacts: The junction between the terminal of
the axon of one neuron and another neuron is called a "synapse".
... ... *somatic calcium transients: The main part of the nerve
cell, the central part from which projections arise, is called
the "soma". When a nerve cell is activated, the permeability of
the surface membrane to various ions undergoes transient changes.
If, for example, the permeability of the surface membrane to
calcium ion undergoes a transient increase, the result is a
transient calcium ion current, increasing the intracellular
concentration of calcium. In this context, a transient increase
of intracellular calcium ion produces fluorescence of a calcium
ion indicator.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
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3. MEDICAL BIOLOGY:
MISREGULATION OF MITOSIS AND HUMAN AGING
The idea that human aging is controlled by genes is not the
end but the beginning of research: Does the aging process involve
one gene, or a fixed group of specific genes, or groups of
various genes at different times and in different individuals?
The term "messenger RNA" refers to the ribonucleic acid
molecule transcribed from DNA that carries the coded information
specifying the sequence of amino acids in a protein. Since the
various messenger RNAs present in any cell can be isolated and
identified, the population of present messenger RNAs can be used
to establish a profile of the active genes of that cell.
Fibroblasts are a type of connective tissue cell secreting
structural proteins such as collagen, the proteins forming a
matrix in which the fibroblasts become embedded. These cells can
be easily obtained from skin, and they can be easily cultured
outside the body.
Progeria (Hutchinson-Gilford disease; Hutchinson-Gilford
syndrome; premature senility syndrome) is a condition of
precocious aging, with onset at birth or early childhood, the
condition characterized by growth retardation, a senile
appearance with dry wrinkled skin, early occurrence of
*atherosclerosis in blood vessels, and premature death (usually
before the age of 20) due to coronary artery disease. The disease
is apparently genetic, but the details of the etiology are not
clear.
In this context, the term "phenotype" refers to the specific
individuality of an organism as determined by the interaction
during development between its genetic constitution (genotype)
and the environment.
In this context, the term "mitosis" refers to the entire
cell division phase of the cell cycle, with "cell cycle"
referring to the entire life history of a single cell from
mitosis to mitosis, including the sequence of intervening phases.
In this context, the term "postmitotic cells" refers to cells
that normally do not undergo mitosis once they have fully
differentiated (e.g., neurons and muscle cells).
... ... D.H. Ly et al (4 authors at 2 installations, US) report
an analysis of aging correlates in human genetic profiles, the
authors making the following points:
1) The authors measured messenger RNA levels in actively
dividing fibroblasts isolated from young, middle-age, and old-age
humans and humans with progeria. Messenger RNA levels were
analyzed with high-density *oligonucleotide arrays containing
probes for more than 6000 known human genes.
2) The authors report their results suggest that an altered
expression profile of genes involved in mitosis occurs with age,
and that these changes result in increased rates of *somatic
mutation, leading to numerical and structural *chromosome
aberrations and mutations that manifest themselves as an aging
phenotype.
3) The authors suggest that these chromosome pathologies,
which begin to occur in dividing cells relatively early in life
(but in the postreproductive stage), may lead to misregulation of
key structural, signaling, and metabolic genes associated with
the aging phenotype, such as the apparent misregulations
characteristic of *osteoporosis, *Alzheimer's disease,
*arthritis, etc. Misregulation of this sort is expected to
increase in each round of cell division, and it may be propagated
to other normal mitotic cells (e.g., *leukocytes, *epithelial
cells, *glial cells, etc.) and postmitotic cells (e.g., neurons,
muscles, etc.) through changes in the *extracellular matrix and
oxidized fatty acid derivatives that affect signaling pathways.
Aging, the authors suggest, may therefore occur gradually and in
mosaic patterns, rather than as a uniform phenomenon
characteristic of cancerous growth (which is clonal -- deriving
from a single mutated progenitor cell)
4) The authors conclude: "Additional studies are required
before we can understand the aging process in complex organisms,
both in mitotic and postmitotic tissue, but the studies reported
here highlight important mechanisms that may contribute to aging
and age-related problems."
-----------
D.H. Ly et al: Mitotic misregulation and human aging.
(Science 31 Mar 00 287:2486)
QY: Richard A. Lerner, Scripps Research Institute 619-784-1000.
-----------
Text Notes:
... ... *atherosclerosis: "Arteriosclerosis" is a generic term
for several diseases in which the arterial wall becomes thickened
and loses elasticity, and "atherosclerosis" is a form of
arteriosclerosis characterized by patchy thickening (atheroma) in
the subintimal layer (i.e., immediately below the innermost layer
[intima]) of medium and large arteries, the thickening capable of
reducing or obstructing blood flow.
... ... *oligonucleotide arrays: (DNA microarrays) DNA
microarrays are chips containing hundreds or thousands of gene
snippets laid out in precise arrays that provide rapid snapshots
of the expression of whole suites of genes. The general method in
microarray analysis is to a) isolate messenger RNAs (mRNAs)
produced by a genome; b) convert mRNA into complementary DNA
(cDNA); c) add a fluorescent tag to the cDNA for tracking
purposes; d) wash a solution of tagged cDNAs over a DNA
microarray chip. Each DNA snippet on the chip will bind the cDNA
from the corresponding gene, and by measuring the fluorescences
arrayed on the chip, the profile of gene expression is revealed.
... ... *somatic mutation: In general, a mutation occurring in
non-germ cells, which means the mutation is not transmitted to
the next generation of individuals (but is transmitted to the
next generation of cells of that type).
... ... *chromosome: In cells with chromosomes, the
chromosomes are the physical structures into which DNA is
organized and on which genes are carried.
... ... *osteoporosis: A generalized progressive diminution of
bone density (bone mass per unit volume) that causes skeletal
weakness. The ratio of mineral to organic elements is unchanged.
The major clinical manifestations of osteoporosis are bone
fractures resulting from a reduction below the fracture threshold
of the amount of bone available for mechanical support.
... ... *Alzheimer's disease: There are various forms of dementia
produced by various causes. Alzheimer-type dementia (Alzheimer's
disease) is apparently related to what appear to be specific
cellular and histological degenerative processes, with loss of
cells from several specific brain areas, the brain showing
moderate to marked atrophy. Memory loss is the most prominent
early symptom.
... ... *arthritis: In general, inflammation of a joint or a
state characterized by inflammation of joints.
... ... *leukocytes: White blood cells, of which there are
various types.
... ... *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.
... ... *glial cells: Cells of the central and peripheral nervous
system that metabolically support neurons. Such cells also
produce the multiple membrane layers called myelin and enfold
certain nerve cell axons with it.
... ... *extracellular matrix: In general, the extracellular
matrix is a layer consisting mainly of proteins and
glycosaminoglycans that form a sheet underlying endothelial and
epithelial cells. The molecular constituents of the matrix are
secreted by cells in the vicinity. Endothelial cells are the
cells that line blood vessels.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON NEW APPROACHES TO HUMAN AGING
It is probably safe to say that research on the biological basis
of human aging will remain a central interest in the sciences as
long as there is any research at all. However, despite the
general public attention to the subject, it is only during the
past few decades that cell biologists have begun a vigorous
attempt to understand the aging process. A major reason for this
is that the new knowledge provided during the past 40 years by
molecular biology concerning fundamental cellular processes
suggests that an understanding of the biological basis of human
aging is indeed possible. The cell biologist Leonard Hayflick
(University of California San Francisco, US) first demonstrated
in the 1960s that the mortality of biological cells is either
directly or indirectly programmed in each cell, and that this is
an important aspect of the aging of human tissues (see related
background material below). Hayflick now presents a provocative
essay on the subject, the author making the following points:
1) The author points out that in the past 100 years life
expectancy at birth in developed countries has increased from
approximately 48 years to 76 years, the same gain that occurred
over the previous 1900 years. But this progress has neither
advanced nor resulted from our understanding of aging. Instead,
it is the control of infectious diseases of the young that
explains the increase in life expectancy during the 20th century.
2) The author suggests that the failure to distinguish
between the diseases of old age and the aging process is
widespread even in the scientific community. The virtual
resolution of various childhood diseases such as poliomyelitis
and iron-deficiency anemia did not increase our knowledge of
childhood development. Similarly, the resolution of the leading
causes of death in old age -- cardiovascular disease, stroke, and
cancer -- are unlikely to advance our knowledge of the aging
process.
3) The author suggests that one example of the consequences
for science policy of the failure to distinguish research on age-
associated diseases from research on the fundamental biology of
aging is that "it is virtually impossible to raise funds for
research on aging, because in the minds of policy-makers and the
public no one suffers or dies from it." More than half of the
budget of the US National Institute on Aging is spent on
Alzheimer's disease, yet the elimination of this disease "will
have only a trivial impact on life expectancy and will not
advance our knowledge of the fundamental biology of aging." The
author suggests that greater attention must be given to a
question that is rarely posed: Why are old cells more vulnerable
to disease than young cells?
4) The author concludes: "The resolution of all causes of
death currently written on the death certificates of those older
than 65 will result in an increase in life expectancy of only
about 15 years. An increase in our knowledge of how age changes
occur does not put a 15-year limit on what is possible."
-----------
Leonard Hayflick: New Approaches to Old Age.
(Nature 27 Jan 00 403:365)
QY: Leonard Hayflick, Univ. of Calif. San Francisco 415-476-4044.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 14Apr00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CELLULAR AGING: DONOR AGE AND CELLULAR REPLICATION LIFESPAN
Fibroblasts are a type of connective tissue cell, secreting
structural proteins such as collagen, the proteins forming a
matrix in which the fibroblasts become embedded. These cells can
be easily obtained from skin, and they can be easily cultured
outside the body. Normal human fibroblasts have a finite
replicative lifespan in vitro (i.e., they divide a finite number
of times), and this has been postulated to be a cellular
manifestation of aging of the human organism. Several studies
have indeed shown an inverse relationship between donor age (the
age of the persons from which cultured cells are derived) and
fibroblast culture replicative lifespan. But in all cases the
correlation was weak, and with few exceptions the health status
of the donors was unknown. Thus, the relationship between the
replicative lifespan and the age of the donor from which the
cells are derived has remained equivocal (*Note #1).
... ... V.J. Cristofalo et al (5 authors at 2 installations, US)
now report a study of the replicative lifespan of 124 skin
fibroblast cell lines established from donors of different ages.
All donors were medically examined and were declared "healthy"
(according to Baltimore Longitudinal Study of Aging protocols) at
the time the biopsies were taken. The authors report that both
long- and short-lived cell lines were observed in all age groups,
but no significant correlation between the proliferative
potential of the cell lines and donor age was found. A comparison
of multiple cell lines established from the same donors at
different ages also failed to reveal any significant trends
between proliferative potential and donor age. The authors
suggest their results clearly indicate that if health status and
biopsy conditions are controlled, the replicative lifespan of
fibroblasts in culture does not correlate with donor age.
-----------
V.J. Cristofalo et al: Relationship between donor age and the
replicative lifespan of human cells in culture: A reevaluation.
(Proc. Natl. Acad. Sci. US 1 Sep 98 95:10614)
QY: Vincent J. Cristofalo, Center for Gerontological Research,
Alleghany University of the Health Sciences, Philadelphia, PA
19129 US.
-----------
Text Notes:
... ... *Note #1: The possibility that the process of cell aging
and death is under genetic control was first suggested by Leonard
Hayflick in 1961. Hayflick reported that normal human fibroblasts
apparently have an intrinsic limit to the number of times they
can proliferate, with human fibroblasts removed from an embryo
and grown in culture dividing approximately 50 times before they
deteriorate and die. In contrast, human fibroblasts removed from
adults multiply only 15 to 30 times before dying. Also,
fibroblasts removed from young children suffering from Werner's
syndrome (a rare disease that causes premature aging) divide only
2 to 10 times in culture. Further evidence for a relationship
between aging and the replicative capacity of cells was provided
by the discovery that the number of replications in culture is
apparently related to the lifespan of organism. For example,
cultured cells of the Galapagos tortoise, whose maximum life span
is approximately 175 years, divide more than 100 times in
culture, whereas cells from the mouse, whose maximum life
expectancy is only a few years, divide fewer than 30 times in
culture. The correlation roughly holds for other species as well.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 16Oct98
-------------------
Related Background:
BIOLOGY OF AGING: ON TELOMERES AND REPLICATIVE SENESCENCE
Telomeres are defined ends of chromosomes that contain specific
repeated DNA sequences. They are essential for normal chromosome
replication, and since their length shortens a bit with each
replication, they are believed to be involved in the aging of the
cell. Telomerase is an enzyme that repairs damage to telomeres,
and it is thought by some that cancerous cells may have mutant
telomerase, the mutant enzyme conferring immortality on the
cancer cell. ... ... In a review of cell senescence, the aging of
cell cultures, and the immortalization of mammalian cells, John
M. Sedivy (Brown University, US) makes the following points: 1)
Finite replicative lifespan (senescence) of mammalian cells in
culture is a phenomenon that has generated much curiosity since
its description. The obvious significance of senescence to
organismal aging and the development of cancer has engendered a
long-lasting and lively debate about its mechanisms. 2) Three
classical observations are usually cited to argue that in vitro
replicative senescence is a phenomenon with biological
significance: a) the correlation of in vitro lifespan with the
age of the donor; b) the correlation of in vitro lifespan with
the average life expectancy of species; and, c) the reduced in
vitro lifespan of cells from patients afflicted with premature
aging syndromes. 3) Two major theories have been used to explain
limited replicative capacity. The first hypothesis invokes the
gradual accumulation of mutations, and the second hypothesis
invokes the existence of a molecular clock (or clocks) that can
keep track of cell divisions. The second theory is now believed
to be generally true. 4) It is known that cell senescence can be
overcome, because many cell lines in common laboratory use are
quite obviously immortal. Rodent cells can overcome senescence
spontaneously. Human, chicken, bovine, and horse cells rarely, if
ever, immortalize spontaneously. 5) Certain viral or biochemical
interventions in human cell cultures can overcome cell
senescence, typically by causing 20 to 30 extra population
doublings. At the end of this extended lifespan, there is a
decline and death of the culture in 4 to 6 weeks, which has been
termed "crisis". Senescent cells, on the other hand, can be
maintained in vitro in a viable non-proliferative state for very
long periods of time (reports of from 4 to 6 months, and up to 2
years). 6) The author suggests it is amazing that in spite of
very long periods of apparent "immortality", the senescent
program in cells remains intact in cells in which senescence has
been overridden, so that on removal of the overriding agent, the
program is capable of establishing rapid growth arrest. 7) The
current prevailing hypothesis for the nature of the molecular
clock involved in cell senescence is the attrition of telomeres.
*Germ cells, and some key *stem cells, are known to express
telomerase catalytic activity, whereas the majority of somatic
cells lack it. Murine (mouse) embryonic stem cells express
telomerase and are functionally immortal, and elimination of
telomerase eventually results in loss of proliferation. 8) The
author proposes that immortalization of human cells requires a
bypass of both cell senescence and crisis, whereas in rodent
cells cell crisis does not exist and culture lifespan is limited
only by senescence. 9) Evidence indicates that, at least in human
cells, telomere length appears to be linked critically to the
triggering of senescence. The author suggests that although it
remains to be rigorously demonstrated, this strongly implies that
activation of telomerase can result in one-step immortalization.
In conclusion, the author states the two most significant
questions in this field: a) Does cell senescence limit organismal
lifespan? And, b) Is telomerase expression necessary for cancer
progression in vivo?
QY: John M. Sedivy [john_sedivy@brown.edu]
(Proc. Natl. Acad. Sci. US 4 Aug 98 95:9078)
(Science-Week 4 Sep 98)
-----------
Text Notes:
... ... *Germ cells: Any cell from which gametes (sperm cells and
egg cells) are derived. All other cells are called "somatic"
cells.
... ... *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.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 4Sep98
For more information: http://scienceweek.com/swfr.htm
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4. ASTROPHYSICS:
ON THE NEAR DISAPPEARANCE OF THE SOLAR WIND
The surface of the Sun and the regions immediately exterior
to it constitute a domain in which various forces are played out
on an immense scale. The bright surface layer of the Sun is
called the "photosphere", a region a few hundred kilometers thick
at a temperature that ranges from 5770 degrees kelvin at its
innermost part to 4400 degrees kelvin at its outermost part, the
Sun's temperature minimum. Above this is the "chromosphere",
approximately 9000 kilometers thick, where the temperature ranges
from the minimum at the photosphere-chromosphere interface to
approximately 20,000 degrees kelvin. And above the chromosphere
is the "corona", the Sun's faint outer atmosphere, where the
temperature is 2 million degrees kelvin or more.
The solar wind is the steady flow of charged particles,
consisting primarily of protons and electrons, from the solar
corona into interplanetary space. The solar-wind particles have
energies high enough to enable the particles to escape the Sun's
gravitational field, but the wind is influenced by the Sun's
magnetic field, and the particles can be trapped by planetary
magnetic fields.
On 11 May 1999, the particle density of the solar wind
unexpectedly decreased to a remarkably low value -- approximately
0.2 particles per cubic centimeter, compared with a normal value
of 10 particles per cubic centimeter, and a special meeting of
the American Geophysical Union was devoted to the phenomenon last
December.
... ... Alan J. Lazarus (Massachusetts Institute of Technology,
US) presents an account of the special meeting, the author making
the following points:
1) Other instances of low-density solar wind are known, but
this period of more than 27 hours on 11 May 1999 was the longest
known period having a density below 1 particle per cubic
centimeter. At 360 kilometers per second, the speed of the wind
was near its typical value of approximately 400 kilometers per
second, but the pressure exerted by the wind was so low that the
shock front formed by the interaction between the incoming
supersonic wind and the magnetic field of Earth moved outward
from its usual location (approximately 15 Earth-radii in front of
Earth as measured along the Earth-Sun line) to at least 60 Earth-
radii and near the Moon.
2) Normally, the completely ionized solar wind *plasma
compresses the dayside magnetic field of Earth because of the
relatively high conductivity of the wind. The resulting pressure
flattens the magnetic field on the sunward side and drags it out
on the night side into a tail many Earth-radii long. On 11 May
1999, the unusually low pressure resulting from the low-density
wind allowed the magnetic field of Earth to reassert its dipolar
shape over a larger volume.
3) The term "strahl" refers to a particular region of the
solar wind electron velocity distribution, the region forming a
beam that streams in a narrow cone along the magnetic field
lines. On 11 May 1999, the strahl dominated the electron velocity
distribution. Because the solar wind density was so low, the
strahl electrons were relatively unscattered by collisions in the
solar wind, and they arrived near Earth in an unusually intense
and narrow beam that penetrated into the north polar region. The
electron collisions with the atmosphere of Earth over the north
polar region generated the strongest x-ray emissions ever seen
from the polar cap.
4) Why periods of very low density solar wind occur remains
unknown, but such low wind flux periods tend to appear on the
ascending portion of the *solar activity cycle, which is the
period we are in now. The author concludes: "Discussion of low
solar wind flux periods will undoubtedly occupy solar
astrophysicists for years to come."
-----------
Alan J. Lazarus: The day the solar wind almost disappeared.
(Science 24 Mar 00 287:2172)
QY: Alan J. Lazarus [ajl@space.mit.edu]
-----------
Text Notes:
... ... *plasma: In general, in this context, a "plasma" is a
fully ionized gas consisting of ions and electrons moving freely.
... ... *solar activity cycle: The term "solar activity" refers
to all active phenomena on the surface of the Sun, including
sunspots, active prominences, flares, active regions, etc. In
general, solar activity is strongly associated with magnetic
fields which are believed to arise from a dynamo action within
the Sun. Solar activity increases and decreases in a cycle
lasting approximately 11 years.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
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5. MEDICAL PHYSICS:
ON LOCALIZED RADIONUCLIDE THERAPY
The term "radionuclide" refers to any natural or artificial
isotope that exhibits radioactivity, and radionuclide therapy is
the application of such radioactivity to the treatment of
disease, in particular the use of ionizing radiation to kill
malignant biological cells.
The precise mechanism by which ionizing radiation kills
biological cells remains uncertain, although the consensus view
is that electrons dislodged from water or biological molecules
disrupt the bonds between atoms in DNA, resulting in double-
strand DNA breaks, and these breaks in turn kill the cell when it
attempts to divide.
Each year in the US, approximately 200,000 patients receive
some form of therapy with radionuclides, most commonly in the
form of sealed sources introduced locally for the treatment of
gynecological and head and neck cancers, and radiopharmaceuticals
for the treatment of thyroid cancer. The essential idea of this
localized therapy ("brachytherapy"; from the Greek brachys =
near) is to deliver high radiation doses to a tumor but minimal
doses to the surrounding healthy tissue. Radiation sources are
implanted either directly in the tumor or close to it, an
advantage over external beam radiation therapy, in which the
source of radiation is usually approximately 1 meter away from
the patient.
... ... B.M. Coursey and R. Nath (2 installations, US) present a
review of radionuclide brachytherapy, the authors making the
following points:
1) Radionuclides were first used in therapy nearly a century
ago when it was noticed that radium sources brought into
prolonged contact with skin produced burns, and clinicians
devised radiation treatments for surface lesions. By 1915,
therapy with sealed sources of radium-226 or radon-222 had become
widely available. However, following the use of atomic weapons in
the Second World War, there were increasing concerns about the
risks of radiation exposure to medical personnel, and medical
professionals for the most part lost interest in radionuclide
brachytherapy.
2) In the 1950s, remote radiation source-handling with
robotics were introduced and this dramatically reduced medical
personnel exposure. Also at approximately the same time, several
new nuclear reactor-produced radionuclides with better radiation
safety characteristics became available. For example, when
supplies of cobalt-60 (which has a high specific activity, a
half-life of 5.27 years, and which produces high-energy gamma
rays) became available in the 1960s, external beam radiation
therapy with cobalt-60 quickly supplanted external beam radiation
therapy with the lower energy x-ray tubes then in use. Also in
the 1960s, the fission product cesium-137 became available as a
safer alternative to radium-226 for radionuclide brachytherapy,
and cesium-137 is still in active use for treatment of
gynecological cancers.
3) Although other agents that kill biological cells
(cytotoxic agents) exist, radionuclide therapy is in general the
most effective method of controlling the proliferation of
biological cells without unacceptable effects on non-target
tissues, and for many types of cancers radionuclide therapy is
the treatment of choice.
4) Radiation apparently kills cells by damaging cellular DNA
and thereby inhibiting cellular reproduction. To damage DNA, the
energy of the radiation (in the form of photons, electrons, or
heavier charged particles) must exceed 20 or 30 electronvolts.
But if the radiation is delivered from outside the body, as in
external beam radiotherapy, photon energies of several million
electronvolts are required simply to penetrate the tissues and
reach any deep-seated tumor in the body. In contrast,
brachytherapy implants can be successfully performed with
radionuclides that emit photons with energies as low as 20,000
electronvolts. For example, palladium-103, which is used for
prostate implantation, produces radiation with an average energy
of only 21,000 electronvolts. In addition to implanted radiation
sources, radiopharmaceutical therapies also allow a radionuclide
to deliver its decay energy close to or even inside target cells.
In general, localized targeted radionuclide therapy will continue
to have important applications in clinical medicine.
-----------
B.M. Coursey and R. Nath: Radionuclide therapy.
(Physics Today April 2000)
QY: Bert Coursey, National Institute of Standards and Technology,
Gaithersburg, MD US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
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6. COSMOLOGY:
ON COSMIC STRINGS
Theoretical particle physics and theoretical cosmology appear to
lie at two extremes of human thought, but in fact they have much
that binds them together. In both cases, the work of the
theoretician is essentially to pose the same question: What set
of assumptions concerning the real world, subject to appropriate
quantitative analytic methods, will yield quantitative
predictions in accord with observations of the real world? And in
both cases, theoreticians have produced a conception called
"string theory" that has energized both research communities.
In this context, a "string" is a one-dimensional object,
appearing in theories of elementary particles and in theories of
cosmology (cosmic strings). In general, string theory replaces
the idea of a point-like elementary particle (which is used, for
example, in quantum field theory) by a line or a closed string (a
loop), the replacement such that various states of a particle may
be represented by particular wave attributes along the string.
... ... Alejandro Gangui (Observatoire de Paris-Meudon, FR)
presents a review of current research on cosmic strings, the
author making the following points:
1) Current theories of particle physics predict that a
variety of topological defects would almost certainly have formed
during the early evolution of the Universe. The reason is that
the interactions between elementary particles would pass through
distinct phases as the typical energy of the particles decreased
with the expansion of the Universe. New phases would appear in
many places simultaneously, and topological defects would arise
at the borders between these phases. The detection of such
structures in the modern Universe would provide important
information on the events in the earliest instants after the *Big
Bang. On the other hand, an absence of such structures would
force a major revision of current physical theories.
2) As cosmological theorists search for mechanisms that
might have generated the features of the Universe as we know it,
few of their proposals can actually be modeled in the laboratory.
But topological defects can indeed be modeled, and among such
defects the most thoroughly studied are the phenomena called
"cosmic strings".
3) As imagined by physicists, cosmic strings are exceedingly
narrow filaments of primordial material, strands of a trapped
unconverted phase left over from the early moments of cosmic
history. These strings have extraordinary energies (and masses),
move at velocities approaching that of light, and curve space
around themselves. The idea is that through their gravitational
interactions these cosmic strings can draw matter together to
form large-scale structures such as galaxies or clusters of
galaxies, and push matter around to give such structures bulk
drift velocities.
4) Realistic particle-physics models predict that at a
certain stage of their evolution cosmic strings can develop
enormous electric currents, effectively becoming superconducting
wires of astrophysical dimensions. The idea is that such currents
would generate primordial magnetic fields that could give rise to
the currently observed magnetic fields in galaxies. Such electric
currents would also stabilize small loops of cosmic string
against decay, and these loops might form the so-called and yet
undetected "*non-baryonic dark matter" that is thought to occupy
much of the modern Universe.
5) It is believed that the cores of cosmic strings are
extremely small, of the order of 10^(-30) centimeters across, but
their masses are extremely high. If 10^(9) *neutron stars were
squeezed into the size of an electron, the resulting matter-
energy density would still hardly attain the matter-energy
density of cosmic strings.
-----------
Alejandro Gangui: Superconducting cosmic strings.
(American Scientist May-Jun 00 88:254)
QY: Alejandro Gangui [gangui@obspm.fr]
-----------
Text Notes:
... ... *Big Bang: George Lemaitre (1894-1966) proposed that the
expansion of the Universe implied a dense explosive birth of the
Universe at a specific finite time in the past (the event that
came to be called the "Big Bang").
... ... *non-baryonic dark matter: The term "baryonic dark
matter" refers to ordinary matter too dim to be
observed. A baryon is a nuclear particle, e.g., a proton, built
from 3 quarks (fundamental particles that combine to make up
protons, neutrons, and mesons). Non-baryonic dark matter consists
of exotic particles such as "*axions", *neutrinos with mass, or
*weakly interacting massive particles.
... ... *axions: An axion is a hypothetical elementary particle
of very low mass and zero charge, and one of the candidates for
dark matter in the Universe.
... ... *neutrinos: A neutrino is an electrically neutral
elementary particle until recently considered to be always
without mass.
... ... *weakly interacting massive particles: (WIMPS) This
refers to a hypothetical elementary particle that is a candidate
for cosmic dark matter, a stable neutral particle, somewhat
heavier than the neutron, that interacts only weakly with
ordinary matter.
... ... *neutron stars: In an ordinary star, the radiation
pressure produced by the burning of the core of the star is
enough to counteract the inward gravitational force and keep the
star from collapsing; in a star that has exhausted its fuel, such
radiation pressure is severely reduced and the star begins a
gravitational collapse due to its enormous mass. But how far will
the collapse proceed? If, following its terminal stages, the
remnant mass of a star is between 1.4 and 2 to 3 solar masses,
the star will collapse into a neutron star, a body with a radius
of 10 to 15 kilometers, with a core so dense that its component
protons and electrons have merged into neutrons. The average
density of a neutron star is 10^(15) grams per cubic centimeter,
and the weight of an object on the surface of a neutron star
would be 10^(11) its weight on the surface of the Earth. Neutron
stars apparently have an outer shell of iron, but it is iron like
no Earth iron, an iron of 4 orders of magnitude greater density.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 9Jun00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
THEORETICAL PHYSICS: ON STRING THEORY
... ... B.R. Greene et al present a short review of recent
developments in string theory and make the following points: 1)
Particle physics has spent much of this century grappling with
one basic question in various forms: What are the fundamental
*degrees of freedom needed to describe nature, and what are the
laws that govern their dynamics. 2) The current "Standard Model"
of particle physics -- which is nearly 25 years old and which has
much experimental evidence in its favor -- involves 6 *quarks, 6
*leptons, 4 *forces, and the as yet unobserved *Higgs boson. But
this model contains internal indications that it too may be just
another step along the path of uncovering the truly fundamental
degrees of freedom. The Standard Model is valid to distances as
small as 10^(-16) cm, and there is some evidence that the next
level of structure will be detected only at a distance scale of
the order of 10^(-32) cm, which is far beyond our abilities to
measure in the laboratory. 3) A related important issue concerns
the unification of general relativity and quantum mechanics. A
serious problem arises when general relativity is extrapolated to
small distance scales of the order of 10^(-32) cm where quantum
effects must be taken into account: the relevant theoretical
equations produce uncontrollable divergences, and the history of
particle physics suggests this is an indication of a new physics
occurring at these distance scales. 4) String theory offers hope
of addressing both of these issues. There is only one known way
to cure the divergence problem in the quantum-mechanical
expansion of general relativity, and that is to model the
particles in the theory not as points but as one-dimensional
loops of "string". Every consistent such string model necessarily
contains a special kind of particle -- the "*graviton" -- whose
long-distance interactions are described by general relativity.
So in a sense, string theory predicts gravity. 5) An exciting new
frontier was opened during the past few years with the discovery
of "string duality", which predicts equivalences among various
different physical systems. This discovery has its roots in the
properties of "supersymmetry", a novel type of symmetry that all
consistent string theories possess. Briefly, supersymmetry
relates properties of two basic types of particles -- bosons and
*fermions -- which cannot be related by ordinary symmetry. There
is a current belief that supersymmetry will play a role in the
structure of particle physics beyond the Standard Model. One of
the important achievements of string duality has been the
determination of the behavior of the 5 consistent string theories
when interactions become strong. All the consistent string
theories are apparently related to each other, and to an
elaboration known as "membrane theory" (M-theory). String duality
has produced hope that there may be only one possible string-
theoretic model of the universe, and that it may be possible to
eventually predict such features as particle masses and
interaction strengths directly from such a theory. The authors
conclude: "Development has been rapid on many fronts since string
duality was introduced. We may be seeing glimpses of the
underlying principle manifested in these new results. The
challenging task that lies ahead is to discover that principle
and thereby find what may well be the truly fundamental degrees
of freedom in our universe."
-----------
B.R. Greene et al (3 authors at 3 installations, US)
String theory.
(Proc. Natl. Acad. Sci. US 15 Sep 98 95:11039)
QY: David R. Morrison [drm@math.duke.edu]
-----------
Text Notes:
... ... *degrees of freedom: In general, the number of
independent parameters required to specify the configuration of a
system.
... ... *quarks: A quark is a hypothetical fundamental particle,
having charges whose magnitudes are one-third or two-thirds of
the electron charge, and from which the elementary particles may
in theory be constructed.
... ... *leptons: A class of elementary particles. Although they
are affected by electromagnetic and gravitational forces, apart
from that they are involved only with weak interactions, acted
upon by weak forces but not by strong forces, as opposed to
quarks, which are acted upon by strong forces but not by weak
forces. One further difference between leptons and quarks is that
leptons can be isolated as single particles, whereas quarks
apparently cannot. The leptons include the electron, the muon,
the tau, and their associated neutrinos. The mass of the tau is
approximately 3484 times the mass of the electron; the mass of
the muon is intermediate.
... ... *forces: (fundamental forces) According to the *Standard
Model, the fundamental forces comprise the gravitational force,
the electromagnetic force, the nuclear strong force, and the
nuclear weak force.
... ... *Standard Model: In particle physics, the Standard Model
is a theoretical framework whose basic idea is that all the
visible matter in the universe can be described in terms of the
elementary particles leptons and quarks and the forces acting
between them.
... ... *Higgs boson: Higgs fields (named after Peter W. Higgs,
University of Edinburgh, UK) constitute a set of fundamental
theoretical fields that induce spontaneous symmetry breaking. In
general, spontaneous symmetry breaking occurs in systems whose
underlying symmetry state is unstable. A Higgs particle is
associated with a Higgs field in the same way that a photon is
associated with the electromagnetic field. Higgs bosons are
massive mesons whose existence is predicted by certain theories.
Mesons are apparently composed of quark and anti-quark pairs;
they are produced by various high-energy interactions and decay
into stable particles.
... ... *graviton: Several quantum field theories consistent with
both quantum mechanics and special relativity postulate that the
gravitational force between two quantum domain particles is
generated by the exchange of an intermediate particle called a
graviton.
... ... *fermions: Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 16Oct98
-------------------
Related Background:
ON RECENT DEVELOPMENTS IN SUPERSTRING THEORY
Bose-Einstein statistics is the statistical mechanics of a system
of indistinguishable particles for which there is no restriction
on the number of particles that may simultaneously exist in the
same quantum energy state. Bosons are particles that obey Bose-
Einstein statistics, and they include photons, pi mesons, all
nuclei having an even number of particles, and all particles with
integer spin. Fermions (electrons, protons, neutrons) are
particles that obey the Pauli exclusion principle: i.e., no two
fermions of the same kind can occupy the same quantum state.
In particle physics, string theory is a theory of elementary
particles based on the idea that the fundamental entities are not
point-like particles but finite lines (strings), or closed loops
formed by strings, the strings one-dimensional curves with zero
thickness and lengths (or loop diameters) of the order of the
Planck length of 10^(-35) meters. The fundamental forces comprise
the gravitational force, the electromagnetic force, the nuclear
strong force, and the nuclear weak force, and the "grand unified
theories" are theories that aim to provide a mathematical frame-
work in which the electromagnetic forces, strong forces, and weak
forces emerge as parts of a single unified force, with the three
forces related by symmetry. Supersymmetry is an aspect of an
extension of the grand unified theories, an attempt to unify all
the four fundamental forces, i.e., linking gravitation to the
electromagnetic force, the strong force, and the weak force
through a supersymmetry scheme, and superstrings are strings in
this scheme that obey supersymmetry. ... ... John H. Schwarz
(California Institute of Technology, US) presents a brief
overview of some of the advances in understanding super-
string theory that have been achieved in the last few years.
String theories that have a symmetry relating bosons and
fermions, called "supersymmetry", are called "superstring"
theories. Major advances in understanding of the physical world
have been achieved during the past century by focusing on
apparent contradictions between well-established theoretical
structures. In each case the reconciliation required a better
theory, often involving radical new concepts and striking
experimental predictions. Four major advances of this type were
the discoveries of special relativity, quantum mechanics, general
relativity, and quantum field theory. This was quite an
achievement for one century, but there is one fundamental
contradiction that still needs to be resolved, namely the clash
between general relativity and quantum field theory. Many
theoretical physicists are convinced that superstring theory will
provide the answer.
QY: John H. Schwarz, Calif. Inst. of Technology 818-395-6811.
(Proc. Natl. Acad. Sci. US 17 Mar 98)
(Science-Week 10 Apr 98)
-------------------
Related Background:
ON THE EVOLUTION OF STRING THEORY TO MEMBRANE THEORY
... Membrane theory (M-theory) is a recent extension of string
theory in which the fundamental physical entities are considered
as surfaces in a many-dimensional space (membranes) rather than
as lines or loop elements (open or closed strings). Given all of
the above, some caution is necessary: the translation of a highly
abstract mathematical model of physical reality into
non-mathematical language is often an exercise of limited
usefulness, and in this case in particular, we are presenting
only the ghost of the theoretical scheme. String theory was
originally invented in the 1960s as a theory of the strong force,
became overshadowed by the strong force theory of gluons and
quarks, then had a revival in the 1980s -- but with the history
more dependent on new work than on fashion. ... ... M. Duff
(Texas A & M Univ., US), who is active in string theory and
membrane theory, in a review of various aspects of the history
and essentials of string theory and membrane theory, suggests
that future historians may judge the 20th century as "a time when
theorists were like children playing on the seashore, diverting
themselves with the smoother pebbles or prettier shells of
superstrings, while the great ocean of M-theory lay undiscovered
before them."
QY: Michael J. Duff, Texas A & M Univ. 409-847-9451.
(Scientific American February 1998) (Science-Week 23 Jan 98)
For more information: http://scienceweek.com/swfr.htm
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IN FOCUS: ON PHANTOM PROBLEMS IN SCIENCE
"The world is teeming with problems. Wherever man looks, some new
problem crops up to meet his eye -- in his home life as well as
in his business or professional activity, in the realm of
economics as well as in the field of technology, in the arts as
well as in science. And some problems are very stubborn; they
just refuse to let us in peace. Our agonized thinking of them may
sometimes reach such a pitch that our thoughts haunt us
throughout the day, and even rob us of sleep at night. And if by
lucky chance we succeed in solving a problem, we experience a
sense of deliverance and rejoice over the enrichment of our
knowledge. But it is an entirely different story, and an
experience annoying as can be, to find after a long time spent in
toil and effort, that the problem which has been preying on one's
mind is totally incapable of any solution at all -- either
because their exists no indisputable method to unravel it, or
because considered in the cold light of reason, it turns out to
be absolutely void of all meaning -- in other words, it is a
_phantom problem_, and all that mental work and effort was
expended on a mere nothing. There are many such phantom problems
-- in my opinion, far more than one would ordinarily suspect --
even in the realm of science."
-----------
Max Planck: _Scientific Autobiography and other Papers_
(Philosophical Library, 1949)
Max Planck (1858-1947)
-------------------
SCIENCE-WEEK http://scienceweek.com 9Jun00
-------------------
Related Background:
IN FOCUS: ON MAX PLANCK
"Many kinds of men devote themselves to science, and not all for
the sake of science herself. There are some who come into her
temple because it offers them the opportunity to display their
particular talents. To this class of men science is a kind of
sport in the practice of which they exult, just as an athlete
exults in the exercise of his muscular prowess. There is another
class of men who come into the temple to make an offering of
their brain pulp in the hope of securing a profitable return.
These men are scientists only by the chance of some circumstance
which offered itself when making a choice of career. If the
attending circumstances had been different, they might have
become politicians or captains of business. Should an angel of
God descend and drive from the temple of science all those who
belong to the categories I have mentioned, I fear the temple
would be nearly emptied. But a few worshippers would still remain
-- some from former times and some from ours. To these latter
belongs our Planck. And that is why we love him."
-----------
Albert Einstein: from the preface to _Where is Science Going?_ by
Max Planck.
(Original German text 1933, English text Ox Bow Press 1981)
Albert Einstein (1879-1955)
-------------------
SCIENCE-WEEK http://scienceweek.com 7May99
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ERRATUM: REPORT #4, 2 JUNE 00:
In the report in the previous issue of SW on experimental
evolution and aging in fruit flies, the fruit fly Drosophila was
referred to as a "species". This is incorrect. Drosophila is a
genus, a category that includes one or more species; however, the
particular organism used in the experiments was indeed a single
species of fruit fly, Drosophila melanogaster. We apologize for
the error.
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