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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.
April 13, 2001 -- Vol. 5 Number 15
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The first part of the human story is simple:
We rose out of the primeval muck to peer at
the stars. The second part of the story has
yet to be written.
-- Anonymous
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. COSMOLOGY: ON THE MASS OF THE UNIVERSE
The origin of middle-scale cosmological structure is one of the
key issues in cosmology. A plausible assumption is that structure
grows by gravitational collapse of density fluctuations that are
small at early times. One important signature of gravitational
instability is that collapsing structure should generate
"peculiar velocities" that distort the uniform Hubble expansion.
In general, forming superclusters of galaxies should generate a
systematic infall of other galaxies, and this would be evident in
the pattern of recessional velocities, the effect causing an
anisotropy in the inferred spatial clustering of galaxies. Now
researchers report a precise measurement of this clustering of
galaxies, using the redshifts of more than 141,000 galaxies from
the "Two-Degree-Field" (2dF) Galaxy Redshift Survey. The results
favor a low-density Universe with Omega = 0.3.
(J.A. Peacock et al: Nature 8 Mar 01 410:169)
2. PLANETARY SCIENCE:
ON METEORITES, CHONDRULES, AND THE SOLAR NEBULA
Researchers report a study of the metal-rich meteorites Hammadah
al Hamra 237 and QUE94411. These meteorites contain evidence of
highly energetic thermal events resulting in complete
vaporization of a dusty region of the solar nebula. The
chondrules in these meteorites apparently formed under oxidizing
conditions before condensation of iron-nickel metal, at
temperatures greater than or equal to 1500 degrees kelvin, and
were isolated from the cooling gas before condensation of
moderately volatile elements such as manganese, sodium,
potassium, and sulfur. This astrophysical environment is
fundamentally different from conventional models for chondrule
formation by localized, brief, repetitive heating events that
resulted in incomplete melting of solid precursors initially
residing at ambient temperatures below approximately 650 degrees
kelvin. (A.N. Krot et al: Science 2 Mar 01 291:1776)
3. BOHR-HEISENBERG-COPENHAGEN REDUX
The enigma of the Bohr-Heisenberg meeting in Copenhagen in 1941
continues to provoke interest and controversy among both the
public and the physics community. On the surface, the enigma is a
matter of the establishment of a particular history; below the
surface, however, there is the profound issue of the moral
responsibilities of scientists when they find themselves at a
dangerous interface between science and politics. A series of
letters in a leading physics journal now provides some new
information. One letter, from a former colleague of Heisenberg,
states that Heisenberg's purpose at the meeting was to seek
Bohr's opinion about whether the small international community of
nuclear scientists could agree not to work on the bomb. Another
letter, from a physicist who heard Bohr speak about the meeting
in 1960, states that Heisenberg's purpose in 1941 was to persuade
Bohr to join with him in supporting the Nazis and building a new
Europe after a Nazi victory.
(Klaus Gottstein: Physics Today April 2001; Harry J. Lipkin:
Physics Today April 2001)
4. BIOCHEMISTRY: ON CHEMICAL GLYCOBIOLOGY
Oligosaccharides and glycoconjugates (glycoproteins and
glycolipids) have intrigued biologists for decades as mediators
of complex cellular events. With respect to structural diversity,
oligosaccharides have the capacity to far exceed proteins and
nucleic acids. This structural variance allows oligosaccharides
to encode information for specific molecular recognition and to
serve as determinants of protein folding, protein stability, and
pharmacokinetics. Given that glycosylation is one of the most
ubiquitous forms of posttranslational protein modification, the
unexpectedly small number of genes now apparent in the initial
analysis of the human genome provides even more impetus for
understanding the biological roles of oligosaccharides. But
although oligosaccharide functions are now being elucidated in
molecular detail, advances in glycobiology have been slow when
compared to advances in protein and nucleic acid biochemistry.
(C.R. Bertozzi and L.L. Kiessling: Science 23 Mar 01 291:2357)
5. NEUROBIOLOGY: ON THE BRAIN AND VIOLENCE
In human adults, the role of brain damage in violence remains
unclear. A brain lesion by itself is rarely sufficient to cause
violent behavior, and most individuals with brain damage do not
commit criminal acts. But we cannot assume that the brains of
violent individuals are invariably normal. Detailed analysis of
the neurobehavioral aspects of violence is complex. There is the
possibility of a neurogenetic contribution to violent behavior.
Although no single gene for human violence has been discovered,
data from molecular genetics indicate that multiple genes may
interact to predispose individuals to violent behavior. In
general, males are much more likely to commit violent acts than
are females, but genetic factors may not explain this
discrepancy. Socioeconomic and cultural influences play a major
role. Unemployment, lower educational level, alcohol abuse, and
access to firearms all contribute to violent crime among males.
(C.M. Filley et al: The Scientist 2 Apr 01)
6. PHYSIOLOGY: ON RHYTHMIC PROCESSES IN BIOLOGICAL SYSTEMS
The origin and dynamics of rhythmic processes -- once the sole
province of physicians and experimental physiologists -- is
increasingly a research focus of mathematicians and physicists.
Mathematical analyses of physiological rhythms demonstrate that
nonlinear equations are necessary to describe physiological
systems. In contrast to the linear equations of traditional
mathematical physics (e.g., Maxwell's equations, the heat
equation, the wave equation, the Schroedinger equation),
nonlinear equations rarely admit an analytical solution.
Consequently, numerical simulations are one essential feature of
quantitative studies of physiological systems. A complementary
approach is the analysis of the qualitative aspects of simplified
mathematical models of physiological systems. This involves a
mathematical analysis of those features of physiological systems
that will be preserved by classes of models sufficiently close to
the real system. (Leon Glass: Nature 8 Mar 01 410:277)
7. IN FOCUS: ON THE KINGDOMS OF LIFE
8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE DISCOVERY OF ELECTROMAGNETIC WAVES
=-=-=-=-=-=-=-=-=
Section 2
=-=-=-=-=-=-=-=-=
1. COSMOLOGY: ON THE MASS OF THE UNIVERSE
Although galaxies are arranged into gravitationally bound
clusters and superclusters, it is the galaxies themselves that
are usually considered to be the fundamental macroscopic units
of the Universe. One reason is historical, since until recently
no structures larger than a galaxy could be studied. Another
reason is the huge contrast in visible brightness between
galaxies and their surroundings. However, when one uses
instruments sensing the x-ray part of the spectrum rather than
the optical light part of the spectrum, clusters of galaxies are
the most impressive individual structures. Perhaps a good
analogy concerning the distribution of galaxies is to consider a
flask containing a rather dense culture of bacteria in a liquid
medium. With the naked eye, at zero magnification, the density
distribution of bacteria may appear uniform; with moderate
magnification, clustering, filaments, walls, voids in the
culture are evident, each cluster or filament containing
millions of bacteria; at high magnification, with a few dozen
bacteria in the field, there is again a uniformity in
distribution, the larger-scale nonuniformity not apparent.
The most striking aspect of galaxies is their apparent
recession. The term "redshift" (symbol: z) is a
lengthening of the wavelengths of electromagnetic radiation from
a source caused either by the movement of the source away from
the observer (Doppler effect; Doppler redshift) or by the
expansion of the Universe (cosmological redshift). Redshift is
defined as the increase 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.
A Doppler spectral change can be either a shift to the red
(redshift) or a shift to the blue (blueshift) part of the
spectrum: movement of the source toward the observer produces a
blueshift [*Note #1]. Except for local galactic movements, no
cosmological blueshifts are known, and this emphasizes the
difference between Doppler redshift and cosmological redshift:
Usually, when we speak of a Doppler redshift implying a certain
recessional velocity, we mean that the shift is due to the
inherent motion of the source relative to the observer. But
regarding cosmological (galactic) redshifts in such a manner
leads to a picture of all galaxies streaming away from us, such a
picture implicitly placing our Galaxy in the center of some great
explosion, a point of view inconsistent with the "cosmological
principle", which holds that there is no center to the Universe,
that the Universe is everywhere isotropic on the largest scales
(from which it follows that the Universe is also homogeneous).
Thus, the apparent recession velocity of galaxies is something
different from the usual concept of a recession velocity, and in
fact cosmological redshift is due to the properties of space
itself, and since the shift is to the red end of the spectrum,
the implication is that space is expanding everywhere, with every
galaxy seeing every other galaxy receding. This overall motion of
galaxies away from one another is called the "Hubble flow" after
its discoverer Edwin Hubble (1889-1953).
One of the central questions of cosmology is whether this
expansion will continue indefinitely, or whether it will
ultimately be slowed down by the intrinsic gravitational force
that tends to pull the mass of the Universe together. In any
theoretical approach to this question, a critical parameter is
the actual mass density (or more specifically, mass-energy
density) of the Universe. The term "critical density" refers to
the mass-energy density that theoretically stops the expansion of
space after infinite cosmic time has elapsed.
Also central to current cosmological considerations are the
distinctions between the geometries of a "flat" (uncurved;
infinite in both extent and lifetime), "closed" (spherical;
finite in both extent and lifetime), and "open" (*hyperbolic;
infinite and expanding forever) Universe. The term "Omega
parameter" (density parameter) is defined as the ratio of the
actual mass-energy density to the critical 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. The problem
for the past 60 years has thus been to obtain an estimate of the
mass density of the Universe from observations.
In this context, the intrinsic motion of a galaxy due to its
particular responses to forces such as local gravitational
attractions is called the "peculiar motion" of the galaxy, and
its velocity due to such movement is called its "peculiar
velocity". The adjective "peculiar" does not imply that the
velocity is due to anything strange, but refers simply to local
ordinary classical Doppler shifts due to the unique motions of a
given galaxy, as distinct from the overall Hubble flow. The net
redshift of a galaxy is a superposition of the peculiar Doppler
shift upon any cosmological redshift.
... ... J.A. Peacock et al (28 authors at 13 installations, UK,
AU, US) now report a measurement of the cosmological mass density
based on observations of clustering, the authors making the
following points:
1) The authors point out that Hubble demonstrated in 1934
that the pattern of galaxies on the sky is non-random, and since
that time there have been ambitious attempts to map the
distribution of visible matter on cosmological scales. In order
to obtain a 3-dimensional picture, redshift surveys use Hubble's
law, which states that recession velocity is directly
proportional to distance from the observer, to infer approximate
radial distances to a set of galaxies. The first major surveys of
this sort occurred in the early 1980s and were limited to a few
thousand redshifts. In the 1990s, redshift surveys were extended
to much larger volumes, and these studies established that the
Universe was close to uniform in galaxy distribution on large
scales, but with a complex nonlinear supercluster network of
walls, filaments, and voids on middle scales.
2) The authors point out that the origin of middle-scale
cosmological structure is one of the key issues in cosmology. A
plausible assumption is that structure grows by gravitational
collapse of density fluctuations that are small at early times.
One important signature of gravitational instability is that
collapsing structure should generate "peculiar velocities" that
distort the uniform Hubble expansion. In general, forming
superclusters of galaxies should generate a systematic infall of
other galaxies, and this would be evident in the pattern of
recessional velocities, the effect causing an anisotropy in the
inferred spatial clustering of galaxies.
3) The authors report a precise measurement of this
clustering of galaxies, using the redshifts of more than 141,000
galaxies from the "Two-Degree-Field" (2dF) Galaxy Redshift Survey
that began in 1998 and which should be finished at the end of
2001. The authors report their results favor a low-density
Universe with Omega = 0.3.
... ... In a commentary on this work, Marc Davis (University of
California Berkeley, US) makes the following points:
1) The author (Davis) points out that the results of Peacock
et al suggest that the amount of mass associated with galaxy
clustering is approximately 30 percent of the cosmic critical
density, the value at which the mass of the Universe, by the
backward pull of gravity, is just sufficient to eventually stop
the Hubble expansion. This estimated density is consistent with
the result of other methods, suggesting that cosmologists are
finally converging on a reliable estimate of the mean mass
density of the Universe. All indications point to an infinite
Universe that will expand forever.
2) The author (Davis) concludes: "As additional pieces of
the puzzle fall into place, our picture of Big Bang cosmology has
become ever more bizarre. A unifying principle is clearly needed
to explain the many disparate components of the Universe we have
so far discovered. The quest for such unification is likely to
keep us busy for decades to come."
-----------
J.A. Peacock et al: A measurement of the cosmological mass
density from clustering in the 2dF Galaxy Redshift Survey.
(Nature 8 Mar 01 410:169)
QY: John A. Peacock: jap@roe.ac.uk
-----------
Marc Davis: Weighing the Universe.
(Nature 8 Mar 01 410:153)
-----------
Text Notes:
... ... *Note #1: Our own Galaxy and the nearby Andromeda galaxy
are in gravitational association, and the Andromeda Doppler shift
is in fact blue, since the Andromeda galaxy is moving toward us.
... ... *hyperbolic: This is a negative curvature, like the
surface of a saddle, and it is sometimes called a "saddle"
Universe. In such a geometry, the sum of the angles of a triangle
is less than 180 degrees. In a spherical (closed) geometry, the
sum of angles is more than 180 degrees; in a flat geometry, the
sum of angles is exactly 180 degrees.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
LARGE-SCALE STRUCTURES IN THE UNIVERSE
As currently defined, the field of "cosmology" is the study of
the entire observable Universe treated as a single entity. Three
recognized central questions in this field are a) What did the
Universe look like at the dawn of time? b) How did it grow and
develop into what we live in today? c) What forms of matter, both
ordinary and exotic, does the Universe contain? Related to all
three of these questions are relatively recent observations
concerning the large-scale structure of the Universe,
particularly the structure of the distribution of the galaxies.
Each galaxy consists of a relatively local assemblage of hundreds
of millions or billions of stars, with enormous distances between
the galaxies. A cube set down at random in the Universe would
need to have sides 10 million light years long to contain, on
average, one galaxy. Apparently, however, the galaxies are not
distributed randomly in space: most are in groups or clusters,
pulled together by gravity. Some clusters contain many hundreds
of galaxies, and the clusters and groups are themselves arranged
in still larger filamentary or sheetlike structures. The
existence of such large-scale structures is a serious constraint
on cosmological models, and difficult to reconcile with the
"Cosmological Principle", which is the idea that the Universe
overall is homogeneous and isotropic.
... ... Stephen D. Landy (College of William and Mary, US)
reviews current work in the mapping of large-scale structure in
the Universe, the author making the following points:
1) On all scales observed thus far by astronomers, galaxies
appear to cluster and form intricate structures -- presumably
through physical processes that were dominant during the early
expansion of the Universe and later through gravitational
interactions.
2) Over the past several years, technological advances have
enabled astronomers and cosmologists to probe the arrangement of
galaxies at great distances, and the naive notion that at some
scale the Cosmos becomes uniform has been replaced by an
appreciation that the large-scale structure of the Universe must
be understood in terms of random processes: the homogeneity and
isotropicity of the Universe is true only in a subtle statistical
sense.
3) As one moves from our own Galaxy to the entire observable
Universe, clumpiness finally gives way to smoothness. A galaxy is
a lump of stars, gas, dust, and unclassified "*dark matter". It
agglomerates with other galaxies to form galaxy clusters, the
largest bodies in the Universe held together by gravity. The
clusters, in turn, are clumped together into superclusters and
*walls, separated by voids of nearly empty intergalactic space.
Up to some scale, thought to be approximately 100 million light-
years, these progressively larger structures form a *fractal
pattern, i.e., they are equivalently clumpy on every scale. But
between this scale and the size of the observable Universe, the
clumpiness gives way to near uniformity.
4) So-called "cold dark matter models" are now the most
popular explanation for the growth of structure in the
distribution of galaxies. The premise of these models is that
most of the mass in the Universe resides in some unseen ("dark")
and relatively massive type of particle. The particle is "cold"
because it is massive and travels slowly. The particle interacts
with ordinary matter only via the force of gravity, and could
also account for the apparent *missing mass in galaxies and
galaxy clusters. The observed "*power spectrum" of the
distribution of galaxies in the Cosmos generally follows the
predictions of the cold dark matter models. But the power
increases dramatically on scales of 600 million to 900 million
light years, and this discrepancy indicates that the Universe is
much clumpier on those scales than current theories can explain.
-----------
Stephen D. Landy: Mapping the Universe.
(Scientific American June 1999)
QY: Stephen D. Landy, College of William and Mary 757-221-4223
-----------
Text Notes:
... ... *dark matter: In general, 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 at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *walls: In this context, the term "walls" refers to
structured distributions of galaxies, e.g., a clustering 750
million light-years long, 250 million light-years wide, and 20
million light-years thick.
... ... *fractal pattern: A fractal is a geometrical shape whose
structure is such that magnification by a given factor reproduces
the original object. During the past several decades, the idea
that fractal geometry is an appropriate geometry to describe
nature has been proposed by many researchers. The mathematical
constructs involved are appealing because of their symmetries,
and as in the development of many appealing ideas, the use of the
term "fractal" has increased to the point where experimental
observations in all the sciences are being analyzed and
interpreted as examples of systems with apparently fractal
properties. To the mathematician, however, the definition of the
property of "fractality" involves a quantitative requirement of
infinitely many orders of magnitude of power-law scaling of the
parameters of the system -- certainly at least a spanning of many
orders of magnitude.
... ... *missing mass in galaxies and galaxy clusters: In
galaxies, particularly in spiral galaxies, the "missing mass
problem" concerns our inability to account for the motions of
stars at the edges of the galaxy using estimates of galactic mass
based on luminosities of the galaxy members. At the level of
clusters of galaxies, the missing mass problem is more a question
of assumptions concerning the physical basis of nonuniform
distributions of galaxies. In both cases, it is a matter of
asking what one would need to postulate in order to explain
observational data.
... ... *power spectrum: In this context, the term "power
spectrum" is synonymous with frequency spectrum, but the term
"frequency" refers not to a distribution of events in time, but
rather to a distribution of points (galaxies) in space.
Essentially, the same mathematics used to analyze event
frequencies can be used to analyze distribution frequencies. The
power spectrum considered here is a Fourier transform of the
autocorrelation function familiar in event frequency analysis
(e.g., analysis of neuron outputs), but in this case applied to
spatial distribution frequencies. The essential idea is that
given a distribution of a large number of points in space, one
can apply well-known analytic techniques to determine the degrees
of local and global randomness of the distribution. Thus, in this
context, galaxies are treated as points. One of the graphics in
the Landy paper is a map of the distribution of 3 million
galaxies, each galaxy a point which contains billions of stars.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 6Aug99
For more information: http://scienceweek.com/swfr.htm
-------------------
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)
-------------------
Related Background:
EVOLUTION OF COSMIC STRUCTURE: LARGEST SIMULATION TO DATE
One of the important problems in cosmology is to explain the
present structure of the universe, and the evolution of that
structure from the primordial material that came into existence
following the Big Bang. Computer simulations are a significant
part of this research, the idea essentially to calculate from
first principles the properties of a model based on a particular
set of assumptions, compare the results of the calculations with
what is observed in the real universe, and thus, temporarily,
confirm or deny the usefulness of the assumptions that form the
basis of the model. This is the paradigm for most theoretical
model construction in all the sciences, and as a method it is
nothing unique to cosmology. In cosmology, however, the number of
interacting entities is enormous. A new simulation effort was
recently reported, evidently the largest simulation of cosmic
structure to date, the new effort involving consideration of a
simulated cosmos of a billion entities, each of which is
equivalent to about 10 galaxies. The work was carried out at the
Max Planck Institute for Astrophysics (Garching, DE), using a
512-processor Cray supercomputer, and reported at the end of last
month at a cosmology meeting in Paris by Jorg Colberg (MPI-
Astrophysics Garching, DE). The work was also presented a week
ago at the American Astronomical Society San Diego (US) meeting
by August Evrard (University of Michigan, US). This is apparently
the first simulation of how gravity could have gathered post-Big
Bang ripples into large meta-galactic structures -- walls,
clumps, filaments of galaxies -- filling all of space. Some
astrophysicists are saying this work marks a turning point in
numerical cosmology, and they expect this model universe to be a
powerful tool for interpreting data from large surveys of the
real sky. This simulation omits factors other than gravity, such
as pressure and radiation, that also govern galaxy formation. The
calculations have involved two different models, one a model
based on a mass density sufficient to stop cosmic expansion, and
the other (called the "lambda" model) based on a light-mass
universe that will expand forever. Apparently, it is the lambda
model that is producing structures more in agreement with
observations, although both models have difficulty accounting for
some of the more massive and distant galaxy clusters seen in the
real sky.
QY: Joseph Glanz: science_editors@aaas.org
(Science 5 Jun 98 280:1522) (Science-Week 26 Jun 98)
-------------------
Related Background:
AN ARGUMENT FOR A LIGHTWEIGHT UNIVERSE
... A fundamental question in cosmology is whether the expansion
of the universe will continue indefinitely (an open universe) or
eventually cease (a closed universe). According to the current
analytical framework used to describe the universe, the answer to
this question depends on the mass density of the universe. If the
mass density is below a certain calculated value, the "critical
density", there is not enough mass to provide the gravitational
attractions necessary to slow and halt the expansion. This
critical density is equal to 1.9 x 10^(-29)H^(2) grams per cubic
centimeter, which is equivalent to approximately 10 protons per
cubic meter. The (H) indicated is the Hubble constant, the rate
at which the expansion velocity of the universe changes with
distance. Often used is a derived constant Omega(sub m), which is
expressed in units of the critical density, so that a value of
Omega(sub m) = 1 means the mass density is the critical density.
The standard models of the initial expansion of the universe
(*inflation), as well as general arguments not dependent on ad
hoc adjustments of cosmological parameters, predict a flat
universe with the critical density needed to just halt its
expansion. But at the present time, only a small fraction of the
critical density has been detected, even when all the unseen dark
matter in galaxy halos and clusters of galaxies is included.
There is apparently no reliable indication that most of the
matter needed for closing the universe does in fact exist. ...
... Bahcall and Fan (Princeton University, US) present an
analysis of the problem of cosmic mass density. They propose that
several independent measures, especially those using the largest
bound systems known -- clusters of galaxies -- all indicate that
the mass density of the universe is insufficient to halt the
expansion. They also propose that a new method involving the
evolution of the number density of clusters with time provides
the most powerful indication so far that the universe has a
subcritical density. The authors suggest that various techniques
reveal a consistent picture of a lightweight universe with only
20 to 30 percent of the critical density, and thus the universe
may expand forever.
QY: Neta A. Bahcall: neta@astro.princeton.edu
(Proc. Natl. Acad. Sci. US 26 May 98 95:5956)
(Science-Week 26 Jun 98)
-----------
Text Notes:
... ... *inflation: The inflationary model, first proposed by
Alan Guth in 1980, involves the idea that quantum fluctuations in
the time period 10^(-35) to 10^(-32) seconds following the
Big Bang 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 26Jun98
For more information: http://scienceweek.com/swfr.htm
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2. PLANETARY SCIENCE:
ON METEORITES, CHONDRULES, AND THE SOLAR NEBULA
The current consensus view of the origin of the Solar System
proposes that its formation began with the gravitational collapse
of part of an interstellar cloud of gas and dust, the cloud with
an initial mass only 10 to 20 percent larger than the present
mass of the Sun and approximately spherical in shape. As the
cloud revolved about the Galactic center, its collapse caused it
to rotate, the speed of rotation increasing as the cloud
contracted, the increase in accordance with the conservation of
angular momentum. As the cloud contracted, it flattened to form a
disk around a central condensation, this configuration called the
"solar nebula". As gas and dust were pulled in toward the central
condensation, potential energy was converted to kinetic energy
and the temperature of the material rose until ultimately the
temperature became great enough in the interior of the
condensation for nuclear reactions to begin and give birth to a
star -- our Sun. Meanwhile, the material of the rotating disk
collided, coalesced, and gradually formed larger and larger
objects.
The time-scale for the formation of the planets of the Solar
System is believed to have been relatively short. The collapse of
the original interstellar cloud required approximately 10 million
years. The time for the formation of the disk depended on the
size of the solid particles, with a formation-time of 100,000
years for 1-micron particles to only 10 years for 1-centimeter
particles. The increase of the local density at the midplane of
the disk enhanced the opportunities for the growth of particles
by collision, and as they grew, the resulting increase in their
gravitational fields accelerated further growth. Calculations
indicate that objects 10 kilometers in size will form in only
1000 years. Continued growth by accretion led to larger and
larger objects, with the energies released during accretionary
impacts sufficient to cause vaporization and extensive melting,
transforming the primitive material originally produced by the
condensation of the nebula.
At some point after most of the matter in the solar nebula
had formed discrete objects, a sudden increase in the intensity
of the *solar wind is believed to have cleared the remaining gas
and dust out of the system. The larger debris remained, some of
which can still be seen today in the form of asteroids and
comets. The rapid growth of Jupiter apparently prevented the
formation of a planet in the gap between Jupiter and Mars,
leaving in this area thousands of asteroids whose total mass is
less than one=third of the mass of the Moon. The great majority
of meteorites recovered on Earth come from these asteroids and
thus provide many significant chemical and petrological clues to
the conditions and processes in the early solar nebula.
All meteorites are evidently samples broken from outcrops or
rock or metal, which until fairly recently in Solar-System
history were part of asteroid bodies, mostly in the inner region
of the asteroid belt (2.2 to 2.6 *astronomical units). Like rocks
from the Moon, the Earth, or any other similar planetary body,
the present state of a meteorite is determined by the total
effect of events that occurred on its parent asteroid throughout
the entire history of the Solar System. Although there is no a
priori reason why asteroids should be pristine samples of the
primordial solar nebula, the principal driving force behind
asteroid studies has been the belief that small bodies such as
asteroids and comets are the bodies most likely to preserve
evidence of events that occurred in the early Solar System.
Evidence derived from the study of meteorites -- fragments of
asteroids -- supports this conclusion.
In general. there are 3 types of meteorites: stony
meteorites, iron meteorites, and stony-iron meteorites:
Stony meteorites have densities of approximately 3.4 grams
per cubic centimeter and on average contain 42 percent oxygen,
20.6 percent silicon, 15.8 percent magnesium, and 15.6 percent
iron, with no other element exceeding 2 percent. Stony meteorites
are further subdivided in to "chondrites and "achondrites". Of
the meteorites known to fall on Earth, 93 percent are stony
meteorites.
Iron meteorites have densities of approximately 7.8 grams
per cubic centimeter and contain on average 91 percent iron, 8
percent nickel, and 0.6 percent cobalt.
Stony-iron meteorites contain on average 50 percent nickel-
iron and 50 percent stony (mainly silicate) material.
The dichotomy of stony meteorites into chondrites and
achondrites is largely based on the presence or absence of
"chondrules". All meteorites with chondrules are chondrites. A
chondrule is a small rounded particle embedded in most
chondrites. Chondrules are usually approximately 1 millimeter or
less in diameter and consist for the most part of the silicate
minerals olivine and pyroxene. From textural and chemical
relationships, it is apparent that chondrules were formed at high
temperatures as dispersed molten droplets, which subsequently
solidified and aggregated into chondrite masses. This process
evidently occurred in the solar nebula before the accretion of
the planets. However, how the chondrules were melted is not
understood. It is believed that dust particles already in
existence were melted by high-energy events such as high-velocity
collisions, the melts splashed about as droplets that quickly
cooled and crystallized. In general, it is believed that the
formation of chondrules required temperatures of approximately
1500 degrees kelvin.
The abundance of chondrules in most meteorites is at present
taken as evidence of major high-temperature events in the early
Solar System, and in particular it is believed that chondrules
were formed in the solar nebula: local and transient heating
events appear to have been important on a wide-scale in the solar
nebula, but the nature and cause of these events remain unknown.
... ... A.N. Krot et al (6 authors at 4 installations, US UK)
present an analysis of chondrules in two iron-rich meteorites and
an apparent new setting for chondrule formation. The authors make
the following points:
1) The authors point out that chondrules are composed mainly
of ferromagnesian silicates with accessory Fe-Ni metal and minor
sulfides. Although the mineralogy, bulk chemistry, and textural
properties of typical chondrules provide constraints on their
formation, the exact mechanism of chondrule formation remains
enigmatic. Typical chondrules show textures consistent with
crystal growth from a rapidly cooling (100 to 1000 degrees kelvin
per hour) silicate melt. Chondrules often contain relic fragments
of earlier chondrules, suggesting that the chondrule-forming
process was repetitive. Also, chondrules in most chondrites are
surrounded by fine-grained rims. On the basis of these
observations, it is generally believed that chondrules formed in
dusty regions of the solar nebula during localized, brief,
repetitive heating events with peak temperatures of approximately
1800 to 2100 degrees kelvin. This resulted in incomplete melting
of solid precursors initially residing at ambient temperatures
below approximately 650 degrees kelvin, which is the condensation
temperature of silicon. Shock waves and lightning discharges are
currently favored as the most plausible chondrule-forming heating
mechanisms.
2) The authors report a study, using optical and scanning
electron microscopy, electron microprobe and ion microprobe
analyses, and other techniques, of the metal-rich meteorites
Hammadah al Hamra 237 and QUE94411. The authors report these
meteorites contain evidence of highly energetic thermal events
resulting in complete vaporization of a dusty region of the solar
nebula. The chondrules in these meteorites apparently formed
under oxidizing conditions before condensation of iron-nickel
metal, at temperatures greater than or equal to 1500 degrees
kelvin, and were isolated from the cooling gas before
condensation of moderately volatile elements such as manganese,
sodium, potassium, and sulfur. The authors point out this
astrophysical environment is fundamentally different from
conventional models for chondrule formation by localized, brief,
repetitive heating events that resulted in incomplete melting of
solid precursors initially residing at ambient temperatures below
approximately 650 degrees kelvin.
3) The authors conclude: "The energetic and dynamical
astrophysical environment recorded by chondrules and zoned Fe-Ni
metal grains in [these meteorites] indicate that these meteorites
formed earlier than most chondrites, when the protoplanetary disk
was more active. Dating of the formation of these meteorites and
their components should be possible and will determine whether
this is correct."
-----------
A.N. Krot et al: A new astrophysical setting for chondrule
formation.
(Science 2 Mar 01 291:1776)
QY: Alexander N. Krot: sasha@pgd.hawaii.edu
-----------
Text Notes:
... ... *solar wind: 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.
... ... *astronomical units: (AU): 1 AU = the mean distance from
the Sun to the Earth = approximately 93 million miles, and
exactly 149,597,870 kilometers.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
A SHOCK WAVE MODEL FOR THE FORMATION OF CHONDRULES
Chondrites are a type of stony meteorite consisting of an
agglomeration of millimeter-sized globules called chondrules, and
the are thought to be unchanged since the original condensation
out of the nebula from which the sun and solar system formed.
... ... Connelly and Love (2 installations, US) review present
ideas concerning the formation of chondrules in a process of
repeated, localized, brief (minutes to hours) melting of cold
aggregates of mineral dust in the protoplanetary nebula that was
the precursor to the solar system. Astrophysical models of
chondrule formation have been unable to explain the
petrologically diverse nature of chondrites, of which there are
many types. However, a nebular shock-wave model for chondrule
formation is consistent with many of the observed petrological
and geochemical properties of chondrules, and explains various
structural properties of the particles. Identifying the source of
the shock-wave heating is still a problem, but the author
suggests several possibilities.
QY: Harold C. Connelly Jr., California Institute of Technology
818-395-6811 (Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
ORIGIN OF CHONDRULES AND THE FORMATION OF JUPITER
Chondrites are a type of stony meteorite consisting of an
agglomeration of millimeter-sized globules (chondrules) that are
thought to be unchanged since the original condensation out of
the gaseous nebula from which the sun and solar system formed.
Planetesimals are bodies with dimensions of 10^(-3) to 10^(3)
meters that are believed to form planets by a process of
accretion. The term "accretion" refers to an aggregation, an
increase in the mass of a body by the addition of smaller bodies
that collide and adhere to it, provided the relative velocities
are low enough for coalescence. In principle, one can distinguish
"first generation" planetesimals from second (or third, etc.)
generation planetesimals formed by the breakup of first
generation planetesimals, all of the events determined by granule
densities, planetesimal densities, gravitational fields, orbits,
orbital velocities, and so on. In addition, in complex
gravitational systems consisting of many small orbiting bodies
influenced by the gravitational field of a large orbiting body,
there are "resonances" that may arise, amplifications of
gravitational perturbations due to various periodicity
parameters, and these perturbations may play an important role in
the gravitational evolution of the entire system. The term "bow
shock" refers to the shock wave produced by the interaction of
the supersonic solar wind (the continuous flow of hydrogen and
helium gas from the sun) with the magnetic field of a planet (in
this case, Jupiter). The chondrules are considered an important
theoretical link to the origin and early development of our solar
system, and various models have been formulated based on their
chemistry and physical properties. ... ... Now Weidenschilling et
al (3 authors at 3 installations, US IT) present a model for the
production of chondrules by heating of debris from disrupted
first-generation planetesimals, with Jovian resonances exciting
planetesimal orbiting eccentricities enough to cause collisional
disruption and melting of dust by bow shocks in the nebular gas.
The authors suggest the age of chondrules may indicate the times
of Jupiter's formation and dissipation of gas from the asteroidal
region, and that their model reconciles the present apparently
incompatible temporal and dynamical constraints on theory
produced by observations and analysis.
QY: S.J. Weidenschilling: sjw@psi.edu
(Science 30 Jan 98) (Science-Week 13 Feb 98)
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. BOHR-HEISENBERG-COPENHAGEN REDUX
The enigma of the Bohr-Heisenberg meeting in Copenhagen in 1941
(see related background material below) continues to provoke
interest and controversy among both the general public and the
physics community. On the surface, the enigma is a matter of the
establishment of a particular history; below the surface,
however, there is the profound issue of the moral
responsibilities of scientists when they find themselves at a
dangerous interface between science and politics. A series of
letters in a leading physics journal now provides some new
information.
... ... Klaus Gottstein (Max Planck Institute for Physics, DE)
makes the following points:
1) Gottstein points out that he discussed the visit with
Heisenberg himself and also asked Carl von Weizsaecker about the
visit. Weizsaecker had accompanied Heisenberg to Copenhagen and
had talked to Heisenberg immediately after Heisenberg had
returned in despair. Heisenberg was Gottstein's director at the
Max Planck Institute for Physics from 1950 to 1971, and Gottstein
was associated with Weizsaecker from 1974 to 1980 at
Weizsaecker's Starnberg Institute. Gottstein spoke to Weizsaecker
again about the Bohr-Heisenberg meeting in early 2000.
2) Gottstein states that Heisenberg was worried that a
nuclear bomb might be built and used, and Heisenberg wanted
Bohr's opinion about whether the small international community of
nuclear scientists could agree not to work on the bomb. But
Gottstein states that Heisenberg did not get his message across.
Heisenberg told Gottstein that he used very involved language
with Bohr so that, if necessary, he could have given the Gestapo
a harmless interpretation. The reactor project was a military
secret, and telling a foreigner about it was treason, punishable
by death. According the Gottstein, Bohr understood only that
Heisenberg worked on fission and knew in principle how a nuclear
fission bomb could be made, and that, somehow, Heisenberg wanted
to get Bohr involved. Bohr ended the conversation.
3) Gottstein states that David C. Cassidy (see related
background material below) is wrong when he suggests that
Heisenberg went to Copenhagen to convince Bohr that German
victory was inevitable. That Heisenberg wanted Bohr to use his
influence to prevent Allied scientists from building a bomb that
could be used against Germany may be partially true, Gottstein
states, but the full truth is probably that Heisenberg wanted to
talk with his friend about avoiding altogether the construction
of nuclear bombs. According to Gottstein, Bohr was isolated in
occupied Denmark, so Heisenberg "cannot have hoped to learn
anything from him about Allied operations."
... ... In a contiguous letter, Harry J. Lipkin (Weizmann
Institute of Science, IL) points out that he and others at his
institute heard Bohr's version of the Bohr-Heisenberg meeting
when Bohr came to visit the Weizmann Institute around 1960.
Lipkins states that Heisenberg and his colleagues had been
completely surprised by the news that an atom bomb had been
dropped on Hiroshima. Germany had no serious atom bomb program;
the Germans never believed that it was possible. To cover their
embarrassment at having missed this possibility, Heisenberg and
friends invented the story that they had opposed the bomb project
for moral reasons. Bohr was furious at this outright lie and told
Amos de Shalit that Heisenberg's message at the Copenhagen
meeting in 1941 was "You know that we are going to win this war
and we will be building a new high tech Europe based on the
discoveries in quantum physics and nuclear energy. Why don't you
join us?" Lipkin states: "One can imagine Bohr's feelings about
being asked to participate in the building of Adolf Hitler's
'thousand-year Reich' and Heisenberg's insensitivity to such
feelings. The possibility of an atomic bomb was probably not even
discussed, being considered irrelevant at the time."
-----------
Klaus Gottstein: [letter]
(Physics Today April 2001)
QY: Klaus Gottstein: klaus.gottstein@unibw-muenchen.de
-----------
Harry J. Lipkin: [letter]
(Physics Today April 2001)
QY: Harry J. Lipkin: harry.lipkin@weizmann.ac.il
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN BRIEF: BOHR, HEISENBERG, AND COPENHAGEN -- AGAIN.
In the ScienceWeek issue of 19 May 2000 (4:20), we presented an
account of several responses to the new play _Copenhagen_ by
Michael Frayn, a theatrical work dealing with the September 1941
meeting of the physicists Niels Bohr and Werner Heisenberg in
Copenhagen, Denmark. Discussion of the play and the personalities
involved continues in various places, and now the journal Physics
Today presents 3 articles on the Copenhagen meeting, Niels Bohr,
and Werner Heisenberg. These articles offer further details and
should be noted by anyone interested in the subject.
----------
David C. Cassidy: A historical perspective on _Copenhagen_.
Hans A. Bethe: The German uranium project.
Gerald Holton: Werner Heisenberg and Albert Einstein.
-----
(Physics Today July 2000)
-------------------
SCIENCE-WEEK http://scienceweek.com 21Jul00
-------------------
Related Background:
HISTORY OF SCIENCE:
THE PUZZLE OF THE BOHR-HEISENBERG COPENHAGEN MEETING
Niels Bohr (1885-1962) and Werner Heisenberg (1901-1976),
two monumental figures in the history of physics in the 20th
century, are currently the focus of an interesting convergence of
science and the humanities as the result of the new play
_Copenhagen_ by Michael Frayn, the play originally presented in
London two years ago and now also on stage in New York.
Bohr worked in the fields of atomic structure and nuclear
fission, and he proposed the doctrine of complementarity. As
director of the Institute of Theoretical Physics in Copenhagen
from 1920 on, Bohr was the head of what came to be called the
Copenhagen School of Quantum Mechanics, which produced the so-
called "Copenhagen orthodoxy" view of the implications of quantum
mechanics as applied in general to theoretical physics. Bohr was
awarded the Nobel Prize in Physics in 1922.
Heisenberg developed one form of quantum theory (matrix
quantum mechanics) in the late 1920s and formulated the
uncertainty principle, which concerns matter, radiation, and
their reactions, and which places absolute limits on the
achievable accuracy of measurement of physical phenomena in the
quantum domain. Heisenberg was awarded the Nobel Prize in Physics
in 1932.
Bohr was Danish and the son of a professor of physiology;
Heisenberg was German and the son of a professor of Byzantine
history. Early in his career, Heisenberg worked under Bohr in
Copenhagen as a research assistant, and the two men developed a
lasting friendship and an intimate working relationship --
lasting, that is, until 1941, when something happened during a
meeting of the two men in Copenhagen, and the friendship and
working relationship collapsed and was never recovered.
The break between the two men occurred against a background
of political turmoil and war, and what happened during the break
has never been clarified and remains a puzzle. Certain background
facts are salient: a) Germany occupied Denmark in 1940 and
remained in Denmark until 1945. b) The Nazi campaign to
exterminate the Jews was in full swing all over Europe but moved
relatively slowly in Denmark; Niels Bohr, in fact, was half-
Jewish and he remained working at his institute in Copenhagen
until 1943, when he escaped to Sweden. (During the war in
Denmark, and afterward while in Sweden, Bohr helped arrange the
rescue of nearly every Danish Jew.) c) As for Heisenberg, he
remained in Germany during the entire Nazi period, and by 1940 he
was in charge of German research on the atomic bomb. d) In
September 1941, Heisenberg traveled to Copenhagen to meet with
Bohr, and that meeting apparently resulted in the termination of
their friendship.
Physics in the 20th century benefitted especially from
personal interactions of physicists, and the causes and
consequences of a break between two physicists of the stature of
Bohr and Heisenberg are of interest. Indeed, there have been
breaks between other scientists, both in and out of physics, but
this one has been brought to the theatrical stage and it is now
under intense scrutiny.
... ... Bertram M. Schwarzchild (Physics Today, US) presents a
report of a recent symposium of physicists and others held at the
City University of New York, the symposium devoted to the
historical events surrounding the 1941 meeting between Bohr and
Heisenberg and to the theatrical interpretation of these events
by the play _Copenhagen_. Schwarzchild makes the following
points:
1) After the war, during their next meeting in 1947, Bohr
and Heisenberg offered conflicting recollections of the meeting
in 1941. Was Heisenberg, as Bohr remembered it, trying to ferret
out information about Allied efforts to build fission weapons? Or
was Heisenberg, as Heisenberg later claimed, trying to suggest to
the physicists in Britain and America, through Bohr, that both
sides should abandon the search for the means to develop the atom
bomb?
2) At the symposium at the City University of New York, Hans
Bethe, 93 years old and a former member of the Los Alamos team
(he left Germany in 1933), noted in an address that the Germans
failed to realize that graphite was the appropriate moderator for
a uranium reaction because Walter Bothe, the acknowledged German
authority, claimed graphite was unsuitable and, Bethe stated, the
Germans of that era did not challenge authority. In America,
Hungarian refugee Leo Szilard, talking in 1942 to the chemical
engineers who manufactured commercial graphite, discovered that
the offending impurity was boron, and that enough boron could be
removed to make graphite bricks sufficiently pure for reactors.
In his New York address, Bethe asserted that in the Germany of
that time, with its hierarchical culture, "no physicist would
have deigned to consult a chemical engineer."
3) Schwarzchild concludes his report with a note about a
1947 letter from the physicist Max Born (1882-1970) to his son
Gustav. In this letter, Born describes a postwar conversation
with Heisenberg, Born writing of Heisenberg: "His philosophy of
life is definitely somewhat infected by Nazi ideas. He has a kind
of 'biological' creed, 'survival of the fittest', applied to
human relations, and seems to regret more that the Germans have
not turned out to be the fittest, than what we regard to be the
sad and regrettable things."
... ... In a detailed essay on the play _Copenhagen_ in the New
York Review of Books, Thomas Powers, author of a recent biography
of Heisenberg, makes the following points:
1) At the time of the Bohr-Heisenberg meeting in 1941, the
German military, during the first weeks of the war in 1939, had
placed Heisenberg in charge of theoretical work on the
feasibility of atomic bombs, and Heisenberg remained a principal
director of uranium research "until the last shots were fired".
When the war ended, Heisenberg was in southern Germany working on
a small experimental nuclear reactor which never achieved a self-
sustaining chain reaction. It was a tiny program without
scientific or military significance.
2) Powers points out that concerning almost every detail of
the 1941 Bohr-Heisenberg meeting there is more than one opinion,
and long books have been written attempting to sort it all out.
"Frayn [the author of the play] is not trying to establish what
really happened; it is what might, could, or should have happened
that interests him and gives the play its power as a work of
ideas."
3) Powers states that it is possible that in 1941 Heisenberg
wanted to talk to Bohr about the one question posed twice in the
play (at the beginning and at the end): "Does one as a physicist
have the moral right to work on the practical exploitation of
atomic energy?" Powers concludes: "For the scientists who
succeeded where Heisenberg failed, and for the historians who
have recounted their efforts, answering Heisenberg's question is
no simple matter. But once the question is posed there are only
two possible responses -- to ignore the question and to dismiss
his [Heisenberg's] visit to Copenhagen as somehow safe and self-
serving, or to grant him [Heisenberg] the courtesy of an attempt
to reply." [*Note #1].
-----------
Bertram Schwarzchild: Bohr-Heisenberg Symposium Marks Broadway
Opening of _Copenhagen_.
(Physics Today May 2000)
QY: Bertram Schwarzchild [postmaster@aip.org]
-----------
Thomas Powers: The Unanswered Question
(New York Review of Books 25 May 2000)
QY: Thomas Powers [mail@nybooks.com]
-----------
Text Notes:
... ... *Note #1: The question posed by Powers (and posed in the
play), "Does one as a physicist have the moral right to work on
the practical exploitation of atomic energy?" should probably be
replaced by the question "Does one as a physicist have the moral
right to work on the _military_ exploitation of atomic energy?",
since that is what was and remains actually involved. This
revised question has been debated for more than half a century in
our time, and the debate goes on. But it is important to
distinguish this question from the related question, "Does one as
a physicist have the moral right to work on the _science_
underlying the "practical" (or military) exploitation of atomic
energy? The view of the Editor is that the most cogent answer to
this related question was provided by the physicist J. Robert
Oppenheimer (1904-1967), and the following statement by
Oppenheimer appears on the masthead of the ScienceWeek website:
"If you are a scientist you believe that it is good to find out
how the world works, that it is good to find out what the
realities are, that it is good to turn over to mankind at large
the greatest possible power to control the world... It is not
possible to be a scientist unless you believe that the knowledge
of the world, and the power which this gives, is a thing which is
of intrinsic value to humanity, and that you are using it to help
in the spread of knowledge, and are willing to take the
consequences."
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 19May00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. BIOCHEMISTRY: ON CHEMICAL GLYCOBIOLOGY
Carbohydrates are the most abundant organic molecules in
nature, with a wide range of functions that include providing a
significant fraction of the energy in the diet of most organisms,
a storage form of energy in the body, and serving as cell
membrane components that mediate certain forms of intercellular
communication. Carbohydrates also serve as structural components
of many organisms, including the cell walls of bacteria, the
exoskeleton of many insects, and the fibrous cellulose of plants.
The empiric formula for many of the simpler carbohydrates is
[CH(sub2)O](subn), thus the historical name "hydrate of carbon".
The name, however, is a misnomer -- carbohydrates are not
hydrates of carbon.
The term "saccharide" refers to any carbohydrate, especially
to simple sugars. Of saccharides there are 4 general classes:
monosaccharides, disaccharides, oligosaccharides, and
polysaccharides (also called "glycans"), with all cases except
monosaccharides consisting of a simple sugars in a chain of
glycosidic linkages. The term "oligosaccharides" usually refers
to carbohydrates involving 2 to 20 sugars in a chain, although
some authors use "polysaccharide" for any carbohydrate chain
involving more than 10 sugars.
In this context, the term "protein prosthetic group" refers
in general to any specific non-protein component combined with a
protein in stoichiometric proportion, the components usually
essential for some special biological function such as catalytic
activity of an enzyme. Also in this context, the term "antigen"
refers to any chemical moiety that provokes an immune response.
The term "glycoproteins" refers in general to proteins to
which oligosaccharides are attached. The glycoprotein
carbohydrate chains are often branched rather than linear, and
they may or may not be negatively charged. In general, depending
on type, glycoproteins contain highly variable amounts of
carbohydrate. Membrane-bound glycoproteins participate in a broad
range of cellular phenomena, including cell-surface recognition
(by other cells, by hormones, and by viruses), cell-surface
antigenicity (e.g., blood group antigens), as components of the
*extracellular matrix, and as components of various biological
"lubricants" (mucins) of the gastrointestinal and urogenital
tracts. In addition, almost all the globular proteins present in
human plasma (with the notable exception of albumin), and the
secreted enzymes and proteins, are glycoproteins.
In general, the term "glycolipids" refers to compounds in
which fatty acids are attached to sugars. Glycolipids are
essential components of all membranes in the body, but they are
found in greatest amounts in nerve tissue, primarily in the outer
layer of the plasma membrane, where they interact with the
extracellular environment. Glycolipids play a role in the
regulation of cellular interactions, growth, and development.
Glycolipids are usually very antigenic, and they have been
identified as a source of blood group antigens, various embryonic
antigens specific for particular stages of fetal development, and
as a source of tumor antigens. Glycolipids also serve as cell
surface receptors for cholera and diphtheria toxins, as well as
for certain viruses. In general, when cells are transformed into
cancer cells, there is a dramatic change in the glycolipid
composition of the cell membrane. Genetic disorders resulting in
the intracellular accumulation of glycolipids lead to serious
impairment of the nervous system and impairment of fetal
development.
Glycoproteins and glycolipids are considered together as
"glycoconjugates".
The term "endoplasmic reticulum" refers to an intracellular
irregular network of membranes visible only by electron
microscopy, the network occurring in many *eukaryotic cells. The
membranes form a complex meshwork of tubular channels that are
often expanded into cavities ("cisterns"). There are two forms of
endoplasmic reticulum: a) the rough (granular) form, with
*ribosomes adhering to the outer surface; b) the smooth form,
with no ribosomes attached. The functions of the two forms of
endoplasmic reticulum differ: the ribosomes of the rough
endoplasmic reticulum are the sites of protein synthesis, while
the smooth endoplasmic reticulum receives proteins synthesized by
ribosomes and is involved in the synthesis of various important
lipids.
The term "Golgi apparatus" refers to a compound membranous
cytoplasmic organelle of eukaryotic cells, the system consisting
of flattened ribosome-free vesicles arranged in a more or less
regular stack. In general, the Golgi apparatus processes proteins
produced by the ribosomes of the rough endoplasmic reticulum,
such processing including modification of the oligosaccharides of
glycoproteins, and the sorting and packaging of proteins for
transport to a variety of cellular locations. The Golgi apparatus
is also a major site of synthesis of polysaccharides.
... ... C.R. Bertozzi and L.L. Kiessling (2 installations, US)
present a review of current research in chemical glycobiology,
the authors making the following points:
1) The authors point out that oligosaccharides and
glycoconjugates (glycoproteins and glycolipids) have intrigued
biologists for decades as mediators of complex cellular events,
and that with respect to structural diversity, oligosaccharides
have the capacity to far exceed proteins and nucleic acids. This
structural variance allows oligosaccharides to encode information
for specific molecular recognition and to serve as determinants
of protein folding, protein stability, and pharmacokinetics.
Given that glycosylation is one of the most ubiquitous forms of
post-synthesis ("posttranslational") protein modification, the
unexpectedly small number of genes now apparent in the initial
analysis of the human genome provides even more impetus for
understanding the biological roles of oligosaccharides.
2) The authors point out that although oligosaccharide
functions are now being elucidated in molecular detail, advances
in glycobiology have been slow when compared to advances in
protein and nucleic acid biochemistry. The same structural
diversity in oligosaccharides that has captivated biologists has
also frustrated efforts to define oligosaccharide expression
patterns and to correlate structure with function. Some technical
challenges are analytic in nature: determination of the
oligosaccharide sequence in a specific glycoconjugate is still
far from routine. Other difficulties originate in the nature of
glycoconjugate biosynthesis, which is neither template-driven nor
under direct genome (transcriptional) control. Oligosaccharides
are assembled in step-wise fashion primarily in the endoplasmic
reticulum and Golgi apparatus, a process involving significant
microheterogeneity. As a result, it is difficult to obtain
homogeneous and chemically defined glycoconjugates from
biological sources. Without such materials in hand, biological
functions are difficult to unravel.
3) The authors point out that genetic approaches have
contributed significantly to the understanding of oligosaccharide
function. The availability of entire genome sequences has
revealed the multiplicity of enzymes that contribute to
glycoconjugate assembly. Genetic deletion of such enzymes in
model organisms has provided substantial insight. For example,
mice deficient in the enzyme alpha-mannosidase II express an
altered array of nitrogen-linked glycans on their cell-surface
glycoproteins, and such mice are prone to a systemic *autoimmune
response, which suggests that abnormalities in N-glycosylation in
humans may be a factor in the pathogenesis of autoimmune
diseases. Unfortunately, cell-surface presentation of simple as
well as complex glycans requires many genes to be expressed in
concert, and this complicates the analysis of single-gene
mutations.
-----------
C.R. Bertozzi and L.L. Kiessling: Chemical glycobiology.
(Science 23 Mar 01 291:2357)
QY: Carolyn R. Bertozzi: Dept. of Chemistry, University of
California Berkeley, Berkeley, CA 94720 (US).
-----------
Text Notes:
... ... *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.
... ... *eukaryotic cells: In general, cells that contain
internal membrane-bound organelles.
... ... *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.
... ... *autoimmune response: (autoimmune disease) In general,
any pathology that involves a self-immunological process against
the individual's own cells or tissues. Classified as human
autoimmune diseases are rheumatoid arthritis, systemic lupus
erythematosus, rheumatic fever, Addison's disease, multiple
sclerosis, type 1 diabetes mellitus, etc.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
POLYSACCHARIDE ELASTICITY GOVERNED BY CHAIR-BOAT TRANSITIONS
Many common biologically important polysaccharides contain
pyranose rings made of 5 carbon atoms and 1 oxygen atom. These
rings occur in a variety of cellular structures, where they are
often subjected to considerable *tensile stress. The
polysaccharides are believed to respond to this stress by
*elastic deformation, but the underlying rearrangements allowing
such a response remain unclear. It is typically assumed, however,
that the pyranose ring structure is inelastic and locked into a
*chair-like conformation. ... ... P.E. Marszalek et al (4 authors
at Mayo Foundation, US) now report single-molecule *atomic force
microscopy measurements on individual polysaccharides that
identify the pyranose ring as the structural unit controlling the
elasticity of the molecule. The authors report that in particular
they find the *enthalpic component of the polymer elasticity of
*amylose, *dextran, and *pullulan is eliminated once their
pyranose rings are cleaved. The authors suggest their
observations indicate that the elasticity of these
polysaccharides results from a force-induced elongation of the
ring structure and a final transition from a chair-like to a
boat-like conformation. The authors further suggest that the
force-induced deformation they are reporting plays an important
role in accommodating mechanical stresses and modulating ligand
binding in biological systems.
-----------
P.E. Marszalek et al: Polysaccharide elasticity governed by
chair-boat transitions of the glucopyranose ring.
(Nature 17 Dec 98 396:661)
QY: Julio M. Fernandez: fernandez.julio@mayo.edu
-----------
Text Notes:
... ... *tensile stress: In this context, "stress" is defined as
the force per unit area on a body that tends to cause it to
deform, and "tensile stress" is an axial force per unit area
applied to a body that tends to extend it.
... ... *elastic deformation: In this context, elasticity is the
property whereby a molecule changes its shape due to imposed
forces, but recovers its original configuration when the forces
are removed.
... ... *chair-like conformation: (chair-like form; chair form)
The term "conformation" is usually restricted to dynamic spatial
arrangements of atoms or groups in a molecule that may be in
equilibrium with other conformations. Under usual conditions, no
single conformation constitutes a discrete and isolatable
substance (in contrast to configurational isomers). The "chair"
conformation is a particular nonplanar conformation of a cyclic
molecule with more than 5 atoms in the ring (e.g., the chair form
of cyclohexane). The alternative "boat" form (which can also be
assumed by e.g., cyclohexane) is relatively unstable. The two
forms can be visualized as follows (backbone continuous in each
case:
\__ chair \__/ boat
\
... ... *atomic force microscopy: An atomic force microscope is a
type of microscope in which a small probe, consisting of a tiny
chip of diamond, is held on a spring-loaded cantilever in contact
with the surface of a sample. The probe is moved slowly across
the surface, and the tracking force between the tip and the
surface is monitored. The probe is raised and lowered so as to
keep this force constant, and a profile of the surface is thus
produced. Since the instrument can be used with electrically
nonconducting samples, it is useful for biological specimens.
... ... *enthalpic component: Enthalpy, usually denoted as H, is
a thermodynamic state variable defined by H = U + PV, where U is
the internal energy and P and V are the pressure and volume
respectively. For any process which occurs at constant pressure,
the heat absorbed or evolved is equal to the enthalpy change if
the only work is pressure/volume work.
... ... *amylose: A linear polymer component of starch.
... ... *dextran: Any of several polysaccharides that yield
glucose units on hydrolysis.
... ... *pullulan: An extracellular glucan produced by certain
yeast and other fungi. In general, a "glucan" is any
polysaccharide consisting solely of glucose residues.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 26Feb99
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. NEUROBIOLOGY: ON THE BRAIN AND VIOLENCE
Although human violence has been a major focus of research
in psychiatry, psychology, and the social sciences,
neurobiological studies of human violence have been relatively
uncommon. Neurobiology, however, is a major component in our
understanding of human behavior: genetics, environment, brain
structure and brain function are all involved in both ordinary
behavior and in violent behavior.
... ... C.M. Filley et al (3 authors at 3 installations, US)
present a commentary on current research on violence and the
human brain, the authors making the following points:
1) The authors point out that in adults, the role of brain
damage in violence remains unclear. A brain lesion by itself is
rarely sufficient to cause violent behavior, and most individuals
with brain damage do not commit criminal acts. But we cannot
assume that the brains of violent individuals are invariably
normal. The neurologic status of the brains of violent persons
has not been adequately assessed by detailed neurological
examination, neuropsychological testing, *magnetic resonance
imaging, or *functional neuroimaging. Studies of murderers have
suggested a high prevalence of neurologic dysfunction, and some
individuals with traumatic brain injury, epilepsy, dementia, and
sleep disorders have been observed to exhibit excessive violence.
Violence is more likely among those with severe mental illness,
particularly psychosis, and violence is exacerbated by the use of
alcohol and other psychoactive substances.
2) The authors point out that detailed analysis of the
neurobehavioral aspects of violence is complex:
... ... a) The cause of violence is multifactorial, and a direct
correlation between brain dysfunction and a violent act is rarely
possible.
... ... b) Identification of brain lesions is imperfect given the
limitations of diagnostic classifications, the limitations of the
neurologic examination, the limitations of neuroimaging
technologies, the limitations of neuropsychological assessment,
and the limitations of neurochemical analysis.
... ... c) Some subject samples, such as prisoners or those with
severe neurologic or psychiatric disease, are necessarily based
on violent persons who are apprehended or hospitalized.
Conclusions are therefore based only on those whose records are
analyzed, and the potential for violence in the general
population remains unknown.
3) There is the possibility of a neurogenetic contribution
to violent behavior. Although no single gene for human violence
has been discovered, data from molecular genetics indicate that
multiple genes may interact to predispose individuals to violent
behavior. Observations in mouse *knockout models have suggested
that targeted disruption of single genes can induce
aggressiveness in males and diminish nurturing in females.
Aggression in animals and humans is also likely related to genes
regulating central nervous system *serotonin metabolism.
4) In general, males are much more likely to commit violent
acts than are females, but genetic factors may not explain this
discrepancy. Socioeconomic and cultural influences play a major
role. Unemployment, lower educational level, alcohol abuse, and
access to firearms all contribute to violent crime among males.
The *XYY chromosomal disorder serves to highlight difficulties in
establishing an influence of gender on violence.
5) Although no "violence center" exists in the brain, the
*limbic system and the *frontal lobes are areas most implicated
in violence. The limbic system is the neuroanatomic substrate for
many aspects of emotion. The limbic system structure most often
implicated in violent behavior is the *amygdala: placidity has
been described in humans with bilateral amygdala damage, whereas
violence has been observed in those with abnormal electrical
activity in the amygdala. The frontal lobes are apparently the
areas of the most advanced functions of the brain. In particular,
the *orbitofrontal cortices are involved in the inhibition of
aggression: individuals with orbitofrontal injury have been found
to display antisocial traits that justify the diagnosis of
"acquired sociopathy", and some of these individuals have an
increased risk of violent behavior. A balance apparently exists
between the potential for impulsive aggression mediated by limbic
structures, and the control of this drive by the influence of the
orbitofrontal regions.
6) The authors conclude: "Whereas dysfunction of a discrete
brain region, isolated neurochemical system, or single gene will
not likely emerge as a direct cause of violence, all may
contribute."
-----------
C.M. Filley et al: Violence and the brain: An urgent need for
research.
(The Scientist 2 Apr 01)
QY: Christopher M. Filley: University of Colorado 303-492-6694.
-----------
Text Notes:
... ... *magnetic resonance imaging: Magnetic resonance imaging
(MRI) is essentially a technique for examining morphology (as
opposed to _functional_ magnetic resonance imaging, which is a
technique for examining anatomical correlates of function). In
general, MRI involves magnetic coils producing a static magnetic
field parallel to the long axis of the patient or subject,
combined with inner concentric magnetic coils producing a static
magnetic field perpendicular to the long axis. A radio-frequency
coil specifically designed for the head perturbs the static
fields to generate a magnetic resonance image. The interaction
physics in this technique is that between the magnetic fields and
atomic nuclei in brain tissue. "Sliced" views can be obtained
from any angle, and the resolution is quite high and on the order
of millimeters for magnetic field strengths of 1.5 tesla.
... ... *functional neuroimaging: Functional magnetic resonance
imaging (fMRI) is based on the fact that oxyhemoglobin, the
oxygen-carrying form of hemoglobin, has a different magnetic
resonance signal than deoxyhemoglobin, the oxygen-depleted form
of hemoglobin. Activated brain areas utilize more oxygen, which
transiently decreases the levels of oxyhemoglobin and increases
the levels of deoxyhemoglobin, and within seconds the brain
microvasculature responds to the local change by increasing the
flow of oxygen-rich blood into the active area. This local
response thus leads to an increase in the oxyhemoglobin-
deoxyhemoglobin ratio, which forms the basis for the fMRI signal
in this technique. Because of its high spatial resolution
(millimeters) and high temporal resolution (seconds) compared to
other imaging techniques, fMRI is now the technology of choice
for studies of the functional architecture of the human brain.
Positron emission (PET) tomography is a technique for producing
cross-sectional images of the body after ingestion and systemic
distribution of safely metabolized positron-emitting agents. The
images are essentially functional or metabolic, since the
ingested agents are metabolized in various tissues.
Fluoro-deoxyglucose and H(sub2)O(sup15) are common agents used
for cerebral applications, and in cerebral applications of
central importance to the technique is the fact that changes in
the cellular activity of the brains of normal, awake humans and
unanesthetized laboratory animals are invariably accompanied by
changes in local blood flow and also changes in oxygen
consumption.
... ... *knockout models: In general, in this context,
"knockout technology" involves the generation of a mutant
organism (usually a mouse) with a missing specific gene.
... ... *serotonin metabolism: A neurotransmitter substance
involved in nearly everything occurring in the brain, including
psychological states such as anxiety and depression, and
dysfunctions producing migraine and epilepsy.
... ... *XYY chromosomal disorder: Humans ordinarily have 46
chromosomes. Of this number, 44 are not sex-related and are
called "autosomal". Two chromosomes, X and Y, are sex-related. An
individual with two X chromosomes is a female; an individual with
one X and one Y chromosome is a male. Approximately 1 in 1000
males have an extra Y chromosome (total 47 chromosomes), and this
abnormality is denoted as "47,XYY". Such individuals are often
characterized by tallness, severe acne, and sometimes skeletal
malformations and mental deficiency. It has been suggested that
the presence of an extra Y chromosome in an individual may cause
him to be more aggressive and prone to criminal behavior, but
recent studies of the general population have cast doubt on the
validity of this linkage.
... ... *limbic system: In general, this refers to those cortical
and subcortical structures ("cortical" refers to cerebral cortex)
concerned with the emotions. The most prominent anatomical
components of the limbic system are the cingulate gyrus, the
hippocampus, and the amygdala, all "deep brain" structures and
not visible on the exterior surface of the brain.
... ... *frontal lobes: One of the four lobes of the brain. The
other lobes are the parietal lobe, the temporal lobe, and the
occipital lobe. Each hemisphere has these 4 lobes.
... ... *amygdala: A cellular complex in the temporal lobe that
forms part of the limbic system. The major functional correlates
of the amygdala are autonomic nervous system behavior, emotional
behavior, and sexual behavior.
... ... *orbitofrontal cortices: The orbitofrontal cortex lies
directly under the forehead skull.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
6. PHYSIOLOGY: ON RHYTHMIC PROCESSES IN BIOLOGICAL SYSTEMS
Rhythmic processes abound in biology. Many such processes
are exogenous, cued by environmental periodicities, while other
rhythmic processes are endogenous, intrinsic to the organism. The
periodic beating of the heart is an example of an endogenic
rhythmic process. The diurnal cycle in many animals, cued to the
day/night environmental rhythm, is an example of an exogenous
periodicity. Many biological periodicities are evident at the
cellular level, in the nervous system, for example, and in
certain intracellular biochemical cycles. In general, there is no
biological system known which does not exhibit specific rhythms
at some level.
Many biological systems exhibit nonlinear periodicities,
i.e., periodicities that must be described by nonlinear
differential equations, and in analysis of such systems, a number
of concepts are of significance. In general, for a system with
(n) degrees of freedom, the nth-dimensional space mathematically
accessible to the system is called its "phase space", and each
system follows a curve in such a space called its "phase-space
trajectory". In the qualitative analysis of nonlinear systems, a
number of phase-space concepts are useful, particularly
"bifurcation" and "limit cycle".
A bifurcation represents the sudden appearance of a
qualitatively different solution for a nonlinear system as some
parameters are varied: the phase-space trajectory of the system
abruptly departs from its previous path. In a "pitch-fork
bifurcation", for example, a triplet of new paths may abruptly
appear with a variation of certain parameters.
In a phase space, any closed trajectory loop represents a
periodic behavior of the system, and certain nonlinear systems
exhibit trajectories that converge onto a closed loop, such a
loop called a "limit cycle".
The above considerations are quite useful when qualitatively
analyzing the behavior of intractable nonlinear differential
equations. Qualitative analytical methods for nonlinear systems
have been known for more than a century, and such methods have
been intensively applied to various nonlinear biological systems
since the 1950s.
... ... Leon Glass (McGill University, CA) presents a review of
current research on synchronization and rhythmic processes in
physiology, the author making the following points:
1) The author points out that the origin and dynamics of
rhythmic processes -- once the sole province of physicians and
experimental physiologists -- is increasingly a research focus of
mathematicians and physicists. Mathematical analyses of
physiological rhythms demonstrate that nonlinear equations are
necessary to describe physiological systems. In contrast to the
linear equations of traditional mathematical physics (e.g.,
Maxwell's equations, the heat equation, the wave equation, the
Schroedinger equation), nonlinear equations rarely admit an
analytical solution. Consequently, numerical simulations are one
essential feature of quantitative studies of physiological
systems. A complementary approach is the analysis of the
qualitative aspects of simplified mathematical models of
physiological systems. This involves a mathematical analysis of
those features of physiological systems that will be preserved by
classes of models sufficiently close to the real system. For
example, periodic stimulation of a nerve fiber gives rise to a
wide variety of regular and irregular rhythms that can be modeled
by simple as well as complex mathematical systems.
2) The author summarizes the three central concepts of
nonlinear dynamics: bifurcations, limit-cycle oscillations, and
chaos:
... ... a) Bifurcations are changes in qualitative properties of
dynamics. For example, as a parameter changes, steady states can
become unstable and lead to stable oscillations, or a system with
one stable steady state can be replaced by a system with multiple
steady states. Physiological correlates are immediate: Drugs may
lead to changes in control systems so that an abnormal, unhealthy
rhythm is replaced by a more normal rhythm. Mathematically, the
drug induces bifurcation in the dynamics, and as such, the
actions of the drug can be analyzed in a theoretical context.
Often, the same type of bifurcation can be found in a host of
different mathematical equations or experimental systems, and it
is common to consider the universal features of such
bifurcations. Because many diseases are classified and identified
by physicians based on characteristic qualitative features of
dynamics, there is a natural match between the emphasis on
qualitative dynamics in both mathematics and medicine.
... ... b) Stable limit-cycle oscillations are a key feature of
some nonlinear equations. Following a perturbation, a stable
limit-cycle oscillation re-establishes itself with the same
amplitude and frequency as before the perturbation. A
perturbation to a linear oscillation may lead to a new amplitude
of oscillation. For example, there is an intrinsic pacemaker that
sets the rhythm in the human heart. If one or more electric
shocks are delivered directly to the heart near the intrinsic
pacemaker, the heart rhythm is modified transiently but re-
establishes itself with the same frequency as before within a few
seconds.
... ... c) The term "chaos" refers to aperiodic dynamics in
deterministic equations in which there is a sensitivity to
initial conditions. This means that even in the absence of
stochastic processes, irregular rhythms can be generated.
Although it is easy to consider mathematical systems in which all
stochastic influences have been eliminated, in real physical and
biological systems it is impossible to eliminate stochastic
inputs. Thus, although chaotic dynamics is a clear mathematical
concept, applications of this concept to real biological systems
is a difficult undertaking.
-----------
Leon Glass: Synchronization and rhythmic processes in physiology.
(Nature 8 Mar 01 410:277)
QY: Leon Glass: Dept. of Physiology, McGill University, Montreal,
Quebec H3G 1Y6 CA.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 13Apr01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
THEORETICAL PHYSICS: CHAOS AND NONLINEAR DYNAMICS
In general, a nonlinear dynamical system is a system
described by time-dependent differential equations such that the
rates of change of one or more dependent variables of the system
depend in a nonlinear fashion on the variables themselves.
Certain nonlinear dynamical systems, some of which are of great
scientific interest, exhibit "chaotic dynamics". In this context,
the term "chaos" refers to unpredictable behavior arising in a
system that obeys deterministic laws but exhibits
unpredictability. The essential idea is that in certain systems
small perturbations may produce a cascade of larger
perturbations, so that eventually the behavior of such systems
cannot be predicted from prior states no matter if the systems
appear simple and obey deterministic laws. Examples of chaotic
nonlinear dynamical systems are the weather and populations of
organisms, and instances of chaotic dynamics have now been
documented in most scientific disciplines.
Because the differential equations for many nonlinear
systems are often intractable (i.e., no explicit quantitative
solutions are possible), a focus of theoretical research on
nonlinear systems has been on analysis of the qualitative
behavior of such systems, in particular on analysis of the "phase
space" and "trajectories" in the phase spaces of such systems.
The idea is essentially as follows: If the state of a system
depends upon N variables, the instantaneous state of the system
can be viewed as a point (phase point) in an N-dimensional space
(phase space; system hyperspace), and as the state of the system
changes, its phase point can be viewed as describing a trajectory
in its phase space. Qualitative analysis of the possible families
of solutions of nonlinear differential equations can provide
information about such phase space trajectories, and there are
certain real systems for which qualitative analysis of the phase
space trajectories of the system has revealed significant
properties of the system otherwise difficult to delineate.
... ... J.P. Gollub and M.C. Cross (2 installations, US) present
a commentary on recent research on chaotic nonlinear dynamics,
the authors making the following points:
1) The techniques of nonlinear dynamics are well-developed,
but the impact of this field has been largely confined to
phenomena in which there are only a few important time-dependent
quantities. Unfortunately, this excludes a vast range of
important problems in which the behavior of one point in space
can be quite different (though statistically similar) to that at
another location. A particular example is convective behavior.
2) The traditional approach to studying nonlinear dynamical
behavior is to plot the dynamical variables of the system as a
multidimensional phase space graph indicating how the behavior
changes over time. For example, a simplified model of the Solar
System consisting of the Sun and 9 planets would require a phase
space with as many as 60 dimensions (3 position and 3 momentum
coordinates for each body). In the case of a convecting fluid, a
complete description of the flow pattern requires knowledge of
the velocity and temperature at a very large number of locations,
so the number of dimensions of the phase plot are enormous (from
thousands to millions, depending on the desired spatial
resolution). As a result, the methods of nonlinear dynamics are
cumbersome and progress has been slow, even though many
interesting examples of spatiotemporal chaos have been explored
both experimentally and numerically.
3) Recent research (D.A. Egolf et al: Nature 404:733 2000)
involving numerical studies of an accepted model of thermal
convection indicates that the origin of unpredictable motion in
chaotic thermal convective systems, at least in one particular
form of spatiotemporal chaos, lies in what occurs in small
regions of space and over short time-scales. These local changes
in the organization of the flow affect the surrounding regions in
such a way that the entire future evolution is affected. The
authors state: "This is something akin to Ed Lorenz's famous
remark [E.N. Lorenz: J. Atmos. Sci. 20:130 1963] that the
localized flapping of a butterfly's wings might change the
weather dramatically over the entire world a few weeks later."
Although such sensitivity to localized fluctuations has never
been confirmed as the source of the unpredictability of the
weather, it is apparently the origin of chaotic dynamics in
thermal convection.
4) The authors conclude: "The methods used by Egolf et al
should apply to many other forms of chaos in spatially extended
systems (physical, chemical, and biological) for which reliable
model equations are available, so that the key processes leading
to the complex dynamics can be identified. Applications to areas
as diverse as cardiology and atmospheric dynamics might be
expected eventually. Moreover, it is not unreasonable to imagine
that insight into the processes leading to unpredictability will
also lead to progress in modifying or controlling the dynamics of
these systems."
-----------
J.P. Gollub and M.C. Cross: Chaos in space and time.
(Nature 13 Apr 00 404:710)
QY: J.P. Gollub: jgollub@haverford.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Jun00
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS: ON THE KINGDOMS OF LIFE
"Early biologists found it convenient to classify all living
things as either animals or plants. To many people today, this
grouping still seems perfectly adequate. However, examination of
the life-forms that exist on Earth shows that this classification
is unsatisfactory. Although there is a superficial resemblance
between green plants and the fungi, these two groups are divided
by profound biological differences. Unlike green plants, fungi
cannot manufacture their own food from water and carbon dioxide
by the process of photosynthesis. Rather, they require a supply
of organic matter from which they can derive their energy. Fungal
cellular composition is dissimilar from that of green plants, and
the structural polymers of their cell walls are markedly
different. Fungi are therefore now accorded their own status as a
third kingdom. Furthermore, for many years the classification of
microscopic organisms proved to be difficult. Photosynthetic
microbes behave very differently from higher plants. It was
therefore proposed towards the end of the 19th century that
microscopic life-forms should be classified as a fourth kingdom.
This was the kingdom "Protista", proposed in 1866, at a time when
the scientific study of microbiology was in its infancy. This
was, however, almost 200 years after Antonie van Leeuwenhoek
[1632-1723] described 'animalcules' following his development of
the optical microscope. During the 20th century there have been
many advances in microscopy, including the development of the
electron microscope. This has enabled subcellular structures to
be studied in great detail and has revealed that the Protista may
be divided into two major groups. Primitive microorganisms such
as bacteria lack a clearly-defined membrane-bound nucleus and are
called "prokaryotes". This word is derived from two Greek words,
_pro_, meaning before, and _karyon_, a kernel. Prokaryotes are
organisms that evolved before the cell's nucleus, its kernel, was
properly developed. More advanced microscopic life forms, as well
as having a variety of subcellular organelles, also possess a
proper membrane-bound nucleus. These are referred to as
"eukaryotes" because they have a true nucleus (Greek _eu-_, well,
true, or easy). Biologists now reserve the kingdom Protista for
eukaryotic microbial life-forms and separate prokaryotes into
their own kingdom, the "Monera". _Monera_ is 'new' Latin for non-
nucleated protoplasmic masses. Some microbiologists prefer to
refer to this kingdom as the Prokaryotes. Thus, life-forms may be
classified into 5 kingdoms: Animals, Plants, Fungi, Protista, and
Monera (or Prokaryotes)."
-----------
J. Heritage et al: _Introductory Microbiology_
(Cambridge University Press, Cambridge UK 1996, p.1)
-------------------
SCIENCE-WEEK http://scienceweek.com 13Apr01
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8. FROM THE SCIENCEWEEK ARCHIVE:
ON THE DISCOVERY OF ELECTROMAGNETIC WAVES
Sometime around the mid 1880s, the noted physicist Hermann
Helmholtz (1821-1894) suggested to one of his accomplished former
students that the student compete for a prize being offered by
the Berlin Academy of Science for work in the field of
electromagnetics. James Clerk Maxwell (1831-1879), the chief
theoretical architect of electromagnetics, was no longer on the
scene, and the field was in the doldrums. The former student of
Helmholtz had some interest in Maxwell's theoretical equations,
but not much interest in the Berlin Academy prize. Nevertheless,
he went to work to compete for the prize, and in 1888 the former
student of Helmholtz (the student now Professor of Physics at
Karlsruhe), Heinrich Hertz (1857-1894), presented to the world
the first theoretical prediction and experimental demonstration
of what later came to be called "radio waves". Hertz's
experimental demonstration was essentially as follows: If an
oscillating electrical potential is produced in an appropriate
circuit located at point A, that oscillation is propagated
through space and can be detected by an appropriate independent
electrical circuit located at point B, the two circuits having no
direct electrical connection (e.g., no wires) between them. Hertz
was 31 years old at the time of his presentation; what else he
might have done in physics in a full life was never to be known,
for he died before his 37th birthday of a blood disease. Thus
came into being one of the most important technological advances
of modern times -- the physics underlying radio communications,
microwave radiation, radar, satellite telecommunications, and so
on [*Note #1].
... ... Dominique Pestre (Ecole des Hautes Etudes en Sciences
Sociales Paris, FR) presents a biographical essay on Heinrich
Hertz and the discovery of radio waves and the controversy
surrounding the discovery, the author making the following
points:
1) The European physics community of the 1880s was divided
into two camps concerning the physics of electromagnetics. In one
camp were the British "Maxwellians", followers of James Clerk
Maxwell, who supported the idea of electromagnetic radiation
through an ether; in the other camp were the Continental
physicists such as Ernst Mach (1838-1916) and Hermann Helmholtz,
who viewed electromagnetics as an action-at-a-distance
phenomenon. "The first to seize upon Hertz's publications were,
of course, the British Maxwellians, as they were already
convinced that Maxwell was right. They welcomed the news that the
physics professor at Karlsruhe had found ways to produce
electromagnetic waves, to have them interfere, and to measure
their speed of propagation in air, which he found to be the
predicted 300,000 kilometers per second. They announced the
result everywhere and began mounting public demonstrations of the
marvel of the sparks induced at a distance by a Hertzian
generator."
2) In the following months, many physicists throughout the
Western world examined the issues. Studies of the various
parameters of the phenomenon were made, detectors and generators
constructed all over Europe, and new setups and diverse
interpretations conceived and put forward to explain the
phenomenon and certain apparent problems in the measurement of
wave propagation velocity. Many physicists quickly performed
experiments inspired by Hertz, all were able to generate sparks,
and Hertz came to be considered a true genius. "On the other
hand, most people were finally convinced by their own
experiments, their own devices and calculations, their own way of
adjusting proofs and expectations..."
3) The author concludes by noting the accounts of the
phenomenon in textbooks published 1888 to 1890 by Hertz, J.J.
Thomson (1856-1940), and Henri Poincare (1854-1912), with each
author providing a different proof and interpretation of the
observed phenomenon. "Hertz had made a major discovery, no doubt,
but what he had proved, and who had decisively improved our
understanding of this complex phenomenon, remained a matter of
opinion."
-----------
Dominique Pestre: Spark ignites physicists.
(Nature 21 Oct 99 401:745)
QY: Dominique Pestre, Ecole des Hautes Etudes en Sciences
Sociales Paris, FR.
-----------
Text Notes:
... ... *Note #1: In 1896, the first patent for wireless
telegraphy, i.e., the transmission of messages without wires, was
granted to Gugliemo Marconi in the UK. By the following year, the
Wireless Telegraphy Company had been formed to exploit the
invention, and in 1899, the Marconi Wireless Company of America
was set up. American Marconi, as it was called, soon began the
manufacture of wireless equipment for commercial and military
markets. Although Marconi's original invention (based on Hertz's
radio waves) was designed for fixed (point-to-point) and ship-to-
shore message communication, the idea of wireless as a one-way
medium to transmit speech to many people (i.e., "broadcasting")
was quick to follow. On Christmas Eve 1906, 18 years after the
discovery of radio waves by Hertz, Reginald Fessenden made the
first documented broadcast of speech and music from Brant Rock,
Massachusetts. His transmitter was a 1 kilowatt 50 hertz
alternator built by the General Electric Company. The signal was
received clearly in many locations and even on ships at sea. Lee
de Forest made some experimental broadcasts from New York in 1907
and from Paris in 1910. but his 500 watt transmitters were
inherently noisy. However, in 1906 one of de Forest's associates,
Henry Dunwoody, patented a solid-state detector using the newly
invented material carborundum. These crystal detectors soon
became the heart of early radio receivers. Used with headphones
which drew very little power, crystal sets had the great
advantage of not requiring any external source of electricity. In
summary, within a few decades of the discovery by Hertz of radio
waves, practical applications that would dramatically change
society were in place -- all of it an early part of the ongoing
technological upheaval to be witnessed by the 20th century.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 10Dec99
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