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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.
March 23, 2001 -- Vol. 5 Number 12
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Concepts are games we play with our heads; methods
are games we play with our hands, which at times are
so handy they can be played without a head.
-- Frank E. Egler
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Section 1
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Contents of this Issue (Full reports in Section 2):
1. NEUROBIOLOGY:
STRUCTURE OF THE VOLTAGE-GATED SODIUM CHANNEL
Sodium and calcium channels are involved in cell excitation,
neuronal transmission, muscle contraction, and many functions
that relate directly to human diseases. Sodium channels, which
are glycosylated proteins with a relative molecular mass of
approximately 300,000, are responsible for signal transduction
and amplification, and are chief targets of anesthetic drugs and
neurotoxins. Researchers now report the 3-dimensional structure
of the voltage-sensitive sodium channel of the eel Electrophorus
electricus. The structure was determined at 19 angstroms
resolution by helium-cooled cryo-electron microscopy and single-
particle image analysis of the solubilized sodium channel. The
authors report the channel has a bell-shaped outer surface of 135
angstroms in height and 100 angstroms in side length at the
square-shaped bottom, and a spherical top with a diameter of 65
angstroms. Several inner cavities are connected to 4 small holes
and 8 orifices close to the extracellular and cytoplasmic
membrane surfaces. (C. Sato et al: Nature 22 Feb 01 409:1047)
2. ASTROBIOLOGY:
AN ARGUMENT FOR THE UNIVERSAL NATURE OF BIOCHEMISTRY
It is suggested that considering the properties of molecules
likely to be needed to replicate and evolve (two of the
characteristics that define life), it is predictable that life
that we encounter anywhere in the Universe will be composed of
organic chemicals that follow the same general principles as our
own organic-based terrestrial life. The operational definition of
life then becomes: Life is a self-replicating, evolving system
expected to be based on organic chemistry. It is also suggested
that life, wherever we encounter it, will be composed of
macromolecules, and that only two of the natural atoms, carbon
and silicon, are known to serve as the backbone of molecules
sufficiently large to carry biological information. As the
structural basis for life, one of the important features of
carbon is that unlike silicon it can readily engage in the
formation of chemical bonds with many other atoms, thereby
allowing for the chemical versatility required to conduct the
reactions of biological metabolism and propagation.
(Norman R. Pace: (Proc. Natl. Acad. Sci. US 30 Jan 01 98:805)
3. ANTHROPOLOGY:
PALEOLITHIC TECHNOLOGY AND HUMAN EVOLUTION
Paleoanthropologists once considered tool-making to be one of the
defining characteristics of the genus Homo. However, the
diversity of tool-making and tool-using behaviors among
chimpanzees has forced a complete revision of assumptions
surrounding the concept of "man the toolmaker", including
revision of ideas concerning the gender of the first tool users.
Chimpanzees have diverse and regionally varied repertoires of
tool-using, and other "cultural" behaviors. In contrast, Cebus
monkeys are considered prolific tool users but exhibit no
apparent understanding of cause and effect, or of the difference
between appropriate and inappropriate tools. With the appearance
of near-modern brain size, anatomy, and perhaps of grammatical
language approximately 300,000 years ago, the pace of the
evolution of human technology quickened exponentially. A mere
12,000 years separate the first bow and arrow from the
International Space Station.
(S.H. Ambrose: Science 2 Mar 01 291:1748)
4. ASTROPHYSICS: ON THE SHAPES OF PLANETARY NEBULAE
Planetary nebulae are believed to be formed when a slow stellar
wind from the progenitor giant star is overtaken by a subsequent
fast wind generated as the star enters its white dwarf stage. A
shock forms near the boundary between the winds, creating the
relatively dense shell characteristic of a planetary nebula. A
spherically symmetric wind will produce a spherically symmetric
shell, yet over half of known planetary nebulae are not
spherical, but are elliptical or bipolar in shape. A magnetic
field could launch and collimate a bipolar outflow, but the
origin of such a field has until now been unclear, and some
researchers have even suggested that such a field could not be
generated. A new model now demonstrate that terminal giant stars
can indeed generate a strong magnetic field, and that these
fields are strong enough to shape the bipolar outflows that
produce the observed bipolar planetary nebulae.
(E.G. Blackman et al: Nature 25 Jan 01 409:485)
5. MATERIALS SCIENCE: A NEW HIGH-TEMPERATURE SUPERCONDUCTOR
The low-temperature superconductivity theory of Bardeen, Cooper,
and Schrieffer (BCS theory) apparently cannot provide a complete
explanation of the behavior of high-temperature ceramic
superconductors. A version of BCS theory may explain how
superconductivity occurs in certain ceramic materials, but no
complete theory of high-temperature superconductivity in ceramic
materials has yet been proposed. Recently, researchers in
superconductivity were startled when J. Nagamatsu et al (2001)
reported the discovery of bulk superconductivity in the simple
and readily available compound magnesium diboride, with
magnetization and resistivity measurements establishing a
transition temperature of 39 degrees kelvin, the highest known
critical temperature for a non-copper-oxide (non-ceramic) bulk
superconductor. The surprise of condensed-matter physicists at
this new discovery is reminiscent of the surprise of the same
community at the discovery by Mueller and Bednorz in 1986 of
high-temperature ceramic superconductors.
(R.J. Cava: Nature 1 Mar 01 410:23)
6. HISTORY OF PHYSICS: ON THE MEASUREMENT OF TIME
Until the definition of the second in terms of atomic time in
1967, most work in standards laboratories was devoted to
developing secondary standards, such as lumped-element circuits
and quartz crystals, whose resonant frequencies could be
calibrated relative to Ephemeris Time. But frequencies derived
from resonant transitions in atoms or molecules offer important
advantages over macroscopic oscillators. Any unperturbed atomic
transition is identical from atom to atom, so two clocks based on
such a transition should generate the same time. Also, unlike
macroscopic devices, atoms do not wear out, and as far we know
they do not change their properties over time. The adoption of
the International System (SI) second, defined on the basis of
atomic phenomena, as the fundamental time unit, occurred
provisionally in 1964 and finally in 1967. A second is now
defined as 9,192,631,770 cycles of radiation associated with the
transition between the two hyperfine levels of the ground state
of the cesium-133 atom.
(J.C. Bergquist et al: Physics Today March 2001)
7. IN FOCUS: HISTORY OF CHEMISTRY: ON GASES
8. FROM THE SCIENCEWEEK ARCHIVE:
ON PALEONTOLOGY AND EVOLUTIONARY BIOLOGY
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Section 2
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1. NEUROBIOLOGY:
STRUCTURE OF THE VOLTAGE-GATED SODIUM CHANNEL
Electrical signals in the nervous system are generated by
the movement of ions across the nerve cell membrane. These ionic
currents flow through aqueous pores of membrane proteins known as
"ion channels", and these channels vary in ion selectivity, with
some channels specifically permeable to sodium or potassium or
calcium ions.
Certain sodium, potassium, and calcium channels are
"voltage-gated" -- their permeability is switched on and off by
changes in the potential difference across the cell membrane --
and these channels essentially control the dynamic electric
activity of nerve and muscle cells.
The voltage-activated sodium channel from eel electric
organs (of relevance in this report) is a single molecule of
approximately 1800 amino acids, within which are 4 repeating
domains. These domains are architecturally equivalent to the
subunits of other ion channels. Within each domain, there are
apparently 6 membrane spanning regions connected by intracellular
and extracellular polypeptide loops. The eel channel is
apparently representative of a diverse family of channel proteins
present in nerve and muscle fibers.
Fifty years ago, the existence of such specific protein
channels was not even conceived, although it was recognized there
had to be some explanation for specific ion currents in nerve and
muscle cells. Thirty years ago, the existence of such channels
was vaguely proposed, but with hardly any molecular information.
In the past decade, ion channels have become definitive
structural entities.
... ... C. Sato et al (7 authors at 5 installations, JP CH)
present a report on the structure of the sodium-sensitive ion
channel, the authors making the following points:
1) The authors point out that the voltage-sensitive sodium,
potassium, and calcium channels operate together to amplify,
transmit, and generate electric pulses in animals. Sodium and
calcium channels are involved in cell excitation, neuronal
transmission, muscle contraction, and many functions that relate
directly to human diseases. Sodium channels, which are
glycosylated proteins with a relative molecular mass of
approximately 300,000, are responsible for signal transduction
and amplification, and are chief targets of anesthetic drugs and
neurotoxins.
2) The authors report the 3-dimensional structure of the
voltage-sensitive sodium channel of the eel Electrophorus
electricus. The structure was determined at 19 angstroms
resolution by helium-cooled cryo-electron microscopy and single-
particle image analysis of the solubilized sodium channel. The
authors report the channel has a bell-shaped outer surface of 135
angstroms in height and 100 angstroms in side length at the
square-shaped bottom, and a spherical top with a diameter of 65
angstroms. Several inner cavities are connected to 4 small holes
and 8 orifices close to the extracellular and cytoplasmic
membrane surfaces. Homologous voltage-sensitive calcium and
tetrameric potassium channels, which regulate secretory processes
and the membrane potential, may possess related structure.
-----------
C. Sato et al: The voltage-sensitive sodium channel is a bell-
shaped molecule with several cavities.
(Nature 22 Feb 01 409:1047)
QY: Chikara Sato: tisato@etl.go.jp
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
NEUROBIOLOGY: ATOMIC SCALE MOVEMENTS IN POTASSIUM CHANNELS
The functional electrical activity of nerve cells is based
essentially on the rapid movements of ions across the membranes
of these cells, especially the movements of sodium and potassium
ions. These ion movements occur through special pores ("ion
channels") in the cell membrane, and one of the important
problems during the past few decades has been to characterize
these ion channels at the molecular level. Most ion channels are
selective, allowing only ions of a certain type to pass, and an
individual cell has ion channels with various ion selectivities.
The selectivity of an ion channel can be "gated", the channel
effectively opened or closed, and ion channels are said to
"voltage-gated" or "ligand-gated", depending on how the change in
selectivity is provoked. The term "voltage-gated" refers to the
opening or closing of an ion channel by changes in the electrical
potential across the membrane, while the term "ligand-gated"
refers to opening and closing of an ion channel by interactions
between ligands and membrane receptors. It has become apparent
that voltage-gated ion channels are transmembrane proteins
consisting of 4 identical subunits, each of which contains 6
transmembrane segments. Studies of the potassium ion channel have
identified two segments that contain several charged protein
residues, and these charged residues apparently sense changes in
the potential difference across the membrane and form part of the
membrane "voltage sensor". Although these regions apparently
undergo conformational changes in response to changes in membrane
potential, little is known about the nature of these changes.
... ... A. Cha et al (4 authors at 2 installations, US) report a
study of molecular movements of the voltage-sensing region in a
potassium channel, the authors making the following points:
1) The authors used *lanthanide-based fluorescence resonance
energy transfer to measure distances between *Shaker potassium-
channel protein subunits at specific residues. Voltage dependent
distance changes of up to 3.2 angstroms were measured at several
sites near one of the charged protein segments (S4). These
movements directly correlated with electrical measurements of the
voltage sensor, establishing a link between physical changes and
electrical charge movement.
2) The authors suggest that the measured distance changes
indicate that the region associated with the S4 segment undergoes
a rotation and possible tilt, rather than a large transmembrane
movement, in response to voltage.
3) The authors conclude: "These results demonstrate the
first in situ measurement of atomic scale movement in a
transmembrane protein."
... ... In a contiguous and related report, K.S. Glauner et al (4
authors at University of California Berkeley, US) present a study
of voltage-sensor movements of a potassium channel, the authors
making the following points:
1) The authors used fluorescence resonance energy transfer
as a "spectroscopic ruler" to determine distances between S4
subunits in the Shaker potassium channel in different gating
states.
2) The authors conclude their experimental evidence is
consistent with the S4 subunit being a tilted helix that twists
during activation. The authors propose that helical twist
contributes to the movement of charged side chains across the
membrane electric field and that this movement is involved in
coupling voltage-sensing to gating.
-----------
A. Cha et al: Atomic scale movement of the voltage-sensing region
in a potassium channel measured via spectroscopy.
(Nature 16 Dec 99 402:809)
QY: Francisco Bezanilla: fbesanil@ucla.edu
-----------
K.S. Glauner et al: Spectroscopic mapping of voltage sensor
movement in the Shaker potassium channel.
(Nature 16 Dec 99 402:813)
QY: E.Y. Isacoff: eisacoff@socrates.berkeley.edu
-----------
Text Notes:
... ... *lanthanide-based fluorescence resonance energy transfer:
The term "fluorescence resonance energy transfer" (also called
"fluorescence energy transfer") refers to energy transfer between
two fluorophores (chemical groups or molecules capable of
fluorescence). If the two fluorophores are attached to a molecule
at different positions, observations of fluorescence energy
transfer between them can be used to determine the distance
between the two attachment positions. Lanthanide-based resonance
energy transfer is a modification of conventional fluorescence
resonance energy transfer in which a long-lived lanthanide donor
transfers energy in a distance-dependent manner to a conventional
organic fluorescent acceptor. This technique has previously been
used to measure angstrom-scale conformational changes in
proteins.
... ... *Shaker potassium-channel protein: The term "Shaker" here
refers to a mutant of the fruitfly Drosophila, the mutant
exhibiting intense shaking of the legs and body in response to
exposure to a volatile anaesthetic. Genetic analysis of the
mutation some years ago led to the sequencing of a Drosophila
gene expressing a potassium-channel protein, the shaking of the
insect apparently resulting from a mutation in this gene, with
the mutation producing long-lasting potassium ion currents when
nerve fibers are activated. Once the Shaker gene was sequenced, a
conventional procedure was developed in the 1980s to have this
gene expressed in frog egg cells (oocytes) in order to advance
the study of the behavior of potassium ion channels. Thus, the
potassium ion channels in this report are ion channels derived
from the fruitfly Drosophila and expressed in the frog Xenopus
laevis egg cell. The essential aspects of frog oocytes is that
they are large cells (up to 1 millimeter in diameter), and the
introduction of foreign *messenger RNA into the egg cell readily
results in the production of the protein encoded by the
introduced messenger RNA.
... ... *messenger RNA: (mRNA) The ribonucleic acid molecule
transcribed from DNA that carries the coded information
specifying the sequence of amino acids in a protein.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 11Feb00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
STRUCTURAL REARRANGEMENTS IN POTASSIUM CHANNEL GATING
Ion channels are protein channels in cell membranes that allow
ions to pass from extracellular solution to intracellular
solution and vice versa. Most ion channels are selective,
allowing only certain ions to pass, and an individual cell has
ion channels with various ion selectivities. The selectivity of
an ion channel can be "gated", the channel effectively opened or
closed, and ion channels are said to voltage-gated or ligand-
gated, depending on how the change in selectivity is provoked.
The opening of a previously closed ion channel produces a sudden
increase in transmembrane conductance for that ion, and the
process is called "activation". The gating of the movements of
ions through ion channels is of considerable importance for
various processes in all living systems, and forms the basis of
the electrical activity of all nervous systems. Recently (see
background material below), an important advance in ion-channel
research occurred with the experimental determination of the
crystal structure of a potassium channel (KcsA) in the bacterial
species Streptomyces lividans. The structure involves a
tetrameric complex with a centrally located pore framed by the
apposition of individual subunits, each subunit with 2
transmembrane helices (TM1 and TM2) flanking a "selectivity
filter". Intensive studies of this potassium channel in *planar
lipid bilayers have been in progress in a number of laboratories.
... ... E. Porozo et al (3 authors at University of Virginia, U)
now report a study of the structural rearrangements underlying
activation gating in this potassium channel, the study using
*spin-labeling methods and *electron paramagnetic resonance
spectroscopy. The authors report that a comparison of the closed
and open conformations of the channel revealed periodic changes
in spin-label mobility and intersubunit *spin-spin interaction
consistent with rigid-body movements of the two transmembrane
helices TM1 and TM2. These changes involve translations and
counterclockwise rotations of both helices relative to the center
of symmetry of the channel. The movement of TM2 apparently
increases the diameter of the permeation pathway along the point
of convergence of the four subunits, thus opening the pore.
Although the extracellular residues flanking the selectivity
filter remained immobile during gating, small movements were
detected at the *C-terminal end of the pore helix, and the
authors suggest this has possible implications for the gating
mechanism.
-----------
E. Perozo et al: Structural rearrangements underlying
K(+)-channel activation gating.
(Science 2 Jul 99 285:73)
QY: Eduardo Perozo: eperozo@virginia.edu
-----------
Text Notes:
... ... *planar lipid bilayers: The cell membrane consists of a
lipid bilayer and associated proteins, the ensemble approximately
75 to 100 angstroms in thickness. Similar membranes are also
found within a cell surrounding various organelles. Lipid
bilayers are spontaneously forming self-organizing bimolecular
layers of certain molecules (lipids) with long nonpolar chains
terminated by a polar group. In addition to their presence in
cell membranes, such molecules (surfactants) are also found in
soaps. A variety of artificial lipid bilayer membrane systems can
be investigated in the laboratory.
... ... *spin-labeling methods: A "spin-label" is a synthetic
paramagnetic organic free radical incorporated in a macromolecule
or assemblage of macromolecules and used, in particular, in
electron paramagnetic resonance spectroscopy.
... ... *electron paramagnetic resonance spectroscopy: (ESR) This
technique is used to investigate paramagnetic centers in a
molecular system. Only electrons whose spin is not paired with
the oppositely directed spin of another electron give an ESR
signal. With this technique, information can be obtained about
certain transitional ions, free radicals, and free electron
centers. A probe giving an ESR signal can be incorporated into
membrane lipids or attached to proteins to enable otherwise
inaccessible systems to be studied. Through analysis of ESR
spectra, rates of molecular motion and relative orientation of
spin-labeled molecules whose motion is restrained by surrounding
molecules can be determined. Measurements of rates of molecular
motion and molecular orientation have proved to be important in
the study of a variety of biological problems.
... ... *spin-spin interaction: In this context, an interaction
of two neighboring paramagnetic entities, the interaction
producing a change in ESR signal.
... ... *C-terminal end: In general, this refers to the end of
any polypeptide chain at which the 1-carboxy function of a
constituent alpha-amino acid is not attached in peptide linkage
to another amino acid residue.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 13Aug99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ANALYSIS OF POTASSIUM ION MEMBRANE CHANNEL STRUCTURE
... The potassium ion channel from the prokaryotic soil bacterium
Streptomyces lividans is an integral membrane protein with
sequence similarity to all known potassium ion channels,
particularly in the pore region. ... ... Doyle et al (8 authors
at Rockefeller University, US) report an x-ray analysis (data to
3.2 angstroms) of the Streptomyces lividans potassium channel
reveals four identical subunits create an inverted cone cradling
the selectivity filter of the pore in its outer end. The narrow
selectivity filter is only 12 angstroms long, whereas the
remainder of the pore is wider and lined with hydrophobic amino
acids. The selectivity filter is apparently held open by
structural constraints to coordinate potassium ions but not
smaller sodium ions. The authors suggest the architecture of the
pore establishes the physical principles underlying selective
potassium ion conduction.
QY: Roderick MacKinnon: mackinn@rockvax.rockefeller.edu
(Science 3 Apr 98) (Science-Week 17 Apr 98)
-------------------
Related Background:
SIMILAR STRUCTURE OF PROKARYOTIC VS. EUKARYOTIC K(+) CHANNELS
Toxins from scorpion venom are known to interact with potassium
ion channels in eukaryotic cell membranes. Mackinnon et al (5
authors at Rockefeller University, US) report the use of resin-
attached mutant potassium ion channels from the bacterium
Streptomyces lividans to screen scorpion venom, and the toxins
that interact with the channel were identified by mass
spectrometry. The authors suggest their results indicate that the
prokaryotic potassium ion channel, whose structure has now been
revealed, has the same pore structure as eukaryotic potassium ion
channels, and that this structural conservation, through the
application of their techniques, offers a new approach to
potassium ion channel pharmacology.
QY: Roderick MacKinnon: mackinn@rockvax.rockefeller.edu
(Science 3 Apr 98) (Science-Week 17 Apr 98)
For more information: http://scienceweek.com/swfr.htm
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2. ASTROBIOLOGY:
AN ARGUMENT FOR THE UNIVERSAL NATURE OF BIOCHEMISTRY
In contrast to the view that extraterrestrial life may involve a
chemistry and physics not yet imagined on Earth, there is the
view that the fundamentals of extraterrestrial life will be very
similar to the fundamentals of life on Earth. Although this
latter view is considered by many to be too conservative [see
related background material below], it has the marked practical
advantage, in the context of an actual search for
extraterrestrial life, of familiarity: at least we know what we
are looking for.
... ... Norman R. Pace (University of Colorado Boulder, US)
presents a review of the apparent constraints of biochemistry on
the nature of extraterrestrial life, the author making the
following points:
1) The author suggests that considering the properties of
molecules likely to be needed to replicate and evolve (two of the
characteristics that define life), it is predictable that life
that we encounter anywhere in the Universe will be composed of
organic chemicals that follow the same general principles as our
own organic-based terrestrial life. The operational definition of
life then becomes: Life is a self-replicating, evolving system
expected to be based on organic chemistry.
2) The author suggests that the basic drive of life is to
make more of itself. The chemical reactions required for the
faithful propagation of a free-living organism necessarily
require high degrees of specificity in the interactions of the
molecules that carry out the propagation. The author suggests
that such specificity requires information in the form of complex
molecular structure -- large molecules. The molecules that serve
terrestrial organisms typically are very large -- proteins and
RNAs with molecular weights of thousands to millions of daltons,
or even larger, as in the case of genetic DNA. The author
suggests that it is predictable that life, wherever we encounter
it, will be composed of macromolecules.
3) The author suggests that only two of the natural atoms,
carbon and silicon, are known to serve as the backbone of
molecules sufficiently large to carry biological information.
Thought on the chemistry of life has generally focused on carbon
as unique. As the structural basis for life, one of the important
features of carbon is that unlike silicon it can readily engage
in the formation of chemical bonds with many other atoms, thereby
allowing for the chemical versatility required to conduct the
reactions of biological metabolism and propagation. The various
organic functional groups, composed of hydrogen, oxygen,
nitrogen, phosphorus, sulfur, and a host of metals, such as iron,
magnesium, and zinc, provide the enormous diversity of chemical
reactions necessarily catalyzed by a living organism. Silicon, in
contrast, interacts with only a few other atoms, and the large
silicon molecules are monotonous compared with the combinatorial
universe or organic macromolecules.
-----------
Norman R. Pace: The universal nature of biochemistry.
(Proc. Natl. Acad. Sci. US 30 Jan 01 98:805)
QY: Norman R. Pace: nrpace@colorado.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ASTROBIOLOGY: ON INTELLIGENT LIFE IN THE UNIVERSE
... ... J. Cohen and I. Stewart (University of Warwick, UK)
present an essay on current ideas concerning intelligent
extraterrestrial life (aliens), the authors making the following
points:
1) The authors point out that it is possible to imagine the
existence of forms of life very different from those found on
Earth, occupying habitats that are unsuitable for our kind of
life. Some of those aliens might be technological, because
technology is an autocatalytic process, and it follows that some
aliens might possess technology well in advance of our own,
including interstellar transportation. So much is clear, but this
train of logic begs the obvious question of where these
intelligent non-humanoid aliens might be.
2) The authors point out that the subject area of this
discussion is often called "astrobiology", although in science
fiction circles (where the topic has arguably been thought
through more carefully than it has been in academic circles) the
term "xenobiology" is favored. The authors suggest the difference
is significant: Astrobiology is a mixture of astronomy and
biology, and the tendency is to assume that the field must be
assembled from contemporary astronomy and biology; in contrast,
xenobiology is the biology of the strange, and the name
inevitably involves the idea of extending contemporary biology
into new and alien realms.
3) The authors ask: Upon what science should xenobiology be
based? The authors suggest that the history of science indicates
that any discussion of alien life will be misleading if it is
based on the presumption that contemporary science is the
ultimate in human understanding. Consider the position of science
a century ago. We believed then that we inhabited a newtonian
clockwork Universe with absolute space and absolute time; that
time was independent of space; that both were of infinite extent;
and that the Universe had always existed, always would exist, and
was essentially static. We knew about the biological cell, but we
had a strong feeling that life possessed properties that could
not be reduced to conventional physics; we had barely begun to
appreciate the role of natural selection in evolution; and we had
no idea about genetics beyond mendelian numerical patterns. Our
technology was equally primitive: cars were inferior to the
horse, and there was no radio, television, computers,
biotechnology or mobile phones. Space travel was the stuff of
fantasy. If the past is any guide, then almost everything we now
think we know will be substantially qualified or proven wrong
within the next 25 years, let alone another century. Biology, in
particular, will not persist in its current primitive form. At
present, biology is at a stage roughly analogous to physics when
Newton (1642-1727) discovered his law of gravity. "There is an
awfully long way to go."
4) The authors point out that evolution on Earth has been in
progress for at least 3.8 billion years. "This is deep time --
too deep for scenarios expressed in human terms to make much
sense. A hundred years is the blink of an eye compared with the
time that humans have existed on Earth. The lifespan of the human
race is similarly short when compared with the time that life has
existed on Earth. It is ridiculous to imagine that somehow, in a
single century of human development, we have suddenly worked out
the truth about life. After all, we do not really understand how
a light switch works at a fundamental level, let alone a
mitochondrion."
-----------
J. Cohen and I. Stewart: Where are the dolphins?
(Nature 22 Feb 01 409:1119)
QY: Jack Cohen: Mathematics Institute, University of Warwick,
Coventry CV4 7AL, UK.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 9Mar01
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
3. ANTHROPOLOGY:
PALEOLITHIC TECHNOLOGY AND HUMAN EVOLUTION
The evolution of the use of stone tools by ancient humans
has been demarcated by anthropologists into the following stages
in a time-frame from the beginning of stone-tool technology
approximately 2.5 million years ago to the end of the "Old Stone
Age" approximately 40,000 years ago.
a) The earliest stone-tool technology is based on simple
chopping tools made by knocking several flakes off a small
cobble, the "core" stone.
b) The second stage is characterized by tools that require
more extensive conceptualization and preparation, such as
bifacial handaxes.
3) In the 3rd stage, large stones were preshaped by the
removal of large flakes and these were then used as a source of
more standardized flakes that were retouched to produce a large
range of artifacts.
4) In the 4th stage, stone-tool technology was characterized
by narrow stone blades struck from a prepared core.
5) The 5th stage consisted of microlith technology involving
the production of small and delicate artifacts.
The above is only a general schema, with some categories
overlapping when earlier technology persists after the appearance
of later technology. Differences in the African and Eurasian
records are believed to reflect the dynamics of the origin and
migration of anatomically modern humans. Of significance, for
example, is the evidence that blade tools were produced in Africa
nearly 250,000 years ago, but did not enter the Eurasian record
until 40,000 years ago. Categorizations are also dependent on
differences in the histories of the sciences of archeology and
anthropology.
The term "Paleolithic" (Old Stone Age) is essentially an
archeological term applied to Eurasia with approximate time-frame
segments as follows:
Upper Paleolithic: from 40,000 to 8000 years ago.
Middle Paleolithic: from 200,000 to 40,000 years ago.
Lower Paleolithic: from 2.5 million to 200,000 years ago.
The term "hominid" refers, in general, to any primate in the
human family.
... ... Stanley H. Ambrose (University of Illinois Urbana
Champaign, US) presents a review of Paleolithic technology in the
context of human evolution, the author making the following
points:
1) The author points out that paleoanthropologists once
considered tool-making to be one of the defining characteristics
of the genus Homo. However, the diversity of tool-making and
tool-using behaviors among chimpanzees (Pan troglodytes) has
forced a complete revision of assumptions surrounding the concept
of "man the toolmaker", including revision of ideas concerning
the gender of the first tool users. Chimpanzees have diverse and
regionally varied repertoires of tool-using, and other "cultural"
behaviors. In contrast, Cebus monkeys are considered prolific
tool users but exhibit no apparent understanding of cause and
effect, or of the difference between appropriate and
inappropriate tools.
2) The earliest direct evidence of hominid technology dates
to 2.5 million years ago in the Ethiopian Rift Valley, the
artifacts including sharp-edged slivers and lumps of stone,
hammer stones and anvils, and bones with hammer marks and cut
marks from butchery and marrow extraction. This simple technology
is called the "Oldowan Industrial Complex", after excavation
localities in Olduvai Gorge, Tanzania. Early hominids apparently
possessed an excellent empirical understanding of the mechanical
properties of lithic raw materials, fracture mechanics, and
geometry.
3) *Homo habilis is usually considered the first tool maker.
Cranial internal-cavity casts (endocasts) of these fossils show
that its left brain hemisphere has an impression of Broca's area,
the cortical area involved in speech and language, an area that
is adjacent to and probably derived from the area for precise
hand-motor control. The author points out that approximately 90
percent of humans are right-handed, and hand preference is
strongest in skilled tool use involving a precision grip.
Individual chimpanzees exhibit long-term consistency of hand
preference mainly for complicated tool-using tasks, but there is
no overall preference among chimpanzees for right-handedness at
the population level.
3) Large cutting tools, typically approximately 10 to 17
centimeters long, were apparently added to the Oldowan toolkit
approximately 1.5 million years ago, and this marks the advent of
the so-called "Archeulean Industrial Complex". Archeulean
technology is associated with the fossils of H. erectus and H.
heidelbergensis, in the time-frame 1.5 to 0.3 million years ago.
Large flakes, slabs, and cobbles were shaped into large cutting
tools by bidirectional or unidirectional invasive trimming of
lateral edges. Discovered handaxes from this period typically
have a tear-drop-shape and a lenticular cross-section. Cleavers
have a sharp, thin, usually unmodified edge transverse to the
long axis. Picks and knives have convergent tips, like handaxes.
4) Technological and cultural evolution accelerated
approximately 300,000 years ago, during the Middle Paleolithic
period in Eurasia, and during its sub-Saharan African correlate,
the "Middle Stone Age". These advances were made by Neanderthals,
by late archaic humans, and by anatomically modern humans.
Regional stylistic and technological variants are clearly
evident, suggesting the emergence of true cultural traditions and
culture areas. Large cutting tools were supplanted by smaller
tools, and there is evidence of a sophisticated technology for
producing relatively standardized artifacts, which may reflect
more complex cognitive abilities. Stone-tipped spears, knives,
and scrapers mounted in shafts and handles represent a
significant increase in technological complexity.
5) Although blade-based lithic technologies occurred
throughout the Middle Paleolithic period, more sophisticated
technologies appeared approximately 50,000 years ago in East
Africa and the Levant. Blade production substantially increased
the number of usable sharp edges that could be obtained from a
core. Standard blade blanks were shaped into a diverse array of
functionally and stylistically distinct tool types, often as
components of tools of greater complexity. Of greater
significance are ground, polished, drilled, and perforated bone,
ivory, antler, shell, and stone, shaped into projectiles,
harpoons, buttons, awls, needles, and ornaments. Such artifacts
are extraordinarily rare in Middle Paleolithic sites, but are a
consistent feature of Upper Paleolithic and Later Stone Age sites
after 40,000 years ago.
6) The author concludes: "Did the challenges posed by the
increasingly variable, severe, and risky environments of
glacial/interglacial cycles over the past 800,000 years, as well
as more dramatic short-term climatic events, influence behavioral
and biological evolution? Or were changes increasingly
autocatalytic, driven by language and by cultural systems of
knowledge and understanding of nature and society? With the
appearance of near-modern brain size, anatomy, and perhaps of
grammatical language approximately 300,000 years ago, the pace
quickens exponentially... A mere 12,000 years separate the first
bow and arrow from the International Space Station."
-----------
Stanley H. Ambrose: Paleolithic technology and human evolution.
(Science 2 Mar 01 291:1748)
QY: Stanley H. Ambrose: ambrose@uiuc.edu
-----------
Text Notes:
... ... *Homo habilis: In 1964, an early fossil *hominin (1.9 to
1.6 million years before the present) was found in Olduvai,
Tanzania, the brain apparently intermediate in size between the
earliest known Homo fossil *Homo erectus and the
*Australopithecus group. This new fossil was denoted as a new
species by its discoverers and named Homo habilis. The original
set of H. habilis fossils included a relatively complete hand,
its structure apparently compatible with an ability to make and
use tools. (Homo habilis literally means "handy-man".)
Considerable controversy in the paleoanthropology community
concerning H. habilis has continued from 1964 until the present.
... ... *hominin: In general, any human-related fossil group.
... ... *Homo erectus: First discovered by Eugene Dubois in 1891
in Indonesia, this fossil group is currently viewed as the
closest precursor to H. sapiens. Formerly called "Anthropithecus
erectus" and "Pithecanthropus erectus". Pithecanthropus erectus
and Sinanthropus erectus ("Peking man", discovered in 1927) were
in 1951 subsumed under the single category Homo erectus, which
was then recognized as a widespread species exhibiting
significant geographical variation.
... ... *Australopithecus: A now extinct genus believed to have
existed between 4.4 and 1 million years ago, and believed to have
been precursors of the genus Homo. All australopithecines are
apparently characterized by an ape-like form, rather than the
human-like form of the Homo genus.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON MODERN HUMAN ORIGINS
The study of human origins, the field called paleoanthropology,
has intrinsic difficulties because of the relative scarcity of
data, but these difficulties are magnified enormously by the
simple fact that paleoanthropology, in essence, represents a
species attempting to reconstruct its own early history. As might
be expected, an objective reconstruction, one without biases and
preconceptions, is far from easy. The human group we call the
"Neanderthals" lived in much of Europe, part of Asia, and the
Middle East between 150,000 to probably less than 30,000 years
ago. Neanderthals were the first fossil humans to be discovered,
and they have long been the focus of anthropological
investigation. More bones of Neanderthals are known than for any
other human-related (hominine) fossil group, including 30 nearly
complete skeletons, so the preoccupation of the anthropology
community with the Neanderthals is perhaps understandable. One of
the important questions concerning the Neanderthals is what
happened to them? Hypotheses have shifted back and forth since
the first discovery in 1856 of Neanderthal bones, with two major
views. One view is that the Neanderthals were the direct
ancestors of modern Europeans. The other view regards the
Neanderthals as a side branch of human evolution, with extinction
as their fate. This latter view is apparently the majority view
in the paleoanthropology community.
... ... G.A. Clark (Arizona State University, US) presents a
review of current research controversies and methods concerning
the transition from early humans to modern humans that apparently
occurred during the period from 50,000 to 10,000 years ago
(Middle-Upper Paleolithic transition). A central question is
whether the transition occurred abruptly or gradually. The author
makes the following points:
1) Insufficient data is only part of the reason the question
of human origins remains unresolved. Researchers in this area
come from various research traditions, and in each of these
traditions different assumptions about the remote human past
determine what is considered relevant data, which questions are
asked of the data, and how the data are interpreted. More data do
not remove the paradigmatic bias implicit within each research
tradition, and in consequence people from the different relevant
fields fail to communicate effectively.
2) The disciplines that contribute to the field (archeology,
human paleontology, and molecular biology) tend to be discovery-
driven and focused on methodology. The result is a common absence
of concern for the logic of inference underlying claims of
knowledge. European archeological studies of modern human origins
are a particularly good example of such epistemological naivete.
These studies are based on a century-old typological systematics
that emphasizes retouched stone tools, coupled with a set of
biases and preconceptions concerning the relationships between
developments in tool-making and developments of cultures.
3) On the surface, the voluminous literature produced by the
debate concerning modern human origins suggests an informed and
sophisticated interdisciplinary research in which data are
absorbed and digested, arguments assimilated, and methodologies
understood, compared, and evaluated. The author suggests "this is
a gross simplification of a much more complex reality."
4) The author concludes: "We are, in effect, consumers of
one another's research conclusions, but we select among
alternative sets of research conclusions in accordance with our
biases and preconceptions. These biases and preconceptions must
be subjected to critical scrutiny. As long as there is no
explicit concern with the logic of inference -- how we know what
we think we know about the past -- there can be no consensus."
-----------
G.A. Clark: Highly visible, curiously intangible.
(Science 26 Mar 99 283:2029)
QY: G.A. Clark: gaclark@asu.edu
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 28May99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: MAN AS THE CHEMICAL ANIMAL
"The human species has been set apart from other animals by our
ability to make and utilize tools, leading Kenneth P. Oakley to
coin the species-defining phrase 'man the tool-maker'. The
production of tools, whether of wood, bone, or stone, originally
involved only physical manipulations, sometimes of considerable
delicacy and leading to aesthetically refined products. Numerous
other species, however, can create simple tools, from birds that
use twigs to uncover insects in bark to otters that use stones to
break open shellfish. The human species... became further
distinguished from our fellow animals when we learned to
manipulate the environment chemically. The controlled use of fire
first provided protection and warmth, and later a means to
preserve food. Eventually fire was used to rework the molecules
of clay into pottery, of sand into glass, and of various minerals
into metals and alloys. Grains were chemically transformed into
bread and beer, skins and fibers into leather and linen. Resins
from trees became adhesives, perfumes, and means for embalming,
and bitumen became tar and pitch for sealing wooden surfaces of
boats. All these discoveries and inventions were meeting the
needs of our species thousands of years ago. Because humans
improved tools by chemical modification (and fire certainly is a
tool), perhaps another description of the human species is 'the
molecular transformer' or, more simply, 'the chemical animal'".
-----------
Joseph P. Lambert: _Traces of the Past_
(Helix Books, Reading MA 1997)
-----------
[Joseph P. Lambert is Clare Hamilton Hall Professor of Chemistry
at Northwestern University, US]
-------------------
SCIENCE-WEEK http://scienceweek.com 28Apr00
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
4. ASTROPHYSICS: ON THE SHAPES OF PLANETARY NEBULAE
The term "solar wind" refers to the flux of particles,
primarily protons and electrons, that are accelerated by the high
temperatures of the outer region of the Sun ("solar corona") to
velocities large enough to allow the particles to escape the
gravitational field of the Sun. At a distance of one astronomical
unit (AU) (the mean distance between the Earth and the Sun),
during a relatively quiet period, the solar wind contains
approximately 1 to 10 protons per cubic centimeter moving outward
from the Sun at velocities of 350 to 700 kilometers per second.
This creates a positive ion flux of 10^(8) to 10^(9) ions per
square centimeter per second, each ion having an energy equal to
at least 15 electron volts. During solar flares, the proton
velocity, flux, ionized gas ("plasma") temperature, and
associated turbulence increase substantially.
The term "planetary nebula" refers to any of a class of
bright nebulae that may resemble planets when viewed through a
small telescope but are in fact expanding shells of luminous gas
far outside the solar system. There are an estimated 20,000
objects called "planetary nebulae" in our Galaxy, each planetary
nebula representing gas expelled relatively recently from a
central star very late in its evolution. (In contrast, diffuse
and dark nebulae are clouds of gas from which young stars form.)
Planetary nebulae are relatively small, having a typical radius
of one light-year and containing a mass of gas equivalent to
approximately 0.3 solar-mass. Such nebulae assume various shapes,
and one research problem is to explain their formation and
dynamics.
The Hertzsprung-Russell diagram is a plot of stellar
absolute magnitude against spectral type, and is perhaps the most
useful diagrammatic aid in astrophysics. It allows the portrayal
of the evolution of a star as occurring along various paths in
the diagram. The "Main Sequence" is a region on the Hertzsprung-
Russell diagram where most stars lie, including our own Sun. The
evolution of a star can be diagrammed as a movement along the
Main Sequence and an eventual branching off the Main Sequence to
regions associated with various types of old stars, such as red
giants and white dwarfs.
A so-called "giant star" is a large and highly luminous old
star that lies above the main sequence. Giants represent a late
phase in stellar evolution, when the central hydrogen supplies
have been exhausted and the star is burning other nuclei in
concentric shells near its core. An "asymptotic giant branch
star" is a star that occupies a strip in the Hertzsprung-Russell
diagram that is almost parallel to, and just above, the giant
branch. Stars evolve from the horizontal giant branch to the
asymptotic giant branch when they have exhausted the helium in
their cores and are burning it in a shell.
The term "white dwarf star" refers to any of a class of
relatively faint stars representing the endpoint of the evolution
of intermediate- and low-mass stars. White dwarf stars are
characterized by a relatively low luminosity, a mass on the order
of that of the Sun, and a radius comparable to that of the Earth.
Because of their large mass and small dimensions, such stars are
dense and compact objects with average densities approaching
10^(6) times that of water. White dwarfs apparently evolve from
stars with an initial mass of up to approximately 3 to 4 solar-
masses. Considering the evolution of an ordinary Sun-like star,
after quiescent phases of hydrogen and helium burning, the phases
separated by a "first red-giant phase", the star becomes a red
giant for a second time. Near the end of the second red-giant
phase, the star loses its extended envelope in a catastrophic
event, leaving behind a dense, hot, and luminous core surrounded
by a glowing spherical shell -- a planetary nebula. During the
course of its evolution, the star loses a major fraction of its
original mass through stellar winds in the giant phases and
through its ejected envelope. In a star with an original Sun-like
mass, the hot planetary-nebula nucleus left behind will have a
mass of 0.5 to 1.0 solar-mass, and the star will eventually cool
down to become a white dwarf. After several billion years, the
white dwarf will stop radiating, reaching the final stage of the
evolution of this type of star and becoming a cold and inert
stellar remnant (sometimes called "black dwarf").
... ... E.G. Blackman et al (5 authors at 2 installations, US)
present a study of the origins of the shapes of planetary
nebulae, the authors making the following points:
1) The authors point out that planetary nebulae are believed
to be formed when a slow wind from the progenitor giant star is
overtaken by a subsequent fast wind generated as the star enters
its white dwarf stage. A shock forms near the boundary between
the winds, creating the relatively dense shell characteristic of
a planetary nebula. A spherically symmetric wind will produce a
spherically symmetric shell, yet over half of known planetary
nebulae are not spherical, but are elliptical or bipolar in
shape. A magnetic field could launch and collimate a bipolar
outflow, but the origin of such a field has until now been
unclear, and some researchers have even suggested that such a
field could not be generated.
2) Using a nonlinear dynamo model, the authors demonstrate
that an asymptotic giant branch star can indeed generate a strong
magnetic field, the field having as its origin a dynamo at the
interface between the rapidly rotating core of the star and the
more slowly rotating envelope of the star. These fields are
strong enough to shape the bipolar outflows that produce the
observed bipolar planetary nebulae. Magnetic "braking" of the
stellar core rotation during this process may also explain the
puzzling slow rotation of most white dwarf stars compared to the
rapid rotation of asymptotic giant branch stars
-----------
E.G. Blackman et al: Dynamos in asymptotic-giant-branch stars as
the origin of magnetic fields shaping planetary nebulae.
(Nature 25 Jan 01 409:485)
QY: John H. Thomas: astro.me.rochester.edu
-------------------
Related Background:
ASTROPHYSICS: ON THE COMPLEXITY OF THE DEATH OF STARS
In astronomy, the term "nebula" was originally applied to any
astronomical object that appeared fuzzy and extended in a
telescope, and over 100 such objects had already been catalogued
in the 18th century. The majority of these objects were later
identified as galaxies and star clusters. At the present time,
the term "nebula" (i.e., cloud) refers to a region of
interstellar gas and dust. "Emission nebulae" are bright diffuse
nebulae that emit light and other radiation as a result of
ionization and excitation of gas atoms by ultraviolet radiation,
the source of the UV usually one or more hot stars. A "planetary"
nebula is a nebula formed when a *red giant or *supergiant star
sheds its outer layers in the last stage of its evolution,
leaving a hot core that ionizes the expunged gas. The size of
planetary nebulae range from approximately the diameter of our
solar system to a light year across. Their lifetime is only about
10,000 years, and they are all expanding with speeds of
approximately 20 kilometers/second. In this context, the term
"planetary" has nothing to do with planets: the term is
historical, the first planetary nebulae discovered so named
because they gave the impression of planetary disks around stars
when viewed in small telescopes. The term "protoplanetary nebula"
has two separate meanings. In the context of this report, the
term refers to an early mass of gas and dust that will become a
planetary nebula. In the context of studies of planetary
formation, the term refers to the mass of gas and dust
surrounding young stars, the material from which planets will be
formed.
... ... Yervant Terzian (Cornell University) presents a review of
current research concerning planetary nebulae produced by the
death of stars, the author making the following points:
1) The images of planetary and protoplanetary nebulae
provided by the *Hubble Space Telescope have revealed previously
unsuspected morphological complexities of nebulae, and these new
complexities represent a challenge for researchers who study the
mechanisms of star death.
2) When a star similar to the Sun, with a mass of up to a
few solar masses, reaches the last stages of its evolution, the
star expands and becomes a relatively cool (e.g., 2500 degrees
kelvin) red giant, with a size so large that its outer perimeter
would include the orbit of Mars. Such a star loses mass in the
form of *stellar wind, followed by a more intense mass loss that
results from a "superwind". The star loses a substantial fraction
of its mass, and the ejected material forms a planetary nebula.
Following this, the stellar core contracts and becomes a "white
dwarf star", a star approximately the size of the Earth, with a
density of approximately 10^(7) times the density of the Earth,
and with a surface temperature of approximately 10^(5) degrees.
3) The material ejected from such a dying star initially
consists for the most part of atomic and molecular gas, which
partly coalesces shortly after ejection to form warm dust
particles. Eventually, the ultraviolet radiation of the hot white
dwarf ionizes the nebula, and the nebula becomes an emission
nebula. Within a few tens of thousands of years, the expanding
planetary nebula diffuses into the interstellar medium with a
velocity of approximately 20 kilometers per second.
4) The general stages of late stellar evolution are
reasonably well understood, but the mechanisms by which these old
stars eject their envelopes remain unknown, and researchers have
been unable to understand what factors contribute to creating any
particular nebular morphology. Not one of the images provided by
the Hubble Space Telescope shows a simple expanding bubble: most
objects have complex bipolar structures with central torii,
multipolar bubbles, jet-like filaments, globules, and in some
cases sets of circular rings. Many nebula show a detailed point
and mirror symmetry that can extend as much as 100,000
*astronomical units from the central star.
5) Hydrodynamical and magnetohydrodynamical simulations of
such dying stars show a multitude of morphologies, but the models
require magnetic field strengths and stellar rotations that are
not well understood in the context of present theory.
6) The author concludes: "The Hubble Space Telescope
observations have greatly enriched our knowledge about the death
of sun-like stars. The new discoveries illustrate the complexity
of stellar explosions near their deaths and pose a multitude of
new questions for observers and theorists alike."
-----------
Yervant Terzian: The complexity of stellar death
(Science 15 Oct 99 286:425)
QY: Yervant Terzian: terzian@astrosun.tn.cornell.edu
-----------
Text Notes:
... ... *red giant: A "red giant star" is a star in a late
stage of evolution, the star having exhausted the hydrogen fuel
in its core. It has a surface temperature of less than 4700
degrees Kelvin and a diameter 10 to 100 times that of the Sun.
... ... *supergiant star: A supergiant star is an extremely
luminous star of large diameter and low density. The diameter can
be as large as 1000 times that of our Sun.
... ... *Hubble Space Telescope: The Hubble Space Telescope was
launched from a space shuttle in 1990 into a 600-kilometer
low-Earth orbit and has been providing extensive imaging and
spectroscopic observations critical for the development of
astronomy and astrophysics. The new information has concerned hot
stars, stellar chromospheres and coronas, the interstellar
medium, galaxies and galactic clusters, quasars, etc. -- all of
it information uncorrupted by the Earth's atmosphere, which is
the problem for ground based telescopes.
... ... *stellar wind: In general, the term "stellar wind" refers
to the outflow of gas from the surface of a star. The Sun, for
example, loses approximately 10(-14) of its mass each year via
such a wind (solar wind).
... ... *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 24Dec99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON PLANETARY NEBULA AND THE DEATH OF STARS
... In a review of planetary nebulae, Sun Kwok (University of
Calgary, CA) makes the following points: 1) During the final
10,000 years of their life, stars with masses up to 8 times the
mass of the Sun pass through a stage in which they produce
planetary nebulae. Such nebulae are not only bright in visible
light, but they are also radio, infrared, and x-ray sources.
Immediately preceding the first planetary nebula formation, there
is a progenitor phase called the "protoplanetary nebula", and
this entity has recently come into its own as a focus of
research. 2) A full-grown planetary nebula is completely ionized
by the UV light from its central hot star. The central star of a
protoplanetary nebula, however, is relatively cool and does not
emit UV radiation, so the nebula is not ionized and shines by
reflected light only (i.e., the protoplanetary nebula is not yet
an emission nebula). 3) The first protoplanetary nebulae were
observed in the 1970s, when it became apparent that many terminal
stage stars are obscured by dust and can be found only by
searching for emissions at mid-infrared wavelengths. More than
2000 mid-infrared stars in our Galaxy were thus identified. 4)
The classification of planetary nebula is based not on appearance
by on their emission spectra. Because planetary nebula expand
with time, their radio surface brightness decreases as the nebula
ages and becomes more diffuse. The youngest planetary nebula are
thus small and radio bright. 5) While all protoplanetary nebula
have similar infrared characteristics, they differ greatly from
one another in their visual brightness. But since protoplanetary
nebula are not ionized, whatever brightness they possess arises
from starlight reflected off the surrounding dust. A bright
central star typically outshines the small faint protoplanetary
nebula, so that protoplanetary nebulae are best identified when
the system is observed edge-on. 6) Planetary nebula often have
bipolar shapes, and the origin of this form has been a focus of
research. In 1978, Kwon et al proposed a stellar fast wind
hypothesis which has been successful in simulations and has had
some observational support. The author concludes: "While
planetary nebulae have been well-known objects for more than 200
years and have fascinated generations of astronomers, the nature
of their immediate progenitors [protoplanetary nebulae] was not
known until recently. At last, we are now filling in this
missing-link in our understanding of stellar evolution."
QY: Sun Kwon, University of Calgary, CA.
(Sky & Telescope October 1998)
-----------
Text Notes:
... ... *red giant: See notes to previous report.
... ... *supergiant star: See notes to previous report.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 4Sep98
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
BIRTH AND EARLY EVOLUTION OF A PLANETARY NEBULA
White dwarf stars are extremely dense and compact stars that have
undergone gravitational collapse. White dwarfs are of great
interest to cosmologists, because it is believed their masses and
luminosities have little variance and they can thus be used as
"standard candles" to estimate distances. The final expulsion of
a gas by a star as it forms a planetary nebula (the ionized shell
of gas often observed surrounding a young white dwarf star) is
one of the most poorly understood stages of stellar evolution.
Particularly puzzling is how a spherical star can produce a
highly asymmetric nebula with collimated outflows (outflows
aligned parallel to a particular axis). ... ... Bobrowsky et al
(4 authors at 4 installations, US IN ES) now report optical
observations of the nebula surrounding the star He3-1357 (called
by the authors the "Stingray nebula"), a nebula that has
evidently become an ionized planetary nebula within the past few
decades. The authors find that the collimated outflows are
already evident, and they have identified the nebular structure
that focuses the outflows, and have also found a companion star,
which reinforces previous suspicions that binary companions play
an important role in shaping planetary nebulas and in changing
the direction of successive outflows. The authors suggest the
Stingray nebula demonstrates how far the nebular structure can
develop by the time the nebula becomes ionized, and that no other
planetary nebula in this phase of its evolution has been
previously identified.
QY: Matthew Bobrowsky: mattb@cta.com
(Nature 2 Apr 98) (Science-Week 24 Apr 98)
For more information: http://scienceweek.com/swfr.htm
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
5. MATERIALS SCIENCE: A NEW HIGH-TEMPERATURE SUPERCONDUCTOR
At temperatures close to absolute zero (-273.15 degrees
celsius), the thermal, electric, and magnetic properties of many
substances undergo dramatic changes. One such phenomenon is
superconductivity, which occurs below a critical temperature
specific for each substance that exhibits the effect.
Superconductivity was discovered in 1911 by Heike Kamerlingh
Onnes (1853-1926), who was awarded the Nobel Prize for Physics in
1913 for his low temperature research. Kamerlingh Onnes found
that the electrical resistance of a mercury wire suddenly
disappears when the wire is cooled below a temperature of
approximately 4 degrees kelvin. Similar behavior (but at widely
varying critical temperatures) has been found in approximately 25
other chemical elements, including lead and tin, and in thousands
of alloys and chemical compounds. Apart from these known
superconducting materials, all other substances investigated to
within fractions of a degree of absolute zero show normal (non-
superconducting) resistance to the flow of electric currents.
For almost 50 years after the discovery of superconductivity
by Kamerlingh Onnes, there was no successful fundamental theory
that could explain the phenomenon. Finally, in 1957, an
apparently satisfactory theory of superconductivity was presented
by John Bardeen (1908-1991), Leon N. Cooper, and John R.
Schrieffer, who all shared the Nobel Prize for Physics in 1972.
The theory is now called the BCS theory of superconductivity.
The essential aspect of BCS theory is the grouping of
electrons in superconductors in pairs ("Cooper pairs"), with the
motions of all the Cooper pairs within a single superconductor
correlated, i.e., the population of Cooper-pair electrons
constituting a system that functions essentially as a single
entity. (In quantum mechanical terms, each Cooper pair consists
of electrons of opposite spins, thus forming a spin-zero single
*boson, and the population of bosons form a *Bose-Einstein
condensate described by a single wave function.) Application of
an electric voltage to the superconductor causes all Cooper pairs
to move, the movement constituting a current. When the voltage is
removed, current continues to flow indefinitely because the
Cooper pairs (as members of a coherent condensate) are not
scattered by the atomic lattice. As a superconductor is warmed,
its Cooper pairs separate into individual electrons, and the
material becomes non-superconducting.
Such was the theory of superconductivity for nearly 30
years, the theory successfully predicting the behavior of
superconducting materials with critical temperatures close to
absolute zero. In 1986, Karl A. Mueller and J. Georg Bednorz
discovered that certain materials exhibit superconductivity at
temperatures as high as 35 degrees kelvin, and compounds
retaining superconductivity at temperatures as high as 160
degrees kelvin have since been found. Mueller and Bednorz were
awarded the Nobel Prize in Physics in 1987 for their work with
high-temperature superconductors. Such high-temperature
superconductors all contain copper and oxygen atoms that form
planes or chains of atoms in the crystal, and it is believed that
anisotropy is an important factor in their superconducting
behavior. These materials are ceramic oxides, and because they
are superconducting at temperatures easily obtainable with liquid
nitrogen, great effort has been expended to find applications for
these substances. But problems of brittleness, instabilities, and
the aggregation of impurities at surfaces have slowed progress.
Nevertheless, in contrast to superconducting ceramics,
superconducting metals and their alloys must be cooled to near
absolute zero with liquid helium, a process much more expensive
than cooling with liquid nitrogen. Superconducting ceramics thus
remain an important frontier of research in materials science.
In terms of theory, what is significant is that BCS theory
apparently cannot provide a complete explanation of the behavior
of high-temperature ceramic superconductors. A version of BCS
theory may explain how superconductivity occurs in certain
ceramic materials, but no complete theory of high-temperature
superconductivity in ceramic materials has yet been proposed.
Recently, researchers in superconductivity were startled
when J. Nagamatsu et al (5 authors at 2 installations, JP)
(Nature 1 Mar 01 410:63) (in a paper consisting of only 3 short
paragraphs) reported the discovery of bulk superconductivity in
the simple and readily available compound magnesium diboride
[MgB(sub2)], with magnetization and resistivity measurements
establishing a transition temperature of 39 degrees kelvin, the
highest known critical temperature for a non-copper-oxide (non-
ceramic) bulk superconductor. [Editor's note: The surprise of
condensed-matter physicists at this new discovery is reminiscent
of the surprise of the same community at the discovery by Mueller
and Bednorz in 1986 of high-temperature ceramic superconductors.
See related background material below.]
... ... Robert J. Cava (Princeton University, US) presents a
commentary on this new discovery, the author making the following
points:
1) The author points out that in the ideal case of
superconductivity, the zero-resistance state is absolute:
electrons flowing in a continuous loop of superconducting wire
below the critical temperature could theoretically flow for the
age of the Universe and never lose any energy. But in the real
world there are losses, e.g., from microscopic inhomogeneities,
and the ideal is never obtained. Nevertheless, devices made with
superconducting materials have resistances orders of magnitude
lower than those of devices made with conventional conductors.
This low resistance to current means that large currents (on the
order of 10^(6) amperes per square centimeter of wire cross-
section) can be passed without significant heating. For example,
the magnets in magnetic resonance imaging instruments now in
common use are made from metal-alloy superconducting wires, and
these magnets are cooled below the critical temperature of the
metal-alloy by immersion in liquid helium at 4.2 degrees kelvin.
2) The author points out there are two reasons for the
current excitement concerning the discovery of superconductivity
in magnesium diboride: a) Early indications are that magnesium
diboride becomes superconducting by the BCS mechanism, so that
unlike high-temperature copper-oxide superconductors, magnesium
diboride appears to be a "conventional" superconductor. Magnesium
diboride has the highest critical temperature known for a
chemically stable, bulk compound of this type, and this suggests
the possible existence of even higher superconducting critical
temperatures in conventional and readily available materials yet
to be investigated. b) The second reason for excitement is that
it has proved so difficult to make useful wires of
superconducting ceramics. This new report by J. Nagamatsu et al
raises the possibility that superconducting materials based on
magnesium diboride may eventually be able to carry more current
than copper oxide superconductors. With a critical temperature of
39 degrees kelvin, there is also the possibility that magnesium
diboride superconductors would not need to be cooled by liquid
helium, but could be cooled by electrical refrigerators. The
author concludes: "How much this discovery changes the path of
materials physics depends on whether magnesium diboride is a
solitary example of a new way of making high-temperature
superconductors or whether it represents only the tip of an
iceberg."
-----------
Robert J. Cava: Genie in a bottle
(Nature 1 Mar 01 410:23)
QY: Robert J. Cava: rcava@princeton.edu
-----------
Text Notes:
... ... *boson: According to current physics, all particles in
nature are either fermions or bosons, with fermions (always
elementary particles) having half-integer spin (spin-states
characterized by half-integer multiples of Planck's constant
divided by 2ã), and bosons (all other particles) having integer
spin (spin-states characterized by integer multiples of Planck's
constant divided by 2ã). In general, bosons are particles that
obey *Bose-Einstein statistics, and they include photons, *pi
mesons, all nuclei having an even number of particles, and all
particles with integer or zero spin.
... ... *Bose-Einstein statistics: Bose-Einstein statistics is
the statistical mechanics of a system of indistinguishable
particles for which there is no restriction on the number of
particles that may simultaneously exist in the same quantum
energy state. Particles that obey Bose-Einstein statistics are
called "bosons".
... ... *Bose-Einstein condensate: In general, "Bose-Einstein
condensation" is a phenomenon occurring in a macroscopic system
consisting of a relatively large number of bosons at a
sufficiently low temperature (microkelvins down to nanokelvins)
in which a significant fraction of the particles occupy a single
quantum state of lowest energy (the ground state). In an atomic
Bose-Einstein condensate, several thousand atoms essentially
become a single atom, a "superatom", and this effect has been
observed experimentally with atoms of rubidium and lithium, the
atoms trapped and cooled by special methods.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
CONDENSED-MATTER PHYSICS: ON CORRELATED ELECTRON SYSTEMS
One sure thing in science is that whenever the prevailing
authorities in a field announce that nearly all problems have
been solved and that everyone ought to pack up and go home, that
is the time you need to bet all your capital that within a short
time an important discovery or technological innovation will
suddenly open an entire reservoir of new problems that make the
field young again. In science, "maturity" in a field is usually
doomed to be ephemeral, and every scientist knows examples of
this in his own domain. An instance was the so-called "maturity"
of solid-state physics in the 1970s, when independent electron
approximations worked well for most semiconductors and metals,
the phase transition problem seemed solved, and the fundamentals
of magnetism, ferroelectricity, and superconductivity appeared to
be known. Within a short time, however, as if to slam the
authorities who had pronounced solid-state physics a closed book,
there came discoveries of a variety of new materials whose
behavior could not be understood at all with traditional ideas.
These materials have in common the apparent dominant role played
by electron-electron interaction effects, and such systems are
categorized under the general rubric of "highly correlated
electron systems". Examples of such systems are transition metal
oxides, including copper oxide high-temperature superconductors,
*heavy fermion metals, organic *charge transfer compounds, and
one- and two-dimensional electron gas systems. In addition to
intriguing possible technological applications, the behaviors of
these systems appear to present profound challenges in
fundamental physics.
... ... In a recent group of articles on the new frontier of
correlated electron systems, various authors made the following
points:
1) R.J. Birgenau and M.A. Kastner (Massachusetts Institute
of Technology, US) point out that correlated electron systems are
typically characterized by the coexistence of various types of
order, including charge and orbital and *spin density waves,
together with superconducting and magnetic order. In such
systems, these different kinds of ordering, which are believed to
compete with each other in conventional systems, are often
synergistic. The authors state: "Clearly, highly correlated
electron systems present us with profound new problems that
almost certainly will represent deep and formidable challenges
well into this new century."
2) Y. Tokura and N. Nagaosa (University of Tokyo, JP) point
out that in a solid, an electron bound to or nearly localized on
a specific atomic site has three attributes: charge, spin, and
orbital. The orbital represents the shape of the electron cloud
in the solid. In transition-metal oxides with anisotropic d-
orbital electron distributions, the Coulomb interaction between
the electrons ("strong electron correlation effect") is important
for understanding metal-insulator transitions and properties such
as high-temperature superconductivity and *colossal
magnetoresistance. But the orbital degree of freedom occasionally
plays an important role in these phenomena, its correlation
and/or order-disorder transition causing a variety of phenomena
via strong coupling with charge, spin, and lattice dynamics.
3) Subir Sachdev (Yale University, US) points out that small
changes in an external parameter can often lead to dramatic
qualitative changes in the lowest energy quantum mechanical
ground state of a correlated electron system. In anisotropic
crystals, such as the high-temperature superconductors where
electron motion occurs primarily on a 2-dimensional square
lattice, the quantum critical point between two such lowest
energy states has nontrivial emergent excitations that control
the physics over a significant portion of the phase diagram. The
author concludes: "The availability of a large number of 2-
dimensional correlated electron systems (including the high-
temperature superconductors), along with the highly nontrivial
theoretical framework necessary to describe them, makes this one
of the most exciting research areas in condensed matter
physics... The interplay between theory and experiment promises
to be mutually beneficial, in the best traditions of physics
research."
-----------
R.J. Birgenau and M.A. Kastner: Frontier physics with correlated
electrons.
(Science 21 Apr 00 288:437)
QY: Robert J. Birgenau, Mass. Inst. of Technol. 617-253-1000.
-----------
Y. Tokura and N. Nagaosa: Orbital physics in transition-metal
oxides.
(Science 21 Apr 00 288:462)
QY: Y. Tokura, Dept. of Applied Physics, University of Tokyo,
Bunkyo-ku, Tokyo 113-8656, JP.
-----------
Subir Sachdev: Quantum criticality: Competing ground states in
low dimensions.
(Science 21 Apr 00 288:475)
QY: Subir Sachdev [subir.sachdev@yale.edu]
-----------
Text Notes:
... ... *heavy fermion metals: Fermions (electrons, protons,
neutrons) are particles that obey the Pauli exclusion principle:
i.e., no two fermions of the same kind can occupy the same
quantum state. "Heavy fermion systems" are *intermetallic
compounds whose electrical behavior is theoretically described by
postulating "quasiparticles" whose effective masses at low
temperatures are several hundred times the free-electron mass.
... ... *intermetallic compounds: (electron compounds; Hume-
Rothery compounds) In general, an "intermetallic compound" is an
alloy of two metals in which a progressive change in composition
is accompanied by a progression of phases that differ in crystal
structure.
... ... *charge transfer compounds: In general, a "charge-
transfer compound" is a compound in which electrons move between
molecules.
... ... *colossal magnetoresistance: (giant magnetoresistance)
The term "magnetoresistance" refers to a change in the electrical
resistance of a conductor or semiconductor upon the application
of a magnetic field, a property of certain systems. Giant
magnetoresistance is a quantum mechanical effect observed in
magnetic thin-film structures composed of alternating
ferromagnetic and nonmagnetic layers.
... ... *spin density waves: In quantum mechanics, "spin" is the
intrinsic angular momentum of a subatomic particle. Spin states
are quantized, multiples of h/2ã, where h = Planck's constant,
and each particle is characterized by a quantum spin number which
is the multiple factor. "Spin density waves", in general, are
propagating collective spin-variation excitations associated with
certain magnetic systems.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 7Jul00
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
ON THE DISCOVERY OF HIGH TEMPERATURE SUPERCONDUCTIVITY
One of the operating tenets of 20th century "big science" is that
important breakthroughs in science can be more or less engineered
if appropriate conditions are constructed and appropriate
individual researchers placed in those conditions. When this
approach produces a success, the various bureaucrats who support
the idea feel reaffirmed; when various counter-examples to the
approach occur, it is the turn of the doubters to feel
reaffirmed. A cogent instance of a counter-example was provided
in 1986 by the Bednorz-Mueller discovery of high-temperature
superconductivity -- a discovery of signal importance in
experimental physics made by two relatively unknown researchers
working in what can be characterized as a backwater and poorly-
equipped laboratory. Not only was the discovery of high-
temperature superconductivity totally unexpected by the
international physics community, but the discovery of the
phenomenon by outsiders under "little science" conditions caused
a degree of shock in the science policy system. Ordinary
superconductivity is a property of many metals, alloys, and
chemical compounds at temperatures near absolute zero, at which
temperatures (their "critical temperatures") their electrical
resistivity vanishes and they become strongly diamagnetic.
(Diamagnetic substances such as the alkalis and alkaline earth
metals, the halogens, and the noble gases are repelled by magnets
and tend to position themselves at right angles to the magnetic
lines of force.) High-temperature superconductors were unknown
until 1986, but at present there are some known high-temperature
superconductors with critical temperatures greater than 100
degrees kelvin. The accepted theory of ordinary superconductivity
is the Bardeen-Cooper-Schrieffer theory (BCS theory) (1957). At
the present time, a successful theory of high-temperature
superconductivity has not been developed, in spite of a great
deal of effort. Johannes Georg Bednorz (1950- ) and K. Alexander
Mueller (1927- ) shared the Nobel Prize in Physics in 1987 for
their discovery of high-temperature superconductivity in a
ceramic oxide (lanthanum-barium-copper) alloy at 30 degrees
kelvin, at that time the highest superconductivity temperature
ever observed, the work having been carried out at the IBM Zurich
Research Laboratories at Rueschlikon.
... ... Helga Nowotny (Swiss Federal Institute of Technology, CH)
presents an essay on innovation in research and the modern
partnership between basic research and applied science, the
author making the following points:
1) One of the most exciting recent success stories of
science began in September 1986 with the appearance in the
_Zeitschrift fur Physik_ of an article with the cautious title,
"Possible high-T(subc) superconductivity in the Ba-La-Cu-O
system." A few weeks later, the names of the two authors,
Alexander Mueller and Georg Bednorz, and their discovery hit the
front pages of _The New York Times_ and researchers around the
world were caught in an unprecedented frenzy, attempting to
replicate and surpass the findings of the initial breakthrough.
The race for high-temperature superconducting systems was on.
2) The discovery of high-temperature superconductivity was
unexpected in terms of its discoverers, the place of its
discovery, and the scientific ideas involved. It contradicted
conventional wisdom and the expectations of peers and research
administrators. Mueller and Bednorz were outsiders, Mueller a
specialist on perovskites (a type of oxide mineral) and Bednorz a
crystallographer. They benefitted from the novice effect, but
they also enjoyed a degree of autonomy that allowed them to
prepare for the unpredictable. Of the 3 superconductivity
laboratories of IBM, the Rueschlikon laboratory where the two
researchers were based was by far the most modestly equipped. And
the discovery contradicted long-held views, not only overturning
certain established empirical rules concerning superconductivity,
but also unveiling previously unknown phenomena not accounted for
by the classic Bardeen-Cooper-Schrieffer theory.
3) The author points out that even if we knew how to create
conditions under which creativity can flourish, and how to favor
the occurrence of what cannot be planned, the problem remains of
how to turn highly individualistic bursts of scientific
creativity into socially desired techno-scientific outcomes. "For
the most disturbing paradox is this: there has been a relative
decline in the importance of the individual creative act, while
its proliferation is encouraged. Individual scientific creativity
has become a necessary, but no longer sufficient, precondition in
a long, branching sequence of possibilities."
-----------
Helga Nowotny: Innovation machine on the boil.
(Nature 28 Oct 99 401:859)
QY: Helga Nowotny, Swiss Federal Institute of Technology, ETH-
Zentrum, CH-8092 Zurich, CH.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 5Dec99
For more information: http://scienceweek.com/swfr.htm
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6. HISTORY OF PHYSICS: ON THE MEASUREMENT OF TIME
Although a verbal definition of time (other than a purely
operational definition) presents philosophical difficulties, from
the standpoint of physics, time is the most accurately measured
physical quantity. In general, there are two independent and
fundamental time scales: a) the dynamical time scale, which is
based on the regularities of the motions of the celestial bodies
fixed in their orbits by gravitation; b) the atomic time scale,
which is based on the characteristic frequency of electromagnetic
radiation emitted or absorbed in quantum transitions between
internal energy states of atoms or molecules.
The first known device for indicating the time of day was
the "gnomon", which appeared in approximately 3500 BC. This
instrument consisted of a vertical stick or pillar, the length of
the shadow cast by the stick or pillar providing an indication of
the time of day. By the 8th century BC, more precise devices were
in use. The earliest known sundial still preserved is an Egyptian
shadow clock dating at least from the 8th century BC, and which
consists of a straight base with a raised crosspiece at one end.
On the base is inscribed a scale of 6 time divisions. The base is
placed in an east-west direction with the crosspiece at the east
end in the morning and at the west end in the afternoon. The
shadow of the crosspiece on the base indicates the time.
The Babylonian hemispherical sundial (hemicycle), apparently
invented by the astronomer Barosus in approximately 300 BC,
consisted of a cubical block into which was cut a hemispherical
opening. To the opening was fixed a pointer whose end lay at the
center of the hemispherical space. The path traveled by the
shadow of the pointer was approximately a circular arc whose
length and position varied according to the seasons. An
appropriate number of arcs were inscribed on the internal surface
of the hemisphere, each arc divided into 12 subdivisions. Each
day, reckoned from sunrise to sunset, had 12 equal intervals or
"hours". Since the length of the day varied according to the
season, these hours were known as "temporary hours".
The Greeks developed and constructed sundials of
considerable complexity in the 3rd and 2nd centuries BC,
including instruments with either vertical, horizontal, or
inclined dials, indicating time in temporary hours. The Romans
also used sundials with temporary hours, and some of these Roman
sundials were portable. The Arabs increased the variety of
sundial designs, and at the beginning of the 13th century AD the
Arabs wrote on the construction of sundials with cylindrical,
conical, and other surfaces.
In general, a "clock" is a device that performs regular
movements in equal intervals of time, the device linked to a
counting mechanism that records the number of movements. The
first public clock that struck the hours was made and erected in
Milan (IT) in 1335. The oldest surviving clock is that at
Salisbury Cathedral, which dates from 1386. In approximately
1500, small portable clocks driven by a spring appeared, the
dials with an hour hand only. The pendulum was applied as a time
controller in clocks beginning in 1656, although Galileo had
already suggested this in 1582. The familiar subdivision of the
day into 24 hours, the hour into 60 minutes, and the minute into
60 seconds is of ancient origin, but these subdivisions came into
general use in approximately 1600 AD. When the increasing
accuracy of clocks led to the adoption of the "mean solar day",
which contained 86,400 seconds, the "mean solar second" became
the basic unit of time. The adoption of the International System
(SI) second, defined on the basis of atomic phenomena, as the
fundamental time unit, occurred provisionally in 1964 and finally
in 1967. A second is now defined as 9,192,631,770 cycles of
radiation associated with the transition between the two
hyperfine levels of the ground state of the cesium-133 atom. The
number of cycles of radiation was chosen to make the length of
the defined second correspond as closely as possible to that of
the previous standard, the astronomically determined second of
"Ephemeris Time" (defined as 1/(86,400) of the mean solar day).
... ... J.C.Bergquist et al (3 authors at National Institute of
Standards and Technology, US) present a review of current
research on precision atomic clocks, the authors making the
following points:
1) The authors point out that although a unit of time can be
constructed from other physical constants, time is usually viewed
as an arbitrary parameter to describe dynamics. The frequency of
any periodic event, such as the mechanical oscillation of a
pendulum, or the quantum oscillation of an atomic dipole, can be
adopted to define the unit of time, the second.
2) For centuries, the mean solar day served as the unit of
time, but Earth's period of rotation is irregular and slowly
increasing. In 1956, the International Astronomical Union and the
International Committee on Weights and Measures recommended
adopting Ephemeris Time, based on Earth's orbital motion around
the Sun, as a more accurate and stable basis for the definition
of time. This recommendation was formally ratified in 1960 by the
General Conference on Weights and Measures.
3) Until the definition of the second in terms of atomic
time in 1967, most work in standards laboratories was devoted to
developing secondary standards, such as lumped-element circuits
and quartz crystals, whose resonant frequencies could be
calibrated relative to Ephemeris Time. But frequencies derived
from resonant transitions in atoms or molecules offer important
advantages over macroscopic oscillators. Any unperturbed atomic
transition is identical from atom to atom, so two clocks based on
such a transition should generate the same time. Also, unlike
macroscopic devices, atoms do not wear out, and as far we know
they do not change their properties over time.
4) The basic idea of most atomic clocks is straightforward:
a) First, identify a transition between two non-degenerate energy
states of an atom. b) Then, create an ensemble of these atoms
(e.g., in an atomic beam or storage device). c) Next, illuminate
the atom with radiation from a tunable source that operates near
the transition frequency. d) Sense and control the frequency
where the atoms absorb maximally. e) When maximal absorption is
achieved, count the cycles of the oscillator: a certain number of
elapsed cycles generates a standard interval of time. But
although the general idea of an atomic clock is straightforward,
in practice there are a number of experimental difficulties that
limit accuracy. The latest atomic clocks use a single ion to
measure time with an anticipated precision of one part in
10^(18).
-----------
J.C. Bergquist et al: Time measurement at the Millennium.
(Physics Today March 2001)
QY: James C. Bergquist: National Institute of Standards and
Technology, Boulder CO, US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
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7. IN FOCUS: HISTORY OF CHEMISTRY: ON GASES
"The early history of gases is related in some ways to the
history of certain chemical operations and, in particular, to
distillation. In distillation, a liquid is converted by heat into
a vapor, the vapor then condenses in a receiver. However, in some
operations, a chemical reaction would take place, and sometimes
(our modern knowledge tells us) a gas would be produced.
Operators did not understand this. Anything we would call a 'gas'
would be regarded as irrelevant, no more than a will-of-the-wisp.
What concerned them most was the immediate practical consequence
of any buildup of pressure in the system: the retort might
explode. The apparatus would be damaged beyond repair, and the
operator himself might well have been injured. Therefore,
operators adopted the practice of making a hole near the receiver
to allow any 'spirits' to escape. This expedient saved the
apparatus and allowed the distillation to be successfully
concluded. What was lost seemed of no importance. As one later
writer [Sigaud de la Fond (1785)] expressed it:
'Very far from suspecting what they might have gained from
the work, they preferred to safeguard their apparatus. They were
happier to abandon a product the value of which they did not
understand than to risk losing the fruit of the different
operations they were carrying out.'
"If 16th century 'chemists' had been questioned about any
possible loss, on reflection they might have spoken of a 'spirit'
or even a 'wild spirit'. This brings us to the famous definition
of [Jan Baptiste] van Helmont [1579-1644]: 'I call this Spirit,
unknown hitherto, by the new name of Gas, which can neither be
contained by Vessels, nor reduced into a visible body...'"
-----------
Maurice Crosland: "Slipper Substances: Some Practical and
Conceptual Problems in the Understanding of Gases in the Pre-
Lavoisier Era."
(in: F.L. Holmes and T.H. Levere (eds.): _Instruments and
Experimentation in the History of Chemistry_
MIT Press, Cambridge 2000, p.80)
-------------------
SCIENCE-WEEK http://scienceweek.com 23Mar01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: ON THE FOUNDATION OF MODERN CHEMISTRY
"It is interesting that it was not until the early years of the
17th century that the word 'gas' was used. This word was invented
by a Belgian physician, J.B. van Helmont (1579-1644), to fill the
need caused by the new idea that different kinds of 'airs'
existed. Van Helmont discovered that a gas (the gas that we now
call carbon dioxide) is formed when limestone is treated with
acid, and that this gas differs from air in that when respired it
does not support life and that it is heavier than air. He also
found that the same gas is produced by fermentation, and that it
is present in the Grotto del Cane, a cave in Italy in which dogs
were observed to become unconscious (carbon dioxide escaping from
fissures in the floor displaces the air in the lower part of the
cave). During the 17th and 18th centuries, other gases were
discovered, including hydrogen, oxygen, and nitrogen, and many of
their properties were investigated. It was not until nearly the
end of the 18th century, however, that these three gases were
recognized as elements. When Lavoisier recognized that oxygen is
an element, and that combustion is the process of combining with
oxygen, the foundation of modern chemistry was laid."
-----------
Linus Pauling: _General Chemistry_
(W.H. Freeman, San Francisco 1970, p.306)
-------------------
SCIENCE-WEEK http://scienceweek.com 18Jun99
For more information: http://scienceweek.com/swfr.htm
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8. FROM THE SCIENCEWEEK ARCHIVE:
ON PALEONTOLOGY AND EVOLUTIONARY BIOLOGY
Paleontology, the study of the history of life as recorded by
fossil remains, is one of the interfaces between the two fields
of biology and geology. The fossil record includes a diverse
class of objects ranging from molds of microscopic bacteria in
rocks more than 3 billion years old to unaltered bones of human
fossils only a few thousand years old. The quality of
preservations ranges from the occasional occurrence of soft parts
to barely decipherable impressions made by shells in soft mud
that later hardened to rock. Concerned as it is with the physical
record of the history of life, the sister discipline of
paleontology is evolutionary biology.
... ... David Jablonski (University of Chicago, US) presents a
review of the common ground between paleontology and evolutionary
biology, the author making the following points:
1) The next wave of research in paleontology apparently will
build on an array of empirical and methodological advances that
will foster a paleontology more interdisciplinary than ever.
Among the opportunities opened by new developments, four key
interrelated research questions have emerged at the interface
between paleontology and evolutionary biology:
... ... a) What are the rules that govern biodiversity dynamics,
and do these rules apply at all temporal and spatial scales? The
overall trend of plant and animal biodiversity -- whether
measured in terms of taxa, range of body forms, or modes of life
-- has been one of net increase through geologic time. However,
this general trend has been anything but smooth. Diversification
has occurred episodically, has been interrupted by extinction
events, and has had (e.g., in the oceans) at least one prolonged
episode of little net change.
... ... b) Why are major evolutionary innovations unevenly
distributed in time and space? One of the most striking patterns
to emerge from the fossil record is that biological innovations
-- the breakthroughs that open new ecological opportunities and
evolutionary pathways -- do not arise randomly. Regardless of
when the major lineages actually split, the *Cambrian explosion
represents a uniquely rich and temporally discrete episode of
morphological invention for the *metazoan phyla. Smaller pulses
follow mass extinctions, for example, the exuberant *Cenozoic
radiation of mammals after 100 million years of monotonous
morphologies in the shadow of the dinosaurs.
... ... c) How does the biosphere respond to environmental
perturbations at global and regional scales? Life has been
buffeted by asteroid impacts, rapid climate changes, shifts in
oceanic and atmospheric chemistry, continent-scale biotic
interchanges, and a host of other perturbations. The fossil
record provides the basis for a comparative calibration of biotic
responses to different types and magnitude of disturbance.
... ... d) How have biological systems influenced the physical
and chemical nature of the Earth's surface and vice versa? In the
long and complex path from the anoxic, exclusively microbial
*Archean sea to the highly heterogeneous modern system, the major
biological, sedimentary, and geochemical transitions are roughly
coincident in time. Combining the biological record with
increasingly high resolution geochemical methods, researchers are
beginning to focus on time intervals when significant changes
occurred in biological materials and biogeochemical cycling, on
how steady states are maintained, and on the roles of biological
innovations in perturbing and stabilizing those cycles.
2) The author concludes: "Paleontology sits squarely at the
interface between the earth and life sciences. The most powerful
contributions will emerge from analysis of evolutionary dynamics
at different scales and hierarchical levels over deep time and of
the diverse ways life has driven, and been driven by, changes
in the Earth's atmosphere, oceans, and lithosphere."
-----------
David Jablonski: The future of the fossil record.
(Science 25 Jun 99 284:2114)
QY: David Jablonski: djablons@midway.uchicago.edu
-----------
Text Notes:
... ... *Cambrian explosion: The geological period known as the
Cambrian is the time frame from about 505 million years ago to
545 million years ago. Its most outstanding aspect is the rather
sudden appearance of numerous invertebrate fossils, so numerous
that some have termed it an explosion of evolutionary processes.
Many of the life forms that existed during the Cambrian are long
extinct, but their fossils are numerous, and through their
fossils the various Cambrian species have been the subject of
much study by paleobiologists. The Cambrian explosion of life
forms has been a long-standing puzzle for paleobiologists, and at
present there is apparently no single generally accepted
explanation. Among the ideas proposed have been, 1) that the
explosion of new forms resulted from a sudden increase in
atmospheric oxygen; 2) that the explosion is only apparent, and
the Precambrian, the period previous to the Cambrian, lacks
fossils because of heat and pressure associated with important
geological changes; 3) that living forms evolved mostly in
freshwater areas, and are therefore absent in Precambrian
sediments, which are primarily marine; 4) that changes in the
shape and extent of shorelines produced by continental drift
dramatically transformed climate and environment; 5) that the
previous evolution of DNA recombination and regulatory genes
culminated in and sparked the diversity and anatomical complexity
manifested in the explosion; 6) that an exponential increase of
species could become significant only after attaining a threshold
value at the start of the Cambrian; and, 7) that once
multicellular organisms appeared, the intrinsic possibilities for
variation increased enormously with a resultant explosion of
evolved forms. Unfortunately, there is no evidence to suggest a
selection of one of these proposals, although some of them are
less convincing than others. And of course the truth may be that
more than one factor was involved. No matter the origin, the
Cambrian explosion is apparently accepted by most paleobiologists
as a real discontinuity, a period that saw the sudden emergence
of dozens of new orders and phyla, including sponges, *annelids,
*crustaceans, *hemichordates, *brachiopods, and *mollusks.
... ... *annelids: Soft-bodied, metamerically segmented coelomate
worms, e.g., earthworms. The term "coelomate" refers to the
possession of a body cavity.
... ... *crustaceans: A class of Arthropods, including shrimps,
crabs, water fleas, etc.
... ... *hemichordates: A group of marine invertebrates,
including the acorn worms.
... ... *brachiopods: Bivalve coelomate invertebrates that live
attached to the sea-bed (e.g., lamp shells).
... ... *mollusks: (Mollusca) A phylum of bilaterally symmetrical
unsegmented invertebrates. Includes aquatic bivalves such as
mussels and clams, terrestrial slugs and snails, octupi and
squids.
... ... *metazoan phyla: In general, a "phylum" is any major
group. A "metazoan" is any multi-cellular animal.
... ... *Cenozoic radiation: The Cenozoic is the present geologic
era, extending from about 65 million years ago to the present.
Also called "the age of mammals". In this context, the term
"radiation" refers to the spread of a group of biological
entities into new environments with consequent diversification.
... ... *Archean sea: (Archaean; Archeozoic) In general, the
earliest biotic geological era, from approximately 3.9 billion
years ago to approximately 2.6 billion years ago.
-------------------
Summary & Notes by SCIENCE-WEEK http://scienceweek.com 10Sep99
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
IN FOCUS: ON INVERTEBRATE PALEONTOLOGY
"Little work of importance was done in paleontology until the
1700's, at which time both vertebrate and invertebrate fields
began to assume importance. Intensive work in the invertebrate
area arose from recognition of the fact, first clearly seen by
William Smith, an English civil engineer and amateur geologist of
the period, that a given set of beds tended to contain the same
species of shells over vast and widely separated areas. Accurate
determination of fossils could thus be of great practical use to
the stratigrapher; as a result, invertebrate paleontology tended
to develop not as an independent science, but as a handmaiden to
the geologist -- a working tool for the stratigrapher looking for
oil or ores or coal. The fossil shells were rarely thought of as
the remains of once-living organisms, but merely as convenient
markers for the identification of successive formations, and
would have been as useful had they been identifiable mineral
inclusions or distinctive assortments of nuts and bolts... With
this background, the invertebrate workers of Darwin's day not
merely lacked interest in evolutionary ideas, but were inclined
to view them with suspicion as detrimental to their work. For
clear-cut stratigraphic work, the species in a given formation
should be stable entities, clearly distinguishable from those in
the strata above and below. The idea of gradual change and of
transitional forms was abhorrent... With this to contend with, it
is apparent why Darwin was thrown on the defensive in his
treatment of the fossil record. He could not call on the
paleontologists for support; the most he could do was to attempt
appeasement, to show that it was at least possible to interpret
the geological story in evolutionary terms, and that there was no
insuperable objection."
-----------
-- A.S. Romer: "Darwin and the Fossil Record"
(in S.A. Barnett [ed.]: _A Century of Darwin_, 1958, Chap. 6)
(Science-Week 25 Jun 99)
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