ScienceWeek

SCIENCEWEEK

Briefs in the Sciences

May 16, 2003

Vol. 7 - Number 20B

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Contents:

1. On Standard Candles and the Expanding Universe

2. On the First Stars

3. On Quantum Entanglement

4. On Molecular Electronics

5. Chloroplasts and Genes

6. On Neuronal Connectivity

7. On the AIDS Pandemic

8. Prenatal Hormone Exposure and Sexual Variation

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1. ON STANDARD CANDLES AND THE EXPANDING UNIVERSE

Physics Today 2003 April

The following points are made by Saul Perlmutter:

1) In principle, the expansion history of the cosmos can be
determined quite easily, using as a "standard candle" any
distinguishable class of astronomical objects of known intrinsic
brightness that can be identified over a wide distance range. As
the light from such beacons travels to Earth through an expanding
universe, the cosmic expansion stretches not only the distances
between galaxy clusters, but also the very wavelengths of the
photons en route. By the time the light reaches us, the spectral
wavelength has thus been redshifted by precisely the same
incremental factor by which the cosmos has been stretched in the
time interval since the light left its source. That time interval
is the speed of light times the object's distance from Earth,
which can be determined by comparing its apparent brightness to a
nearby standard of the same class of astrophysical objects.

2) The recorded redshift and brightness of each such object thus
provide a measurement of the total integrated expansion of the
universe since the time the light was emitted. A collection of
such measurements, over a sufficient range of distances, would
yield an entire historical record of the universe's expansion.

3) Conceptually, this scheme is a remarkably straightforward
means to a profound prize: an empirical account of the growth of
our universe. A spectroscopically distinguishable class of
objects with determinable intrinsic brightness would do the
trick. In the discovery by Edwin Hubble (1889-1953) of the cosmic
expansion in the 1920s, he used entire galaxies as standard
candles. But galaxies, coming in many shapes and sizes, are
difficult to match against a standard brightness. They can grow
fainter with time, or brighter -- by merging with other galaxies.
In the 1970s, it was suggested that the brightest member of a
galaxy cluster might serve as a reliable standard candle. But in
the end, all proposed distant galactic candidates were too
susceptible to evolutionary change.

4) As early as 1938, Walter Baade (1893-1960), working closely
with Fritz Zwicky (1898-1974), pointed out that supernovae were
extremely promising candidates for measuring the cosmic
expansion. Their peak brightness seemed to be quite uniform, and
they were bright enough to be seen at extremely large distances.
In fact, a supernova can, for a few weeks, be as bright as an
entire galaxy. Over the years, however, as more and more
supernovae were measured, it became clear that they were a rather
heterogeneous group with a wide range of intrinsic peak
brightnesses.

5) In the early 1980s, a new subclassification of supernovae
emerged. Supernovae with no hydrogen features in their spectra
had previously all been classified simply as type I. Now this
class was subdivided into types Ia and Ib, depending on the
presence or absence of a silicon absorption feature at 6150 A in
the supernova's spectrum. With that minor improvement in
typology, an amazing consistency among the type Ia supernovae
became evident. Their spectra matched feature-by-feature, as did
their "light curves" -- the plots of waxing and waning brightness
in the weeks following a supernova explosion.

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2. ON THE FIRST STARS

Nature 2003 422:825

The following points are made by Timothy C. Beers:

1) Late last year, the discovery of the most iron-deficient star
yet identified, HE0107 5240, was announced. This star has a
measured abundance of iron less than 1/200,000 that of the Sun.
Its significance is that it seems to be a relic from the early
Universe, and astronomers are now busy considering how to
interpret it.

2) Researchers have presented various interpretations of HE0107
5240. Each of these interpretations centers on whether this star
exhibits properties that might reveal the likely range of mass
that should be associated with the so-called "population III"
stars -- objects that are presumed to have formed shortly after
the Big Bang, and which are thought to have produced the first
elements heavier than H, He and Li, as well as the "first light"
in the Universe. "Population II" stars are objects that formed
after population III stars, and which incorporated the metals
created by this previous generation. Our Sun, and other (younger)
metal-rich stars in the Galactic disk, are referred to as
"population I" objects.

3) To the astronomer, metals include all elements heavier than
He, and they are thought to be produced only by nuclear reactions
that take place during the lifetimes, or at the deaths, of stars.
Stars such as our Sun have inherited the net production of metals
by all of the previous generations that lived (and died) before
it. Stars with the lowest observed abundances of heavy elements,
such as HE0107 5240, must therefore have been born before other
stars, because the gas clouds from which they formed had only the
slightest traces of these heavy elements. Thus, regardless of the
outcome of debate about the nature of the very first stars,
HE0107 5240 remains chemically the most primitive object yet
discovered, and is a crucial "laboratory" for tests of the
origins of the first elements in the Universe.

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3. ON QUANTUM ENTANGLEMENT

Physics Today 2003 April

The following points are made by B.M. Terhal et al:

1) Erwin Schroedinger (1887-1961) coined the word entanglement in
1935 in a three-part paper (Naturwiss. 1935 48:807; 49:823,844;
Engl. trans.: Proc. Am. Philos. Soc. 1980 124:323) on the
"present situation in quantum mechanics." His article was
prompted by Albert Einstein, Boris Podolsky, and Nathan Rosen's
now celebrated "EPR paper" that had raised fundamental questions
about quantum mechanics earlier that year.

2) Einstein and his coauthors had recognized that quantum theory
allows very particular correlations to exist between two
physically distant parts of a quantum system; those correlations
make it possible to predict the result of a measurement on one
part of a system by looking at the distant part. On that basis,
the EPR paper argued that the distant predicted quantity should
have a definite value even before being measured if the theory
were to claim completeness and respect locality. However, because
quantum mechanics disallows such definite values prior to
measuring, the EPR authors concluded that, from a classical
perspective, quantum theory must be incomplete.

3) Schroedinger's 1935 perspective comes closer to the modern
view: The wavefunction or state vector gives us all the
information that we can have about a quantum system. About
entangled quantum states, he wrote, "The whole is in a definite
state, the parts taken individually are not," which we now
understand as the essence of pure-state entanglement. In that
same 1935 article, Schroedinger also introduced his famous cat as
an extreme illustration of entanglement: A cat physically
isolated in a box with a decaying atom and vial of cyanide
represents a quantum state having macroscopic degrees of freedom.
If the atom were to decay and trigger the release of cyanide, the
cat would die. The quantum-mechanical description of the system
is a coherent superposition of one state in which the atom is
still excited and the cat alive, and another state in which the
atom has decayed and the cat is dead. The isolated cat-trigger-
atom-cyanide system as a whole is in a definite entangled state,
even though the cat itself exists as a probabilistic mixture of
being alive or dead.

4) For the three decades following the 1935 articles, the debate
about entanglement and the "EPR dilemma" -- how to make sense of
the presumably nonlocal effect one particle's measurement has on
another -- was philosophical in nature, and for many physicists
it was nothing more than that. The 1964 publication (J.S. Bell:
Physics 1964 1:195) by John Bell changed that situation
dramatically. Bell derived correlation inequalities that can be
violated in quantum mechanics but have to be satisfied within
every model that is local and complete -- so-called local hidden-
variable models. Bell's work made it possible to test whether
local hidden-variable models can account for observed physical
phenomena. Early and ongoing recent experiments showing
violations of such Bell inequalities have invalidated local
hidden-variable models and lend support to the quantum-mechanical
view of nature. In particular, an observed violation of a Bell
inequality demonstrates the presence of entanglement in a quantum
system.

Notes:

A "hidden variables theory" is one of a class of physical
theories which deny that the quantum state of a physical system
is a complete specification. The hidden variables are those
components of the hypothetical complete state that are not
contained in the quantum state.

"Bell's inequality", formulated by John Bell (1928-1990) in 1964,
is one of a family of inequalities concerning the probabilities
of joint occurrence of certain events in two well-separated parts
of a composite system, the inequality implied by any hidden
variables theory that satisfies an appropriate locality
condition. In this context, in general, a locality condition is a
condition such that no interaction between two entities can occur
in less time than the time required for light to travel from one
entity to the other. For example, any apparent instantaneous
effect of one entity upon the other entity implies locality is
not obeyed.

"Bell's theorem" is the theorem that no hidden variables theory
satisfying an appropriate locality condition can make statistical
predictions in complete agreement with those of quantum
mechanics. In other words, there are situations in which quantum
mechanics predicts a violation of Bell's inequality. Another
formulation is that any hidden variables theory that forbids
instantaneous interactions cannot make predictions in complete
agreement with those of quantum mechanics.

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4. ON MOLECULAR ELECTRONICS

Physics Today 2003 May

The following points are made by J.R. Heath and M.A. Ratner:

1) Molecules have not historically played a prominent role in
electronic devices. Ten years ago, chemical applications were
limited to the use of small molecules such as silanes and
germanes as thin-film precursors or as the components of etching
processes, resist precursors, packaging materials, and the like.
Engineered inorganic insulators, semiconductors, and metals were
the heart of the industry, and the fundamental knowledge that
gave birth to the integrated circuit was appropriately credited
back to the fundamental solid-state physics that was largely
developed in the mid-20th century.

2) Over the past decade, the picture has not changed much.
Conducting polymers have emerged as a real, albeit still minor,
technology. However, over the next 10-20 years, molecules may be
increasingly viewed not just as the starting points for bulk
electronic materials, but as the active device components within
electronic circuitry. Although this possibility is hardly a
foregone conclusion, a number of fundamental issues favor the
development of a true molecular-based electronics.

3) Consider a lattice of nanowires. Each wire is 5 nm in
diameter, and the lattice constant is 15 nm. At a typical doping
level of 10^(18) atoms of boron or arsenic per cubic centimeter,
similar 5-nm diameter, micron-long segments of silicon wires
would have 15-20 dopant atoms, and a junction of two crossed
wires would contain, on average, approximately 0.1 dopant atom.
Consequently, field-effect transistors fabricated at these wiring
densities might exhibit non-statistical and perhaps unpredictable
behavior. Other concerns, such as the gate oxide thickness, power
consumption (just from leakage currents through the gate oxide),
and fabrication costs, also highlight the difficulty of scaling
standard electronics materials to molecular dimensions. At device
areas of a few tens of square nanometers, molecules have an
inherent attractiveness because of their size, because they
represent the ultimate in terms of atomic control over physical
properties, and because of the diversity of properties -- such as
switching, dynamic organization, and recognition -- that can be
achieved through such control.

4) Although molecular electronics has been the subject of
research for some time, over the past few years a number of
synthetic and quantum chemists, physicists, engineers, and other
researchers have sharply increased the ranks of this field.
Several new molecular-electronic systems, analytical tools, and
device architectures have been introduced and explored. As a
result, the basic science on which a molecular electronics
technology would be built is now unfolding. For example, current
research is using molecules in such electronics applications as
interconnects, switches, rectifiers, transistors, nonlinear
components, dielectrics, photovoltaics, and memories.

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5. CHLOROPLASTS AND GENES

Current Biology 2003 13:R314

The following points are made by Paul Jarvis:

1) Chloroplasts, like mitochondria, evolved from a free-living
prokaryotic organism that entered the eukaryotic lineage through
endosymbiosis. During the course of their evolution, chloroplasts
relinquished most of their genes to the nucleus, and so became
subservient to the eukaryotic host. Today, more than 90% of the
3000 or so proteins present in chloroplasts are encoded in the
nucleus, translated in the cytosol and imported into the
organelle post-translationally. The remainder are encoded and
synthesized within the organelle itself by an endogenous genetic
system.

2) One of the consequences of this partitioning of genetic
information is that processes which take place inside
chloroplasts necessarily require input from two different
compartments. For example, the photosynthetic complexes of the
thylakoid membranes comprise core subunits encoded by the
chloroplast genome, and peripheral subunits encoded by the
nuclear genome. To ensure that these complexes are assembled in
stoichiometric fashion, and to enable their rapid reorganization
in response to changing environmental cues, the activities of the
nuclear and chloroplast genomes must be closely coordinated
through intracellular signalling.

Notes:

The term "thylakoid" refers to a sac-like vesicle containing the
photosynthetic pigments in photosynthetic organisms. In
prokaryotes, the thylakoids are of various shapes and are
attached to the plasma membrane; in eukaryotes, the thylakoids
are flattened and located in chloroplasts; in the chloroplasts of
higher plants, the thylakoids form dense stacks called "grana".
Isolated thylakoids preparations can carry out photosynthetic
electron transport and associated phosphorylation.

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6. ON NEURONAL CONNECTIVITY

Current Biology 2003 13:R264

The following points are made by Alain Ghysen:

1) The establishment of a reproducible pattern of connectivity is
as essential to a neuron's function as its physiological
properties. Yet our understanding of neuronal connectivity is
much less advanced than that of neuronal physiology, perhaps
because connectivity has to be apprehended within the complexity
of the entire central nervous system (CNS). Taking advantage of
the relative simplicity of the fly sensory system, Zlatic et al.
(Neuron 2003 37:41) have elucidated the molecular mechanism that
drives a given type of sensory neuron to extend its terminal
branches along a specific fascicle in the central nervous system
of Drosophila larvae. They report that, contrary to expectations,
the axon does not recognize a given fascicle -- rather, it
recognizes the position at which this fascicle runs. This is
because the axon responds to the same positional cue that
directed the formation of the fascicle.

2) The sense organs of insects are a favorable system for
studying neuronal development. This is because they are
innervated by a fixed number of neurons -- one in the case of the
mechanosensory bristles -- and because they often occupy
stereotyped positions on the body, such that one can deal with
identified neurons. Analysis of sensory projections in adult
flies revealed that the axons recognize and follow pre-existing
pathways which differ for different types of sensory neuron. The
identification and experimental analysis of these pathways was
hindered, however, by the fact that axonogenesis occurs during
metamorphosis, when the central nervous system itself is
massively remodeled. Recent work on adult sensory neurons has
revealed that their axons are at least partly guided towards and
within the CNS by the neurites of persistent larval neurons.

3) In summary: The mechanism that allows a sensory neuron to
extend its terminal branches along the appropriate fascicle
within the CNS turns out to be the same as that which positioned
the fascicle earlier on, and the gene that controls this position
is the same as that which determined the neuron's identity.

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7. ON THE AIDS PANDEMIC

New Engl. J. Med. 2003 348:1800

The following points are made by William J. Clinton:

1) The future is not hopeful for the nearly 40 million people
living with HIV or AIDS in the developing world. There we have
failed. Infection rates range as high as 38 percent, and 14,000
more people become infected every day. Twenty-five million people
have died from AIDS-related illnesses, and 8500 more people die
each day. AIDS has orphaned 10 million children, and 33 million
more will be orphaned by the end of the decade unless we take
immediate and decisive action. Although there is much talk about
treatment, the reality on the ground is that very few people are
currently receiving medication. In Africa, fewer than 30,000 of
the estimated 4.5 million people who need treatment are getting
it. Biomedical researchers have provided us with the tools that
we need to turn the tide on this crisis. But the political
leaders and citizens of the world, have not yet done enough to
see that they are put to use.

2) Although the deaths of 25 million people should be cause
enough for concern, this is not simply a humanitarian crisis.
This disease has devastated not only the families of those who
are infected, but the political, social, and economic structures
of entire nations. When 80 percent of persons dying from AIDS are
between 20 and 50 years of age, societies are left with the young
and the old, with few to support them.

3) Educational systems are falling apart. Teachers are dying
faster than more can be trained, and children are staying home to
look after their dying parents. Meanwhile, funding for education
is being cut to compensate for the rising costs of health care.
Security forces are weakening. At a time when there is increasing
international instability, trained soldiers and police officers
are dying faster than they can be replaced. Agricultural
production is dwindling. Farmers and farm laborers are dying, and
farms do not have the workers they need to produce at high
levels. No segment of society is left untouched. Doctors, nurses,
factory workers, business people, and government officials are
all dying in alarming numbers. In today's increasingly integrated
world, if these nations are allowed to disintegrate, the
economic, political, and strategic consequences will be felt far
beyond their borders. Today, this is happening in Africa.
Tomorrow, it could happen elsewhere. The fastest-growing rates of
infection are in the former Soviet Union, the Caribbean, India,
and China.

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8. PRENATAL HORMONE EXPOSURE AND SEXUAL VARIATION

American Scientist 2003 91:218

The following points are made by John G. Vandenbergh:

1) X and Y chromosomes are only the beginning of sex
determination. Biologists have recently taken a closer look at
the events between fertilization and sexual maturity that
establish an individual's sexual characteristics. These events
help explain variability among individuals in sexual anatomy,
physiology and behavior. For example, an XX individual can
exhibit some masculine traits if hormone production or
sensitivity is abnormal during early development.

2) In fact, there is almost a continuum of sexual traits between
male and female. The ability of environmental influences during
development to produce such a continuum demonstrates that another
classical dichotomy, between nature and nurture, is in fact a
synergy: Genes and environment work together to produce an
organism. Specific genes are only turned on when the environment
of the cell, tissue, organ or organism calls for them. In the
case of sexual characteristics, one mechanism by which
environmental variables exert their influence is through the
activity of hormones.

3) Hormones are substances released by cells into the
bloodstream, where they travel throughout the body and influence
the function of other, distant cells. Hormone molecules
themselves, or the enzymes that produce those molecules, are
encoded by the genome, but hormone concentrations can be
modulated by a wide array of environmental factors, including
stress, food consumption, temperature, and time of year. In turn,
hormone concentrations modulate the expression of genes in a
variety of different cell and tissue types, producing anatomical,
physiological and behavioral differences.

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