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<jats:p>The World Health Organization (WHO) has implemented a plan to eradicate poliovirus that is widely viewed as having made enormous progress. If all goes as planned, polio will be eradicated on this planet by the year 2003. However, there is a debate, as highlighted in a pair of Policy Forums in this issue, over when vaccination against polio can be stopped. Dove and Racaniello believe that the reliance of the WHO on the live Sabin oral poliovirus vaccine (OPV) means that there will be a continuing threat of release of potentially pathogenic virus into the environment. They are also concerned that the planned destruction of all wild-type polio stocks will not be possible. They therefore recommend a switch to the inactivated polio vaccine (IPV). In response, Hull and Aylward explain why a switch from OPV is not necessary and describe the studies being sponsored by the WHO to determine how and when immunization can safely be ended.</jats:p>

<jats:p>The World Health (WHO) has implemented a plan to eradicate poliovirus that is widely viewed as having made enormous progress. If all goes as planned, polio will be eradicated on this planet by the year 2003. However, there is a debate, as highlighted in a pair of Policy Forums in this issue, over when vaccination against polio can be stopped. Dove and Racaniello believe that the reliance of the WHO on the live Sabin oral polio virus vaccine (OPV) means that there will be a continuing threat of release of potentially pathogenic virus into the environment. They are also concerned that the planned destruction of all wild-type polio stocks will not be possible. They recommend a switch to the inactivated polio vaccine. In response, Hull and Aylward set out the reasons for thinking that a switch from the OPV is not necessary and describe the studies being sponsored by the WHO to determine how and when immunization can safely be ended.</jats:p>

<jats:p> Optical spectroscopy of conjugated molecules is described by using collective electronic coordinates, which represent the joint dynamics of electron-hole pairs. The approach relates the optical signals directly to the dynamics of charges and bond orders (electronic coherences) induced by the radiation field and uses only ground-state information, thus avoiding the explicit calculation of excited molecular states. The resulting real-space picture is reminiscent of the normal-mode analysis of molecular vibrations and offers a unified framework for the treatment of other types of systems including semiconductor nanostructures and biological complexes. Spatial coherence displayed in two-dimensional plots of the five electronic normal modes that dominate the optical response of poly( <jats:italic>p</jats:italic> -phenylene vinylene) oligomers with up to 50 repeat units (398 carbon atoms) in the 1.5- to 8-electronvolt frequency range suggests a saturation to bulk behavior at about five repeat units. </jats:p>

<jats:p> Corundum, α-Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> , is the thermodynamically stable phase of coarsely crystalline aluminum oxide, but syntheses of nanocrystalline Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> usually result in γ-Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> . Adsorption microcalorimetry, thermogravimetric analyses, and Brunauer-Emmett-Teller adsorption experiments, coupled with recently reported high-temperature solution calorimetry data, prove that γ-Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> has a lower surface energy than α-Al <jats:sub>2</jats:sub> O <jats:sub>3</jats:sub> and becomes energetically stable at surface areas greater than 125 square meters per gram and thermodynamically stable at even smaller surface areas (for example, 75 square meters per gram at 800 kelvin). The results are in agreement with recent molecular dynamics simulations and provide conclusive experimental evidence that differences in surface energy can favor the formation of a particular polymorph. </jats:p>

<jats:p>The pressure dependence of deuterium-hydrogen (D-H) fractionation in water to 500°C and 200 megapascals has been calculated from high-temperature, high-pressure spectroscopic data. Pressure effects have a maximum at the critical temperature of water (20 per mil between 22 and 200 megapascals). Even larger effects are predicted for vaporlike densities from molecular dynamics simulations and molecular orbital calculations. Pressure effects explain many of the large discrepancies in published mineral-water D-H fractionation curves. Possible applications to natural examples include mineral-water isotope geobarometry.</jats:p>

<jats:p>Compressive paleostresses, as recorded by twinned calcite in carbonate rocks that cover cratonic northwestern North America, are perpendicular to the orogenic front of the Late Cretaceous to Early Cenozoic Sevier fold-thrust belt. Inferred differential stresses decrease from ∼100 megapascals (MPa) at the orogenic front to ∼20 MPa up to 2000 kilometers inland. New analyses near the Late Paleozoic Appalachian front refine earlier results from the eastern Midcontinent. The Appalachian and Sevier stress data in North America's continental interior are remarkably similar in spite of distinctly different tectonic properties. This suggests that continental interior stresses are largely insensitive to tectonic characteristics of compressive plate margins and that far-field stress transmission is filtered by deformation styles in mountain belts.</jats:p>