Causality and the speed of sound

A usual causal requirement on a viable theory of matter is that the speed of sound be at most the speed of light. In view of various recent papers querying this limit, the question is revisited here. We point to various issues confronting theories that violate the usual constraint.Comment: v2: additional discussion on models that appear to have superluminal signal speeds; version to appear in GR

Using light to predict fuels-reduction and group-selection effects on succession in Sierran mixed-conifer forest

Many semi-arid coniferous forests in western North America have reached historically unprecedented densities over the past 150 years and are dominated by shade-tolerant trees. Silvicultural treatments generally open the canopy but may not restore shade-intolerant species. We determined crossover-point irradiance (CPI) (light at which the height growth rank of pairs of species changes) for seedlings in Sierra Nevada mixed-conifer forest and used these to interpret light environments produced by fuels-reduction thinning and group selection with reserved large trees. Nine of 21 species pairs had well-defined CPIs. The CPI of the most common shade-tolerant and intolerant species (white fir (Abies concolor (Gordon & Glendl.) Lindl. ex Hildebr.) and ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson)) was 22.5 mol m^(-2) day^(-1) or 41% of full sun. Median understory irradiance increased from 9.2 mol m^(-2) day^(-1) (17% full sun) in pretreatment forest to 13 mol m^(-2 day^(-1) (24% full sun) in lightly and 15.5 mol m^(-2) day^(-1) (28% full sun) in moderately thinned stands and 37 mol m^(-2) day^(-1) (67% full sun) in group-selection openings. We estimate that 5%-20% of ground area in lightly to moderately thinned stands would have enough light to favor shade-intolerant over shade-tolerant growth compared with 89% of ground area in group-selection openings. The CPI provides a tool to assess regeneration implications of treatment modification such as increasing heterogeneity of thinning to enhance regeneration or reserving large trees in group-selection openings to maintain wildlife habitat

Studies of plasma performance and transport in the advanced toroidal facility (ATF)

An overview of recent ATF experimental results and program plans is presented, with emphasis on the role of magnetic configuration controls in transport studies. The ATF operating space is bounded by a density limit that effectively sets a limit on the energy confinement time {tau}{sub E}. Although this limit is not solely due to impurities, it has recently been raised by improved cleanliness following titanium gettering. This has led to collapse-free neutral beam injection (NBI) discharges with global {tau}{sub E} {approx} 16 ms. Preliminary experiments show that stored energy and bootstrap current are sensitive to details of the magnetic configuration. 13 refs., 8 figs