27 research outputs found
A Geometrical Characterization of the Twin Paradox and its Variants
The aim of this paper is to provide a logic-based conceptual analysis of the
twin paradox (TwP) theorem within a first-order logic framework. A geometrical
characterization of TwP and its variants is given. It is shown that TwP is not
logically equivalent to the assumption of the slowing down of moving clocks,
and the lack of TwP is not logically equivalent to the Newtonian assumption of
absolute time. The logical connection between TwP and a symmetry axiom of
special relativity is also studied.Comment: 22 pages, 3 figure
Macromolecular theory of solvation and structure in mixtures of colloids and polymers
The structural and thermodynamic properties of mixtures of colloidal spheres
and non-adsorbing polymer chains are studied within a novel general
two-component macromolecular liquid state approach applicable for all size
asymmetry ratios. The dilute limits, when one of the components is at infinite
dilution but the other concentrated, are presented and compared to field theory
and models which replace polymer coils with spheres. Whereas the derived
analytical results compare well, qualitatively and quantitatively, with
mean-field scaling laws where available, important differences from ``effective
sphere'' approaches are found for large polymer sizes or semi-dilute
concentrations.Comment: 23 pages, 10 figure
True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces
Abstract: Rock outcrops of the sedimentaryâstratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sedimentâwater or sedimentâair interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentaryâstratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these shortâterm, transient, smallâscale features are counterâintuitively abundant within a 3.8 billion yearâlong sedimentaryâstratigraphic record that is known to be inherently timeâincomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the timeâlength scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional byâproducts of the accrual of the sedimentaryâstratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentaryâstratigraphic record. They should be recognized as providing highâdefinition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentaryâstratigraphic record is unrepresentative of the geological past
The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?
EFFECT OF SOME PHENOLIC COMPOUNDS ON THE OXIDATION OF 4-METHYL CATECHOL CATALYZED BY AVOCADO POLYPHENOLOXIDASE
Acoustic analysis and playback experiments do not support the taxonomic revision of the Central and Western Canary Islands subspecies of the Eurasian Stone-curlew Burhinus oedicnemus distinctus
Capsule: Acoustic analysis does not support the elevation of B. o. distinctus to full species.
Aims:To verify whether the vocal repertoires of B. o. oedicnemus and B. o. distinctus show biologically significant quantitative and qualitative differences.
Methods: Integration of acoustic analysis of some of the most frequently uttered call types recorded in Italy and in Canary Islands with playback experiments.
Results: The vocal repertoires of the individuals belonging to the two subspecies were rather similar, but the quantitative analysis of acoustic parameters evidenced some differences between the considered populations. In particular, the three most used call types showed higher frequency and faster utterance rhythm for B. o. distinctus than for B. o. oedicnemus. Playback experiments indicated that individuals from the nominate subspecies responded in the same way to the playback of calls of individuals belonging to both subspecies.
Conclusion: Acoustic analysis supports the distinctiveness of Stone-curlew populations from Central and Western Canary Islands, thus confirming the available morphological and genetic data. These results, however, do not suggest the elevation of B. o. distinctus to full species