The Clock Paradox in a Static Homogeneous Gravitational Field1

The gedanken experiment of the clock paradox is solved exactly using the general relativistic equations for a static homogeneous gravitational field. We demonstrate that the general and special relativistic clock paradox solutions are identical and in particular that they are identical for finite acceleration. Practical expressions are obtained for proper time and coordinate time by using the destination distance as the key observable parameter. This solution provides a formal demonstration of the identity between the special and general relativistic clock paradox with finite acceleration and where proper time is assumed to be the same in both formalisms. By solving the equations of motion for a freely falling clock in a static homogeneous field elapsed times are calculated for realistic journeys to the stars. ¹ Both authors contributed equally to this paper.     

New high temperature polymers. I. Wholly aromatic ordered copolyamides

Wholly aromatic ordered copolyamides of unusually high thermal stability were prepared by the condensation of aromatic diacid chlorides with symmetrical diamines containing preformed aromatic amide units in an ordered arrangement. The preservation of order in the condensation step was assured by using interfacial or solution polymerization techniques at temperatures below 50°C. Each polymer contains units derived from aminobenzoic acids, arylene diamines, and arylene diacids. By use of para- and meta- phenylene units, eight different polymers are possible; all were prepared. Differential thermal analyses and thermogravimetric analyses showed these polymers to have melting points or decomposition temperatures in a range from 410°C. for the all-meta polymer to 555°C. for the all-para one. Substitution of the internal N-hydrogens of the diamines with methyl groups or phenyl groups leads to additional ordered copolymers. Several were prepared, but their melting points were much lower than those of the parent polymers limiting their usefulness in high temperature applications. Tough pliable films were prepared from all eight unsubstituted polymers, and crystalline fibers with tenacities of ca. 6 g./den. were prepared from three of the polymers. The properties of the fibers were retained to a high degree even when determined at temperatures up to 400°C. Fibers aged at 300°C. for extended periods of time showed remarkable retention of fiber properties.     

Where organometallics and dendrimers merge: the incorporation of organometallic species into dendritic molecules

This review will describe the ongoing efforts being made to incorporate organometallic fragments into the framework of dendrimers. While purely dendritic organic molecules are well known and well studied, species incorporating organometallic moieties potentially offer many benefits that are not available to only organic containing dendrimers. For example, catalytic or redox active organometallic functions can be included in the dendritic framework and impart these characteristics onto the dendrimer. This report will give an overview of the latest developments in this field by highlighting selected examples that detail novel synthetic strategies or dendrimer construction methodologies, interesting practical applications or address specific problems associated with organometallic dendrimers.     

ESR spectra of AsF2−6 and SbF2−6

ESR spectra observed in γ-irradiated CsAsF₆ and CsSbF₆ matrices have been assigned to the free radicals AsF^2?₆ and SbF^2?₆ respectively. The large, isotropic central-atom hyperfine interactions and isotropic g-values of these species suggest that they possess ²A_tg ground-states in O_h symmetry. The contribution to the semi-occupied orbital of the central-atom valence s atomic orbital is ≈ 0.6, greater than in the halogen hexafluorides (≈0.4) and the chalcogen hexafluoride anions (≈ 0.5). Variation of the central-atom hyperfine interaction with temperature is interpreted in terms of a second-order Jahn-Teller effect involving a low-lying ²T_1u excited state.     

Advances in Acoustical Holography

The most recent branch of holography, acoustical holography employing acoustical radiation, presents a variety of new and highly interesting possible applications: Thus objects can be imaged in a turbid liquid medium (marine research); the technique can be used for non-destructive investigations of the internal structure of objects that are opaque to light (testing of materials) and is possibly also a potential tool for the three-dimensional imaging of biological structures (biomedical diagnosis).