Spatial Directions, Anisotropy and Special Relativity

The concept of an objective spatial direction in special relativity is investigated and theories assuming light-speed isotropy while accepting the existence of a privileged spatial direction are classified. A natural generalization of the proper time principle is introduced which makes it possible to devise experimental tests of spatial isotropy. Several common misunderstandings in the relativistic literature concerning the role of spatial isotropy are clarified.Comment: 24 pages, 2 figures. Final enlarged and slightly corrected version. Keywords: relativity, very special relativity, spatial isotropy, reciprocity, differential aging; Foundations of Physics, 201

Simultaneity as an Invariant Equivalence Relation

This paper deals with the concept of simultaneity in classical and relativistic physics as construed in terms of group-invariant equivalence relations. A full examination of Newton, Galilei and Poincar\'e invariant equivalence relations in $\R^4$ is presented, which provides alternative proofs, additions and occasionally corrections of results in the literature, including Malament's theorem and some of its variants. It is argued that the interpretation of simultaneity as an invariant equivalence relation, although interesting for its own sake, does not cut in the debate concerning the conventionality of simultaneity in special relativity.Comment: Some corrections, mostly of misprints. Keywords: special relativity, simultaneity, invariant equivalence relations, Malament's theore

Modellizzazione frattale di immagini SAR del mare

L’oggetto del presente lavoro di tesi è stato quello di sviluppare un valido modello di tipo frattale per la rappresentazione di immagini SAR di scenari marini. Il modello proposto si basa sull’idea di riprodurre separatamente la componente spettrale di rumore, dovuta alle elaborazioni effettuate dal sistema SAR, e la componente spettrale informativa dovuta alla particolare configurazione della superficie marina. Tramite l’applicazione del modello proposto ad alcune immagini SAR ERS-1 ed ERS-2 è stato possibile verificarne la reale applicabilità a dati reali

On Selleri's "Weak Relativity''

F. Selleri's main arguments for the restoration of absolute simultaneity in physics are analysed and shown to be faulty. In particular both the classical Sagnac effect and the recent so-called linear Sagnac effect can be dealt with within special relativity in a natural way. The appeal to the conventionality of simultaneity thesis is also shown to be ineffectual. Other arguments, such as the two spaceships' argument and the block universe argument are briefly examined. Notwithstanding a negative overall assessment, the importance of keeping alive the research on the foundations of relativity is emphasized and Selleri's role in this undertaking appreciated.Comment: 27 pages, 8 figure

MIMO noise radar - matched filters and coarrays

Copyright © 2008 IEEEThe noise radar concept can be extended to an array of K transmit antenna and M receive antenna. When independent noise sources are transmitted from each antenna the approach may be viewed as a special case of MIMO radar and matched filters may be derived. In this contribution statistical properties of matched filters for MIMO noise radar are derived. For any array the concepts of the sum and difference coarrays are useful and important tools for understanding an arraypsilas beamforming properties particularly for sparse arrays. Element space matched filters are shown to be related to the concept of the sum coarray and some examples of the advantages of this for sparse transmit/receive array geometry are provided. A variation of this result for beam space matched filters is also presented.Douglas A Gray, Amerigo Capri

Percolation of single-walled carbon nanotubes in ceramic matrix nanocomposites

The percolation of carbon nanotubes (CNT) in an electrical insulating ceramic is studied for the first time. The in situ synthesis of the CNT (0.2–25 vol%) by a CCVD route allows to achieve their homogeneous distribution in the spinel matrix. Up to 11 vol% CNT, the DC electrical conductivity (σ) is well fitted by the scaling law of the percolation theory σ=k(p−pc)t with a low percolation threshold pc=0.64 vol%. At the threshold, σ jumps over seven order of magnitude (from 10−10 to 0.0040 S cm−1) and then reaches a maximum at 8.5 S cm−1. The results are discussed in relation with the characteristics of the CNT, their damaging during the hot-pressing at 1300 °C and the microstructure of the composites. CNT-ceramic composites become attractive materials not only for their enhanced mechanical properties, but also for the possibility to tailor the electrical conductivity through the CNT content