A review of policies and strategies affecting the peri-urban interface

As discussed elsewhere (Adell, 1999; Allen, 1999), whilst there is no accepted definition of what precisely constitutes the "peri-urban interface", the project team has identified at least three different approaches from where it has been conventionally conceptualised. These approaches may be classified according to the set of variables they choose to emphasise: physical attributes, such as proximity to the city and poor infrastructure; socio-economic variables; or urban-rural flows (of people, energy, goods)

Configuring the cancellation of optical near-fields

The characteristic near-field behavior of electromagnetic fields is open to a variety of interpretations. In a classical sense the term 'near-field' can be taken to signify a region, sufficiently close to some primary or secondary source, that the onset of retardation features is insignificant; a quantum theoretic explanation might focus more on the large momentum uncertainty that operates at small distances. Together, both near-field and wave-zone (radiative) features are fully accommodated in a retarded resonance propagation tensor, within which each component individually represents one asymptotic limit - alongside a third term that is distinctly operative at distances comparable to the optical wavelength. The propagation tensor takes different forms according to the level of multipole involved in the signal production and detection. In this presentation the nature and symmetry properties of the retarded propagation tensor are explored with reference to various forms of electric interaction, and it is shown how a suitable arrangement of optical beams can lead to the complete cancellation of near-fields. The conditions for such behavior are fully determined and some important optical trapping applications are discussed

Optically induced multi-particle structures: multi-dimensional energy landscapes

Recent quantum electrodynamical studies on optically induced inter-particle potential energy surfaces have revealed unexpected features of considerable intricacy. The exploitation of these features presents a host of opportunities for the optical fabrication of nanoscale structures, based on the fine control of a variety of attractive and repulsive forces, and the torques that operate on particle pairs. Here we report an extension of these studies, exploring the first detailed potential energy surfaces for a system of three particles irradiated by a polarized laser beam. Such a system is the key prototype for developing generic models of multi-particle complexity. The analysis identifies and characterizes potential points of stability, as well as forces and torques that particles experience as a consequence of the electromagnetic fields, generated by optical perturbations. Promising results are exhibited for the optical fabrication of assemblies of molecules, nanoparticles, microparticles, and colloidal multi-particle arrays. The comprehension of mechanism that is emerging should help determine the fine principles of multi-particle optical assembly

On the low energy limit of one loop photon-graviton amplitudes

We present first results of a systematic study of the structure of the low energy limit of the one-loop photon-graviton amplitudes induced by massive scalars and spinors. Our main objective is the search of KLT-type relations where effectively two photons merge into a graviton. We find such a relation at the graviton-photon-photon level. We also derive the diffeomorphism Ward identity for the 1PI one graviton - N photon amplitudes.Comment: 14 pages, 1 figure. Final version to be published in Physics Letters

Medellín’s low-carbon Metrocables. Lifting informal barrios out of poverty

Medellin's first two aerial cable-car (Metrocable) lines to hilly informal low-income districts are now widely regarded as the inspiration for a tide of similar schemes in a dozen cities in Latin America and beyond. These lines were a bid by city authorities to provide a fast and efficient connection to the overground mass-transit system. Public transport systems will continue to be essential for the equitable and efficient operation of Latin American cities. Medellín’s experience shows that, with appropriate complementary investments, cable-cars can make a significant contribution to the wellbeing of the urban poor while helping with cities' low carbon growth goals