What we don't know about time

String theory has transformed our understanding of geometry, topology and spacetime. Thus, for this special issue of Foundations of Physics commemorating "Forty Years of String Theory", it seems appropriate to step back and ask what we do not understand. As I will discuss, time remains the least understood concept in physical theory. While we have made significant progress in understanding space, our understanding of time has not progressed much beyond the level of a century ago when Einstein introduced the idea of space-time as a combined entity. Thus, I will raise a series of open questions about time, and will review some of the progress that has been made as a roadmap for the future.Comment: 15 pages; Essay for a special issue of Foundations of Physics commemorating "Forty years of string theory

Spacetime and the Holographic Renormalization Group

Anti-de Sitter (AdS) space can be foliated by a family of nested surfaces homeomorphic to the boundary of the space. We propose a holographic correspondence between theories living on each surface in the foliation and quantum gravity in the enclosed volume. The flow of observables between our ``interior'' theories is described by a renormalization group equation. The dependence of these flows on the foliation of space encodes bulk geometry.Comment: 12 page

Asymptotic Analysis of Generative Semi-Supervised Learning

Semisupervised learning has emerged as a popular framework for improving modeling accuracy while controlling labeling cost. Based on an extension of stochastic composite likelihood we quantify the asymptotic accuracy of generative semi-supervised learning. In doing so, we complement distribution-free analysis by providing an alternative framework to measure the value associated with different labeling policies and resolve the fundamental question of how much data to label and in what manner. We demonstrate our approach with both simulation studies and real world experiments using naive Bayes for text classification and MRFs and CRFs for structured prediction in NLP.Comment: 12 pages, 9 figure

On the existence of supergravity duals to D1--D5 CFT states

We define a metric operator in the 1/2-BPS sector of the D1-D5 CFT, the eigenstates of which have a good semi-classical supergravity dual; the non-eigenstates cannot be mapped to semi-classical gravity duals. We also analyse how the data defining a CFT state manifests itself in the gravity side, and show that it is arranged into a set of multipoles. Interestingly, we find that quantum mechanical interference in the CFT can have observable manifestations in the semi-classical gravity dual. We also point out that the multipoles associated to the normal statistical ensemble fluctuate wildly, indicating that the mixed thermal state should not be associated to a semi-classical geometry.Comment: 22 pages, 2 figures. v2 : references added, typos correcte

The Library of Babel

We show that heavy pure states of gravity can appear to be mixed states to almost all probes. Our arguments are made for $\rm{AdS}_5$ Schwarzschild black holes using the field theory dual to string theory in such spacetimes. Our results follow from applying information theoretic notions to field theory operators capable of describing very heavy states in gravity. For certain supersymmetric states of the theory, our account is exact: the microstates are described in gravity by a spacetime ``foam'', the precise details of which are invisible to almost all probes.Comment: 7 pages, 1 figure, Essay receiving honorable mention in the 2005 Gravity Research Foundation essay competitio

Environmental supply chain management in the construction sector: theoretical underpinnings

Environmental implications of a product/project are typically spread across its supply chain, from design through to end-of-life. This is particularly acute for construction sector because of the inherent nature and complexity of its supply chain. The sector, therefore, could benefit from the application of environmental supply chain management (ESCM), a systematic and integrated approach towards restraining the adverse environmental impacts emanating from the supply chain. Unfortunately, uptake of ESCM in construction vis-à-vis other sectors is limited and fragmented. This formed the motivation, which using 39 interviews across 20 firms in the UAE construction sector, provides systemic insights on the various facets of ESCM namely, core and facilitating environmental practices, internal and external drivers (pressures/motives) and barriers/challenges affecting environmental practices’ implementation, and relevant environmental performance measures across different firm types and stakeholders with the support of several established/emerging theories. The findings are expected to enhance the application of ESCM in construction

Foreign versus local firms: implications for environmental sustainability

Purpose – Managing environmental consequences while sustaining economic development necessitate strong commitment and participation of all firms across sectors. However, the environment-related role of foreign and local firms is unclear from previous research. With increasing trade liberalization and entry of foreign firms, this question has become particularly relevant. The purpose of this paper is to contrast the roles and contributions of foreign and local firms from an environmental sustainability perspective. Design/methodology/approach – Using data collected through a structured survey (395 responses) and semi-structured interviews (19 numbers) from the United Arab Emirates (UAE) construction sector (research setting), the study analyses and understands the hypothesized differences between foreign and local firms on three key environmental sustainability aspects: the extent of environmental practices implementation, the strengths/influences of drivers and barriers affecting the implementation, and the environmental, cost-related, and organizational performance benefits derived. Findings – Foreign firms were found to implement environmental practices to a greater extent, have a greater internal drive to implement these practices, and face lower barriers to implementation than local ones. Local firms, though were found to be not far behind foreign ones with regards to the environmental, cost-related, and organizational performance benefits derived. Practical Implications - Findings from the study are expected to help policymakers and practitioners develop policies/interventions that ensure all firms irrespective of their nature of ownership contribute equitably to environmental sustainability. Originality/value – This study is arguably the first comprehensive attempt to understand how various environmental sustainability aspects are perceived and performed by local and foreign firms

Green supply chain management: the case of the construction sector in the United Arab Emirates (UAE)

Restraining the negative environmental impacts of the construction sector constitutes one of the major challenges of the twenty-first century. However, efforts to address it have been largely fragmented. With environmental consequences of a construction project typically dispersed across its life cycle, i.e. from design through to end-of-life, greening this sector requires a supply chain wide focus inclusive of all key stages and stakeholders; also, all relevant aspects such as the nature of green practices implemented and associated drivers, barriers and performance implications need to be considered. This forms the focus of the present study where a comprehensive, green supply chain management (GSCM) oriented understanding of the construction sector is developed through the context of the UAE construction sector, and incorporating inputs from all key stakeholders, i.e. Developers, Architects/ Consultants, Contractors and (material) Suppliers. The study contributes to improving the efficiency and effectiveness of greening of the construction sector