Is a time symmetric interpretation of quantum theory possible without retrocausality?

Huw Price has proposed an argument that suggests a time-symmetric ontology for quantum theory must necessarily be retrocausal, i.e. it must involve influences that travel backwards in time. One of Price's assumptions is that the quantum state is a state of reality. However, one of the reasons for exploring retrocausality is that it offers the potential for evading the consequences of no-go theorems, including recent proofs of the reality of the quantum state. Here, we show that this assumption can be replaced by a different assumption, called $\lambda$-mediation, that plausibly holds independently of the status of the quantum state. We also reformulate the other assumptions behind the argument to place them in a more general framework and pin down the notion of time symmetry involved more precisely. We show that our assumptions imply a timelike analogue of Bell's local causality criterion and, in doing so, give a new interpretation of timelike violations of Bell inequalities. Namely, they show the impossibility of a (non-retrocausal) time-symmetric ontology.Comment: Accepted for publication in Proc. Roy. Soc. A., 18 pages, 3 figures, revTeX4.

Logical pre- and post-selection paradoxes are proofs of contextuality

If a quantum system is prepared and later post-selected in certain states, "paradoxical" predictions for intermediate measurements can be obtained. This is the case both when the intermediate measurement is strong, i.e. a projective measurement with Luders-von Neumann update rule, or with weak measurements where they show up in anomalous weak values. Leifer and Spekkens [quant-ph/0412178] identified a striking class of such paradoxes, known as logical pre- and post-selection paradoxes, and showed that they are indirectly connected with contextuality. By analysing the measurement-disturbance required in models of these phenomena, we find that the strong measurement version of logical pre- and post-selection paradoxes actually constitute a direct manifestation of quantum contextuality. The proof hinges on under-appreciated features of the paradoxes. In particular, we show by example that it is not possible to prove contextuality without Luders-von Neumann updates for the intermediate measurements, nonorthogonal pre- and post-selection, and 0/1 probabilities for the intermediate measurements. Since one of us has recently shown that anomalous weak values are also a direct manifestation of contextuality [arXiv:1409.1535], we now know that this is true for both realizations of logical pre- and post-selection paradoxes.Comment: In Proceedings QPL 2015, arXiv:1511.0118

Resonant interaction of fast particles with Alfven waves in spherical tokamaks

The Spherical Tokamak (ST) concept has become one of the main avenues in magneticnuclear fusion research since STs successfully demonstrated plasma operationat [Beta] = 2P[mu]0=B2~1. Next step ST machines aiming at achieving burning plasmaconditions in high [Beta] plasmas are being planned, such as the Spherical TokamakPower Plant (STPP) and the Component Testing Facility (CTF). Instabilities offast particle-driven Alfven eigenmodes are often observed in present-day STs. Suchinstabilities, driven by fusion-born alpha particles as well as by fast ions producedwith auxiliary heating schemes, in the next step STs may pose a major problem asthese instabilities may affect confinement and losses of the fast ions.A theory of compressional Alfven eigenmodes (CAE) with frequencies above thedeuterium cyclotron frequency,[omega] > [omega]cD, is developed for plasma parameters of aSTPP, and modes in the ion-ion hybrid frequency range, [omega]cT < [omega] < [omega]cD, are alsoinvestigated in order to assess the potential of diagnosing the deuterium-tritium(D-T) ratio. For the 1-D character of a STPP equilibrium with [Beta]~1 , a `hollowcylinder toroidal plasma model is employed for studying CAEs with arbitrary valuesof the parallel wave-vector k[||] = k[.]B/|B|. The existence of weakly-damped CAEs,free of mode conversion, is shown to be associated with the `well in the magneticfield profile, B = B (R), that can exist at the magnetic axis.A significant part of this thesis focusses on the experimentally observed effectsof resonant wave-particle interaction between Alfven waves and fast particles in theMega Amp Spherical Tokamak (MAST) device at the Culham Laboratory, UK, andin the LArge Plasma Device (LAPD) in the University of California, Los-Angeles,USA. New robust experimental scenarios for exciting CAEs in the MAST spherical tokamak are developed, and interpretation of the observed CAEs in the frequencyrange [omega]cD/3 < [omega] < [omega]cD is given in the context of the 1-D ST model and the Dopplershifted cyclotron resonance. The e ciency of the Doppler resonance between coand counter directed fast ions and left and right hand polarised Alfven waves isfurther assessed experimentally on the LAPD device, with probe ions injected inthe presence of Alfv en waves launched by an external antenna.The developed theory of CAEs is then applied to a calculation of the linear kineticdrive of CAEs in the MAST experiments. A model representation of the fast iondistribution function, produced by neutral beam injection (NBI), is used by fittingto the TRANSP Monte-Carlo NBI modelling results. The main free energy sourcesassociated with temperature anisotropy and bump-on-tail are estimated analytically,and the CAE stability boundary is qualitatively assessed.In order to explain the experimentally observed difference between steady-stateand pulsating Alfvenic modes, the non-linear theory of fast particle driven modesnear marginal stability is extended to include dynamical friction (drag). For thebump-on-tail problem, the drag is shown to always give an explosive amplitudeevolution in contrast to diffusion in velocity space in the vicinity of the wave-particleresonance. This is then extended to the case of experimentally observed NBI-driventoroidal Alfven eigenmodes (TAEs) in the MAST machine. The experimentallyobserved differences between TAEs driven by fast ions produced with ion cyclotronresonance heating (ICRH) and NBI are then interpreted. The problem of dragdominated collisions for modes excited by fusion-born alpha particles in burningplasmas such as a STPP and ITER is underlined.Imperial Users onl

California Voting and Suburbanization Patterns: Implications for Transit Policy, MTI Report 12-05

Public transit is an environmentally friendly transportation mode that usually focuses on transporting people within and to the city center. However, over the last 60 years, population and employment has been suburbanizing. As the median voter lives further from the city center, and thus enjoys fewer benefits from accessing public transit, does this reduce such a voter’s propensity to support public investment in public transit improvements? We analyze voting patterns on 20 transit-related ballot propositions from state-wide elections in California between 1990 and 2010. Controlling for demographic, socio-economic and political ideological factors, we focus on the role of suburbanization as a possible causal factor in determining public support for public transit investment. The results provide a rich picture of the attitudes towards transportation policy among California voters, and will help policy makers to better understand citizen preferences and to better predict how future trends will shift support towards or against transit. Finally, we suggest ways policy makers can use urban land markets to increase support for trans

Establishing the Imperial Oilfield Case Study: Part 1 - Reservoir Appraisal and Characterisation

Imperial Users onl

The Impact of Center City Economic and Cultural Vibrancy on Greenhouse Gas Emissions from Transportation, Research Report 11-13

Urban planners and scholars have focused a great deal of attention on understanding the relationship between the built environment and transportation behavior. However, other aspects of the urban environment – including the vibrancy and quality of life in urban areas – have received little attention. This report seeks to close this gap by analyzing the effects of both land-use and urban vibrancy on transportation patterns. Analysis of data from a variety of sources suggests that in addition to the built-environment, the vibrancy of the urban environment also affects transportation behavior. Moreover, vibrancy affects land-use patterns. By integrating objective measures of center-city quality of life into transportation choice models, our new statistical results inform public policy. We discuss specific public policy options for reducing greenhouse gas emissions and increasing public transit use