Optimal time travel in the Godel universe

Using the theory of optimal rocket trajectories in general relativity, recently developed in arXiv:1105.5235, we present a candidate for the minimum total integrated acceleration closed timelike curve in the Godel universe, and give evidence for its minimality. The total integrated acceleration of this curve is lower than Malament's conjectured value (Malament, 1984), as was already implicit in the work of Manchak (Manchak, 2011); however, Malament's conjecture does seem to hold for periodic closed timelike curves.Comment: 16 pages, 2 figures; v2: lower bound in the velocity and reference adde

Observationally Indistinguishable Spacetimes: A Challenge for Any Inductivist

Results on the observational indistinguishability of spacetimes demonstrate the impossibility of determining by deductive inference which is our spacetime, no matter how extensive a portion of the spacetime is observed. These results do not illustrate an underdetermination of theory by evidence, since they make no decision between competing theories and they make little contact with the inductive considerations that must ground such a decision. Rather, these results express a variety of indeterminism in which a specification of the observable past always fails to fix the remainder of a spacetime. This form of indeterminism is more troubling than the familiar indeterminism of quantum theory. The inductive inferences that can discriminate among the different spacetime extensions of the observed past are here called "opaque," which means that we cannot readily see the warrant that lies behind them

Would Two Dimensions be World Enough for Spacetime?

We consider various curious features of general relativity, and relativistic field theory, in two spacetime dimensions. In particular, we discuss: the vanishing of the Einstein tensor; the failure of an initial-value formulation for vacuum spacetimes; the status of singularity theorems; the non-existence of a Newtonian limit; the status of the cosmological constant; and the character of matter fields, including perfect fluids and electromagnetic fields. We conclude with a discussion of what constrains our understanding of physics in different dimensions.Comment: 31 pages, 1 figur

Global Spacetime Structure

Here, we outline the basic structure of relativistic spacetime and record a number of facts. We then consider a distinction between local and global spacetime properties and provide important examples of each. We also examine two clusters of global properties and question which of them should be regarded as physically reasonable. The properties concern "singularities" and "time travel" and are therefore of some philosophical interest

Sequential Intercept Model: Framework for a ‘Wicked Problem’

This article also appeared on p. 7–8 of the Spring 2018 print edition.The Sequential Intercept Model offers conceptual points at which a person with serious mental illness could be diverted from the criminal justice system and into community-based treatment. This article reviews the 2015 book "The Sequential Intercept Model and Criminal Justice" (New York: Oxford University Press), which looks at the success of programs along the intercept continuum. A workshop on the model sponsored by the Alaska Department of Corrections will be held in Anchorage in May 2018.[Sidebar:] Sequential Intercept Model workshop in Anchorage / Reference

Causally simple inextendible spacetimes are hole-free

It is shown that causally simple inextendible spacetimes are hole-free, thus confirming the expectation that causal simplicity removes holes from spacetime. This result is optimal in the sense that causal simplicity cannot be weakened to causal continuity. Physically, it means that if there is some partial Cauchy hypersurface which, for some reason, does not fully develop its influence, then there is some discontinuity in the causal relation.Comment: Revtex4, 9 pages. v2: minor correction

Parole in Western Australia: an analysis of parole cancellations of female offenders

Foreword The number of prisoners in Australian prisons has been increasing over the past decade. In Western Australia the number of female offenders has increased by 40 percent over the past five years. One contributing factor to this increase may be the re incarceration of parolees who have violated parole. This research used the publicly available decision documents from the Prisoners Review Board in Western Australia to investigate the background details of offences, and the details of the parole violations of 41 women released in 2013–14. Data revealed that a high proportion of women returned to prison after a very short time in the community as a result of illicit drug use. The high cost of re-incarceration is considered against a background of rehabilitation and extra support in the community that might assist released women negotiate their complex lives on release without resorting to further drug use. The paper includes a number of recommendations to consider in an effort to reduce the recidivism of female offenders

Can We Know the Global Structure of Spacetime?

Here, we briefly review the notion of observational indistinguishability within the context of classical general relativity. We settle a conjecture given by Malament (1977) concerning the subject and then strengthen the result considerably. The upshot is this: There seems to be a robust sense in which the global structure of every cosmological model is underdetermined