Journal Article

The Extent of Computation in Malament–Hogarth Spacetimes

P. D. Welch

in The British Journal for the Philosophy of Science

Published on behalf of British Society for the Philosophy of Science

Volume 59, issue 4, pages 659-674
Published in print December 2008 | ISSN: 0007-0882
Published online December 2008 | e-ISSN: 1464-3537 | DOI: http://dx.doi.org/10.1093/bjps/axn031
The Extent of Computation in Malament–Hogarth Spacetimes

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We analyse the extent of possible computations following Hogarth ([2004]) conducted in Malament–Hogarth (MH) spacetimes, and Etesi and Németi ([2002]) in the special subclass containing rotating Kerr black holes. Hogarth ([1994]) had shown that any arithmetic statement could be resolved in a suitable MH spacetime. Etesi and Németi ([2002]) had shown that some ∀ ∃ relations on natural numbers that are neither universal nor co-universal, can be decided in Kerr spacetimes, and had asked specifically as to the extent of computational limits there. The purpose of this note is to address this question, and further show that MH spacetimes can compute far beyond the arithmetic: effectively Borel statements (so hyperarithmetic in second-order number theory, or the structure of analysis) can likewise be resolved:

Theorem A. If H is any hyperarithmetic predicate on integers, then there is an MH spacetime in which any query ? nH ? can be computed.

In one sense this is best possible, as there is an upper bound to computational ability in any spacetime, which is thus a universal constant of that spacetime.

Theorem C. Assuming the (modest and standard) requirement that spacetime manifolds be paracompact and Hausdorff, for any spacetime there will be a countable ordinal upper bound, , on the complexity of questions in the Borel hierarchy computable in it.

Introduction

1.1

History and preliminaries

Hyperarithmetic Computations in MH Spacetimes

2.1

Generalising SADn regions

2.2

The complexity of questions decidable in Kerr spacetimes

An Upper Bound on Computational Complexity for Each Spacetime

Journal Article.  6900 words.  Illustrated.

Subjects: Philosophy of Science ; Science and Mathematics

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