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The impact of models of a physical oracle on computational power†

Published online by Cambridge University Press:  06 September 2012

EDWIN J. BEGGS ,
JOSÉ FÉLIX COSTA  and
JOHN V. TUCKER
EDWIN J. BEGGS
Affiliation:
School of Physical Sciences, Swansea University, Singleton Park, Swansea, SA2 8PP, Wales, United Kingdom Email: E.J.Beggs@Swansea.ac.uk; J.V.Tucker@Swansea.ac.uk
JOSÉ FÉLIX COSTA
Affiliation:
Centro de Matemática e Aplicações Fundamentais, University of Lisbon, 1649-003 Lisboa, Portugal and Instituto Superior Técnico, Technical University of Lisbon, 1049-001 Lisboa, Portugal Email: jose.felix.costa@ist.utl.pt
JOHN V. TUCKER
Affiliation:
School of Physical Sciences, Swansea University, Singleton Park, Swansea, SA2 8PP, Wales, United Kingdom Email: E.J.Beggs@Swansea.ac.uk; J.V.Tucker@Swansea.ac.uk
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Abstract

Using physical experiments as oracles for algorithms, we can characterise the computational power of classes of physical systems. Here we show that two different physical models of the apparatus for a single experiment can have different computational power. The experiment is the scatter machine experiment (SME), which was first presented in Beggs and Tucker (2007b). Our first physical model contained a wedge with a sharp vertex that made the experiment non-deterministic with constant runtime. We showed that Turing machines with polynomial time and an oracle based on a sharp wedge computed the non-uniform complexity class P/poly. Here we reconsider the experiment with a refined physical model where the sharp vertex of the wedge is replaced by any suitable smooth curve with vertex at the same point. These smooth models of the experimental apparatus are deterministic. We show that no matter what shape is chosen for the apparatus:

  1. (i) the time of detection of the scattered particles increases at least exponentially with the size of the query; and

  2. (ii) Turing machines with polynomial time and an oracle based on a smooth wedge compute the non-uniform complexity class P/log* ⫋ P/poly.

We discuss evidence that many experiments that measure quantities have exponential runtimes and a computational power of P/log*.

Type
Paper
Information
Mathematical Structures in Computer Science , Volume 22 , Special Issue 5: Computability of the Physical , October 2012 , pp. 853 - 879
Copyright
Copyright © Cambridge University Press 2012

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