Gandy: "machine Computation", Discrete, Deterministic, and Limited To "local Causation" By Light Spe
Robin Gandy's influential paper titled Church's Thesis and Principles for Mechanisms appears in Barwise et al. Gandy starts off with an unlikely expression of Church's Thesis, framed as follows:
- "1. Introduction
- "Throughout this paper we shall use "calculable" to refer to some intuitively given notion and "computable" to mean "computable by a Turing machine"; of course many equivalent definitions of "computable" are now available.
- "Church's Thesis. What is effectively calculable is computable.
- " ... Both Church and Turing had in mind calculation by an abstract human being using some mechanical aids (such as paper and pencil)"
Robert Soare (1995, see below) had issues with this framing, considering Church's paper (1936) published prior to Turing's "Appendix proof" (1937).
Gandy attempts to "analyze mechanical processes and so to provide arguments for the following:
- "Thesis M. What can be calculated by a machine is computable."
Gandy "exclude from consideration devices which are essentially analogue machines ... .The only physical presuppositions made about mechanical devices (Cf Principle IV below) are that there is a lower bound on the linear dimensions of every atomic part of the device and that there is an upper bound (the velocity of light) on the speed of propagation of change". But then he restricts his machines even more:
- "(2) Secondly we suppose that the progress of calculation by a mechanical device may be described in discrete terms, so that the devices considered are, in a loose sense, digital computers.
- "(3) Lasty we suppose that the device is deterministic: that is, the subsequent behavior of the device is uniquely determined once a complete description of its initial state is given."
- "Now we come to the most important of our principles. In Turing's analysis the requirement that the action depended only on a bounded portion of the record was based on a human limitation. We replace this by a physical limitation which we call the principle of local causation. Its justification lies in the finite velocity of propagation of effects and signals: contemporary physics rejects the possibility of instantaneous action at a distance."
Read more about this topic: History Of The Church–Turing Thesis
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