Physical Indeterminacy in Digital Computation

41 Pages Posted: 23 Dec 2019 Last revised: 11 Apr 2021

See all articles by Carl Hewitt

Carl Hewitt

Massachusetts Institute of Technology (MIT)

Date Written: September 25, 2019


[Church 1931] and [Turing 1936] developed equivalent models of computation based on the concept of an algorithm, which by definition is provided an input from which it is to compute a value without external interaction. After physical computers were constructed, they soon diverged from computing only algorithms meaning that the Church/Turing theory of computation no longer applied to computation in practice because computer systems are highly interactive as they compute, which inspired the development of the Actor Model in 1972 to characterize all digital computation. Indeterminate computation can be thousands of times faster than the parallel λ-calculus (cf. [Takahashi 1989]) or pure logic programs [Hewitt 1969].

This article presents metatheory for a universal theory of digital computation that is more general and powerful than Church/Turing computation [Church 1935, Turing 1936]. Meta[Actors] (meta theory of Actors) proves that theory Actors has the following properties:

• The theory Actors is uniquely categorical, i.e., it characterizes Actors (including types) up to an unique isomorphism.

• The theory Actors is model sound, i.e., every theorem of Actors of is true in the unique up to isomorphism model.

• The unique up to isomorphism model has partial predicates and propositions because of indeterminacy.

• Proof checking in the theory Actors is computationally decidable.

• There are no “monsters” [Lakatos 1976] in models of the theory Actors such as the ones in 1st-order theories of computation in which there can be Zeno-like computations, which have an infinite number of computational steps between two steps. Consequently unlike 1st-order theories, the theory Actors is not subject to cyberattacks using “monsters” in models.

• The theory Actors is algorithmically inexhaustible, i.e., it is impossible to computationally enumerate theorems of the theory thereby reinforcing the intuition behind [Franzén, 2004]. Contrary to [Church 1934], the conclusion in this article is to abandon the assumption that theorems of a theory must be computationally enumerable while retaining the requirement that proof checking must be computationally decidable.

Thesis: Any digital system can be directly modeled and implemented using Actors.

Keywords: Uniquely Categorical Theories, Strong Types, Universal Intelligent Systems, Actor Model Of Computation, Gul Agha, Joe Armstrong, Russ Atkinson, Jeremy Avigad, Steve Awodey, Henry Baker, Nicolas Bourbaki, John Burgess, Will Clinger, Per Brinch Hansen, Alonzo Church, Thierry Coquand, Haskell Curry

Suggested Citation

Hewitt, Carl, Physical Indeterminacy in Digital Computation (September 25, 2019). Available at SSRN: or

Carl Hewitt (Contact Author)

Massachusetts Institute of Technology (MIT) ( email )

77 Massachusetts Avenue
50 Memorial Drive
Cambridge, MA 02139-4307
United States

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