Today, the 8th of September 2016, we had a very interesting discussion about a theorem, due to Selim G. Akl, pointed to me in a tweet by Andy Adamatzky. Such theorem has, according to Akl, the consequence that the Church-Turing thesis, a basic tenet of theoretical computer science, is false. Of course, surprising statements require solid arguments: is Akl’s solid enough?
First of all, let us recall what the Church-Turing thesis is, and what it is not. Its statement, as reported by the Stanford Encyclopedia of Philosophy, goes as follows:
A function of positive integers is effectively calculable only if recursive.
Here, for a calculation procedure to be “effective” means the following:
- it has a finite description;
- it always returns the correct output, given any valid input;
- it can be “carried on by pencil and paper” by a human being; and
- it requires no insight or ingenuity on the human’s behalf.
One model of effective procedures is given by the recursive functions; another one, by the functions computable by Turing machines; a third one, by the functions which are representable in Church’s -calculus. Alan Turing and Stephen Cole Kleene proved that the three classes coincide: thus, in ordinary practice, the Church-Turing thesis is often stated with “Turing-computable” in place of “recursive”.
The class of Turing machines has the advantage of containing a universal element: a special Turing machine and an encoding from the set of Turing machines to the set of natural numbers exists such that, when the special Turing machine is provided the encoding of an arbitrary Turing machine and a valid input for the latter, it will return the value of the encoded Turing machine on the provided input.
Now that we have written down what the Church-Turing thesis is, we can examine Akl’s theorem.
In his 2005 paper, Akl defines a universal computer as a system having the following features:
- It has means of communicating with the outside world, so to receive input, and where to send its output.
- It can perform every elementary arithmetic and logic operations.
- It can be programmed, according to the two previous rules.
- It has unlimited memory to use for input, output, and temporary values.
- It can only execute finitely many operations (evaluating input, producing output, performing an elementary operation, etc.) at each time step.
- It can simulate any computation performed by any other model of computation.
The statement of the theorem, which does not appear explicitly in the original paper but is written down in the one from 2015 which clarifies the idea and addresses criticism, is hereby reported verbatim:
Nonuniversality in Computation Theorem (NCT): No computer is universal if it is capable of exactly operations during time unit of computation, where is a positive integer, and is finite and fixed once and for all.
The main argument is that no such computer can perform a computation which requires more than operations at some time . Explicit examples happen in parallel computation, a field Akl is a master of, where the number of operations that can be performed in a time unit grows linearly with the number of processors: for instance, reading values in input can be done in time by a parallel machine with processors, but not by any machine with processors.
Such requirement, however, does not appear in the notion of universality at the base of the original, and actual, Church-Turing thesis. There, to “simulate” a machine or algorithm means to be able of always reproducing the same output of the algorithm, given any valid input for it, up to an encoding of the input and the output. But no hypothesis on how the output is achieved from the input is made: a simulation in linear time, such that each step of the simulated algorithm is reproduced by exactly operations of the Turing machine, is as good as one where the simulation of the th step takes operations from the Turing machine, or where no such regularity appears.
Among the (counter)examples provided by Akl are:
- Computations with time-varying variables.
- Computations with time-varying computational complexity.
- Computations whose complexity depends on their placement on a schedule.
- Computations with interacting variables, e.g., states of entangled electrons.
- Computations with uncertain time constraints.
None of these, however, respect the definition of computation from the model of recursive functions: where the values of the variables are given once and for all, and can possibly change for recursive calls, but not for the original call. They can be seen as instances of unconventional models of computation: but by doing this, one changes the very notion of computation, which ceases to be the one at the basis of the Church-Turing thesis.
So my guess is that Akl’s statement about the falsity of the Church-Turing thesis actually falls in the following category, as reported in the humorous list by Dana Angluin:
Proof by semantic shift: Some standard but inconvenient definitions are changed for the statement of the result.
Actually, if we go back to Akl’s definition of a universal computer, it appears to be fine until the very last: the first two points agree with the definition of effective computation at the basis of the actual Church-Turing thesis, the next three are features of any universal Turing machine. The problem comes from the last point, which has at least two weak spots: the first one being that it does not define precisely what a model of computation is, which can be accepted as Akl is talking of unconventional computation, and it is wiser to be open to other possibilities. But there is a more serious one, in that it is not clear
what does the expression “to simulate” mean.
Note that the Stanford Encyclopedia of Philosophy reports the following variant of the Church-Turing thesis, attributed to David Deutsch:
Every finitely realizable physical system can be perfectly simulated by a universal model computing machine operating by finite means.
Deutsch’s thesis, however, does not coincide with the Church-Turing thesis! (This, notwithstanding Deutsch’s statement that “[t]his formulation is both better defined and more physical than Turing’s own way of expressing it”.) Plus, there is another serious ambiguity, which is of the same kind as the one in Akl’s definition:
what is “perfectly simulated” supposed to mean?
Does it mean that every single step performed by the system can be reproduced in real time? In this case, Akl is perfectly right in disproving it under the constraint of boundedly many operations at each time unit. Or does it mean that the simulation of each elementary step of the process (e.g., one performed in a quantum of time) ends with the correct result if the correct initial conditions are given? In this case, the requirement to reproduce exactly what happens between the reading of the input and the writing of the output is null and void.
Worse still, there is a vulgarized form of the Church-Turing thesis, which is reported by Akl himself on page 172 of his 2005 paper!, and goes as follows:
Any computable function can be computed on a Turing machine.
If one calls that “the Church-Turing thesis”, then Akl’s NCT is absolutely correct in disproving it. But that is not the actual Church-Turing thesis! It is actually a rewording of what in the Stanford Encyclopedia of Philosophy is called “Thesis M”, and explicitly stated not to be equivalent to the original Church-Turing thesis—and also false. Again, the careful reader will have noticed that, in the statement above, being “computable by a Turing machine” is a well defined property, but “computable” tout court definitely not so.
At the end of this discussion, my thesis is that Akl’s proof is correct, but NCT’s consequences and interpretation might not be what Akl means, or (inclusive disjunction) what his critics understand. As for my personal interpretation of NCT, here it goes:
No computer which is able to perform a predefinite, finite number of operations at each finite time step, is universal across all the different models of computation, where the word “computation” may be taken in a different meaning than that of the Church-Turing thesis.
Is mine an interpretation by semantic shift? Discussion is welcome.
- Selim G. Akl. The Myth of Universal Computation. Parallel Numerics ’05, 167–192.
- Selim G. Akl. Nonuniversality explained. International Journal of Parallel, Emergent and Distributed Systems 31:3, 201–219. doi:10.1080/17445760.2015.1079321
- The Church-Turing Thesis. Stanford Encyclopedia of Philosophy. First published January 8, 1997; substantive revision August 19, 2002. http://plato.stanford.edu/entries/church-turing/
- Dana Angluin’s List of Proof Techniques. http://www.cs.northwestern.edu/~riesbeck/proofs.html