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Church-Turing thesisIn computer science, the Church-Turing thesis states in its most common form that every effective computation or algorithm can be carried out by a Turing machine. Any computer program in any of the conventional programming languages can be translated into a Turing machine, and any Turing machine can be translated into most programming languages, so the thesis is equivalent to saying that the conventional programming languages are sufficient to express any algorithm. The thesis, which is now generally assumed to be true, is also known as Church's thesis or Church's conjecture (named after Alonzo Church) and Turing's thesis (named after Alan Turing).
The thesis might be rephrased as saying that the notion of effective or mechanical method in logic and mathematics is captured by Turing machines. It is generally assumed that such methods must satisfy the following requirements:
The notion of "effective method" is intuitively clear but is not formally defined since it is not exactly clear what a "simple and precise instruction" is, and what exactly the "required intelligence to execute these instructions" is. (See for example effective results in number theory for cases well beyond the Euclidean algorithm.)
In his 1936 paper On Computable Numbers, with an Application to the Entscheidungsproblem Alan Turing tried to capture this notion formally with the introduction of Turing machines. In that paper he showed that the 'Entscheidungsproblem' could not be solved. A few months earlier Alonzo Church had proven a similar result in A Note on the Entscheidungsproblem but he used the notions of recursive functions and Lambda-definable functions to formally describe effective computability. Lambda-definable functions were introduced by Alonzo Church and Stephen Kleene (Church 1932, 1936a, 1941, Kleene 1935) and recursive functions by Kurt Gödel and Jacques Herbrand (Gödel 1934, Herbrand 1932). These two formalisms describe the same set of functions, as was shown in the case of functions of positive integers by Church and Kleene (Church 1936a, Kleene 1936). When hearing of Church's proposal, Turing was quickly able to show that his Turing machines in fact describe the same set of functions (Turing 1936, 263ff).
Since that time many other formalisms for describing effective computability have been proposed such as register machines, Emil Post's systems, combinatory definability and Markov algorithms (Markov 1960). All these systems have been shown to compute essentially the same functions as Turing machines; systems like this are called Turing-complete. Because all these different attempts of formalizing the concept of algorithm have yielded equivalent results, it is now generally assumed that the Church-Turing thesis is correct. However, the thesis does not have the status of a theorem and cannot be proven; it is conceivable but unlikely that it could be disproven by exhibiting a method which is universally accepted as being an effective algorithm but which cannot be performed on a Turing machine.
In fact, the Church-Turing thesis has been so successful, that it is now almost moot. In the early twentieth century, mathematicians often used the informal phrase effectively computable, so it was important to find a good formalization of the concept. Modern mathematicians instead use the well-defined term Turing computable (or computable for short). Since the undefined terminology has faded from use, the question of how to define it is now less important.
The Church-Turing thesis has some profound implications for the philosophy of mind. There are also some important open questions which cover the relationship between the Church-Turing thesis and physics, and the possibility of hypercomputation. When applied to physics, the thesis has several possible meanings:
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