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📘 Turing asked what a computation can be

In 1936, Turing asked a startling question: what does it mean for a problem to be solved by a mechanical procedure?

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. From procedure to machineExplain how Turing formalized mechanical calculation with a tape, head, states, and rules.The machine model turns a procedure into local, explicit transitions and lets you see how data and instructions can be represented.
  2. Universality and undecidabilityState the universal-machine idea and halting-problem result accurately.One machine can simulate encoded machines, but no universal decider can correctly determine whether every arbitrary machine will halt.
  3. Scope and legacyPlace the result in the Church–Turing thesis and distinguish formal limits from practical analysis.Equivalent formalisms support a careful thesis, while undecidability limits universal guarantees rather than all useful computation.

Questions this course answers

What are the main parts of Turing's abstract machine?

The model uses a tape, head, internal states, and exact transition rules.

What does the halting problem show?

The impossibility concerns a complete decider for all machines and inputs.

Why is a universal Turing machine important?

Universality lets one fixed machine interpret descriptions of other machines.

Grounded in trusted sources

  • A. M. Turing, On Computable Numbers, with an Application to the Entscheidungsproblem, Proceedings of the London Mathematical Society 42 (1936), 230-265: https://doi.org/10.1112/plms/s2-42.1.230
  • A. M. Turing, On Computable Numbers, with an Application to the Entscheidungsproblem: A Correction, Proceedings of the London Mathematical Society 43 (1937), 544-546: https://doi.org/10.1112/plms/s2-43.6.544
  • Alonzo Church, An Unsolvable Problem of Elementary Number Theory, American Journal of Mathematics 58 (1936), 345-363: https://doi.org/10.2307/2371045
  • Stanford Encyclopedia of Philosophy, The Church-Turing Thesis: https://plato.stanford.edu/entries/church-turing/
  • Alan Turing Digital Archive, On Computable Numbers publication record: https://turingarchive.kings.cam.ac.uk/publications-lectures-and-talks-amtb/amt-b-7
  • Wikimedia Commons, Turing machines category and image records: https://commons.wikimedia.org/wiki/Category:Turing_machines

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