Computer Science Illuminated, Third Edition


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Chapter 17: Limitations of Computing

Alan Turing

Time magazine chose Alan Turing as one of its 100 most influential persons of the 20th Century. Their biography of Turing said:
   For what this eccentric young Cambridge
   don did was to dream up an imaginary
   machine--a fairly simple typewriter-like
   contraption capable somehow of scanning,
   or reading, instructions encoded
   on a tape of theoretically infinite length.
   As the scanner moved from one square
   of the tape to the next--responding to
   the sequential commands and modifying its
   mechanical response if so ordered--the output of
   such a process, Turing demonstrated, could replicate
   logical human thought.

The device in this inspired mind-experiment quickly acquired a name: the Turing machine, and so did another of Turing's insights. Since the instructions on the tape governed the behavior of the machine, by changing those instructions, one could induce the machine to perform the functions of all such machines. In other words, depending on the tape it scanned, the same machine could calculate numbers or play chess or do anything else of a comparable nature. Hence his device acquired a new and even grander name: the Universal Turing Machine. . . . So many ideas and technological advances converged to create the modern computer that it is foolhardy to give one person the credit for inventing it. But the fact remains that everyone who taps at a keyboard, opening a spreadsheet or a word-processing program, is working on an incarnation of a Turing machine.18

Alan Turing was born in June of 1912 to Julius Mathison Turing, a member of the Indian Civil Service, and Ethel Sara Stoney, the daughter of the chief engineer of the Madras railway. His father and mother spent most of their time in India, while he and his older brother were in various foster homes in England until his father's retirement in 1926.

The British Public (read Private in American English) school system of the day did not foster original thinking, so Turing had trouble fitting in. He was criticized for his handwriting, struggled in English, and even in mathematics didn't produce the expected conventional answers. At Sherborne School, which he had entered at 13, the headmaster said that if he was solely a scientific specialist, he was wasting his time at a public cchool. Yet a public school education was terribly important to his mother, and so he persisted. Two things sustained him during this period: his own independent study and the friendship of Christopher Morcom, who was a student a year ahead of him in school. Morcom provided vital intellectual companionship, which ended after two years with Morcom's sudden death.

In 1931, Turing entered King's College, Cambridge, to study mathematics. The atmosphere at King's College encouraged free-ranging thought, providing him with an intellectual home for the first time. He graduated in 1934 and was elected a fellow of King's College in 1935 for a dissertation "On the Gaussian Error Function," which proved fundamental results in probability theory.

Turing then began to work on decidability questions, based on a course he had taken on the foundations of mathematics with Max Newman. In 1936 Turing published a paper in which he introduced the concept of what we now call a Turing machine. These concepts were introduced within the context of whether a definite method or process exists by which it could be decided whether any given mathematical assertion was provable. Alonzo Church's work at Princeton on the same subject became known at the same time, and so Turing's paper was delayed until he could refer to Church's work. As a result, Turing spent two years as a student at Princeton working with Church and von Neumann.

At the outbreak of World War II, Turing went to work at the Government. Again we quote from the Time Magazine text:
   Turing, on the basis of his published work, was
   recruited to serve in the Government Code and
   Cypher School, located in a Victorian mansion
   called Bletchley Park in Buckinghamshire. The task of
   all those so assembled--mathematicians, chess
   champions, Egyptologists, whoever might have
   something to contribute about the possible permutations
   of formal systems--was to break the Enigma
   codes used by the Nazis in communications between
   headquarters and troops. Because of secrecy restrictions,
   Turing's role in this enterprise was not
   acknowledged until long after his death. And like the
   invention of the computer, the work done by the
   Bletchley Park crew was very much a team effort. But
   it is now known that Turing played a crucial role in
   designing a primitive, computer-like machine that
   could decipher at high speed Nazi codes to U-boats
   in the North Atlantic.

Turing was awarded the Order of the British Empire in 1945 for his contributions to the war effort.

After a frustrating experience at the National Physical Laboratory in London, where he was to build a computer, he returned to Cambridge where he continued work and write. The war-time spirit of cooperation that had short-circuited bureaucracy had faded, and the ACE (Automatic Computing Engine) was never built. In 1948, Turing became a Deputy Director of the computing laboratory at Manchester University. The vague title reflected its meaninglessness, and Turing spent the next years working and writing on a variety of different subjects.

In 1950 he published an article reflecting one of his major interests: Can machines think? From this article came the well-known Turing test. He also became interested in morphogenesis, the development of pattern and form in living organisms. All the while he continued his research in decidability and quantum theory.

On June 7, 1954, Turing died of cyanide poisoning, a half-eaten apple laying beside his bed. His mother believed that he accidentally died while conducting an experiment; the coroner's verdict was suicide. A few years ago, the award-winning oneman play Breaking the Code was performed in London's West End and on Broadway, giving audiences a brief glimpse of Turing's brilliant, complex character.

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