Alan Turing: The Enigma The Centenary Edition

by Andrew Hodges

  ALAN TURING: THE ENIGMA

  Andrew Hodges is Tutor in Mathematics at Wadham College, Oxford University. His classic text of 1983, since translated into several languages, created a new kind of biography, with mathematics, science, computing, war history, philosophy and gay liberation woven into a single personal narrative. He is an active contributor to the mathematics of fundamental physics, as a follower of Roger Penrose. See www.turing.org.uk for further material.

  TO THEE OLD CAUSE!

  The dedication, epigraphs, and epitaph are taken from the Leaves of Grass of Walt Whitman.

  ‘Alan Turing was by any reckoning one of the most remarkable Englishmen of the century. A brilliant mathematician at Cambridge in the ’30s, Turing discovered that his was precisely the kind of intelligence needed by Britain during the war and became the presiding genius at Bletchley Park, the boffin centre which cracked the German Enigma code. (A character in McEwan’s The Imitation Game was loosely based on him.) There he became obsessed by the notion of machine intelligence and was, in effect, the father of the modern computer. Mistrust and bureaucracy, however, frustrated many of his plans after the war, when Turing was to discover that though he was the master of his own sphere, politically he remained as his was in 1941 − a servant. A homosexual, Turing found his own morality and scientific ideas increasingly at odds with the values of the state which he served. Eventually, he committed suicide. Andrew Hodges’s book is of exemplary scholarship and sympathy. Intimate, perceptive and insightful, it’s also the most readable biography I’ve picked up in some time’

  Richard Rayner, Time Out

  ‘Researched and written extraordinarily well. It is a first-class contribution to history and an exemplary work of biography’

  Nature

  ‘Life and work are both made enthralling by Hodges, himself a scientist’

  Sunday Times

  ‘This rather shadowy figure has now finally been lifted into the light of day … it has to be said that Andrew Hodges has put together an extraordinary story’

  Sunday Telegraph

  ‘This book has a great deal to offer: clear technical descriptions set against their backgrounds; the story of a man largely at odds with the system he lived in: and the puzzle of Alan Turing himself’

  Times Higher Education Supplement

  ‘Andrew Hodges, in this fine biography Alan Turing: The Enigma, brings Turing the thinker and Turing the man alive for the reader and thus allows us all to share in the privilege of knowing him’

  Financial Times

  ‘This is not a book to be argued about. It is a book to be read’

  New Scientist

  ‘A major work at any level. Recommended’

  Personal Computing World

  THE CENTENARY EDITION

  With a foreword by Douglas Hofstadter and a new preface by the author

  ANDREW HODGES

  Alan Turing: The Enigma

  Published in the United States by Princeton University Press, 41 William Street, Princeton, New Jersey 08540 press.princeton.edu

  First published in 2012 by Vintage, Random House, 20 Vauxhall Bridge Road, London SW1V 2SA www.vintage-books.co.uk

  Copyright © 1983 by Andrew Hodges

  Preface to the 2012 Centenary edition copyright © 2012 by Andrew Hodges

  Foreword copyright © 2000 by Douglas Hofstadter

  All Rights Reserved

  First published by Burnett Books Ltd in association with

  Hutchinson Publishing Group, 1983

  Unwin Paperbacks edition, 1985

  Reprinting, 1985 (twice), 1986, 1987 (twice)

  First published by Vintage in 1992

  Library of Congress Control Number 2012935958

  ISBN 978-0-691-15564-7

  Printed on acid-free paper. ∞

  Printed in the United States of America

  1 3 5 7 9 10 8 6 4 2

  Contents

  List of Plates

  Foreword by Douglas Hofstadter

  Preface to the 2012 Centenary edition

  PART ONE: THE LOGICAL

  1 Esprit de Corps to 13 February 1930

  2 The Spirit of Truth to 14 April 1936

  3 New Men to 3 September 1939

  4 The Relay Race to 10 November 1942

  BRIDGE PASSAGE to 1 April 1943

  PART TWO: THE PHYSICAL

  5 Running Up to 2 September 1945

  6 Mercury Delayed to 2 October 1948

  7 The Greenwood Tree to 7 February 1952

  8 On the Beach to 7 June 1954

  Postscript

  Author’s Note

  Notes

  Acknowledgements

  Index

  List of Plates

  Alan’s father, Julius Turing (John Turing)

  Alan Turing with his brother John, St Leonard’s, 1917 (John Turing)

  Alan with his mother in Brittany, 1921 (John Turing)

  Colonel and Mrs Morcom with Christopher, 1929 (Rupert Morcom)

  Alan Turing with two school contemporaries, 1931 (Peter Hogg)

  Alan Turing in 1934 (John Turing)

  Alan Turing with his parents, 1938 (John Turing) Sailing at Bosham, August 1939 (John Turing)

  The naval Enigma machine

  A Colossus machine in operation at Bletchley Park, 1944-5 (HMSO)

  The Delilah terminal, 1945 (HMSO)

  Finish of a three-mile race, 1946 (King’s College, Cambridge) The Pilot ACE computer in 1950

  The prototype Manchester computer, 1949 (Department of Computer Science, University of Manchester)

  Alan Turing at the console of the Ferranti Mark I computer, 1951 (Department of Computer Science, University of Manchester)

  Robin Gandy in 1953 (Robin Gandy)

  Alan Mathison Turing, Fellow of the Royal Society, 1951 (King’s College, Cambridge and The Royal Society)

  Foreword

  Is a mind a complicated kind of abstract pattern that develops in an underlying physical substrate, such as a vast network of nerve cells? If so, could something else be substituted for the nerve cells − something such as ants, giving rise to an ant colony that thinks as a whole and has an identity − that is to say, a self? Or could something else be substituted for the tiny nerve cells, such as millions of small computational units made of arrays of transistors, giving rise to an artificial neural network with a conscious mind? Or could software simulating such richly interconnected computational units be substituted, giving rise to a conventional computer (necessarily a far faster and more capacious one than we have ever seen) endowed with a mind and a soul and free will? In short, can thinking and feeling emerge from patterns of activity in different sorts of substrate − organic, electronic, or otherwise?

  Could a machine communicate with humans on an unlimited set of topics through fluent use of a human language? Could a language-using machine give the appearance of understanding sentences and coming up with ideas while in truth being as devoid of thought and as empty inside as a nineteenth-century adding machine or a twentieth-century word processor? How might we distinguish between a genuinely conscious and intelligent mind and a cleverly constructed but hollow language-using facade? Are understanding and reasoning incompatible with a materialistic, mechanistic view of living beings?

  Could a machine ever be said to have made its own decisions? Could a machine have beliefs? Could a machine make mistakes? Could a machine believe it made its own decisions? Could a machine erroneously attribute free will to itself? Could a machine come up with ideas that had not been programmed into it in advance? Could creativity emerge from a set of fixed rules? Are we − even the most creative among us �
� but passive slaves to the laws of physics that govern our neurons?

  Could machines have emotions? Do our emotions and our intellects belong to separate compartments of our selves? Could machines be enchanted by ideas, by people, by other machines? Could machines be attracted to each other, fall in love? What would be the social norms for machines in love? Would there be proper and improper types of machine love affairs?

  Could a machine be frustrated and suffer? Could a frustrated machine release its pent-up feelings by going outdoors and self-propelling ten miles? Could a machine learn to enjoy the sweet pain of marathon running? Could a machine with a seeming zest for life destroy itself purposefully one day, planning the entire episode so as to fool its mother machine into “thinking” (which, of course, machines cannot do, since they are mere hunks of inorganic matter) that it had perished by accident?

  These are the sorts of questions that burned in the brain of Alan Mathison Turing, the great British mathematician who spearheaded the science of computation; yet if they are read at another level, these questions also reveal highlights of Turing’s troubled life. It would require someone who shares much with Turing to plumb his life story deeply enough to do it justice, and Andrew Hodges, an accomplished British mathematical physicist, has succeeded wonderfully in just that venture.

  This biography of Turing, painstakingly assembled from innumerable sources, including conversations with scores of people who knew Turing at various stages of his life, provides a picture as vivid as one could hope of a most complex and intriguing individual. Turing’s was a life that merits deep study, for not only was he a major player in the science of the twentieth century, but his interpersonal behavior was unconventional and caused him great grief. Even today, society as a whole has not learned how to grapple with Turing’s brand of nonconformism.

  Hodges’s rich and engrossing portrait is not the first book about Turing; indeed, Turing’s mother, Sara Turing, wrote a sketchy memoir a few years after her son’s death, presenting an image of him as a lovable, eccentric boy of a man, filled with the joy of ideas and driven by an insatiable curiosity about questions concerning mind and life and mechanism. Although that little book has some merits and even some charm, it also whitewashes a great deal of the true story. Andrew Hodges explores Turing’s mind, body, and soul far more deeply than Sara Turing ever dared to, for she wore conventional blinders and did not want to see, let alone say, how poorly her son fit into the standard molds of British society.

  Alan Turing was homosexual − a fact that he took no particular pains to hide, especially as he grew older. For a boy growing up in the 1920s and for a grown man in the subsequent few decades, being homosexual − especially if one was British and a member of the upper classes − was an unmentionable, terrible, and mysterious affliction.

  Atheist, homosexual, eccentric, marathon-running English mathematician, A. M. Turing was in large part responsible not only for the concept of computers, incisive theorems about their powers, and a clear vision of the possibility of computer minds, but also for the cracking of German ciphers during the Second World War. It is fair to say that we owe much to Alan Turing for the fact that we are not under Nazi rule today. And yet this salient figure in world history has remained, as the book’s title says, an enigma.

  In this biography, Andrew Hodges has painted an extraordinarily detailed and devoted portrait of a multifaceted man whose honesty and decency were too much for his society and his times, and who brought about his own downfall. Beyond the evident empathy that Hodges feels for his subject, there is another level of depth and understanding in this book, one that makes all the difference in a biography of a scientific figure: scientific accuracy and clarity. Hodges has done an admirable job of presenting to the lay reader each idea in detail, and most likely this is so because, as is obvious to a reader, he himself is passionately intrigued by all the ideas that fascinated Turing.

  Alan Turing: The Enigma is a first-rate presentation of the life of a first-rate scientific mind, and given that this particular mind was attached to a body that had a mind of its own, the full story is an important document for social reasons as well. Alan Turing would probably have shuddered had he ever suspected that the tale of his personal life would one day be presented to the public at large, but he is in good hands: it is hard to imagine a more thoughtful and compassionate portrait of a human being than this one.

  Douglas Hofstadter

  Preface to the Centenary Edition

  On 25 May 2011, the President of the United States, Barack Obama, speaking to the parliament of the United Kingdom, singled out Newton, Darwin, and Alan Turing as British contributors to science. Celebrity is an imperfect measure of significance, and politicians do not confer scientific status, but Obama’s choice signalled that public recognition of Alan Turing had attained a level very much higher than in 1983, when this book first appeared.

  Born in London on 23 June 1912, Alan Turing might just have lived to hear these words, had he not taken his own life on 7 June 1954. In that very different world, his name had gone unmentioned in its legislative forums. Yet in the secret world, over which Eisenhower and Churchill still reigned, and in which the newly reorganised NSA and GCHQ were the holy of holies, their names to be whispered, Alan Turing had a unique place. He had been the chief backroom boy when American power overtook British in 1942, with a scientific role whose climax came on 6 June 1944, just ten years before that early death.

  Alan Turing played a central part in world history. Yet it would be misleading to portray his drama as a power play, or as framed by the conventional political issues of the twentieth century. He was not political as defined by contemporary intellectuals, revolving as they did around alignment or non-alignment with the Communist party. Some of his friends and colleagues were indeed party members, but that was not his issue. (Incidentally, it is equally hard to find money-motivated ‘free enterprise’, idolised since the 1980s, playing any role in his story.) Rather, it was his individual freedom of mind, including his sexuality, which mattered − a question taken much more seriously in the post-1968 and even more in the post-1989 era. But beyond this, the global impact of pure science rises above all national boundaries, and the sheer timelessness of pure mathematics transcends the limitations of his twentieth-century span. When Turing returned to the prime numbers in 1950 they were unchanged from when he left them in 1939, wars and superpowers notwithstanding. As G. H. Hardy famously said, they are so. This is mathematical culture, and such was his life, presenting a real difficulty to minds set in literary, artistic or political templates.

  Yet it is not easy to separate transcendence from emergency: it is striking how leading scientific intellects were recruited to meet the existential threat Britain faced in 1939. The struggle with Nazi Germany called not just for scientific knowledge but the cutting edge of abstract thought, and so Turing’s quiet logical preparations in 1936-8 for the war of codes and ciphers made him the most effective anti-Fascist amongst his many anti-Fascist contemporaries. The historical parallel with physics, with Turing as a figure roughly analogous to Robert Oppenheimer, is striking. This legacy of 1939 is still unresolved, in the way that secret state purposes are seamlessly woven into intellectual and scientific establishments today, a fact that is seldom remarked upon.

  The same timelessness lies behind the central element of Alan Turing’s story: the universal machine of 1936, which became the general-purpose digital computer in 1945. The universal machine is the focal, revolutionary idea of Turing’s life, but it did not stand alone; it flowed from his having given a new and precise formulation of the old concept of algorithm, or mechanical process. He could then say with confidence that all algorithms, all possible mechanical processes, could be implemented on a universal machine. His formulation became known immediately as ‘the Turing machine’ but now it is impossible not to see Turing machines as computer programs, or software.

  Nowadays it is perhaps taken rather for granted that computers can rep
lace other machines, whether for record-keeping, photography, graphic design, printing, mail, telephony, or music, by virtue of appropriate software being written and executed. No-one seems surprised that industrialised China can use just the same computers as does America. Yet that such universality is possible is far from obvious, and it was obvious to no-one in the 1930s. That the technology is digital is not enough: to be all-purpose computers must allow for the storage and decoding of a program. That needs a certain irreducible degree of logical complexity, which can only be made to be of practical value if implemented in very fast and reliable electronics. That logic, first worked out by Alan Turing in 1936, then implemented electronically in the 1940s, and nowadays embodied in microchips, is the mathematical idea of the universal machine.

  In the 1930s only a very small club of mathematical logicians could appreciate Turing’s ideas. But amongst these, only Turing himself had the practical urge as well, capable of turning his hand from the 1936 purity of definition to the software engineering of 1946: ‘every known process has got to be translated into instruction table form …’ (p. 326). One of Turing’s 1946 colleagues, Donald Davies, later developed such instruction tables (as Turing called programs) for ‘packet switching’ and these grew into the Internet protocols. Giants of the computer industry did not see the Internet coming, but they were saved by Turing’s universality: the computers of the 1980s did not need to be re-invented to handle these new tasks. They needed new software and peripheral devices, they needed greater speed and storage, but the fundamental principle remained. That principle might be decribed as the law of information technology: all mechanical processes, however ridiculous, evil, petty, wasteful, or pointless, can be put on a computer. As such, it goes back to Alan Turing in 1936.

  That Alan Turing’s name has not from the start been consistently associated with praise or blame for this technological revolution is due partly to his lack of effective publication in the 1940s. Science absorbs and overtakes individuals, especially in mathematics, and Alan Turing swam in this anonymising culture, never trying to make his name, although frustrated at not being taken seriously. In fact, his competitive spirit went instead into marathon running at near-Olympic level. He omitted to write that monograph on ‘the theory and practice of computing’ which would have stamped his name on the emergent post-war computer world. In 2000 the leading mathematical logician Martin Davis, whose work since 1949 had greatly developed Turing’s theory of computability, published a book1 which was in essence just what Turing could have written in 1948, explaining the origin of the universal machine of 1936, showing how it became the stored-program computer of 1945, and making it clear that John von Neumann must have learnt from Turing’s 1936 work in formulating his better-known plan. Turing’s very last publication, the Science News article of 1954 on computability, demonstrates how ably he could have written such an analysis. But even there, on terrain that was incontestably his own discovery, he omitted to mention his own leading part.