The Turing biopic, The Imitation Game, is based on the classic biography 'Alan Turing: the Enigma'. We talk to its author Andrew Hodges, fellow and tutor in mathematics at Wadham College, University of Oxford.
Q What was it that gave you the inspiration to write your biography of Alan Turing?
A As a mathematics student at Cambridge in the late 1960s, I came across Alan Turing while reading around the subject that we now call the Foundations of Computer Science. Then his name came up in quite a different context in 1972 through my involvement in the gay rights movement.
I was in personal contact with friends of his and was told this extraordinary story of how his death in 1954 had followed from an arrest, conviction and very harsh treatment. Very few people knew about this and it had come to me in a very unusual way through oral history. There was nothing written down about this at all. But then at the end of 1976, the BBC showed a television series called 'The Secret War' and in the last episode there was the first serious documentary account of what had happened at Bletchley Park, which had quite a good account, for the time, of the codebreaking method for the Enigma.
The thing that struck me was that his death in 1954 wasn't the death of a typical gay man. Turing was the individual with the utmost secret Anglo-American crypto knowledge. He knew everything of that kind that happened in the Second World War. Indeed, it transpired that he had continued to work for GCHQ after 1948 until that was broken off with his arrest in 1952. What happened in the years 1952-54 must have involved his security status, and his death must be seen in that light. From that point up until the 1990s, it was important to the government and secret organisations to exclude homosexual people. It was one of the criteria for positive vetting. His quite extraordinary adventures at this time, including a trip to Greece in 1953, must have been of great interest to them.
Q Turing received a posthumous royal pardon recently for his conviction. What do you think of that?
A I was never terribly keen on it. At his trial, it was put forward that he was a national asset by Hugh Alexander who was the scientific chief at GCHQ. And so the plea was that Turing was a very important person. The effect of the pardon really is to say that pulling rank is established as the right criterion for judging someone. Well, that's not a very good principle. Justice should be the same for everyone: it shouldn't be different for famous or important people.
I'm sure Turing himself wouldn't have minded if the whole thing had been hushed up, which it certainly could have been. But I'm sure this wasn't the kind of attitude he had: he thought the law was wrong.
When I took up the story, it was on the basis that his case gave an extreme illustration of what's going to happen in society if you have such laws. It's going to create a great historical embarrassment. But the formal pardon doesn't really do anything for him: he's dead.
Q The blue plaque at Alan Turing's birthplace that you unveiled in 1998 describes Turing as 'code-breaker and pioneer of computer science'. Are these six words a good crystallisation of the man, or do we need to expand upon them?
A Turing would have described himself as a mathematician. I think it's fair to unpack that and describe some of the things he did. The two things he did which are most distinctive are that he founded the whole concept of computer science, upon which everything in computer science theory is now based. And the other thing was his work during the Second World War, which was extremely important cryptanalysis.
Although what he did often seems abstruse, he was unusual in that he was very alive to engineering and the concrete application of difficult ideas. The best example of that is in his code-breaking work. But you can see it in everything he did. Computer science is all about linking logical possibilities with the physical reality. There are lots of paradoxes in Turing's life, but this is the central theme.
Q How would you describe Turing's influence on the outcome of the Second World War?
A The central most distinctive thing he did was the breaking of the naval Enigma messages. That was something that he alone took on after the more basic form of the Enigma had been dealt with. You have to make that distinction: the Enigma machine was used by all branches of the German Armed Forces, and indeed by diplomatic and other departments as well. But it was the use by the German Navy that was the most difficult to break. They used it much more carefully than the others.
Turing was taken on by the government as a part-time consultant in 1938 and only started full-time in September 1939 immediately on the outbreak of war. He, with Gordon Welchman and helped by very important information coming from Polish mathematicians, really dealt with the basic Enigma problem very quickly.
The crucial thing about what Turing did – what makes him distinctive – is that he took on the naval problem, which was commonly thought of as impossible. That's what he attacked doggedly, and with very little to go on. There was a breakthrough when, in February 1941, the British captured codebook papers. Afterthat Turing's advanced methods were very successful and he recruited a team of very brilliant young mathematicians to help. His group at Hut 8 was perhaps the most distinctive of the war in what it achieved.
Q But it was more complicated than that...
A It's a complicated story because that success was cut off completely on 1 February 1942, when the German U-boat system went over to an advanced Enigma with four rotors. You don't really capture what Turing did without realising that there was this drama to it. It wasn't just a case of breaking the code, and then it was done. There was this constant knife-edge situation where a small change in the Enigma system could make a huge difference in the deciphering business.
1942 was a very fraught year: they were still able to decipher other naval signals but the Atlantic U-boat system, which was the most important, was a crisis. In fact, Turing's role went on from there. The US production of the cipher-breaking Bombe machines played a crucial part in the latter part of the war, and Turing was the top-level person who made the liaison with the US between November 1942 and March 1943. After that, Turing was the top consultant to all aspects of the code-breaking business both at Bletchley Park and the US. He was the most important person on the scientific side at Bletchley Park during the Second World War.
Q You have described Turing as "an isolated and autonomous mind". Can you describe how this kind of mind led to his achievements?
A He had a particular type of mind that combined very abstract things with things that were very down to earth as well. You can see that in his big 1936 work, his paper 'On Computable Numbers'. That took off from the most abstruse and abstract problem of mathematics, which he resolved by giving this concrete picture of a teleprinter-like machine. He asks what can such a machine do. What can't it do? He argues that this model encompasses everything that you could possibly call computation. Turing was the person who linked that theoretical question with something that was physical, concrete.
The outcome of that was the concept of a Universal Machine as something, again, that you could actually construct. He must have thought about doing that in the late 1930s, and that's now what we call the principle of the digital computer; the principle of the stored program; the principle that programs are themselves a form of data so that you can copy them, translate them, do anything you like with them, just as you can do anything like that with numbers. There's no real distinction.
That was very much his own thing: it didn't come from reading what other people had done. There are hardly any citations in that paper: it just took off from one really vital thing, which was what Kurt Gödel had done in 1931. But it had this characteristic of being both completely up with abstract mathematics of the time, and introducing this new down-to-earth engineering quality. He was very keen on turning things into engineering.
Q What do you think he would have made of the laptop computers that we use today in our everyday lives?
A He wrote a lot about what computers could do. But he also had to do a certain amount of campaigning for the principle of the computer right away in 1945.
Everything seemed to be going extremely well: his ideas were not neglected and he was taken onto the National Physical Laboratory, which was the premier government scientific laboratory, and they were very keen on rivalling the US with a completely new type of computing. His ideas were adopted, but he had to make a case for it. Part of the case was that a machine of the kind that we now recognise as a computer was something where you only write new software and you don't have to do anything to the hardware inside. It was completely revolutionary to say you had the same machine and it didn't matter whether you are doing numerical analysis or code-breaking, or whether it was doing what we now call data processing.
For that reason I don't think he would be surprised by the scope of computers. But I do think that everyone from that era would have been surprised by the cheapness and miniaturisation that has been developed in the past 70 years.
Q How did the famous Turing Test work?
A A lot of people do assume that there's some immaterial mind, soul, or something special about the human brain that could never be replicated by any other form of object. Turing wanted to challenge that. But he didn't want to challenge it just by saying "that's not true" he wanted to challenge it by giving a test to see whether it was true or not.
He devised a test that resembled a competition, with the idea of making an objective test of whether a computer was doing as well as a person does when thinking, by having both the output of the computer and the output of the human judged by a third party who can't see which is which. Basically, it's a rationalist idea going back to Descartes and the Enlightenment, and it's not really, in itself, very revolutionary. But the way he expresses it, I think, is very characteristic. He makes a game out of it. And although his writing is all about the theory of computers, he is also saying: "I'm human."
Q Can you speculate on what Turing might have gone on to achieve had he not been tragically cut short in his prime?
A In the last two years of his life he continued busily on his theories of mathematical biology. That was again something which combined considerable mathematical sophistication with his down-to-earth aspect. He was very determined to keep on with that, and had he lived there is no question that he would have continued. He had also begun to work in a modern research group mode, in a way that in the succeeding decades now dominates research activity.
In his last two years there is a lot of interest in fundamental physics, returning to what he was interested in when young. One of the things that I think might have been of particular interest to him is what we call quantum computing. One of the first things Turing studied was von Neumann's 'Mathematical Foundations of Quantum Mechanics', which he read in the original German in 1932. That is something to which he would have liked to return, I think, because it connects logic and axioms with the physical world. That was very much up his street. Nobody did very much on this until the 1970s and I think that is something that would have been very interesting to him.
Q Do you think that he would have made a good advocate for the public understanding of science?
A He wrote an article in 1954 for Penguin Science News, which was the precursor to the New Scientist. It was a popular article and the whole point of it was to reach the public. It was about recent advances in the theory of computability. That's another way, I think, in which he could have gone, as a communicator of science to the public.
Of course, the irony of his position was that the central thing to his career, his experience at Bletchley Park, which had formed a terrific bridge between the logic of the 1930s and electronics of the 1950s, was the one thing that he could not talk about. It is notable that he does not discuss in his papers where his ideas come from. Although you can see the urge to communicate, there are various reasons why he remains an enigma.
A special paperback movie tie-in edition of Andrew Hodges' 'Alan Turing: the Enigma' is published by Vintage, £8.99