Why Alan Turing was the right choice for the £50 note

Please note: The Alan Turing £50 note display closed on Sunday 13 March 2022.

This is so much more than a salute to Turing’s enduring genius: the move, which will be celebrated in an exhibit at the Science and Industry Museum in Manchester, salutes precious ideals of human equality, and is a wonderful reaffirmation of how our society has changed for the better since Turing was convicted in 1952.

The range and impact of Turing on Britain and our way of life first came home to me in 2012 when, as a Trustee of the Science Museum Group, I learned about his extraordinary life in a new exhibition, Codebreaker.

Turing is best known for his wartime codebreaking exploits at Bletchley Park, where he devised ways to crack German ‘Enigma’ messages on an industrial scale. The intelligence uncovered at Bletchley was central to Britain’s war effort and may have shortened the conflict by up to two years.

But the award-winning exhibition showed many more facets to his genius. My colleague Roger Highfield, the Science Museum Group’s Science Director, reminded me that perhaps his most profound contribution to modern life dates back to a paper he wrote in 1936, when he was just 24.

The paper on an abstruse theoretical problem carried a theoretical description of a ‘universal computing machine’, which is now seen as the conceptual basis of today’s stored-program computers. Turing himself wrote one of the first practical designs for a stored-program computer, later realised as the ‘Pilot ACE’, which you can see today on display in the Science Museum’s Information Age gallery.

Alongside his work in cryptanalysis and computing, Turing also sketched out the ‘Turing test’ in his seminal 1950 paper ‘Computing machinery and intelligence’, which has become an influential concept in artificial intelligence.

And he was interested in understanding the codes of life too. Morphogenesis – the development of patterns in living things – occupied his thoughts for his last years as he ran computer simulations of the mathematics and chemistry of life itself. (A few years ago, the Science and Industry Museum in Manchester did an experiment to investigate his predictions with sunflowers, which provided support but revealed new patterns too.)

But Turing would come to break the then codes of society. At Cambridge University, where Turing studied in the 1930s, and at wartime Bletchley Park, his homosexuality was relatively tolerated. But the Cold War amplified anxieties of the age, as gay people were assumed to be at risk of blackmail, and thus a risk to national security; Turing’s time at Bletchley had given him access to some of the nation’s most secret knowledge.

In 1952, following what was then deemed an unlawful sexual relationship, Turing was tried and convicted of ‘gross indecency’ under the anti-homosexuality legislation of the day.

He was stripped of his security clearance and lost his post-war consultancy to the Government Communications Headquarters (GCHQ), the post war successor to Bletchley, which the Science Museum covers in a new exhibition, Top Secret. He was offered a choice of imprisonment or a one-year course of hormone treatment -chemical castration – and chose the latter, becoming impotent and developing breasts.

On 7 June 1954, Turing ingested a large amount of cyanide solution at his home in Wilmslow, Cheshire and was found dead the next day. The coroner recorded a verdict of suicide, opining that Turing’s ‘mind had become unbalanced’ but the full circumstances of his death remain a mystery.

At the end of 2012, I joined Professor Stephen Hawking, Astronomer Royal Lord Rees and Science Museum advisor Sir Paul Nurse, and fellow Trustees Lord Faulkner and Doug Gurr, among others to sign a letter in the Daily Telegraph to call on the government to grant a posthumous pardon and “formally forgive this British hero”.

“He led the team of Enigma codebreakers at Bletchley Park, which most historians agree shortened the Second World War. Yet successive governments seem incapable of forgiving his conviction for the then crime of being a homosexual, which led to his suicide, aged 41.”

Denying that it would set a precedent, we added: “It is time his reputation was unblemished.” In December 2013, Turing was granted a posthumous pardon by the Queen, an unusual move. Three years later, a law was passed to posthumously pardon all homosexual and bisexual men convicted of subsequently abolished sexual offences. Appropriately, it became known as ‘The Alan Turing Law’.

Turing was my favoured candidate among a very strong field when the Bank of England announced it wanted an eminent scientist to adorn the £50 note  at an event in the Science Museum. Today’s announcement and the exhibit in Manchester are is a wonderful way to recognise this British hero. Alan Turing was a pioneering codebreaker, the architect of artificial intelligence and an inspirational figure who has changed our world for the better.