Alan Turing’s legacy is helping to answer some of life’s most fundamental questions

There are currently two valid £50 notes in circulation. The old paper version features scientist James Watt and industrialist Matthew Boulton – famous together for developing and marketing state of the art steam engines during the industrial revolution.

The new note features Alan Turing – the Second World War hero, immortalised in the entertaining, but largely inaccurate, 2014 film The Imitation Game starring Benedict Cumberbatch.

The polymer-based 50 has been in circulation since 23 June last year, released on what would have been Turing’s 109th birthday. But there are fewer than 100 days left to spend any of the old paper £50 notes that you might have saved in your piggy bank or tucked away in your purse.

From then on, it will be Turing all the way, although if you’re like me you probably won’t have seen one of the old £50s and are unlikely to see the new one either!

Turing is probably best-known for his pioneering code breaking work during the Second World War. Alongside his team at Bletchley Park, he made the first breakthroughs into the German naval Enigma code, which eased the passage of allied ships across the Atlantic. He was also instrumental in creating a machine called the Bombe that could routinely crack Enigma. Turing’s war work, for which he was awarded an OBE, saved countless lives and is believed to have significantly shortened the war.

During his short academic career, Turing made towering contributions to a diverse range of areas, from pure mathematics to the theory of artificial intelligence. He introduced the idea of the stored programme computer years before such machines existed.

Eventually, electronic technology became sufficiently advanced to allow Turing’s ideas to make the leap from his brilliant mind into the real world. Although no one person can claim to have invented the computer, the descendants of Turing’s theoretical machine sit in billions of offices, homes and pockets around the world.

In 1952, aged 40, Turing wrote a lesser-known paper in a new area, which was no less brilliant than his preceding work. In The Chemical Basis of Morphogenesis, Turing proposed a mechanism known as “diffusion-driven instability” by which patterns might form in the early embryo. The same mechanism, he realised, might account for a multitude of patterns in nature including those seen on animal coats, suggesting a mechanism for how the leopard got its spots.

Turing’s demonstration of the use of maths to shed light on the secrets of life was highly influential in the development of what is now my own specialism – mathematical biology. At the heart of our rapidly growing subject is the attempt to represent biological systems of interest mathematically or computationally using models.

Today, Turing’s legacy – the idea of taking a quantitative approach to biology – is helping to answer some of life’s most fundamental questions. Mathematical biologists are attempting to understand how things can go wrong during the development of an embryo and, as we have seen throughout the pandemic, to plan for, understand, and respond to disease outbreaks.

Turing’s recognition on a bank note marks a significant step forwards, not just because of his academic achievements, but also due to the discrimination he endured to achieve them. He is the first member of the LGBT+ community to be recognised in this way.

Turing was a gay man in a time when “homosexual acts” were outlawed in the UK. In 1952, he was charged with “gross indecency”. At trial, Turing pleaded guilty, insisting that he saw nothing wrong with his actions. Consequently, he was convicted and offered the choice between prison or probation with a course of the hormone stilboestrol for a year: a so-called “chemical castration”.

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The “treatment” was designed to reduce libido and had the effect of rendering Turing impotent and causing him to grow breasts. If this were not humiliation enough, Turing’s security clearance was revoked, barring him from his ongoing work for the government’s communication headquarters, (GCHQ).

On 7 June 1954, Turing died of cyanide poisoning. Next to his bed lay a half-eaten apple. It was speculated (since Snow White was one of Turing’s favourite fairytales) that he had laced the fruit with cyanide before consuming his own “poisoned apple”. The inquest into his death recorded a verdict of suicide.

In 2009, then prime minister Gordon Brown, apologised on behalf of the British government for the appalling way Turing was treated and in 2013 Turing received a posthumous royal pardon. In 2017, the Alan Turing law came into force, retrospectively pardoning all men convicted of “gross indecency” for acts no longer considered offences.

In light of his courage, scientific prowess, technological foresight, far-reaching computational influence and stoicism in the face of injustice, I will be delighted to carry a picture of this pioneering war hero around in my wallet. That is, if I ever get to see a new £50!

Kit Yates is a senior lecturer in the Department of Mathematical Sciences and co-director of the Centre for Mathematical Biology at the University of Bath