On Wednesday 23 June the Bank of England releases a new polymer 50 pound banknote featuring mathematician, computer pioneer and codebreaker Alan Turing (1912-1954). The banknote contains lots of geeky features from Turing’s pioneering work in mathematics, computer science, code breaking and even mathematical biology. CWI researchers and professors Lynda Hardman and Jurgen Vinju comment on the meaning of Turing’s work for present day computer science and society.

In 2019 Alan Turing was selected from a shortlist of British scientists to be featured on the new banknote. The release date 23 June was chosen because it is Turing’s birthday. CWI researcher professor Jurgen Vinju is delighted with the Alan Turing-banknote: “It emphasizes the fact that information technology in general and computers in particular are such fundamental infrastructures for the modern society. The honour is due to Turing who gave the field its theoretical foundation and the corresponding motivation to make digital computers a reality.”

Turing’s work is still highly relevant, says Vinju. “I just finished writing an academic paper in which I refer to the Turing machine from 1936. In the paper I designed a new programming language. I was looking for the limits to calculate certain numbers, and the limits are defined by the Turing machine.”

The Turing machine is in fact a mathematical model: any problem that can be calculated, can be calculated on a universal Turing machine. Our personal computers, laptops, tablets and smartphones are all simpler versions of a universal Turing machine. In 2012, during the celebration of Alan Turing’s 100th birthday, two members of Vinju’s CWI-group, Jeroen van den Bos and Davy Landman, actually built a working Turing Machine from Lego: www.youtube.com/watch?v=FTSAiF9AHN4. The Lego Turing machine demonstrates the most fundamental principles of any computer in an attractive and simple way for a wide audience.

*Specimen of the Turing side of the new banknote. Credit: Bank of England.*

Let’s take a closer look at the new banknote. As is traditionally the case, Queen Elizabeth is on the front side of the banknote. It is the back side that is fully devoted to Alan Turing and shows many geeky illustrations of Turing’s pioneering work:

- The most eye-catching is Alan Turing’s portrait, which is based on a photo taken in 1951, now part of the Photographs Collection at the National Portrait Gallery in London.

- Bottom right of the portrait we see Turing’s signature which has been taken from the visitor’s signature book on display at Bletchley Park Trust in 1947, where Turing did his codebreaking work during the Second World War.

- Just below his signature, we see a quote from Turing, given in an interview to
*The Times*newspaper on 11 June 1949: “This is only a foretaste of what is to come and only the shadow of what is going to be.”

- On the left-hand side of the portrait, ending at his shoulder, we see a ticker tape depicting Turing’s birth date (23 June 1912) in binary code.The ticker tape is a part of the Turing machine-model that he developed in 1936.

- Left of his head, we see a mathematical table and formulae from Turing’s groundbreaking 1936 paper ‘
*On Computable Numbers, with an application to the Entscheidungsproblem*’. This paper is widely recognised as being foundational for computer science.

- Against the background of the table we see a photo of the Automatic Computing Engine (ACE) Pilot Machine which was developed at the National Physical Laboratory based on Turing’s pioneering design. The ACE was one of the first electronic stored-program digital computers. There are also a series of background images depicting technical drawings from The ACE Progress Report.

- Spread across the banknote we also find some technical drawings for the British Bombe, the codebreaking machine specified by Turing, used to break Enigma-enciphered messages during the Second World War.

- Finally, we can see a sunflower-shaped red foil patch in the upper right corner, with the letters AT (Alan Turing) in the middle. This item is linked to Turing’s study of the question: ‘How does an organism know how to grow?’ To answer this question Turing developed in the early 1950’s mathematical-biological models and used the Mark 1 computer to calculate them.

The fact that Alan Turing was selected to be on the new 50 pound note is also an important symbol of diversity. In 1952 Turing was arrested and convicted of homosexual activities, an official crime at that time. He was given the choice between a prison sentence and hormone therapy for a year, and chose the hormone therapy. Only two years later, Turing died at the age of 41. He most likely committed suicide by eating a poisoned apple. In 2009 prime minister Gordon Brown officially apologised on behalf of the British Government for Alan Turing’s 1952 conviction for homosexuality: “[...] we are sorry. You deserved so much better.”

The British CWI-researcher professor Lynda Hardman is also happy with the new Alan Turing banknote. “Our digital communication nowadays came into existence really thanks to Alan Turing and Tim Berners-Lee, the inventor of the World Wide Web. Without them we wouldn’t be talking via Zoom now.”

Hardman sees an interesting parallel between the research in the CWI-group *Human**-Centered Data Analytics*, of which she is part, and the famous Turing test: “In the Turing test, the question is whether a machine can be intelligent. The research we are doing in the Digital Humanities is actually a kind of reverse Turing test: much of what people have produced in terms of culture over the centuries has now been digitized. The question for we researchers then is: how do we extract meaningful human information from all the digital data?”

Hardman ends with telling a personal anecdote illustrating the importance of Alan Turing as a public figure: “When our daughter Anna was in high school, age 15, she was asked to choose one of the great British people to make a video about, and she decided for Alan Turing. At CWI we have an original Enigma-coding machine, a German machine that Turing helped breaking during the Second World War. Anna once came to the institute and was allowed to briefly touch the Enigma. She found it absolutely amazing.”

**About CWI**

Founded in 1946, Centrum Wiskunde & Informatica (CWI) is the national research institute for mathematics and computer science in the Netherlands. It is located at Amsterdam Science Park and is part of the Institutes Organisation of NWO. The institute is internationally renowned. Over 150 researchers conduct pioneering research and share their acquired knowledge with society. Over 30 researchers are also employed as professors at universities. The institute has generated twenty-seven spin-off companies.