What’s the most famous pedestrian crossing in the world? Probably, this:

Yes, the Abbey Road zebra crossing, made famous after the lead singer of Wings, one of the Travelling Wilburys, Yoko Ono’s husband and the voice of Thomas the Tank Engine (once, collectively known as The Beatles, I’m told) decided to feature its tarmacadamic striations on their final proper album.

But there’s another pedestrian crossing in town with less frivolous claims to world attention. Here it is:

We’re on the corner of Southampton Row and Russell Square in leafy Bloomsbury. There’s no plaque to mark the event but this is the unlikely spot where the nuclear chain reaction was first conceived.

Hungarian scientist Leó Szilárd was a theoretical physicist working in Berlin. His credentials were excellent, having trained under Planck, Einstein and von Laue. But the forces of Nazism were on the rise and in 1933 the Jewish Szilárd fled to London to escape possible persecution. Our great city was quick to inspire him. On the morning of 12 September 1933, Szilárd was waiting at this crossing for the lights to change, mulling over a recent speech by Ernest Rutherford who’d dismissed the possibility of atomic energy as ‘moonshine’.

As he stepped off the kerb, so the story goes, Szilárd was hit by an idea (and not by the number 59 bus, which might have happened were he to absent-mindedly cross at the time of my photo). Perhaps it was the momentary alignment of vehicles, or the jostling of pedestrians eager to get across the road. He began to think about the power of collisions. If a nucleus could be hit by a neutron to force the release of two further neutrons, and then each of these could go on to instigate similar ejections from other nuclei, a chain reaction would ensue that would liberate huge amounts of energy. In other, slightly hyperbolic words, Szilárd had invented the atomic bomb.

While the humble street crossing was an unlikely place to make one of the most important imaginative leaps in human history, the idea’s first practical implementation occurred in an equally unusual setting. On 2 December 1942, Szilárd and other scientists watched the first ever self-sustaining chain reaction in a squash court beneath the athletics stadium at the University of Chicago.

Two and a half years later, (and to Szilárd’s horror) a quarter of a million Japanese citizens were dead in the wake of the atomic strikes on Hiroshima and Nagasaki. Szilárd’s brainwave, initiated by the chance movements of cars at a Bloomsbury pedestrian crossing, had led to a weapon of unprecedented power.

And thus did the gentle streets of Bloomsbury, home to Virginia Woolf, Dorothy Sayers, Charles Dickens and William Butler Yeats, also inspire that darkest of eras, when the world hovered only a button-press away from mutually assured destruction and nuclear holocaust.

Previous instalments of the Scientific Tourist in London.