Dave Dobson’s past is not a secret. Not technically, anyway – not since the relevant US government intelligence documents were declassified and placed in the vaults of the National Security Archive, in Washington DC. But Dobson, now 65, is a modest man, and once he had discovered his vocation – teaching physics at Beloit College, in Wisconsin – he felt no need to drop dark hints about his earlier life. You could have taken any number of classes at Beloit with Professor Dobson, until his recent retirement, without having any reason to know that in his mid-20s, working entirely as an amateur and equipped with little more than a notebook and a library card, he designed a nuclear bomb.