The dotcom bubble. The financial crisis of 2008 and 2009. The oil price spiral of the 1970s. The launch of the single currency. It would be fun, in a nerdish kind of a way, to debate which was the most seismic economic event of postwar history. But in fact the answer would be this: the ‘Nixon shock’, a fateful day when the final link between gold and the money you carry around in your pocket, or on your bank card, was finally severed.