There are times when music can seem a solitary experience. That goes for the listener, sectioned-off from the world in headphones or listening solo and glorying in “their” music through speakers. It also goes for performers, so often portrayed as ego-driven – and indeed some soloists can only survive the nerves, the lonely travelling life and the terrifying exposure through an assertion of ego. In a cacophonous age, the choice of music can define and anchor the individual. But there are times, of collective crisis or celebration, when music can remind us what a society is. And on Friday November 22, 1963, in a concert by the Boston Symphony Orchestra that happened to also be a WGBH radio broadcast and so was captured for posterity, the BSO’s revered music director Erich Leinsdorf broke some unimaginable news to a crowded symphony hall. What was to have been a routine concert became…