When the Whole Room Couldn't Help but Sing

On a Tuesday evening in March 2019, cellist Sheku Kanneh-Mason stepped onto the stage at London's Royal Festival Hall and began playing Elgar's Cello Concerto. The audience sat in practiced silence — this was, after all, a classical concert with unwritten rules. But something happened in the second movement. A woman in the third row began swaying. A teenager in the balcony closed his eyes and let his head fall back.…