KEITH JARRETT After four decades, a piano giant still plucks inspiration from thin air By Jeff Tamarkin Jazz is in part the art of improvisation—and legendary pianist Keith Jarrett takes the concept to its extreme. Up until the moment he presses down the keys, he hasn’t a clue as to how he will begin or what will follow. “There’s this nanosecond, or maybe it’s an eternity, between sitting at the piano ready to play something and actually...