You know the kind of music you sometimes hear in the background of a movie when the heroine opens the door to an attic filled with mysterious objects, light from a window streaming in through the dusty air, the whole scene enigmatic but not especially menacing or foreboding. Silence. A quiet piano arpeggio in no recognizable key over a hushed string chord. Silence. Repeat. Silence. Repeat. Now turn the movie off and let the music go on like that by itself for 80 minutes. That's Morton Feldman's "Piano and String Quartet."