no, it would not be silence. I don't mean to be all snooty here-- but since we are talking about it-- it would be whatever sounds occur during that time. He was inspired by those Rauschenberg paintings ( that appear to be blank canvasses, and are but are functioning as shadow catchers- real-time spontaneous art framers, if you will)- same with 4'33". It's about whatever happens in that space in that particular moment. It is a kind of unconscious spontaneous group improv, sort of. Interestingly, improvised solos over tunes aren't copyrightable- bandleaders ripped them off players all the time.