“Bridge Over Troubled Water” began as Paul Simon’s “humble little gospel hymn song.” But upon its release, it quickly took on a life of its own. Simon’s inspirational words and music, Art Garfunkel’s spine-tingling vocal and Larry Knechtel’s majestic piano all contributed to a work that resonated deeply, as a both an epitaph for the 1960s and a reassuring affirmation for a new decade’s beginning. Believe it or not, “Bridge Over Troubled Water” is 40 years old; what’s hard to grasp isn’t that