Imagine all the cultural artifacts the world would be without if The Beatles had never existed. The band scored Nike Air commercials and inspired those T-shirts with four first names stacked atop one another. They’re the basis of a supervillain group in “The Powerpuff Girls” and an entire Cirque du Soleil show. Then there’s that poster from your college dorm room. And Monty Python’s “Life of Brian.” Mop tops. Wu-Tang Clan’s “The Heart Gently Weeps.” That one “Mad Men” episode.