Katy Perry and Justin Trudeau are both big names. But they’re not at the same level of celebrity. If you’re a political junkie, you know Trudeau. If you are…well, even marginally knowledgeable about pop culture, you know Katy Perry. You might know ...

Mr. Marche is the author, most recently, of “The Next Civil War: Dispatches From the American Future.” The former Canadian prime minister Justin Trudeau and the pop star Katy Perry are now publicly a couple: They acknowledged it, naturally enough, by ...