When I think about New York City in the aftermath of 9/11, I think about those “missing” persons posters taped by desperate relatives to streetlights and storefronts and scaffolding. I think about the candlelight vigils that filled Union Square night after night, and about the proliferation of American flags: Every corner bodega seemed to have one and every taxicab, too. I recall how scary it was to get on the subway (for fear of a bomb), and how scary it was to open the mail (for fear of anthrax).

It was all so raw, so terrifying—and it was all so analog.

After all, this was three years before Facebook, four years before YouTube, five years before Twitter, six years before the iPhone, and nine years before Instagram. So there were no anguished tweets or status updates from those trapped on the upper floors of the Twin Towers. There were no…