You’re looking at a photograph of my son John holding his daughter Emma. They’re on the rooftop of their Manhattan apartment, on the south side of 15th Street, between 7th Avenue and 8th. It’s a pleasant fall evening in 1997, and my wife and I are visiting from St. Louis. Emma is ten months old.

But maybe you aren’t looking at John and Emma. Your eyes probably focus instead on those two gray monoliths in the background. Before it means anything else, the picture may remind you of a disaster. Who would have thought?

Even I can’t look at this family photo with simply a warm memory of the moment, though I am looking at my son and my granddaughter. Those towers in the background can’t be overlooked. Back then, it seemed to me, they stood impervious, dominant, like Gulliver among the Lilliputians. In the photo, they stand like innocent bystanders, irrelevant to the purposes of the people in the photo or the person behind the camera. Now they are too relevant.

For a few years I had the picture on a library shelf, nicely framed, prominently displayed. But by January 1, 2002, when I retired, I could no longer look at it without thinking immediately of the World Trade Center. At that time, all my feelings involved rage, sympathy, and uncharacteristic xenophobia. For a long time after 9/11, I watched—again and again—the towers’ inferno, their collapse, the rolling cloud of gray dust scattering New Yorkers, the mound of debris and pulverized human bodies. I wondered what the victims’ last thoughts were—God? family? The picture on my bookshelf became unbearable. I took it down and stored it away. There were already too many reminders of 9/11.

I took it out again last winter, when Emma turned eleven. She lives in St. Louis now, and of course has no memory of her brief time on 15th Street. The picture is also in my son’s family scrapbook, and when Emma looks at it, she chides her father for his scruffy beard and for holding her too close to the edge of the rooftop. She doesn’t even notice the skyline. Emma is tall and lean like her parents. She has a genuine gravitas when she talks about environmental issues, and she’s in the third year of a vegetarian diet. But 9/11, Iraq, and Afghanistan are only part of her school vocabulary. They lack historical context, they stir no emotion, and they have no clear consequence in her life. I worry that 9/11 and its aftermath will soon have the same remoteness for children her age as the Civil War, the World Wars, and Vietnam.

I wonder how many others have a similar photo: a casual picture of someone or something in New York that coincidentally includes the Twin Towers. And I wonder if it provokes in them the same strange emotions I’ve experienced. There must be thousands—hundreds of thousands—of these personal images, with a story behind each one, a story now overshadowed and secondary to the story of 9/11.