Vulnerability in an open city

At 2:50 ON MONDAY afternoon, with a spring sun casting crisp shadows across Boylston Street, two bombs detonated in close succession, turning the city's largest annual public gathering into a tableau of glass and blood. Exhausted marathoners wheeled in shock, stumbled, collapsed. Doctors expecting to treat leg cramps found themselves running trauma units instead.

The disbelief curdled into anger "who could have done this?" and a profound sense of worry. Was there enough security? Did someone carrying a bomb really walk undisturbed so close to the finish line of the city's most famous annual event? A feeling of vulnerability coursed through Boston like an aftershock. If it could happen at the Boston Marathon, it could happen anywhere in the city.

The Marathon is an exceptional moment, of course: tens of thousands of people lining the streets, cheering behind barricades, clustering together, exposed. But an urban tragedy like the bombing unsettles us more generally, because it wakes us up to precisely how much we have to fear every time we set foot on a city street. Simply to live or work in a city is to open ourselves, daily, to almost unimaginable risk. We stand unprotected in crowds; we travel underground, beneath water, or 300 feet in the air. We buy weird food from strangers in trucks. City life means violating every rule your parents taught you, every day.

After events like Monday's, it can be easy to conclude that there is just something wrong about cities. Perhaps people were never meant to live so close together, to be so unguarded in large groups. It's inarguably safer to disperse, each of us behind a lawn and a driveway and a "beware of dog" sign. But there is another way to think about it: that what we felt this week, the collective vulnerability, is an exact reflection of what makes cities work in the first place, what makes them productive, and vital, and almost unimaginably resilient.

After decades in which Americans flooded out of cities as soon as they could afford it, there is a reason that people have begun flocking back. The exceptional safety of suburbs, their vanishingly low rates of crime and risk, turned out to come with a steep price. Increasingly we are recognizing that when new things happen, they happen in cities: the places where people stand shoulder to shoulder, meet strangers, have conversations they didn't expect. Where they accept unpredictability. Where they leave themselves open to something going wrong. Increasingly we are recognizing that when new things happen, they happen in cities: the places where people meet strangers and have conversations they didn't expect. Where they accept unpredictability. Where they leave themselves open to something going wrong.

It is possible to see dynamism and risk as flip sides of the same coin. Every once in a while, that openness comes with a horrible cost. Boston has been lucky in this regard: Not since a courthouse explosion sent 22 people to the hospital in 1976 has anyone been seriously hurt in a public bombing here. Now, sadly, Patriots Day will commemorate the moment that streak ended.

But in the wake of a tragedy like last week's, it also matters to remember that within the openness that leaves us vulnerable lie the seeds of recovery. Some of it will take years. Some of it we have already seen.

In the videos following the Marathon bombing, amid the horror, the screaming, the concussion of the bomb and the confused runners, something else is visible as well: people running not out, but in. They muscled apart barriers to allow pedestrians to escape the chaos; they tore what fabric they had into tourniquets. They sensed that a rift had been opened. The risks were still there; there could just as easily have been a third bomb awaiting them. But the city had already begun to close around the rift.

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LONG-DISTANCE RUNNING is a metaphor for isolation, for individual achievement and willpower. The stories of marathon runners every year focus on their private resolve and superhuman dedication.

But anyone who has run the Boston Marathon, or lives along its route, knows that the actual feeling of being there is precisely the opposite. It's an expression of collective energy so total you can feel it. Once a year, a small city spontaneously arises just to take care of the crowd flowing through Greater Boston. In town after town, people set up chairs and sit on their lawns to hand water to strangers. They crank inspiring music at full volume for the benefit of people they have never met, and will never see again.

To run the Marathon is to be greeted by a nearly unbroken string of spectators, kids pedaling on trikes alongside you, an entire college trying to slap your hand. You could eat a case of oranges, hand-quartered, offered to you by children. There is almost no event that so perfectly encapsulates what it means to be a citizen.

Planting a bomb at an event like the Boston Marathon seems almost unimaginably perverse. If it's a symbol, it's a symbol of what people are willing to do for one another: Dick Hoyt, who pushes his wheelchair-bound son to the finish line every year. The hundreds who run for leukemia research, or the Jimmy Fund. Exceptionally engineered humans arrive from Kenya, Ethiopia, Germany, Japan, and the city enfolds them like homegrown champions. Cosmas Ndeti won the Marathon three times and gave his son, a boy born in Kenya, the middle name Boston.

The spirit we see at work on Marathon Monday has become a vogue in sociology. It goes by the name social capital—the rich capacity of people to support one another productively, to build networks, to cooperate. And we need these opportunities to build all the social capital we can. The statistics show that trust is actually declining in America, in cities as much as anywhere. An event like the Marathon, a parade of strangers cheering for strangers in a grand civic spectacle, somehow pulls us out of that dive.

Economists and planners have become fascinated by cities' ability to bring us together in spite of ourselves—and by what happens when they do. "People create ideas by literally bumping into each other and finding each other. That's the wellspring of ideas. That's what advances humanity," says Columbia University development professor Vishaan Chakrabarti. He has calculated that an astonishing 90 percent of America's gross domestic product comes from just 3 percent of its landmass—that is, from the densest cities.

What happens there requires care, cultivation, even planning. A century ago the visionary landscape designer Frederick Law Olmsted designed spaces allowing for people of all classes to mingle, those beloved city parks that include Central Park and the Back Bay Fens. Today modern urban planners have returned to that idea, eschewing privacy in favor of plazas, boulevards, openness—features that allow what Chakrabarti calls "the spark of urban serendipity," or, more touchingly, "public joy."

The sound of those two explosions abruptly ended the feeling of easy serendipity and public joy in Boston, at least for a while. In the days afterward, the Back Bay, one of the nation's most beautiful urban neighborhoods, lay barricaded off. If we think of the city as a dense grid of people, and not just streets and buildings, it is fair to say that a chunk of the city went temporarily missing. The bombing removed it.

But perhaps it is more accurate to say it was dispersed. When the news began to spread, shared documents arose online: People who lived in suburbs around Boston, with beds and sofas free, began to invite strangers visiting the city to their homes. It is easy to imagine that an event like this would fracture trust the way it fractures glass, slicing apart the thin web that holds a city together. Instead the web spread outward. "I can drive to pick you up," wrote one person. "It's an Aerobed, but it's a comfortable one!" wrote another.

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AS OF THIS WRITING, we don't yet know who planted the bombs on Boylston Street. And, importantly, nobody knows how officialdom will react over this next year. As much as we worry about safety, we also worry about blame, and crackdowns, and rightly so: Humans are prone to excess in reaction. New York City returned to normal in many ways after the 9/11 attacks, but in some ways it did not. The immense concrete bollards that went up around government buildings remain; Boston has those, too. All over the United States, we are scanned and detained and filmed in ways that we have never been before.

Openness and security, we're forced to remember, are a trade-off. The city of Boston has taken plenty of heat for its insularity and guardedness, but in the full sweep of its story, Boston is a stake planted in the ground for freedom. The first school in the country is still here, and still public. The green heart of the city is still called, simply, the Common.

Nothing, no virtue of cities, can make up for what the victims of the Marathon bombing lost on Monday. But no amount of safety, no security regime, would be worth exchanging for what we have gained from this way of life over the centuries: the shifting web of human connection, the knowledge and prosperity that were born here—not in spite of our vulnerability, but because of it.

There will be another Marathon next year, the 118th. It will be different. Boston will be locked down more securely before the event. Perhaps there will be metal detectors on city streets; there will be more surveillance cameras, more dogs. We will be less vulnerable, more guarded.

But look around, when it happens, for signs of a stronger spirit at work. People will cheer on their lawns as 25,000 strangers run by. They will throng the barriers in Brookline to cheer for people whose names they can't pronounce. They will hand water to strangers, and their children will proffer carefully sliced oranges, and the exhausted runners will take them with trust and gratitude. The city will be enfolding them again, risk or no. And to anyone who loves the city, it will look a lot like public joy.

Stephen Heuser is the editor of Ideas. E-mail sheuser@globe.com.