Life expectancy, walking, and that giant city a hundred miles north of me.

Why New Yorkers Last Longer, from New York magazine. (Thanks to Jess Haralson.)

The article mentions how life expectancy is calculated, and says:

The math works like this. Imagine that one man dies of AIDS at age 25. Since he was statistically supposed to live to 78.6 years, he’s died about 50 years too early, so he shaves 50 years off the city’s overall pool of life. If one Wall Street guy collapses of a heart attack at age 65, he shaves only ten years off. You’d have to have five Wall Streeters die at that age to equal the impact of one AIDS victim. By the same logic, one infant’s dying during childbirth—77.8 years too early—is equal to ten people’s succumbing to lung cancer at age 70. It is a very weird form of horse trading. The more you’re able to prevent young people—folks in their twenties and thirties—from dying, the more rapidly you boost a city’s overall life expectancy.

Although I'm not sure if the math works out exactly like that -- I've mentioned this before -- it certainly seems reasonable that it does. And a lot of the time, the "value" of a certain public health policy is computed in terms of number of years of potential life lost, not in terms of number of potential lives lost. This makes sense; younger people's lives are more valuable simply because what we're really valuing is the remainder of the life. And countries in sub-Saharan Africa that have been hit hardest by HIV do, in general, show the lowest life expectancies.

The article also makes the claim that New Yorkers live longer because they walk more. This seems reasonable to me. (And New Yorkers walk faster than the average person, too.) I don't live in New York -- I live in Philadelphia, which has a deep-seated inferiority complex in relation to New York. But we walk fast here, too -- I know this because people who move here from other places often say that we walk fast here, but rarely say that we walk slow. Walking fast is my own personal extreme sport, because I happen to walk faster than the average person (I have long legs) and so I actually end up weaving between people in order to go at my preferred speed. Am I healthier than average? I'm not sure.

Sometimes I wonder what my weaving through crowds would look like if time were made into a third spatial dimension -- or what a crowd of people in general would look like. People manage to not crash into each other. And when I'm walking, I will walk somewhere where someone else was a moment ago, where someone will be in a moment -- but we don't collide. Our world lines don't (indeed, can't) intersect. (World lines are usually thought of as positions in four-dimensional spacetime, but I'm assuming that my position is in two dimensions, thus a three-dimensional spacetime.) You'd see ordinary pedestrians' world-lines moving along in clumps. I go faster, so my world-line would be less steep (assuming time is the vertical dimensions). Clumps of tourists go slower and are probably more tightly clumped than ordinary people. (Don't believe me? Go down to Independence Hall. My theory is that tourists are afraid of being infiltrated by the natives.) Is it possible to identify tourists just by looking at their world-lines? And does a tourist alone be identified, or only a large clump of them? I would argue that the lone tourist doesn't exist, although it's difficult to articulate why -- perhaps because the lone tourist probably makes more of an effort to fit in with the crowd around them. All I know is that when I've been traveling alone in strange cities, people have actually come up to me and asked for directions. Somehow I manage to look like I belong.