ALL THE WEB geeks I know are playing around with Twitter. Basically, Twitter is an online tool for updating your friends about your life on a minute-by-minute basis using mobile devices. You sign up for a Twitter account and build a friends list; then you read and write little things about your day with friends from your mobile or the web. Most people seem to use it with their mobiles, though, and indeed Twitter's claim to originality is that it integrates mobiles and the web so seamlessly.

Originally designed as a kind of event-organizer to help you meet up with your friends via I'm-here-now posts, Twitter has quickly become more than that. People use it to hold time-shifted conversations with their friends throughout the day, creating long, elaborate discussions in tiny bite-sized chunks of text. The result is a Twitter "time line," showing comments from various people over time so that you can travel along the thread of a conversation, watching it develop and change like friendships themselves do.

I opened a Twitter account and quickly grew bored—it's more twitchy and spastic than blogging, and I didn't want constant updates about my friends on my phone. But I still get about three or four Twitter-friend requests per day, and people keep asking me what I think about the hyperactive little app.

I wasn't sure how to explain Twitter's bizarre popularity until I read an article published this week in the Proceedings of the National Academy of Sciences that used data from hundreds of cities to create a mathematical model suggesting that the "pace of life" in urban areas speeds up exponentially relative to population size. What that means is that if your population grows at rate n, your pace of life grows at the rate of n-squared. In other words, really freakin' fast.

"Pace of life," according to first author of the study Luis Bettencourt, includes everything from technological innovation to wealth and the speed of walking traffic. So you would expect that as the population of cities grows, for example, the speed of communications technologies like Twitter should grow exponentially faster.

Twitter, primarily an urban phenomenon, makes perfect sense if you look at Bettencourt's model. More than half the world's population lives in cities, and many city centers like the Bay Area are growing. As these populations grow, tech innovation grows far more quickly: thus the move from daily newspapers to blogs to Twitter in just 10 years.

Twitter's popularity reflects the accelerating pace in cities; people use Twitter as they stroll around with mobiles, and the rapidity of their updates reflects a sense that new, exciting things are happening to them every minute, not just every few hours (blog time) or every day (newspaper time).

So as our cities grow more dense, our social relationships are moving into Twitter time. Bettencourt and his co-authors would claim that this is neither good nor bad—it's simply the inevitable outcome of urban growth. Other aspects of city life, such as gas consumption and road construction, proceed at a more leisurely pace—they grow at the same speed as the population, or even (in the case of construction) more slowly than the population does.

The question is, how can a city of limited resources (water, gas, optical fiber) sustain an exponential growth in technologies, innovations and wealth? In other words, will maintaining ourselves in Twitter time—constantly growing the population, constantly using resources—kill us?

Bettencourt and his colleagues say that's a very real possibility. One outcome of their model proposes that a society in Twitter time will collapse when it uses up all its resources. The population will drop off precipitously, and as it drops, our "pace of life" will slow exponentially.

Of course, there's another way. An equally real possibility is that urban cultures go through phases of Twitter time, then slow down again for a while, essentially "resetting" the model. This "resetting," according to the authors, happens after a singularity where wealth changes the society so much that new resources are needed—and new technologies are implemented to create those resources. As populations grow, however, those singularities come at a more rapid clip. Can humans innovate new, sustainable technologies in time to prevent urban collapse? Only time will tell.

Annalee Newitz ([email protected]) is a surly media nerd who lives in book time, with occasional forays into blog time.

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