Ideas: The Life Blood Of Cities
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Ideas: the lifeblood of cities
* 23 May 2007 * NewScientist.com news service * Dana Mackenzie
http://www.newscientist.com/channel/being-human/mg19426051.400-ideas-the-lifeblood-of-cities.html
"HOW in the image of material man, at once his glory and his menace, is this thing we call a city," said the architect Frank Lloyd Wright in a 1904 speech. He proceeded to elaborate on his metaphor of a city as a living organism:
"Thousands of acres of cellular tissue, the city's flesh, outspreads layer upon layer, enmeshed by an intricate network of veins and arteries radiating into the gloom, and in them, with muffled, persistent roar, circulating as the blood circulates in your veins, is the almost ceaseless beat of the activity to whose necessities it all conforms..."
Do cities actually work like biological entities? In his rather Gothic description, Wright emphasised the physical functions of a city: distributing goods, conveying people and removing waste. It's the same view that economists traditionally take to explain our insatiable desire to live in the same place as everybody else. Cities evolve, the theory goes, because they benefit from economies of scale and manage resources more efficiently than decentralised communities. Biology works similarly: large animals expend energy more efficiently than small ones, so the metabolism of an elephant is slower than that of a mouse.
A new view of cities is emerging, however, which goes beyond conventional economics or biology. Cities are where ideas are born, say Luis Bettencourt of the Los Alamos National Laboratory and Geoffrey West of the Santa Fe Institute, both in New Mexico - and that is a far more powerful growth stimulant than economies of scale. "The presence of qualified professions and entrepreneurs constitutes a reason for a place to grow," says Bettencourt. "If you can create a place that is exciting intellectually, that tends to attract more people."
The findings are surprising because they suggest that cities follow growth trajectories that have no biological counterpart. This year, for the first time, more people will live in cities than in rural areas, according to UN projections. At this tipping point in human history, it is worth trying to understand the mechanisms behind urbanisation and where it is headed.
Cities have fascinated social scientists for more than a century. The English economist Alfred Marshall is often credited with being the first to study cities from an economic point of view. In his 1890 book Principles of Economics he wrote: "The large towns and especially London absorb the very best blood from all the rest of England; the most enterprising, the most highly gifted, those with the highest physique and the strongest characters go there to find scope for their abilities."
During the 20th century, many researchers studying urban growth focused on economies of scale and their effect on wages. In 1974, Vernon Henderson of Brown University in Rhode Island proposed that cities reach an optimal size by growing until their workers' per capita income reaches a maximum; when it starts to decline, workers leave for other cities. More recently, researchers including West have tried to identify deeper mechanisms behind these societal patterns. Though West is a physicist by training, his reputation stems mostly from his pioneering and controversial work on scaling laws in biology - how things change with size.
What is all the fuss about scaling laws? "Physicists are used to thinking about extremely large systems of identical particles," says Steven Strogatz, a mathematician at Cornell University in Ithaca, New York. Take a piece of iron: at high temperatures, the spins of the particles jiggle around in random directions. If you gradually lower the temperature, the spins stay random until you reach a critical point - then they suddenly line up, and you have a ferromagnet.
This switch from disorder to order is called a phase transition. In the 1960s, physicists noticed that phase transitions follow certain universal patterns, called power laws, even if they have nothing in common physically. Kenneth Wilson of Cornell showed in the 1970s that these power laws come about through the growth of fractal structures, work which won him the Nobel prize in 1982. Since then, Strogatz says, "When physicists see a power law, they think in terms of phase transitions, and they smell Nobel prizes. They are like sharks with blood in the water."
Biologists, meanwhile, had known about a mysterious power law for decades. In 1932, the Swiss physiologist Max Kleiber showed that the amount of metabolic energy used by animals increases with size, as the three-quarters power of their body weight: so for example, a dog that is 16 times as large as a rat will use only about eight times as much energy. Alternatively, an animal's metabolism per kilogram slows down in proportion to the one-quarter power of its weight. That is why a mouse has to eat half its body weight in food every day, while humans can get by on a far smaller proportion of theirs.
When he heard about Kleiber's law, West thought about it as a physicist. Was there a fractal structure involved? Well, yes, an animal's circulatory network has branches upon branches upon branches. Was there a large assembly of identical units? Yes, the capillaries are more or less the same size in an elephant as in a mouse.
In 1997, West's team showed that the quarter-power law follows from these observations, along with an assumption that the circulatory network optimises the use of energy (Science, vol 276, p 122). By working out the properties of the network, they derived the power law for metabolic rate and also showed that other quantities such as the size of blood vessels and the length of an animal's life scale with body size according to power laws. West's team has since extended these ideas to natural ecosystems (New Scientist, 1 May 2004, p 38). Their model, however, is still not widely accepted by biologists (see "Power struggle").
Undaunted, West set his sights on another large system of identical units - the modern city. In particular, he wanted to know whether the imagery of a city as a living organism could be backed up by quantitative data. "My first strategy was to ask the same question that had been answered in biology: are there scaling laws? Surprisingly, for cities in particular, no one had looked at this question."
With help from their collaborators at Arizona State University, Tempe, and Dresden University of Technology in Germany, West and Bettencourt tracked down all sorts of information about the "metabolism" of cities, including the number of gasoline stations and laundries, electrical power usage and the total wages earned. Their database, assembled with the aid of the internet from hundreds of cities across the US, Europe and China, is the first important outcome of the project. "The type of data they obtained would not have been possible to get 20 years ago," says Sidney Redner, a physicist from Boston University who has written on migration to and from cities.
The team plotted each variable versus city population and looked for an overarching pattern. "When I started, I thought everything would be like biology," says West. If cities followed biological laws, he reasoned, their metabolism per capita ought to slow down as they get bigger, and the scaling should follow a power law. "Had we seen quarter powers, we would have said, 'Fantastic! Cities are just big biological organisms'," West says. "But it just wasn't true."
Instead, they found that the variables fell into two distinct groups (Proceedings of the National Academy of Sciences, vol 104, p 7301). Quantities related to a city's infrastructure, such as the number of gas stations and the length of paved roads, did scale "sublinearly", meaning that the larger the city, the less of these were required per capita. But measures of economic output and innovation - the number of patents, total wages, GDP, even the pace of walking - scaled "superlinearly", showing increasing returns with size (see Graphs). "The scaling laws say that on average, as a city grows you can predict its output and input, its energy consumption, its wealth creation, its level of crime," says Bettencourt.
The results suggest that bigger cities have a faster pace of life, fuelled by wealth and new ideas. It's a phenomenon that has no biological counterpart, says West, but it fits with the common perception of the big city. What's more, the consequences for growth over time are clear. While biological organisms have a built-in mechanism for keeping their size under control, cities may not, which means their growth can accelerate out of control. Indeed, Bettencourt and West's work strongly suggests that there is no maximum per capita income; big cities just get richer, and rich cities get bigger.
This has major implications for urban sustainability. The researchers have shown in their models that if sublinear growth dominates, a city's population will gradually approach its optimum size and then stabilise. By contrast, a city growing superlinearly has no maximum size, so in theory its population can keep growing infinitely. Of course, no real city can sustain such growth, so any superlinear boom must be followed by a bust.
New York is a perfect example. Throughout its history, West says, the city has experienced waves of accelerating growth: from 1800 to 1850, from 1860 to 1890, from 1900 to 1920, from 1930 to 1940 and from 1950 to 1960. (After 1960, the pattern is harder to discern.) Each boom was followed by a bust, and the cycles are getting shorter; in fact, they are getting too short for the decennial census to track (see Graph).
That is no accident. "Superlinear scaling gives a natural explanation for why the cycles have to get tighter," says West. In his models, if you reset superlinear growth at the time of a bust, the duration until the population would grow infinite again gets shorter. As West points out, examples of this are all around us. "The timescale of an innovation, as measured by a product lifetime, is now significantly less than a human lifespan. That's a new phenomenon," he says. For instance, pen and ink lasted for hundreds of years. The typewriter, a few generations. The personal computer, one generation. Now we have iPods, cellphones and other mobile devices. Will they even last one generation? Urban organism
"If you take it to its logical conclusion, you'll need a major innovation every year, or every few months," says West. "That is obviously not sustainable. What is the nature of the end stage? We certainly do not have an answer."
Richard Florida, an economic geographer at George Mason University in Fairfax, Virginia, echoes West's concerns. "I worry if we push the speed of the urban organism past that of the human organism," he says. "Already we're hiring personal trainers and coaches to keep us going, and boosting our memory with computers." He adds that as cities continue to get larger, the contrast between the "talent-attracting haves" and the "talent-exporting have-nots" will increase, and the fault lines will not run between the developed and developing world, as they used to, but will split countries internally.
The picture is not very encouraging for the have-nots either. Take the US cities of Buffalo, Pittsburgh and Cleveland, whose populations have been steadily diminishing since 1960. "These are cities that stopped growing because they haven't found the next innovation cycle, and were left with something stagnant or collapsing," Bettencourt says.
So what is the lesson for urban development? Is there a third choice besides stagnation or increasingly frantic cycles of boom and bust? As a matter of fact, there is. You can invest the fruits of the city's economy in something other than growth, says Bettencourt. "The assumption of our paper is that as you create resources, you put them back into the city to create more population," he says. "That's not necessarily true. You can use them to change the shape of the city, to add infrastructure or to change the type of economic activity."
The next step is to explain why an idea-based economy scales superlinearly. It might have to do with the network of contacts between people. If a city has n inventors, the number of contacts between inventors can grow roughly as fast as n2. The power of 2 represents a superlinear growth rate, whereas a power of 1 would be linear. From the data it looks as if the actual power is neither 1 nor 2, but around 1.2. The researchers hope that by working with Strogatz, an expert on networks, they will flesh out an explanation.
Bettencourt and West admit that their quest for an underlying theory has only just begun. One can hope that a final model of how cities work will be as vivid as Wright's original description: "The poisonous waste is drawn from the system of this gigantic creature by infinitely ramifying, thread-like ducts, gathering at their sensitive terminals matter destructive of its life, hurrying it to millions of small intestines to be collected in turn by larger, flowing to the great sewers, on to the drainage canal, and finally to the ocean." “Top 5 megacities Tokyo, Japan - 35 million Mexico City, Mexico - 20 million Mumbai, India - 19 million São Paulo, Brazil - 19 million New York-Newark, US - 19 million” Dana Mackenzie is a science writer based in Santa Cruz, California From issue 2605 of New Scientist magazine, 23 May 2007, page 48-51 Power struggle
As Geoffrey West of the Santa Fe Institute, New Mexico, and his collaborators try to extend power laws to social science, they are still fending off criticism about applying these laws to biology. Last February, at a meeting of the American Association for the Advancement of Science (AAAS), West and Carlos Martinez del Rio of the University of Wyoming, Laramie, exchanged arguments.
Martinez del Rio criticised the "universalist" doctrine by which all organisms obey the same quarter-power rule for metabolic rate versus size (see heart-rate Graph). The specific law, he said, may depend on whether one is comparing individuals, species or other taxa. Biologists treasure diversity and rebel against any theory that lumps all animals together under one law; many biologists have an "allergic reaction" to it, he claimed.
West defended his model, which attempts to explain biological power laws, just as vigorously. The quarter-power law, he said, is a "straw man", and by focusing their attacks on that, opponents of the theory are missing the point. "It is the dynamics of networks which give rise to that equation that are universal," he said, adding that the model's critics ignore the wide variety of variables that it explains.
Other researchers support the physics-inspired approach. Speaking from the audience at the AAAS session, Jim Collins, head of the National Science Foundation's directorate for biological sciences, pronounced, "This is a wonderful way to do biology." Steven Strogatz, a mathematician at Cornell University in Ithaca, New York, agrees: "I have no patience with the nitpickers. I think that it's a beautiful programme of research that West and his collaborators have had."
