If I don't ask you you know what a city is, but if I ask you you don't. This happens in many areas where there are many possibile definitions that you really never consider in everyday life but as you start to think about it then you see that many interpretations are possibile.
A city can be an area defined by historical tradition, "the city of London", an aggregate of buildings and a mesh of infrastructures that are surrounded by a "void" (but where do you place the boundary of the city, since they get fuzzier as you move from the center to the periphery) and in this case Yokohama and Tokyo are just one city. You might decide that a city is what the municipality defines... And many more.
A city may also be seen as an aggregate of people, and the word "aggregate" may assume different meanings: proximity, density, commonality/complementarity of activities, culture, language. Chinese quarters may be consider as cities within a city, as an example.
The social aspects that characterise a city and that may eventually be taken as a measuring stick for defining a city have been difficult to pinpoint. But today the data generated by everyday interactions may provide a new tool to explore the concept of city.
This is what Bin Jiang and Yufan Miao at the University of Gävle in Sweden have been doing.
Just by having a cell phone (a powered one...) you are providing location data. And by using a social network you provide information on the proximity of your acquaintances.
The two researchers have analysed location data of people in the US on BRightkite, a social network, over a period of three years. Just by looking at location data they created an avalanche of dots on the US map. Than they joined nearby dots by segments forming triangles and averaged the size of the resulting triangle. At this point they visualised only the triangles whose size was below the average and voila: natural cities emerged from the map.
The availability of an avalanche of data and their correlation (Big Data) will provide in the future a new way to look at and to define what a city is. And these cities will continually evolve providing real time information to city planner and to citizens. The work being done in the Urban Life and Mobility along with the one in Future Cloud are going to change the way we think about a city and will eventually morph our everyday behaviour into our instantaneous perception of our city. We will be deciding on the spot, by feeling the instantaneous city, which way to go, what transportation to use and quite often where to go and what to do.
This is something we are doing every day from millennia in our social relation: you change your behaviour, your thinking and your plans depending on who you happen to meet and what the social interaction creates. It is natural to have this instantaneous behaviour "creation" in social interaction. Well, my bet is that we are going to have also instantaneous behaviour "creation" in our interaction with our city. And that will change our idea of city forever!