River deep, mountain high: Of long-run knowledge trajectories within and between innovation clusters

We bring together the topics of geographical clusters and technological trajectories, and shift the focus of the analysis of regional innovation to main technological trends rather than firms. We define a number of inventive clusters in the US space and show that long chains of citations mostly take place between these clusters. This is reminiscent of the idea of global pipelines of knowledge transfer that is found in the geographical literature. The deep citations are used to identify technological trajectories, which are the main directions along which incremental technological progress accumulates into larger changes. While the origin and destination of these trajectories are concentrated in space, the intermediate nodes travel long distances and cover many locations across the globe. We conclude by calling for more theoretical and empirical attention to the "deep rivers" that connect the "high mountains" of local knowledge production.