Culture is transferred through interactions between cultural groups. As a result, cultural exchange is a function of the rate of interaction between groups. We can think of the rate of transfer as a sort of flow, mediated by the topology of interactions. This topology can be geographical topology (geographical features that impede movement will reduce interaction, and so reduce cultural flow), communications topology (a phone network that spans a continent increases cultural flow across the continent), and social topology (media bubbles may lead to divergent evolution of geographically proximate cultures). Say that a feature of the topology has “high impedance” if it slows the flow of culture, and low impedance if it allows for more rapid flow of culture.
Over time, the influences on the topology of interaction change. Early on, geography exerted a strong influence. Even today, populations with mountains between them will tend to have less cultural exchange than we might otherwise expect from for groups that are physically proximate as the bird flies. For a long time, oceans had a high impedance, but as ocean-navigating technology improved, oceans became low impedance, and lower in particular than certain feature of land terrain. Moreover, the impedance of oceans does not scale linearly with distance (the fixed costs of crossing a straight and crossing an ocean are similar).
Air travel further affected the topology of interaction. Large urban centers that became travel hubs had more interaction with each other, and physical proximity mattered less. Communication technology further reduced the significance of physical proximity, and culture often flows with lower impedance across the internet than it does between physically proximate populations.
The change in culture flow can lead to significant turbulence: changing topology through the introduction of the internet effectively opened the floodgates between population who had very little flow prior to that change. When very different cultures suddenly have very low impedance between them, the result is a steep cultural gradient, resulting in turbulence in the form of hatred, violence, political turmoil, bigotry, etc.
One final thing to note is the ascendance of social topology in mediating culture flow. The role of media bubbles, class divisions, assortative mating, and language have all increased significantly in shaping culture, as these tend to dictate who interacts with who and how culture is passed between groups. But social topology is much more fluid than previously dominant aspects of topology. It also has strong feedback effects, so that culture shapes social topology, which shapes culture flow, which shapes culture etc.