I won’t bore you with the details of the conference I’ve been at in Dresden, but I will mention this. As an example of a complex network, one of the speakers showed a little animation of movement of commercial aircraft across Europe. The animation consisted of a map of europe, with little dots showing the position of every commercial aircraft over the course of one 24 hour period (speeded up of course). I assume this data came from air-traffic control.
The effect it generated can probably be mimicked quite well at home (in summer). Take an atlas, open it at a map of Europe, place big dabs of honey or jam at cities like London, Frankfurt, Rome, Amsterdam etc, and little dabs at smaller places like Lisbon, Dresden, Palermo, etc etc, and leave it in a nice sunny place by an ants nest. Then observe the effect an hour or so later.
How air traffic control can cope with so many aircraft movements is incredible.
(NB if you are wondering what this has got to do with physics, it’s to do with the pattern of connections between airports. There are a few global ‘hub’ airports (e.g. Frankfurt) which connect to hubs in other continents, and many smaller local hubs that connect to the big hubs, etc. He was using this as an analogy to the map of connections that exist between neurons in the brain.)