Worldgate complexes
(NB: The term "complex" is used to describe any group of worldgates that can be demonstrated (by analysis of their relative power or control structures) to be naturally related to one another. The term does not indicate anything about the number of gates: a complex can be as few in number as two, though routinely there are more than that in one of these groupings. Complexes have been seen to have a slight bias toward odd-numbered groupings, groupings based on prime numbers, and groupings of eleven.)
Worldgate complexes occur in two basic forms -- natural and artificial. The second type is often associated with or based on an incidence of the first.
Worldgates tend to occur naturally, over time, in places of unusually high population concentration. The minds of living beings have an effect on the structure of the physical universe around them; the constant pressure and desire of their minds for things they want and don't have tends to "fray" universal and sub-universal structure in the area. The more beings that are packed together in one space, the more tightly they are packed, and the longer the packing is sustained, the more likely that the local structure of spacetime will fray sufficiently to tear open -- either temporarily or permanently -- and allow access to other realities. Odd disappearances of objects, or (more usually) of people, is normally the first evidence that gating is starting to happen in a given area.
The population pressure required to spawn or maintain a worldgate is normally expressed by an equation known as the demobaric formula or gating pressure formula, primarily involving two interacting varables: population concentration per square meter, and and percentage of population expressed as a function of planetary population. (There are other variables of much lesser importance in the equation which, for simplicity's sake, will not be discussed here.) The relationship between these variables is expressed in the threshold constant. Each planet has its own version of the formula and the constant: the relationship between them changes over time as the population of any given planet shifts. For example:
One of the earliest spontaneously-occurring worldgate complexes is at Chur in southeastern Switzerland. Chur grew extremely quickly after its founding in the early Bronze Age, positioned as it was at the crossing of two major trade routes associated with Alpine passes. It was, however, a very small city, so that the inflowing population were very tightly packed inside its walls. When the city's population reached a significant percentage of the Earth's population at that time (the actual amount does not have to be very high to be statistically significant) as balanced against the appropriate concentration of human beings per square meter, it reached the threshold constant, and the first worldgate manifested itself near Welschdoerfli. (This also demonstrates a poorly-understood phenomenon called "offset", in which the first-manifesting gate of a complex will routinely appear near a significant population concentration but not actually in the middle of it.) However, Chur's gating complex could probably not have come into being much later than the Bronze Age, as its population as compared to that of the rest of the Earth would then have been too small. The larger the Earth's population becomes, the more difficult it becomes for new gates to manifest, as even very large cities have trouble achieving the threshold constant unless they are skyscraper-heavy. However, once the initial gate appears, later ones may appear (as Chur's Calanda gate did in the 1400's) without reference to lower population numbers: the tendency of a gate to spawn is usually a function of its power structures and the way they interact with the local environment, not the local population's growth or decline.)