Topological inversion

So despite the relative recency of the world-behind-the-mirror concept, it's already overused. Let's twist it a bit instead.

Consider a world which, effectively, has usable space where we have impermeable material, and impermeable material where we have usable space. The exact mechanism is not important: perhaps there is a race of xornlike creatures to whom air is toxic and corrosive; perhaps the beings-in-the-walls actually exist ‘above’ the world, ‘phased out’ slightly in a manner that isn't possible in a region of space with such low energy density as open air. All that's necessary is that the boundary be impermeable both ways.

Now if the boundary really is completely impermeable, there would be no way of knowing of the other side's existence. (Well, modulo the fact that, a. the boundary isn't immutable because objects aren't, and b. oh hey, physics doesn't work like that. But we shall handwave both of these away.) However, if the bounadry is only mostly impermeable, we have interesting consequences.

Transportation of goods will almost certainly be cheaper on one side or the other. Which is which depends on fiddly irrelevant details of the technobabble, each side's social development, and plot requirements; but we'll call the one where transportation is more expensive A-side, and where it is cheaper B-side. Given the ability to transport goods across the A-B boundary at reasonably available locations and relatively low amortized prices (possibly ‘anywhere, anytime’; possibly only at fixed but constructible portals), one can save money on the transportation of A-side goods between points in A-side by transporting them through B-side.

(TBC)