Fair point. I guess there's another assumption, that the rings are built at a constant rate. In fact, if we make the typical assumption that the economy grows e

Fair point. I guess there's another assumption, that the rings are built at a constant rate. In fact, if we make the typical assumption that the economy grows exponentially (which might hold for all but the longest term models), it could be possible to build newer rings at an exponentially faster speed by constructing more and more of them simultaneously.