>Use of a dense matrix is an artificial constraint you've imposed yourself, but that only disproves feasibility of your proposed solution, not the entire problem in general.

The use of a dense matrix is the traditional way of solving the problem. The issue is that it solves the wrong problem. You need a dense tensor which requires more storage than the world currently has for an economy of 20 people.

Social interactions are sparse until they aren't. If you think otherwise try to estimate what every Europeans interaction with Gavrilo Princip was on 27 June 1914 vs 28 June 1914.

As for gravitation: I'm very happy for the planets and asteroids out there. Unfortunately the economy isn't a solar system.