[ben_w]

> Take a tube and spin it around you in space, it does nothing to you because there's no other forces working on you.

Yes, it does — if you are in contact with the structure, the force you feel is the outside acting against your inertia to keep you in uniform circular motion. There is a layer of air in contact with the structure at any moment, so it ends up co-rotating, so anywhere inside except the axis of rotation itself will feel a force proportional to the distance from the axis.

> When we're talking about a generation ship, we're talking about something a little larger than the Gravitron.

Naturally. You can still spin them along and axis: https://en.m.wikipedia.org/wiki/O%27Neill_cylinder

We are a long way from been able to build such structures, and I have doubts about the suitability of human political psychology given the travel times involved and how long countries last for on average, but the physics of spin-gravity is fine, even though there may be noticeable Coriolis effects depending on scale.