GR allows for the possibility of travel in time by the space-dragging effect of an infinitely long, dense, rotating cylinder. Q: Why does the cylinder have to be infinitely long? A: It doesn't, an extremely long cylinder will do - long enough to eliminate "edge effects". Q: Why even extremely long? Are we talking dozens/thousands/millions of light years? Would one light year be too short a cylinder? Ten thousand miles? My question is less about how long it needs to be and more about why it has to be so long? i.e. How the does vast length of the cylinder affect the um ... effect?