First: Yes, equal pull from a shell in all directions creates a mathematic '0' but only if the shell is an idealized form, and the only thing that exists in spacetime.
True. Assuming that we're talking about a sphere 1AU in radius, and construction is such that area density is within 1Kg/m
2, then the internal gravity from the shell might be as much as 0.0000000000000000000000001G. My apologies for the imprecision.
Gravity, also, is non-zero for any body making actual contact with the shell, especially if held in contact with the shell through centrifugal force.
Also correct. But if the spin is regulated so that objects "fall" towards the surface at 9.8m/s
-1, this results in a force of 1G, even if that force is not gravitational in nature. It's the same context as when fighter pilots are exposed to "high G's" during manoeuvres.
Furthermore, as soon as more than one person/being began walking/swimming around the interior 'floor' of the sphere the gravitational profile would no longer perfectly equal. Unless all the beings within this sphere walked around in some parrallellized cosmic synchronized-dance routine (which actually is in itself an interesting story idea) the equilibrium of the sphere would fail. This too is basic physics.
And when people walk on Earth, it changes the length of both the day and the year by some immeasurably small amount. Does that mean that planets are also unstable? Given that a Dyson Sphere would outmass the Earth by many orders of magnitude, it's not unreasonable to say that the effect would be literally unmeasurably small.
Besides, people don't need to be walk ing around in parallel loops for them to cancel each other out; they just need to be walking around at random; for everyone heading west, there's (on average) someone else with the same eastward momentum.
Second: I misspoke, by "center" I meant the central equator. Sorry.
Unless there's some thickening of the material at the equator, this would not happen. The gravitational attraction (unnoticeable as it is) is not towards the plane of rotation, but towards whatever part of the surface had a manufacturing defect that makes it thicker / more dense than the rest of the structure.[/quote]
Third: I perhaps invited the 'towers of pizza dough' with my analogy, but I was still thinking of the actual materials described in the story. Of course materials of sufficient resilience may exist for this undertaking, they simply weren't the ones described in the story.
Nor, of course, was a Dyson Sphere. If we're changing the basic structure, it seems only fair to also change the materials it's made of. The Globus Cassus is indeed flattened out, and has low enough tensions that the construction materials described would work admirably.