Now if you’re talking quantum vacuum thrusters, exploring exactly how that works should also give you a good idea of what space is! ;-)
The conundrum is: what is the rotation without the relative perspective of the stationary observer? What pulls the rope taut?
Ok, this seems to be a clash of intuitions explained by reference frames —
When you imagine an object in deep space or its own empty universe, you intuitively use an “inertial reference frame”, where “the physics of a system in an inertial frame have no causes external to the system”. I.e. you do not see any “fictitious” forces like the centrifugal force, so the rope would not be taut.
https://en.m.wikipedia.org/wiki/Inertial_frame_of_reference
If you are imagining the rope to be taut, you are imagining a force pulling it taut, but this force that you’re imagining is only explained by physics if you’re using a rotational (non-inertial) reference frame.
So if your system has unexplained “centrifugal” forces, those forces can be modelled as the system rotating within an inertial reference frame, without having any evidence of any “non-rotating” matter.
It just so happens that deep space is pretty close to an inertial reference frame, so your mind sort of makes that assumption.
Or, in other words, you could measure and perceive the rate of the rope’s rotation by analyzing the forces in system, without having any physical external point of reference.
Eh? You measure the rotation against the thing that performed the “rotate the rope” step in the experiment.
Thought experiments that contain the step “now magically remove X from the observable universe” should be treated with caution! :-)
If you change it to just “There exists a universe empty except for a rotating set of rocks connected by a rope”, rotation needs no meaning as it’s one of the axioms. Also, there is no observer in that universe, so you’re making a pretty dangerous assumption about the ability to observe a universe without being a part of it.