In this question I asked about the possibilities of what a boundary might be like, with emphasis on the storytelling.
Now I'd like to investigate what a hard boundary would mean in quantum mechanics, in a more “hard SF” manner.
Imagine a bubble of walled-off spacetime that occurred in the lab so it could be examined up close an personal. Whether the inside is in stasis, destroyed, censored, or whatever is not important here. The interesting thing is that a boundary exists and quantum-mechanical wave functions are prohibited from entering the region contained by the boundary.
Imagine, perhaps, that it's an energy well of arbitrary height. Or, I'm intrigued by @Beta’s remark, “The mirror is 100% legal; all fields and space-time curvature are symmetrical at the boundary, or equivalently the boundary condition allows no normal components of anything.” Or, it just somehow prevents a wavefunction collapse from ever choosing that position.
The phenomena resulting from this should be benign. It needs to interact with normal matter! The bubble won’t just fall through the Earth like a neutrino, or fly off at the speed of light. Rather, it needs to be massive so it can stay put in the room, follow the standard path through spacetime like a massive object, and be held and pushed by normal matter. E.g. it could be placed in a stand and stay put in the lab, even as the Earth turns under it.
I'm thinking that something like Pauli Exclusion could be made to work: the electrons of matter would feel the excluded bubble nearby and distort the shape of the wave function, requiring higher energy. Making it a hard solid object is the main issue!
Second, what might be happening very close to the edge? If it's pushed and must move like a billiard ball, but it is not a point-mass, the force must somehow be communicated around the entire space. If it's infinity rigid there would be problems with motion due to relativistic effects.
Anybody up to communing with the Hamiltonian?