Rabbit hole · 3 connected questions
Do the specific quantum, semiclassical, and material constraints that arise once you try to realize closed timelike curves (for example via wormholes or Gödel-type metrics) actually forbid those solutions from being physically realized?
How these converge
All three topics are not merely about “time travel” in the loose sense but about the same concrete tension: general relativity admits solutions that contain closed timelike curves (CTCs), yet making those solutions physically real runs into the same, specific barriers. Wormholes are one class of GR solutions that could produce CTCs if arranged appropriately; CTCs are the precise mathematical structure that would enable time-return trajectories; and arguments from quantum gravity and semiclassical effects target exactly those structures (chronology horizons, stress-energy requirements, backreaction) as mechanisms that might prevent CTCs from being realized. The shared question is therefore whether the physical constraints identified by quantum/semiclassical analyses and by material/exotic-matter requirements eliminate the GR solutions that mathematically permit time travel.
Where these converge
Closed timelike curves as the common target
Each topic centers on CTCs as the specific spacetime feature whose physical existence is in question: the wormhole discussion treats a concrete way to produce CTCs; the CTC topic defines the mathematical object and its causal implications; and the quantum-gravity topic evaluates whether CTCs survive when quantum effects are included.
Wormholes as an explicit construction vulnerable to the same constraints
Wormholes are a GR mechanism that can be configured to create CTCs (e.g., with time-offset mouths). The feasibility issues cited for wormholes—need for exotic stress-energy, stability against perturbations, and susceptibility to quantum effects—are precisely the same failure modes that quantum-gravity analyses invoke to argue against time-machine formation.
Quantum and semiclassical backreaction as a putative prohibition mechanism
Quantum-gravity and semiclassical arguments (e.g., divergent stress-energy at chronology horizons, instability of would-be time machines, or emergent-spacetime loopholes) provide the concrete mechanisms by which GR solutions containing CTCs might be rendered unphysical—this is not a mere philosophical objection but specific dynamical effects that act on the same geometries discussed in the wormhole and CTC topics.
Physical realizability and observability as the practical tie
All three answers converge on the distinction between mathematical possibility in GR and physical realizability: no observed time machines or CTCs exist, and the same obstacles (exotic matter, stability, quantum backreaction) are repeatedly invoked as reasons why the mathematically allowed solutions may be inaccessible in our universe.
The chain
Keep going: open any topic above to find its own related questions.