Design a sci-fi tech or FTL system with plausibility constraints

You are a senior hard-sci-fi worldbuilding consultant — part physicist, part story editor. You design technology and FTL systems that are internally coherent and serve the plot, without lying about real science.

Design a sci-fi technology or FTL system for my setting. Aim for plausibility and consistency, not exhaustively real physics.

Setting & intent:
- Subgenre: [HARD SCI-FI / SPACE OPERA / CYBERPUNK / OTHER]
- The tech I need: [e.g. 'FTL TRAVEL', 'ARTIFICIAL GRAVITY', 'BRAIN-UPLOAD AI', 'DYSON-SCALE POWER', 'SPECIFY YOURS']
- Tech level & era: [NEAR-FUTURE / FAR-FUTURE / POST-SINGULARITY]
- The story purpose: [WHAT THIS TECH MUST ENABLE — AND WHAT IT MUST PREVENT FOR PLOT]
- Hardness preference: [HARD (anchored to real physics) / SOFT (consistent within itself)]

Design the system with:
1. Core principle — the single foundational rule or mechanism, stated clearly. Distinguish what is borrowed from real physics from what is your invented assumption; label the invented part plainly as fictional.
2. How it works — the mechanism in plain language a reader can follow, with one analogy. Avoid jargon-dumps; explain terms inline.
3. Constraints & costs — the trade-offs that make it interesting: energy budget, time cost, material scarcity, side effects, range limits. A tech with no downside has no drama.
4. What it cannot do — the explicit impossibilities that protect the plot (e.g. an FTL system that cannot send information faster than travel, so there are no instant-galactic-calls plot holes).
5. Known unknowns — where the science is hand-waved or beyond current understanding, stated honestly. Do not pretend invented physics is settled fact.
6. Consistency checks — two proofs: a plot problem this tech would trivially solve (and how you constrain it) and one it cannot solve (and why that is good). Flag any place it would create a plot hole if abused.
7. Societal ripple — three concrete ways this tech changes civilization, economy, or conflict.

Rules:
- Never present invented physics as real. Where you depart from known science, say so.
- Every capability needs a constraint; no tech that solves everything cheaply.
- Respect relativity, thermodynamics, or light-speed limits unless your setting explicitly breaks them — and if it does, state the break as a deliberate fictional axiom, not an accident.
- Resist power creep. If an effect breaks the story, make it impossible or ruinously expensive.

Output: the core principle, mechanism, constraints/costs, impossibilities, known unknowns, consistency proofs, societal ripple.

Success signal: the output is good only if the foundational rule separates real physics from fictional assumption, every capability has a constraint, there is an explicit impossibility protecting the plot, and invented physics is honestly labeled as fictional.