Self-replicating nanobots converting planets into Dyson swarms or mega-computers is a science fiction staple — the default future in stories featuring artificial superintelligence. But according to LessWrong essayist SurvivalBias, physics throws two major obstacles into the plan. First: material composition. Rocky planets like Mars are mostly iron and magnesium. Icy moons are water and hydrogen. No single nanobot design works on both. You'd need different models for different worlds — which then face the same problem within each world, since composition varies locally. You'd end up needing roads, trucks, and factories anyway. At that point, why nanobots? Second: energy. You need enormous energy to break rock into its constituent elements. Chemical reactions can't provide it — rock is already energetically exhausted. Nuclear fusion at nano-scale is magic-level technology. Solar power is the only realistic option. Even at Mercury, the solar system's best location, with perfect efficiency — and ignoring the cost of reassembling materials and launching them into orbit — nanobots could eat through rock at about two point four meters per year. Mercury's radius is roughly twenty-four hundred kilometers. Do the math: billions of years. Conclusion: planetary disassembly via self-replicating nanobots isn't just difficult; it's physically implausible on useful timescales.

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