Writing on Centauri Dreams, metallurgist Peter Marinko discusses von self-replicating von Neumann probes. Those hypothetical automata move from one star system to the next, mining the system’s resources to build more replicates of themselves.
Marinko’s original post is here, and here is a follow-up. He argues that the problems such probes would encounter to mine local resources and build copies of themselves are much harder than commonly anticipated, and may be so hard that this is actually impossible.
Impossible? But isn’t this exactly what life on earth is already doing? Marinko acknowledges this, and here is his reply:
Life is not a counterexample to the closure problem; life is what closure looks like when you pay its actual price. A cell does not manufacture bearings to micron tolerance, does not need vacuum, does not require phase-pure silicon or reference metrology, and above all does not need to specify its output. It tolerates enormous error, discards most of its offspring, and lets selection curate the survivors — over billions of years, in a medium (liquid water, at moderate temperature, with an atmosphere and a gravity well) that supplies concentration and transport for free. Biology bought replication by abandoning precision, determinism, and speed, and by spending geological time as its currency.
A von Neumann probe cannot make that trade. It must arrive at a specified place, build a specified artifact to specified tolerances, and do so in decades. The moment we relax those requirements enough for a biological strategy to work — accept vast error, accept mostly-failed offspring, accept deep time — we no longer have an engineering project; we have seeded a biosphere and lost the ability to say what it will become.
There may be an immovable Pareto frontier that makes it impossible for a system to be highly coherent, but also flexible enough to self-replicate under a wide range of conditions. This is the same coherence tradeoff I’ve written about here.
Nehaveigur 