Taking Feynman’s Path to nanotech, or even studying it seriously, would require finding answers to a number of open questions. These questions, however, are quite important and knowing the answers will be invaluable in understanding the envelope of possibilities for future manufacturing technology.
- Is it in fact possible to build a compact self-replicating machine using macroscopic parts fabricators and manipulators? We know that a non-compact one is possible — the world industrial infrastructure can replicate itself — and we know that a compact microscopic replicator can work, e.g. a bacterium. But the bacterium uses diffusive transport, associative recognition of parts by shape-based docking, and other complexity-reducing techniques that are not available at the macroscale.
- Not quite the same question: how much cheating can we get away with? In KSRM theory, it’s common to specify an environment for the machine to replicate in and some “vitamins,” bits that the machine can’t make and have to be provided, just as our bodies can’t synthesize some of the molecules we need and must get them pre-made in our diets. For the purpose of the Feynman Path, we can cheat in a very legitimate way: any part or capability that can be provided on whatever scale it’s needed from the outside, is fine. We start with control signals — no autonomous AI necessary! If we synthesize molecular gears by chemistry, wonderful. If we can provide single-crystal chunks of silicon or diamond to carve, do it. If we can polish surfaces with low-angle e-beams, wonderful. What’s left for out machine to have to make?
- What are the roadblocks we’ll have to invent our way around? The classic example is that electromagnetic motors work poorly at small scales, so we have to shift to electrostatic ones. Here are some others (by no means a complete list):
- Near the atomic scale, we hit an “ontological phase boundary” where we have to quit thinking of our material as continuous (and able to assume arbitrary shapes, like circles) and start treating it as discrete and lumpy.
- Gravity essentially vanishes, but the adhesion of parts to each other increases tremendously.
- Lubrication by oily fluids quits working.
- Heat dissipation rates increase as surface/volume ratios do.
… and so forth.
- What kinds of forming techniques will work? There are a number of solid free-form fabrication technologies at the macroscale, which can in theory be adapted to smaller-scale forms where molten droplets, or whatever, are replaced by mechanochemical deposition tips. At larger scales, for decent surface tolerance, SFF must be followed by surface finishing. Will this work all the way down?
- And finally, how do we see what we’re doing? It’s all very well for Heinlein to talk about scanners, but existing techniques like scanning probes only work over relatively flat surfaces, not in the crowded interior of a milling machine or the like. We’ll probably have to build physical surface scanners — but existing shop practice makes extensive use of physical contact measurement anyway, so this probably won’t be such a major change.