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When introducing APM to a nontechnical audience what often comes up is the question:
Why not go further down in size right down to the nucleons (protons and neutrons) and build with them instead?

There a great number of reasons why going down further to nucleons is (for all practical purposes) not possible:

  • We can only interact with nuclei in statistical ways (scattering experiments) shooting high energy particles
  • Putting nucleons together at normal conditions (like present on earth) does not allow formation of complex structures like molecules. For all we know it always equilibrates to a (sometimes somewhat deformed) ball. (fluctuating much more comlicated than the electron hull of an atom - there are no analytic solutions analogous to atomic orbitals - solutions which form the basis of directed covalent bonds and the basis for molecules)
  • At normal conditions (like present on earth) adding nucleons to a nucleus goes well just up to a point (the end of the periodic table of elements ~94 protons) after that the "assembled" nucleus falls apart (nuclear decay). Really big ones really fast. (Emitting destructive radiation).

Neutron stars

The only place where bigger stable conglomerations of nucleons are possible are neutron stars. But even there molecule like structures are very unlikely to be present when looked at it from an anthropic principle viewpoint, that is there is no need for nucleons being capable of forming molecules to make life in our universe possible.

More importantly neutron stars cannot yet be investigated experimentally in great detail, since they are still very far beyond our current reach of spaceflight.

Our understanding of neutron stars is still extremely limited. Its all still in a vague research state. No highly reliable models are available and highly reliable models are of absolute necessity for conducting predictive exploratory engineering. So beside SciFi for entertainment something like femtotechnology is not amenable to targeted design. Of couse one can never exclude that one day we'll find something interesting on/in neutron stars but this is in the realm of lucky discoveries. There's only an accidental path with the target "femtotechnology" that may not exist (in the femtomachinery sense). Or better: is extremely unlikely to exist (gut feeling of the author).

(Side-note 1: This pattern can be generalized. See: "Visions not yet amenable to targeted design")

(Side-note 2: There's a SciFi novel called Dragon's_Egg about a civilization of a femtoscale life-form on a neutron star.)

Moving within

A recurring futurologist topic is escaping the "malthusian trap" (running out of resources by civilization growth ending up in a zero sum game). Beside the obvious idea of spreading out into space (the big) there is the quite radical idea of going into the small instead (possibly simulated worlds) run on more and more miniaturized computers.

Currently the atomic scale is the absolute limit beyond which we cannot see any potential practical machine like use. The nuclear scale lies well beyond that (current) limit.

Subatomic but not nuclear

Instead of going down to nucleons to pack more functionality in the same volume there's an other option too. While atoms have a fixed size, in stiff machine phase nanosystems they can be placed at discernible location increments that much smaller than atoms. This works the better the lower the temperature is since low temperatures make the amplitudes of thermal vibrations smaller. At some point quantum effects are bound to show up but stiff machine phase nanosystems are prone to behave non-quantum mechanically so in typical monolithic gemstone based cog-and-gear nanomachinery this will only happen at extremely low temperatures.

Positioning is analog (angle of an axle, shift of a rod) so information can't be encoded in a digital combinatorial way (8 levels correspond to only 3 bits 2*2*2 = 8 and not 8 bits 2^8 = 256) and high cooling effort may not justify the little gain.

To pack more computing power in a given space one of course could design nanomechanics deliberately for quantum effects (low-inertia, tightly constrained - beside cold). With this we finally end up with quantum computing. But it seems rather likely that non mechanical approaches are better much suited for quantum computing (less extreme colling requirements due to much lower mass).
Note that quantum computing is weaker than matching full parallelism (that physically cant be implemented).

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