Alexander Bolonkin has proposed the possibility of manipulating nucleons to produce stable, macroscopic structures of nuclear matter at zero pressure (which he calls "AB-matter"), by analogy with the nanotech ideas of directly manipulating atoms to build high-tech materials.
The basic claim is that an unbounded number of alternating protons and neutrons can be arranged in a fiber held together by residual nuclear force and a small contribution from magnetism due to the nucleon magnetic moments, and prevented from collapsing and held rigid by electrostatic repulsion. Superstrong macroscopic structures can then be built by combining these basic nuclear matter needles.
Bolonkin is a legitimate scientist (PhD in aerospace engineering), but not a nuclear physicist, and has gotten papers on this stuff published, but not in physics journals (for example, "Femtotechnology: Nuclear AB-Matter with Fantastic Properties" in American Journal of Engineering and Applied Sciences and "Femtotechnology: Design of the Strongest AB Matter for Aerospace" in Journal of Aerospace Engineering). Furthermore, nobody else seems to have published anything on this topic, all of which makes me rather skeptical of his claims.
So, ignoring the issue of how you'd construct it in the first place (assume we find some helpful Cheela to do it for us or something), could a linear arrangement of alternating protons and neutrons at zero pressure (i.e., not confined in a neutron star or something) remain stable and not collapse into one big nucleus, or segment itself into a bunch of individual nucleii? And does it make any difference if the fiber is kept under tension by some external means?
Best Answer
Everything that's physically possible occurs naturally. Atoms arrange themselves naturally into one- and two-dimensional structures (lipid and polymer chains in 1D, graphene and nanotubes from soot in 2D) and so we're able to envision techniques to mass-produce those. But there's no evidence that nucleons form chains in nature (at least outside of neutron stars), and so no reason to believe we could construct such a phase.
In fact we can be a little more quantitative. Different nuclei have different shapes; the sign of their quadrupole moment tells you whether they are primarily cigar-shaped ("prolate") or coin-shape ("oblate"), and octupole and higher moments describe more complicated shapes than ellipsoids. There are a small number of nuclei which are super-deformed in their ground states, with their long elliptical axis two or three times the length of their short axes. But those superdeformed nuclei tend to have masses $A\approx 80\mathrm{-}100$, so an ellipsoid with 3:1:1 axis ratio is a pretty long way away from a one-dimensional chain.
You have to remember the reason that the periodic table has finite size: the nuclear force, which holds nucleons together, is a contact force with a potential like $$ V \sim \frac \alpha r e^{-r/r_0}. $$ For the attractive part of the nuclear interaction the range $r_0$ is set by the pion mass to about 1 fm. If you separate two protons by several femtometers, as in a heavy nucleus, the attractive interaction is exponentially weakened relative to the electrical repulsion. I see no reason to expect this to be any different for nucleons in a hypothetical chain.