Originally published August 17, 2016

Will There Ever Be a Material to Replace Steel?

We constantly hear about new or exotic materials which are stronger than steel, but for many uses it turns out that steel is still the best stuff available to use. When is one of these new materials actually going to be able to replace steel?

Quite possibly never. Fancy materials like kevlar and carbon-fiber and even titanium alloy are only stronger than steel by weight. Their sole advantage over steel is lightness. If you compare strength by volume, they do much more poorly. This means that if you were to, for instance, try to make a sword out of titanium, it would have a much fatter blade than a steel one, in order to have the same strength and heft. And it would be inferior at holding an edge. You’d have to, like, insert a separate bit of tungsten carbide or something along the edges, and have a way to replace parts of that when they get chipped.

(Such a design might be a pretty good way to make a sword, actually. A Japanese katana is a bit like this: whereas a western sword is hardened and then tempered, so the whole blade is springy and tough but not as hard as it could be, a katana is glass-hard at the edge but soft along the spine. If it’s damaged, the edge will chip, but the back part won’t even spring back into shape if bent. It has to be hammered straight again. Then the edge has to be ground down where it chipped. A titanium-plus-tungsten-carbide design would make the middle of the sword about as tough as tempered steel, but give it an edge able to cut notches into any metal. While it lasted.)

If talking about swords sounds too exotic and arbitrary, let’s instead talk about crowbars. The tip of a prying tool has to be strong, hard, tough, and thin. It has to be rugged enough to exert thousands of pounds of force while fitting into very narrow gaps. You can’t make a crowbar out of carbon fiber, and even the best titanium alloys won’t be any improvement over steel.

Any materials that really are stronger than steel are generally not hard or tough, and materials harder than steel, such as diamond, are generally brittle and easily shattered. It’s entirely possible that there’s no such thing as an exotic material that can outdo steel, even for such a mundane application as making nuts and bolts — that no possible combination of atoms can get there.

Which, to a science fiction reader like me, begs the question of whether you could make something that is not based on atoms. Is there some kind of exotic substance or field in the far reaches of physics that could replace ordinary chemical elements as building materials? Old-timey SF is full of improbable superstrong materials invented by advanced technology. Could any of them ever exist?

As far as we can currently tell, the answer is no. We might be able to make some incremental improvements, such as putting alloys into a glasslike amorphous state instead of a crystalline one, but that’s probably all.

But I did once see a physics paper which described a theoretical solid state far stronger than ordinary matter. All you need to do to create it is subject ordinary hydrogen to a magnetic field of a billion gauss (100,000 tesla) or more. Heavier elements would also work, but would require more magnetic field strength as atomic weight increases. The paper speculated that this substance might exist on the surfaces of neutron stars. In such a field, the electron clouds around the hydrogen nuclei elongate and finally merge with each other, so the atoms form a kind of polymer. The resulting substance is very dense — far heavier than any metal, though far lighter than neutronium — and very strong. As best I can recall without being able to access the text of the paper, sideways to the magnetic field the strength was calculated to be somewhat proportional to the density, but lengthwise along the field lines, it would be way stronger than that.

There are three problems with this idea. First is that it’s impossible to make a magnetic field like that to order, or to shape it for the convenience of the objects you want to create. Second is that the effects of such a field on all the other stuff around the superstrong material would make it impossible to fit in amongst anything else made of ordinary matter. It would, for instance, be lethal to any living thing in the area. And the third is that this paper has not received much followup as far as I have been able to find, and what I’ve been able to track down in later work often criticizes the assumptions of earlier authors, and says their calculated numbers may have substantial errors. It appears that “linear molecules” in intense magnetic fields are an accepted concept, but whether it would be superstrong in proportion to its density, or only in proportion to normal matter, is not clear to me. The key value is probably the binding energy per atom, and I’m seeing estimates of that all over the map, from a few times that of common materials to around a hundred thousand times. In newer calculations the smaller numbers seem to be predominating.

The most comprehensible modern article I could find so far about neutron star linear molecules is this one. As far as I can understand it, it seems to imply that in a billion gauss field, linear molecules would have about twenty times the tensile strength per weight of steel, and to boost that to a hundred times would take a field of hundreds of billions of gauss. Such substances could remain solid at temperatures approaching a million degrees, or ten million at the higher magnetic field strength. Density would be a few thousand times that of steel at the lower strength, and a hundred times greater still at the higher intensity. So the strength rises only slightly faster than the density.

It might be interesting to calculate the amount of electric current you’d have to circulate around a cylindrical space to maintain linear-molecule conditions inside. I suspect that you’d end up with such a furious mass of rotating electrons that the synchrotron radiation alone would blast anything around it into ruin, and this leakage would require a titanic power supply to replenish so the field wouldn’t collapse in a moment. My rough understanding is that to create a billion gauss, each meter of length enclosed by the coil might have to be carrying something like ten trillion ampere-turns. That’s about a kilogram of electrons per second. So yeah, I don’t think that’s gonna happen.

I mentioned neutronium. What about that? Unfortunately for our dream, it’s not a solid, it’s a superfluid. Not to mention that it can only exist under extreme pressure, and would otherwise first explode, then undergo rapid beta-decay over the next minute. Unlike “linear molecules”, it apparently has no resistance at all to flying apart.

But there is a phase of high-density matter which precedes neutronium, which is known among scientists as “nuclear pasta”. In this phase, protons and neutrons bind into supersized nuclei, which form clumps and strings and sheets within a fluid matrix of ions or unbound protons and neutrons. These clumps, which according to their shapes are called “gnocci”, “spaghetti”, or “lasagna”, are super-rigid: the hardest substance there is. In terms of resistance to lateral deformation, it would be ten billion times as hard as steel. But this does not mean it has any tensile strength, or would hold together if removed from the titanic pressure which formed it.

Another possibility which sounds attractive on paper is “exotic atoms”. The idea is to make atoms in which some or all of the orbiting electrons are replaced with other negatively charged particles. Since all other such particles are much heavier than electrons, their orbitals will pull much closer to the nucleus, making the whole atom smaller. This in turn makes molecular bonds much tighter, increasing their strength because the attractive forces involved obey the inverse square law, weakening with distance.

The trouble comes when you ask what particle to use. You can’t invent new ones; the universe gives you a fixed menu. Of the available choices, the least bad is probably the muon. The problem is that it decays in two microseconds, and the tightly bound environment of an atom doesn’t help — it makes them break down faster, rather than helping stabilize them, as happens in neutronium. And if a molecule has too many muons, adjacent nuclei might pull so close together that they undergo cold fusion. But at least this raises the possibility that though it’s a terrible building material, it might be an excellent power source, right? Probably not: anything that provides copious muons will almost certainly consume more power than the fusion could produce.

Perhaps someday we might meet an advanced alien civilization — possibly one so advanced that they don’t even regard us as intelligent life, and can’t even remember what it was like to ever not know the answer to a question about science. We might expect that their stuff would be made of magically wondrous materials, but then end up finding that like us, they still have to make things out of steel.

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