What is superconductivity, and when will we all get maglev trains and unlimited electrical power?

 

 Superconductivity is one of those concepts — like electron spin or time dilation — that seems somewhat esoteric, but which, if mastered through technology, could truly revolutionize the world. It’s a concept we already use heavily today, in various applications, but an ability to create it in ever less-hospitable environments could be the key to bringing many of the dreams of science fiction to life.
What is superconductivity? Put simply, superconductivity is the property of having zero (not almost zero, not vanishingly close to zero, but zero) resistance to the movement of electrons. It’s more than just really really low resistance, because in order to have true superconductivity in, say, a wire, one end of that wire needs to be able to receive 100% of the energy put in at the other. This means that if we pump some electricity into a closed superconducting loop, that loop would hold its charge indefinitely. The electrons will simply go round and round and round, never stopping, and never losing any of their energy to resistance, magnetic interference, or even heat loss.

A sample of the exotic superconducting material bismuth strontium calcium copper oxide (BSCCO-2223).

Yet there’s one big problem with the closed-loop thought experiment: it implies that superconductivity is a state in which a material can simply be. However, all superconductors currently known have to be actively kept in that state through the input of energy; we have to keep them below a certain critical temperature, and often supplement this by applying a magnetic field to knock out any few remaining internal forces. The temperature thresholds are incredibly low, and thus incredibly expensive to maintain. Aluminum, for instance, has a superconducting temperature threshold of 1.2 Kelvin, or -271.95 °C.
The physics involved are either quite simple or quite complex, depending on the material. In pure metals or simple metal alloys, superconductivity comes about basically when the atoms of that material have been cooled (slowed) to the point that electrons are not scattered as they try to move through the lattice of metal atoms. That’s great, but stopping atomic movement (heat) is very difficult, as mentioned. More complex materials, some of which can achieve superconductivity above cryogenic temperatures, are decidedly within the realm of quantum weirdness, and have to do with transient interactions between electron pairs.
This means that our infinite-energy-loop could only exist so long as we’re expending significant energy to keep the loop in a superconducting state, and that sort of defeats the point of lossless energy storage, now doesn’t it?

The world’s first superconducting cable. Not even bleeding edge science can look particularly advanced, on a construction site.

The current applications for superconductors are all limited by their temperature requirements. MRI machines are incredibly expensive, largely because they require exotic substances like liquid helium to cool metal coils to the point that they can conduct enough electricity to create the strengths of magnetic field required to bulk-reorient the molecules of the human body. Much of the shocking expense of the Large Hadron Collider came from the same source. Even research into fusion power is being slowed by the almost unbelievable expense and difficulty of creating huge magnetic tokamak rigs for plasma confinement.
This is why our Holy Grail is not superconductivity, which has been achieved in everything from super-cooled porcelain to super-cooled diamond, but practical superconductivity. This is also referred to as high-temperature superconductivity or (for the truly ambitious) room-temperature superconductivity. The threshold of “high temperature” is technically around 30K, but in conversation these days, it’s generally locked to the limitations of real-world application. A high temperature for a superconductor is, basically, any temperature that scientists can create for an acceptable energy cost. If we could suddenly cool a superconducting material to 29 Kelvin with very little trouble, 29 Kelvin would effectively become a high temperature, for our purposes.

Maglev trains would be the logical choice in almost every case, if not for how prohibitively expensive they are.

Ask yourself: What are the technological barriers to making Africa into humanity’s all-powering electrical battery? There are, in a general sense, two. One is the ability to collect and store a large enough portion of the sunlight falling on that desert continent, and the other is the ability to actually get that stored energy around the world, to the homes, offices, and factories where it’s needed. With affordable-enough and practical-enough superconducting material, we could ship our electrons across the Atlantic. We could turn municipal transit lines into magnetically levitated bullet trains. Hospitals could have more MRI machines than they require, and lend some out for home use. In general, it could allow the large-scale application of technologies previously only possible on the small scale, or in special, well-funded labs.
We’re nowhere near those thresholds, today. Different crystal structures can do the work (diamond works, as mentioned), but what scientists have found is that they can achieve the same results in complex mixed materials — though the physics of precisely why that is are currently unclear. The best superconductors ever created are cuprates, or cooper-ion containing substances, but the most advanced of these still require cooling to -140 °C, and are quite difficult and expensive to produce.

A diagram of the internal workings of the superconducting cable in Essen, Germany.

That’s not to say there haven’t been any successes. Considering simple electrical efficiency, which accounts for a loss of roughly 6% of electricity in power transmission, the German city of Essen recently installed a kilometer-long superconducting cable for transferring grid power. This cable uses liquid nitrogen to achieve a working temperature of 60K, or -206°C. That’s very impressive, and the use of liquid nitrogen for cooling makes it at least somewhat affordable, but we’ll need far better to start mass-replacing the electrical infrastructure of the entire world.
Superconductivity is a major area of research for both academics and industrial scientists, but it’s very possible that an eventual solution will be found on a blackboard first, and in the laboratory second.