42 | NewScientist | 8 May 2010

Miracle matterSuperconductors, superfluids and supersolids

It is tempting, faced with the full-frontal assault of quantum weirdness, to trot out the notorious quote from Nobel prize-winning physicist Richard Feynman: “Nobody understands quantum mechanics.”

It does have a ring of truth to it, though. The explanations attempted here use the most widely accepted framework for thinking about quantum weirdness, called the Copenhagen interpretation after the city in which Niels Bohr and Werner Heisenberg thrashed out its ground rules in the early 20th century.

With its uncertainty principles and measurement paradoxes, the Copenhagen interpretation amounts to an admission that, as classical beasts, we are ill-equipped to see underlying quantum reality. Any attempt we make to engage with it reduces it to a shallow classical projection of its full quantum richness.

Lev Vaidman of Tel Aviv University, Israel, like many other physicists, touts an alternative explanation. “I don’t feel that I don’t understand quantum mechanics,” he says. But there is a high price to be paid for that understanding – admitting the existence of parallel universes.

In this picture, wave functions do not “collapse” to classical certainty every time you measure them; reality merely splits into as many parallel worlds as there are measurement possibilities. One of these carries you and the reality you live in away with it. “If you don’t admit many-worlds, there is no way to have a coherent picture,” says Vaidman.

Or, in the words of Feynman again, whether it is the Copenhagen interpretation or many-worlds you accept, “the ‘paradox’ is only a conflict between reality and your feeling of what reality ought to be”.

WHAT DOES IT ALL MEAN?

FORGET radioactive spider bites, exposure to gamma rays, or any other accident favoured

in Marvel comics: in the real world, it’s quantum theory that gives you superpowers.

Take helium, for example. At room temperature, it is normal fun: you can fill floaty balloons with it, or inhale it and talk in a squeaky voice. At temperatures below around 2 kelvin, though, it is a liquid and its atoms become ruled by their quantum properties. There, it becomes super-fun: a superfluid.

Superfluid helium climbs up walls and flows uphill in defiance of gravity. It squeezes itself through impossibly small holes. It flips the bird at friction: put superfluid helium in a bowl, set the bowl spinning, and the helium sits unmoved as the bowl revolves beneath it. Set the liquid itself moving, though, and it will continue gyrating forever.

That’s fun, but not particularly useful. The opposite might be said of superconductors. These solids conduct electricity with no resistance, making them valuable for transporting electrical energy, for creating enormously powerful magnetic fields – to steer protons around CERN’s Large Hadron

Collider, for instance – and for levitating superfast trains.

We don’t yet know how all superconductors work, but it seems the uncertainty principle plays a part (see “Something for nothing”, page 39). At very low temperatures, the momentum of individual atoms or electrons in these materials is tiny and very precisely

known, so the position of each atom is highly uncertain. In fact, they begin to overlap with each other to the point where you can’t describe them individually. They start acting as one superatom or superelectron that moves without friction or resistance.

All this is nothing in the weirdness stakes, however, compared with a supersolid. The only known example is solid helium cooled to within a degree of absolute zero and at around

25 times normal atmospheric pressure. Under these conditions, the bonds between

helium atoms are weak, and some break off to leave a network of “vacancies” that behave almost exactly like real atoms. Under the right conditions, these vacancies form their own fluid-like Bose-Einstein condensate. This will, under certain circumstances, pass right through the normal helium lattice – meaning the solid flows, ghost-like, through itself.

So extraordinary is this superpower that Moses Chan and Eun-Seong Kim of Pennsylvania State University in University Park checked and re-checked their data on solid helium for four years before eventually publishing in 2004 (Nature, vol 427, p 225). “I had little confidence we would see the effect,” says Chan. Nevertheless, researchers have seen hints that any crystalline material might be persuaded to perform such a feat at temperatures just a fraction above absolute zero. Not even Superman can do that. n

Michael Brooks was the Science party candidate for the constituency of Bosworth in the UK general election this week (www.scienceparty.org.uk)

” At room temperature, helium is normal fun. Close to absolute zero, though, it becomes super-fun”