Superstuff: When quantum goes big

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In the coldest labs in the universe, bucketfuls of liquid flow uphill and solids pass through one another

FOR centuries, con artists have convinced the masses that it is possible to defy gravity or walk through walls. Victorian audiences gasped at tricks of levitation involving crinolined ladies hovering over tables. Even before then, fraudsters and deluded inventors were proudly displaying perpetual-motion machines that could do impossible things, such as make liquids flow uphill without consuming energy. Today, magicians still make solid rings pass through each other and become interlinked - or so it appears. But these are all cheap tricks compared with what the real world has to offer.

Cool a piece of metal or a bucket of helium to near absolute zero and, in the right conditions, you will see the metal levitating above a magnet, liquid helium flowing up the walls of its container or solids passing through each other. "We love to observe these phenomena in the lab," says Ed Hinds of Imperial College, London.

This weirdness is not mere entertainment, though. From these strange phenomena we can tease out all of chemistry and biology, find deliverance from our energy crisis and perhaps even unveil the ultimate nature of the universe. Welcome to the world of superstuff.

This world is a cold one. It only exists within a few degrees of absolute zero, the lowest temperature possible. Though you might think very little would happen in such a frozen place, nothing could be further from the truth. This is a wild, almost surreal world, worthy of Lewis Carroll.

One way to cross its threshold is to cool liquid helium to just above 2 kelvin. The first thing you might notice is that you can set the helium rotating, and it will just keep on spinning. That's because it is now a "superfluid", a liquid state with no viscosity.

Another interesting property of a superfluid is that it will flow up the walls of its container. Lift a bucketful of superfluid helium out of a vat of the stuff, and it will flow up the sides of the bucket, over the lip and down the outside, rejoining the fluid it was taken from.

Though fascinating to watch, such gravity-defying antics are perhaps not terribly useful. Of far more practical value are the strange thermal properties of superfluid helium.

Take a normal liquid out of the refrigerator and you find it warms up. With a superfluid, though, the usual rules no longer apply. Researchers working at the Large Hadron Collider at CERN, near Geneva, Switzerland, use this property to help accelerate beams of protons. They pipe 120 tonnes of superfluid helium around the accelerator's 27-kilometre circumference to cool the thousands of magnets that guide the particle beams. Normal liquid helium would warm up considerably if used in this way, but the extraordinary thermal properties of the superfluid version means its temperature rises by less than 0.1 kelvin for every kilometre of the beam ring. Without superfluids, it would have been impossible to build the machine that many physicists hope will reveal the innermost secrets of the universe's forces and building blocks.

The LHC magnets have super-properties themselves. They are made from the superfluid's solid cousin, the superconductor.

At temperatures approaching zero kelvin, many metals lose all resistance to electricity. This is not just a gradual reduction in resistance, but a dramatic drop at a specific temperature. It happens at a different temperature for each metal, and it unleashes a powerful phenomenon.

For a start, very little power is needed to make superconductors carry huge currents, which means they can generate intense magnetic fields - hence their presence at the LHC. And just as a superfluid set rotating will keep rotating forever, so an electric current in a superconducting circuit will never fade away. That makes superconductors ideal for transporting energy, or storing it.

The cables used to transmit electricity from generators to homes lose around 10 per cent of the energy they carry as heat, due to their electrical resistance. Superconducting cables would lose none.

Storing energy in a superconductor could be an even more attractive prospect. Renewable energy sources such as solar, wind or wave power generate energy at an unpredictable rate. If superconductors could be used to store excess power these sources happen to produce when demand is low, the world's energy problems would be vastly reduced.

We are already putting superconductors to work. In China and Japan, experimental trains use another feature of the superconducting world: the Meissner effect.

Release a piece of superconductor above a magnet and it will hover above it rather than fall. That's because the magnet induces currents in the superconductor that create their own magnetic field in opposition to the magnet's field. The mutual repulsion keeps the superconductor in the air. Put a train atop a superconductor and you have the basis of a levitating, friction-free transport system. Such "maglev" trains do not use metal superconductors because it is too expensive to keep metals cooled to a few kelvin; instead they use ceramics that can superconduct at much higher temperatures, which makes them much easier and cheaper to cool using liquid nitrogen.

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