Superstuff: When quantum goes big
- 16 January 2012 by Michael Brooks
- Magazine issue 2847. Subscribe and save
- For similar stories, visit the Quantum World Topic Guide
The world of superstuff is a cold one, existing only within a few degrees of absolute zero (Image: Julien Pacaud)
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 heliumor 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.
A tale of two particles
These are strange behaviours indeed, so what explains them? Both superfluidity and superconductivity are products of the quantum world. Imagine you have two identical particles, and you swap their positions. The physical system looks exactly the same, and responds to an experiment exactly as before. However, quantum theory records the swap by multiplying their quantum state by a "phase factor". Switching the particles again brings in the phase factor a second time, but the particles are in their original position and so everything returns to its original state. "Since switching the particles twice brings you back to where you were, multiplying by this phase twice must do nothing at all," says John Baez at the Centre for Quantum Technologies in Singapore. This means that squaring the phase must give 1, which in turn means that the phase itself can be equal to 1 or -1.
This is more than a mathematical trick: it leads nature to divide into two. According to quantum mechanics, a particle can exist in many places at once and move in more than one direction at a time. Last century, theorists showed that the physical properties of a quantum object depend on summing together all these possibilities to give the probability of finding the object in a certain state.
There are two outcomes of such a sum, one where the phase factor is 1 and one where it is -1. These numbers represent two types of particles, known as bosons and fermions.
The difference between them becomes clear at low temperatures. That is because when you take away all thermal energy, as you do near absolute zero, there aren't many different energy states available. The only possibilities to put into quantum theory's equations come from swapping the positions of the particles.
Swapping bosons introduces a phase change of 1. Using the equations to work out the physical properties of bosons, you find that their states add together in a straightforward way, and that this means there is a high probability of finding indistinguishable bosons in the same quantum state. Simply put, bosons like to socialise.
In 1924, Albert Einstein and Satyendra Bose suggested that at low enough temperatures, the body of indistinguishable bosons would effectively coalesce together into what looks and behaves like a single object, now known as a Bose-Einstein condensate, or BEC.
Helium atoms are bosons, and their formation into a BEC is what gives rise to superfluidity. You can think of the helium BEC as a giant atom in its lowest possible quantum energy state. Its strange properties derive from this.
The lack of viscosity, for instance, comes from the fact that there is a huge gap in energy between this lowest state and the next energy state. Viscosity is just the dissipation of energy due to friction, but since the BEC is in its lowest state already, there is no way for it to lose energy - and thus it has no viscosity. Only by adding lots of energy can you break a liquid out of the superfluid state.
If you physically lift a portion of the superatom, it acquires more gravitational potential energy than the rest. This is not a sustainable equilibrium for the superfluid. Instead, the superfluid will flow up and out of its container to pull itself all back to one place.
Superconductors are also BECs. Here, though, there is a complication because electrons, the particles responsible for electrical conduction, are fermions.
Fermions are loners. Swap them around and, as with swapping your left and right hand, things don't quite look the same. Mathematically, this action introduces a phase change of -1 into the equation that describes their properties. The upshot is that when it comes to summing up all the states, you get zero. There is zero probability of finding them in the same quantum state.
We should be glad of this: it is the reason for our existence. The whole of chemistry stems from this principle that identical fermions cannot be in the same quantum state. It forces an atom's electrons to occupy positions further and further away from the nucleus. This leaves them with only a weak attraction to the protons at the centre, and thus free to engage in bonding and other chemical activities. Without that minus sign introduced as electrons swap positions, there would be no stars, planets or life.
So how do the electrons in superconductors form into BECs? In 1956, Leon Cooper showed how electrons moving through a metal can bind together in pairs and acquire the characteristics of a boson. If all the electrons in a metal crystal form into such Cooper pairs, these bosons will come together to form, as in superfluid helium, one giant particle - a BEC.
The main consequence of this is a total lack of electrical resistance. In normal metals, resistance arises from electrons bumping into the metal ions bouncing around. But once a metal becomes a superconductor, the electron-pair condensate is in its lowest possible state. That means it cannot dissipate energy and, once the Cooper pairs are made to flow in an electrical current, they simply keep flowing. The only way to disturb superconductivity without raising the temperature is to add energy another way, for example by applying a sufficiently strong magnetic field.
Though superfluids and superconductors are bizarre enough, they are not the limit of the quantum world's weirdness, it seems. "There is yet another level of complexity," says Ed Hinds. That complexity comes into play below 1 kelvin and at more than 25 times Earth's atmospheric pressure, when helium becomes a solid. This form of helium plays havoc with our notions of solidity. Get the conditions right and you can make solids pass through each other like ghosts walking through walls.
Such an effect was first observed in 2004 by Moses Chan and Eunseong Kim at Penn State University in University Park, Pennsylvania. They set up solid helium in a vat that could rapidly rotate back and forth, inducing oscillations in the solid helium. They observed a resonant vibrational frequency which they interpreted as indicating that there were two solids in the vat, which were passing through each other.
Admittedly the two solids do not fit our usual definitions. One was made up of "vacancies", created when helium atoms shake free of the lattice that forms solid helium. The gaps left behind have all the properties of a real particle - they are so like real particles, in fact, that their quantum states can lock together to form a BEC. The solid helium is also a BEC, and it is these two condensates that pass through each other.
Chan and Kim's observation is still somewhat controversial; some researchers think there is a more prosaic explanation to do with deformations and defects in the helium lattice. "There is a lot of activity, several theory notions and experiments of interest, but no real agreement," says Robert Hallock of the University of Massachusetts at Amherst.
Nonetheless, even the fact that it might be possible to create solids that aren't really solid shows just how odd superstuff can get. And it's all because the world has a fundamental distinction at its heart. Everything, from human beings to weird low-temperature phenomena like liquids that defy gravity, stems from the fact that there are two kinds of particles: those that like to socialise, and those that don't. Sound familiar? Perhaps the quantum world isn't that different from us after all.
Extreme SuperatomsSuperfluids, superconductors and supersolids owe their bizarre behaviour to the formation of a sort of superatom inside them, known as a Bose-Einstein condensate (BEC).
But might it be possible to create such a state outside of a liquid or solid? It took researchers many years, but in 1995 a team at the University of Colorado at Boulder and the US National Institute of Standards and Technology finally succeeded in coaxing a gas of rubidium into a BEC, its lowest possible quantum state. The breakthrough won team leaders Carl Wieman and Eric Cornell, together with Wolfgang Ketterle at the Massachusetts Institute of Technology, the 2001 Nobel prize for physics.
When Wieman and Cornell made their condensate, their lab briefly became home to the coldest place in the universe, just 20 nanokelvin above absolute zero. It wasn't the only BEC in the cosmos, though, even discounting superfluid or superconductor experiments that might have been taking place at exactly the same time.
Last year the Chandra X-ray telescope discovered that the core of a neutron star called Cassiopeia A, which lies 11,000 light years away from Earth, is a superfluid. One teaspoon of neutron star material weighs six billion tonnes and the intense pressure from the outer layers is enough to squeeze the core into a BEC. Yet, despite the name, the core of a neutron star isn't exclusively made of neutrons; it contains a portion of protons too, which also form a BEC. You can think of this as a superfluid or, because the protons carry electrical charge, a superconductor.