Table-top astrophysics: How to build a multiverse THE heavens do not lend themselves to poking and prodding. Astronomers therefore have no choice but to rely on whatever data the cosmos deigns to throw at them. And they have learnt a lot this way. Thus you can even (see article) study chemistry in space that would be impossible in a laboratory. Some astronomers, though, are dissatisfied with being passive observers. Real scientists, they think, do experiments. It is impossible - not to mention inadvisable - to get close enough to a star or a black hole to manipulate it experimentally. But some think it might be possible to make meaningful analogues of such things, and even of the universe itself, and experiment on those instead. Ben Murdin of the University of Surrey, for example, has been making white dwarfs. A white dwarf is the stellar equivalent of a shrunken but feisty old-age pensioner. It has run out of fuel and is contracting and cooling as it heads towards oblivion - but taking its time about it. As they shrink white dwarfs pack a mass up to eight times the sun's into a volume the size of Earth. A consequence of stuffing so much matter into so little space is that white dwarfs have powerful magnetic fields. Many aspects of a white dwarf's mechanics, including how long it will last, are thought to depend on its magnetism. But it is hard to measure. To make estimates, scientists examine the light a white dwarf emits for telltale patterns left by stellar ingredients like hydrogen. They then compare this spectrum with a theory, based on calculations from first principles, of how magnetic fields effect light emitted by hydrogen. The predictions agree with experiments up to the strongest fields mankind can muster - about 1,000 tesla, generated in a thermonuclear detonator. The problem is that the theory puts white dwarfs" magnetic fields at 100,000 tesla or more, well beyond humanity's reach. Dr Murdin built his own little white dwarf to see if the theory looked good. It consists of a silicon crystal sprinkled with phosphorus atoms. A silicon atom has four electrons in its outer shell. In a crystal, all four are used to bind it to neighbouring atoms. Phosphorus has five outer electrons. Insert a phosphorus atom into the silicon lattice and you are left with an unused electron. Since phosphorus also has one more proton in its nucleus than silicon does, taken together the extra particles resemble a hydrogen atom: a single electron tethered to a single proton. However, the extra electron is much less tightly held by the extra proton in this pseudo-hydrogen than it would be in real hydrogen. This weaker grasp means that it takes much less magnetism to make a given change in the pseudo-hydrogen's spectrum than it would for real hydrogen. So when Dr Murdin placed the crystal in a 30-tesla magnet at Radboud University in the Netherlands (his lab in Guildford lacks the necessary kit), he was mimicking the conditions in a 100,000-tesla white dwarf. And the spectrum came out looking just the way the theory predicted. A black hole in a bath... Creating a star in a laboratory is small beer compared with creating a black hole. This is an object that is so massive and dense that not even light can flee its gravitational field. Looking inside one is therefore, by definition, impossible. All the more reason to try, says Silke Weinfurtner of the International School for Advanced Studies, in Trieste, Italy. Dr Weinfurtner plans to make her black hole in the bath. The bath in question, properly called a flume, is a water-filled receptacle 3 metres by 1.5 metres and 50cm deep, across which carefully crafted trains of ripples can pass. In the middle of the tank is a plug hole. If the water going down the hole rotates faster than the ripples can propagate, the ripples which stray beyond the aqueous "event horizon" (a black hole's point of no return) will not make it out. They are sucked down the drain. Then the researchers will check whether the simulacrum affects water waves in a way analogous to that which general relativity predicts for light - itself a wave - approaching an astrophysical black hole. According to Albert Einstein's theory, a region immediately outside the event horizon of a rotating black hole will be dragged round by the rotation. Any wave that enters this region but does not stray past the event horizon should be deflected and come out with more energy than it carried on the way in. To detect this super-radiant scattering, as the effect is called, Dr Weinfurtner will add fluorescent dye to the water and illuminate the surface waves with lasers. The waves, often no bigger than one millimetre, can then be detected using high-definition cameras. Stefano Liberati, Dr Weinfurtner's colleague in Trieste, reserves the greatest enthusiasm for another aspect of the experiment. It might, if the researchers are lucky enough, offer clues to the nature of space-time. Could the cosmic fabric be made up of discrete chunks, atoms of space if you like, rather than being continuous, as is assumed by relativity? This problem has perplexed physicists for decades. Many suspect black holes hold the answer, because they are sites where continuous relativity meets chunky quantum physics. Waterborne holes serve as a proxy. Water is, after all, made up of just such discrete chunks: molecules of H₂O. As wavelengths fall - equivalent to rising energy - waves reach a point where the size of molecules may begin to influence how they behave. If Dr Weinfurtner and Dr Liberati observe some strange behaviour around their event horizons, theorists will be thrilled. ...and home-brewed universes Even benchtop black holes, though, are nothing compared with the ambitions of Igor Smolyaninov of the University of Maryland. For Dr Smolyaninov wants to create entire universes. The way light travels through the four dimensions of space-time is mathematically akin to how it moves through "metamaterial." These are substances with features measured in nanometres, or billionths of a metre, which let them bend light in unusual ways. For example they can force light to skirt along the outside of an object, hiding it from view as if behind an invisibility cloak. Space-time, too, bends light, in ways that depend on how mass is distributed within it. In principle, then, metamaterials ought to be able to mimic how light moves not just through the space-time scientists on Earth are familiar with, but also other possible space-times to which they do not, and never will, have access. Two years ago Dr Smolyaninov suggested an experiment with various metamaterials, corresponding to universes with different properties lashed together into a home-brewed multiverse. In a paper to be published in Optics Express, he and his colleagues report that they have succeeded. Rather than fine-tune metamaterial to exact specifications, which is finicky and expensive, the researchers used nanoparticles of cobalt, which are relatively easy to get hold of, and suspended them in kerosene. They then applied a magnetic field which, thanks to cobalt's ferromagnetic nature, arranged the particles into thin columns. In space-time terms the length of the columns is time and the two axes perpendicular to the length represent the three spatial dimensions in a real universe. To build his multiverse, Dr Smolyaninov added slightly less cobalt to the kerosene, about 8% by volume, than was needed to maintain stable nanocolumns. Natural fluctuations in the density of the fluid then lead to the spontaneous erection of transient nanocolumns - equivalent to space-times popping up only to fizzle and re-emerge elsewhere in the multiverse. They could be detected by their effect on polarised light shone through the material. Whether all this ingenuity unravels any cosmic truth is uncertain. Cliff Burgess, a theorist at Perimeter Institute for Theoretical Physics in Ontario, has his doubts. But he thinks that such experiments are nevertheless worth pursuing. "Like tap-dancing snakes," he says, "the point is not that they do it well, it is that they do it at all."