The Square Kilometre Array: Divide and rule THE reputation of physics as the queen of sciences is reflected in the amount of money that governments are willing to spend on it. The Large Hadron Collider, Europe's latest particle smasher, cost around $9 billion and took a decade to build. But, just occasionally, other fields get to play with some big, taxpayer-funded kit of their own, too. On May 25th it was the astronomers" turn in the limelight. For several years two groups of countries, one consisting of Australia and New Zealand, and the other of several sub-Saharan nations led by South Africa, have been polishing their rival bids to host the Square Kilometre Array, a gargantuan, €1.5 billion ($1.9 billion) radio telescope first proposed in 1991 and designed to be the most sensitive ever constructed. After months of deliberation, the SKA's funding nations announced their decision: that the telescope would be split, and both groups would host a bit of it. In astronomy, as in particle physics, bigger kit is better kit. A larger telescope can gather fainter signals and produce sharper images. Radio astronomers already have a few supersized instruments to have fun with, notably the 305-metre-wide Arecibo telescope, carved into a Puerto Rican hillside in 1963, and the 100-metre Robert C. Byrd instrument in West Virginia, which, unlike the Arecibo dish, can be steered to point at different parts of the sky. Yet the physics of radio astronomy means that such mighty machines are, nevertheless, fairly crude. The resolving power of a telescope is determined by the ratio of its size to the wavelength of the radiation it is collecting. A typical optical telescope has a diameter a few million times the wavelength of visible light. Applying that sort of ratio to the SKA, which is designed to work with wavelengths measured in metres, would require a dish thousands of kilometres across. Dishes of the day Building such a dish is obviously impossible. But what is possible is to build many smaller dishes spaced a long way from one another, and to link them with clever computer algorithms so that they behave as if they were a single giant telescope. This is called interferometry, and is not a new idea. Many of the world's radio telescopes are linked in this way, providing far better resolution than any of them could alone. And several countries have already built collections of small, cheap dishes and networked them into more powerful "virtual" instruments. What makes the SKA special is its sheer scale. The design calls for around 3,000 individual receivers arranged rather like a spiral galaxy, with most of the telescopes concentrated in an inner core, and the rest arranged into a set of arms up to 3,000km (about 2,000 miles) long. Fibre-optic cables will link each of these dishes to a central processing area, where supercomputers will stitch their data together. When it is fully up and running (by 2024, assuming no big delays), the SKA will be more than 50 times more sensitive than any other radio telescope, and able to survey the sky thousands of times faster. That power will be used to investigate some of the biggest outstanding questions in astronomy. The SKA will join the hunt for gravitational waves - ripples in the structure of space predicted by Albert Einstein's general theory of relativity. It will probe the mysterious magnetic field that exists between the stars. It will allow astronomers to peer back into the universe's Dark Ages, a period roughly 400,000 to 800m years after the Big Bang, during which things cooled enough to allow the first large-scale objects, such as galaxies and clusters of galaxies, to form. And its resolving power will help with the search for extrasolar planets. But all that is a long way off, and the telescope will have to be built first. That the funding nations felt able to split the telescope in half reflects how closely matched the two bids were. Both consortia had constructed precursor telescopes that could be integrated into the SKA itself, and each bid had its advantages. The African bid, whose core will be in Northern Cape province, did well in the technical stakes, chiefly because the geography of the area allows a more efficient layout for the telescope, and also because electricity was thought likely to be cheaper there. The SKA will use about 110 megawatts when up and running, so power bills will be a significant expense. The Australasian bid, centred on the virtually unpopulated Shire of Murchison, in Western Australia, scored better for radio quietness (important to prevent interference), and on non-scientific factors such as political stability and the quality of the working environment. Nevertheless, many people - including Naledi Pandor, South Africa's science minister (pictured), who said as much in an official statement - reckon that the decision had more to do with politics than science. As with all such big projects, questions of national prestige intruded upon the technical judgments. And that was particularly so in Africa, where the SKA is seen as a good-news story for a continent still struggling to overcome its image as a violent and chronically unstable place. Building on two separate sites is possible because the SKA is really three telescopes in one, with different sets of receivers designed for low-, medium- and high-frequency work. The new plan calls for the low-frequency antennae to be given to the Aussies and the Kiwis, with the other types being built in Africa. Doing it that way will cost more, if only because the bidders will each need to construct their own fibre-optic network to link their antennae together. But although the funding nations may grumble, the prospect of a bit of extra money seems unlikely to go down too badly with the legions of radio astronomers who are, at last, going to see their dream machine built, nor with South Africa's scientific establishment, which will have a chance to show that it is up to the task of running a big project of this sort. South Africa has an impressive history of astronomy. The first permanent observatory in the southern hemisphere was built, in 1820, near Cape Town. If this project is equally successful, the country will have a strong future in the subject, too.