Matter (Culture, #8)

by Iain M. Banks

The Shellworlds were mostly hollow. Each had a solid metallic core fourteen hundred kilometres in diameter. Beyond that, a concentric succession of spherical shells, supported by over a million massive, gently tapering towers never less than fourteen hundred metres in diameter, layered out to the final Surface. Even the material they were made from had remained an enigma – to many of the galaxy’s Involved civilisations at least – for over half a billion years, before its properties were fully worked out. From the start, though, it had been obvious that it was immensely strong and completely opaque to all radiation.

In an Arithmetic Shellworld, the levels were regularly spaced at fourteen-hundred-kilometre intervals. Exponential or Incremental Shellworlds had more levels close to the core and fewer further out as the distance between each successive shell increased according to one of a handful of logarithm-based ratios. Arithmetic Shellworlds invariably held fifteen interior surfaces and were forty-five thousand kilometres in external diameter. Incremental Shellworlds, forming about twelve per cent of the surviving population, varied. The largest class was nearly eighty thousand kilometres across.