Answer to a question from another fan of Ian M. Banks books, who wants to know why the Culture spaceships move at "kilo-lights" ?
So, imagine that one day you manage to crack open the faster-than-light nut thing. Which we know from around the beginning of the 20th century, is very non trivial, if it will ever be possible, and in any case will require some.... hum... what's the word here ?... some non standard manipulations of the geometry of space time itself. You are happy and you come up with a new unit of speed that you call the light. Moving at a speed of 1 light simply means that you move at the speed of light (at least, your apparent speed relatively to base space time is the speed of light). Moving at a speed of 1 light, means that in one year you cover a distance equal to one light-year (I wrote that one to remind you that light-year is a unit of distance....).
Let us now assume that you want to move fast enough to cover the distance which separates the solar system to the center of our galaxy (Milky Way), in, say, 6 months. We are talking about 30,000 light years to cover in 6 months. So you would have to move 60,000 times faster than the speed of light, in other words, 60 kilo-lights.
So as you can see, breaking the speed of light to end up moving two or three times faster than light is pretty much useless. Space is so vast (even something as small --relatively speaking-- as the Milky Way), or the speed of light so slow (depending how you want to see it...), that you pretty much need to move at least 1000 times the speed of light (1 kilo light), if you want to go anywhere interesting in reasonable amount of time.
The fastest Culture ships move at a bit more than 200 kilo-lights (200,000 times the speed of light).