It's just struck me that a civilisation propagating through space can be envisaged a bit like light-cones, as in GR, except that the velocity factor isn't c, it

It's just struck me that a civilisation propagating through space can be envisaged a bit like light-cones, as in GR, except that the velocity factor isn't c, it's b (the maximum speed at which beings can move), which kinda depends on the beings. But I think the cones thing is still interesting; it means that for any given maximum travel speed, there must be other civilisations that can never know anything about you, nor you about them.