“History too has an inertia. In the four dimensions of spacetime, particles (or events) have directionality; mathematicians, trying to show this, draw what they call “world lines” on graphs. In human affairs, individual world lines form a thick tangle, curling out of the darkness of prehistory and stretching through time: a cable the size of Earth itself, spiraling round the sun on a long curved course. That cable of tangled world lines is history. Seeing where it has been, it is clear where it is going – it is a matter of simple extrapolation. For what kind of Δ v would it take to escape history, to escape an inertia that powerful, and carve a new course? The hardest part is leaving Earth behind.”
―
Kim Stanley Robinson,
Red Mars
