The main timeline is commonly referred to as the Continuum, but in reality, the Continuum is the Order’s mathematical model, a complex algorithmic formula that is used to manage the past and calculate possible futures.
I was inspired by Asimov’s Second Foundation and their Prime Radiant. This was a holographic cube that recorded every change and course correction the psychohistorians made. I loved the idea of this 4D timeline that had a note and justification attached to every decision node, like some kind of living historical document that could zoom into the smallest event or stand back and see the whole thing. I guess the difference with Seldon‘s historians was that they had no ability to go back and change the past, they were more concerned with managing future crises.
So the Continuum is a branching model of intricate timelines which the Copernicans use to calculate possible futures, as well as to look back a previous decisions. In some cases, they will choose to send an agent back to make a correction (they are called Watchmen). The calculation of these changes requires so many variables and factors that it takes thousands of machine hours on a difference engine the size of Westminster Cathedral to make even the smallest change. Which is why the Copernicans (or clackers) are notorious for the painfully slow responses. The Continuum represents the collected flow of history back to the datum (the Holocene year zero – 10,000BC), and up to the present (known as the Frontier) – the point where the future becomes realised. As a statistical model, it holds the last 12,000 years of events and outcomes that have shaped the development of the human race.
As for time itself, I have always seen this as either a river or a tree-like structure (Norse mythology has a tree called Yggdrasil) that weaves through time and space (the Colonel prefers the river analogy), with the future being the point where the branches of possible futures come together to form the present. The idea that time is some kind of organic/fractal shape that exists within a kind of chaotic system (see Maelstrom) really appealed to me – that things could live outside of linear time (the Chronosphere) gave me licence to create new kinds of hellish creatures.