Some argue that the so-called Big Bang that created the universe was actually a Big Crunch, and that the Chaos that was Before was condensed and contained and constricted, named Azathoth by its creations. Regardless, where before there was No Thing, there now was Some Thing, and that thing had 11 dimensions – three spatial dimensions, two temporal dimensions, and six “higher” dimensions. The totality of these dimensions defines the “Cartesian Plane” of reality.
A time machine is simply a vehicle that can travel in five dimensions rather than three.
The two temporal dimensions are Causality and Probability. Human languages struggle with presenting these dimensions without analogizing them to spatial dimensions, so forgive our metaphor. Causality is Event A leading to Event B; Probability is the likelihood of event B being Potentiality A, B, C, or D. All Potentialities exist within time – so in the Cartesian field of the cosmos, there is a timeline where Event A reified Potentiality A, a timeline where Event A reified Potentiality B, and so on. As event piles upon event, these lines move around the plane, Potential Points converging into Causal points and diverging into new lines. A Temporal Nexus is any point where multiple significant timelines converge or diverge.
Once a conscious entity has existed through the quantum collapse that converts Potential Points into Causal Points, that point is now “hardened” for them. That is to say, if you know about a section of timeline (event A led to event B1), it becomes very difficult to change it for yourself (event A leading to event B2). In many ways, ignorance is a powerful tool in the time traveler’s kit.
A Fixed Point in Time (or Temporal Node) is a Temporal Nexus that the Time Lords have “hardened” so that it is very difficult (or impossible) to move to any but a specific Causal Point from one’s current place on the timeline. In a sense, they have made the universe aware of its own future at these points. The Web of Time is the metastructural framework created by the manipulation of temporal space by the Gallifreyans to create Fixed Points in Time that guarantee the evolution and existence of The Time Lords. Traveling outside of the web in a TARDIS is very difficult; the Time Lords would insist it is impossible.
Known Temporal Nodes include the destruction of Pompeii by Mt Vesuvius in AD 79, the American raid on the coastal town of Innsmouth, MA in AD 1928, and the destruction of Bowie Base One on Mars in AD 2058. Entities from outside of the Web of Time can enter the Web of Time more easily at these Temporal Nodes but only the most powerful or skilled time-travelers can “jump the track” from the Fixed Point into a timeline that has not already been spun into the Web.
If you are a time traveler posting in this thread, please do not “spoil” the future for our more linear posters.