There is nothing too lore-heavy or speculative for this post, just something I thought about after listening to a video about the Three-Body Problem of physics, and how it relates to the relationship between Vex and paracausality.
What makes the Vex dangerous is their ability to perfectly simulate the physical world. A Vex unit was able to create 227 perfect simulations of the Ishtar Collective team, and the Infinite Forest simulates countless realities in the Vex's efforts to find a path forward where they win. In a sense, the Vex simulations result from solving Chaos Theory, or at least brute-forcing a solution.
To explain, "chaos" is not about "randomness", but it is about the [b]unpredictability of a system[/b]. We can predict what will happen in a system up to a point, but we can't predict it past the predictability horizon. For Vex simulations to be so precise, to even be able to perfectly simulate a conscious mind, they'd have to solve for this. Or, in the brute force scenario, they perfectly capture the initial conditions and set the simulation off on its own, given the sentience that Vex simulations demonstrate.
This is where paracausality messes up the Vex and their simulations. At the moment, the Vex cannot simulate paracausal forces, though things might change post-Final Shape. Because they cannot simulate them, they cannot take into account the influence they have on a system's initial conditions. For instance, Quria couldn't simulate Oryx, only make a bootstrap of who Oryx once was, Aurash.
In other words, Light and Darkness add new variables to reality the Vex are incapable of accounting for, adding new levels of unpredictability to a system's initial conditions. The same goes for the Worm and the Ahamkara, given how they're able to drastically change the conditions of a system through the Anthem Anatheme.
This can even be extended to the Final Shape and the Taken power. "The combination of a chosen past and limitless future into a perfect forever"; "imposing a singular origin and all decisions that follow"; "reforming matter in a self-contained reality, where the creator defines past, present and future".
In others, the complete elimination of unpredictability and the imposition of an absolute, deterministic system.
Again, nothing too speculative.
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Almost. They changed the Vex to allow a partial simulation of paracausality in base D2 and we saw that in Curse and other times we’ve been in the Forest. In Curse we saw them simulating a Guardian fireteam and they were using their Light. We’ve also seen them make Hive who use their abilities. They just waved their hands and made it happen. That’s why the threat they supposedly posed to us in Curse was never explained or hinted at. Their inability to harness Darkness hasn’t been explained yet but their use of one of those Ishtar clones to interact with Clovis Bray could be a clue. If Exo Minds, which are just digital copies, can harness power then these digital clones should be able to as well. That’s a way to make them scary without really making anything new