Circumstantial simultaneity is a concept that is responsible for a great deal of Weird Plot Shit in Homestuck and, depending on interpretation, may or may not be a type of time shenanigans. Doc Scratch the concept as follows:
Circumstantial simultaneity is a concept more complex than its temporal analogue, and is valuable for examining the properties of paradox space. It is the agent responsible for the major cosmic event which pre-extinction Alternians came to refer to as The Great Undoing. The same concept rules the innumerable lesser events by which this critical moment shall be catalyzed, including the break, my employer's arrival, the detonation of a very powerful bomb, and my own death. It is an abstraction weaving together the fortunes of otherwise perfectly disparate chronologies, such as those bound to a pair of distinct sessions.
Rather frustratingly, he concludes that
It's not fully comprehensible to a mortal mind, and the length I will go to explain it to you will not extend beyond this sentence.
Circumstantially simultaneous events generally are not temporally simultaneous for the simple reason that if they take place in different continuities of time, as they frequently do by being in different regions of paradox space (e.g. different sessions), it is not possible to make meaningful temporal comparisons. However, events that are circumstantially simultaneous often have some kind of similarity of circumstance, hence the name.
Examples of circumstantially simultaneous events:
- Rose and Vriska, both Heroes of Light, Jack Noir. Furthermore, both are seeking to defeat him in part to avenge the deaths of those he has murdered: Rose avenging John, his Dad and her Mom; Vriska avenging Karkat, Terezi and probably the other remaining trolls. Finally, it is worth noting that both are killed as a direct or indirect result of the duels; Rose is directly killed by Jack (although ), while to prevent the outcomes she has foreseen in Vriska's actions. In fact, depending on the interpretation of Doc Scratch's clock, Spades Slick (who is the trolls' version of Jack) may have even contributed to Vriska's death by .
- The deaths of Karkat and Terezi by Gamzee's hand in an Aradiabot's , and by Jack Noir's hand in Vriska's .
- According to Doc Scratch, his own and ; he of paradox space as that which contains both events.
- kids' universe by Jack Noir attacking Bilious Slick with the Red Miles, and the destruction of the trolls' universe by Spades Slick shooting Snowman. These two events were circumstantially synchronized in the Tumor, providing it with the mass-energy required to create the Green Sun. of the
- Caliborn, the Red Miles on post-scratch Earth and post-scratch Derse are circumstantially simultaneous; the Red Miles on Earth are in fact Jack's from the aforementioned destruction of Bilious Slick. Unusually for circumstantial simultaneity, the events also happen to be temporally simultaneous, at least from Dirk's perspective.
Examples of circumstantially simultaneous countdowns:
- The Mysterious Countdown and the countdown on B1 session's Tumor. They both are first seen at 10 hours 25 minutes, and end with circumstantially simultaneous events.
- If the journeys of three years of both the Prospitian battleship with John, Jade and the planets onboard and the trolls' Ectobiology Lab with Dave, Rose and the remaining trolls onboard are seen as countdowns, they are circumstantially simultaneous: both begin with circumstantially simultaneous events and are believed to end with temporally simultaneous events. John indirectly acknowledges this when discussing with Jade about never seeing Dave or Rose when dreaming; he first claims that they do not sleep "at the same times" then realizes that "at the same times" doesn't make sense because they are in two different universes.
Another case that might possibly constitute a type of circumstantial simultaneity is Calliope intentionally striving to ensure that the session she shares with Caliborn is in some sense synchronised with the session of the post-scratch kids.
It is also possible that many of the callbacks and recurring gags could be explainable in-universe as examples of circumstantial simultaneity.