Curate, connect, and discover
I’ve been hearing these words timelines and AUs being thrown around and use interchangeably as if they are one and the same when that’s anything further from the truth. Which is why my pretentious ass is gonna try to explain the difference a timeline and an Alternate Universe as well as how to tell the difference.
First off, we have to mention that Undertale clearly possesses a multiverse -an Underverse if you will- in which every Universe resides in and within each universe is a timeline. Unlike the multiverse though, which can contain an infinite number of alternate universes, a universe may only have one timeline active at a once. The only way for a new timeline to become active is through reloading or resetting back to a previous point in time. Meanwhile, a new universe is always created due to the actions of transdimensional beings who reside outside of the Under-verse (aka: us). It is impossible for anyone that resides within the Under-verse to create a new universe by themselves without the help of a transdimensional being. At best, they can create new timelines, but that’s it.
So since we establish the basics, let’s get down to the key difference. Timelines are essentially the variables of a universe. They can be radically different from one another, but so long as they are a possibility within a universe, they can exist. For example: Chara never falling into the underground, Asriel and Chara both surviving, or the war never occurring in the first place. While the results may sometimes be radically different from another, they are all possible.
As for alternate universes, they are essentially the non-possibilities of a universe, no matter what, it is physically impossible for them to exist as a timeline in Undertale. Generally, Anything that operates under different mechanics and universal laws then Undertale or radically alters or swaps the souls of characters counts as an AU. Note: just because characters have a different personality than in canon, that does not always mean their soul: IE, their subconscious, inner self, or whatever, has been altered as well.
For example, let’s say Alphys decides to build a body for Napstablook alongside Mettaton as well. This event will effect and alter their personalities from that point forth, but who they are inside remains the same, plus that action is a possibility so it would count as being an alternate timeline.
If it was something like Sans always had the personality of Papyrus, vice-verse, or whatever. So long as they possess zero or barely any similarities to their Undertale self, then they exist within an alternate universe. Like if Monster Kid now acts exactly like Joseph Stalin or if Toriel acts like a crazy Florida lady that hates children, then that change would count it as an AU.
Now I mentioned before how only one timeline can be active in a universe at a timeline. Which may have some of you thinking “Doesn’t that mean all the different fanworks of Undertale are alternate Universes then even if the change is rather minor?”
Just because only one timeline can be running at once, it doesn’t mean that other timelines can’t exist alongside it just in a sort of limbo mode. Take for example a race between twelve different horses. The people within the active timeline perceive horse #3 as the winner, while we can perceive any of the horses winning or none at all, we just have to choose the outcome we want to see and we’ll see it.
So no, not every fanwork is an alternate universe. Some are AUs and others timelines, which are which, you decide because I honestly don’t have the time to go through every single one because there are literally thousands. I’m just doing this because I find it fun applying logic and rules to a world with spaghetti loving Skeletons and Metrosexual ghost-bot David Bowie.
By the way, do you want a theory/view on souls or determination next?