Everything is just roughly made based on what we discussed. Feel free to modify any information through proposal feature
Build three connected worlds together:
A fantasy world(Name TBD) that runs on magic.
A sci-fi world(Name TBD) that runs on physics.
A merged world(Name TBD) where the two have collided and now coexist.
Both worlds used to be one. They split apart, and now they are coming back together. The merge threatens to destroy both of them. Our job as a club is to build each world deeply enough that we can tell the main story: the merge, and the people caught in it.
The universe consists of two worlds with different fundamental laws.
Fantasy world | Sci-fi world | |
|---|---|---|
Governing principle | Magic | Physics |
Medium filling space | Aether | Vacuum(?) |
Fundamental particles | Magical carrier particles | Standard particles |
Native beings | Elves, dragons and other magical beings | Alien species |
Shared beings | Humans, ordinary animals | Humans, ordinary animals |
The two worlds share a common origin, so their fundamental particles and forces are broadly similar. Because the split was relatively recent on a cosmological scale, the particles of each world can still interact with those of the other.
Beyond these two worlds lie other realms, which are distant and unconnected. These two worlds are unusual because they sit close together, a consequence of having once been one.
The large-scale structure of each world, including whether the sci-fi world is a single galaxy and the shape of the fantasy world, is not yet established in canon.
(not confirmed It can be changed) Before the split there was a single world. It held both magic and physical law, and it was ruled by a family of gods. Humans and many ordinary animals already lived there at that time.
(not confirmed It can be changed) A conflict among the ruling gods split the world in two. According to the most widespread account, the science god left and took physical law with him, which created the sci-fi world. The world that remained, the fantasy world, had no physics of its own and came to sustain itself entirely through magic.
Anything that existed before the split, including humans, retained its nature in both worlds. Anything that arose afterward is native to only one. Elves, for example, are a product of magic and have no counterpart in the sci-fi world.
(not confirmed It can be changed) After the split, the two worlds developed independently. The sci-fi world became a galaxy of space-faring civilizations, empires and corporations. The fantasy world developed its own peoples, kingdoms and magical traditions.
The length of this period is not established. Around 100,000 years has been proposed.
In the present era, the two worlds are drifting back toward each other. The merge is a gradual process, not a single event. Its effects include:
Glimpses of the other world becoming visible
Portals opening where the worlds come into contact
Aether leakage from the fantasy world into the sci-fi world
The merge is expected to end with the two worlds fully combined into one sci-fi and fantasy world.
Humans
Ordinary animals
Magical beings
Alien species
The gods of the original world are, by most accounts, real, but they are not omnipotent. They are described as an extremely powerful race, not as transcendent beings, and they rarely intervene in mortal affairs. Within either world, there is no definitive proof of their existence. Different cultures worship different gods, and some of the gods that are worshipped may not exist at all. [this is still discussed though]
The fantasy world
The sci-fi world
sff world
Note: organized by AI, but I asked to keep people's word as it is.
source: discord and what we discussed in our meeting so far
Things that got an explicit "yes" in the meeting or were decided by majority.
Main conflict = Science vs Magic
Duskkitte: "Can we create a main conflict for this world building project?" → Austin: "Science vs magic!!" → this became the project's name and overall frame.
How the three worlds relate (Kaya's original premise)
"originally it was one world... Then it split to two, and now it's crashing back together."
"the two universes were one, they turned into two, and then they went back to being one."
Endgame: "the systems will merge. And then that will become a sci-fi and fantasy world." → the merged world has to exist. ("We need to have a sci-fi and fantasy world, otherwise the fairies are never going to get to mine their oil.")
No doom ending
"Guys, sorry, majority rules. We're not doing doom or world building." / "Yeah, we're not doing doom."
An ending where the worlds just blow up was rejected. ("what if we just have the universes collide and blow up?" → "But then we're never gonna have a sci-fi and fantasy world.")
The gods are not central to the plot
"Do we, like, agree that the gods don't have to be, like, central to the plot then?" → "Yes." / "Yes." / "Absolutely."
"They're very far removed from the plot" / "we should focus on the man rather than the god."
"the god should not have too much day-to-day influence over the life of our main character."
The gods are real, but people in-world don't know for sure
"They are real, but it's like they don't actually know." / "ambiguously, yeah."
Kaya: "I think we should have, like, an actual absolute truth. But the people in the universe can believe what they want."
This answers the "canonically correct religion" concern Duskkitte raised on Discord: multiple religions can coexist.
Working principle: hand-wave for now
"if it works at a quick glance, it works." / "If you can hand wave it at a glance, leave it be."
"We can just hand wave, like, it, for now at least." / "we can figure it out later."
How magic interacts with the science world: the aether model (agreed at the end of the meeting)
The meeting wrapped with "Oh my God, we figured it out." / "we figured out how the magic... is going to work within, like, the scientific world".
The magic universe is "an alternate physics engine" with "similar fundamental particles and similar fundamental forces", collectively called "magical carrier particles."
"The magical universe has an aether and the physical universe does not. Magical particles decay when there is a vacuum and there's no aether."
When the worlds collide, aether leaks into the sci-fi universe: "the aether will want to spread into the rest of the universe" → on the magic side, "the magical carrier particles start having more malfunctions"; on the science side, "anything that comes into contact with some aether is either, like, damaged or destroyed by it."
Worst case: "the aether content in the combined universe would be too high for regular matter to exist and too low for magical particles to exist" → "a state of perfect disentropy" → "And that's what they're trying to stop."
Kaya's summary for non-physics people: "the space that makes up the magic universe has a property that allows magic to exist... the collision would allow this stuff to go into the sci-fi universe... but only for a period of time because then it'll disperse like water vapor."
Still unsolved: "We still haven't figured out how they're gonna stop it."
The room mostly went along with these, but there was no explicit vote. A quick check at the next meeting could move them to "Confirmed."
What the gods are = "a very powerful species"
Kaya: "I don't think they should be transcendent. I think they should be, like, actual, like, corporeal entities. Like, they're less gods and more like... a species."
Austin: "I think that explanation's a good compromise. Just say that it was a very powerful species that created the simulation." → "Yes, okay. Okay, done. Done."
Condition: "if this was true, this should never ever be revealed in the plot of the story." / "Under no circumstances." → "a fun headcanon or behind the scenes idea"
Note: someone also preferred to "have the gods be ambiguous whether or not they exist."
Why the world split (Kaya's genesis idea)
"in the beginning, there were, like, these creator gods, and these creator gods made a bunch of realms. And one of these realms... had the gods that ruled over this realm had a fight, and that's why it split."
"the science god was like, 'Okay, you guys suck. I'm outta here.' And, like, yoinks the science away from the magic world."
Other realms are far away, but "these two are really close together because they used to be one."
Where the realms live = an extra dimension: "That extra dimension can be where all the different realms are."
How the worlds drift back together = Jin's binary star model
"What if they are, like, binary stars, and sometimes these universes get, like, close to each other... they know that each other exists because they see, like, glimpses of the other"
Kaya: "And over time, the glimpses became, like, you actually see the other universe." → "And then that turns into, like, things, holy shit, everything's going to explode."
Later Austin accepted it too: "we can do Jin's model, the binary star model"
Scale: the heroes don't flip the whole universe
"the heroes are trying to solve a much smaller problem... situated within a broader context in the universe."
Their impact comes "through a butterfly effect or something" → "Yeah, for sure."
"I would like this one storyline to be well-known enough within the universe that almost everyone knows about it, but... doesn't necessarily have to affect everyone."
"we've established that we don't want, like, super powerful characters."
Proposal: "we focus on, like, one specific country from each world for now."
Theme = "changing your destiny"
Kaya: "I like, um, changing your destiny, because I think it's pretty broadly applicable, but it's also fairly specific."
After the meeting, Discord was already running with it: Duskkitte "On the theme of changing destiny...", Mobius Crow "On the theme of resisting/succumbing to destiny, consider: trans character"
Jiin: "like what you're supposed/expected to become vs what you want to become"
Magic becomes carrier particles in the science world
Austin's first proposal: "in the magic universe, how it works is that magic has a carrier particle... the smaller magic universe, it slowly starts radiating out carrier particles for magic." → "I actually really like that one. It's a good compromise."
Kaya later: "I much prefer your original idea of the magical carrier particle" + "magic turns into magical carrier particles when it's, like, transferred from one universe to another."
This merged into the aether model (Section 1).
Simulation theory itself: "the gods are actually people who are making simulated universes in some sort of computer" → Kaya: "In that case, I strongly disagree with this. I don't want to do that." / "I also disagree." (Reason: "a lot of people are going to find it a boring explanation") → replaced by the "very powerful species" compromise.
Both worlds collide and everyone dies (doom): "majority rules. We're not doing doom."
Gods as protagonists: "The gods would make really cool protagonists" → "I hate having an omniscient being as a main character. There's no character development." + agreed that gods aren't central to the plot.
Revealing the simulation / the truth about the gods in the story: "this should never ever be revealed in the plot of the story."
Kaya's 5th-dimension FTL is not canon yet: in her own FAQ, "is this canon" → "no lmao, we'll probably have a larger conversation about this during our meeting next sunday" (in the meeting: "Has anyone... determined how we're gonna do FTL or no?" → "No.")
The meeting argued about these the longest without landing anywhere. Kaya's wrap-up: "as for the story, we, like, didn't talk about that."
Q1. What is the main story / main conflict? (the biggest open question)
Cosmic catastrophe (Kaya): the threat of "Complete annihilation of both worlds" and people trying to stop it. "The magical cataclysms in the sci-fi world are literally eating away at the sci-fi world... it gives people that sense of urgency." / "I like the destruction idea better."
Political / invasion (Austin): "the sci-fi universe attempts to invade the fantasy universe's resources", "I'm taking Avatar as my main inspiration... like the RDA in Pandora" (someone: "It's Avatar meets Avatar.") / "The problem will be purely political, not cosmological."
Middle ground (Jin): "sci-fi world, portal opens up. They put trash in there... people from the fantasy world come in and they're like, 'Stop trashing our planet.' And they low-key go to war. But then the main character... save the day by stopping the war."
Another pitch: "Project Hail Mary-esque, where it's just a bunch of different people from the other worlds communicating, trying to solve this giant apocalypse."
Sides: "there should be sci-fi plus fantasy versus sci-fi plus fantasy." vs "I think just to keep things simple for the introduction to the world."
Kaya: "I'm not sure I like the, like, fantasy versus sci-fi thing. I don't think they have to be going against each other."
Q2. How do they save the world? (the fix for the aether problem)
"We still haven't figured out how they're gonna stop it."
Option: "We can just, like, leak it out very slowly" → "hotspots in this galaxy where the magic is allowed to slowly proliferate" → "these portals are gonna stop becoming portals and become actual reality."
Option: move aether into low-pressure areas: "we can just, like, osmosis it over and over."
Austin's alternative: "a race against the clock to find ways to distort the spacetime enough to keep themselves safe" + "a detection for whenever a portal is imminent" → Kaya: "I still think it's way too complicated."
Q3. How do the two spacetimes actually merge?
Austin: "the two universes colliding in four-dimensional space would not cause the space-time of either of each universe to merge together" → "that one is something that would require another deep think." Dark energy idea: "that dark energy is what attracts the aether."
Kaya: "We don't have to do that right now. It's completely fine." / "The space-time continuum can just, like, meld in seamlessly."
Austin's balloon/relativity explanation (gravity reaching across universes): "I really like the problem" vs "Guys, if you're not a physics major, one, you don't gaf, two, this is too complicated."
Night sky: "some of the stars are distorted, some of the stars are in different locations, there's new stars" vs "look at the night sky and there's twice as many stars in it."
Q4. Who/what can cross through portals?
Kaya's original: humans only. "the soul information of the human is inscribed in both worlds" / "The things that existed in the original universe when they were both combined can have their information in both universes."
Pushback: "It's very anthropocentric, though. Like, the idea that humans are, like, the chosen race."
Variant: "maybe people think only humans can cross over, but there are, like, important caveats" → "Cats."
Austin: "when one entity goes to the other universe, it becomes its closest approximation." (the Spanish–Italian analogy)
Centaurworld approach: "they slowly shape into something... as they spend more time in that world" → pushback: "there can't ever be an instance... where there is magic in the science world."
"elves... because of the rules of the science world, they'll just, like, explode" / "You just make elves into nukes."
Access: "only nobles and people who have frequent access to the portals" / "only the select group via prophecy stuff can cross over"
Key objection: "if even as the worlds mix together, the species can't cross over, then what's the point of the worlds even mixing together?"
Q5. Is the fantasy world soft magic or hard magic?
Kaya: "the way anything in this universe works is by, like, belief." (people breathe "because the people in the world believe that they have to breathe") / "For now, I think it's okay to leave the magic a little vague."
Austin: "We need to pick a lane. Either both universes are soft magic, or both universes are hard magic."
Discord: Jiin "For our world imagination and belief/conviction could both be factors", Fate "tbh still not sure how the magic in this world works"
Jin: "We don't need to define everything" / "We can't get world builder's disease, gang." vs "But we're a world-building club, we should, like, world build this."
Q6. Do we use a prophecy?
Against: "I also just hate the idea of a prophecy in the first place." / "prophecy takes away agency from the characters." / "The power of the ordinary people is, like, the best thing."
Compromise (Kaya): "what if we have a prophecy that can't be broken, but the prophecy is that everything will collide and everything will be destroyed, and the main characters are helping to solve the problem by breaking the prophecy"
"Just have the prophecy given by people without the gods actually agreeing with it."
Q7. Is the fantasy world one planet or many?
Kaya's torus: "a flat Earth, but it's like a torus" (inspired by Ted Chiang's Tower of Babylon), "if you keep going up, you'll come out from the bottom", "The entire fantasy universe is, like, one flat plane."
Many planets: "they're also space-faring civilizations. They just use magic instead of physics", "they ride dragons to other planets", nether portals between planets → "Their logistics are gonna be amazing"
Pushback: "I thought we were just doing one fantasy planet because making that multiple planets makes it dissimilar to sci-fi." → "That's a very good point."
Q8. Time scale
From split to merge: "tens or hundreds of thousands of years" (Kaya) / "100,000 is fine" (Austin)
From first reports to completed merge: "At the very least, decades" / "Thousands of years."
Q9. Keep relativity?
Kaya: "are we keeping relativity then?" → "gee i hope not because that sucks", "I was low-key gonna copy Star Wars and their idea of not having relativity"
Compromise idea: "maybe there's no relativity in the magical universe and there is in the physical universe."
Q10. How big is the universe?
Kaya's proposal: "the entire universe is one small galaxy. Like, there's nothing beyond the galaxy. There are other realms."
Posted in #fantasy-world. None of these were voted on in the meeting, so they're all still proposals. "Reactions" = replies in the channel.
Name | Proposed by | Core idea (original words) | Reactions |
|---|---|---|---|
Social media (bird constructs) | Mobius Crow | "Bird constructs that fly when summoned as a message. They fly to people who lay out birdseed labeled with the sender's name. Each message duplicates into the number it needs to reach all the followers" | — |
Terrorpins | Mobius Crow | "a species related to turtles and tortoises that is a nocturnal predator of humans and other sentient lifeforms. They have pin like teeth. They are fast. Run" | — |
Potions (anyone can brew) | Big Mo | "Anyone who is magic or non-magic can brew magical potions... magical due to the properties of the ingredients acting alone or chemically working with other ingredients." | Duskkitte: "I like the 'anyone can brew potions' part" |
Carving runes with magic stone | Duskkitte | "anyone can use magic stone to carve ruins [runes] that cast spells they normally wouldn't be genetically capable of casting" | Add-on to Potions |
Mushroom clouds | Big Mo | "clouds that rain down mushroom spores... a nuisance for those living in villages or cities, but are great for forest-dwelling civilizations" | Mobius Crow: "a society that lives in hobbit hole-like storm bunkers and takes advantage of the harvest" |
Nocti | Duskkitte | "Corvid and owl people with a culture based around guilds: every family has its type of craft... Changing professions is considered a dishonor to the family name." Being an actor is "worse than being a janitor" even though noble society relies on plays and portraits. Homes have "landing platforms instead of stairs and are sometimes carved into cliff sides" | — |
Soules | Mobius Crow | "Souls are attached to the bottom of a person's feet"; the gods "just decided to go with" a desert culture's theology. "jumping is a large part of religious experiences because it separates the foot from the foot's shadow", "solemates", "walking a mile in their shoes" | Mentioned in the meeting: "we already have set up the ideas of souls" |
The Empathy (paladins) | sableavyon | "a huge collective of paladins who swear themselves to a certain platonic ideal of good". Main power "become who you are meant to be"; 3–4 of Grace / Shine / Flight / Adaption / Shapeshifting. "eight tenets"; break your core tenet and you lose your powers permanently | Fate: "I love the different abilities based on desires to do good!" + asked "what about the 'bad'?" and "what should be the limit" |
The Zodiac Witches | Fate | "12 zodiac witches... take turns to watch over the birth of human beings and give blessings to children born in their respective month". "demigods who ascended", "were alive before the great split", take apprentices | Austin: "the fantasy world wouldn't have the same stars as we do" |
Make our own constellations / zodiac | Fate | "maybe we can make our own zodiacs and constellations then?", "the zodiac constellations can be named after those creatures too" | kaya: "CAN WE MAKE CONSTELLATIONS OMG", Jiin: "I love stars... yessssssss", Austin: "Mm hmm" — the most enthusiasm in the channel |
Visualization / belief magic | Jiin | "a faith/belief-based magic system where the better you can visualize stuff the better you're at making it happen" → "imagination and belief/conviction could both be factors" | Fate: "so it's really about who has a stronger imagination?" (compared to Frieren) |
Mind flayer species | Duskkitte | "can only sustain themselves by eating sentient beings (maybe there's some unknown alternative)", "all their philosophies and religions are wrong because it's just evolution", a subplot about finding a cure, "only eating bad people" | Mobius Crow "Oooh", bureaumantic "ough.. the angst...". In the meeting: "cool to have as a subplot in the main party"; also suggested as a species that existed in the original pre-split world |
Elves | (meeting) | "elves don't exist in the sci-fi world... because they were created as a result of the magic world" | Came up as an example in the Q4 debate, not confirmed |
Magic-side space travel ideas from the meeting: "you just get eaten by a worm, and the worm travels super fast... then poops you out where you wanna go", "I wanna have hyperspace whales", "this cave system where the caves have portals to other planets".
From #sci-fi and the meeting. Austin said in the meeting that "There's civilizations that have existed in the sci-fi universe before the thing even happened", so existing work shouldn't be affected much if the premise changes.
Name | Proposed by | Core idea (original words) | Reactions |
|---|---|---|---|
Living machines | Austin | "living machines" / "the epitome of life. perfection" — "highly based on the covenant and the phyrexians" | — |
Argentelle-Silica Collective | Austin | "a vast interstellar empire spanning thousands of star systems in the Reverent Galaxy, one of the nearest galaxies to the great breach". Argentelles ("silver exoskeletons") + Silica ("silicon-based life forms") → "a super race of artificially enhanced and bioengineered living computers". "savior ideology", "Von Neumann Probes" | Still in progress ("This is what i have so far") |
Mining planet | (meeting) | "this planet here is a microcosm for the issues that are happening throughout the entire galaxy. Corruption, capitalism". To the empire it's "a little side project"; to the people living there it's survival (War of 1812 analogy) | Austin pitched it as "the perfect place" for the story, but "we're not gonna have one planet anymore" etc. — undecided |
WEM / MSU scores | Austin | WEM: "The total amount of energy a weapon can dispense within 100 seconds of standard operation". MSU: "Imagine the collective military power of your world is normalized to 10,000" | bureaumantic: "ya sure!" |
FTL via 5th dimension | kaya | "ftl travel using a 5th dimension, the same 5th dimension that separates all the 'realms'", "kinda like the nether in minecraft". Magic and tech use "the exact same medium" but different techniques. 4th dimension = "time" | bureaumantic: "i like this!" / Austin: "Wormholes only need 4 dimensions to work, though?" / kaya herself: "is this canon" → "no lmao" |
Gauss guns | Austin | "Just learned about gauss guns" | ARandom_F-22: "you heard of a gauss flayer?" |
Sun lens weapon (sun gun) | Austin | "the lens works like a magnifying glass under the sun", "mounted on a ship", "this is a WW1 era tech level civilization" | Ballast drain suggestion: "many opened grates, like a shower drain". Note: empire of the sun / Arctic order / EDC / HNS / Dominion are examples from Austin's own existing setting — need to confirm whether they're part of this project |
GreenCat's OC lore | GreenCat | "i kind of want to integrate my oc lore into this world" | No details yet |
Ideas about where the two worlds meet. Only things not already covered in Sections 1–4.
Portals / how contact happens
Trash portal: "a random country in the fantasy world that somehow opened up a portal and just using it to, like, throw their garbage in... that's where the magic first leaks into the sci-fi world." (Later version: "the corporations in the sci-fi world... using portals... as trash cans")
Portals of different sizes: "some of them are small enough to be one nation's trash disposal, but other ones can get entire solar systems in."
Whole planets crossing over: "the portals become so big that an entire planet could pass through" → "One day you wake up and then you're just in a different universe." / "You look up at the night sky and the stars are different."
"Sometimes there's a portal so big you don't even know there's a portal."
Intersection portals: "the two universes are passing by each other, and the points of intersect... are where a portal will open up", "over time, the frequency of the portals appearing should be bigger and bigger"
Portals as warnings: "a portal forming near places where there's a lot of inhabitants... the portal itself indicates the oncoming of a calamity."
A distorted third space (Kat): "distorted time and spaces... where technology and magic can, quote, unquote, coexist... technology where the rules of physics becomes glitchy, and then the magic becomes unpredictable" / people there "will be mutated in some way."
"what if instead of annihilation, it just caused, like, glitches and bugs and morphed stuff?"
Keep the portal mechanism a mystery: "I think maybe it's best left as a mystery... an event that you get to hear a lot of" (compared to Iron Lung's Quiet Rapture) vs "they're probably going to do a lot of back and forth between the realms... it probably would be important to specifically outline"
Probability: "like Hawking radiation, these portals are opening up because of random" / "There's a rare chance of possible bullshit happening... maybe this world is setting up for that"
Consequences of the collision
Magic dying out in the fantasy world: "places in the fantasy world where magic stops working. And that has cultural consequences because their religion stops functioning, and then that causes societal collapse." → "That's a great idea."
A planet with no sun: "Their planet is eternal night, and their heat is bleeding off into space" + "huge, like, star feeders that feed on the nebulae" (Lovecraftian)
Blame: "the empire to blame all the rebellions for the sudden blowing up."
Who found the first portal
"the first person who found the first portal was someone from the sci-fi world... saw that this place is like, you can make anything happen... they're gatekeeping that information."
"a Wizard of Oz thing where someone from the sci-fi world gets stuck in the fantasy world and pretends to be some magic person."
Factions
"the UN of the people of the sci-fi world, the UN of the people of the fantasy world, they agree that they're helping each other, but there are specific factions because they're not actually united on it."
Thanks to portals the fantasy side is strong defensively: "on the defense, it'd be very strong" / but "bad on the offense because there's no portals on the sci-fi world"
After the merge: "now we have a bunch of sci-fi planets and we just have magic planets, and they have to learn to get along with each other." → "Or don't get along with each other and go to war."
Story & character ideas
Duskkitte's mind flayer as the fantasy-side MC: "their reasons for exploring the science side once the worlds collide could be to try and find a cure."
Jin's take on "changing your destiny": "people cross the portal. They're supposed to fight each other and cause huge havoc. But eventually they figure it out and do diplomacy and try to live peacefully."
Mobius Crow: "On the theme of resisting/succumbing to destiny, consider: trans character" / Jiin's trans Ralsei theory example
sweet.dreams.timaeus: "Will it cause a crisis of faith? Will people simply shift their beliefs?... what happens to the power structures when humanity's idea of the gods is shifted"
Mobius Crow on religion: "the 'true religion' be this long dead minor religion", "have the gods pick and choose random stuff to basically turn from headcanon to canon"
Duskkitte: "a godless world where people still believe gods are real (as in maybe they mistake spirits for gods)"
Note: the meeting transcript labels speakers only as "Speaker 0, 1, 2...". Quotes are attributed to Kaya, Austin, or Jin only where their names came up in the conversation, so some attributions may be off.
core concepts. Highest principles that can be applied to both 3 worlds.
Soul archive system(?) (I remember kaya mentioned about this)
Everything is just roughly made by we talked about

Aether is a pressurized substance that fills the space of the magic universe. It is the property of that space that allows magic to exist. The physical (sci-fi) universe has no aether.
The magic universe is an alternate physics engine. It has similar fundamental particles and similar fundamental forces to the physical universe. All the particles in the magic universe are collectively called magical carrier particles.
Gravity and the strong force roughly work the same way in both universes.
There are slight differences that lead to interesting consequences. For example, there may be no relativity in the magical universe, while there is in the physical universe.
The two universes haven’t diverged so much that magical particles and physical particles can’t interact. The difference is that magical particles carry magic.
Magic turns into magical carrier particles when it is transferred from one universe to the other.
Magical carrier particles decay in a vacuum with no aether. This is why magic can’t survive in the physical universe on its own.
When the two universes collide, the aether wants to spread into the rest of the universe. Aether is pressurized and the vacuum of the physical universe is empty, so by the laws of thermodynamics it leaks out.
In the sci-fi world:
Areas form where magic is allowed to exist, but only for a period of time. Then the aether disperses like water vapor.
Aether interacts poorly with the particles of the physical universe. Anything that comes into contact with aether is damaged or destroyed.
In the fantasy world:
The aether becomes thinner and thinner.
Magical carrier particles start having more and more malfunctions.
In some places magic stops working altogether. This has cultural consequences: religions stop functioning, which can lead to societal collapse.
If the leak continues long enough, the aether content of the combined universe becomes too high for regular matter to exist and too low for magical particles to exist. The result is a state of perfect disentropy: the whole universe becomes uniformly aether and energy, and all particles are destroyed.
Preventing perfect disentropy is the central threat that the characters are trying to stop.
Because aether is a physical substance, the threat is a physical problem: how to keep a liquid from leaking out of the other universe.