# My setting has a multiverse, how could the agency in charge of naming them and documenting them tell them apart?

They'd be documented and named similar to how NASA give names to planets or stars. Since there is literally an infinite number of universes, I think they'd have a name followed by a Roman numeral. Maybe universes could be categorized by some influential event.

The problem with labeling seperate universes, is that it's not immediately apparent that you are within a different one. You could end up in a universe identical to ours, but where something like tardigrades don't exist in the one you've entered. Maybe you're currently in a universe where an obscure village somewhere in Europe has a different name. The possibilities are literally limitless.

In my setting, black holes and white holes serve as nexuses for multiversal travel. Entering a black hole will take you to the location where the black hole should be in the parallel universe, which is instead a white hole. Obviously, anything can enter a black hole, but nothing can escape, while on the reverse anything can escape a white hole, but nothing can enter.

1. Temporary names and categories grouping by observed properties

If not every detail of a multiverse item is known, then it's logically impossible to know exactly which universe you're in. As an extension of that, you're probably not going to be able to give one a unique, descriptive name to any given universe. But if multiverse monikers have something like a hashed name representing different features, perhaps organized from general things (like fundamental rules of physics) to specific things (like names of villages at distinct points in time), a name can categorize which broad "group" of universe you're in.

Critically, if universes are fractally detailed and your ability to observe all of their details is limited, then there is going to be a maximum level of precision which you can apply to identifying them-- you'll never be able to fully specify even one, let alone all of them together.

2. A function which takes information about a universe as inputs

If the agency has some protocol for observing, classifying, and identifying universes, the idea of a specific name for a universe may not be necessary. You feed observations into the function, and it tells you "where" you are. Depending on how exactly your travelling between universes works, it may not be feasible to say "I want to go to Universe 8213213246HSXIAZ"-- it may be the best you can do to simply identify where you happen to be, and compare that to records other travelers have left.

3. It's a work in progress, and always will be

If the possibilities are literally endless, and the level of detail distinguishing universes from one another is effectively infinitely, fractally complex, then the idea of constructing a complete catalog is probably not what this agency would have in mind. Instead they'll get to work, knowing that their labors will never end, and updating their classification schemes at need (instead of trying to account for infinite possibilities at the outset).

4. Names are arbitrary, if you can't tell the difference universes might be the same, and the agency started somewhere

The agency starts in one universe (well, infinite universes), and coordination will not be possible across all analogous agencies (since infinite instances of the agency will also mean infinite, arbitrarily different naming and classification approaches). So each agency might as well just start numbering the universes they encounter, labeling their own universe as Universe 1 (or Universe A).

This idea reminds me of the old TV show Sliders, where arbitrary "coordinates" were (eventually) assigned to parallel universes with infinite possible variations. The information wasn't part of a generally accessible catalog, it was strictly a reference for specific travelers.

5. Close enough!

There is a philosophical approach suggesting that, if two things do not differ in any of their properties, then they are the same thing. If the ability of your agency to observe the details of universes is limited, either practically (there are so many new kings to memorize) or inherently (some details simply cannot be observed, because they lack the technology or something), then the agency's ability to state that they are in a new universe is limited.

The effective set of universes it can observe, document, and name is going to be limited. There will be many universes (well, infinite universes) which the agency just can't distinguish between. It may be aware of this, and respond as in (1), or they may not, and they will call many, many universes by the same name out of ignorance. Either way, it really cuts down on the complexity of the task of cataloging them, though infinite universes will still present infinite challenges and infinite work.

## Kill some cats!

In the many-worlds interpretation of quantum mechanics, the multiverse exists because of probability. Every time a random subatomic event happens, this interpretation would argue that the universe splits into near-identical copies - one for each possible outcome. For example, the random decay of a Uranium atom would produce a new universe in which it never decayed. All possible trajectories for the history of the Universe will therefore occur. We get one universe for every "yes" and another for every "no" that happens on the quantum scale, and over time, those add up.

You're probably familiar with Schrödinger's Cat, a "paradox" posed by quantum superposition. The gist is that a random quantum event will determine whether our cat is poisoned or not. If our sample decays, the cat is poisoned; if it doesn't the cat lives. Until we observe the cat, both outcomes are paradoxically true. The MWI interpretation resolves this paradox by saying the outcome we don't observe still occurs in a parallel universe. We might live in Universe-Cat-Living, while our doppelgangers now live in the newly created Universe-Cat-Dead. Both outcomes still exist.

What's great about the MWI multiverse is that after the moment of divergence, each world is no longer identical, since quantum events are random. Therefore, if you killed a second cat in each Schrödinger-world, they would have yet another completely random chance of surviving.

Let's list all possible outcomes of a second cat-trial. We would have (1) Cat1-Living-Cat2-Living, (2) Cat1-Living-Cat2-Dead, (3) Cat1-Dead-Cat2-Living, and (4) Cat1-Dead-Cat2-Dead. You could also express this in binary. If we assign 0 to living and 1 to dead, we get 00, 01, 10, 11. We've just numbered four universes that look almost exactly the same!

I propose that in order to label each universe, your researchers "kill some cats". They can conduct an arbitrary number of quantum observations to generate a random number unique enough to identify each universe. As universes age and continue to diverge, extra observations can be used to tack on extra digits. Maybe a new observation is made every second, or every day, depending on how fast the universes tend to diverge.

The irony in applying quantum mechanics here is that every time scientists make an observation to catalog a universe, they'll make at least one new universe in the first place just by observing.

I hope you have a lot of cats.

• It doesn't have to be cats. It could be inanimate objects (a lot more humane). – Galactic Feb 29 at 5:06
• @galactic_analyzer Of course. I chose cats because it's a familiar example to a lot of people and it makes for an interesting answer title, but the main point is summarized by conduct an arbitrary number of quantum observations. – Zxyrra Feb 29 at 9:30