Ooooh! A namespace question. Do you want a good answer, or a plausible answer (what people would actually come up with)? Because I've already got a 9-page answer half-written with citations to studies on corporate-naming rights...

This sounds like finding hotel room numbers in Hilbert's Infinite Hotel.

PS We live in the Coca-Cola™ universe, formerly known as Universe 107:68-cHA Amalgam Dolomite Stratosphere universe cluster, candidate universe 6ca88fe4. You'd think there'd be many Coca-Cola™ universes, but it turns out that in all the rest it's just not successful enough to have bought the naming rights from the UN (or Galactic Federation, or Council of Ricks).

@JohnO Definitely a good answer. This will probably go in a book one day, and even if it's a little unrealistic, I don't want to start any sentence with, "Back in Universe 36A_ver7;Toyota_claim803"

Futurama considered this problem, and found a workable solution

@user535733 "Whelp, sorry everyone. Looks like some idiot set up a new portal, and now this entire universe, the Earth and all the uncountable stars and infinite complexity, is now collectively called 'The Fighting Mongooses.' Look, I didn't make the rules."

Is it possible to open up a portal between the same two universes at a different place on the earth simultaneously? Or is every portal a new portal to a new, different world?) In which case, would the portal universe have the same name in both instances? And would the two portals decouple synchronously, or each go their own way (That is, could a person who entered at new portal A in the attic and a person who entered at simultaneously new portal B in the basement, meet in an identical world on the first floor at some later time?)

In many cultures, a new baby is named after the first thing the mother sees after birth.

@JustinThymetheSecond No, it's not possible for any portals to link universes already in existence. Each new portal activation strictly doubles the current number of universes.

I spent an hour typing up a good answer, but kept revising myself so much I realised I just want to see what other people come up with at this point.

What happens when a politician in one world doesn't like the name that was given to their world? It seems that it is the person who opened the portal and created the new world, who names the new world, without any regards to the wishes of everyone else in that world? Suppose everyone else BUT the person who opened the portal wants to keep the name of their world as 'Earth', on BOTH sides of the new portal? And how do they keep up with the name changes? Yesterday I am from Earth, now I an from Earth-Becky, tomorrow I am from Earth-Becky-Anrandy-Mom?

@TitaniumTurtle I know the feeling. :) @ JustinThymetheSecond Hopefully there's a numeric system so people don't get emotionally invested. Nobody I know of cares what their country's 3-digit phone code is.

Okay, my problem now is 'causality'. In opening up a new portal, do you clone the entire history of the history of universe A onto universe B? So someone in B, when they look for their great-great-grandfather, would now find him in the histories of world B and not the original world A? Or do all of the cloned worlds share the same history BEFORE the clone? You don't suddenly create a clone of all of the worlds BEFORE the clone, otherwise causality just goes to hell. Something is 'caused' by an event that never occurred until after the clone.

All worlds are perfectly duplicated at the moment a portal opens. That means that both are identical at that moment, including everything that happened before. They have identical histories, yes. Then they can have different futures from that point on.

Here is the problem with causality. You might need a map. Alice spawns Bob. But previously, Alice spawned Jane. So you are saying that, at that point, the new clone Bob ALSO spawned a previous Jane, called Jane-from-Bob because it is not Jane-from-Alice? So that you have Jane previously spawned by Alice, and Jane previously spawned by Bob? Both IN THE PAST? So Jane-from-Alice can now spawn a different universe called Sussie that is not spawned from the Jane-from-Bob? What is the causality for Sussie, when Jane-from-Bob wasn't spawned until Alice was spawned?

Something happened before the causal event?

@JustinThymetheSecond I have literally no idea what you're talking about. There's no causality issue here. It's just that from the moment in time when the portal opens, the two parallel universes start the same but are allowed to take different paths. There's no spawning or anything, it's just 2 copies of the same universe. You're not accessing new universes, you're cloning existing ones.

I don't want to start any sentence with, "Back in Universe 36A_ver7;Toyota_claim803" But there is a universe where that naming convention really excites you! :-)

In your scenario of earth 1, 2, 3, and 4. After creating the portal in your attic, earth 3, if you go to your living room and create a portal into an earth 6, would a portal in the living room of the house in earth 3 suddenly get a portal into earth 6, without anyone doing anything on earth 3? What about the living room in earth 4? How does someone on earth 2 get to earth 6, otherwise, if their earth does not have a portal, and the earth they were cloned from (their earth 1?) didn't have a portal?

When did WWI get its name? When WWII started. Before that, it was called The Great War. There was no need to call it WWI when there was no WWII.

@JustinThymetheSecond Re-read the 3rd paragraph of the question. Opening a new portal -doesn't* create portals in existing Dupliverses, but it does duplicate all existing Dupliverses alongside their portals. The first portal created in Universe "►01" creates its mirror, "◄02". Creating a second portal in "►01" doesn't create one in "◄02" - but the new universe, "◄03", will contain a copy of the original portal, which connects to "►04", a mirror-copy of "◄02" as of the moment that Portal "01↔03" was created. Effectively "◄02" and "►04" are Dupliverses without a currently open portal.

@JustinThymetheSecond The interesting question is: would it then be possible to punch a portal between "◄02" and "►04" at the exact same location as the portal between "►01" and "◄03", without creating new Dupliverses, to create a Universe-loop?

@Chronocidal I'm not sure what you mean by "punch a portal". If you mean it like "punch a hole in something," as I've said many times now, there are no loops. Any time you create a portal, it generates a copy universe. It never leads back to a universe already in existence.

the long earth series solves the problem with numbering them east and wast, (earth west 212) since people already operate with the cardinal directions in mind. with datum earth being the one everyone starts on (earth 0).

@GiladM did you consider to simply use the numbered path from the original universe? / is the origin, /1 is the 1st duplicate, /2 the 2nd, and so on. If Someone opens a portal in the 2nd duplicate then it will be /2/1. Like...well... folders in your file system. In this way the uniqueness is given by the full path and each alt universe does not need to know how many other universes are out there (just its own origin). You may even give a (pseudo!) scientific explanation (like...I don't know...each number in the path is the count of quantum spins in the universe lattice...

20 portals (even in the worst case scenario) give you a manageable name (and you may alternate digits and numbers to drop the separator, something like: /1/2/3/1 is A2C1.

@AdrianoRepetti cyber101 had an answer below which suggested the same thing (though without the file analogy, which I quite like). My only concern with it is that name changes will still be required after each portal opens, from the perspective of the "copy" universe. Read through the comments on that answer to see what I mean, hopefully my back-and-forth with cyber got the point across

"if you have a portal you set up last week in your basement (let's say, to an "Earth 2"), and you open a new portal in your attic to an Earth 3, then of course your house in Earth 3 has a basement with a portal in there that leads to an Earth 4." Wouldn't instead Earth 2 open up a portal to Earth 4? Also, if the Earth 2 has deviated enough from Earth 1 in the meantime, it might happen that the person in there does not open a portal or that they do it slightly earlier or later.

@FabianRöling No. The first portal in Earth 1 mirrors Earth 1 to create Earth 2 (1|2), an then connects them (1↔2). The second portal, also in Earth 1, mirrors the Earth 1/Earth 2 pair to create Earth 3 and Earth 4 (4↔3|1↔2), and then connects those (4↔3↔1↔2) Earth 4 is a mirror of Earth 2, but it is not connected - and, apparently, never can be connected,

Okay, something for OP to think about - but not necessarily to answer here (or at all). Suppose I create a huge Portal (think "Barn Door" size). Then I build a room through the Portal - perhaps just shove a large shipping container partway through and leave it there. Finally, I construct a new door frame halfway through (so that half the frame is in Universe µ, and the other half is in Universe φ) and turn it on. Does it work? Does anything weird happen? Does the intersection point count as the diagonal-universes (µ and mirror-φ or φ and mirror-µ) being connected?

@Chronocidal Already considered it. This is actually completely doable, and in general, portals can move through other portals with no paradox. It completely derails most naming schemes, though, and since it's a very rare occurrence in my story, I figured I'd avoid the chaos and just not mention it. :)

Does a portal create a mirror image, so that everyone who was left handed is now right handed? Or does it create a 180 degree rotation, so on both sides of the portal, the portal is on the person's left (or right, whichever)?

@Fabian Röling Indeed you have identified my problem with causality and the nature of 'past-present-future' in the scenario. Events that happened in the past re-happen in the past because something happens in the present. That is, Earth 4 cloned sometime in the past from Earth 2 because a new clone of the entire clone tree was made today. The question becomes, when does Earth 4 become decoupled from Earth 2 - when the original clone for Earth 2 happened, or at the time when the new Earth 4 was cloned during the current universe clone? When did Earth 4 become desynched from Earth 2?

@Chronocidal Oh no, we're both right and that makes it even worse: On one day, world 1 opens a portal to world 2, so fine, so good. On the next day, world 1 opens a portal to world (let's say) 3 AND world 2 opens a portal to world 4 (unless they were desynced enough in the meantime). But both world 3 and world 4 already include the old portal, which leads to yet another different world each, which could be called 5 and 6.But those two worlds will have opened a new portal just now, leading to world 7 and 8.But those two world already include the old portal, leading to 9 and 10…Instant ∞ worlds!

Can a new portal connect to an existing world, If I am in earth 1 and earth 2 already exists can I build another portal connecting 1 and 2?

@Chronocidal they will need to agree on some kind of prime, an arbitrary starting point, just like how we have a prime meridian for longitude, this will need to happen no matter what scheme you use. since each world start in pairs. likely you start with the two worlds with the first portal east 1 and west 1.