# How to tag each parallel universes?

In a distant future humanity developed ways to traverse between parallel universes. It makes sense to mark our current universe in case we got lost, but how to do it? There is a possibility of an identical pair of universes where all the fundamental physical constraints are an exact copy!

Bonus Question

(Warning: Not for the faint of heart!)

The above scenario assumes that each universe is a separate object, however humanity realized that universes are constantly popping in and out of existence. An analogy might be producing bubbles inside of boiling water, except some of the bubbles are long lived while many suffer premature death before getting to the surface. There is also the possibility that two or more bubbles merge and form a bigger bubble which in turn is then enveloped by yet another big bubble.

Is it possible to tag each and every bubble (/universe) that ever existed, assuming time travel is childs play?

Difficulty: Fiendish!

• If you could elaborate on how this wondrous traversal technology works, it would greatly improve our ability to provide a non-opinion-based answer. Commented Dec 7, 2015 at 5:19
• @Frostfyre: the technology is similar to HHG2G! Commented Dec 7, 2015 at 5:23
• Just carry the entire universe with you. Duh. Commented Dec 7, 2015 at 8:13
• See some of Leonard Suskind's videos. In one he uses a bit string to describe the different branching bubble universes, because it's part of the math that analyses the situation. However you can't give each a unique symnol for real because there are an infinite number. Commented Dec 7, 2015 at 16:50
• "Everyone grab a length of wire!" en.wikipedia.org/wiki/The_Farnsworth_Parabox Commented Dec 7, 2015 at 16:55

If you look at mainstream physics multiverse interpretations such as the Many-worlds interpretation (MWI), you can get a lot of ideas from the current scientific hypothesis.

Unfortunately there are a number of challenges your fictional universal graffiti artists (a.k.a. "taggers") will face:

# Other "you" comes up with the same idea

A major problem with "tagging" your universe, however, lies in this:

Paul Steinhardt has famously argued that no experiment can rule out a theory if it provides for all possible outcomes.[5]

The MWI (as well as some other similar multiverse models, even expanded universe models (c.f. Hubble volumes)) implies infinite, or at the very least, uncountable universes. What happens when a possibly infinite group of other "you"s come up with the exact same idea, and even the exact same identifier for their universe?

# My suggestion for your Homing Device

As others have already suggested, but I hope I'll be able to provide much more detail about, is quantum entangled objects. This is a complex and subtle topic, so it bears some explanation.

It's possible to use some sort of quantum entanglement between a device you carry with you, and a device you leave at home. The key here is that these devices must be created ahead of time—if your device is lost or stolen, you will be completely, possibly permanently cut off from your universe!, which I expect would bring up some interesting potential sub-plots.

These objects also have a lifetime and are very sensitive to interference. So far, we've only been able to keep quantum entangled experiments running for a few minutes to a few hours in very carefully controlled tests. Even accounting for significant advances in science, this device—perhaps second only to your ship's life support system in terms of importance—would likely be rather delicate and sensitive.

Quantum entanglement (QE) and superpositions can't be used directly for your homing device, since quantum entanglement cannot actually propagate information over distances! Read that again. That would violate causality in a pretty big way. You cannot use this device for communication.

So what's it good for then? QE essentially guarantees you and your favorite Earth will receive the same stream of random data. Happily, it is resistant to eavesdropping—as a consequence of physics, no one else will be able to intercept that random data. To be useful, combine the QE with quantum key distribution.

You and Earth then take that random data, converted to ordinary binary bits, and obtain a unique shared secret (like a password) that both of you know. At this point, you must then use some ordinary (non-quantum) inter-dimensional communication channels (your creative skills will be required there).

Your shared secret can be used with the one-time pad, (OTP) which will effectively guarantee you're talking to the desired Earth. In the real world, OTP has been rigorously proven unbreakable when used correctly, and it is absolutely the right tool for the job in this situation.

# "Bonus" question: tagging more than one universe

Absolutely. You can simply extend my answer to include multiple pairs of entangled homing devices. One end stays on your Earth (or maybe your spaceship, if that's your base of exploration?) and the other end is planted in the desired universe.

If you tag many universes, storage may become an issue, since a pair of physical devices is required for each universe. In that case, you could instead create a network of paired devices, similar to peer-to-peer computer networks. In general, graph theory will give you the building blocks to come up with a decent fictional entanglement network.

# How big are these things?

I don't have a good answer for that, unfortunately. Our experiments to date have not really been concerned with size (one of the successful experiments required a particle accelerator!) The actual entangled matter would be just a small group of subatomic particles, but even with a futuristic advanced fictional version, I'd expect it would need some kind of powered containment vessel, plenty of shielding, and plenty of redundancy.

Personally, I wouldn't go any smaller than a coffee mug, but if you want to make it central to the story, you have a lot of freedom on this point. Make it small enough to steal? Big enough to require a lot of power? Smaller and more vulnerable? Bigger, but less room for more of them?

# What have we learned?

1. Quantum entanglement must be done before you leave home, and the devices must be kept safe, or communication will be impossible, given infinite (or just plain "many") worlds
2. Quantum key distribution basically gives you a long "password" to prove it's you
3. One-time pad encryption/decryption is what you would use to actually send/receive a message, but you must use more conventional (non-quantum) communication methods

# Why is this all necessary?

(Further discussion, since MWI/multiverse presents a big problem where identification is concerned.)

While you may have nefarious villains (or desperate good guys) who would try to trick you, get you to fly your valuable ship into their universe, etc., the real problem is just that there are so many universes. There might be an infinite number of universes where you cut yourself shaving that morning. And an infinite number where you didn't.

Even if your number of "reachable" universes only lies in the dozens that are similar enough to warrant this question you asked, that's still enough to preclude any sort of human-knowledge-based tagging system.

Say you generate a random tag, "puppies56831X", and call it a day. Some alternate you (even infinite alternate yous), might be similar enough to generate the same random tag and otherwise be indistinguishable under close scrutiny.

Quantum superpositions are the only potentially valid answer I know of, which is why I have focused my answer on them even though there is at least one other answer that mentions them. I did not get the idea from that answer, nor did I borrow any of the specifics—I barely skimmed it. I just hope my treatment of the topic, as it relates to your question, is helpful to you.

• +1 for a very detailed answer and for the Why is this all necessary section which brings up some interesting issues and hopefully leads the OP to additional questions. The one that I'm currently interested in is "Why would you need to know what world you are in?" If it is close enough to recognize you and accept you in the role you expect to play, who cares if it is the actual universe where you nicked yourself shaving yesterday, or if it is one where you didn't. Plane-ism, like its precursor nationalism, might become an unnecessary burden to a highly-mobile world(s) traveler. Commented Dec 7, 2015 at 13:34

If they are "hard" sci-fi parallel universes:

Most sci-fi has what I would call "soft" parallel universes. This is where everything is nearly the same.

Real parallel universes would likely be vastly more divergent. There's billions of years in which chaos events could have happened and propagated. Even if you somehow are finding parallel earths - which would not always exist - it's extremely unlikely that you'll find humans. Maybe something close, but probably not that could interbreed. And if you do find humans, it's unlikely that they'll have similar cultures or that you'll find, for example, a parallel "you".

So in this case, just name them by order of discovery, or by whimsy. Call us Universe 1, the first parallel Universe 2, and so on. Or if we find, say, Dinosaur Earth, call it that.

If they are soft sci-fi parallel universes:

This is the standard, "Earth but slightly different." In this case, I would recommend naming them off of the point of of divergence. So:

• Universe/Prime - us
• Universe/1945 - diverged at the atomic events near the end of WW2
• Universe/1776 - diverged near the US's revolutionary war.
• Universe/1776B - diverged near the US's revolutionary war, but with a different, third result.

And so on.

• A similar naming scheme was used in Heinlein's books. He decided that one of the most important divergences in his worlds was who landed on the moon first, so Time Corps members refer to their timelines by names (our timeline would be Neil Armstrong) Commented Dec 7, 2015 at 5:43

If quantum-entangled particles remain entangled across planes, then each plane could be linked to two other planes by two pair of entangled particles. From a large enough collection of these links, a bi-directional mesh could be assembled and used as a map.

The trick would be to use one of each plane's paired particles as a sender and the other as a receiver. Then all you need to do is watch a very precise clock. For example, if plane 11 has a sending particle, whose paired receiving particle is on plane 14 and if plane 11's receiving particle is paired with a sending particle on plane 3; you could have devices attached to the sending particles to invert their spin on a specific second of each minute. Therefore, plane 11 would invert its particle on the 14th second and plane 3 would perform its inversion during the 11th second. The receiving particles on each plane would therefore see their receiving particles invert on the second appropriate to their place in the map of planes.

And when your transdimensional empire expands past 60 planes, change all the clocks to run on specific seconds within each hour. That will cover your expansion out to 3600 planes.

• Your answer suggests using QE to communicate by essentially "flipping bits" on the entangled particles. Unfortunately, quantum mechanics forbids that. See the No-communication theorem: "In very rough terms, the theorem describes [...] two people, each with a radio receiver, listening to a common radio station: it is impossible for one of the listeners to use their radio receiver to send messages to the other listener." But this is for fiction, so if your idea makes a better story, I'll be happy for you! :-) (Cont'd... Commented Dec 7, 2015 at 8:13
• ... cont'd) You'll note I based my answer on QE as well, and submitted mine some time after yours. I had not read your answer at all, and I hope you'll agree the similarities are fairly superficial in any case&mdash;it looks like I took it in a different direction and depth. (I see we did both suggest networks, though I think we described them rather differently.) What I'm trying to say is, if you feel like I cheated you out of any ideas, I'm sorry. I promise that wasn't the case. Commented Dec 7, 2015 at 8:24
• No problem on the similarities. Even if they were more similar than they are, I wouldn't care. What is important is providing the OP and the site with good answers. Thanks for the bit on No-communication theorem. I didn't know anything about that. The knowledge will improve my future efforts on this site. Thanks again! Commented Dec 7, 2015 at 13:13

I'm not entirely sure how you get to the new parallel universe, but there must be some sort of an entry point. I can't imagine you would just magically "appear" there.

So if you tethered yourself to the original universe you came from and went to the parallel universe(s), the tether would still be attached and you could follow your "life line" back home.

I'm assuming the tether would not get destroyed, because the human traveling would also get destroyed if that was the case.

An issues with this method would be if the tether is basically anchored to the entry point. That would give you limited movement based on the length of your tether.

In the story Rumfuddle by Jack Vance, the problem was solved by having a gateway between the worlds through which you had to pass as if a door. These gateways can be expanded to any size, and additional gateways "pinched off", thereby making multiple gateways to the same parallel universe.

Within each universe, the gateway can be moved about without affecting its position within the other universe. (This is physically problematic because you could, for example, drain an ocean from one world into another, or create a perpetual motion machine).

However the machine which made the gateways could never recreate a lost gateway, because the number of universes is infinite, so even with the greatest accuracy possible, the coordinates would only find a neighbouring, almost identical, universe.