What is a universe? (taboo the word "exist")

Question

It seems to me that "universe" and "exist" are defined cyclically. When not talking about many worlds or parallel universes, it's clear what it means to say whether or not a thing exists. All definitions are eventually cyclic, but this is something we intuitively understand and can point to real life examples of, eg.

Things that exist: My laptop, Clouds, Beluga whales, Post-lunch lethargy

Things that don't exist: Unicorns, Halting oracles (for a Turing-complete language), Perpetual motion machines

We have concrete evidence of things in the former list. The things in the latter list have well-defined and straightforward behavior and we know what evidence we might expect to see in the world if they existed, so it seems likely they don't. But this same intuition for determining whether or not a thing exists doesn't seem to apply when introducing "many worlds" into the discussion. By definition, things that "exist" in another universe don't impact our world in a way we can observe. So what does it mean to say that it "exists"?

Is there an alternate way to define "universe" and "exist" so that they're not circular?

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Note 1: I'm referring to "parallel" universes such as those hypothesized under MWI; not fictional universes (eg. Marvel cinematic universe) which I have no difficulty with the definition of.

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Note 2 (Only read this if it's unclear to you what an answer to this question might look like; the following is a "false" answer; something that has approximately the right structure, but fails to hold up under further scrutiny):

Structure of an answer: This is a solution I thought of that sounds right initially, but doesn't quite work (and I'll explain why it doesn't work at the end), but perhaps it serves as a good template for the sort of answer I'm looking for:

Alice and Bob are friends who live on the edge of each other's cosmological event horizon and have been chatting for the last couple million years. Alice's subjective experience has been that each time she sends a message to Bob, it takes significantly longer to get his response than it did last time. But Bob's experience mirrors Alice's. It's taking longer for her responses to reach him as well. Their experiences are symmetrical and each can compute the date at which they will no longer be able to communicate and even chat about their experience of losing touch forever but neither one anticipates ceasing to exist.

Now let's imagine that decoherence (the process under MWI by which Everett branches cease to be able to interact with each other) is a gradual process rather than an instantaneous one. Even if this is false, it's necessarily true for large complex structures like people built out of many atoms. Alice has been chatting with her Everett clone who's switched gender identities and taken to calling himself Bob. That is, Alice and Bob are composed of different quantum states of the same molecules and they are interfering with each other because particles do that sometimes. As Bob is fading from existence, Alice says to him, "Oh no! You're fading away!"

Under MWI, we might expect Bob to say "Well it looks to me like you're fading away!". Their experiences are symmetrical just as they were in the cosmological event horizon example. Neither views themselves as being in the process of disappearing. They're just losing track of each other.

Conversely, in a one-universe model, Bob might notice himself fading from existence (like Marty at the "Enchantment Under the Sea" dance in Back to the Future).

Initially, this feels like an answer to me, but the problem is that I don't think anyone subscribes to what I claimed above was the MWI model. Bob behaves as if he's still interacting with his universe and it's still solid. Suppose Bob's hands have faded away in Alice's universe. So Alice can no longer interact with Bob's hands. She asks Bob to clap his hands and tell her if they make noise. He claps his hands and claims that they do make a noise (because that's what he's experiencing). This means that the molecules in his hands, while no longer directly affecting Alice's universe by bouncing light into her retinas, are still indirectly affecting Alice's universe by sending pressure waves to Bob's ears, transforming into electrical signals in his brain, and then causing him to speak words which become pressure waves that reach Alice's ears and then become electrical signals in her brain.

But decoherence isn't about things becoming "invisible" in that they impart some effects on your universe but not others, it's about things ceasing to impact your universe in any way whatsoever.

Answer 1

Wittgenstein proposed a view on this in his Tractatus Logico Philosophicus, which is to say that a world is a totality of facts, rather than of things. We understand possible worlds not in terms of the potential states of their objects (which only “exist” because of their combination in structured states of affairs), but rather in terms of logically possible alternative *ways things could be”. Some proposition that does not correspond to a fact, but is instead only a possibility, does not give rise to things that “exist in potentia” because the possibilities are non-factual.

This type of approach also shifts our perspective away from trying to identify particles or individuals across possible worlds and more towards the idea that we might have “counterfactual counterparts” - there is no “me” in a scientifically possible non-actual world, but there might be someone who is similar to me to all intents and purposes that matter in the present context. I can talk about them in ways that might seem interesting in discussing notions of counterfactual possibility, using the terminology of modal logics to do so.

So the delineation is about which statements do we consider to correspond to facts and which do we treat as mere possibilities. A scientific realist has a clear answer to this - the facts are only determined by stuff in the world, and what we work with is our best-and-under-review model of what that stuff is as determined through rigorous evidence-seeking practice. But this view does also allow for other models of factivity.

Answer 2

Helmuth Plessner developed a philosophy of nature with input from biology, most explicitly laid out in his book The Levels of the Organic and the Human (1928). One could say that our universe, which he would call "world", can be defined as

The sum of interactions with our living processes (both within and outside of our physical body) that are consciously represented (within a given cultural sphere, not individually).

This view is obviously not essentialist or realist in the traditional sense. It basically accounts for the fact that every living being does only have "its" surroundings insofar it is able to perceive and interact with it - a relation which can also be mediated (eg. by instruments) - as well as our ability of cultural reproduction and symbolic representation, which constitutes a world which is intersubjectively shared (see Tomasello's theory of shared intentionality for a recent empirical validation of this theoretically derived aspect). Furthermore, it has no qualms with the development and cultural differentiation of life-worlds/universes.

The theory is also quite Kantian insofar it rejects any "existence" that transcends our abilities of interaction, ie. it does not make sense to speak of a transcendent reality or "other universes" but as a metaphorical manner of speech, even though Plessner does argue that we necessarily do so - it is one of his three foundational anthropological laws.

As a more direct answer to your second note from the viewpoint of this theory: Obviously, as soon as all culturally interacting individuals are categorically unable to interact with a certain entity, they will agree that this entity is not part of their universe (anymore). But, as I said, this would be due to the living processes continuing to happen and not having points of interaction (anymore). There is no symmetrical relation of physical existence, we need both the intentional opening to the outside (a "reaching out" or "attempt of interaction" from) and a perceivable "feedback" to living processes. Plessner adopts Uexküll's concept of a "life-circle" here. But it is the life-form which just is (in the sense of being in the process of living) and points of interaction which exist for it insofar there is interaction.

For example, to a normal chimpanzee, a book without pictures is nothing more than something it can throw, hit with, rip apart, etc. and only has existence as an object in this sense for it (just like to toddlers). The existence as a book and possible holder of knowledge etc. for us is only due to us being able to understanding it to be a book which can be read, ie. our abilities of interaction. This has nothing to do with species or the like, just our biological abilities plus cultural upcoming (as demonstrated by some studies on apes and monkeys).

Answer 3

Old Q, but missing an easy A, so:

In physics, a universe is the set of all processes which can affect the process under examination. The universe is the set of all processes which can affect any process examinable.

Answer 4

A universe is the space-time, regardless of whether there is something in it or not. It is the stage, if you will. It doesn't depend on the observer. The only thing that differs between observers is what the "observable universe" is, which is by definition the part of the universe that is observable to a specific observer. The observer is always in the middle of that, but the universe as a whole is (thought to be) infinite and has no middle point.

All of that has NOTHING to do with Quantum mechanics and it's interpretations (MWI). This ONLY has to do with space-time expanding and light speed being constant, which causes a finite horizon for the observable universe.