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For the purpose of this questions let's assume that the physics of our universe can be fully described by a complete non-contradictory theory (i.e. that theory of everything exists). Then our universe can be described by that theory and an initial state, i.e. a pair Universe0 = (ToE0, S00) in a sense that (assuming for simplicity the physics of our universe can be described by an evolution of a state in discrete or continuous time) any future state can (in math sense) be uniquely determined from S00 by applying the laws ToE0. Imagine some other non-contradictory theory and an initial state Universe1 = (ToE1, S10). E.g. a huge deterministic virtual machine with discrete time, or a universe with more familiar spacetime, but with 5+1 instead of 3+1 dimensions. For simplicity assume that no full simulations of Universe1 were ran in Universe0 and vice versa.

All of us, including all the readers of that question on StackExchange are parts of Universe0, and the neurons in our brains react to this question exactly as follows from ToE0 and the initial state. Most of us believe that we exist.

If the laws of ToE1 and the initial state S10 lead to complex structures being part of Universe1 being able to wonder about its fundamental laws and maybe even ask each other questions similar to this one, then such structures might believe that they exist for the same reason we do (why wouldn't they?). In other words it appears that Universe1 can be as subjectively real for such structures as Universe0 is subjectively real for us.

It appears that the situation is symmetric. Universe0 for structures in Universe1 is no more than Universe1 for humans in our universe (Universe0): in both cases the other universe is just a non-contradicting theory describing its evolution and the initial state. Does that mean that Universe1 actually exists in the same sense our universe exists? Or is there something which makes us and our universe more objectively physically real than Universe1 or any other theoretically possible universe? And if there is no such objective difference, isn't that the answer to "Why is there something instead of nothing?"?

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  • You seamlessly passed from "universe can be described by a non-contradictory theory" in paragraph 1 to "universe is a non-contradictory theory" in paragraph 4. An apple can be described by abstractions, but you can't put abstractions in your mouth. That's the objective difference. You also passed from "imagine some other non-contradictory theory" to "the other universe is". If you are concluding "existence" from "is" your argument is circular, and if it is from "imagine" that would be the difference between imagination and reality.
    – Conifold
    Dec 3 '20 at 0:43
  • @Conifold Thank you, edited to clarify. In the case of physics which can be formulated as a deterministic evolution of a state in discrete or continuous time (this is not a very restrictive assumption: both GR without closed time-like curves and quantum mechanics can be formulated as such), by "described" I mean that in Universe0 any future state is uniquely determined from any past state by applying the laws ToE0 (in mathematical sense). By "fully described" I mean that states of the universe are in 1:1 relationship with some elements of a rigorously defined set (in the sense of math).
    – fiktor
    Dec 3 '20 at 1:35
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    The difference between a description, no matter how "fully" it is one, and what it describes remains, existence of one has nothing to do with existence of the other.
    – Conifold
    Dec 3 '20 at 1:39
  • The question is only about the existence of the other (what it describes), existence of a description is understood in the sense of mathematics (e.g. an existence of a full description of Pac-Man simulation can be verified by writing down the program and a mathematically rigorous description of the language), and the (math) existence of ToE for our universe is the main assumption (although quite a reasonable one given our success record in fundamental physics).
    – fiktor
    Dec 3 '20 at 2:11
  • "Theoretically possible" is the same as mathematical existence. Our universe, existence or non-existence of ToE for it, and determinism are entirely moot, any description is always a ToE of what it describes. So you might as well take any description, deterministic or otherwise, and ask whether consistency makes what it describes exist non-mathematically (it doesn't). In sum, it is unclear what you are asking.
    – Conifold
    Dec 3 '20 at 5:10
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That is an excellent question! The right kind,1 as Socrates would put it, because, in order to answer it, we would have to, in a way, solve Philosophy first ;) -- answer the most basic questions, like the nature of reality, or the meaning of truth!

(... and yes, we can -- in fact, as you can see below, we always knew the answers... it's remembering them recently became like
Socrates above: Humans! Beware of busy life!
The novel "civilization": ... 😑
The novel "civilization": i'll show you "busy life"...)

Anyway, you were saying:

In other words it appears that Universe1 can be as subjectively real for such structures as Universe0 is subjectively real for us.

And so it does, no argument here. At least it would appear that way IF "subjectively real" was, actually, a thing... which it isn't -- it simply cannot be.

"Subjectively real" -- or "true" -- is an oxymoron: in what sense something "is true", if it is only true for you, yet false for others?

And yes, your question was about "real", not "true", but that IS what "true" actually means -- it means it is real! Or, if you want, it is a factual statement, one that accurately describes the One and only -- the objective reality, which we all share and are a part of. And which existence we must assume1, well, for that very reason -- having the concept of truth, or, yes, being able to tell "real" from "possible", or "possible" from "impossible". Which, in turn, we need to know our options, to have a conscious choice, and, hence, the freedom to act -- our agency.

The latter, btw -- the agency -- is what consciousness is about: beyond mere awareness, it means the capacity to act consciously. And that's what Philosophy is ultimately about, because "To be is to do" -- that's Socrates. And I don't care if it wasn't, that's still him... This one is from Plato's "Gorgias":

"Do not take what I say as if I were merely playing. For you see, the subject of our discussion -- and on what subject should even a man of slight intelligence be more serious? -- namely, what kind of life should one live..."

Summing up the above, it's not about what's possible -- it's about what we can possibly afford.

 

1 In fact, it is so on point, I can't help but wonder if you knew the answer already ;) Either way, thanks a bunch for asking it!

2 The other assumption we must make is that this realty is, fundamentally, explainable through what the Ancient Greek referred to as λόγος, or -- in a rather narrow sense -- the logic and reason.

"Through [the Lógos] all things were made; without it nothing was made that has been made. In it was life, and the life was the light of men. And the light shineth in darkness; and the darkness comprehended it not." -- John 1:3, the Gospel's opening verses.

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