[bobdihi (OP)]

Lets assume there is more than one Universe. Looking up from this planet, how do we know that we not looking
into another universe. Maybe they overlap? Surely there is no borders between Universes. Or how would we know where one Universe ends and other one starts?
  Sorry if this question was asked already.

[Kryptid]

That would depend on how you define a "universe": https://en.wikipedia.org/wiki/Multiverse#Classification_schemes

[Bill S]

That would depend on how you define a "universe"

Absolutely.  John Gribbin’s usage does help a little.

Cosmos = everything that exists, or can exist.
Universe = our (in principle) observable portion of spacetime and its contents.
universe = any other universe that may, or may not, exist.

[Zer0]

A Link which provides an explanation n sheds light on the Subject. 👌

https://www.thenakedscientists.com/articles/features/do-we-live-multiverse

It showcases a viewpoint on Mutiverses from three different theories namely:
1) Astrophysics.
2) String Theory.
3) Quantum Mechanics.

Conclusion (from within the article) 👍

All three of these theories may point to other universes, suggesting we live in a multiverse. If we live in an infinite universe, there are exact replicas of ourselves and every possible scenario is played out somewhere in space. If string theory is correct and we live in more than three spatial dimensions, there could be other universes out there, perhaps similar to ours or perhaps entirely different. And finally, according to quantum physics, every time a decision is made, every outcome happens as the universe splits into parallel universes, and we are just one of many similar universes. If any of these theories are correct the implications are complicated. What determines which universe we live in? What is the significance of our own existence if there are other universes?

P.S. - Yes, most questions asked are similar in nature n related to fundamental concepts such as time, space, gravity, black holes etc etc.
But no need to be apologetic for asking a similar question cause Scientific Knowledge is Dynamic in nature.
Laws are theorised, made concrete n then modified or changed over a period of time.(not all laws are meant to be broken)
So a question asked in the past might not necessarily have the same answer in the future. ✌

Could someone spend a few braincells n shed some light of wisdom on the concept of a multiverse from a thermodynamic & entropy point of view.
Q - Should we stop considering the universe as a closed/isolated system?

[Bill S]

If we live in an infinite universe, there are exact replicas of ourselves

Everything that can happen, does happen.  That, in itself, is quite misleading, but I would then ask: What evidence do we have that the occurrence of "exact replicas of ourselves" is even possible? 

What is an "exact replica"?

[Bill S]

This is, possibly, more relevant to the OP.

  Amanda Gefter (New Scientist, 14.07.2007. Pp 30-34) says this of extra dimensions.  “To have gone unnoticed all this time, these dimensions must be curled up like submicroscopic origami.  What’s more, the number of forms that this origami can take is truly vast, with each one corresponding to a different universe with different particles and different fundamental constants.”

 Perhaps “rolledupness” is relative.  A dimension that is rolled up in the RF of one observer could be fully extended in the RF of another.  This must lead us to consider the possibility that all the other universes intermesh with ours because they exist within dimensions that are tightly rolled up in our RF, but expanded in their own.  Such a concept would necessarily imply that our Universe is present in all these other universes in the same way.  In their RFs we are “submicroscopic origami”.
 
Aligning this with Deutsch’s interpretation of the double slit experiment could be somewhat problematic.

[evan_au]

necessarily imply that our Universe is present in all these other universes in the same way.

The idea of "rolled up" dimensions in string theory is that the properties of fundamental particles is affected by how large or small the dimensions are rolled up.

It is possible that a small change in the "diameter" of one dimension could mean that neutrons are stable, but protons and electrons are not (for example). This would mean that none of our familiar elements would be stable, and we could not exist.

String theorists see that string theory can produce many universes with wildly different properties - but they haven't (yet) found one that looks like our universe, with electrons, protons and neutrons, etc.
See: https://en.wikipedia.org/wiki/String_theory#Extra_dimensions

[Zer0]

Hi bobdihi ☺

I found information which relates to the Subject.

Where does one Universe stop and another start?
Here is a link which certainly would prove a good read on the subject.  👌

https://www.ras.org.uk/news-and-press/2982-new-survey-hints-at-exotic-origin-for-the-cold-spot

A little glimpse of what the link contains:  👍

"The cosmic microwave background (CMB), a relic of the Big Bang, covers the whole sky. At a temperature of 2.73 degrees above absolute zero (or -270.43 degrees Celsius), the CMB has some anomalies, including the Cold Spot. This feature, about 0.00015 degrees colder than its surroundings, was previously claimed to be caused by a huge void, billions of light years across, containing relatively few galaxies."

"The researchers instead found that the Cold Spot region, before now thought to be underpopulated with galaxies, is split into smaller voids, surrounded by clusters of galaxies. This 'soap bubble' structure is much like the rest of the universe, illustrated in Figure 2 by the visual similarity between the galaxy distributions in the Cold Spot area and a control field elsewhere."

"Perhaps the most exciting of these is that the Cold Spot was caused by a collision between our universe and another bubble universe. If further, more detailed, analysis of CMB data proves this to be the case then the Cold Spot might be taken as the first evidence for the multiverse – and billions of other universes may exist like our own."

Thanks & Credits - Royal Astronomical Society.
✌

[Zer0]

If we live in an infinite universe, there are exact replicas of ourselves

(1) Everything that can happen, does happen.  That, in itself, is quite misleading, but I would then ask: (2) What evidence do we have that the occurrence of "exact replicas of ourselves" is even possible? 

(3) What is an "exact replica"?

Hi Bill, hope you are doing well & good. ☺

(1) Considering the theory of infinite multiverses, everything/something/anything/nothing can n will happen.
Might sound misleading but consider a dice rolls 6 6 6 three times in sequential order.
Although it might be a rare occurrence but if we multiply the dice * rolls * infinity the chances or percentage of getting a 666 sequence would increase.

(2) In Subjective opinion there can be no evidence of it at all.

(3) An "exact replica"  would be a copy/reproduction/clone of an Original.

P.S. - Maybe someone else might be able to explain (1) in a much better/simpler/logical manner.
But I completely agree with you on (2) cause the multiverse theory hasn't been proven as yet, n even if it does become a scientific fact one cannot conclude that a replica/copy/clone would exist, but only an Identical entity would.
As far as (3) is concerned I don't suppose anyone would try to change my view on the definition of an exact replica cause then they would be wrong in doing so...and even if they try which they surely can, but certainly they shouldn't expect me to change my stance orelse both them & me would be wrong.
✌

[Bill S]

Thanks for the good wishes, Zer0.

….. consider a dice rolls 6 6 6 three times in sequential order.
Although it might be a rare occurrence but if we multiply the dice * rolls * infinity….

How can you multiply by infinity, if infinity is not a number?  OK, I know what you mean, but it is this sort of fuzzy thinking about infinity that, I think, leads to a lot of misleading ideas.

An "exact replica" would be a copy/reproduction/clone of an Original.

Would exact replicas of a conscious being have the same thoughts, at the same time?

[yor_on]

The answer that makes sense to me is Bills.
Amanda Gefter

It's theorizing, then again

=

one thing though, I don't think you need to propose 'rolled up' dimensions for it. In some sense I would go for symmetries instead

[Zer0]

@ Bill S

I don't consider Infinity as a number, I look at it from a symbolic point of view.   ~
Still I won't even try explaining it cause I'm way too small, n infinity just seems never ending.

Q) Would exact replicas of a conscious being have the same thoughts, at the same time?
A) Do identical twins or clones have similar behaviour/likes/dislikes/choices, nope not really.

But considering the theory/thoughts on MultiVerses, just maybe there is a real possibility an Identical Entity would/could have 'same thoughts' provided they exist in a identical/similar/mirror universe and that's where I hear Infinity whisper, " Its Possible ". ~

[Bill S]

Q) Would exact replicas of a conscious being have the same thoughts, at the same time?
A) Do identical twins or clones have similar behaviour/likes/dislikes/choices, nope not really.

Identical twins are,of course, not exact replicas.  I worked for some years with George Newmark; https://www.kirkusreviews.com/book-reviews/john-george-newmark/to-the-zoo-in-a-plastic-box/
and had no problem distinguishing between him and his brother, John.

But considering the theory/thoughts on MultiVerses, just maybe there is a real possibility an Identical Entity would/could have 'same thoughts' provided they exist in a identical/similar/mirror universe and that's where I hear Infinity whisper, " Its Possible "

Be that as it may; I prefer to try to arrive at some sort of logical conclusion.  Even if it is wrong, it is easier to correct than some vague thoughts about what might be possible.

Taken to its "logical" conclusion the idea of multiple, identical, beings results in an infinity of such entities.  Quite apart from any doubts I might have about the validity of talking about infinite numbers of anything, I would look for some clarity regarding what is implied by the term "exact replicas". 

Is the use of that term similar to the the way in which people often use "nothing", when they really mean something which they regard as outside the scope of the scenario they are considering?

[Bored chemist]

Isn't it a bit like asking where the Earth and the Moon meet?

[Bill S]

I think I need you to be a bit more specific about the similarity before I have a go at answering that.

[Bored chemist]

Why assume the different universes stop and start at all?
If they do, why assume  that one stops where the other starts?

[Bill S]

Why assume the different universes stop and start at all?
If they do, why assume  that one stops where the other starts?

Thanks, that clarifies which aspect of the thread is relevant to the question.

I would certainly make neither of those assumptions. 

A lot must depend on the particular interpretation of the broader multiverse concept in question.  I try to keep an open mind on the question of the physical reality of multiple universes, especially those purported to pop into existence at the drop of a hat – or less, but grave doubts are never far away..