identical universes

[Copernicus]

There are some people proposing that there would be infinite identical universes in a multiverse world. Lets say there are infinite universes. Hypothetically if time can be compared to one revolution of a circle, but there are no increments on that circle, could there actually ever be one identical universe?

[Cougar]

Hypothetically if time can be compared to one revolution of a circle....

But it doesn't appear that time will ever revolve around and start repeating itself!

And really, I care little about whether the universe is 'infinite' or not. Judging from current conditions, all the stars will eventually 'burn out', then the novelty of "whatever can happen, will happen" will rather burn out as well....

[malaidas]

are you referring to alternate timelines here?

if so then we are pretty much beyond the bounds of science. If however we aee talking different branes that are the same as ours, then I would guess this would depend again upon what you mean. Minor differences would make no real difference upon observation but presumably would not preclude the two existing, however I would doubt this would yield infinte, although it would be a very large number if all possibilities were to exist

[Swift]

This section of the forum is for astronomy and space exploration questions with straightforward, generally accepted answers.

Questions that are likely to lead to extended discussion about the correct answer, or that have no clearcut correct answer, should be posted in the forum most appropriate to the topic of the question.

This thread does not seem to meet the Q&A criteria. Moved from Q&A to S&T. Please don't make me have to move it to ATM.

[Copernicus]

are you referring to alternate timelines here?

if so then we are pretty much beyond the bounds of science. If however we aee talking different branes that are the same as ours, then I would guess this would depend again upon what you mean. Minor differences would make no real difference upon observation but presumably would not preclude the two existing, however I would doubt this would yield infinte, although it would be a very large number if all possibilities were to exist

I was talking about some work by Tegmark talking about how far we would have to go to find an exact universe like ours. The wikipedia reference https://en.wikipedia.org/wiki/Multiverse

"Level I: An extension of our Universe[edit]
A prediction of chaotic inflation is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical laws and physical constants. In regard to configurations such as the distribution of matter, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon, there will eventually be Hubble volumes with similar, and even identical, configurations. Tegmark estimates that an identical volume to ours should be about meters away from us.[19]
Given infinite space, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe.[50] This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique."

It is also interesting to note the following information that may indicate Tegmark believes the Hubble volume to be the extent of our universe. Which I am partial to.

[malaidas]

personally, i don't think (to my knowledge) there is reasonable cause to give any particular size to the universe except that it will be a finite number at least equal to and most likely significantly greater than the observable universe. I am also far from clear why this would effect the number of equivalent universes (ETA: sorry but have to add: the moment we start talking about infinity, then by definition factors like size of universe become irrelevant, for instance 10% of infinity is infinity. it might be theoretically a smaller number but it's still infinite)

Anyway, i'll read up on the reference and get back to you if this remains in S&Tr

[Ken G]

Given infinite space, there would, in fact, be an infinite number of Hubble volumes identical to ours in the universe.[50] This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique."

That argument strikes me as putting the purpose of the cosmological principle on its head. We don't have a cosmological principle because we have some reason to believe it is true, such that we then go out and make all our other theories consistent with it. Instead, we have that principle simply because we have no reason not to, but as scientists, we are always looking for the reasons not to. As such, it is no kind of principle to be extrapolated to absurd extents, it is a principle to be tested skeptically.

[malaidas]

here's my take on this. it all depends upon whether 2 universe's with completely identical nature can exist. if they can then yes given in theory there are an infinite number of universe's then the number of copies must be infinite. however if something like the Pauli exclusion principle applies then given a universe only has a finite number of states (extremely large but finite) then there can only be a finite number of copies of it.

[Copernicus]

That argument strikes me as putting the purpose of the cosmological principle on its head. We don't have a cosmological principle because we have some reason to believe it is true, such that we then go out and make all our other theories consistent with it. Instead, we have that principle simply because we have no reason not to, but as scientists, we are always looking for the reasons not to. As such, it is no kind of principle to be extrapolated to absurd extents, it is a principle to be tested skeptically.

What I posted was from Wikipedia. I had assumed it was something from max tegmark.

[Ken G]

What I posted was from Wikipedia. I had assumed it was something from max tegmark.

It's certainly fine from the perspective of "here are the ramifications of the cosmological principle", but we must be careful not to use it to say "this is what the universe should be like." Observations tell us what the universe is like, not theory!

[malaidas]

precisely Ken, we have string theory (hypothesis ) that suggests that other universes are possible,; we have the weak anthropic principle that suggests that this gives an answer to the boundary conditions.... but we have no evidence whatsoever to actually support or deny the claim. All of this is pure suppositiono

[Copernicus]

precisely Ken, we have string theory (hypothesis ) that suggests that other universes are possible,; we have the weak anthropic principle that suggests that this gives an answer to the boundary conditions.... but we have no evidence whatsoever to actually support or deny the claim. All of this is pure suppositiono

I didn't want to even get into are there other universes. I was looking for mathematical possibilities that would show that no two universes would be exactly alike. Even if there were infinite universes.

[malaidas]

I would personally say that this will inevitable come down to just what you mean by the same? if we are talking 11 dimensional space i see no reason to suggest that 2 apparently similar universes couldn't exist, but apparently similar is not 'the same'.

[Copernicus]

I would personally say that this will inevitable come down to just what you mean by the same? if we are talking 11 dimensional space i see no reason to suggest that 2 apparently similar universes couldn't exist, but apparently similar is not 'the same'.

Some of the people proposing other universes propose that there are an infinite number of universe identical to our universe in every way. Please see this article. http://www.space.com/18811-multiple-...-theories.html

[John Mendenhall]

Speculation, speculation.

Personally, I think there are an infinite number of universes on the head of a pin.

(Your underlying assumption has nothing to support it. We can't even tell if this Universe is infinite. GIGO. The question is meaningles. You can have any anawer you want.)

[malaidas]

ok skipping 1-3 for a second. 4 is an inaccurate understanding of many worlds and 5 is something more suited to the reality thread, it's mathematical realism not science. ok 1 this depends utterly on an infinite expanse of space but whilst possible the evidence of expansion not just being matter but space itself would tend to argue against this. 2) yes possible 3) already covered.

equally with 1 it has a very geocentric nature as written in your citation. one of the huge arguments against the observable universe being significant beyond the restrictions imposed by C and the age of our 'universe' is that this would place us at the centre for which there is completely no reason to uphold indeed every reason not To

[Copernicus]

Speculation, speculation.

Personally, I think there are an infinite number of universes on the head of a pin.

(Your underlying assumption has nothing to support it. We can't even tell if this Universe is infinite. GIGO. The question is meaningles. You can have any anawer you want.)

I also think there are an infinite amount of universes on the head of a pin. I am just wondering if there mathematical hypothesis that would show conditions under which it would be impossible for there to even one identical universe.

[Jens]

here's my take on this. it all depends upon whether 2 universe's with completely identical nature can exist. if they can then yes given in theory there are an infinite number of universe's then the number of copies must be infinite. however if something like the Pauli exclusion principle applies then given a universe only has a finite number of states (extremely large but finite) then there can only be a finite number of copies of it.

Thinking logically, I would agree that if there are an infinite number of finite-sized universes, then yes each one will be repeated infinitely. It becomes tricky if they are infinite in size, however. Then I think it's equivalent to dividing by zero, so I think it becomes an invalid question. There would not be any meaning to "identical" with an infinitely sized one.

[malaidas]

oh and bubble universes suggests a finite number.... large but finite because no process like that could yield infinity. you cannot get infinity unless you start with infinity, by definition.

[malaidas]

Jens, yes if the size of the universe is infinite then all bets are off and we are in the realm of mathematical absurdity

unless of course case 1 in Copernicus's citation is true, in which case you have an infinite number of Sub-universes.

[malaidas]

I also think there are an infinite amount of universes on the head of a pin. I am just wondering if there mathematical hypothesis that would show conditions under which it would be impossible for there to even one identical universe.

if there are an infinite number of universes, in whatever sense then I agree if there can be a copy there must logically be an infinite number and by extension alternate timelines play out independently of our own universe etc. The issue as you say is, can there be even 1 copy. but i don't think anyone is qualified to make an assertion either way, scientifically. we are well into the realms of pure philosophy. Mathematically i don't think there is a reason it couldn't be true, but as Ken says Maths is not the same thing as empirical science and there is nothing to go on here. It's quite probable that a model could be hypothesized both ways, without evidence the question even mathematically is pure speculation

[malaidas]

as an aside i will leave it to other better qualified people on here to explain just why the many worlds explanation of Quantum is not the same as multiverse and why therefore it's not pertinent to the discussion.

[Copernicus]

if there are an infinite number of universes, in whatever sense then I agree if there can be a copy there must logically be an infinite number and by extension alternate timelines play out independently of our own universe etc. The issue as you say is, can there be even 1 copy. but i don't think anyone is qualified to make an assertion either way, scientifically. we are well into the realms of pure philosophy. Mathematically i don't think there is a reason it couldn't be true, but as Ken says Maths is not the same thing as empirical science and there is nothing to go on here. It's quite probable that a model could be hypothesized both ways, without evidence the question even mathematically is pure speculation

This is my question. One has a layer of atoms. In comes a photon. The layer of atoms is not perfectly smooth. How many angles can that layer of atoms be impacted with. My guess is that it is infinite angles. Is it wrong to think that if that layer can be impacted with infinite angles, because it is not completely flat, that exact outcomes can never be the same? And therefore, there will never be identical universes.

[DaveC426913]

Jens, yes if the size of the universe is infinite then all bets are off and we are in the realm of mathematical absurdity.

It doesn't have to be infinite, merely arbitrarily large.

There was a Sci Am article a few years back that calculated how often, in an arbitrarily large universe, our observable universe would start repeating.


They started with a "universe" of only four pixels, and showed how far you could tile the volume before you had to repeat yourself.
Then they "just" scaled it up by the number of units and possible states in our observable universe. (it's a big number, but not a complex calculation).

All you had to do was go X number of petametres in any direction, and you'd arrive at a universe identical to ours.

[DaveC426913]

This is my question. One has a layer of atoms. In comes a photon. The layer of atoms is not perfectly smooth. How many angles can that layer of atoms be impacted with. My guess is that it is infinite angles. Is it wrong to think that if that layer can be impacted with infinite angles, because it is not completely flat, that exact outcomes can never be the same? And therefore, there will never be identical universes.

Well, that probably involves the Planck unit - the smallest possible unit of volume. So you don;t have an infinite number of states.

[malaidas]

This is my question. One has a layer of atoms. In comes a photon. The layer of atoms is not perfectly smooth. How many angles can that layer of atoms be impacted with. My guess is that it is infinite angles. Is it wrong to think that if that layer can be impacted with infinite angles, because it is not completely flat, that exact outcomes can never be the same? And therefore, there will never be identical universes.

we're onto the question here of whether or not the universe is continuous or discrete? every scientific theory/hypothesis clothes it in a discrete nature which would leave the number of angles very high but not infinite as Dave illustrates in the post below yours

the problem with a finite number however is it doesn't allow the same logical conclusions as an infinite one. so yes in terms of Dave's example there could be a very large number of copies but it doesn't alliw the same conclusions as if it were infinite.

[malaidas]

It doesn't have to be infinite, merely arbitrarily large.

There was a Sci Am article a few years back that calculated how often, in an arbitrarily large universe, our observable universe would start repeating.


They started with a "universe" of only four pixels, and showed how far you could tile the volume before you had to repeat yourself.
Then they "just" scaled it up by the number of units and possible states in our observable universe. (it's a big number, but not a complex calculation).

All you had to do was go X number of petametres in any direction, and you'd arrive at a universe identical to ours.

I don't argue with what you are saying, but if the number is finite you are left with no reason to assert that a particular copy exists.

ETA: the immediate question is, can a separate sub universe be different to ours? is so then in potentia all copies, (bar our own universe of course), could be of it and none of ours. only if the number of copies is infinite can this not be the case, unless the probability of a copy of our particular universe exusting was zero, which is kind of paradoxical in and of itself, seeing as we exist.

[malaidas]

It doesn't have to be infinite, merely arbitrarily large.

There was a Sci Am article a few years back that calculated how often, in an arbitrarily large universe, our observable universe would start repeating.


They started with a "universe" of only four pixels, and showed how far you could tile the volume before you had to repeat yourself.
Then they "just" scaled it up by the number of units and possible states in our observable universe. (it's a big number, but not a complex calculation).

All you had to do was go X number of petametres in any direction, and you'd arrive at a universe identical to ours.

I don't argue with what you are saying, but if the number is finite you are left with no reason to assert that a particular copy exists.

[malaidas]

apologies for the accidental copy

however putting things into context, if there were a google copies in this many sub universe array, but the chance of our universe existing, (for whatever reason), within in which was 1/googleplex it would hardly be surprising to find only 1 or 2 copies of it extant. This would blow any philosophy about multiple copies of our universe out of the water, there has to be a lot of copies, in fact tending towards infinite at very least for it to have any real play in what is being hypothesised.

The true answer may be in fact unknown here, indeed probably lies outside of science to determine, but we can at least establish what the variables are as it were.

[DaveC426913]

I don't argue with what you are saying, but if the number is finite you are left with no reason to assert that a particular copy exists.

No, that's exactly opposite to what I'm saying.

I'm saying it is possible to calculate just how large the universe needs to be before a given observable universe has to start repeating itself.
It may be large, but it is a far cry from infinite.

The SciAm article showed a trivial example of a "observable universe" with a mere four locations and four states (colours).
Place another one next to it, and give it a different set of states.
You can make about 2 dozen "observable universes" before you run out of unique combinations.
If your universe is ore than 2 dozen OUs in extent, you must have repeating OUs.

Scale that up to the size and possible state of our IRL observable universe, and you only need a certain size of universe to hold them all. It's large, but not infinite.


Actually, I think the SciAm article may not have been quite so ambitious. They may have been just posited a duplicate "Earth". How any planets must be observed before one runs through all possible combinations pf planets, and must start repeating them? Again, it's large, but not infinite.