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### Parallel universes either don't exist or can't be accessed

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Let's take a look at the highly optimistic version of the parallel universes theory; the one that says any time something can happen more than one way, the universe/time branches for each and every way it could possibly happen. This is great and all, and it means that there are infinite parallel universes, and infinite of each kind you would specifically want.

The problem with this, as I see it, is that that means there should be infinite parallel universe where they found a way to travel through time between various parallel universes. This means that we should have already seen infinite universe-jumping time travelers, even if they arrived by mistake. And I really do mean infinite. They would occupy every space in our universe. Only, they haven't, which means one of four things:
1. There are no other parallel universes
2. Parallel universe theory is wrong and there aren't infinite parallel universes
3. It is not possible to travel between parallel universes
4. It is not possible to travel back in time and no parallel universes have invented time travel yet

And before you say "maybe we're one of those infinite parallel universes that doesn't get any visitors", I think that's giving time and space a little too much credit; if it is possible to travel between parallel universes, I think that would fall outside the branch-for-every-possibility mechanism.

What are your thoughts?
 And I really do mean infinite. They would occupy every space in our universe.

Ignoring the universes thing, this is a misunderstanding of the nature of infinities. Hilbert's Hotel is a good place to start: http://en.wikipedia.org/wiki/Hilbert's_paradox_of_the_Grand_Hotel
Isn't it possible for us to be in a universe where "universe-jumping time travelers" never come to our universe? If there exists a universe for every possible event, it would make sense that there is at least one universe where no one ever jumps from another universe to theirs.
The Grand Hotel applies to infinite countable sets. It's not clear what the cardinality of the set of all possible universes would be. It depends on whether space-time is continuous or discrete, and finite or infinite.

 And before you say "maybe we're one of those infinite parallel universes that doesn't get any visitors", I think that's giving time and space a little too much credit; if it is possible to travel between parallel universes, I think that would fall outside the branch-for-every-possibility mechanism.
This preemptive rejection seems unjustified to me. First you say that there must be at least one universe where every chain of events occurs, then you say that a particular chain of events can't occur in one particular universe.

Suppose I throw a fair coin once per second. If heads, some trans-universal traveler arrives anywhere in the universe during that second. If tails, none does. If the number of throws is infinite, then a sequence of 157.78476e+15 tails in a row is certain to occur sooner or later.
If you now do the same with infinitely many coins all independent of each other and thrown at the same time, the probability of that sequence appearing immediately in at least one of those coins is 1. In fact, the probability of at least one coin turning up infinitely many tails is also 1. Why? Because the set of all those infinitely many infinite sequences is exactly as large as the set of real numbers, and that last probability is equivalent to the probability of finding one particular real in the set of all reals.
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I never understood the point behind parallel universes, other than that it makes for some semi-interesting Sci-Fi stories.

Disregarding the notion that parallel universes don't make any sense at all... if these parallel universes can't be observed, and their behavior has no impact on our universe... then what's the point in even theorizing that they exist? What does their existence imply?

Basically: why should I care enough to pursue the idea further? Because right now it seems pointless to me.
Disch wrote:
Because right now it seems pointless to me.

Everything in this realm of "study" (as in guessing, and being unable to prove right or wrong your guesses), almost everything is pointless to the real world.

The way I see parallel universes is that they only exist in our minds, not in actual physical manifestations in some other space time continuum. We can theorize what the world would be like if B happened instead of A, but that doesn't mean that there is another universe somewhere where it actually happened that way. All talk of this is merely for theoretical fun; an interesting mind problem with no real solution. Nothing more.
L B wrote:
this means that we should have already seen infinite universe-jumping time travelers

That's just a variant of http://en.wikipedia.org/wiki/Fermi_paradox (there are so many extraterrestrials, why don't they visit Earth?)

Disch wrote:
If these parallel universes can't be observed, and their behavior has no impact on our universe... then what's the point in even theorizing that they exist?

Based on L.B's description, I think this is about the many-world interpretation of quantum mechanics, which is, of course, not a theory (it's an interpretation), and which sometimes comes up with potential future experiments that would demonstrate inter-world communication. (personally, I don't think quantum mechanics needs an interpretation, it works just fine as it is. When pressed, I go with path integrals)
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I believe there are parallel universes and to think there isn't would be narrow minded of us. It would be the same as thinking we are alone in the universe. This is a point I've brought up numerous times on other forums. I've always pointed out that what is science fiction to us would be reality in one of the parallel universes. Hercules and the gods exist, the United Federation of Planets exist, the Force is with you, etc. In some realities our lives may very well be their soap operas.

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The one thing we have to think about though, is that for every parallel universe, there is a parallel version of us. We could be rich in one, poor in one, dead in one, casanova in one, single, a killer, etc.
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 I think this is about the many-world interpretation of quantum mechanics

Well I suppose I don't know enough about quantum mechanics to intelligently comment.

 I believe there are parallel universes and to think there isn't would be narrow minded of us.

Skepticism is not narrow-mindedness. I don't see any reason to blindly believe [seemingly] crackpot notions without a shred of proof.

This sounds to me like some stoners came up with the idea:

- "Woah dude, what if there were like, another universe where everything was the same except I was a millionaire."
- "Woah dude, that'd be awesome. What if there were like an infinite number of those universes!"

It just seems so incredibly far fetched... and I haven't seen anything to really suggest it could be true.

But as I said to Cubbi, I don't know anything about Quantum Mechanics. Perhaps there's some deeper science behind it that I'm unaware of, but from where I stand right now the whole concept is anecdotal at best.
You believe many other things with just as much proof, such as that your sensory input is correlated with an external experience, or that other people are not unconscious automata (aka p-zombies). Where's the harm in one more unprovable belief?

 This sounds to me like some stoners came up with the idea: [...]
Surely you can come up with something better than an appeal to ridicule.
There may not exist parallel universes since we can only ever experience a single branch, and are unable to see the branching as it happens. How and why do we "choose" a branch? Maybe we don't, because maybe they don't exist at all.

 4. It is not possible to travel back in time and no parallel universes have invented time travel yet

http://www.dedoimedo.com/physics/time.html
helios wrote:
Surely you can come up with something better than an appeal to ridicule.

Nope. That's the best I've got.

With that, I'll bow out. =P
Quite a while back I read an article in New Scientist, that proposed parallel universes could be created by "folding" a piece of space-time fabric back onto itself, like folding a sheet of paper in half. They were saying that a parallel universe could exist millimetres away from our universe.

I think that was the nuttiest thing I had ever read, and wondered whether the authors were totally stoned when they wrote it.

To create such a piece of space-time fabric such as this would require a completely impossible arrangement of mass.

One of the things about General Relativity is the way that the space-time fabric drawings can be abused big time.
 One of the things about General Relativity is the way that the space-time fabric drawings can be abused big time.

More that one of the things about GR is that attempts to explain it in terms of everyday objects lead to abuses because GR is simply not like everyday objects.

http://xkcd.com/895/

Disclosure; I did not do well in my GR exam :p
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Maybe no universe figured out how to universe-jump...
Across infinite universes there would be infinite time (unless we're assuming that they all "came into existence" at the same time) and with infinite time, the probability of every event is either zero or one, i.e., everything that can happen eventually does happen. Therefore if jumping between universes was possible, it would have been done.

It is also possible that, assuming there are infinite universes and only a subset of them (possibly even only one) has a species capable of travelling to different universes, they just haven't come to ours yet (and perhaps they never will).

The problem with this is saying "subset of infinity"... surely infinity divided by any other number is still infinity. Otherwise you'd be able to produce infinity by multiplying natural numbers. And this is all assuming "infinity" is even a number, which it probably isn't.
Infinite universes does not mean everything that can happen has happened. It just means everything that can happen has OR will happen. Just because cross-universe hopping hasn't occurred (to our knowledge) doesn't mean it's impossible. Infinite universes is still limited to time. There has only been X amount of time that humanity has been around.
I just view multiple universes, as an array:

multiverse = new universe[?/0][?/0][?/0]...; // or something along those lines.

I'm not going to get into my thoughts on devision by zero.
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As ResidentBiscuit says, infinite universes does not mean everything is possible. If there were infinite universes with a light-speed limit, none of them would contain faster than light travel, for example.

As an aside, I don't subscribe to the infinite universes theory. I view it as a bad attempt to explain away the apparent non-deterministic nature of quantum mechanics that has itself been picked up and run away with (this is not the same as suggesting there is only one "universe"; I just don't subscribe to the branching "every choice separates into separate universes" idea).
 Across infinite universes there would be infinite time
What do you mean "across"?

 The problem with this is saying "subset of infinity"... surely infinity divided by any other number is still infinity. Otherwise you'd be able to produce infinity by multiplying natural numbers. And this is all assuming "infinity" is even a number, which it probably isn't.
First of all, make up your mind. Is infinity a number or a set? If it's a number, then you can't talk about a subset of a number. If it's a set, then you can't talk about dividing a set by a number, and of course you can't make a set by multiplying two numbers, because that's just silly.
Second, what's the problem with saying "subset of an infinite set"?
 It is also possible that, assuming there are infinite universes [set A] and only a subset of them [set B] (possibly even only one) has a species capable of travelling to different universes, they just haven't come to ours yet (and perhaps they never will).
How we reason from here depends on the cardinality of A and B. If A is countable, B can be either finite or infinite, but it will be countable. If A is uncountable, B could be finite, or infinitely countable, or uncountable. Just saying "a subset" doesn't mean much, because a subset of an infinite set can be as large as the original set.

 As an aside, I don't subscribe to the infinite universes theory. I view it as a bad attempt to explain away the apparent non-deterministic nature of quantum mechanics that has itself been picked up and run away with (this is not the same as suggesting there is only one "universe"; I just don't subscribe to the branching "every choice separates into separate universes" idea).
I agree completely.
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