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

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closed account (1yR4jE8b)
Just because there is an infinite amount of rational numbers between 0 and 1, doesn't mean one of them is 4.
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 Infinite universes does not mean everything that can happen has happened. It just means everything that can happen has OR will happen.
 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.
 Just because there is an infinite amount of rational numbers between 0 and 1, doesn't mean one of them is 4.
You guys are trying to refute the converse of the argument L B is making.
He's not saying "there are infinite universes, therefore there is one universe for every possibility". He's saying "every possibility branches into a new universe, therefore there are infinite universes".
helios wrote:
Across infinite universes there would be infinite time
What do you mean "across"?

When I say "across infinite universes" I mean taking the entire set of them into account. Unless each one starts and stops existing at exactly the same time, even if there is a significant amount of overlap, it could/would cover an infinite amount of time (assuming time is a meaningful concept when talking about multiple universes).

helios wrote:
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.

Yeah, I mixed up terms a lot there. I couldn't tell you if infinity was a number but I don't think it's a set. When I said subset of infinity I meant subset of infinite universes. Sorry for the confusion.

helios wrote:
Second, what's the problem with saying "subset of an infinite set"?

I don't see how infinity can be divided by a natural number and produce anything but infinity. I can't really conceptualise infinity as a number. When you say "infinite set" I don't think of a set which has infinity elements, I think of a set which has an arbitrary and uncountable number of elements. In other words, I don't think of infinity as having a concrete value.

Moschops wrote:
If there were infinite universes with a light-speed limit, none of them would contain faster than light travel, for example.

That's not really the same thing. I said everything would have a probability of 0 or 1, i.e., completely impossible or completely certain.

Moschops wrote:
I just don't subscribe to the branching "every choice separates into separate universes" idea

I don't believe or disbelieve it, but I think it's an interesting idea.

@ResidentBiscuit,
Yes, you're right, even if the probability of an event was 1, it wouldn't necessarily have happened at a given point in time.
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 Unless each one starts and stops existing at exactly the same time
I have no idea what you're saying now.
A universe is a closed region of space-time (i.e. it's not possible to leave the region by merely moving in any of those dimensions) and the things inside, correct? Then what does it mean for a universe to "start at the same time" as another?

 When you say "infinite set" I don't think of a set which has infinity elements, I think of a set which has an arbitrary and uncountable number of elements.
Well, that's just you, then. "Infinite set" has a very specific meaning: A set whose cardinality is larger than any natural number.
An "arbitrarily large" quantity is not necessarily infinite, either. For example, the gap between two consecutive primes can be arbitrarily large, but not infinite.
"Countable" also has a specific meaning: A set is countably large it's finite or if it can be put in a one-to-one correspondence with the set of natural numbers. This is commonly rephrased to "is as large as N". An uncountable set is of course not countable. For example, the set of reals.

 When I said subset of infinity I meant subset of infinite universes
Before we go any further, you'll have to clarify what an "infinite universes" is. I don't see how it's different from a set of universes. Mostly since you take a subset of it.

 I don't see how infinity can be divided by a natural number and produce anything but infinity.
I didn't mention infinity in that sentence. I assume you meant to write "infinite set" and are muddling the terminology again.
We'd first have to define the division of a set by a natural number, since it isn't trivial.
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helios wrote:
I have no idea what you're saying now.
A universe is a closed region of space-time (i.e. it's not possible to leave the region by merely moving in any of those dimensions) and the things inside, correct? Then what does it mean for a universe to "start at the same time" as another?

Yeah, that didn't make sense, forget it. I was speaking as if the Big Bang for each parallel universe would happen at a different time, but that doesn't really make sense since the concept of time doesn't have any meaning outside of a/the universe.

helios wrote:
Before we go any further, you'll have to clarify what an "infinite universes" is. I don't see how it's different from a set of universes. Mostly since you take a subset of it.

The thread is about the hypothetical existence of an infinite number of parallel universes, right? That's what I was talking about.

helios wrote:
An "arbitrarily large" quantity is not necessarily infinite, either.

I said arbitrary and uncountable, and if it's uncountable then it's infinite (unless you take "uncountable" to mean "too impractical to count", but I didn't mean that).

helios wrote:
I didn't mention infinity in that sentence. I assume you meant to write "infinite set" and are muddling the terminology again.

No, I did mean to say infinity, not infinite set. I don't see how dividing infinity by a natural number can produce anything but infinity. For example, what is half of infinity?
 I can't really conceptualise infinity as a number

This is because infinity isn't a number. It's just a concept.
 I said arbitrary and uncountable, and if it's uncountable then it's infinite (unless you take "uncountable" to mean "too impractical to count", but I didn't mean that).
I think I see what you mean, now.
What I think you mean by "uncountable" is that the quantity is so large that no matter how many more "+1" you add to "1+1+...+1", you'll never reach a value higher than the quantity. Am I right? This by itself is the typical definition of an infinite quantity: a quantity so large that it's larger than any natural number. No need to get "arbitrarily large" involved.
But the way you were saying it just leads to confusion. "Uncountable" has the meaning I stated above. It's a property of sets, not of quantities.

 The thread is about the hypothetical existence of an infinite number of parallel universes, right? That's what I was talking about.
So the answer to my question is "yes"?

 No, I did mean to say infinity, not infinite set. I don't see how dividing infinity by a natural number can produce anything but infinity. For example, what is half of infinity?
Okay, let's take a step back, because this is getting confusing.
Going back to that other post, you said
 The problem with this is saying "subset of infinity infinite universes"... 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.
The underlined sentence is the problem. In the sentence "subset of infinitely many universes", there's nothing to imply that the subset is to have any particular size.
For example, let's take N={1,2,3,...}. Suppose we define a division operation of a set s that returns m sets of equal size. So N/2 = {{2,4,6,...},{1,3,5,...}}. In this case, you're right. The size of N is infinite, and the size of both the sets returned by N/2 is also infinite. But {5} is no less a subset of N than {2,4,5,...}. In fact, you said it yourself:
 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
So, again, what the problem?
(responding to earlier posts) I think the guy named chris and our residential biscuit have missed or disregarded part of my point. You say that id something can happen, it has or will. That means that if time travel the way we ideally envision it could happen, it would, and then if universe jumping were also possible, it would lead to use seeing universe-jumping time travelers. Saying "it just hasn't happened yet" doesn't make very much sense to me; with infinite time travelers, what is there to not happen? Doctor Who sure makes light work of it.
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Again, you are looking too narrowly at this. If universe jumping has been solved in one universe you are forgetting there are an infinite number of universes to jump between and the chances of our universe being visited by them is minimal in that contrast.
There's just as many universes to jump to as there are to jump from.
If the "set of universes" branches out for every possibility, then the possibility that Universe X jumped to our Universe should be in that set. But that possibility somehow spawned a different "our Universe", splitting our Universe in "our Universe where X did not jump to us" and "our Universe where X did jump to us". Where does that leave us? Jumping to our "unjumped" Universe should also be in the set, shouldn't it?

Let's say Universe X can jump, but not target. The destination is random. How many Universes are spawned?
@ResidentBiscuit,
Yeah, that's what I'm saying. I don't think of it as a number.

helios wrote:
I think I see what you mean, now.
What I think you mean by "uncountable" is that the quantity is so large that no matter how many more "+1" you add to "1+1+...+1", you'll never reach a value higher than the quantity. Am I right? This by itself is the typical definition of an infinite quantity: a quantity so large that it's larger than any natural number. No need to get "arbitrarily large" involved.
But the way you were saying it just leads to confusion. "Uncountable" has the meaning I stated above. It's a property of sets, not of quantities.

Yes, that's what I meant.

helios wrote:
The underlined sentence is the problem. In the sentence "subset of infinitely many universes", there's nothing to imply that the subset is to have any particular size.

I think what I was trying to say is that it's non-trivial to divide an infinite set into subsets because if you divide infinity by a number you get infinity (does that make it prime?) but as you showed you can have a subset of an infinite set where the subset has a definite size.

In my defence I just started university and haven't been sleeping enough (I slept through both of my lectures this morning, but I'm massively ahead on the programming module so I'll have time to catch up on the others).

@L B,
You're forgetting that our universe hasn't/won't exist for infinite time. That means that even if time travel is possible, it might never be figured out.
@chrisname So, what about other parallel universes that invent time travel? This seems to prove my point. I would like to be wrong, though...
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Time travel wouldn't allow them to travel between universes though.
i once heard a cool explanation for the big bang it was a higher energy universe colliding with ours...how cool an idea as that, ten thousand years ago that would sound soooo abstract

i also thought that science is missing something deeply fundemental...like mathematicians have got the concept of 1 so so wrong
 ten thousand years ago that would sound soooo abstract

Ten thousand years ago the wheel was an abstract idea -_-
@chrisname This whole thread is (or was meant to be) about being able to access parallel universes. If both time travel and travel between parallel universes are possible, we should see signs. If time travel isn't, we may have to wait for signs (probably unlikely). If travel between parallel universes is not possible or they don't exist, then maybe we never invent time travel.

My point is that one or both has to be impossible/nonexistent or I'm missing some fundamental explanation for why we haven't seen any signs.
So, basically it either has to be impossible/nonexistent... or it doesn't?
If by "it doesn't" you mean "there's a reason we don't see proof", then yes you are correct.
closed account (1yR4jE8b)
 If both time travel and travel between parallel universes are possible, we should see signs. If time travel isn't, we may have to wait for signs (probably unlikely).

What if time travel is possible, but only into the future? What if Time Travel is possible but it is not possible to detect?
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