# If the universe is infinite, does that mean that everything exists somewhere?

## Main Question or Discussion Point

If the universe is infinite, does that mean that everything exists somewhere, besides obviously impossible things like a star that contains oxygen but doesn't contain oxygen or a 4-sided triangle?

mgb_phys
Homework Helper
It would if it was but it isn't

Chalnoth
If the universe is infinite, does that mean that everything exists somewhere, besides obviously impossible things like a star that contains oxygen but doesn't contain oxygen or a 4-sided triangle?
Well, consider this by way of analogy.

The set of all even numbers is infinite. I can go on counting even numbers for ever and ever and never reach an end.

But clearly the set of all even numbers does not include all possible numbers. It doesn't include, for instance, the number pi.

So even if the universe is infinite (we don't know whether or not it is), then that doesn't necessarily mean that all possibilities are realized.

However, there may be other reasons to believe that all possibilities are realized, mainly stemming from quantum mechanics, where we find, for instance, that if there is the possibility of matter inhabiting a region of space, then particles of that sort of matter will necessarily pop in and out of the vacuum. Another way of saying this is that in quantum mechanics, there mere possibility of existence forces existence. So it is not unreasonable to suspect that perhaps all possibilities must actually be realized.

This doesn't mean that anything and everything we can imagine occurs, of course. We can imagine quite a lot of impossible things, as you mention above. But we can also imagine a great many things that are not obviously impossible, and yet may turn out to be upon deeper inspection.

Entropee
Gold Member
Well so far we think that the Universe is finite, but with no boundaries. I really hate the word infinity >.< It's so unbelievably unfathomable, and people just toss it around as if it's just a large number. Good thoughts on the subject though.

Chalnoth
Well so far we think that the Universe is finite, but with no boundaries.
I don't know who this "we" is, but so far as I am aware there is no consensus on this. Currently there just is insufficient evidence to say anything more than, "the universe is very, very big."

So there might not be another Earth where I'm dating Jennifer Aniston? Dang.

Chalnoth
So there might not be another Earth where I'm dating Jennifer Aniston? Dang.
Well, just because you can imagine it doesn't mean it's possible. Consider, for a moment, how many opportunities you have had to get to know a beautiful actress. Probably not very many.

If, in this parallel world, you were the sort of person that had a life where you were in at least occasional contact with beautiful actresses, would your life be so different that that person even count as being you in the first place?

Entropee
Gold Member
I don't know who this "we" is, but so far as I am aware there is no consensus on this. Currently there just is insufficient evidence to say anything more than, "the universe is very, very big."
Okay okay.. by "we" I mean Stephen Hawking haha

And if our anti particles formed anti people on an anti earth we could have identical twins down to the last particle...maybe... ;P

Chalnoth
Okay okay.. by "we" I mean Stephen Hawking haha
I'm pretty sure if you asked him straight up he'd say basically the same thing I just did. He did, of course, present the no boundary proposal (where the universe has no boundary either in time or in space), but that doesn't mean he would go so far as to claim that he knows it's true. He may think it likely (it's his idea, after all), but I doubt he'd go that far.

And if our anti particles formed anti people on an anti earth we could have identical twins down to the last particle...maybe... ;P
Well, we don't have anti-particles. That's one of the requirements of our cosmological observations: that early-on, there was a very small breaking of the symmetry between matter and antimatter.

Entropee
Gold Member
Well yes I agree with you thats why i said its what he "thinks" not knows, and would YOU go so far as to say that you "know" we dont have anti-particles? There may not be symmetry between matter and antimatter but It's still somewhere.

Chalnoth
Well yes I agree with you thats why i said its what he "thinks" not knows, and would YOU go so far as to say that you "know" we dont have anti-particles? There may not be symmetry between matter and antimatter but It's still somewhere.
Yes, because we've actually looked for them. They're not out there. Basically, if the matter and anti-matter were physically separated, then you'd occasionally get clumps of matter running into clumps of anti-matter, causing rather large explosions. We don't see any of that.

Furthermore there's the problem that around the time of the emission of the CMB, our universe was extremely uniform, so that there was no way that normal matter and anti-matter could have been out of contact with one another.

Entropee
Gold Member
Thats true, but why then was the early universe so hot?

Also an unrelated question maybe you can answer for me, if most of the universe is hydrogen, why didnt all the hydrogen undergo nuclear fusion when the universe was as big as a baseball? Was it because the particles were different at the time?

Chalnoth
Thats true, but why then was the early universe so hot?
It appears to be a result of the end of inflation. Basically, whatever it was that drove inflation had to have a whole lot of energy. When it decayed, it reheated our universe to a tremendous temperature.

Also an unrelated question maybe you can answer for me, if most of the universe is hydrogen, why didnt all the hydrogen undergo nuclear fusion when the universe was as big as a baseball? Was it because the particles were different at the time?
Right, if the expansion rate would have been slower then, it would have. It would have progressed all the way to producing iron and that'd be the most common element.

But this takes time. First the protons and neutrons condense out of the quark-gluon plasma, so you have hydrogen right away. Then you start to make helium from the hydrogen. Then you start to make heavier and heavier elements.

As it turns out, the expansion rate was such that the universe cooled to where the nuclear fusion basically stopped by the time there was only around 25% helium sitting around, and long before more than trace amounts of anything else formed. Incidentally, this is even faster than it sounds, because much of the helium stemmed from the neutrons that were around early-on (when you have nothing but protons around, fusion takes a heck of a lot of energy, as you have to convert protons to neutrons, and because they repel one another, but when you have lots of neutrons sitting around it's much easier).

Entropee
Gold Member
I thought we didnt have protons and neutrons in the early universe, wasn't there lots of particle decay so they would be different now?

Chalnoth
I thought we didnt have protons and neutrons in the early universe, wasn't there lots of particle decay so they would be different now?
Oh, well, in the very early universe there weren't any. But when the quark-gluon plasma cooled, well, protons and neutrons were the particles they condensed into: they're the lightest baryons. Heavier baryons are unstable and would have quickly decayed into protons and neutrons.

Entropee
Gold Member
About how long did it take for the quark-gluon plasma to cool?

Chalnoth
About how long did it take for the quark-gluon plasma to cool?
Well, either way I don't know off the top of my head and you might be able to find it yourself as quickly as I could off of Google, but what specifically do you mean by this?

That is, are you asking how long the process of nucleosynthesis took, from the time the protons/neutrons condensed out of the plasma to the time that fusion stopped? Or are you asking how long after the end of inflation that this occurred?

Entropee
Gold Member
How long after the end of inflation is more what i meant. But yeah i could google it lol.

Chalnoth
How long after the end of inflation is more what i meant. But yeah i could google it lol.
Hehe :) Yeah, I actually looked it up to. It's about three minutes ;)

Entropee
Gold Member
Wow nice haha, did you see my post on your profile?

Well, just because you can imagine it doesn't mean it's possible. Consider, for a moment, how many opportunities you have had to get to know a beautiful actress. Probably not very many.

If, in this parallel world, you were the sort of person that had a life where you were in at least occasional contact with beautiful actresses, would your life be so different that that person even count as being you in the first place?
lol, you sure know how to pour water on a nice thought!!!!!

just kidding

Chalnoth
lol, you sure know how to pour water on a nice thought!!!!!

just kidding
Haha, well, I think that it still can be extremely interesting.

Imagine, for a moment, just walking down the street. If we just take the part of your wave function that you know about today, and imagine all of the future parts (using the many worlds interpretation, of course), then those future parts will likely follow nearly the same but slightly different trajectories, for a while. I imagine it like a blurring of myself, some parts slightly ahead, some slightly behind. Some slightly to the left, some slightly to the right, etc. So the different parts of my wave function are slowly, very slowly dispersing. It might take many trips outside the house before anything interesting happens.

But then something interesting does happen: a car, going too fast, almost hits me. Well, it almost hits the "me" that I see, but there are other me's that were in slightly different places: some of them were just a little bit too close to the car, and got smacked. Suddenly, what were once very similar worlds become very different.

In another situation, something very similar is happening, but the event that causes the divergence is, say, I'm not paying attention to where I'm going, and I almost run into a pretty girl. I manage to apologize for the incident, strike up a conversation, and we start dating. Of course, that's just the "me" that I observe: some of the me's in other worlds either are far enough away that they don't almost run into her, or are close enough that they actually run into her and just end up pissing her off. In some others, the conversation takes a slightly different turn and we never see each other again. Etc. etc.

So there might well be people out there who, when I was a child, were still part of my wave function, the part that I remember, but who today have extremely different lives.

If the universe is infinite, does that mean that everything exists somewhere, besides obviously impossible things like a star that contains oxygen but doesn't contain oxygen or a 4-sided triangle?

You must define the word 'infinite' before you can validly ask this question, and before anyone can validly answer it.

Does that make sense?

And if you're able to define it to your satisfaction, then you'll have answered your own question, I believe. :)

Chalnoth
You must define the word 'infinite' before you can validly ask this question, and before anyone can validly answer it.
That's easy: if the universe is infinite in space, then even if one could travel much faster than the speed of light, one could travel forever without ever crossing one's path.

Another way of saying it is that if you could write down coordinates for the entire universe, then you'd never reach a number in those coordinates that was "beyond" the universe.

Haha, well, I think that it still can be extremely interesting.

Imagine, for a moment, just walking down the street. If we just take the part of your wave function that you know about today, and imagine all of the future parts (using the many worlds interpretation, of course), then those future parts will likely follow nearly the same but slightly different trajectories, for a while. I imagine it like a blurring of myself, some parts slightly ahead, some slightly behind. Some slightly to the left, some slightly to the right, etc. So the different parts of my wave function are slowly, very slowly dispersing. It might take many trips outside the house before anything interesting happens.

But then something interesting does happen: a car, going too fast, almost hits me. Well, it almost hits the "me" that I see, but there are other me's that were in slightly different places: some of them were just a little bit too close to the car, and got smacked. Suddenly, what were once very similar worlds become very different.

In another situation, something very similar is happening, but the event that causes the divergence is, say, I'm not paying attention to where I'm going, and I almost run into a pretty girl. I manage to apologize for the incident, strike up a conversation, and we start dating. Of course, that's just the "me" that I observe: some of the me's in other worlds either are far enough away that they don't almost run into her, or are close enough that they actually run into her and just end up pissing her off. In some others, the conversation takes a slightly different turn and we never see each other again. Etc. etc.

So there might well be people out there who, when I was a child, were still part of my wave function, the part that I remember, but who today have extremely different lives.

I'm a layman, just getting into physics, but that makes sense. So are there infinite universes, or just one universe that is infinite. I saw a special on the Discovery channel where they said there were an infinite number of universes in the "multiverse".

How was that proven (or derived)?