# Infinite Universe and Parallel Universes

1. Jun 8, 2015

### RyanH42

If we suppose universe is infinte than there will be no parallel universes.So I know that If $Ω_k$→Universe will be infinite.Then there will be no parallel universes cause there's one universe.
I am confused.Is that mean the parallel universe idea is wrong ?

2. Jun 9, 2015

### Chalnoth

It's wrong on a few counts.

1. There's no way to know whether or not the universe is infinite. For example, we can't measure a curvature of identically zero: there will always be some error in our measurements, so we can never say that the curvature is perfectly flat. And even if we could, there are ways to write down a universe that is perfectly flat but is, nevertheless, finite (such as the toroidal universe).
2. It's perfectly plausible to write down models where our universe is infinite, but that there are other, disconnected regions which our universe cannot communicate with. One such model would be from string theory, where our universe is a 4-dimensional brane. This brane could be infinite in extent, and there could be another infinite brane that is separated from our own across a fifth direction that we can't access within our four dimensions of space-time. This sort of universe would be parallel in the literal meaning.
3. The quantum multiverse occupies the same space and time, but is separated into "worlds" that cannot communicate with one another.

3. Jun 9, 2015

### tom aaron

Being in the sciences my first response is the same as most issues. The need to define a term. What do you mean by 'universe'. What is meant by 'Infinite'...what is meant by 'parallel', etc.

The above question needs to be posed with reference to some author's work.

4. Jun 9, 2015

### RyanH42

Think a box, box is space time.Its not important box is finite or infinite.If we call universe is infinite I mean our universe is equal size as box.If I said finite universe it means our universe is a sphere in that box and there can be other spheres (parallel universes).So I mean If our universe is infinte then there will be no place to make a new unverse.But If our universe is finite then there will be a place. $Ω_k=0$ Means our universe is infinte so size will be equal as box and there will be no place to new universe.There one universe and its ours.

This make sense
I am talking about this situation. Parallel universe.If our universe is infinte then there will be no thing here a pic.If you look the pic I am askig If
$Ω_k=0$ Means our universe is infinte so size will be equal as box and there will be no place to new universe.There one universe and its ours. Is this true

I know Chalnoth gave the answer but I want to clarify my question

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5. Jun 9, 2015

### Chalnoth

In quantum mechanics, systems can be in a superposition of states. For an overly-simplified example of a system with two states:

$$\langle\Psi | = \langle 1| + \langle 2|$$

This is analogous to the "both alive and dead" cat in the Schrodinger's cat thought experiment. If the two states have different energies, then the system will oscillate between them. Sometimes it'll mostly be in state $\langle 1|$, sometimes it'll mostly be in state $\langle 2|$. But if we ever perform an observation on this system that distinguishes between the two states, we'll only ever see one or the other.

Let's say that we've put this system into another system in state $\langle A|$. The way we'd write this is:

$$\langle\Psi|\langle A| = \langle 1|\langle A| + \langle 2|\langle A|$$

What this does is it changes the oscillation. Previously, we might go between the two states in microseconds. But now we've combined this state with another, and for the system to oscillate we have to wait until the oscillation of the 1-2 system is in sync with the A system's oscillations to see any oscillation. If the state $\langle A|$ is complex enough (say, hundred to thousands of atoms at room temperature), then the oscillation time will quickly grow to larger than the age of the universe. This means that this system, even though it is still in both states $\langle 1|$ and $\langle 2|$, those two states can no longer interact with one another and each behaves almost entirely as if the other state doesn't exist.

This is the many worlds of quantum mechanics. The entire wavefunction occupies the same space, it's just that observers in some branches cannot observe what's going on in other branches.

6. Jun 9, 2015

### RyanH42

This is the quantum multiverse idea.This idea and my Pic 2 (1) is same thing.or different ??

7. Jun 9, 2015

### Chalnoth

That picture isn't specific enough for me to say.

8. Jun 9, 2015

9. Jun 9, 2015

### RyanH42

There's many universes but we cannot contact with other universes

10. Jun 9, 2015

### Chuckstabler

Wait, is there ANY evidence that there are "multiple parallel" universes that exist? Is it even a testable hypothesis?

11. Jun 9, 2015

### Chronos

By most accounts the multiverse hypothesis is untestable in principle. That is not make it wrong. The better question. scientifically speaking, is whether any evidence exists to suggest the need for a multiverse. The multiverse hypothesis does not suffer a lack of motivation, just none sufficient to elevate it above the level of fringe science, IMO.

12. Jun 9, 2015

### Chalnoth

There are multiple different multiverse concepts, and their testability varies. In particular, most multiverse ideas are part of a larger framework, and that framework has other features that are very much testable.

The quantum multiverse, for example, is a direct consequence of wavefunction dynamics. Those wavefunction dynamics are testable and have been tested in a wide variety of experiments. It is possible to add assumptions to get rid of the extra "worlds", but there is no reason whatsoever to do that. So absent other evidence, the most reasonable conclusion is that the many worlds of quantum mechanics exist.

Edit: Also, I should mention that the decoherence procedure that leads to the many worlds has been observed.

13. Jun 10, 2015

### Quds Akbar

Think of the universe as a balloon, it expands and you are a bug at the 2D surface (3D or 4D in our case but 2D to make it easier to imagine), it is infinite to you, because wherever you go, you will not find an edge, and as the balloon expands the distances between places will expand too. This idea is called the balloon analogy. And that means that for the balloon to expand extra dimensions are needed and from this point it gets a bit more complicated.

14. Jun 10, 2015

### Chalnoth

There's a difference between being infinite and having a horizon. What you've described is having a horizon. This isn't a feature of any expanding universe, by the way: it only appears due to the accelerated expansion. If there were no cosmological constant (or dark energy), then the expansion would continuously slow such that there would always be new galaxies to pass by if you were traveling at a high rate (well, until the universe got old enough that galaxies no longer existed, or the universe recollapsed).

15. Jun 10, 2015

### julcab12

We only have interpretation(indecisive) for quantum multiverse/worlds not evidence or at least not yet or it will never be. We can't really say much about. I can't deny that it is a direct assumption from some observable phenomenon in QM -- multiplicity(abstract). Exist is a strong word for QM's worlds -- perhaps silent?. I'm always advice to be very careful of the full observable ensemble. For instance, the Einstein cross. We can directly assume that quasars really do exist in multiple space at the same time and add multiple universes from direct interpretation standpoint. Luckily, we have lensing that produces this weird natural phenomenon and we have 1 quasar projected in multiple images when computed/realized. It's not about adding assumption to get rid of something we understand from one perspective but consider other interpretations and what it might lead.

16. Jun 10, 2015

### tom aaron

Not really. The most reasonable approach is 'not' to add assumptions of extra 'worlds' but to try and explain the Universe through the known properties of matter and energy.

Yes, there may be multiple universes, parallel, etc. but at this stage there is no scientific basis to add them...science is about a testable hypothesis. Adding a multiverse theory is like adding a god variable...nice to speculate, philosophize about...but at the end of the day...all untestable and no more than great conversation disguised by pseudo-science gobbledygook that sounds important.

I enjoy reading about alternative universes but, being in the sciences, I have a file in my brain marked 'fun stuff'.

17. Jun 10, 2015

### Chalnoth

Sure we can. We can test and measure the way quantum wavefunctions behave.

Except the extra worlds of quantum mechanics are not an assumption. They're a consequence of the way that quantum wavefunctions behave. You get them automatically from quantum mechanics unless you add extra rules to quantum mechanics to get rid of them.

Without evidence there's no reason to add the extra rule or rules to quantum mechanics that gets rid of the other worlds.

18. Jun 10, 2015

### OCR

19. Jun 10, 2015

### Chalnoth

20. Jun 11, 2015

### julcab12

... Evidence? Not quite. Common misconception on wavefunction. Some would argue that the quantum state is just a probability distribution others objectively real(Hilbert space). To be precise we are not certain on what were directly looking at (to some extent). The same reason i mentioned about Einsteins Cross, under any circumstances that we don't have any knowledge on lensing/SR we could easily fall for 'multiplicity enigma' surely because it looks that way and we could easily proposed Quasars being in places at the same time, assign some statistical distribution and remain valid. But we all know for a fact that the image is just a projection http://physics.stackexchange.com/qu...itational-lensing-account-for-einsteins-cross.

.. I find the idea and limit of SR to be convincing relating to CI interpretation on behavior of 'things'. For instance, "when objects travel the speed of light. It just happens that quantum uncertainty is more prevalent when it comes to particles traveling the speed of light, and it is negligible for objects with mass that don’t. Then if it is impossible to describe a particle accurately when it is traveling the speed of light using The Special Theory of Relativity and the theory itself breaks down, then if it was a truly accurate theory, it would mean that classical physics as we know it would break down. Say an object is traveling the speed of light at a constant speed. It is traveling at a constant speed so it assumes that it has a velocity of zero. Then from that frame everything else is traveling the speed of light. Everything contracts to zero. Then it would appear to the object that it is everywhere at once. Then it would seem like it had infinite speed, but from the other frame of reference it is only traveling at about 300,000 km/s. The theory breaks down. The two observers do not agree on a single state of reality. Then it just so happens that particles traveling the speed of light can be measured to travel at 300,000 km/s, and those particles can also have an action at a distance instantaneously through things like entanglement at the same time. Then the solution is simple. Both states of realities are actually happening at the same time, and as a direct consequence we observe this type of non-classical behavior. Particles that travel the speed of light can also have an action at a distance that is instantaneous at the same time.

After all. MWI will remain indecisive, interpretation(MW "I" --stands for anyways)..