# String Theory and Multiverses

1. Mar 9, 2014

### fsujoseph

I am familiar with String theory and the Multiverse theory, but I didn't know that apparently String Theory points to a multiverse? Can anyone give insight to this?

Also what do you guys think about the Multiverse theory? Personally it is far-fetched, but I am not dismissing it. I don't want to repeat the mistakes we once made.

Last edited: Mar 9, 2014
2. Mar 10, 2014

### andrien

It has to do with the size and shape of the calabi-yau manifold with the help of which the compactification is done.you need something extra to avoid the moduli problem for which you need fluxes,which somehow give it a definite size and it will now take energy to change the shape of the manifold.These fluxes are somekind of threading lines through the holes of the manifolds.A calabi-yau in 6 dimensions has around 500 holes,and fluxes themselves are quantized(say take value from 0 to 9) which gives around $$10^{500}$$ possibilities.Each one of which corresponds to different vacuum,one of which is ours corresponding to almost zero cosmological constant.That is the main idea behind relation between string theory and multiverse.

Last edited: Mar 10, 2014
3. Mar 10, 2014

### Chalnoth

The short, basic version of it is that string theory seems to have a gigantic number of possible low-energy states, and there's good reason to believe that a significant number of the states are meta-stable and can, therefore, act as the basis for a tremendous variety of low-energy physical laws. From this, it's difficult for string theory to be true and for there not to be a multiverse where different regions have different low-energy physical laws.

Philosophically, I generally think that a multiverse is a much more reasonable proposal than a unique universe (where the low-energy physical laws are the same everywhere). First, just in general it's simpler to fully-specify a set of entities than a single entity from a set. Secondly, there is the fact that our universe has life, which has some rather specific requirements (formation of dense structures, complex chemistry, stable environments, to name a few). It just seems utterly implausible to me that the fundamental laws have to be such that the only possible way the universe can turn out is one that supports life (ours).

4. Mar 10, 2014

### fsujoseph

That's fascinating! But is the answer really more satisfying than to say the Universe has an infinite number of galaxies?

5. Mar 10, 2014

### Chalnoth

That doesn't answer the question of, "Why these physical laws?" For example, why does gravity have the strength i does? Why do we have electricity and magnetism, and why does that force have the strength it does? Why are there protons and neutrons? Why do we have stars that forge heavier elements in their cores? Why are those heavier elements often stable?

After all, all of the galaxies that we can see play by the exact same rules that we see in action here on Earth.

It is by no means clear that the answers to these and other questions can possibly be due to the fundamental behavior of the universe. It may well be that there are regions out there with a very different strength of gravity, without any force that looks like the electromagnetic force (but instead have different forces with different behaviors), and without any stars or planets at all. Some may have nothing but black holes. Some may have matter that is spread apart too far for it to ever coalesce into a star. If there is this proliferation of different sets of low-energy physical laws, if many things do happen (though not anything at all....there must still be some basic rules), then our own universe is relatively easy to explain: we see the laws we do because the low-energy laws allow us to exist.

This is essentially the same argument for why we live on Earth rather than Mars. Any possible planet didn't have to be like Earth at all. Instead there are many planets, around many stars, with many different environments. On some of them, like Earth, life is possible. And beings like ourselves, who can ask these questions, obviously cannot exist on a barren wasteland like Mars. The same may potentially be true of the low-energy physical laws we observe.

6. Mar 10, 2014

### phinds

As Chalnoth has said different galaxies are just part of THIS universe.

So, the two (multiple galaxies / multiple universes) have nothing to do with each other. It is believed that the laws of physics are uniform throughout our universe. This gets a bit problematic when you move outside the observable universe since we'll never be able to know through direct evidence that it's true.

7. Mar 11, 2014

### julcab12

...The Multiverse scenario is something you can play with and hope something comes up IMO. Think of it as one of the ingredients that you put in a conventional dishes. It can either taste good or bad depending on your preference. Personally, I don't like multiverse in my any of my dishes lol. On a serious note, Multiverse is commonly used to reconcile a sense on some of the problems in cosmology (cosmic and chaotic inflation- Bekenstein bound , string landscape/false vacua, superposition in QM, and some mathematical implication(most if not all)) mentioned by Chalnoth. It's not a solution(lack of physicality) but a consequential derivation and interpretation relative to notion of suggesting multiple systems varying in different states. However, some postulate can also suggest on a singular basis(UNI-verse)-eternal bounce-cyclic of which our universe is all there is going on a stages of transitional parametric bounces that goes on forever and the condition of our universe today is one of the stages- Imagine It like the passing of seasons and our universe is in it's summer (universe condition) and soon it will achieved critical and bounce to winter and goes back again^^. I find this concept much stable than the latter IMO.^^

8. Mar 11, 2014

### fsujoseph

So you mean the universe could possibly have different physical laws than the ones we observe?

9. Mar 12, 2014

### julcab12

According to them. All possible physical laws/configurations/patterns/information or whatever you want to call it are embedded/within the system itself(UNIiverse). To get an approximate picture and analogy. Imagine that our universe is like a slot machine(lack of a better example) and each time you pull the lever(bounce mechanism). It produces combination(condition of universe). Each combination infers to different physical laws and It would take a hefty amount of tries(bounces) in order to get the same result or combination again.

Here are some layman friendly links^^:

http://www.youtube.com/watch?v=A7ZvVkboMfY- Seeing Through the Big Bang into Another World-Penrose

http://wn.com/roger_penrose__cyclic_universe_model-- anything related to cyclic model

http://www.physics.princeton.edu/~steinh/vaasrev.pdf

http://wwwphy.princeton.edu/~steinh/cycliccosmology.html

10. Mar 13, 2014

### Nugso

Is there any Multiverse theory that does not include String Theory?

11. Mar 13, 2014

### Chalnoth

Yes. See the many worlds of quantum mechanics, for example.

But string theory (including M-theory) really is our only current candidate for the fundamental laws of nature, so it shouldn't be terribly surprising that it's included in a lot of multiverse ideas.

12. Mar 13, 2014

### Nugso

Thanks, Chalnoth.

Well, I'm currently reading a book about String Theory(against it, to be more accurate), hence I wanted to know if it was possible!

13. Mar 14, 2014

### craigi

Perhaps the most compelling evidence for a multiverse (or alternative, which I won't mention here), is Weinberg's prediction of the cosmological constant. You're not alone in thinking that it's far-fetched, but Weinberg sucessfully predicted it with a remarkable degree of accuracy, when (perhaps all) others were predicting a value of exactly zero.

This gives a firm foundation for a new type of reasoning in physics and we could be seeing a paradigm shift in the field of cosmology and quantum physics. People like Everett and Wheeler are being taken far more seriously at the moment. It may be that this paradigm shift is exactly what is needed to successfully create a theory of quantum gravity.

I'd recommend Barrow and Tipler's book, particularly for the historical context. It's nearly 30 years old now and there have been a lot of developments since then. With the increasing interest, I'm sure that it's due an update, though Tipler's subsqeuent conclusions may make this difficult.

The biggest problem with multiverse theories is that they can universally, be considered a safety net explantion for anything that is a necessity, but that we have yet no other explanation for, which leaves us with a dilema about where lies the burden of proof.

For the most part, the reasoning involved is simply common sense, but the implications can seem fantastical. Despite discourse on this dating back thousands of years and almost a hundred years in the field of physics, physicists are going to need some time to get over it.

Last edited: Mar 14, 2014
14. Mar 15, 2014

### Chalnoth

This may be unpalatable to many, but it is logically sound. We simply have no reasonable expectation to be surprised about there being aspects of the observable universe that may seem unlikely but are required for life to exist. Life can, after all, only exist in certain environments, and can only exist with certain sets of physical laws. We need no more than a set of fundamental laws that explores a wide variety of low-energy physical laws to explain this.

This doesn't mean that everything can be explained by this selection effect. If there is an instance of apparent fine tuning that is not necessary for life to exist, then that instance of apparent fine tuning demands a different explanation. But as long as it's required for life, there's no reason to seek an explanation beyond the selection effect that is the weak anthropic principle.

15. Mar 15, 2014

### julcab12

https://www.physicsforums.com/showthread.php?t=612943-- Here is a discussion mainly philosophical but tackles most of the deep aspect of the subject and probably one of the best I've read so far.

...... It is somewhat straight-forward for me. It is not needed to introduce large number of sets(universes) to explain why a particular set-N(present universe) has that given threshold. A function-(bounce/s) is sufficient enough as a condition producing factor in a single dynamic system i.e cycles of possible thresholds. In a simple system such as a clock; numbers represent Physical laws, intervals-time, arrow-bounce. We can assume that a 12 separate clocks is unneeded to make sense of 7,8, or 2 is conditioned for life.

In the end, I also have reasons to consider multiverse but not as much as universe does.

http://arxiv.org/ftp/arxiv/papers/1308/1308.0932.pdf-Cyclic [Broken] models of the relativistic universe: the early history

http://www.lanl.gov/projects/cosmology/sf11/Tirthabir_Biswas_sf11.pdf- Overview of cyclic inflation-

Last edited by a moderator: May 6, 2017