Physics in the Multiverse (alpha)

Discussion in 'Physics & Math' started by BenTheMan, Nov 30, 2007.

  1. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    http://arxiv.org/abs/0711.4460

    Like it or not, string theory (as it is currently formulated) seems to have a huge (HUGE) number of solutions.

    String theory, in and of itself, has only one dimensionful parameter---the string coupling. All other dimensionfull parameters in the low energy effective field theory are derived from that one parameter. This is a true statement in ten dimensions. When one goes from ten dimensions to four dimensions (by compactifying on a special six dimensional space), one introduces a large bit of arbitraryness. For example, the six dimensions are compact in the sense that they are periodic---you can travel along one of the sixth dimensions and end up back where you started from. But how far do you have to travel---i.e. what is the radius of the compact dimension?

    The two main problems that string theorists have is that there is nothing that really tells them why the compact space should be six dimensional, and there is no mechanism that will tell them HOW the dimensions became compact. Related to these problems is the challenge of deriving the radius of the compact directions, or the volume of the compact space. These problems are typically grouped under the category of moduli stabilization.

    Currently, a large number of string theorists believe (I am personally on the fence on the issue!) that there is this idea of a multiverse. One can think of the multiverse as the set of all solutions to string theory. In the absence of a way to stabilize moduli, we must look at all of the solutions and see if we can make some general predictions about nature.

    Typically, these large number of solutions tend to be (actually) a GOOD thing, when it comes to solving things like the cosmological constant problem. The cosmological constant is small and positive---understanding the cosmological constant as a manifestation of the zero point field (the most naive assumption) leads to a prediction that is off by 120 orders of magnitude. (This error is MUCH bigger than the ratio of the observable universe to the Bohr radius.) There is hope that such a small and positive cosmological constant may be understood in terms of the large number of solutions to string theory---if we assume that the cosmological constant is more or less random in all of the solutions, there are enough solutions to generate a small cosmological constant some of the time.

    Anyway, in this thread we will discuss the phsycis of the multiverse, and one way that people are currently trying to get predictions from string theory. The reference linked to above looks to be a fairly pedagogical introduction to the subject, and it's only a few pages long.

    Note, off-topic replies will be deleted, as this is an alpha thread.
     
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  3. Reiku Banned Banned

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    Oh dear Ben... what's wrong...? Was the truth a bit hard to swallow...

    ...String Theory is unfalsifiable and so is the Multiverse Theory... Oh No!!!!

    ... Hardly off topic... but just to please your poor sodding mind:

    ''Like it or not, string theory (as it is currently formulated) seems to have a huge (HUGE) number of solutions.''

    Yes... well... we both know why that is don't we...? It's because nothing can be narrowed down. It's as free as a dove, and as believable as a yeti.

    ''you can travel along one of the sixth dimensions and end up back where you started from.''

    Never heard that before... I've heard that for the fifth dimension... If you move or try to move out of that dimension, you end up exactly where you began. Why do you mention the sixth dimension?

    ''The two main problems that string theorists have is that there is nothing that really tells them why the compact space should be six dimensional''

    Ha! So much for a professional String Theorist, you don't even know the contending theories.
    The leading idea for the Compactification of dimensions is found best exampled in the Hyperspace Theory... Describing a sudden cataclysmic event making the dimensions compactify. The theory which compliments this best is Ekpyrotic Theory.

    ''and there is no mechanism that will tell them HOW the dimensions became compact''

    Can anyone say Bullshit?

    ''Currently, a large number of string theorists believe (I am personally on the fence on the issue!) that there is this idea of a multiverse.''

    On the fence are we? I simply don't believe in them. It's proposterous really.

    '' There is hope that such a small and positive cosmological constant may be understood in terms of the large number of solutions to string theory---''

    Hope? String Theory is a bloody hope... That's all it is based on.

    ''if we assume that the cosmological constant is more or less random in all of the solutions, there are enough solutions to generate a small cosmological constant some of the time.''

    Your gonna have to be clearer here. It's so silly of me, but it sounds as though you are saying the CC varies over spacetime?

    ''Note, off-topic replies will be deleted, as this is an alpha thread.''

    No problems there then eh?
     
    Last edited: Dec 1, 2007
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  5. QuarkHead Remedial Math Student Valued Senior Member

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    Ben, I just read the linked reference. It's a lot of fun, and, even though I understood almost none of your post (my shortcoming, not yours!), I saw very little in the link to disagree with.

    In fact, I find the idea of a multiverse highly attractive, intuitively. Is the notion regarded as crackpot? You seem to be suggesting not
     
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  7. Reiku Banned Banned

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    The notion isn't quite ''crackpot,'' but it is a very unbelievable theory at best.
     
  8. QuarkHead Remedial Math Student Valued Senior Member

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    Why do you say that, Reiku? I once read a book by Sir Martin Rees, called something like Six Magic Numbers, where he points out that, if any one of these constants differed from their actual value in the n-th decimal place, our universe would not exist in its present form, or at all.

    That is, the probability of our universe existing in its present form is remote to say the least. It seems to me there are only 3 possibilities; our universe is one of an infinite number of "creation" experiments, of which it is the only survivour; all such experiments coexist in some abstract sense; or there is a God.

    The last of these, I find intellectually lazy, but I shan't press the point, for fear of offending others. Now, whether there are arguments for or against either of the other other 2 possibilities is not for me to say.
     
  9. Reiku Banned Banned

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    By the way... to remove the obsurdity of the splitting and merging effect, one needs to resort to a finite sum of universes. There needs to be no more than 10^100, but there are no existing theories with a finite number of universes to this count.

    Currently, the smallest known (I think), is predicted by string theory Landscape, with about 10^500 universes... But still, many problems concerning the validity of multiple universes still turns out to be, unfalsifiable.
     
  10. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Reiku---further violations of the alpha rules will result in warnings. Continued attempts to derail this thread will result in a ban.

    Consider this a warning.


    ======================

    String theory is a ten dimensional theory. In order to get four dimensional physics from it, six of the ten dimensions must be compactified. The compact manifold is six dimensional, with all six dimensions being periodic.

    There is very little contnet in the rest of your posts.
     
  11. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    This idea is natural in the context of Linde's chaotic inflation scenarios. In those models, at the big bang, there are a bunch of ``bubbles'' which are of order the Planck scale. During inflation, the Planck scale bubbles get blown up to roughly the size of a solar system. They continue to expand, and one of the bubles, specifically ours, develops structure and planets and physics forums. The set of all of the bubles is called the multiverse.

    The challenge is to make some general statements about physics in the multiverse---for example, the problem that most people tend to attack is the cosmological constant. In our universe, the CC is small and positive. But the question is, why?

    So you could imagine some sort of smoothly varying function over the entire multiverse. Somewhere that function will be small and positive. And because we NEED a small and positive CC in order for us to be here, our universe must live at one of those places.

    These arguments are called ``anthropic'', and I think they're fun to make

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    For example, what would happen if the weak force were just a bit stronger? Well, the weak force mediates nuclear decays. Nuclear decays tell the stars how to burn. So if we made the weak force stronger, stars wouldn't burn as long, making the appearance of intelligent life (at least as WE know it) less likely.

    Now, all of this to say that I hate the anthropic principle. I think it is a last resort for us, but at least now it is hard to see how we will escape it.
     
  12. Reiku Banned Banned

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    Well Dr Hawking admitted at a conference earlier this year that we might actually be looking at an Anthropic answer after all. I don't have a link before you ask.
     
  13. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Hawking and the Cambridge people have always been anthropic people. Just look at papers by Don Page for example. In this paper he estimated the height of a giraffe using anthropic reasoning.
     
  14. Reiku Banned Banned

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    Fine.
     
  15. QuarkHead Remedial Math Student Valued Senior Member

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    Ben, you assume a lot of your readers. What does this mean, for example?
    or this.
    Not only why, but what is the cosmological constant? Why must it be "small and positive"?
    Sorry to appear dim, but I don't get it.
     
  16. Reiku Banned Banned

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    The Chaotic Inflation, is when you have microcopic systems, like universes, that inflate - universes are a bit like the spontaneous bubbling of quantum foam.

    It may not need to be small, but if it is, it flies in the face of everything we know about physics. This is why Ben keeps analyzing a vacuum error calculation found by renormalizing the CC with by saying it is the same as the ZPE. It needs to be small because if it isn't, then we have magnitudes of error (about 122 time larger, which is more energy than what is contained in all of reality, which is about 10^80 particles) from mathematical calculations...

    ... I'm still a bit shakey on it all though. I prefer the negative large Constant. I just think we are missing the bigger picture, or something is wrong with the calculus.

    By the way, before Ben erased my work to you QH, my explanation to your question was in short:

    ''Each universe is unique, as there maybe several outcomes to a certain event, but only one individual outcome is allowed in any single universe. Thus, whenever i flip a coin and observe what side it has landed on, i become apart of the splitting of the universe, and my body is projected into two me's - one in this universe looking and observing a heads, let's say, and another me in the 'newly born' universe observing a tails. However, this easy-creation of universes disturbs some scientists. The idea is, if you flip a coin in 100 tosses, you create something equivalent to 1,267,650,600,228,229,401,496,703,205,376 universe-possibilities real; that is a little over 10^30. If every 6 billion-odd souls on earth simply stopped to flip a coin a hundred times, you could imagine the amount of universes that would split off from our own.''
     
  17. QuarkHead Remedial Math Student Valued Senior Member

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    This is no better - what is "quantum foam", for pity's sake?

    I can make no sense of this. I'm lost again. You guys are too clever for me, so I'm out of this thread.

    In fact, I'm going to the pub! Cheers!
     
  18. Reiku Banned Banned

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    ''Quantum foam'' is the spontaneous bubbling of energy at the smallest level and time possible, which is called the ''planck time'' which is 5.8 x 10^-44 and the space has a square measurement of 1.616 x 10^-33 called ''the Planck Space''. This is how all matter and energy spontaneously comes into existence, and is said to come out of the vacuum, which is intrinsic to the Zero-Point Energy, or as i shortened it, ZPE.
     
  19. zephir Banned Banned

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    The multiverse concept is not concept of just string theory, it's a concept comming out from quantum mechanics (the Everett's "many worlds" interpetation of it in particular). While the string theory is quantum mechanics based (between many others...), it's evident, the multiverse concept is much more general, then some string theory.

    Furthemore, the multiverse concept is poorly defined at the scope of string theory itself.

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    On the above picture we can see, how the single event (flash of radiation, for example) influences a two different remote places of space-time independently. Is that an example of multiverse consistent with string theory?

    If yes, why? If not, why?
     
  20. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    You are all mistaking the concepts of the multi-verse. In string theory, the concept of multi-verse has absolutely nothing to do with Everett's many worlds interpretation.

    To QuarkHead---

    Ok, I was hoping to stimulate some discussion, but unfortunately Reiku and zephir are spreading mis-information.

    The cosmological constant is some constant energy density that lives in the vacuum.

    You are familiar with the Doppler effect, right? You listen to a train blowing it's whistle while coming towards you and the frequency increases, the frequency decreases as it travels away from you. Well, the same thing happens to light---as things travel away from you, the light becomes red-shifted. So, measuring the light coming at us from distant galaxies, we find that there is a relationship between how much the light is redshifted and how far away the galaxy. This is the cosmological constant.

    You can think of it like the surface of a baloon---if you draw two points on the surface, the rate at which the two points separate from each other is proportional to the distance between the two points.

    So we know that the distance between our galaxy and distant galaxies is getting bigger, and at an accelerating rate. This is exactly the opposite effect that gravity would have---so in some sense, you can think of the cosmological constant as negative gravity. This is the cosmological constant---some mysterious thing that has to act to push things in an opposite direction as gravity.

    Why must it be small and positive? Well, it must be small in order to avoid too much expansion. If the cosmological constant isn't small, then the universe becomes empty very quickly. If the cosmological constant were negative, we'd see the light from distant galaxies blue shifted.

    We can measure the CC (using the redshift data) and calculate a number. Reiku was talking about a sero point field. In essence, what we do in quantum field theory is turn each point into a harmonic oscillator. When you treat a harmonic oscillator quantum mechanically, you find the energy eigenvalues are

    \(E_n = (n+1/2)\hbar \omega, \forall n \supset \mathbb{Z} \geq 0\).

    What this formula says (forgive the abuse of notation!) is that the lowest energy is not zero. When we do quantum field theory, this means that the energy of the vacuum is not zero! This is what Reiku is talking about when he talks about the zero point field.

    This has all of the properties of the cosmological constant, except for the fact that it scales as the number of points in space-time. So, this number is at the best very large, and at the worst infinite. So, treating the CC as zero point fluctuations of a quantum field is definitely wrong, and has been called the ``worst prediction in theoretical physics''---this prediction is about 120 orders of magnitude wrong.

    What we CAN do is argue that we can scale these fluctuations away---it turns out that you can always add a constant (in this case!) when it is convenient. If you have ever worked a conservation of energy problem, you KNOW that you are free to choose what ``zero'' means. You can define a book (mass m) sitting on a table (height h) to have potential energy 0 or mgh, whatever helps you calculate. Well, the dynamics here are very similar---you can get rid of the energy stored in all of these harmonic oscillators by just adding a constant.

    So we know why the cosmological constant could be big, and we know why it could be zero. What we don't know is why it should be small.

    Hopefully that clears some things up. I still owe you a decent explanation about chaotic inflation---in short, think of the head on your beer. I had a long explanation written the other day, and accidentally hit refresh on my browser.
     
  21. zephir Banned Banned

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    Don't be surprised, the "many words" hypothesis is one of the quantum mechanics interpreations and the string theory is completelly quantum mechanics based. Furthemore, the "many words" hypothesis was elaborated later in Pribram's holograpics theory of universe - well, and the Hooft's, Susskind's and Maldacena's holographics reformulation is just "hot topic" of string theory...

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    This conceptual continuity is still present and quite live here.
     
  22. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    zephir---perhaps I didn't get the memo they sent to all the other string theorists... I can assure you that the multi-verse that the paper above talks about (I assume you actually read it?) has nothing to do with the Everett interpretation.
     
  23. zephir Banned Banned

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    Just a quote from the introductory article quoted by You at the very beginning of this topic:

    "These are good reasons for considering seriously the many-worlds interpretation of Hugh Everett. Every possible outcome to every event is allowed to define or exist in its own history or universe, via quantum decoherence instead of wavefunction collapse. In other words, there is a world where the cat is dead and another one where it is alive. This is simply a way of trusting strictly the fundamental equations of quantum mechanics. The worlds are not spatially separated but more kinds of ”parallel” universes. This tantalizing interpretation solves some paradoxes of quantum mechanics but remains very vague about how to determine when splitting of universes happens. This multiverse is complex and, depending on the very quantum nature of phenomena leading to other kind of multiverses, it could lead to higher or lower levels of diversity. "

    You didn't read this at all, did you? ...Wrong!

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