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String Theory and Eternal Inflation

  1. Dec 4, 2011 #1
    I've been hearing that String Theory tied to eternal inflation seems to be providing support for the multiverse. But, I'm missing something with this connection.

    (1) Eternal inflation seems to "predict" a truly infinite number of other Universes or, at least as time progresses, a limit tending towards a truly infinite number of Universes.

    (2) String Theory had a problem related to how the particular shape of the Calabai-Yau space of the extra compactified dimensions determines the "properties of the Universe". The problem being that there are on the order of 10^500 possible shapes and nothing in the theory saying which shape to pick, as valid for our Universe. So, I believe it's being claimed that String Theory is pointing to a multiverse, with all 10^500 shapes valid, each for a different Universe.

    Assuming (1) and (2) are correct, I don't see the "rock solid" connection here.

    As Carl Sagan showed us in Cosmos, a googleplex (10^100^100) is as close to infinity, as the number 1 is.

    Now, 10^500 is much smaller than a googleplex, so it seems to me that String Theory is predicting a number that is MUCH smaller than eternal inflation seems to "predict", or at least allow.

    So, is it just me, or does this whole business seem a bit contrived, and without a very solid foundation?
  2. jcsd
  3. Dec 4, 2011 #2


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    String 'theory' is a work in progress. It makes no useful predictions, so, it is no more than a hypothesis in my mind.
  4. Dec 4, 2011 #3
    String theory is a Theory Of Nothing (TON). It says nothing about our universe.

    The multiverse is a non-scientific hypothesis.
  5. Dec 5, 2011 #4
    No doubt I lean towards what you guys are saying.

    But, unfortunately, this is being touted as the next possible "revolution" in physics, and I sure don't see any better ideas right now.
  6. Dec 5, 2011 #5


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    The primary takeaway here is that string theory predicts a large number of different ways that the universe can be, and that when you have eternal inflation, there is a mechanism by which all of these various possibilities are explored. So you have a prolific theory: a theory that predicts that all of these 10^500 possibilities come about.
  7. Dec 5, 2011 #6


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    I don't think anybody within physics touts it as the next "revolution". But string theory is generally considered to be an extremely interesting, sometimes even likely, possibility. The main reason why there is so much excitement over string theory is because it predicts quantum gravity: quantum gravity just falls right out of the equations.

    So when you have a theory from which the solution to the most challenging unsolved problem in physics just falls out of the theory automatically, well, that is an exceedingly compelling result.
  8. Dec 5, 2011 #7
    Strangely enough, that isn't true. If you have a bizillion universes then you can start doing statistics and make falsifible statements.
  9. Dec 5, 2011 #8
    Loop quantum gravity. Entropic gravity. There are also some theory independent principles that you can use to make some predictions without having to commit yourself to a particular theory.
  10. Dec 5, 2011 #9


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    Unfortunately, in practice this often ends up being a difficult proposition, because if you have an infinite number of universes, there's no unique way of doing the probabilities. But I don't think it's an unsolvable problem: already there have been significant strides forward made with respect to the holographic universe.
  11. Dec 5, 2011 #10
    Brian Greene?
  12. Dec 5, 2011 #11


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    Do you have any specific examples? Because I'd be a bit surprised if he used wording like this. Somewhat less surprised if it's a popular outlet (scientists unfortunately have a tendency to sort of "spice up" scientific topics when talking to popular audiences).
  13. Dec 5, 2011 #12
    This is what serious physicists call just hype.
  14. Dec 5, 2011 #13
    Except that mental statistics in thought experiments do not count as scientific... as even string theorists do not cease to emphasize.
  15. Dec 5, 2011 #14
    Well, I would not take his views seriously... :wink:
  16. Dec 5, 2011 #15
    Yep, that was kinda my point. And, that is just where he said this last - his latest popsci show, based off his latest book.

    But, what's the big deal on the wording, really?

    If the multiverse turns out to be true, this would be HUGE. I don't think calling it "revolutionary" would be unreasonable, at that point.

    He's just doing it too early, and that's what we should have issue with, imho.
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