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Multiverse theory

  1. Jan 18, 2007 #1
    if there are other universes according to the multiverse theory, where are these universes "located"? also, more importantly, does that imply that out universe is bounded (finite) because doesnt out universe have to end for another to begin?
    i know the idea behind string theory and the idea of p branes and d branes. does that have anything to do with the answer?
    thank you
     
  2. jcsd
  3. Jan 18, 2007 #2
    Who knows, I'm definitely not a religious person, but if we can assume that our universe magically appeared, what prevents other universes from doing the same thing? If another universe began like our own, I would speculate that it is completely independent of ours. However there are theories that suggest multi universes that are connected by some means.
     
  4. Jan 18, 2007 #3

    Garth

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    Multiverse? Show me one other universe and I will believe you.

    Garth
     
  5. Jan 18, 2007 #4

    hellfire

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    It depends what you mean with multiverses. There are different meanings for this term as you can read here. The most popular idea, at least according to my knowledge, is the multiverse theory based on eternal inflation. This happens in an infinite background space-time in which tiny inflationary bubbles appear and grow leading to "island universes".

    The main motivation for this idea is to explain fine tuning and coindicences (such as e.g. the cosmological constant problem) making use of the mediocrity principle applied to our universe with its laws of physics. The physics of multiverses based on inflation is not necessarily related to supersting theory and branes but does not exclude the possibility of a relation, especially with the idea of the string theory landscape.

    There are many good and readable books about this subject, like e.g. Vilenkin's "Many Worlds in One" or Leonard Susskind's "The Cosmic Landscape: String Theory and the Illusion of Intelligent Design".
     
    Last edited: Jan 18, 2007
  6. Jan 18, 2007 #5
    Theories which presuppose or 'prove' the existence of other universes, are extrapolated from abstractions of abstractions of abstractions of an abstract formal logic (mathematical) system. Maths do not inherently represent an external reality -- instead, we use the language of maths to model or describe our perception of physical, tangible reality.

    So, when you posit the existence of other universes, be weary that those claims are not empirically derived. This is to say, we have not verified through experiment and observation, the possible existence of such abstract spaces. We model these other universes, on abstract manifolds, which do not have any 'true' representation in nature.

    It is not known whether or not mathematics and geometry exists objectively in nature, as Plato described, or if we simply project our own subjective perception of mathematics and geometry onto reality. As to the question of ‘where these universes are located,’ one must quickly refer to how knowledge is derived. We can not transcend our models and associations of our perceptions and experiences with reality, so it is impossible to truly visualize how our universe ‘rests’.

    This is why we derive these results through logic systems (such as mathematics) as our experience is often times, counter-intuitive to the results we discover.

    However, one can use crude examples and analogies to visualize a rough approximation of how our universe might rest. A very simple example is the room that you are sitting in as you type. If you can imagine yourself as the ‘universe’ and the room that you are in as ‘a higher dimensional space’ you can get a crude idea how one object, can rest in a higher dimensional space. There are much better examples for visualizing this, but that’s the quickest and easiest that I can imagine.

    Also, it might be important to understand that it’s impossible to visualize these manifolds that string theory describes, with 11-dimensions. We exist spatially, in 3-dimensions, constituting six degrees of freedom. It is impossible to go beyond our perception and experience of existing in three dimensions to imagine the existence of others, and I don’t think that one would benefit much since these extra dimensions are only large enough for quantum entities to ‘travel’ along. Topology is the branch of mathematics that helps us model and describe these abstract spaces.

    At least that is my cognitive construction of reality. I haven't started my undergrad physics yet, just my maths -- this is all based on personal reading.
     
    Last edited: Jan 18, 2007
  7. Jan 18, 2007 #6
    What is "the multiverse theory" you are referring to in the first place?

    There are theories, like the theory of (cosmic) inflation, which (amongst others) predict more as one universe may spring off from the background spacetime foam.

    But that is - more or less - a side effect of the theory used. Other universes are more or less unobservable, so there is no direct proof possible for these other universes.

    The theory of cosmic inflation described the early stage of the universe, and explains why there was a Big Bang (it was the 'Bang' of the Big Bang).
    Cosmic inflation (of which there are several distinct theories) has made pretty good testable predictions, so in that respect it stands on more or less solid ground. It is not just 'wild' speculation.
    But there are assumptions involved, like for instance the assumption of the existence of a scalar field with some specific properties, which have not been observed.

    As to you question as "where are they" I believe the answer is that they are not in a location which can be defined in our local spacetime coordinates. They are - what is called - in a 'seperate' spacetime bubble. As to that, it can not be properly said as that these universes are 'somewhere' (at least not in terms of our 'local' space time coordinates).

    It requires one to have some level of abstraction and knowledge about mathematical defined spaces and manifolds, to grasp this idea.
     
    Last edited: Jan 18, 2007
  8. Jan 19, 2007 #7
    It would seem that only a multiverse can allow a pre-destined universe to become uncertain.
     
  9. Jan 20, 2007 #8
    "Fast everything is a problem of belief, a lot of things need scientic confirmation. You can know how is Universe begin, but you need believe it yet. I believe in multiverse theory, because i agree with Michio Kaku physical understanding of parallel worlds."
     
  10. Jan 21, 2007 #9
    thanks for your good answers.
    i guess what i meant originally was the multiverse theory as an interpretation(explanation) for wavefunction collapse in quantum mechanics. but on second thought you are all correct there are several different multiverse theories.
    my main question was this:
    regardless of what theory concerning many universes we are talking about and regardless of whether they are right or wrong, i was just curious if any one of them went further than claiming there are other universes period. specifically, for example, where would these hypothetical universes be located? can different universes interact like galaxies? etc.

    thanks!
     
  11. Jan 21, 2007 #10
    I dont think two universe can co-exist in the same time frame. therefore there is no "outside". and since time is infinite, so is how many universes there are. i think they interact like frames in a movie, the previous frame attributes to the current frame, which attributes to the next frame, and this happens at the speed of light which is the speed of the film.

    I wish i could explain it in math form.
     
  12. Jan 21, 2007 #11
    Why can't you explain it in math form? I didn't really understand it in word form.
     
  13. Jan 22, 2007 #12
    If there were really another universe, I would think that it would be totally independent of our own. If our universe consist of light, matter, energy, atoms, etc, there would not be another "universe" within our own, because if it were observable, then most likely it wouldn't be considered a universe. But like everyone else has said, there is no proof yet for an alternate universe, and if there are any other universes, chances are that they will not ever be observable, or even "exist" in our point of view.

    Something I find cool to think about is the "edge" of space, the "edge" of the universal expansion. I wonder what that looks like...
     
  14. Jan 22, 2007 #13
    The notion of other universes is counter-intuitive to our experience, so I would think only through logically rigorous mathematics and geometry can we transcend our experience and construct postulates and axioms capable of describing these extremely abstract and complex models of reality.

    When you start working with other universes, you are approaching an undefined boundary of abstraction.
     
  15. Jan 23, 2007 #14
    anthropic principle

    Nobody is mentioning the anthropic principle as the "evidence" of multiverse. While this is not evidence in a traditional hard scientific way, neither is dark matter or dark energy. Those are extrapolations from facts via proven to be sufficiently reliable methods of inference. I think the anthropic principle is one of those methods and it has to be considered, unless you have some good logical reason to rule it out a priori.

    Here's, in a nutshell, what I would argue:

    1. Fact: intelligent life as we know it [necessarily] depends on a number of universal constants taking on certain values
    2. Fact: as we know today, there is no explanation why those constants take on those certain values (no theory of everything that gives us one equation that would predict all the constants)
    3. Fact: as we know today, there is nothing that [theoretically] prevents those constants from taking on a huge number of other different values, with equal probability
    4. Therefore, all things being equal, life is very unlikely in an arbitrary run of a Universe from the big bang to its end (whatever it might be)

    5. Assumption: a rational person would assign credence to the theory that predicts a more likely outcome than not (if statistically there is a higher chance of getting killed in a bus than in a plane, all things (costs) being equal, a rational person will fly to the destination, if he wants to live)
    6. Therefore, given 4 & 5, If I have two theories, one of which says you're a result of a very unlikely unique and specified event (dumb luck), and the other - you are a result of a unique event out of gazillions of unspecified events, rationality dictates it is the latter theory you should assign credence to.

    In other words, rationality dictates that there are Universes with different constant values, given what we know today. That is, let's not jump into metaphysics and start proposing there is a magic hand, that there is a formula of theory of everything awaiting to be "discovered", and other millions of dead end cop-outs. Let's deal with the information we have today.

    Pavel.
     
  16. Jan 23, 2007 #15
    currently the visible universe appears to be flat- which suggests a universe with infinite spatial extent- this is evidence for a Level I multiverse in the Tegmark scheme:

    "Level I: A generic prediction of inflation is an infinite ergodic universe, which contains Hubble volumes realizing all initial conditions - including an identical copy of you about 10^{10^29} meters away. "

    more here: http://arxiv.org/abs/gr-qc/0102010
     
  17. Jan 24, 2007 #16

    Chronos

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    I object to the mulitiverse conjecture on, IMO, strictly logical and scientific grounds. Any region of the 'universe' that is not casually connected [i.e., has no observational consequence in our region] is no more relevant than an imaginary friend. Any region that is causally connected to our region is an extension, hence a part of the same universe.
     
  18. Jan 24, 2007 #17
    The notion of "casually connected" has no meaning within Planck's scale, as I understand it. Does that mean physics should give up searching for the answers in the time before gravity separates from the other forces? Or should it find another tool (theory) for categorizing knowledge without the traditional notion of causality. Where do you draw the line and say "this is NOT a good method of inference, but THAT one is". I think there are plenty of examples now, especially in QM, where you can't rely on the classical notions and consider anything else an "imaginary friend". I personally don't believe that the fact that the anthropic principle predicts something not causally connected (by established and well known means) is sufficient to rule it out as an inadequate method of inference. But that's just my opinion.

    If I got too philosophical for this thread and should go to a different section with this, I apologize in advance.

    Pavel.
     
  19. Jan 27, 2007 #18
    What "multiverse" theory are you objecting?

    The only theory I know that somehow comes up with the concept of there being "multiverses" is called the theory of eternal chaotic inflation.

    This theory is definately scientific, and it itself falsifiable.

    That it predicts 'effects' which might not be observable, does not perse imply that we should discredit the theory.

    I suppose you compare this "multiverse" theory with the theory that somehow there are undectable gremlins sitting on my desk. There is no way I can ever detect them, and they do in no way cause anything to be different then when they were not there.

    Now, we do discredit this theory because it is senseless. Wether or not invisible and indetectable gremlins exist, does not matter anyway, so that is why we can discredit this theory.

    This is different with a theory of cosmological inflation, since the theory predicts certain outcomes that are observable and falsifiable, not withstanding the aspect that the theory also predicts there be "other" universes with in our current understanding are not falsifiable.
     
    Last edited: Jan 27, 2007
  20. Jan 27, 2007 #19
    right behind you!!

    :biggrin:


    PS.
    No, turning your head won't help, it stays behind you!
     
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