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A question about time and universe expansion

  1. Aug 14, 2011 #1
    I've been looking all over the internet for an answer to this question, and I am yet to find one, so I've decided I'll just have to ask someone. I wasn't sure exactly which forum this thread should go in, but since it involves the expansion of the universe, I figured this would be the best forum for it.

    Anyway, could someone, preferably with a good grasp of cosmology explain to me what exactly is meant when it is said that the universe is expanding AND YET we live in a block universe, how can a tenseless universe expand? Wouldn't expansion of the system require time to exist outside the system as opposed to being a feature of the system.

    I did assume that the expansion was a referrence to the shape of the universe, in the sense that the further along the time axis you are, the larger the spatial dimensions would be, increasing the potential states for energy and matter to occupy and thus explaining the increase in entropy from low when space was constrained at the big bang to high when space is big and cold (so to speak) and subsequently the apparent arrow of time, etc. However, lately I've gotten the impression from reading up on things that this model isn't the accepted view, so now I'm just lost. Can someone please steer me back on track? Would be appreciated, thanks :smile:
     
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  3. Aug 16, 2011 #2
    This is actually a little bit philosofical. You may want to read up on the B theory. The B theory is in short the idea that the future is not less real than the past, just because we know less about it. It does not use tenses, but rather describes events as sooner, before or after or simultaniously. It doesn't mean that nothing happens or things just "freeze". Time is, in such, the forth dimension. Special relativity. So time doesn't have to be outside the system, as you say.

    The universe does not simply drift apart. It does so at an accelerated rate. Dark energy is said to fill up the holes in the universe for about 70% and seems to have a great deal of negative pressure.
     
  4. Aug 16, 2011 #3

    bapowell

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    Where did you hear this terminology: "block universe" and "tenseless"? Neither of these words/phrases is accepted terminology in modern cosmology.
     
  5. Aug 16, 2011 #4
    Thanks for your reply. I get what you're saying, but the thing is, I'm already fairly familiar with the block universe model, and although I'm aware that's more of a philosophical way of looking at the universe than scientific, I was under the impression that most mainstream cosmologists and particle physicists took it to be an accurate representation of the universe, at least based on what we currently know. I'd tend to agree, since most of the arguments against it seem to either be born out of an anti-deterministic desire to maintain freewill, or just plain old arguments from analogy (e.g. the map is not the territory), which whilst poetic and all that, are hardly useful tools for understanding the ontological nature of reality.

    My main issue is with the idea that the universe can be both tenseless and yet at the same time be expanding. How would those two ideas be reconciled? Would it be the case that the universe rather than expanding is merely bigger, or perhaps wider, the further one is along the axis of the fourth dimension. And thus when it's said that the expansion is accelerating, wouldn't it be more accurate to say that the angle at which the universe gets wider along the fourth dimension is getting steeper?

    If that's not the case then maybe you could tell me where I'm going wrong and help me to visualise it a bit more accurately, thanks again :smile:
     
  6. Aug 16, 2011 #5
    The terminology is besides the point, as the principle is accepted, regardless of what terminology is used.

    And if you're now going to tell me that the principle is not accepted, then all I can say is that scientists need to start being more open about where current research is leading and stop giving mixed messages, it's very off putting to us laymen.
     
  7. Aug 16, 2011 #6

    bapowell

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    The terminology is not besides the point -- you are using language that is meaningless within the framework of modern cosmology. This is a science forum where we discuss mainstream science. Feel free to indicate what current research you are referring to, and where I might be able to find an authoritative description of the block universe as a proposed model for the physical universe. Mixed messages? I can't answer for that, but there are plenty of resources available in print and on the web that provide information on real science going on in the field of cosmology.
     
  8. Aug 16, 2011 #7

    berkeman

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    This thread is closed temporarily for Moderation....


    EDIT -- re-opened.
     
    Last edited: Aug 16, 2011
  9. Aug 16, 2011 #8

    berkeman

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    Geodesic, bapowell is the PF Science Advisor who is trying to help you here. Please re-read the Rules link at the top of the page (you agreed to those rules when you joined here). You do indeed need to post links to the terms you are using, and the science we discuss here is mainstream science. You state that the "principle is accepted" -- we will judge that from the links you post.

    The PF is a great learning resource. Please post within the rules. Thank you.
     
    Last edited: Aug 16, 2011
  10. Aug 16, 2011 #9
    I beg to differ, it's just another good old boys club, I'm out, bye.

    Have fun voting republican.
     
  11. Aug 16, 2011 #10

    berkeman

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    Your loss. The rules are the way they are for a reason.
     
  12. Aug 16, 2011 #11

    cepheid

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    It seems like a shame that we lost a member due to such a drastic failure in communication. He/she thought we were trying to ban certain topics from discussion, when in reality, we were just trying to establish what the heck the topic of discussion was in the first place. Maybe if somebody had said:

    We actually don't understand what you mean when you say "block universe" and "tenseless."

    or,

    We don't really know what you are talking about, even though you seem to be under the impression that we all should. Could you please clarify?"

    Then perhaps this thread could have gone somewhere...

    EDIT: On the other hand, Geodesic Worm displayed a certain lack of maturity and closed-mindedness, so maybe it wouldn't have helped anyway.
     
    Last edited: Aug 16, 2011
  13. Aug 16, 2011 #12

    bapowell

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    I asked him directly where he heard these phrases. He had ample opportunity to establish a discussion on these topics. He instead chose to be rude and confrontational. The block universe is a philosophical model, and as such, has no place in this forum. There are too many legitimately curious and friendly people on these forums to spend time dealing with people who don't know the difference between science and philosophy, and who are too busy blaming others for the shortcomings in their understanding.
     
  14. Aug 16, 2011 #13

    cepheid

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    I didn't know that the block universe was a philosophical model. I thought we were still at the stage of trying to establish what he was referring to, and that he was misinterpreting our request for clarification as a dismissal of his (as yet still undefined) term. So...nevermind.
     
  15. Aug 16, 2011 #14

    Chronos

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    GW was just spoiling for a philosophical argument and didn't bother to do even the most basic research to correctly phrase whatever 'question' was being attempted. That is a classic 'move the goal posts' strategy. Redefine your terms as the arguments emerge.
     
  16. Aug 16, 2011 #15
    He posted in the wrong forum -- should have been in GR where they're familiar with the terms "block universe" and "tenseless universe". But I thought his question was clear enough:

    It can't. So, if the universe is indeed expanding, then it would be wrong to interpret GR as implying that we live in a block or tenseless universe. (And, yes, interpretations of physical theories are philosophical exercises.)

    I'm drawn to threads with the word 'time' in them. :smile: I've found that lots of people take it to be a whole lot more mysterious than it needs to be, and I think that that's where the OP was coming from.

    Anyway, no big deal. Just too bad that a curious layperson using good syntax and punctuation (they're harder to come by than you might think :smile:) felt the need too leave PF (if that's what happened). One thing I've learned is that one can't have too thin a skin if one is intent on badgering physics professionals with layperson questions. He/she could have clarified matters from the start by simply specifying the meaning of the terms he/she was using. There's probably dozens of threads about a 'block/tenseless' universe in the PF archives. It's certainly at Wiki. And an abstract search at arxiv.org on "block universe" will get a few hits.
     
  17. Aug 16, 2011 #16

    Chronos

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    The skin of ignorance is thicker than you may imagine. Refusal to accept observational evidence, mathematical rigor or acknowledge widely accepted concepts is symptomatic.
     
  18. Aug 17, 2011 #17
    I'm not sure what you're referring to wrt this thread. The OP clearly had a problem understanding how a timeless interpretation of GR could be reconciled with our common experience, and the observation that our universe is expanding/evolving. The problem being that there are theoretical physicists and philosophers of science who subscribe to the idea that our experience of time is an illusion. It's a problem of unwarranted reifications of mathematical theories. Depending on one's interpretation, one can get a theory jumping through all sorts of hoops, and then the general public is left wondering what the heck these people are talking about.
     
  19. Aug 17, 2011 #18
    So, I suppose the physical content of his questions was something like: Is space-time (as opposed, e.g., only space) expanding? and What is the (local/global) four dimensional geometry of the universe? To which I might answer: "globally, http://en.wikipedia.org/wiki/Friedmann%E2%80%93Lema%C3%AEtre%E2%80%93Robertson%E2%80%93Walker_metric" [Broken] is increasing; which does not make sense because no such concept can be applied directly to space or extended objects, in general). Perhaps if one only considers a finite region of the cosmos your idea would make sense (but in the real world this could be invalid since over small enough scales (10's of millions of light-years or less) the universe may not expand (indeed, we know in some regions it contracts))."
     
    Last edited by a moderator: May 5, 2017
  20. Aug 17, 2011 #19

    bapowell

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    You seem to be missing the point that these are manifestly philosophical ideas -- eternalism, while perhaps of interest to philosophers and some cosmologists, is not a testable model of the physical world. Almost all cosmologists are well-versed in GR, and I'd say that perhaps the most well rounded amongst us have heard of the block universe, but I can definitively say that none of them are conducting serious scientific research into eternalism. You're essentially saying that a neuroscientist, for example, should be well-versed in moral psychology or phrenology. Not so.

    Also, you probably aren't seeing the post that was deleted by the mediator in which the OP called me a "patronizing asshat" simply because I asked him for references to these ideas. If you think that's the proper way to behave on these forums, then perhaps you too should reread the rules...
     
  21. Aug 17, 2011 #20

    bapowell

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    Any cosmologist proposing alternative theories of the universe have to do one of two things: 1) establish the predictions of the theory and confirm with observation/experiment, or 2) absent the ability to do, caveats his theory as speculation and takes care not to dupe the public into considering it more relevant and supported than it is.

    Now, the block universe people seem to be selling a philosophical model as a physical cosmology. If so, this is irresponsible and I do indeed feel bad for a hapless public that buys into it.
     
  22. Aug 17, 2011 #21
    Ok. They're interpretations of the mathematical theory. But interpretations are mostly what laypersons get in the popular literature. The problem that the OP was having seems like a common problem. This or that theorist has a particular way of talking about what this or that theory means. And sometimes the way that a theory is interpreted/translated into ordinary language just doesn't make any sense. The OP's specific problem was reconciling the block time/universe idea (which might be inferred from an interpretation of GR) with the idea of an evolving/expanding universe (which is inferred from observation and predicted by GR). My understanding is that they're irreconcilable, and that most physicists disregard the block time/universe interpretation just as most physicists disregard, say, the Many Worlds interpretation of the quantum theory.

    Regarding philosophical ideas, a physical mathematical theory (such as GR or QM or some cosmological model) is a philosophy of how the world, or some aspect of it, works that's based on experimentation/observation and that's expressed in the language of mathematics. The math (a shorthand method of quantifiable expression/communication), while an integral part of the scientific method, isn't the science (the observation and experimentation) part. It's the philosophy part. And part of the entire scientific process involves interpreting the meaning of theories which are supported by observation and experimentation regarding what might be inferred from them about the world and expressed in ordinary language. And of course some interpretations, such as the block time/universe via GR, are untestable.

    Regarding semantics and communication, it seems that lots of people at PF attach a negative connotation to the word 'philosophy' as though it necessarily and only involves and refers to untestable statements. But of course that's not the case.

    No, I didn't see that post. No excuse for that stuff if he was here to learn. You guys/gals do a great job here at PF, and have helped lots of people including me. The OP was just too sensitive and unnecessarily impatient.

    Anyway, thanks for the feedback. If you disagree with anything I said about the philosophy part of science, or the irreconcilability thing, then criticism is welcomed. I hope it's clear that I haven't been saying anything such as that "a neuroscientist, for example, should be well-versed in moral psychology or phrenology."
     
    Last edited: Aug 17, 2011
  23. Aug 17, 2011 #22

    marcus

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    Hi Thomas! I have not been following this thread and just now looked at post #17 at the top of the page. Saw that bit which I quoted. I think it is a good point.

    I'm not familiar with the concept of "Eternalism" but I've seen the term "block universe" used a lot---I think in a non-reified way.

    I don't have a coherent position or viewpoint on this, that I can present. Just value your comments and think they are enlightening. I was reminded of this article in the journal General Relativity and Gravitation by a worldclass cosmologist George Ellis (co-author with Hawking of The Large Scale Structure book.) I will get the link to Ellis' article:

    http://arxiv.org/abs/gr-qc/0605049
    Physics in the Real Universe: Time and Spacetime
    George F. R. Ellis
    (Submitted on 9 May 2006)
    The Block Universe idea, representing spacetime as a fixed whole, suggests the flow of time is an illusion: the entire universe just is, with no special meaning attached to the present time. This view is however based on time-reversible microphysical laws and does not represent macro-physical behaviour and the development of emergent complex systems, including life, which do indeed exist in the real universe. When these are taken into account, the unchanging block universe view of spacetime is best replaced by an evolving block universe which extends as time evolves, with the potential of the future continually becoming the certainty of the past. However this time evolution is not related to any preferred surfaces in spacetime; rather it is associated with the evolution of proper time along families of world lines
    28 pages, including 9 figures.

    I think it's important to note that George Ellis explicitly rules out getting involved in the contentious philosophical issues of ontology. He explictly says he is talking about the mathematical models used to describe the universe. The various block models are not "re-ified" as you say---not "thing-ified." He says this at the end as a kind of afterthought or caveat. It should be clear that in the main body of the article he is doing cosmology and not getting himself mired in broader issues of interpretation, though cosmology can serve as an input or basis for subsequent philosophical reasoning.

    ==quote Ellis==
    5.4 Issues of Ontology
    The hidden issue underlying all this discussion is the question of the ontological nature of spacetime: does spacetime indeed exist as a real physical entity, or is it just a convenient way of describing relationships between physical objects, which in the end are all that really exist at a fundamental level? Is it absolute or relational? Could it after all be an emergent property of interacting fields and forces (Laughlin 2005), or from deeper quantum or pre-quantum structure (Ashtekar 2005: Chapters 11-17)?
    I will not pursue this contentious point here (for discussions, see e.g. Earman 1992, Hoeffer 1998, Huggett 2006). Rather I emphasize here that the discussion in this paper is about models or representations of space time, rather than making any ontological claims about the nature of spacetime itself. However I do believe that the kind of proposal made here could provide a useful starting point for a fresh look at the ontological issue, and from there a renewed discussion of the degree to which our representations of the nature of spacetime are an adequate representation of its true existential nature.
    ==endquote==
     
    Last edited: Aug 17, 2011
  24. Aug 17, 2011 #23
    @ marcus

    Thanks the feedback, links and references.
     
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