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Isn't space expansion logically required?

  1. Jun 25, 2015 #1
    If spacetime is a single entity, shouldn't it be simply logical that space expands?

    We know that spacetime is one single entity, there is no space AND time, but just spacetime.

    As we move towards the future, more time gets 'created', the more we get into the future away from the Big Bang, the more time there is from the Big Bang. Time expands as the future unfolds.

    So, is it not simply logical and unavoidable that also space must expand, so that the more we move away from the Big Bang there is not just more time but also more space?

    Expecting otherwise would be destroying the unity of space and time, we should not expect only the time dimension of spacetime to grow and the space dimensions to stay the same, should we?
     
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  3. Jun 25, 2015 #2

    phinds

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    Since space is just geometry, what's your point? Space-time grows and things on a cosmological scale get farther apart but that's just geometry and since space isn't "something", it's just distance, there's no need to "create" more of it.
     
  4. Jun 25, 2015 #3
    Space is more than just geometry, space is where fields reside. Are you saying that the space fields occupied 5 billion years ago had the same dimensional extension as the space which fields occupy at the present?
     
  5. Jun 25, 2015 #4

    phinds

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    No, I'm saying space is just the framework in which the fields and matter exist. Space isn't a "thing", it's just dimensions. Stuff, on a cosmological scale, gets father apart but that doesn't mean more space is "created" in between the stuff ... it's just stuff changing position in the framework

    EDIT: I see that I'm just repeating myself, so I guess I'm not explaining it in a way that is helpful to you. I'd suggest a forum search for something like "space time geometry" and/or "metric expansion" and check out the links at the bottom of this page.
     
  6. Jun 25, 2015 #5
    I'm a bit taken aback by the statement space-time is just dimensions, like coordinates we wrote on stuff, and so, not really a thing.

    Doesn't GR show that really geometry is the fundamental thing, in terms of our experience. I mean to say the travelling twin just had some run-ins with some of his or someone else's, "notions of geometry"... doesn't help his now older brother. And just because our approach of writing coordinates on real stuff to figure out what the principles of how stuff behaves are, ends up causing fits of confusion doesn't means it's just our coordinate writing that is confused. It actually means the geometry of our world, is a thing, and it is weird.
     
  7. Jun 25, 2015 #6
    Well I guess you are one the few ones believing that space does not expand. If you say that stuff gets farther apart but space does not expand (no new space gets created), then you believe that space does truly stretch. At any rate, 1 Km eventually becomes 2 Km, no matter which way you think of it. I can agree that spacetime is only a set of relationships governing how spacetime events can relate to each other, but the properties of these relationships hold true, call it whatever you like. The magnitude of spacetime dimensions grow larger with time, you like it or not.
     
  8. Jun 25, 2015 #7

    phinds

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    Yes, the magnitude of the dimensions get larger, as I have said all along, but no space is "created", things just get farther apart in the framework of space and in that sense there is definitely an expansion but this "fabric" and "stretching" stuff is crap propagated by pop-science.
     
  9. Jun 25, 2015 #8
    I think the problem is in thinking of space as a substance.
    Space is not any kind of substance, so it can't be 'created'.
    Space can contain things which are substantial, ie things which have a mass.
    Space can also contain various kinds of fields, (energy essentially).
    Space itself though is not a form of matter or energy, It is Phinds pointed out, a 'framework' we use to describe things like distances, velocity, position and so on.
     
    Last edited: Jun 25, 2015
  10. Jun 25, 2015 #9

    PeroK

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    One can create confusion, yet confusion isn't a substance.
     
  11. Jun 25, 2015 #10
    Sure, the term 'created' can be applied to abstract entities, but I think the OP is envisiging that expansion of space implies that something physical is being created.
     
    Last edited: Jun 25, 2015
  12. Jun 25, 2015 #11

    PeroK

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    One could argue that distance is physical rather than abstract.
     
  13. Jun 25, 2015 #12
    I won't disagree with that.
    Yes you can say that expansion means distance is being created, it's a valid statement, though it sounds like odd way of saying 'distances are increasing'
    I had the impression though I could be wrong, that the OP envisages 'Stuff; that is either a form of matter or energy coming newly in to existence..
     
    Last edited: Jun 25, 2015
  14. Jun 25, 2015 #13
    Spacetime is just a container. It's geometry labels, It's measure merely abstract... Unbelievable.

    By what thing do you move and age? Something else, something real?

    Quit calculating for a second, and look around...it's not just puzzles, this is the stuff the world is made of.
     
  15. Jun 25, 2015 #14
    Yes spacetime is a geometric framework, (well actually there a number of geometric frameworks that can describe space.)
    Yes I am moving and ageing within the context of the framework, How is that unbelievable?
     
  16. Jun 25, 2015 #15
    OMG.

    Honestly though I love nerds. I aspire to be a real one.
    :wink:
     
    Last edited: Jun 25, 2015
  17. Jun 25, 2015 #16

    phinds

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    No, it is the framework in which exists "the stuff the world is made of". 99+% of atoms are space but that doesn't make space "stuff", it's just distance. Quarks and electrons and photons, etc, are stuff.
     
  18. Jun 25, 2015 #17

    Chalnoth

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    Nope. There is no such requirement. It is perfectly possible for space to either expand or contract. Doing neither is only possible in perfectly empty space with no cosmological constant (if there's any matter or other energy such as photons, then a universe that is neither expanding nor contracting is unstable: it will very soon start doing one or the other).

    This isn't an accurate representation of spacetime in General Relativity. In General Relativity, the entire past and future is part of the same manifold. This is more or less required by the fact that different observers disagree on what the definition of "now" is.

    If I'm standing here on Earth in the Milky way, and I have a clock that reads 12:01 today, and another person is standing on some other planet off in the Andromeda galaxy but that clock reads 12:03 on the same day according to one observer, then another observer could look at both clocks and see the same time, or see that my clock is faster than than the clock of the person on Andromeda. There's no way to say who has the right answer.

    To bring it all together, if we can't agree on "now", then we can't agree on what times at different locations are in the future. And if it's not possible to agree upon which times at different locations are in the future, it makes no sense to say that the future is newly-created.
     
  19. Jun 25, 2015 #18
    Not so fast friend. I know of what you speak but I'm far enough into Susskind's QM for dummies to know that just whether or not matter and energy, or geometry comes first, is not a matter entirely settled.
     
  20. Jun 25, 2015 #19

    phinds

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    I'm not following you. What do you mean "comes first" and why does it matter? The discussion is about whether or not space has a material existence.
     
  21. Jun 25, 2015 #20
    And yet the overall a(t) of the universe, it's expansive shape, has to also be accounted for when considering the relativity of simultaneity right?
     
  22. Jun 25, 2015 #21
    Well if matter and energy are emergent properties of space-time geometry (there are papers over in BTSM) then the question of which is a more capable primary description is open. It can be argued, I believe, that geometry may be "primary". In other words the primary "material" is quanta of space-time: "Vacuum state" properties, and algorithms.

    Clearly that's 'out there' thinking, but it's not mine, I'm not making it up.
    Aren't 'strings' an example? Aren't they a fundamental geometric object, from which matter and energy, and vacuum are made?

    Bottom line, to my thinking re the material aspect of raw space-time. One twin gets old, one doesn't. They are both made of matter, but we describe what happened to them in terms of the geometry of space-time their paths took. And what happens to them is pretty material if you ask me. So what's the difference between that and the case of expansion? In neither case do we really know what is going on QM wise, between matter and changing geometry. The temperature or energy density of the CMB decreased because someone forgot to move our temperature framework labels, or because "space grew"? Seems fair to say "space grew", and mean it.
     
    Last edited: Jun 25, 2015
  23. Jun 25, 2015 #22
    You don't need QM to understand that if you have an expanding volume of 'stuff', then less 'stuff' exists in one cubic metre of it.
    Less stuff means less energy, so not surprisingly the temperature is less.
     
    Last edited: Jun 25, 2015
  24. Jun 25, 2015 #23
    And yet if you ask "how" and then how that, then how that... it's turtles all the way down. And so I would contend that, we actually do need QM to truly understand any thing...fully.

    Not that we can't make iPhones and cars and the internet and rockets and pizzas and gravity boots and blow ourselves up without truly understanding everything fully. Though we probably will need QM for the gravity boots...

    :cool:
     
  25. Jun 25, 2015 #24

    phinds

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    I don't follow your logic at all, but I do get that you seem to believe space "grows" in the same sense that flowers grow and I completely disagree. I think things just get farther apart and I don't see how any of the above contravenes that.
     
  26. Jun 25, 2015 #25
    Unless string theory or something like that proves to be a real 'theory of everything', then you are right.
    Science is in the meanwhile a method of uncovering the layers of turtles and discovering what the nature of the turtles are in each layer.
    This sometimes produces useful new technologies.
    (I would definitely consider anti gravity boots to be useful technology, but the operating manual might be hard to understand)
     
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