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What is space?

  1. Nov 16, 2011 #1
    I'm brand-new to this site, an English teacher, certainly not a physicist, but I'd sure appreciate some (gentle) help.

    The short version of my question:

    How can the universe expand into infinite space?

    Are there two kinds of space? The first is the distances between galaxies in our universe, and the second, space, is something else?

    Where did space come from? Was it there before the Big Bang?

    If there is space and space, why isn't there time and time?
     
  2. jcsd
  3. Nov 16, 2011 #2
    Hey Philip welcome to the Forum.

    I have been full circle round this question here in this thread where I got lost on older Physics concepts such as the aether:

    https://www.physicsforums.com/showthread.php?t=487911&highlight=properties+of+space


    My conclusion is that space has no objective existence whatsoever in the same way that ponderable matter does. By itself it is just a measurement of the distance or the separation between two physical objects, similar to the separation between two points in time. It is also a framework in which ponderable matter can "hang". It is also a void through which force carriers can propagate, but it is the characteristics of the force carriers which determine the way in which they behave and interact with ponderable matter. I believe that this is the most modern view and it replaces the older concepts of space being an aether with properties of its own.
     
    Last edited: Nov 16, 2011
  4. Nov 16, 2011 #3
    I'm reading The Hidden Reality by Brian Greene. Here's his explanation (buried in a note at the bottom of p. 138): "If space were infinitely big, you might wonder what it means to say that the universe is larger now than it was in the past. The answer is 'larger' refers to the distances between galaxies today compared with the distances between those same galaxies in the past . . . . In the case of an infinite universe, 'larger' does not refer to the overall size of space, since once infinite always infinite. But for the ease of language, I will continue to refer to the changing size of the universe, even in the case of infinite space, with the understanding that I'm referring to changing distances between galaxies."

    While physicists and cosmologists are usually precise, I can't grasp these two uses of the word space. One kind of space means one thing, and another space means something else.

    Furthermore, I understand (I think) that if all matter and energy were removed from the universe, then time and space would vanish, too. But why wouldn't space disappear? And since General Relativity is about spacetime, why isn't there spacetime?
     
  5. Nov 16, 2011 #4
    The distance between distant galaxies is increasing and accelerating due to dark energy. The universe is to al intents and purposes infinite, whilst the observable universe is finite and getting smaller all the time, due to expansion making the most distant parts no longer observable. I am not sure what you are confused about?
     
  6. Nov 16, 2011 #5

    phinds

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    "to all intents and purposes" begs the question of IS it infinite or not, and that is a heavy discussion, not to be dismissed lightly even though you ARE right that it is irrelevant to our direct experiences.
     
  7. Nov 16, 2011 #6

    phinds

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    I don't think I could add anything to Tanelorn's excellent comments on space, I just wanted to add a note about the Big Bang. It is a highly controversial subject as to what, if anything, came before the big bang. The currently accepted model of cosmology basically says that we know a lot about what happened AFTER the singularity and absolutely nothing about the singularity itself or any possible "before" ("before" doesn't really exist in the standard cosmological model).

    So you're next question HAS to be "how can something come out of nothing?". I can only suggest that you stock up heavily on headache pills and read about Quantum Mechanics.
     
  8. Nov 16, 2011 #7
    The--or maybe I should say "our"--universe is infinite? I'm paraphrasing from the Brian Green book, but one of the things we know pretty certainly is that the expansion of the universe is speeding up. That's what Perlmutter et al just got a Nobel Prize for.

    Researchers have been able to calculate the cosmological constant--the amount of dark energy in the universe necessary to account for this expansion--by the simple algebra of flipping E=mc2 around to M=e/c2. The data from the faster-than-expected expansion requires a cosmological constant of just under 10-29 grams in every cubic centimeter of space.

    The number of cubic centimeters in our universe is big, but it is not infinite.
     
  9. Nov 16, 2011 #8

    phinds

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    You should not make definitive statements on this forum that you cannot back up with science. What you have stated is an unsupportable personal opinion. You MAY be right, but you may be wrong, and either way, you can't prove it, so stating it as fact is not a good idea.
     
  10. Nov 16, 2011 #9
    Hi Philip7575

    Another way of getting a really good headache is to Google “Cosmic Topology.”
     
  11. Nov 16, 2011 #10
    phinds:

    Jeez, I'm an English teacher. How could I possibly have a personal opinion about the cosmological constant or the size of the universe in cubic centimeters?

    I'm paraphrasing Brian Greene's The Hidden Reality, p. 138 and p. 337 n. 8 and 9. Greene is a professor of physics and mathematics at Columbia University, and a well-respected populizer of tough science like this. His first book, The Elegant Universe, was a finalist for the Pulitizer Prize. And I'm getting that information from the book's dust jacket, so it's not "unsupportable personal opinion," either.

    The point is that if researchers can calculate the cosmological constant by plugging the total amount of dark energy in the universe into that calculation, the total amount of dark energy must be finite. Put another way (and this is me, not Greene), if the universe were infinite, then the total amount of dark energy in it would also have to be infinite. Put still another way (me again, not Greene), if the universe were infinite how could it be expanding?
     
  12. Nov 16, 2011 #11
    Makes sense to me.

    Not necessarily. 2 times infinite is still infinite.
     
  13. Nov 16, 2011 #12
    Imax: "2 times infinite is still infinite."

    To cite Greene again, "In the case of an infinite universe, 'larger' does not refer to the overall size of space, since once infinite always infinite."

    This is one of the things that led to my initial question. If the universe is infinitely large, then it can't get any bigger. So physicists talk about space in our finite universe at the same time they say our finite universe is expanding into infinite space.

    It's these two uses, space vs. space that I don't understand.
     
  14. Nov 16, 2011 #13

    phinds

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  15. Nov 17, 2011 #14
    Welcome to the forum Philip, I too am new at this and sometimes catch myself making factual statements that really need to be prefaced with "My Opinion is"

    First of all there is only one kind of space. The space between galaxies is the same as the space inside atoms. There may be differences like temperature, density, fields etc. but space is space.

    I believe the universe works in cycles so yes there would have been space before the Big Bang. Space from the last cycle would have condensed down to a singularity, during which time there would not have been any space.

    I think the universe is formed from an finite number of extremely small particles. These particles may be condensed into matter as we know it or diffused to the point we can't see them. As they move in relation to each other they each form electromagnetic fields. I believe it is the combination of these fast moving particle and their fields that make space as we know it.

    As for your first question, the universe is not expanding into space. Space IS what expands.
     
  16. Nov 17, 2011 #15
    There’s only one space. The space in our universe isn’t expanding into some other outside space. If the universe is finite, then travelling a straight line in any direction can bring you back to where you started. It’s just like here on Earth. The big difference is that space has 3 dimensions, whereas here on Earth, gravity restricts up and earth down.

    Brian Greene gave a good description of expansion:

    Mathematically, a finite space can expand, remain static, or contract. Our Universe seems to be expanding.
     
  17. Nov 17, 2011 #16
    I believe it expands and contracts but never static or the cycle would stop. And yes our universe does seem to be expanding but I wonder?
     
  18. Nov 17, 2011 #17
    Hi Philip, from one boffin to another, I'll give it a go at explaining. Not that long ago, I had a similar confusion.. what is expanding into what.. and I also ran into the "universe is infinite" conundrum. It eventually made sense to me, even though I haven't seen any of the math (and probably would struggle with it if I did).

    What is finite? Our observable universe (OU) is finite. It is a spherical distance beyond which we cannot see. There are two reasons for this. The first is that the light from stars beyond that have not yet reached us. The second is (and this will be difficult, but more on it later) that the universe is expanding, faster than light, meaning that some of the light from beyond our OU will never reach us.

    What is infinite? The whole universe, not just our OU, is believed to be infinite, according to the more popular models. As background to this, people also get confused with the Big Bang, thinking of it like an explosion that happened in one place and spread beyond that centre. The name, Big Bang, was given to it by one of its early detractors. The actual model says that when the BB occurred, it happened everywhere (infinitely), in the same instant.

    What is expansion? Expansion, as has been said in this thread, is the growing distance between galaxies, but more specifically, between matter that is not bound by gravity to other matter. Just as our Moon is tied in orbit to us, and us in orbit to the Sun, so are galaxies tied to other galaxies. Expansion is putting more distance between galaxies that are not so bound.

    An important intermediate point is to be understood about light speed (c). It is believed that nothing can travel faster than light, even galaxies relative to each other. Yet in this expansion, the distance between some galaxies is growing at faster than light speed. Sounds like a contradiction? I'll explain.

    The distance between such distant matter is growing in two ways. The first way is their actual movement, relative to each other, a concept we readily understand from everyday experiences. The second way that the distance is growing is that dark energy is impacting on space-time in a way that effectively puts more distance between the two objects. Still confused? So was I.

    The best way that cosmologists have attempted to represent, what is essentially a mathematical model, is by using a 2-dimensional analog of 3 dimensional space, by asking us to imagine that the universe is like a balloon. Travel anywhere on that balloon's surface and you will find no end point. Objects on the surface of the balloon will move relative to each other. Add air to that balloon, and the distance between objects grows, in part from their own movement and in part from the expansion of that balloon. So, add the slower than light movement of the objects, relative to each other, to the expansion, and you can see how the growth in distance can exceed the light speed limit.

    As I say, this is my boffin explanation, much of which I've learned from the patient ones here. Our Earth-bound paradigms make it difficult to comprehend some of these ideas, but as they've rolled around in my head, I could see the logic.

    Does that help at all?

    Oops.. I just looked up boffin.. the online dictionary uses words like "scientist" and "expert". :blushing: I'm not a scientist, nor an expert - just someone who asks, listens and learns about this fascinating subject.
     
    Last edited: Nov 17, 2011
  19. Nov 17, 2011 #18

    phinds

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    Narrator, that's a great explanation. Just to expand one point a little, it's VERY important to remember that the balloon analogy is JUST and analogy, and it's even more important to remember that you can only consider the surface of the balloon. People often look at the balloon analogy and start talking about the fact that it is a hollow shell with a center. In the analogy, you cannot consider the center; there IS no center, there is ONLY the surface.
     
  20. Nov 17, 2011 #19

    phinds

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    So you dispute the evidence that is believed by essentially all physicists? You might want to read up on the evidence a bit before you make such a cavalier statement.
     
  21. Nov 17, 2011 #20
    With the observation of the acceleration in the expansion of the universe, I don't see how it could slow down and contract, since this observation shows the exact opposite.
     
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