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Thread: Textbook Explanation?

  1. #1
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    Textbook Explanation?

    Being relatively new to this board, this probably ties into some other thread of which I am unaware, so please enlighten me if it does.

    I am not an astronomer (my background is chemsitry) but while reading Horizons: Exploring the Universe 7th ed. (copyright 2002) by Michael Seeds, (which I guess would be considered an introductory astronomy text and I am assuming Dr. Seeds is a mainstream astronomer), I ran across the following passage:

    ďWhy do we say the universe canít have an edge or center?

    The quick answer is that an edge or a center would violate the cosmological principle, which says that every place in the universe is similar in its general properties to every other place. Then a place at the edge or center would be different, so there can be no edge and no center. Of course, this isnít critical thinking; it is only an appeal to the arbitrary authority of cosmological principle. What is the real reason we conclude that there can be no edge and no center?

    Imagine that the universe had an edge and you went there. What would you see? Of course, you might imagine an edge to the distribution of matter with empty space beyond, but that would not be a real edge. Imagine an edge beyond which there is no space. Could you stick your head beyond the edge and look around? Thinking about an edge to the universe leads us to logical problems we cannot resolve. From this, we conclude that an edge is impossible [emphasis mine]. Of course, if there is no edge, then there is no center because we define the centers of things by reference to their edges.Ē
    To flatly state that because we cannot comprehend or understand what it would mean for the universe to have an edge, there cannot be an edge seemed like such an odd thing for a scientist to say. Isnít it possible that our understanding of the universe could improve to the point that an edge to the universe would no longer present logical problems? Am I just not understanding something or is this an example of scientific hubris?

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    I feel this is an example of "scientific hubris"; but I don't think I am worthy of weighing the premis on the limited facts that are posted here.
    If the good Doctor is quoted correctly, then someone may wish to email a similar query to him.
    And; (to start a sentence with a conjunction),ItmayhavebeensaidbutImissedit----- Tbutyl, welcome to the board.

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    I still say the edge is time. We can't see past the 'future' fence.

    I think the edge and center aren't there in 3D space. So we have to expand our concept of what makes an edge or a center. Then we might be able to get past the difficulty of conceptualizing what the Universe space actually is.

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    Re: Textbook Explanation?

    Quote Originally Posted by Tbutyl
    To flatly state that because we cannot comprehend or understand what it would mean for the universe to have an edge,
    Please define 'edge'.

    Quote Originally Posted by Tbutyl
    there cannot be an edge seemed like such an odd thing for a scientist to say. Isnít it possible that our understanding of the universe could improve to the point that an edge to the universe would no longer present logical problems? Am I just not understanding something or is this an example of scientific hubris?
    I understand the sort of edge, boundary, border, Dr. Seeds is talking about -- and the Universe, as he said, can't have one of those -- but I don't understand the word edge as you use it.

    Thanks.
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

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    Quote Originally Posted by beskeptical
    I still say the edge is time. We can't see past the 'future' fence.

    I think the edge and center aren't there in 3D space. So we have to expand our concept of what makes an edge or a center. Then we might be able to get past the difficulty of conceptualizing what the Universe space actually is.
    And here we go..............

    In my experience; (compounded by my personal form of madness), I believe I have seen the 'future' in "daymares".

    Spacetime is relevant; but only in context.

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    Re: Textbook Explanation?

    Quote Originally Posted by Tbutyl
    To flatly state that because we cannot comprehend or understand what it would mean for the universe to have an edge, there cannot be an edge seemed like such an odd thing for a scientist to say. Isnít it possible that our understanding of the universe could improve to the point that an edge to the universe would no longer present logical problems? Am I just not understanding something or is this an example of scientific hubris?
    There are two easy ways of looking at the universe which are currently, I believe, considered valid. I don't remember which one is currently thought to be holding the trump card, though...

    You have infinite, and you have finite and unbounded.

    The latter one is obvously edge free, since it has no boundries.
    The former has no centre (and hence, no reachable edge), since it is infinite in extent.

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    I'm not so sure. The 'Horizons - Exploring the Universe' is an introductory text as you suggest and the topic of cosmology probably generates more heated & lengthy debate than any other. Encapsulated in that single statement that you highlighted are nearly a century of argument and counterargument, which would possibly be inappropriate for a basic text to explore in greater depth. What such a text should do is inspire deeper research into the topic (which evidently it has! :wink: )

    The technique of reductio ad absurdum has its place in scientific discussion. The Universe by definition is everything that is - space, time and everything therein. An edge by definition is a boundary. What then does it mean to say that there is a boundary to the Universe when there can be nothing beyond that edge that is not part of the Universe by definition? What does it mean to stand at such an edge? What happens if you walk 10 feet beyond it?

    Don't get me wrong - I am not sure I agree with the result that Seeds has proffered. I just don't believe that his statement should just be dismissed simply as hubris.

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    Quote Originally Posted by Pinemarten
    I feel this is an example of "scientific hubris"; but I don't think I am worthy of weighing the premis on the limited facts that are posted here.
    If the good Doctor is quoted correctly, then someone may wish to email a similar query to him.
    And; (to start a sentence with a conjunction),ItmayhavebeensaidbutImissedit----- Tbutyl, welcome to the board.
    Thank You for the welcome.

    As soon as I get a chance, if I can locate an address for Dr. Seeds, I will send him an email to see if he would care to comment.

    Quote Originally Posted by 01101001
    Please define 'edge'.
    I guess my mental image of the edge of the universe is the boundry between the universe and what we are expanding into (probably from seeing the "balloon with dots on it being inflated" as an analogy for the expanding universe a few too many times).

    Relatively recently, didn't some scientists report a size for the universe? IIRC, it was something like 100+ billion light years across. I could probably find the reference, but I am sure many (if not most) of you folks are more up on this than I am and could find a better reference than I could and much faster. If I am remembering correctly, and some reputable scientists did offer a size for the universe, what did they use as "edges" to come up with the reported dimensions?

    Quote Originally Posted by AGN Fuel
    The technique of reductio ad absurdum has its place in scientific discussion. The Universe by definition is everything that is - space, time and everything therein. An edge by definition is a boundary. What then does it mean to say that there is a boundary to the Universe when there can be nothing beyond that edge that is not part of the Universe by definition? What does it mean to stand at such an edge? What happens if you walk 10 feet beyond it?

    Don't get me wrong - I am not sure I agree with the result that Seeds has proffered. I just don't believe that his statement should just be dismissed simply as hubris.
    You could be correct, that Dr. Seeds chose that wording he did because it was an introductory text and it was not the place for a more in depth analysis (I hope I can get an email to Dr. Seeds and get his imput).

    I certainly have found myself pondering those very questions that you pose. I guess I tend to believe that as our understanding of the universe increase, we may need to change or modify our definition of universe as it applies to our surroundings. I sometimes wonder if we do not in fact choose definitions in such a way as to avoid having to deal with questions and problems that our current understanding of physics is unable to handle. Likewise, is it possible that the various models of the universe that say there is no center and no edge, do so because these things are just too troubling to deal with?

    Quote Originally Posted by Ut
    You have infinite, and you have finite and unbounded.

    The latter one is obvously edge free, since it has no boundries.
    The former has no centre (and hence, no reachable edge), since it is infinite in extent.
    Since my understanding of cosmology is still in its infancy, I find I have trouble with both of these. If the universe had a beginning (my understanding of the Big Bang Theory) and it is between 10 and 15 billion years old, how can it be infinite? If the universe expanded from the original singularity in all directions, what if we ran history backwards, wouldn't the universe collapse back down to that singularity? Couldn't that point be consisdered the center of the universe?

    The description of the universe as finite but unbounded always gives me problems trying to get a mental picture of what is going on and I can't avoid thinking about what lies outside this unbound but finite universe (which I see is being discussed on another thread).

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    If the universe had a beginning (my understanding of the Big Bang Theory) and it is between 10 and 15 billion years old, how can it be infinite?
    A fair question. Also, one that I, unfortunately, can't answer. I suggest you ask another question, though: If you take the universe to be everything, how can it be finite?

    If the universe expanded from the original singularity in all directions, what if we ran history backwards, wouldn't the universe collapse back down to that singularity?
    Define "expanded from" here. Singularities are ugly stains, which physicists have been trying to scrub out of existance for some time now. Some hope quantum gravity will remove the need for such a concept.

    Couldn't that point be consisdered the center of the universe?
    No, because we've never left it. That "point" has expanded, and now encompasses everything, just as it did previously.

    The description of the universe as finite but unbounded always gives me problems trying to get a mental picture of what is going on and I can't avoid thinking about what lies outside this unbound but finite universe (which I see is being discussed on another thread).

    I can easily tell you what lies outside it: Nothing. The universe encompasses all, by definition. Anything you can imagine to be outside it is actually inside it.

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    Quote Originally Posted by Ut
    The description of the universe as finite but unbounded always gives me problems trying to get a mental picture of what is going on and I can't avoid thinking about what lies outside this unbound but finite universe (which I see is being discussed on another thread).
    It's trite, and I'm sure I've seen it on BABB many times, but the ol' two-dimensional me on the surface of a huge, huge (expanding) sphere, works for me. The sphere is so large that any measurement I might make says my locale is flat as flat can be. Yet, if I traveled this sphere of finite size, I could travel endlessly, peridocally passing through my starting position. Thus, it is finite, but unbounded.

    Now the fun part: just bump everything up one dimension.

    Yeah, it's simple, too simple, but it gets me over the mental hurdle.

    Edit: fix quoting.
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

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    Here is a link to a were astronomers say they have determined that the univese is approximately 156 billion light years wide.

    http://www.space.com/scienceastronom...ay_040524.html

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    Quote Originally Posted by Tbutyl
    Here is a link to a were astronomers say they have determined that the univese is approximately 156 billion light years wide.

    http://www.space.com/scienceastronom...ay_040524.html
    How did you wrap your mind around this part:

    All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.
    They say that "starting point" is one end of a 156 billion ly diameter to us. The same would be true for observers anywhere considering the oldest light now reaching them. So, everywhere is the center of such a diameter. So, nowhere is near an edge.
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

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    Perhaps the edge of the universe is the point where science meets philosophy?

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    Quote Originally Posted by 01101001
    How did you wrap your mind around this part:

    All the pieces add up to 78 billion-light-years. The light has not traveled that far, but "the starting point of a photon reaching us today after travelling for 13.7 billion years is now 78 billion light-years away," Cornish said. That would be the radius of the universe, and twice that -- 156 billion light-years -- is the diameter. That's based on a view going 90 percent of the way back in time, so it might be slightly larger.
    They also seem to talk about the radius of the Visible Universe.

  15. #15
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    Jeez, somewhere I recall someone explaining the expanding universe as sitting in a movie theater where every chair was one foot apart. then you move all the chairs apart by a foot. from each chair, it appears that you are the center and the universe is expanding away from you.

    personally, i prefer the 'expanding balloon' idea, where we are all essentially dots on thesurface of a balloon as it's being blown up and as it expands the distnaces between the individual dots increase at the same rate. and again, it appears that each point is the center.

    an interesting thing about the balloon demo is that there is a 'center', but its not in the same plane as the 'universe'. (because the universe is two dimensional on the balloon and the 'center' is an imaginary point within the balloon'. Its very possible that with our current knowledge we simply can't make a solid statement about this - we need more facts. another interesting observation is that our 'visible' universe is only a small fraction of the real universe. from any point on the balloon, we can only see a fraction of the whole balloon. so there's that to consider.

    lastly, we should also consider with the balloon analogy that we are riding the 'shockwave' of the creation of our universe (i.e the skin of our balloon). In short, what I'm saying is that we may be really off base at this point in time (like trying to figure out what the elements are without a knowledge of the atom - or medicine without germ theorey) because we're so dumb. It would be really presumptious to think that in 2005 we could speak intelligently about everything in our universe.

    Oh, and last i'd heard, Dr. Seeds was teaching at "Franklin and MArshall" college in PA. Great school, btw, have a friend who is an alum there.

    John

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    I honestly havent given this topic much thought, because the way I see it, it doesnt really matter what is outside of the visible universe, because there are just so many possibilities, and, we will never know if we are correct. Its fun to speculate, but it doesn't really matter. consider an ant...going about its life...not really knowing or caring about anything else....its surrounding environment of like....a tree...some rocks...and some other bugs composes the visible universe to it. If you could tell it about the crazy things going on in our visible universe, or even just the rest of the earth, it would probably be completely dumbfounded and shocked...I think we would be the same way if we were to learn about the whole universe and what may lay beyond it.

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