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Size of all things in the universe

  1. Aug 12, 2013 #1
    If all humans were between 5-6 inches tall we would not know we are small.

    If all humans were between 50-60 feet tall we would not know we are tall.

    Size is relative. We need a point of reference.


    How big is the universe? Could it be real small?
     
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  3. Aug 12, 2013 #2

    PeterDonis

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    If size is relative, why do you care? The universe is a lot bigger than you are.
     
  4. Aug 12, 2013 #3
    Size is relative, and so is speed of change in relation to it... something large like a galaxy has causal influences that may take hundreds of thousands of years to pass from one end to the other, while small things like quantum entities are interacting among themselves untold zillions of times per second.

    As far as a point of reference, we "sort of" have c, which relates time and distance, and the apparent order of scale below which classical concepts begin to fail... not sure how well defined that is, but it's "sort of there", too.

    That is to say, if humans were between .5 and .6 angstroms, we would know that we were smaller than this classical / quantum boundary scale... as things are, we know we are above it...
     
  5. Aug 12, 2013 #4

    HallsofIvy

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    This makes no sense without more detail l If all humans were between 5-6 inches tall and everything else were the same size it is now, we would definitely know! Especially the first time your dog licked your face!

    If all humans were between 50- 60 feet tall and the earth were the same size, we would not be able to move!

    For one thing, you need to say what "an inch" or "a foot" could mean without a reference! Before anyone can answer your question "Could the universe be real small?" you need to say what you mean by "real small".
     
  6. Aug 12, 2013 #5
    Don't you see that clearly he is considering the scenario that were humans 5-6 inches tall that the entire universe of all things were scaled likewise... so that "we would not know"? His idea is of an absolute universal change in size of everything, and since all relative sizes remain in proportion, we don't know know how big or small we "really" are.

    What I would wonder is if this change in size of "everything" would include the general boundary between classical and quantum - as in, what about size itself heralds the transition? Would that size be "invariant" across "different size scale" universes?
     
  7. Aug 12, 2013 #6

    russ_watters

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    That's self-contradictory and more or less meaningless: yes, words like "small" and "tall" are relative and so we'd need to measure with respect to other things. At 6", we'd be "small" next to a dog and at 60 feet, we'd be tall next to a giraffe. So what?
    90 billion light years.
    Relative to us, no.
     
  8. Aug 12, 2013 #7

    micromass

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    The problem is that if everything were scaled likewise, then so would the concept of an inch. So we would not be 5-6 inches tall.
     
  9. Aug 12, 2013 #8

    russ_watters

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    It isn't clear to me because if that's the case, nothing has actually changed except the scale of the units. It's like saying I'm 6' tall, but you're 1.8 m tall so I'm taller than you. At best, it's just wrong.
     
  10. Aug 12, 2013 #9

    phinds

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    NOTE: this refers to the OBSERVABLE universe, NOT to the whole universe. We have no idea how big the whole universe is.
     
  11. Aug 12, 2013 #10
    Why is that? Or are you assuming all man made structures are also the same?
     
  12. Aug 12, 2013 #11

    chiro

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    One idea might involve looking at anthropological argument (but not simply applied to humans, but all living things, and the stability of physical and dynamical systems).

    There have been arguments made that for example if the gravitational constant were changed even slightly then things would be severely screwed up.

    I'd suggest to look into similar ways to answer this question to understand why the scales are the way they are (at least in terms of making an inference with some sort of justification and evidence like in the above example).
     
  13. Aug 12, 2013 #12
    Thank you for explaining this better than I did.

    Obviously if we were real small we could say we are only a few atoms long, but we would not know if we are big or small.

    The size of the universe is large in relationship to us, but I wonder if our universe is real tiny in the greater scheme of things. A galaxy may be as small as an amoeba.


    I assume that our current size is dictated by gravity directing human evolution. If we were a 1/4 inch in height perhaps we could jump as high as a cricket.
     
  14. Aug 12, 2013 #13

    micromass

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    What greater scheme of things? The universe is all that there is.
     
  15. Aug 12, 2013 #14

    Chronos

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    The Planck length is a convenient yardstick. It is about 1.6E-35 meters. The observable universe is about 1E27 meters in diameter. We humans are considerably closer in size to the diameter of the universe than to a planck length. It would be truly remarkable if we were smack in between. A fertilized human ovum is, however, just about the right size - see there, we are special after all.
     
  16. Aug 13, 2013 #15
    As far as I know, whether a system behaves classically does not depend on it's size but on the amount of particle interactions in that system.
     
  17. Aug 13, 2013 #16

    russ_watters

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    Huh? We know how big a galaxy is and we know how big an amoeba is. A galaxy is not as small as an amoeba.

    This all sounds like gibberish to me. You don't seem to be applying logic here.
     
  18. Aug 13, 2013 #17

    phinds

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    +1 on that.
     
  19. Aug 13, 2013 #18
    Guys don't take him so literally, what he means is that an ant is stupidly large compared to an atom, the world is stupidly large to an ant and the observable universe is mind blowingly large compared to the earth. What he means is that the observable universe might appear so large to us, but compared to the actual size of the entire universe, it may be like an atom compared to earth.

    So in actual fact the observable universe is actually quite tiny. I don't like talking about this subject because it's just basically saying "what if, what if, what if" which tends to lead to philosophy which leads to a closed topic. Bottom line is, the observable universe in comparison to the entire universe could be as small as an atom compared to earth... then again it could all be controlled by a 8 headed dragon riding a pony wielding a trident making stars explode.
     
  20. Aug 13, 2013 #19
    Thank you. I think i can sleep better now.
     
  21. Aug 13, 2013 #20
    See the poster above. He seems to explain it a bit better.
     
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