Notices
Results 1 to 9 of 9

Thread: So what tells us the Universe can't be infinite?

  1. #1 So what tells us the Universe can't be infinite? 
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,046
    The question of an infinite universe came up on another thread just a little bit ago. I'm curious how strong the evidence is against it. The questions of being bombarded by infinite starlight, or having all the matter gravitationally collapse together are easy to settle, but I'm pretty sure there are other things.

    (A tapered universe allows for that pretty easily, because most models that predict either of those problems require the assumption of an approximately uniform distribution of matter, which would be false in a tapered universe.)

    I've heard Quasars and radio galaxies (which are apparently the same thing) are only seen to occur at really far distances, and therefore at very long times into the past. The nearest known I think is within 800 million light years, I think?

    (Maybe Quasars occur in galaxy clusters similar to the way super massive black holes occur in galaxies? Maybe there's usually only one per cluster? I know spotting a quasar usually makes it easier to find galaxies nearby. We know other clusters are far away.)

    Anyone know of other objections?

    1) - Infinite light

    2) - Everything would collapse together

    3) - Quasars only occur very far away

    4) - ???


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    4)- Infinite energy


    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  4. #3  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,046
    Yes, but if the infinite energy is spread out over an infinite area, then how do we decide which one is supposed to dominate?

    In a tapered universe, the suggestion is that the ratio of matter/energy to area gets smaller the larger the structure we look at. A galaxy cluster has less matter per area than a galaxy. A super cluster has less matter per area than a cluster. The bigger the amount of space we look at, the larger the construct.

    This is why this is such an interesting topic for me. It's the very idea of infinity that's confusing.

    The concept of an object traveling away from a planet or star at escape velocity evokes a lot of the same concepts. If it never encounters any other gravity wells, intuition tells us that it must eventually slow down and begin to return.

    Physics tells us that it won't.
    Reply With Quote  
     

  5. #4  
    Forum Ph.D. Leszek Luchowski's Avatar
    Join Date
    Jun 2008
    Location
    Gliwice, Poland
    Posts
    807
    Quote Originally Posted by kojax
    This is why this is such an interesting topic for me. It's the very idea of infinity that's confusing.

    The concept of an object traveling away from a planet or star at escape velocity evokes a lot of the same concepts. If it never encounters any other gravity wells, intuition tells us that it must eventually slow down and begin to return.
    That's because our intuition was formed on Earth, where it's impossible for a moving but unpropelled object to keep moving indefinitely by sheer inertia.

    If there were air in outer space, your escaping object would eventually lose its velocity, and then, even at four dozen gazillion trillion kilometers, the star's gravitational pull would claim it back. In vacuum, this is not the case.

    Of course, in reality, the space is not a _perfect_ vacuum, but it's so close to perfect that your object will have met many other stars before it can suffer any perceptible drag from (extremely rarefied) interstellar gas.

    Best, Leszek.
    Reply With Quote  
     

  6. #5  
    Forum Professor
    Join Date
    Apr 2007
    Location
    In the circuitous haze of my mind
    Posts
    1,028
    If there were air in outer space, your escaping object would eventually lose its velocity, and then, even at four dozen gazillion trillion kilometers, the star's gravitational pull would claim it back.
    I doubt that; after a certain point you are completely unaffected by your sun's gravity.
    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"

    -Einstein

    http://boinc.berkeley.edu/download.php

    Use your computing strength for science!
    Reply With Quote  
     

  7. #6  
    Time Lord
    Join Date
    Apr 2008
    Posts
    5,305
    Quote Originally Posted by Cold Fusion
    after a certain point you are completely unaffected by your sun's gravity.
    Quantify that? I doubt gravity knows fences.

    Anyway.

    1) - Infinite light

    The volume is infinite, not the density. The composition needn't be different. A guppy can live in a lukewarm fishbowl or a lukewarm lake - same water.


    2) - Everything would collapse together

    You kinda answered that: "if the infinite energy is spread out over an infinite area, then how do we decide which one is supposed to dominate?"

    To what center would everything collapse?


    3) - Quasars only occur very far away

    Polar caps and active volcanoes only occur very far away too - same explanation perhaps.


    4) - Infinite gravity

    See (1). It's not like this is getting made from nothing and just accumulates * la perpetual motion.



    Something I think worth considering about infinite space, often overlooked, is that normal usage means "infinitely large in scale". This I find is what people imagine. However a corollary to allowing boundless larger scales, I believe, is boundless smaller scales. Otherwise, the universe would have a "floor", with us located just a smidgen "above the floor". A full-on infinite provides no positional references.


    Quote Originally Posted by Kojax
    In a tapered universe, the suggestion is that the ratio of matter/energy to area gets smaller the larger the structure we look at. A galaxy cluster has less matter per area than a galaxy. A super cluster has less matter per area than a cluster. The bigger the amount of space we look at, the larger the construct.
    That's manifestly true whatever model we imagine. You're talking about trends along shift in scale (frame). For example smaller things have a higher velocity - even a person walking around on Earth must have a higher velocity than Earth. These rules are so numbingly obvious that we don't think much about them... same as we didn't used to think much about gravity, I guess.
    Reply With Quote  
     

  8. #7  
    Forum Professor leohopkins's Avatar
    Join Date
    Dec 2006
    Location
    Dulwich, London, England
    Posts
    1,417
    If you tried to pull two particles apart which are connected via the strong nuclear force, in the end you would end up creating another particle because the force between them gets stronger, the further you move them apart. It follows the inverse square law, just in the opposite way to which gravity does. anyway.........

    You could carry on pulling these quarks apart, thusly creating new quarks, ad infinitum. so why shouldnt the universe be infinate ? (in terms of empty vacuum)

    I have heard people say that before the BB (or EOS) that there was nothing. WHAT??? Not even a perfect vacuum??? - No they say, not even a vacuum? - well what then?, a sodding brick wall ? lol No, there was absolutely nothing, not a brick wall, not a vacuum, just absolutely nothing.

    When I think about that I feel like punching someone.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

    www.leohopkins.com
    Reply With Quote  
     

  9. #8  
    Forum Professor sunshinewarrior's Avatar
    Join Date
    Sep 2007
    Location
    London
    Posts
    1,526
    Quote Originally Posted by leohopkins
    I have heard people say that before the BB (or EOS) that there was nothing. WHAT??? Not even a perfect vacuum??? - No they say, not even a vacuum? - well what then?, a sodding brick wall ? lol No, there was absolutely nothing, not a brick wall, not a vacuum, just absolutely nothing.

    When I think about that I feel like punching someone.
    Only if you hold 'someone' responsible for creating that situation. Scientists are quite good at following Wittgenstein's dictum: Whereof we cannot speak, thereof we must remain silent.

    When they say there was 'nothing', all they mean is 'nothing that connects our universe to any possible pre-Big Bang', and they refuse to speculate too much - though the many-universes hypotheses have spin-offs including the idea that universes beget universes so that a Big Bang for one universe may just be a black hole for another. But that's as far as they go because any such hypothesis is not, with our current theories, testable...
    Reply With Quote  
     

  10. #9  
    Forum Ph.D. Leszek Luchowski's Avatar
    Join Date
    Jun 2008
    Location
    Gliwice, Poland
    Posts
    807
    Quote Originally Posted by Cold Fusion
    If there were air in outer space, your escaping object would eventually lose its velocity, and then, even at four dozen gazillion trillion kilometers, the star's gravitational pull would claim it back.
    I doubt that; after a certain point you are completely unaffected by your sun's gravity.
    In the real universe, after a certain point the gravity of other stars etc will be much stronger than that of your "mother" star, and will make it negligible by comparison.

    But if there were no other stars, nothing in all of space, just your "mother" star, you in your runaway spaceship, and the rarefied interstellar gas that saps your momentum (assuming the gas is evenly distributed so ITS gravity doesn't compound the equation), then yes, no matter how far away you have flown prior to stopping, the gravity of your sun will pull you back. The farther you have escaped, the gentler and slower the pull will be, but as unescapable as value-added tax.

    Cheers, Leszek.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •