1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question: are there proportions in infinity?

  1. Aug 15, 2012 #1
    Let me preface this by saying Math is not my strongest subject.

    I am curious if there is any relationship between proportionality and the term infinity.

    This question stems from an online discussion about whether or not the physical universe is infinite or not. (Obviously its not, but I have a hard time explaining this to the new agers).

    Lets say that on average, for every 100 stars in the universe, there is a corresponding black hole giving us a 100:1 ratio.

    Now lets say that the universe is infinite. Meaning there would be an infinite number of stars and black holes.

    Could you then say there is an infinite number of stars and black holes at a proportion of 100:1?
    I know how non-sensical that sounds, even to a math layman such as myself.

    But is there anything in the mathematics of infinity that has to do with proportionality?

    Could you describe the rate at which numbers appear, that are divisible by 2 versus the rate at which numbers appear that are divisible by 3 on an infinite number line?

    Thanks in advance!
  2. jcsd
  3. Aug 15, 2012 #2

    Stephen Tashi

    User Avatar
    Science Advisor

    Some forum members fearlessly answer questions like "What is the proability that a positive integer selected at random is even?" - as if it were possible to make such a selection and as if the infinity of positive integers had a fraction of even integers equal to 1/2.

    However, I don't know of any mathematical theory that correctly deals with such questions except by using limits of finite quantities. For example if you say:

    Let p(n) = the probability that an integer selected at random from the first n integers is even
    Let p(n) = the fraction of the first n integers that are even

    then you can sensibly talk about taking "The limit of p(n) as n approaches infinity". In that phrase, the word "infinity" has no independent meaning. The whole phrase has a mathematical definition, but the individual terms like "infinity" or "approaches" have no precise definition.

    By the way, Benoit Madelbrot, in one of his books on fractals, remarks that the standard arguments that the universe is finite (such as "if it were infiinite then the sky would be infinitely bright") are incorrect if we consider certain fractal distributions of stars.
  4. Aug 15, 2012 #3
    Thank you for the clarification and an easily understandable explanation!

    I have read about the fractal placement of stars as a possible solution to olbers paradox.

    Which confuses me even more. Have we even noticed anything remotely close to a fractal pattern in the placement of stars? Or maybe its difficult to discern from our scale/point of view.

    But, wouldn't an infinite universe = infinite mass = infinite gravity?
  5. Aug 15, 2012 #4

    Stephen Tashi

    User Avatar
    Science Advisor

    I don't think cosmology can settled by simplistic logic - but I'm not an astronomer. I don't know of any physical or mathematical laws that confirm your statement. (I take it that by "=", you don't mean "equal", but rather "imply".)
  6. Aug 15, 2012 #5
    I also have questioned the meaning of "approaches" as in "The limit of p(n) as n approaches infinity". The implication is that to approach infinity n must increase. But is 100 or 1 million any closer to infinity than is 1? Wouldn't the phrase have more meaning if it were stated as "The limit of p(n) as 1/n approaches zero".
  7. Aug 15, 2012 #6
    The limit as n approaches infinity has a well-defined technical meaning. The distance to infinity is not relevant to limits. We say that [itex]p(n)\rightarrow x[/itex] as n approaches infinity if for all [itex]\varepsilon >0[/itex], there exists an N such that for all [itex]n\geq N[/itex] holds that the distance between p(n) and x is smaller than [itex]\varepsilon[/itex].

    With this definition, we don't need to talk about infinity or distance to infinity.
  8. Aug 15, 2012 #7

    Yes, its probably not wise to use such simplistic logic for such an immense question.

    And yes I should have used the word imply rather than =.

    Thanks for the help. I was unsure which math forum to pose the initial question in.

    Maybe I should continue this in one of the physics/cosmology forums.

    Again, I appreciate that you took the time to answer my questions.
  9. Aug 18, 2012 #8
    Some people say infinity is infinity. There is nothing greater than infinity not even infinity + 1. But, others argue that there is a greater infinity of real numbers between 0 and 1 than there are integers between 0 and infinity. It is all how you interpret it.
  10. Aug 18, 2012 #9


    User Avatar
    Science Advisor

    There are different classes of infinities: the idea of limits helps establish things like this.

    The thing you have to remember is that infinity in no way acts like a finite number. A finite number has a distinct definition in that it can be compared to any other number that has some kind of ordering or ranking.

    Infinities don't work like that because they are not defined the same way.

    You can use some tools to rank different infinities but it requires a different way than ranking finite numbers. The first person to look at this in serious detail (in modern mathematics) was George Cantor and you can read his work on the subject (and later work from others that followed).

    There are things like limit laws that deal with very precise definitions involving infinity so it's completely misleading to say that it's all down to "interpretation".
  11. Aug 18, 2012 #10
    Check out Cantor's diagonal argument
    http://en.wikipedia.org/wiki/Cantor's_diagonal_argument . It is very convincing that there are larger infinities. Specifically, it shows how there is an infinite set larger than the countably infinite set aleph-naught that we are used to seeing such as the set of all natual numbers {1, 2, 3,...}.
  12. Aug 18, 2012 #11
    Yes, but it is still debated between many people (I personally am on the side of different sizes of infinity).
  13. Aug 20, 2012 #12
    Of course there is, since there is more than one infinity.

    That is why the ratio of two (different) infinities can be deduced as a real number eg

    Wallis' formula.
  14. Aug 21, 2012 #13


    User Avatar
    Science Advisor

    Maybe you'll find this interesting; it is somewhat related (If I understood your

    question correctly) :


    Look up the section on multiplication and division--which you may relate

    to proportion-- but remember that cardinal

    arithmetic does not follow the standard rules that everyday numbers follow.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook