Can we even find examples of infinity in nature?
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19Can we even find examples of $1/2$ in Nature? – Gerry Myerson Jun 05 '12 at 13:09
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1Don't we see $\pi$ in nature? So why not 1/2? – zzzzzzzzzzz Jun 05 '12 at 13:13
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8That doesn't answer the question. Anyway, where do we see $\pi$ in Nature? Have you ever seen a circle in Nature? – Gerry Myerson Jun 05 '12 at 13:17
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I don't think we necessarily need a circle to find $\pi$. How about the presence of $\pi$ in "describing fundamental principles of the universe" http://en.wikipedia.org/wiki/Pi#Physics ? – zzzzzzzzzzz Jun 05 '12 at 13:19
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4Actually, we don't even know if there are an infinite number of particle in nature. – Vittorio Patriarca Jun 05 '12 at 13:24
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3Did you really mean to use $\aleph_1$ rather than the continuum? – Harry Altman Jun 05 '12 at 13:53
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1I would like to see concrete examples of sets with cardinality $\aleph_{1}$ which exist outside of the machinery of large cardinals or ordinals -- because otherwise I am inclined to doubt the alefs -- the beths on the other hand, I get immediately. – graveolensa Jun 05 '12 at 14:10
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2Why does it matter that we can find examples of infinity in nature? If $g = 10^{100}$ denotes a googol, and $G = 10^g$ denotes a googolplex, it is highly doubtful that one could find examples of $G^G$ in nature. – user642796 Jun 05 '12 at 14:30
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I think finding infinity – zzzzzzzzzzz Jun 05 '12 at 14:57
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2Define Nature. – lhf Jun 06 '12 at 07:08
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1@GerryMyerson Can we even find examples of 1/2 in Nature? What if you fold something? – draks ... Jun 06 '12 at 07:17
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6@draks: then you have a folded object, not a 1/2. – Jun 06 '12 at 08:31
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So: does it make sense to ask whether time is discrete or continuous? – GEdgar Jul 03 '12 at 21:35
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If by nature mean physical universe, note that there is no proof that an infinite set exists. While some of us would like to believe that universe is infinite, this cannot be proved. You also have to remember that while we wish to perceive reality as a continuous process, we cannot measure it in a precise accuracy. Given two pieces of metal of length 1m (by the same ruler) they might differ by a few atoms, which means an actual increase in length since while very very small atoms still have a physical size. So we cannot really point out an infinite object in the universe, but we also fail pointing out something which is "exactly half" of another thing, etc.
If, on the other hand, you mean "in mathematics outside of set theory", then $\aleph_1$ is somewhat hidden. The reason is that in non-set theoretic constructions there is often little importance to cardinalities but rather countability; the cardinality of the continuum; and perhaps the power set of that.
However there are still mild appearances of $\aleph_1$. For example Shelah proved in 1980 that there is a group of size $\aleph_1$ such that every proper subgroup of it is countable. Another example (although slightly more set theoretic in nature) is the solution for Whitehead problem's for $\aleph_1$. If we assume in addition to ZFC a certain set theoretical assumption then every $W$-group of size $\aleph_1$ is free; if we prove a different assumption (again, with ZFC) then there is a $W$-group of size $\aleph_1$ which is not free.
Somewhat related:
Asaf Karagila
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I agree. Yes, sets of specific objects in nature are in principle finitely countable. However, in practice, it depends on how you specify the object. "How many 'yellow' bricks ?" raises the question "exactly what is 'yellow' ?", which gives rise to infinitely uncountable subsets since the definition relies on an impossibly accurate measurement. A little besides the point, but no numbers in nature are static as objects evolve and disappear all the time - even stars, not to say electrons. – Jens Jul 05 '17 at 13:35
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Your comment makes no sense. How did you get from "finite" to "uncountably many subsets" is beyond me. – Asaf Karagila Jul 05 '17 at 14:56
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1Is the set of "All frequencies" (colors) finitely countable, infinitely countable or just uncountable? – Jens Jul 06 '17 at 12:59
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I don't know. The collection of frequencies that we can measure and distinct, however, is most certainly finite. – Asaf Karagila Jul 06 '17 at 13:10
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How about this : the set of the possible repartition of probability of presence of, say, an electron is a continuum (well at least in most models!), while the set of its possible energy levels is countable.
Albert
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3I think you give the game away when you write, "well, at least in most models". Models are mathematics, not nature. – Gerry Myerson Jun 06 '12 at 00:07
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The question as asked is somewhat ill-defined. I'm taking the word "nature" to mean "the thing described by physics", but what does it mean to say that some mathematical object is found "in nature"?
Such a phrase can only have meaning relative to some scheme of associating mathematical objects to physical "things". It's easy to construct such a scheme: e.g. if I consider an interpretation where I associate "$\aleph_1$" to the apple sitting on the table and don't assign meaning to any mathematical objects, then I have found $\aleph_1$ in nature: it's sitting on the table.
Maybe more interesting is a more 'standard' interpretation associated with a physical theory. The interpretations typically seen in a physical theory don't tend to associate cardinal numbers to objects. In particular, they don't assign $\aleph_1$ to any sort of object, and this fact is a banality.
But, in this case, we can turn to higher-order concepts. e.g. a version of Newtonian physics based upon ZFC+CH says that the number of points in any region of space is $\aleph_1$.