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I have trouble with the mathematical notion of infinity.

Example: Consider all of the natural numbers. It has a beginning, therefore it is bordered, therefore it cannot be infinity.

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  • What kind of research have you done for infinity? You could start with: en.wikipedia.org/wiki/Infinity. The infinity symbol "Infinity (symbol: ∞) is a concept describing something without any bound or larger than any natural number." – mavavilj Sep 20 '18 at 10:43
  • But not larger than infinite ray of natural numbers?.. – Josef Klimuk Sep 20 '18 at 10:46
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    The succession of natural numbers havo ne end (there is no "greatest" number) but has a beginning : 0 (or 1, according to the definition). – Mauro ALLEGRANZA Sep 20 '18 at 10:47
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    Additionally, taking that loose interpretation of "bordered", you would probably agree that a closed interval (beginning and end) on the real line is bordered, right? Well, there are an infinite amount of points in that interval, so why would it being bordered lead to there being only a finite amount of points? Both of your inferences, that having a beginning leads to being "bordered", and that being "bordered" means it cannot be infinite, seem to be both unsound and invalid. Can you defend these views instead of just asserting them so we can understand why you believe they should be the case? – Not_Here Sep 20 '18 at 11:55
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    In-finite means not finite, rather than not bordered. The interval [0,1] or a closed circle are also "bordered", yet infinite. – Conifold Sep 20 '18 at 22:29
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The usual word for what you're calling "bordered" is "bounded". A set of numbers is bounded if we can find an upper and a lower bound. The set of natural numbers has a lower bound, but no higher bound. Therefore, it's unbounded, and infinite.

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  • Just to play devil's advocate, what about the set of complex numbers. I don't see how the idea of "upper" or "lower" bound apply there. – Frank Hubeny Sep 20 '18 at 16:28
  • @FrankHubeny why is the devil’s advocate? You haven’t contradicted David Thornley – ChristopherE Sep 20 '18 at 18:59
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    @FrankHubeny Your comment makes no sense. The complex numbers are obviously infinite, so taking David's tentative definitions, there isn't any contradiction to be found. Again by these tentative definitions presented in this answer, if C doesn't have an upper bound or a lower bound then it obviously doesn't have both which means it isn't bounded. What are you trying to point out? – Not_Here Sep 20 '18 at 19:34
  • @ChristopherE David defines bounded as a set where "we can find an upper and a lower bound". Consider a bounded set of complex numbers, say, {i, -i, 1, -1} It is a bounded set. Where is the upper and lower bound? – Frank Hubeny Sep 20 '18 at 19:55
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    @FrankHubeny: Yes, the concept does need to be changed if we're talking about complex rather than natural numbers. Complex sets can still be bounded, but in somewhat different ways. Also, I should have been more clear about an upper bound not having to be a least upper bound. In the set you mention, 1 is an upper bound, but thee is no least upper bound. – David Thornley Sep 20 '18 at 20:02
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If your definition of an "infinity" is "an ordered set that has neither a maximum or a minimum", then the natural numbers would indeed not be an "infinity".

However, it is also true that the natural numbers are an "infinite set" (or stated in more detail, "a set whose cardinality is infinite").

There is no contradiction here, because "Klimuk infinity" has very little to do with "infinite set".


My general advice regarding matters of the infinite is to ignore any conclusions on the topic that do not come from mathematical contexts, or otherwise heavily draw from the mathematical notion.

It took thousands of years for people to realize that there were a whole multitude of different ideas, notions, and objects that had previously all been called "infinity", so most history on the topic consists of all of these different things jumbled together into a confused mess. And even when someone could coherently discuss one specific idea they were calling "infinity", there is no guarantee it has any bearing on what someone else was calling "infinity".

Without really knowing much about what you're thinking, the idea you specifically have in mind is probably best captured in the notion of a "compact topological space".

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Consider all of the natural numbers. It has a beginning, therefore it is bordered, therefore it cannot be infinity.

Nope. It's not really (for some definition of "really"), as you say, "bordered". Perhaps you're already familiar with the trivial demonstration that the even numbers are equinumerous with all numbers: just take 2,4,6,8,... and divide each number by 2, whereby you get 1,2,3,4,... So, you already see where I'm going with this???...

...It's pretty much equally easy to map 1,2,3,4,... into ...-4,-3,-2,-1,0,1,2,3,4,... as it was to map 2,4,6,8,... into 1,2,3,4,... For any natural number n, just map it to n/2 for even n, as above, and to -(n-1)/2 for odd n.

So, 1,2,3,4,... are fundamentally just syntactic symbols. Interpreted one way, the usual way, they're the natural numbers. But interpreted our way above, 1,2,3,4... are just a different set of labels/symbols for the entire set of integers, which aren't "bordered".

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Re @FrankHubeny's comment, implicit in the above answer is a slightly more mathematical elaboration that I circumvented, thinking the details aren't of much interest here. (But I'd also have thought it would've been immediately obvious to Frank, with an ms in math and professional programming background, as per his profile.)

The underlying issue is the syntax-vs-semantics distinction I briefly alluded to. It's not 1,2,3,4,... which is bounded/"bordered", per se, but rather the poset ordering, in this case a total order, 1<2<3<4<... that lets you arrange them in a way that imposes a (more-or-less artificial) "beginning" on an otherwise unordered set. And, what's important here is that the cardinality of that set is independent of any imposed (partial or total) order.

Without the natural-number ordering, 1,2,3,4,... are just a collection of strings, "1","2","3","4",...,"998","999",... Programmers frequently generate such "collections" using Backus-Naur form (bnf), a notation for context-free grammars. Indeed, without the natural-number semantics/order imposed on them, our "1","2","3","4",... strings are just a context-free grammar typically called "numerals", and generated by bnf as illustrated at https://courses.cs.vt.edu/~cs1104/Compilers/Compilers.100.html (see the example towards the bottom of that page). Actually, the lecturer there fails to account for strings like "0123","000123",etc with any number of leading zeros. The usual textbook bnf exercise asks for a grammar that excludes them (which I'll also leave as an "exercise for the reader":).

So the upshot is that, as a collection of elements/objects, the natural numbers 1,2,3,4,... are identical/isomorphic to the strings "1","2","3","4",... And then "1" can't really be called the "first string" (although there's yet another "artificial" ordering called the lexicographic order, essentially alphabetic, where it would be first). What you've ultimately got is a very,very,very... big bucket of strings, with no beginning,middle,end; just a big jumbled-up collection. And you know that collection is countably infinite by an analysis of the underlying grammar that generates it. Then, if you want to impose a semantics and order on that collection, it's your business. But it won't affect the cardinality.

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  • You could also map the natural numbers to a bounded set in (0,1] by taking n to 1/n. I don't see how these mappings answer the question. – Frank Hubeny Sep 21 '18 at 7:50
  • @FrankHubeny I already said "Interpreted one way..." versus "Interpreted another way...". The op >>already<< illustrated a "bounded set" interpretation. You simply illustrated yet another bounded one. And I simply illustrated an unbounded one, trying to demonstrate to the op that bounded/"bordered" or not is >>irrelevant<< to cardinality. There's no "answer", per se; rather, it's not relevant. (And I edited the answer adding an entirely different, more elaborate discussion, whereby an analysis of the context-free grammar generating numerals establishes their cardinality, independent of order.) – user19423 Sep 21 '18 at 13:08
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Well, let us look at the concept of ‘infinity’ from a physicist point of view.

There exist two separate assumptions: “infinitely big” and “infinitely small.” By infinitely big, one means that space can have infinite volume, that time can continue forever, and that there can be infinitely many physical objects.

By infinitely small, it represents the continuum—the idea that even a liter of space contains an infinite number of points, that space can be stretched out indefinitely without anything bad happening, and that there are quantities in nature that can vary continuously.

The two assumptions are closely related, because inflation, the most popular explanation of our Big Bang, can create an infinite volume by stretching continuous space indefinitely.

The theory of inflation explains how a subatomic speck of matter transformed into a massive Big Bang, creating a huge, flat, uniform universe, with tiny density fluctuations that eventually grew into today’s galaxies and cosmic large-scale structure—all in beautiful agreement with precision measurements from experiments such as the Planck and the BICEP2 experiments.

But by predicting that space isn’t just big but truly infinite, inflation has also brought about the so-called measure problem, which I view as the greatest crisis facing modern physics.

The problem is that whatever experiment you make, inflation predicts there will be infinitely many copies of you, far away in our infinite space, obtaining each physically possible outcome; and despite years of teeth-grinding in the cosmology community, no consensus has emerged on how to extract sensible answers from these infinities. So, strictly speaking, we physicists can no longer predict anything at all!

This means that today’s best theories need a major shakeup by retiring an incorrect assumption. Which one? Here’s my prime suspect: ∞. Infinity Doesn’t Exist

A rubber band can’t be stretched indefinitely, because although it seems smooth and continuous, that’s merely a convenient approximation. It’s really made of atoms, and if you stretch it too far, it snaps. If we similarly retire the idea that space itself is an infinitely stretchy continuum, then a big snap of sorts stops inflation from producing an infinitely big space and the measure problem goes away.

Without the infinitely small, inflation can’t make the infinitely big, so you get rid of both infinities in one fell swoop—together with many other problems plaguing modern physics, such as infinitely dense black-hole singularities and infinities popping up when we try to quantize gravity.

In the past, many venerable mathematicians were skeptical of infinity and the continuum. The legendary Carl Friedrich Gauss denied that anything infinite really exists, saying “Infinity is merely a way of speaking” and “I protest against the use of infinite magnitude as something completed, which is never permissible in mathematics.”

In the past century, however, infinity has become mathematically mainstream, and most physicists and mathematicians have become so enamored with infinity that they rarely question it. Why? Basically, because infinity is an extremely convenient approximation for which we haven’t discovered convenient alternatives.

Consider, for example, the air in front of you. Keeping track of the positions and speeds of octillions of atoms would be hopelessly complicated. But if you ignore the fact that air is made of atoms and instead approximate it as a continuum—a smooth substance that has a density, pressure, and velocity at each point—you’ll find that this idealized air obeys a beautifully simple equation explaining almost everything we care about: how to build airplanes, how we hear them with sound waves, how to make weather forecasts, and so forth. Yet despite all that convenience, the air, of course, isn’t truly continuous. I think it’s the same way for space, time, and all the other building blocks of our physical world. We Don’t Need the Infinite

Let’s face it: Despite their seductive allure, we have no direct observational evidence for either the infinitely big or the infinitely small. We speak of infinite volumes with infinitely many planets, but our observable universe contains only about 1089 objects (mostly photons).

If space is a true continuum, then to describe even something as simple as the distance between two points requires an infinite amount of information, specified by a number with infinitely many decimal places. In practice, we physicists have never managed to measure anything to more than about seventeen decimal places. Yet real numbers, with their infinitely many decimals, have infested almost every nook and cranny of physics, from the strengths of electromagnetic fields to the wave functions of quantum mechanics. We describe even a single bit of quantum information (qubit) using two real numbers involving infinitely many decimals.

Not only do we lack evidence for the infinite but we don’t need the infinite to do physics. Our best computer simulations, accurately describing everything from the formation of galaxies to tomorrow’s weather to the masses of elementary particles, use only finite computer resources by treating everything as finite. So if we can do without infinity to figure out what happens next, surely nature can, too—in a way that’s more deep and elegant than the hacks we use for our computer simulations.

Our challenge as physicists is to discover this elegant way and the infinity-free equations describing it—the true laws of physics. To start this search in earnest, we need to question infinity. I’m betting that we also need to let go of it.

Ref.- http://blogs.discovermagazine.com/crux/2015/02/20/infinity-ruining-physics/#.W6Oe1Hsza1s

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