Sets that have the same size together the collection of herbal numbers are dubbed ‘countably infinite’. Examples encompass the collection of also numbers and the set of rational numbers (numbers that have the right to be composed as fractions).The set of real numbers (numbers the live ~ above the number line) is the very first example the a set that is larger than the collection of smashville247.netanic numbers—it is ‘uncountably infinite’.There is an ext than one ‘infinity’—in fact, there room infinitely-many infinities, every one larger than before!
This is the 2nd topic in ours two-part series on infinity. We recommend you review the an initial in the series—To infinity—before analysis this one.

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Previously, us looked at approaches of comparing the size of sets and discovered that numerous infinite sets could be combine up exactly, such as the collection of smashville247.netanic numbersGLOSSARYnatural numbersThe collection of all positive entirety numbers, which have the right to be used to count individual objects. Relying on which mathematician you ask, this set may or may not include the number 0. Examples: 1, 2, 3, … and also the set of also numbers. These sets both have actually the exact same size, or ‘cardinality’—namely, ℵ0, our familiar ‘infinity’.

Does every infinite collection have the very same size? deserve to they all be suitable up exactly?

Despite intuitions, we have the right to count and also compare the size of limitless sets. Picture source: benjgibbs / Flickr.

For a lengthy time, it appeared as though all infinite sets would have the ability to be paired up through each other—meaning the every infinite collection had the very same size, ℵ0. This intuitive idea was shattered in 1874 through the discovery of larger and larger infinities. In a five-page record that shook the mathematical people (and offered rise to set theory in the process), the German mathematician Cantor laid out his exploration of new horizons ‘beyond infinity’.

How is it possible to build a ‘larger’ infinite set than the collection of all herbal numbers? Clearly, we’re walking to need to add extra ingredient to the mix.

Rational thinking

The an initial way us can broaden our number device is by presenting all the an adverse numbers, forming the set of integersGLOSSARYintegersThe set of all whole numbers, both positive and negative. The number 0 is definitely a part of this set. The natural numbers are a part of the integers. Examples: -2,-1,0,1,2,…, ℤ. However, it shortly becomes clear that this won’t aid us reach a larger cardinality—we’ve properly doubled the set ℕ by including in a negative number because that each confident number, and also we understand from our suffer with combining the odd and even numbers that doubling one infinite set is not going to change its cardinality.

The next thing us can try is to encompass all the numbers that deserve to be written as fractions, creating the set of rational numbersGLOSSARYrational numbersThe collection of every numbers, both positive and also negative, that deserve to be written as a fraction. Equivalently, the set of all numbers whose decimal expansions either terminate or at some point repeat. The natural numbers and the integers space a component of the rational numbers. Examples: -3, 1/3, 0.5, …, ℚ. Us seem to have an awful most numbers in ℚ—for starters, there’s infinitely-many fountain between any type of two whole numbers! Clearly, that going to be daunting to pair off the set of natural numbers:

$$mathbbN ext = , 1, 2, 3, 4, 5, 6, … $$

with the collection of reasonable numbers:

$$mathbbQ ext = 0, frac12,frac13, frac23, frac14, frac34, … , 1, frac112, frac113 … $$

Difficult… but not impossible. Cantor showed exactly how this might be done by thinking around things a little differently. The an essential is come realise the in order to pair turn off the rational numbers with the smashville247.netanic numbers, we just need to discover a means to perform the rational number in some sort of order. We can then pair the very first rational number in our list v the herbal number 1, the second rational in the list through 2, and so on.

On the surface, this appears a tall order—how can we deserve to list the rational numbers, which include fractions, in an stimulate that records every solitary one? yes no such point as the smallest portion bigger than 0, because that starters! yet by law a couple of mental zigzags, it turns out that it is possible to make such a list.

To start, we notification that every fraction is make from a pair of herbal numbers—one top top the optimal of the fraction (the numerator) and also one ~ above the bottom (the denominator). There are two caveats. First, we cannot have 0 on the denominator the the fraction. Second, we can write the same portion in infinitely-many various ways using various pairs of number (for example, 1⁄2 is the exact same as 2⁄4, and also 3⁄6, and also so on).

Since every reasonable number have the right to be believed of together a pair of herbal numbers, we can attract up the complying with infinite table to record every possible rational number:


Above: The begin of an unlimited table that lists every possible portion in its many simplified form. Each fraction is derived by dividing the pillar number (the numerator) by the heat number (the denominator).

We only want each portion to appear once in the table, and at the moment this table captures each portion over and over again, infinitely-many times. So us take care to get rid of all the duplicates by crossing the end each entry in the table wherein the portion can be simplified (all the entries whereby the numerator and denominator share a typical factor are removed).


Having excellent that, we have the right to now zigzag our method through the table to catch every solitary fraction, prefer so:


Above: Zig-zagging through the table the fractions, in stimulate to perform every possible fraction in some sort of stimulate (the stimulate in which we meet them in ours zig-zags).

Then all we need to do then is list every portion in the order us encountered it throughout the zigzag:

Table: one possible bijection between the herbal numbers and also the reasonable numbers.Natural numberRational number

And, similar to that, we’ve made our bijection between the rational numbers ℚ and the smashville247.netanic numbers ℕ! we can examine that every smashville247.netanic number has a partner (the perform of fractions never ever ends) and that every reasonable number has a companion (every rational appears exactly once in the table, after ~ we closely remove the duplicates). Although us didn’t capture all the negative rational number in our mapping, the exact same idea can be conveniently modified to take them right into account.

So it transforms out that the collection of rational number is countable too—it has actually cardinality ℵ0. It might seem choose we’ve run out that options; that there yes, really is just one type of infinity. But Cantor had one final trick increase his sleeve, which he released in his famous 1874 paper. There is one more source the numbers us can attract on.

The real surprise

Apart from smashville247.netanic numbers and rational numbers, over there is one more set that numbers that most civilization are acquainted with—the collection of actual numbersGLOSSARYreal numbersThe collection of every numbers that live top top the number line. The herbal numbers, integers and also rational numbers room a part of the actual numbers—but the real numbers likewise include numbers the cannot be composed as a fraction, favor π and the square root of pi, whose decimal expansions never ever terminate or repeat. Examples: -2, 0, 1/4, √3, log in 2, …, ℝ. The collection of real numbers contains all the entirety numbers and also all the fractions, and also lots of various other numbers that can’t be written as a fraction, favor π (pi) and the square source of 2.

Cantor had the ability to prove that it was difficult to produce a bijection in between the collection of real numbers ℝ and also the collection of herbal numbers ℕ. No issue how difficult you try, you’ll always end up using up every one of the herbal numbers and also still having real number left over, unpaired. There space definitely more real numbers, in a mathematically specific way, than there are natural numbers.

To prove this, Cantor supplied an elegant mathematical method known as ‘proof by contradiction’. Cantor started by assuming that you could create the bijection, listing all real numbers in part order, just like we did for the reasonable numbers, in bespeak to match them through the smashville247.netanic numbers. That then confirmed that no matter just how you had produced the list, he’d have the ability to show girlfriend a actual number that didn’t show up on your list, hence contradicting your result and proving it can not be done.

Even much more incredible, Cantor’s ‘diagonalisation argument’ doesn’t even need all the actual numbers to it is in considered. It turns out that any kind of piece that the number line, no matter how tiny, has ‘more’ numbers in it than all of the smashville247.netanic numbers (and every one of the rationals too)! To highlight Cantor’s argument, we’ll simply take the set of real numbers in between 0 and 1 and show that you can not possibly enhance them up v the set of all herbal numbers.

Suppose the we’ve produced our bijection, creating a perform containing every solitary real number in between 0 and also 1 in some order. Cantor climate hands us another real number in between 0 and 1 the can’t perhaps be on our list. To produce this number, that takes the an initial decimal place of our very first number on our list and also adds 1 come it, and also uses the result as the an initial decimal location of his brand-new number. (If the very first decimal place of our very first number was 9, he changes it to a 0). He then takes the 2nd decimal ar of our second number and also adds 1 to it, using the result as the second decimal ar of his brand-new number. This process continues, causing a actual number that Cantor claims won’t it is in on ours list.

Some infinities are higher than others

No matter exactly how you pair lock together, girlfriend can constantly find a genuine number that isn’t paired v a natural number. This way there are much more real than natural numbers—even though there’s an boundless amount the both!

Use this interaction to pair increase some herbal numbers through the start of genuine numbers. We’ll constantly be may be to generate a brand-new number that’s no on your list. We do this by taking one digit from each actual number (in a diagonal line pattern) and also incrementing that by one.

Natural numberReal number
Unmapped real number 0.13579
Natural numberReal number
Unmapped real number 0.22222
Natural numberReal number
Unmapped actual number 0.33333
Natural numberReal number
Unmapped genuine number 0.25599

Is that right? Well, let’s try and discover Cantor’s number in our list. It can not be the first number, since the first decimal location (at least) doesn’t match up. It can’t be the second number on our list, due to the fact that the second decimal place doesn’t match. It can not be the millionth number on our list, due to the fact that the millionth decimal ar doesn’t match, and also so on. No matter how we go about making the list, us can’t even capture every actual number between 0 and 1!

Cantor’s discussion works because that the actual numbers due to the fact that the decimal development of a real number doesn’t need to have a sample to it. This is in contrast to the decimal expansions of reasonable numbers, i beg your pardon must ultimately repeat themselves.

So at last we have finally discovered a larger infinity than ℵ0! probably not surprisingly, this brand-new infinity—the cardinality that the collection of actual numbers ℝ—is called ℵ1. It’s the 2nd transfinite cardinal number, and our very first example the a bigger infinity 보다 the ℵ0 infinity we know and love.


So although Buzz Lightyear’s goals of travelling ‘to infinity and beyond’ no make sense in the literal sense—infinity is no a place, nor a destination, nor a final point—you could argue the mathematics enables us to explore infinity and beyond. Different infinite sets deserve to have different cardinalities, and some are bigger than others. Beyond the infinity known as ℵ0 (the cardinality that the natural numbers) over there is ℵ1 (which is larger) … ℵ2 (which is bigger still) … and, in fact, one infinite variety of different infinities.

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It may seem esoteric, but the knowledge of infinity—and collection theory—is an essential to understanding the very foundations that mathematics. It’s expertise that was tough won over millennia, and also that knowledge offers us v tools for understanding very real complexities in the world around us.