Wait a second... I didn't realize Cantor's argument was that there are SOME infinite sets that cannot be put on a one to one correspondence with natural numbers. I thought it was that there were NO infinite sets (other than algebraics) that can be put on a one to one correspondence with natural numbers.
Infinite sets of numbers come in nested categories, and people have come up with lots of categories that can be put on a one to one correspondence with natural numbers, and lots of other categories that can't be. Here's a nesting of categories, where each infinite set is a subset of all the later infinite sets:
...
Integral eighth powers
Integral fourth powers
Integral squares
Non-prime numbers
Natural numbers
Integers
Rational numbers
Algebraic numbers
Computable numbers
Describable numbers
Real numbers
Complex numbers
Quaternions
...
All of those sets up through the describable numbers can be put on a one to one correspondence with natural numbers; starting with the real numbers, they cannot be. In addition, you can get an infinite set by deleting one of these sets from one of its supersets. The prime numbers are the natural numbers with the non-primes deleted; the negative numbers are the integers with the natural numbers deleted; the irrational numbers are the real numbers with the rational numbers deleted; and so forth. In particular for our topic, the transcendental numbers are the real numbers with the algebraic numbers deleted. And in general, any time you have a category like that, defined by taking one of the basic categories and deleting one of its smaller subsets, what you have left will have the same number of elements as the larger category you deleted a subset from. So there are exactly as many prime numbers as natural numbers; there are exactly as many negative numbers as integers; there are exactly as many irrational numbers as real numbers; and there are exactly as many transcendental numbers as real numbers.
"Exactly as many" has a defined meaning when we're talking about infinite sets. When set Q has exactly as many elements as set S, it means there exist subsets P and R, where P is a subset of Q and R is a subset of S, such that P can be put on a one to one correspondence with S and Q can be put on a one to one correspondence with R. You don't have to actually put Q and S on a one to one correspondence with each other. That can be technically very difficult for a variety of boring reasons, so we use the subset method. The idea is that when P can be put on a one to one correspondence with S, it means the number of elements in P and S are equal, so since P is a subset of Q, the number of elements in Q must be
greater or equal to the number in S. So if we can do that subset matching in both directions, we get |Q| >= |S| and |S| >= |Q|, and from the two inequalities we deduce |Q| = |S|.
Here's an example. Are there the same number of real numbers R with 0 <= R < 1 as there are infinite strings of decimal digits? On first glance, obviously yes -- they're the same set, right? .358 = .358; .4747... = .4747...; sqrt(1/2) = .70710678118...; and so forth. The problem comes in when we remember that .73999... and .74000... are the same real number, but they're different infinite strings of decimal digits. So if you match them up that way, with .4747... mapped to .4747... and so forth, then you don't have a proper one to one correspondence between the sets. Finding a way to match them up so there are no repetitions is going to be an incredible pain in the ass if it can be done at all. So how can we show the two sets have the same number of elements? The simplest solution is the subset method. For one direction it's trivial: we simply delete the set of strings of decimal digits that end in an infinite string of 9's. So we have a one to one correspondence between the reals from 0 to 1 and a subset of the infinite strings of decimal digits. For the other direction we need a one to one correspondence between infinite strings of decimal digits and a subset of the reals from 0 to 1. There are a lot of ways to do that; here's an easy one: We map each infinite string of decimal digits to the real number with the same expansion
in base 11. So now "999..." will map to .999... in base 11 instead of in base 10, i.e., it maps to 9/11 + 9/121 + 9/1331 + ... This makes the ".73999... = .74000..." problem go away, since they aren't equal in base 11. Of course, base 11 has exactly the same problem with its "a" digit ("a" means ten over some power of eleven), .73aaa... = .74000...; but that won't hurt us since our set of infinite strings of decimal digits doesn't have any "a" digits in it. So we've successfully created a one to one correspondence between infinite strings of decimal digits and a subset of the reals from 0 to 1. Note that this isn't one-to-one between the two sets we care about -- we're missing a lot of reals in this direction. There's no string of decimal digits that maps to 0.0a = 10/121, for instance. But that's okay. There are as many strings of decimal digits as a subset of the reals, and there are also as many reals as a subset of the strings of decimal digits, so Bob's your uncle.
After all, the natural numbers include every possible permutation of digits.
They include every
finite permutation of digits. So all the numbers that can be specified with a finite amount of information can be put in a 1-to-1 correspondence with the natural numbers. That's not just the algebraics. The algebraic numbers are the numbers that can be specified with a finite amount of information
in the language of polynomials. But there are other languages that are more expressive than polynomials. All the familiar transcendental numbers like pi and e can also be specified with a finite amount of information, using notations with summation symbols or integrals or trigonometric functions or whatever. So to put them in a 1-to-1 correspondence with the natural numbers you merely have to encode those operators as digits, the same way we encode polynomial coefficients as digits when we map algebraics to natural numbers. The transcendental numbers you can't put in a 1-to-1 correspondence with the natural numbers are precisely the ones that can't be specified with a finite amount of information
in any language whatsoever -- it's the ones whose digit sequences are
random.
I don't see the pertinence of the diagonal argument to whether or not transcendent numbers outnumber rationals?
If there were the same number of rationals as transcendentals, or if there were more rationals, then you could put all the transcendentals into a 1-to-1 correspondence with the rationals or with a subset of the rationals. (That's how it works with finite sets; that's how it works with ordinary infinite sets like the natural numbers. If |A| >= |B| then there exist a C and an M such that C is a subset of A and M is a 1-to-1 mapping and M(B) = C. For example, if the number of {lion, tiger, bear}=3 is greater or equal to the number of {goat, sheep}=2 then there exist C = {lion, bear} and M = {goat:bear, sheep:lion} such that {lion, bear} is a subset of {lion, tiger, bear} and {goat:bear, sheep:lion} is a 1-to-1 mapping and M({goat, sheep}) = {lion, bear}.)
So the relevance is that since the diagonal argument proves that you can't put all the transcendentals into a 1-to-1 correspondence with the rationals or with a subset of the rationals, it shows that the hypothesis that there are the same number of rationals as transcendentals, or that there are more rationals, implies a conclusion that we know isn't true. True premises don't imply false conclusions. Therefore the hypothesis that there are the same number of rationals as transcendentals, or that there are more rationals, must be false. Therefore transcendental numbers outnumber rationals.
You may be feeling this is wrong because the diagonal argument doesn't say anything about rationals -- it's all about setting up a 1-to-1 correspondence with the natural numbers. But that doesn't make any difference because we already know there are the same number of rationals as natural numbers. We know because we can make a 1-to-1 correspondence between them, as follows:
0 : 0
1 : 1/1
2 : -1/1
3 : 2/1
4 : -2/1
5 : 1/2
6 : -1/2
7 : 3/1
8 : -3/1
9 : 1/3
10: -1/3
11: 3/2
12: -3/2
13: 2/3
14: -2/3
15: 4/1
...