2006-06-22 11:11

Infinite Amateurism on Maths

I stumbled upon Thomas Thurman's post on his blog (here) where he comments about a discussion on The Guardian about how .99(recurring) is or is not the same as 1.

Of course to anyone who knows what a rational number is, .99(recurring) is simply a very long way to write 1. Hell, to anyone who bothered learning his fractions, that should be obvious!

But anyway, one of the comments mentions Hilbert's Hotel, which is a pet toy of mine.

If you are uncomfortable or annoyed by the concept of infinity, you may want to avoid the rest of this post.

Hilbert's Hotel is this paradox (Thanks Wikipedia!):

A hotel with an infinite number of rooms (1, 2, 3 and so on, so it's a numerable infinity) is full. A guest arrives. Yet he still gets a room. How?

The answer is that you ask the guest in room 1 to move to room 2, from 2 to 3 and so on. Then the new guest goes to room 1, which is now free.

Because of the nature of infinity, this works, while on a finite hotel it wouldn't.

  • Unintuitive things about infinity: If you add any number to it, the result is the same infinity.

Now assume an infinite (numerable) number of guests arrives. You have to ask the guest in room 1 to go to room 2, the guest in room 2 to go to room 4, and guest n to go to n*2.

Now you have an infinite (numerable) number of free rooms: all the odd rooms.

  • Unintuitive things about infinity: If you multiply it by any number, the result is the same infinity.
  • Unintuitive things about infinity: If something is infinite, it contains a part the same size as the whole (follows from the previous two things, and is, in fact an "if and only if").

However, here's where it gets tricky. You could get a certain number of new guests and there could be no way to fit them in the rooms even if the hotel was empty.

Because there's infinity, and then there is infinity. You saw that whenever I mentioned the number of rooms I mentioned they were infinite (numerable)? That's because you can put an integer number to each, and number them all.

There are infinite sets of things that are bigger, they are literally uncountable. You can't put a number to each, even with an infinite amount of time (and yes, I know that infinite amount of time there is a big problem).

The simplest set imaginable that large is that of the real numbers. The real numbers are all the numbers you can imagine, allowing for infinite decimals, and allowing that those decimals may not ever be recurring (so you have things like 2, 1/3, and pi).

Showing there are more of those that there are integer numbers is not simple enough for this but go along with me for a while.

  • Unintuitive things about infinity: There are different sizes of infinite. Go blame Georg Cantor.

Now it gets really weird. Suppose we call the size of the infinite in Hilbert's Hotel A0 (I have no idea how to do an Aleph, sorry), and the size of the real numbers C. Cantor showed how to build, once you have an infinite set, a larger infinite set called his power set.

That means we now have a whole infinite (numerable) "sizes" of inifinite things. Those are called the transfinite numbers.

  • Unintuitive things about infinity: There are infinite different sizes of infinite. Go blame Georg Cantor some more.

Which brings a lot of questions:

  • Are there only those? Isn't there something between A0 and C which is some in-between size?
  • Is there an infinite set that's smaller than the integers?
  • Ok, infinite infinites... infinite (numerable) infinites, or infinite (something else) infinites?

Well... I have no idea. And last I checked, which was long ago, and my memory is no good, noone else knew.

This is the kind of things that will tell you whether you could be a mathematician. Do you find all this talk about transfinite numbers intriguing and mysterious, or just dull and boring and impractical?

If you find it dull and boring, it may be my writing, or you may be unsuited for maths.

If you find it intriguing and/or mysterious, it certainly is not my writing, and you would probably enjoy maths in your life. Where else are you going to run into "noone knows that" this quickly?

The problem is, of course, that most of the fun math has already been done at least a century ago, but there is always a chance of something fun and intriguing and new coming along.

The last I know of was Gödel's theorem, which is really simple enough for anyone with knowledge of arithmetic to follow, but weird enough for 99.99% of the people to go crazy about (and for those who don't really understand it to write whole books about it applying it to totally improper subjects).

But you know, noone really had thought of such thing as "larger than infinity" quite as Cantor did, and noone thought about Gödel's subject quite as he did before him.

Maybe we are missing something absolutely simple, incredibly elegant, awesomely shocking somewhere in basic maths. Not likely. But possible. Wouldn't it be fun to find it?

BTW: Gödel starved himself to death and Cantor "suffered poverty, hunger and died in a sanatorium".


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