What is Aleph Zero?
Aleph Zero is the first transfinite number.
If I can list the items in a collection, then I can count them, by assigning the numbers 1, 2, 3 and so on. In this case I say that the collection is countable. Of course I do not need to actually put the items in a list. If I were counting the lakes in the Lake District, I could not move the lakes, but I could write a list of map co-ordinates for an arbitrary point inside each lake. I could then count the items in my list.
How many counting numbers
I can put the counting numbers themselves in a list, and assign numbers to them. 1 is labelled 1, 2 is labelled 2 and so on. So the collection of counting numbers is countable. But the list never comes to an end. Why not? Suppose I got to the end of the list at number 57 say. Then you could find another number, 58, which is not in my list. So my original list was not complete. Even if I told you a really enormous number, such as a googol, you could always find a new number not in my list, by simply adding one to my number. So there is no counting number that represents the total number of all counting numbers. We give this total a special name, aleph-zero.
So if a collection is countable, there are two cases. Either it is finite, and I can find a counting number that represents the number of items in it, or it is infinite, and contains aleph-zero items.
For an explanation of a googol see Rudy Ruckers interesting book 'Mind Tools'
The number of even numbers is aleph-zero
I can list the even numbers, 2, 4, 6, and I can assign a counting number to each. 2 is labelled 1, 4 is labelled 2, and so on. For any even number I can immediately find its label by dividing it by 2. But there are infinitely many even numbers. If I told you any even number, you could always find another one by adding 2. So the total number of even numbers is aleph-zero.
The number of positive rational numbers is aleph-zero
A rational number is one that can be written a/b where a and b are whole numbers, and b is not equal to zero.
Now I can put the numbers in this table into a list by starting at the top left corner and then following the red line up and down the diagonals, assigning counting numbers as I go. Given any positive rational number I can follow the red line until I get to it.
But why do I go up and down the diagonals, instead of just going straight along the first row, and then along the second row, and so on? Because these are infinitely many items in the first row, so I would never get to the second row.
So I have put all the positive rational numbers in a list and assigned counting numbers them. As there are infinitely many positive rational numbers, there must be aleph-zero of them.
So it is tempting to think that aleph-zero is just another name for infinity BUT
There are more than aleph-zero real numbers between 0 and 1
The rational numbers can all be written as decimals. So for example 1/4 is written as 0.25. This has only two figures after the decimal point. But some numbers need infinitely many figures after the decimal point. For example 1/3 is 0.33333333333333333…. with 3s going on for ever.
But there are other numbers that are not rational, like pi, or the square root of 3, which can also be written as infinite decimals.
Suppose all the numbers between 0 and 1 which can be written as infinite decimals are countable. Then I can put them in a list.
Now you can construct a new number as follows. Look at the first number in my list, and look at its first decimal place. If this is zero write down 1, otherwise write down 0, for the first decimal place of your new number.
Now look at the second decimal place of my second number, and if it is zero write 1, otherwise write zero, for the second decimal place of your new number
Continue in this way for each number in my list. So your number is different from my first number in the first decimal place, different from my second number in the second decimal place, different from my nth number in the nth decimal place. So your new number is different from all the numbers in my list. It is an infinite decimal that is not in my list. So my list is not complete. There is no way that I can make a complete list of all infinite decimal numbers. So the collection of infinite decimals is not countable. The number of items in the collection is not aleph-zero.
We give the new name aleph-one, to the number of real numbers. Aleph-zero and aleph-one are called transfinite numbers. It is beyond the scope of this web page to demonstrate that aleph-zero is the smallest transfinite number and that aleph-one is the second.