(Intro music)
Hi, I am Agustin Rayo.
I'm an associate professor[br]of philosophy at MIT,
and today I want to[br]talk about how there are
some infinities that[br]are bigger than others.
So, a first exercise to see why[br]some infinities are bigger
than others is to think[br]about Hilbert's Hotel.
So Hilbert's Hotel is like an[br]ordinary hotel, except that
instead of having finitely many rooms
like most hotels, it has[br]infinitely many rooms.
So we can draw this.
We're just having a very long rectangle
with lots of rooms, and we're going to
number each room with a natural number.
So, the first natural number is[br]zero, after that comes one
and two and three and four[br]and five and so forth.
And I'm going to assume that this[br]hotel is completely full.
So in other words, every[br]room has a person inside it.
So we're going to draw a[br]person inside each room.
And I'm going to assume that these
various persons are numbered, too.
So maybe each of them is holding
a piece of paper with a number.
The person in room zero has a[br]piece of paper with zero on it,
the person in room one[br]has a piece of paper
with the number one in it, and so forth.
So now ask yourself this question:
what would happen if an extra person
were to come to the hotel?
Could they be accommodated?
Now, in a finite hotel, the answer is "No,"
because all the rooms[br]are already occupied,
and we can assume that[br]these are prickly guests
and don't want to share rooms.
But in an infinite hotel, it can be done.
So think of it this way.
We can just ask each person[br]to move one room to the right.
Mr. Zero, who's now in room[br]zero, moves to room one,
and Miss One, who's now in[br]room one, moves to room two,
and two moves to three,[br]and three moves to four,
and four moves to five, and so forth.
So then the result is that[br]all of our original guests
are in rooms, but our[br]first room is vacant,
because now there's nobody in room zero,
so we can welcome our additional guest.
One thing that's amazing[br]and worth emphasizing
is that the hotel was[br]full at the beginning,
and we could still fit in more people.
That's amazing, but true.
Part of the reason that's[br]possible is that there are two
things that come together[br]in the finite case,
but need to be kept apart[br]in the infinite case.
So in the finite case, if you have a set
and you add some extra things to the set,
the set you get is bigger[br]than the original one,
but in the infinite case,[br]that's not necessarily true,
because have your[br]original set just consist
of the original hotel guests,[br]and then add one more.
The new set isn't bigger, in[br]the sense that you can still
put each element of the new set
into a different room in your hotel.
So if you measure size in[br]terms of how big a hotel
you would need to accommodate[br]the members of the set,
then we need to conclude[br]that the set of the original
guests, and the set of the original guests
plus one, are of the same size.
Crazy! It's this amazing thing.
And if you think of it, you[br]can see that we could have
accommodated any finite[br]number of new guests, too,
because suppose we get seven[br]hundred new guests.
Then we can just ask each[br]of our original guests
to move seven hundred[br]rooms to the right.
But wait! What if we got[br]infinitely many new guests?
OK, so suppose that we have[br]infinitely many new guests,
and they too are numbered[br]with the natural numbers,
so the first one is holding[br]zero, and the second one
is holding a one, and the next one
is holding the two, and so forth.
How could we accommodate them?
Well, we can do it like this.
We just ask each of our[br]original guests to move
to the result of multiplying[br]their current room by two.
So the guy in zero goes to two times
zero equals zero, so he stays.
The person in one moves to one times two,
that's two, so she moves one to the right.
The person in two moves to[br]two times two, that's four.
She moves two to the right, and so forth.
So the result of this is[br]that all the old guests
are occupying even-numbered rooms,
and all of the odd-numbered[br]rooms are free,
so we can ask each of the new guests
to go to an odd-numbered room.
Which one?
They can just look at their[br]number, multiply it by two,
and add one, and they can go to that room.
So here's the lesson of this.
The lesson of this is that[br]if we have an infinite hotel,
we can accommodate one copy[br]of the natural numbers,
but we can also accommodate two copies
of the natural numbers, and in fact,
we can accommodate as many copies
of the natural numbers as[br]there are natural numbers.
But before tackling that question,
it's useful to consider a different one,
and this is the first of[br]two wonderful theorems
that were proved by Georg[br]Cantor in the nineteenth century.
The first theorem is that you[br]can put the rational numbers
in one-one correspondence[br]with the natural numbers.
Now, what is a rational number?
A rational number is a number[br]of the form "a/b,"
where "a" and "b" are[br]both natural numbers,
and we assume that "b" is not zero.
So what Cantor did is[br]show that we can pair up
the natural numbers with the rational[br]numbers with no remainder.
Every natural number gets[br]a unique rational number,
and every rational number gets a unique
natural number and nobody's left out.
So here's how to do it.
We simply draw a matrix.
Each column is going to[br]correspond to a numerator,
and each row is going to[br]correspond to a denominator.
So this is how you fill out the matrix.
And so forth.
So you can see that every[br]rational number is on this matrix,
because remember, you[br]get a rational number
by taking a natural number and dividing it
by a natural number different from zero.
Suppose your rational number is[br]seventeen divided by ninety-four.
In order to find the cell that[br]corresponds to that number,
just go to the column that's[br]labeled "seventeen,"
and to the row that's labeled[br]"ninety-four,"
and that's where your[br]number is going to be.
So every rational number is on the matrix.
So what Cantor discovered[br]is that you can assign
a natural number to each[br]cell, and here's how to do it.
Just diagonalize. So[br]have this one be zero,
and this one, one. And[br]two, and three, and four,
and five, and six, and seven,[br]and eight, and so forth.
Using this system, every rational number
got assigned a natural number.
So now we can come back to our hotel.
So remember, our question[br]was "How could you fit
infinitely many infinities[br]into your hotel?"
Well, now we have a way of doing it.
So, think of each column on our matrix
as corresponding to one[br]infinity of new guests.
In order to decide which[br]room to send a person to,
all you need to do is use Cantor's trick.
So, assign a natural number to each cell
in the matrix, and ask the person to go
to the room corresponding to that number.
So there, we fit infinitely new
infinities of guests into our hotel.
Amazing!
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