- How it Feels to study Pure
Mathematics
To see a world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour.
- William Blake
I have a master in pure mathematics (it also includes a substantial amount of applied mathematics and physics). People often wonder what is it that we do? And they visualize
the most complicated mathematics they know, and they try to take it
to the next level mentally. But it is very hard to try to understand
something you know nothing of. People who only have a minimum of
mathematical knowledge guess "Oh, so you multiply really large
numbers", those who had math in high school/college guess "Oh,
so you differentiate and integrate difficult functions?", those
who have a university education where they needed advanced
mathematics guess "Oh, so you solve hard differential equations on
difficult grids/spaces?". Today I will endeavor to give a
picture of how I feel when doing pure math.
For inspiration, I recommend the
following videos. After we have seen them, we will try to do some of
this mathematics. Note that we mention Cantor several times.
(Part 2 if you're interested)
Pure mathematics is that of unlimited
abstraction and precision. We define everything as precisely as
humanly possible, and we try to make everything into abstract
concepts. Understanding something in pure mathematics is often like
walking a tightrope over an abyss, no room for small deviations or
imperfect intuition.
A lot of the problems I think about when working, you would need 4 years of university education just to be able to ask. But now I will use
some examples that you need almost no mathematics education to
understand.
First we will look at sizes of infinity
(this is very close to the paradoxes talked about in the videos
above). To talk about size, we have to define it, and size of what?
Definition
A set contains elements, and an element is contained in a set.
Example
{1,2,3,5} is a set containing the elements 1, 2, 3, and 5.
Example 2 {a,
b, car, boat, 99} is a set containing the elements a, b, car, boat
and 99.
Definition 3
The size of a set is the number of elements it contains
Example 4 The
size of {1,2,3,5} is 4
Example 5
The size of {a, b, car, boat, 99} is 5
Using these definitions, we could have
defined a set to be infinite size if it has no finite size. But to be
able to speak of different infinity sizes, we have to use a more fine
tuned definition.
Definition 6
Two sets are equal if there is a bijection between them
Definition 7 A
bijection is a function which maps all the elements of a set to the
elements of another set, and where no two elements are mapped to the
same.
Now
we can state the first question
Question 1
Are there as many even (positive) numbers as there are (positive)
numbers in total?
Answer 1 (with proof)
Yes. Let E={2, 4, 6, 8, 10, ...} be the set of even numbers, and let
N={1, 2, 3, 4, 5, ...} be the set of all numbers. Then we can
construct a function, f, from E to N by f(x)=x/2. This function is a
bijection. Hence the size of E is the same as the size of N.
How
can this be? Clearly the set N contains the set E, and then some, how
can they be equal? Well we just proved that they were. The only thing
we can conclude form the fact that N contains E is that the size of E
is smaller or EQUAL to the size of N.
What we just did is known as
Hilbert'sparadox of the Grand Hotel, with Infinitely many new guests.
You should look at the link. This is NOT
a paradox, this is a well established mathematical fact. What seems
to be a paradox is only because our intuition is not used to the
concept infinite.
Calculations with infinite
If
you use a modern computer program, it often has inf (infinite) as a
kind of number. It will normally give the following computations:
inf +
1000 = inf
inf –
1700 = inf
inf +
inf = inf
inf*1000
= inf
inf*inf+inf^inf
= inf
1/inf
= 0
1/0 =
inf
inf-inf
= NAN
inf/inf
= NAN
(-1)^inf = NAN
Here
NAN means Not A Number. That is because you are not allowed to do
these operations, the result is undefinable (if you chose a definition
you would end up with a contradiction). The problem with inf-inf is
that we don't know which inf is "largest". In some
applications you may get the answer 0, in others 31, in yet others
you may get inf.
Countable and Uncountable infinity
But
what about the set R of real numbers (all numbers, including pi and
the square root of 2, but not including imaginary numbers), is this
also the same size as Z and N and Q? No. This proof is quite deep,
and took me several days to understand (several years ago). If you
want a challenge, see wikipedia's page on
Cantors diagonal argument.
The Length of the Rationals (the set
of fractions Q)
There is another
very much used notion of size. This is what we use for integration,
and to avoid confusing it with the size of sets from before, we call
this new thing for length.
Definition
The length of an interval on the real line, is the right endpoint
minus the left endpoint.
Example
We write [-3,7] for all the numbers between -3 and 7 including -3 and
7. The length of this interval is 7-(-3) = 10.
We
can generalize this concept of length to other sets than intervals,
for example to the union of intervals. Not surprisingly we get:
Theorem
If one set is contained in an interval, the length of the set is
smaller or equal to the length of the interval.
Supertheorem:
The size of Q is countably infinite, but the length of Q is 0 (on
the real line).
Proof: We
have already seen that the size of Q is countably infinite. What
about its length? Well, write Q as a sequence Q={q1, q2, q3, q4, ...}
where all qi are fractions. Let K>0 be any arbitrary number (for
example 0.000000001). Then the first fraction, q1, is contained in an
interval of length K, namely [q1-K/2, q1+K/2]. The second fraction is
contained in an interval of half that length (namely [q2-K/4,
q2+K/4]). The third fraction q3 is contained in an interval with half
that length again. Let us sum up this:
q1
contained in an interval of length K
q2
contained in an interval of length K/2
q3
contained in an interval of length K/4
q4
contained in an interval of length K/8
q5
contained in an interval of length K/16
...
So Q
must be contained in the union, which will be an set with size
smaller than (smaller because some of the intervals may overlap):
K +
K/2 + K/4 + K/8 + K/16 + ... = 2K
What
do we now know? Q is contained in something of length 2K (you can
choose any K>0). Hence the length of Q is smaller or equal to 2K
(you can choose any K>0). The only possibility is that the length
of Q is 0.
QED.
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Comment by Nok er Nok:
I'll keep beating the same horse as
always, I guess. We have spent some time discussing these questions
and generally agree, but I can't help but object to the following
paragraph. I have mostly the same knee-jerk reaction to it as the
Newcomb's paradox, Hangman's paradox, etcetera. On the whole, I'll
even illustrate my point with an XKCD strip
(gasp!),
http://xkcd.com/169/ , though I'm not sure if the
miscommunication is intentional or not.
"What we just did
is known as Hilbert'sparadox of the Grand Hotel, with Infinitely many
new guests. You should look at the link. This is NOT a paradox, this
is a well established mathematical fact. What seems to be a paradox
is only because our intuition is not used to the concept
infinite."
This is most certainly a paradox. The
mathematical formalism you describe and which mathematicians use is
consistent and useful, yes, that's not what I'm trying to deny. Using
that formalism to claim there are -as many- even numbers as natural
numbers, however, is dishonest; using layman's terms in that manner
-creates- the paradox. Nobody protests against the bijection-wizardry
which is firmly belonging to alternate math-dimension. They do,
however, have issues with mathematicians redifining common English
words to mean something entirely different, and then marveling at how
counterintuitive it is - particularly when they a few moments later
set the trap by mixing real world example, impossible mathematical
constructs, normal English and indistinguishable mathematical
definitions.
Let's deconstruct Hilbert's Hotel to start
with, and let's pretend we are not familiar with the mathematical
formalism for dealing with infinities. Hilbert then claims the
following:
- hotel with infinite number of rooms
-
infinite number of guests at the hotel
- such that all rooms
are occupied, each by a single guest
which is all fine and dandy,
but sets up for the counterintuitive conclusion
- the hotel
can still house another group of guests, exactly as large as the
number which currently occupies every single room
Now, if you
look at that without consulting your English<->Mathematics
dictionary, you will surely conclude that this is perfect nonsense.
Somebody is pulling a cheap parlour trick on you, one of these words
have to mean something else that is appears. You can easily construct
equally (more?) valid arguments than Hilbert presents, to show that
the conclusion is impossible. For instance, it is easy to visualise
that no room will be left unoccupied after you swap any two guests
between their respective rooms, any number of times, and even though
Hilbert does this an infinite number of times, this shouldn't change
anything.
The parlour trick here is, of course, that
occupied does not mean occupied at all, it has to do with bijections,
and infinity does not mean a number you can increment arbitrarily
many times, it is instead some mathematical construct with such
properties that it cannot possibly have anything to do with any
actual hotel. You might say the point of Hilbert's Hotel is that
infinity cannot be treated as just any large number, you claim that
our intuitions are not prepared to deal with infinity, but I strongly
disagree. Hilbert's Hotel only shows that the mathematical lingo he
ends up translating to 'occupied' and 'infinity' has nothing to do
with a normal understanding of these words.
Taken as a story
to accompany the mathematical formalism, to illustrate how it handles
infinities, cardinality and size as something to do with bijections,
it does an okay job. Without that context, it is not the slightest
bit clever or enlightening, but just a load of gibberish. Hilbert's
Hotel says -nothing- -whatsoever- about how hotels of arbitrary size
work; it intentionally mixes mathematical formalism with a real world
example which it then -fails to describe-!
Of course, all
of this comes from the same sort of people who with a straight face
will call f(x) = constant an increasing function - and a
decreasing one, at the same time. Nevermind that increasing is a code
word for non-decreasing, which you cannot know without consulting
your Google-translate English<->Mathspeak, or being familiar
with the tradition of inclusive definitions in mathematics.
I
think inclusive definitions are useful, and I'm not quite decided on
whether using somewhat familiar but inaccurate and misleading terms
is better than inventing new, arbitrary ones. However, I'm certainly
not going to give mathematicians any credit for clever paradoxes
which does nothing but illustrate that the mathematicians themselevs
do not understand that their redefined words cannot be inserted into
common English prose without appropriate and careful
translation.
As an endnote, I feel fairly certain that it
would be very possible to develop a formalism in which the natural
numbers, the even numbers, the prime numbers and so on and so forth
were -not- the same size. Of course, these alternate defintions would
not develop fruitfully into integration and cardinality, like the
current one does, but this alternate mathematics would be able to
present the exact same Hilbert Hotel and the exact opposite
conclusion; the hotel -cannot- accomodate even one more guest, much
less another infinity of them. Or perhaps they would balk the moment
you suggested that every one of the infinite number of rooms is
currently occupied. And if this is true, it should be all the more
obvious why Hilbert's Hotel is, indeed, a paradox of sorts.
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