In his wonderful new book “Proof and the Art of Mathematics”, Joel David Hamkins asks a question about irrational numbers that stopped me in my tracks:
Prove that the irrational real numbers are exactly those real numbers that are a different distance from every rational number.
I reacted to this problem in three stages:
- OK, sure.
- Wait what?
- Oh cool! Why?
It took me a moment to understand what he was asking, then a moment longer to think about how to approach a solution. In the end, what helped me make sense of it was a problem that I’ve used to teach fractions to my 3rd and 4th Graders. These sort of connections between the math of young students and of sophisticated adults is mathematically exciting. It gives me the same sort of “oh man that’s beautiful” buzz that mathematicians sometimes use to describe their realizations.
The question comes from Marilyn Burns’ fraction books: Put two fractions on the number line, maybe 1/4 and 1/2. What number is exactly halfway between them?
Lots of students would say “1/3.” This isn’t how fractions work, though. (The jumps from 1/4 to 1/3 to 1/2 aren’t constant, an early example of non-linear growth.) This problem often catches my students in stages just as the irrational number question caught me: Oh, it’s obvious! Wait what now?
The question mark is half of the distance from 1/4 to 1/2. That makes it 1/8 away from 1/4, and 1/8 + 1/4 = 3/8.
It’s possible to generalize this result. I have good memories of pre-teen students at math camp, huddled around a chalkboard and trying to express this result algebraically.
The actual solution to this isn’t particularly important at the moment. The point is that you can always find the distance between two fractions, and that distance can always be expressed as a fraction. And half of that distance? Again, a fraction. Pick the lower place on the number line, jump ahead by that half-distance and what do you get? Fraction, fraction, fraction. It’s fractions all down the line. As long as you start with two fractions, you get one in the middle.
Fractions are also known as “rational numbers.” So let’s take another step closer to the original problem, which was about the distance between an irrational number and any rational numbers.
Start with a number on the number line. If it’s rational, then you can write it as a fraction. Then reach out in one direction to another fractional/rational number. What’s waiting for you in the other direction?
Again, you can compute the answer using the distance between 2/5 and 1/2, What’s significant is that this other number will also be … wait for it … a fraction. No way out, they’re everywhere. You’ll be able to express the distance between 2/5 and 1/2 as a fraction, you subtract that distance from 2/5 and … you get the picture. Fractions, across the sky. But what does it mean?
This time, start with an irrational number – something like the square root of 2 – and stick it in the middle slot. Then, stretch out in one direction to reach a rational number. What sort of number is waiting for us an equal distance in the opposite direction?
Suppose for a moment that, as in the 4th Grade question, the other number turns out to be a rational number. Then at the left and the right are rationals. Remember the pre-teen version of the question: if the left and right numbers are expressable as fractions, there is a formula for finding a rational number exactly halfway between them. And that would mean the square root of 2 is rational. Which, no. So! That other number is irrational.
Another, more precise way of saying the above: Let m be an irrational number and suppose it is the same distance to two rational numbers. Then m is exactly halfway between two rational numbers, and based on Marilyn Burns that too is rational…hey no way, man, it’s irrational! So irrational numbers are a different distance away from every rational number, which means if a rational number is on an irrational’s right side, there isn’t a rational number an equal distance to the left. And vice versa.
(Strictly speaking this is only half of what Hamkins asked us to prove. The other half is showing that every number that is a different distance to every rational number is irrational.)
A lot of mathematics seems obvious in retrospect, and this was one of those times for me. That’s also the case for the connections to what I teach my elementary and middle school students. Duh, fractions are fractions and this was a problem about fractions. (Fractions, EVERYWHERE. Let that haunt you.)
What I find wonderful is that when you’re teaching you never know what seeds you’re planting. That’s supposed to be a truism about teaching kids, but it also seems true when applied to yourself. Every new idea you share with someone is an idea that might be useful to you, the teacher, in some new and unexpected context. In a very real sense, teaching is also sometimes teaching yourself.
 There is probably a nit to pick about rational numbers being more precise a term than fractions in this context, but I’m going to juuuuuust slip right away from that conversation if you don’t mind.