Pi Day Post on Irrational Numbers
My youngest son is obsessed with . “How much is ,” he asked me one day recently. “I don’t know, a little more than 27,” was my unsatisfactory reply (which, it turns out, is only accurate for large values of “a little”). Google Calculator tells me that it’s (approximately) 36.4621596 but what does it really mean to raise an irrational number to an irrational power?
Despite being inspired by my elementary-school-aged son, this post is not about elementary or middle school math, in my usual style. (For elementary Pi Day ideas, check out last year’s Pi Day post.) But stick with me, if you will, and maybe you will learn something fascinating about irrational numbers.
Let’s step back a moment. What does it mean for a number to be irrational? My son knows that the decimal expansion of pi goes on forever and never repeats. An irrational number is defined to be a number that cannot be expressed as a ratio or fraction (and a rational number is a number that can be expressed as a fraction). All rational numbers have decimal expansions that are either terminating (such as 1/2) or repeating (such as 1/3). Maria over at Homeschool Math has an excellent introduction to irrational numbers and proof that is irrational — the former should be accessible to middle schoolers, the latter high schoolers and beyond. (Pi, as it turns out, is not only irrational but also transcendental, which means that it is not a solution to any polynomial with rational coefficients.)
Alexander Bogomolny at Cut the Knot offers an easy-to-understand proof that it is possible to raise an irrational number to an irrational power and obtain a rational result! Many are familiar with Euler’s famous equation or equivalently . Somehow one can raise an irrational number to an imaginary irrational power (!!!) and get an integer! But what does it mean to raise a number to an irrational power??
Most of us have an intuition about what it means to raise a whole number to a whole number power. means you multiply 3 by itself 7 times: .
What does it mean to take a number to a rational power? , but why is this the case? One intuition is to think that because it’s taken to the 1/2 power, you need 2 of them multiplied together just to get up to the original base, in this case, 3. Or if you’re comfortable with the rules of exponents, you’ll know that . So if times itself equals 3, it must be equal to Similarly
But is there any intuition that can help us undertand what it means to take a number to an irrational power? What could it mean to multiply by itself times? The best explanation I can find is over at Ask Dr. Math, where Doctor Rob explains that we find the value of a number to an irrational exponent by raising the number to successively more precise rational approximations of the irrational exponent, and then using calculus to find where that sequence converges. (Although, that does not explicitly cover the case where the base is also irrational, the same general approach applies.)
Finally, Thomas Christie offers a nice explanation of Euler’s Equation. (This link has been broken for the past few days, but I hope it will be back!) In case Christie’s page doesn’t come back, you might wish to look here or here.
I hope you had fun exploring irrational numbers and powers with me today! Now, go eat some pie.😉 Happy Pi Day!