I am wrong in thinking the circumference or the diameter of a circle has to be rational?
I’m kind of dissatisfied with the answers here. As soon as you talk about actually drawing a line in the real world, the distinction between rational and irrational numbers stops making sense. In other words, the distinction between rational and irrational numbers is a concept that describes numbers to an accuracy that is impossible to achieve in real life. So you cannot draw a line with a clearly irrational length, but neither can you draw a line with a clearly rational length. You can only define theoretical mathematical constructs which can then be classified as rational or irrational, if applicable.
More mathematically phrased: in real life, your line to which you assign the length L will always have an inaccuracy of size x>0. But for any real L, the interval (L-x;L+x) contains both an infinite number of rational and an infinite number of irrational numbers. Note that this is independent of how small the value of x is. This is why I said that the accuracy, at which the concept of rational and irrational numbers make sense, is impossible to achieve in real life.
So I think your confusion stems from mixing the lengths we assign to objects in the real world with the lengths we can accurately compute for mathematical objects that we have created in our minds using axioms and definitions.
…when the mathematician and the philosopher argue, but the engineer just smiles: you are both wrong :-)
There is no circle where the diameter and the circumference are both whole numbers.
No, they don’t have to be rational. It’s counter-intuitive but you can accurately draw a line with an irrational length, even though you can’t ever finish writing that length down.
The simplest example is a right-angled triangle with two side equal to 1. The hypotenuse is of length root 2, also an irrational number but you can still draw it.
Thanks for the answer. I’m confident you’re correct but I’m having a hard time wrapping my head around drawing a line with an irrational length. If we did draw a right angled triangle with two sides equal to 1cm and we measured the hypotenuse physically with a ruler, how would we measure a never ending number? How would we able to keep measuring as the numbers after the decimal point keep going forever but the physical line itself is finite?
how would we measure a never ending number?
You’re talking about maths, maths is theoretical. Measuring is physics.
In the real world you eventually would have to measure the atoms of the ink on your paper, and it would get really complicated. Basically … you can’t exactly meassure how long it is because physics gets in the way (There is an entire BBC documentary called “How Long is a Piece of String” it’s quite interesting).
Is that basically the coastline paradox?
It’s not that it can be measured forever, it’s just that it refuses to match up with any line on the ruler.
For a line of length pi: it’s somewhere between 3 or 4, so you get a ruler and figure out it’s 3.1ish, so you get a better ruler and you get 3.14ish. get the best ruler in existence and you get 3.14159265…ish
…and when you go deep enough you suddenly lose the line in a jumble of vibrating particles or even wose quantum foam, realising the length of the line no longer makes sense as a concept and that there are limits to precision measurements in the physical world.
Irrational numbers can be rounded to whatever degree of accuracy you demand (or your measuring instrument allows). They’re not infinite, it just requires an infinite number of decimal places to write down the exact number. They’re known to be within two definite values, one rounded down and one rounded up at however many decimal places you calculate.
