Finite vs. Infinite
Date: 07/10/97 at 11:39:06 From: Aab Subject: Geometry If a line segment is a measurable part of a line, why is the number of points that make up a line segment infinite? Thanks for your help! Angie
Date: 07/15/97 at 13:44:04 From: Doctor Chita Subject: Re: Geometry Dear Angie, What a provocative question. I'm not sure I can give you the definitive answer, but I'll try to provide one way of thinking about this paradox. One way to begin is to quickly review some ideas from geometry. Euclid defined a "point" as "that which has no part." This means it has no measurable properties, including size. Therefore, a point takes up no space. A line, according to Euclid, is "length without breadth." Further, "The extremities of a line are points," and "A straight line is a line which lies evenly with the points on itself." This suggests that a line has the property of length and contains at least two points. Using these two notions of a point and a line, a good model is the number line, a geometric way to represent points on a line. First, to represent numbers, we have to impose a coordinate system on the line. Start by marking off a point to represent 0, the origin. Then choose a convenient scale that divides the line into equal intervals on either side of 0; the positive numbers to the right (by convention) and the negative numbers to the left. Every point on this line can be matched to a number, and every number can be matched to a point on the line. Since there is an infinite number of numbers, there must be an infinite number of points on the line. But what about a segment - a finite portion of a line (and your question)? The number line houses all real numbers - positive and negative rational numbers (integers, fractions, decimals) and irrational numbers (pi, e, the square root of 2, etc.). Consider the rational numbers for now. Let two rational numbers be the endpoints of a segment, say 0 and 5. The set of numbers between the endpoints can be matched to points that lie on the segment. For any two rational numbers, you can always find a number between them. For example, if a segment has endpoints at 0 and 5, then it has a length of 5 units. The point halfway between 0 and 5 is (0+5)/2 = 2.5. Therefore, you could graph this point on the segment. Find a point one half the distance from 0 to 2.5. That corresponds to 5/4 = 1.25 and can be matched to a point on the segment between 0 and 5. Now find the number that corresponds to the point that is 1/2 the distance between 0 and 1.25. That gives you another number and point. Repeat this process: Bisecting these segments locates points closer and closer to zero. However, they are never at 0 and, therefore, they occupy a "measureless" position on the segment. As you continue this process to infinity (that is, bisecting the set of tiny segments), you'll find you'll never run out of numbers and points. Since points don't take up any "real" space, they all fit infinitely comfortably on this finite length of the line! By the way, we haven't even inserted numbers/points that represent the irrational numbers! Between two given points, there is an infinite number of them, too. The square root of 2 is approximately 1.414213562. Therefore, it lies on the segment between 0 and 5. And so do numbers like sqrt(2)/2, square(2)/4, sqrt(2)/8, etc., where "sqrt(2)" means "find the square root of 2." It's one of those mind-boggling ideas that makes mathematics so fascinating. Don't you agree? -Doctor Chita, The Math Forum Check out our web site! http://mathforum.org/dr.math/
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