Reply to Robin, Diminishing Circle Argument

Jesse L. Yoder

Subject: Reply to Robin, Diminishing Circle Argument

Author: Jesse Yoder <jesse@flowresearch.com >

Organization: Epigone

Date: 30 Oct 1997 10:56:23 -0500

Robin - Thanks very much for your response to the note I posted on calculus and geometry.
You

wrote:

"I'm intrigued, but how does the diameter equal circumference divided
by 2? It seems you have eliminated Pi from the equation. Perhaps this is why Pi figures
in so importantly when computing curved areas within the Cartesian coordinate system.

Good luck with your work. Send me more info any time."

Diminishing Circle
Argument

I concede that the idea that diameter of a circle = the circumference divided by
2 is

counterintuitive. This comes about from what I call the "diminishing circle
argument." This argument goes as follows:

Take any circle E and draw two smaller circles inside the larger circle. The two
smaller circles should be equal in area, with one edge intersecting the outside edge
of the circle E (the point where the diameter intersects the circumference) and the
other edge passing through the radius

of E (this is easier to draw than to describe). This gives you two smaller circles,
A and B, each with a diameter that is half the diameter of the original circle E.
This is because the area of E is (pi x r squared), and the area of circle B is (pi
x (r/2) squared), (where r refers to the radius of A).

This equals pi x r squared/4, or 1/4 the area of the original circle E. Since A and
B are equal in size, they have the same area, which is equal to 1/4 the area of E.
Hence the areas of A and B together equal 1/2 the area of E, the large circle. This
means that the remaining areas, C and D (which are also equal, but not circles) are
together each equal in area to the areas of the smaller circles A and B.

The above shows that circle E is divided into four equal areas: the areas of A, B,
C, and D. The next step shows that the circumference is B = the Circumference of
E/2. The circumference of E = 2 x pi x r. Since the radius of B is 1/2 the radius
of E, the radius of B is 2 x pix r/2 (where r refers to the radius of E). But 2 x
pi x r/2 = pi x r, which is 1/2 the circumference of E. So the circumference

of B (one of the smaller circles) is half the circumference of E. This means that
half way around the circumference of E, which is C/2, equals the entire circumference
of circle B. This also means that halfway around the circumference of B plus halfway
around the circumference of A equals halfway around the circumference of E (since
A and B are equal, their circumferences are equal).

So going from one end of the diameter of E to the other via the circumference of
E is the same distance as going from one end of the diameter of E to the other via
the two circumferences of the smaller circles A and B.

This last result is the main result I need to get the argument that the diameter
of a circle equals the circumference divided by 2. Now we have circle E divided into
four equal areas, the areas of A, B, C, and D. Now imagine further dividing circles
A and B each into four smaller areas in the

same way, by drawing two smaller but equal circles within A and B, each dividing
the areas of A and B into four smaller areas. Again, going halfway around A is the
same distance as traveling along circumferences of the two smaller circles within
A. Continue this process indefinitely. At the end point, we have the diameter consisting
of the smallest allowable points packed up against one another. But the distance
around all these points still equals the distance around the

circumference of E, so the diameter of circle E will equal half the circumference.
The same argument can be repeated for any circle.

At this last stage of the argument, I used to say "Now imagine this process
completed to infinity. At infinity, the circles will be infinitely small, thereby
equaling the points on a line, and the distance along the diameter will equal half
the circumference." Sandy Norman of the University of Texas at San Antonio has
convinced me of the folly of, on the one hand, rejecting the notion of infinity and
then, on the other hand, relying on it when it is convenient to do so. But I don't
believe that I need to rely on the notion of infinity to complete the diminishing
circle argument.

In this geometry I am proposing, the points on a line are densely packed, each with
a specifiable area. In other words, they are not "infinitely small" (whatever
that is supposed to mean). How small the points will vary with what is being measured-but
once the units are specified, they can only be made smaller by re-specifying the
unit of measurement. (This is the only way to avoid

Zen's paradox.) This means that in this geometry, there is a finite number of points
on the number line, not an infinite number, and that I reject the idea that between
any two points, there always lies another.

Of course, I realize that the diminishing circle argument is counterintuitive, since
it certainly seems shorter to walk along the diameter of a circle than to go along
its circumference. The difference is in how you analyze motion along a diameter.
If you analyze it as moving along the circumferences of a very large number of very
small circles (or points), then the distance is in fact the same as the distance
along the circumference of the circle whose diameter you are traversing. On the other
hand, if you analyze the motion along the diameter as motion along the path of a
moving point, giving you motion along a "straight line," then the distance
is less than

traveling along the circumference of the circle whose diameter you are traversing.
But since I reject straight lines, I can't accept this "straight line"
analysis of motion along a diameter.

I hope that this answers your question (if you've gotten this far). Since you asked
about my background, and what level I'm working at, I might as well answer that too.
I have a PhD in philosophy (from U. of Mass. Amherst). While my main area of concentration
is in philosophy of mind, I have also done some philosophy of mathematics. My main
area of interest in this subject is to try to understand if the assumptions that
underlie mathematics are valid, and to try to

suggest alternatives where I feel they are not. My view of philosophy is that it
mainly consists in evaluating the assumptions the other fields of study take for
granted. In the case of mathematics, I have always been bothered by the notion of
infinity, and I object to the idea of analyzing what is continuous as being infinitely
many discrete things. So I am trying to come up with a different way of analyzing
continuous phenomena (such as the number "line") without relying on the
notion of infinity.

I am working as a market analyst for a market research company (Flow Research), writing about industrial applications for flowmeters and pressure transmitters.
I am trying to figure out whether my geometry can be used to improve or change the
ways in which flow rate is calculated, since the fundamental flow equation is Volumetric
Flow = Area x Velocity, and pipe areas typically are around, thereby invoking pi.

Thanks again for your question, Robin, and I welcome any comments you may have on
my response.

Jesse

To view the original post, go to:

http://forum.swarthmore.edu/epigone/geometry-research/froxpalzhand/ac7ma91i33e4@forum.swarthmore.edu

Jesse L. Yoder

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