I don't get why this troll physics is wrong.

LogicX

http://i254.photobucket.com/albums/h...0457745312.jpg

Can someone explain the flaw in this logic?

Integral

I see no proof that the construction will ever exactly equal the circle.

LogicX

Could you be more specific? Just like an integral is the riemann sum of n number of rectangles as n goes to infinity. I assume that is the same reasoning being used here.

EDIT: http://www.axiomaticdoubt.com/?p=504

Makes sense.

Hurkyl

An integral is defined to be the limit of Riemann sums. Is perimeter defined to be the limit of approximating stairsteps?

Grep

haha That's a good one, I'll have to remember that next time I want to mess with someone.

I think of it this way. Since "removing" the corners like that doesn't change the perimeter at all, it will fail to converge on the perimeter of a circle. So it's rather unlike, say, increasing the number of sides of a polygon inside the circle. That one converges on the real perimeter. His example does not.

The fact that the perimeter never changes as he removes the corners is pretty much proof that the technique will never work. It needs to converge to a smooth curve (to it's limit) that equals the perimeter of the circle, which this will never do (being jagged).

Not sure if that's 100% clear, someone else can probably put it better.

FlexGunship

No matter how many times you repeat the process, the circle is only touched tangentially four times (as defined at the start). To find the circumference you must touch it tangentially in all places. At no time will the function's rectilinear surface length resolve to a valid approximation of the circle for this reason (among others).

Furthermore, we know the process never enters the perimeter of the circle, so we can conclude that the final perimeter will have to be larger than the actual circumference. I would guess you'd get an answer ~4ish.

disregardthat

There is no such thing as the "real" perimeter of the circle independent from the (arbitrary or not) definition of it as the limiting sum of the uniformly decreasing length chords. The limit in the example is not the same as the conventional length of the perimeter (or any differentiable curve), rather than the real length of the perimeter.

D H

What do you mean by "real", Jarle? What makes the Euclidean norm more real than the taxicab norm?

What is the circumference of the curve x²+y²=r² using the taxicab norm?

disregardthat

I mean what he calls "real" is more correctly put as "conventional", since there is no pre-existing platonic length of the circle independent of our ways of "finding" it. The choice of the euclidean norm is conventional as a measure of length (which is commonly understood by length unless otherwise is stated), it is not measuring real lengths as opposed to other norms. Similarly, approximating with regular polygons doesn't measure real length as opposed to other limiting sums. They measure different things, but none of them are more real than the other.

Grep

Another thing my friend pointed out is that, the perimeter he's calculating is always 4. Then he concludes that pi = 4. But pi is HALF the circumference. The perimeter of the circle would be 4, which means if 2*pi = perimeter then pi = (perimeter / 2) = (4 / 2) = 2. Silly of me not to notice that obvious problem.

But it sounds better to say pi = 4 for the purposes of confusing people, I suppose.

disregardthat

The diameter in the drawing is set as 1. Besides, the resulting ratio must be larger than pi since the measured length is constantly larger.

Grep

I kind of see your point. But it's, IMO, nitpicking of the highest order. Let's assume I have a perfect unit circle drawn out (or to well within the tolerances with which I will measure). I can also measure the circumference to a certain precision. Which means I can empirically determine PI to whatever the limits of my measurement and my ability to get a perfect unit circle. Won't get me there exactly, but neither will any other method since it's an irrational number. All I can do is compute it to a certain number of decimal points.

It's real in the sense that if I compute it and it disagrees with my empirical measurement within my level of accuracy, I would have to conclude that my computation is wrong. Either that, or define a circle as something which has no relation to the real world, which wouldn't be very useful. And I mean "real" in that sense, and that sense only.

Grep

And? That means the sides of the initial square are of length 1. Which means it's perimeter is 4 throughout. And since the perimeter will always be 4, we would have to derive a pi which is half that, following his logic (which is obviously totally wrong).

disregardthat

It might well be nitpicking, but then again it might not since I believe there is a presupposition of something platonically real about some preferred definitions of distance (and area for that matter). I am not commenting on imprecise mathematical formulations here.

What you physically measure is also based upon your choice of measure. In any case I am referring to the mathematics, not physics.

When one somewhat arbitrarily (though physically applicable) generalize distance to differentiable curves in some particular way one cannot expect (and one can never do so) that this is the "real" way to do it as opposed to alternative ways. They are only different. As is alternative norms.

disregardthat

No, since pi is defined as the perimeter divided by the diameter. In this case the perimeter equals pi, since the diameter is 1. pi is only only half the perimeter if the diameter is 2 (2*r*pi =d*pi = 1*pi = pi).

Grep

Apologies, on that one I have to agree with you 100%. I clearly mixed up r and d. My fault for assuming my friend got it right and not thinking it through for myself (not a regular habit I assure you... ).

Dadface

Consider say the left hand top quadrant of the circle.Now by drawing a series of alternate vertical and horizontal lines construct a series of steps which rises from the bottom of the left side of the quadrant and to the top.One can see that the total vertical distance covered by these steps=d/2 and the total horizontal distance=d/2.From this it is easy to see that the total distance covered by such steps surrounding the circle =4d as is given in the question.It is 4d regardless of how many steps are used and what their sizes are.