Very Minkowski

Geometry Level 5

Consider the Euclidean plane equipped with the rectilinear metric . In this metric, the distance between two points ( a , b ) , ( x , y ) (a,b), (x,y) is defined as

d ( ( a , b ) , ( x , y ) ) = x a + y b . d((a,b), (x,y)) = |x-a| + |y-b|.

We call the resulting geometry as Minkowski geometry . For example, in Minkowski geometry, the points ( 1 , 5 ) (1,5) and ( 3 , 2 ) (3,2) are 3 1 + 2 5 = 2 + 3 = 5 |3-1| + |2-5| = 2+3 = 5 units apart.

Recall that a circle is given by a point x x and a radius r r ; a circle is the set of points that have distance exactly r r from x x .

Consider a circle in Minkowski geometry. It can be proven that the ratio of its circumference to its diameter is constant. Find this ratio.


The answer is 4.00.

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2 solutions

Sharky Kesa
Jul 31, 2016

Note that π \pi is defined as the ratio between the circumference and diameter of a circle. Also, a circle is defined as the locus of all points equidistant from a central point.

In Minkowski geometry, since the distance formula between two points is the sum of the differences of corresponding coordinates. Thus, a Minkowski circle would be in the shape of a Euclidean square oriented 4 5 45^{\circ} with respect to the coordinate axes. The circumference of this circle would be 8 a + b 8|a+b| (measuring using Minkowski, not Euclidean), whilst the diameter would be 2 a + b 2|a+b| . Thus, the value of π \pi would be 4 4 .

I would prefer if the question was "Find the ratio of the circumference to the diameter of the unit circle centered at the origin". There are many definitions of π \pi that are non-geometric. Also, depending on the metric that you choose, the ratio could differ for various circles.

Calvin Lin Staff - 4 years, 10 months ago

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Sure, I guess that would help people.

Sharky Kesa - 4 years, 10 months ago

I edited the problem to reflect that. Thanks.

Agnishom Chattopadhyay - 4 years, 10 months ago

I have changed the problem to remove the reference to π \pi (because π \pi is a number, not the ratio of circle's circumference to its diameter).

Ivan Koswara - 4 years, 10 months ago

in Minkowski geometry a circle is a square , side S, with diagonals along the x-axes and y-axis.
Diameter is the distance between the sides.
Circumference is the perimeter.
So the ratio Circumference/ Diameter =4S/S=4.


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