Antipodal point

Encyclopedia

In mathematics

Mathematics

Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

, the

**antipodal point**of a point on the surface of a sphere is the point which is diametrically

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

opposite to it — so situated that a line drawn from the one to the other passes through the centre of the sphere and forms a true diameter.

This term applies to opposite points on a circle

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

or any n-sphere.

An antipodal point is sometimes called an

**antipode**, a back-formation

Back-formation

In etymology, back-formation is the process of creating a new lexeme, usually by removing actual or supposed affixes. The resulting neologism is called a back-formation, a term coined by James Murray in 1889...

from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

loan word

*antipodes*, which originally meant "opposite the feet."

## Theory

In mathematicsMathematics

Mathematics is the study of quantity, space, structure, and change. Mathematicians seek out patterns and formulate new conjectures. Mathematicians resolve the truth or falsity of conjectures by mathematical proofs, which are arguments sufficient to convince other mathematicians of their validity...

, the concept of

*antipodal points*is generalized to sphere

Sphere

A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

s of any dimension: two points on the sphere are antipodal if they are opposite

*through the centre*; for example, taking the centre as origin

Origin (mathematics)

In mathematics, the origin of a Euclidean space is a special point, usually denoted by the letter O, used as a fixed point of reference for the geometry of the surrounding space. In a Cartesian coordinate system, the origin is the point where the axes of the system intersect...

, they are points with related vectors

**v**and −

**v**. On a circle

Circle

A circle is a simple shape of Euclidean geometry consisting of those points in a plane that are a given distance from a given point, the centre. The distance between any of the points and the centre is called the radius....

, such points are also called

**diametrically opposite**. In other words, each line through the centre intersects the sphere in two points, one for each ray out from the centre, and these two points are antipodal.

The Borsuk–Ulam theorem is a result from algebraic topology

Algebraic topology

Algebraic topology is a branch of mathematics which uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify up to homotopy equivalence.Although algebraic topology...

dealing with such pairs of points. It says that any continuous function

Continuous function

In mathematics, a continuous function is a function for which, intuitively, "small" changes in the input result in "small" changes in the output. Otherwise, a function is said to be "discontinuous". A continuous function with a continuous inverse function is called "bicontinuous".Continuity of...

from

*S*

^{n}to

**R**

^{n}maps some pair of antipodal points in

*S*

^{n}to the same point in

**R**

^{n}. Here,

*S*

^{n}denotes the

*n*-dimensional sphere in (

*n*+ 1)-dimensional space (so the "ordinary" sphere is

*S*

^{2}and a circle is

*S*

^{1}).

The

**antipodal map**

*A*:

*S*

^{n}→

*S*

^{n}, defined by

*A*(

*x*) = −

*x*, sends every point on the sphere to its antipodal point. It is homotopic

Homotopy

In topology, two continuous functions from one topological space to another are called homotopic if one can be "continuously deformed" into the other, such a deformation being called a homotopy between the two functions...

to the identity map

Identity function

In mathematics, an identity function, also called identity map or identity transformation, is a function that always returns the same value that was used as its argument...

if

*n*is odd, and its degree

Degree (mathematics)

In mathematics, there are several meanings of degree depending on the subject.- Unit of angle :A degree , usually denoted by ° , is a measurement of a plane angle, representing 1⁄360 of a turn...

is (−1)

^{n+1}.

If one wants to consider antipodal points as identified, one passes to projective space

Projective space

In mathematics a projective space is a set of elements similar to the set P of lines through the origin of a vector space V. The cases when V=R2 or V=R3 are the projective line and the projective plane, respectively....

(see also projective Hilbert space

Projective Hilbert space

In mathematics and the foundations of quantum mechanics, the projective Hilbert space P of a complex Hilbert space H is the set of equivalence classes of vectors v in H, with v ≠ 0, for the relation given by...

, for this idea as applied in quantum mechanics

Quantum mechanics

Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

).

## External links

- Map Tunnelling Tool — Demonstrates where on the earth is directly opposite a specified point on the earths surface.
- Calculate the other side of the world