This article is about mathematical structures called spaces. For other uses, see Space (disambiguation).

In mathematics, a space is a set (sometimes called a universe) with some added structure.

Mathematical spaces often form a hierarchy, i.e., one space may inherit all the characteristics of a parent space. For instance, all inner product spaces are also normed vector spaces, because the inner product induces a norm on the inner product space such that:

{\displaystyle \left\|x\right\|={\sqrt {\langle x,x\rangle }},}

where the norm is indicated by enclosing in double vertical lines, and the inner product is indicated enclosing in by angle brackets.

Modern mathematics treats “space” quite differently compared to classical mathematics.

History

Before the golden age of geometry

In the ancient mathematics, “space” was a geometric abstraction of the three-dimensional space observed in the everyday life. The axiomatic method had been the main research tool since Euclid (about 300 BC). The method of coordinates (analytic geometry) was adopted by René Descartes in 1637. At that time, geometric theorems were treated as an absolute objective truth knowable through intuition and reason, similar to objects of natural science; and axioms were treated as obvious implications of definitions.

Two equivalence relations between geometric figures were used: congruence and similarity. Translations, rotations and reflections transform a figure into congruent figures; homotheties — into similar figures. For example, all circles are mutually similar, but ellipses are not similar to circles. A third equivalence relation, introduced by Gaspard Monge in 1795, occurs in projective geometry: not only ellipses, but also parabolas and hyperbolas, turn into circles under appropriate projective transformations; they all are projectively equivalent figures.

The relation between the two geometries, Euclidean and projective, shows that mathematical objects are not given to us with their structure.  Rather, each mathematical theory describes its objects by some of their properties, precisely those that are put as axioms at the foundations of the theory.

Distances and angles are never mentioned in the axioms of the projective geometry and therefore cannot appear in its theorems. The question “what is the sum of the three angles of a triangle” is meaningful in the Euclidean geometry but meaningless in the projective geometry.

A different situation appeared in the 19th century: in some geometries the sum of the three angles of a triangle is well-defined but different from the classical value (180 degrees). The non-Euclidean hyperbolic geometry, introduced by Nikolai Lobachevsky in 1829 and János Bolyai in 1832 (and Carl Gauss in 1816, unpublished) stated that the sum depends on the triangle and is always less than 180 degrees. Eugenio Beltrami in 1868 and Felix Klein in 1871 obtained Euclidean “models” of the non-Euclidean hyperbolic geometry, and thereby completely justified this theory.

This discovery forced the abandonment of the pretensions to the absolute truth of Euclidean geometry. It showed that axioms are not “obvious”, nor “implications of definitions”. Rather, they are hypotheses. To what extent do they correspond to an experimental reality? This important physical problem no longer has anything to do with mathematics. Even if a “geometry” does not correspond to an experimental reality, its theorems remain no less “mathematical truths”.

A Euclidean model of a non-Euclidean geometry is a clever choice of some objects existing in Euclidean space and some relations between these objects that satisfy all axioms (therefore, all theorems) of the non-Euclidean geometry. These Euclidean objects and relations “play” the non-Euclidean geometry like contemporary actors playing an ancient performance. Relations between the actors only mimic relations between the characters in the play. Likewise, the chosen relations between the chosen objects of the Euclidean model only mimic the non-Euclidean relations. It shows that relations between objects are essential in mathematics, while the nature of the objects is not.

The golden age and afterwards: dramatic change

According to Nicolas Bourbaki, the period between 1795 (Geometrie descriptive of Monge) and 1872 (the “Erlangen programme” of Klein) can be called the golden age of geometry. Analytic geometry made a great progress and succeeded in replacing theorems of classical geometry with computations via invariants of transformation groups.  Since that time new theorems of classical geometry are of more interest to amateurs rather than to professional mathematicians.

However, it does not mean that the heritage of the classical geometry was lost. According to Bourbaki, “passed over in its role as an autonomous and living science, classical geometry is thus transfigured into a universal language of contemporary mathematics”.

According to the famous inaugural lecture given by Bernhard Riemann in 1854, every mathematical object parametrized by {\displaystyle n} real numbers may be treated as a point of the {\displaystyle n}-dimensional space of all such objects.^[2]:140 Contemporary mathematicians follow this idea routinely and find it extremely suggestive to use the terminology of classical geometry nearly everywhere.

In order to fully appreciate the generality of this approach one should note that mathematics is “a pure theory of forms, which has as its purpose, not the combination of quantities, or of their images, the numbers, but objects of thought” (Hermann Hankel, 1867).  This is a controversial characterization of the purpose of mathematics, which is not necessarily committed to the existence of “objects of thought.”

Functions are important mathematical objects. Usually they form infinite-dimensional function spaces, as noted already by Riemann and elaborated in the 20th century by functional analysis.

An object parametrized by n complex numbers may be treated as a point of a complex n-dimensional space. However, the same object is also parametrized by 2 n real numbers (if c is a complex number, then c = a + b i, where a and b are real), thus, a point of a real 2n-dimensional space. The complex dimension differs from the real dimension. This is only the tip of the iceberg. The “algebraic” concept of dimension applies to vector spaces. For topological spaces there are several dimension concepts including inductive dimension and Hausdorff dimension, which can be non-integer (especially for fractals). Some kinds of spaces (for instance, measure spaces) admit no concept of dimension at all.

The original space investigated by Euclid is now called three-dimensional Euclidean space. Its axiomatization, started by Euclid 23 centuries ago, was reformed with Hilbert’s axioms, Tarski’s axioms and Birkhoff’s axioms. These axiom systems describe the space via primitive notions (such as “point”, “between”, “congruent”) constrained by a number of axioms. Such a definition “from scratch” is now not often used, since it does not reveal the relation of this space to other spaces. The modern approach defines the three-dimensional Euclidean space more algebraically, via vector spaces and quadratic forms, namely, as an affine space whose difference space is a three-dimensional inner product space.

Also a three-dimensional projective space is now defined non-classically, as the space of all one-dimensional subspaces (that is, straight lines through the origin) of a four-dimensional vector space.

A space consists now of selected mathematical objects (for instance, functions on another space, or subspaces of another space, or just elements of a set) treated as points, and selected relationships between these points. It shows that spaces are just mathematical structures of convenience. One may expect that the structures called “spaces” are more geometric than others, but this is not always true. For example, a differentiable manifold (called also smooth manifold) is much more geometric than a measurable space, but no one calls it “differentiable space” (nor “smooth space”).

More at:  https://en.wikipedia.org/wiki/Space_(mathematics)

(Contributed by Richard Branam.)