Hyperbolic geometry

In mathematics, **hyperbolic geometry** is a non-Euclidean geometry, meaning that the parallel postulate that defines Euclidean geometry isn't true. On a hyperbolic plane, lines that started out parallel will become further and further apart.

Replacing this rule means that hyperbolic geometry is different from ordinary flat plane geometry. For example, triangles will have angles that add up to less than 180 degrees, meaning that they are too pointy and will look like the sides are sinking into the middle.

Many real objects look like hyperbolic planes. For example, some types of coral and lettuce are shaped like pieces of hyperbolic planes. Other people say that it's easier to draw a map of the Internet when your map isn't flat, because there are a lot of computers around the edges but very few in the center. Some physicists even think our universe is a little bit hyperbolic.

## Formal definition

The parallel postulate in Euclidean geometry says that in two dimensional space, for any given line _l_ and point _P_ not on _l_, there is exactly one line through _P_ that does not intersect _l_. This line is called _parallel_ to _l_. In hyperbolic geometry there are at least two such lines through _P_. As they do not intersect _l_, the parallel postulate is false. Models have been constructed within Euclidean geometry that obey the axioms of hyperbolic geometry. These models prove that the parallel postulate is independent of the other postulates of Euclid.

Because there is no hyperbolic analogue to Euclidean parallel lines, the hyperbolic use of _parallel_ and related terms varies among writers. In this article, the two limiting lines are called _asymptotic_ and lines that have a common perpendicular are called _ultraparallel._ The simple word _parallel_ may apply to both.

## Non-intersecting lines

An interesting property of hyperbolic geometry follows from the occurrence of more than one parallel line through a point: there are two classes of non-intersecting lines. If a line A is parallel to another line N, and they both get arbitrarily close to intersecting, but never actually intersect, A and B are **asymptotic** **lines**. More formally, if A is rotated at all towards B, and A and B are asymptotic, then A and B will intersect. If A is flipped around a line perpendicular to B (or the other way around), they are still asymptotic. Any other pair of lines that never intersect but aren’t asymptotic are considered **ultraparallel** (or **disjointly parallel**). Hyperbolic geometry has a negative curvature, and this allows an infinite number of ultraparallel lines, since if you are slightly off from going in the same direction as another line, you are still “pushed” away.

Thus we have this modified form of the parallel postulate: In hyperbolic geometry, given any line _A_, and point _P_ not on _A_, there are exactly two lines through _P_ which are asymptotic to _A_, and infinitely many lines through _P_ ultraparallel to _A._

The differences between these types of lines can also be looked at like this: the distance between asymptotic lines run to zero in one direction and grows without bound in the other; the distance between ultraparallel lines increases in both directions. The ultraparallel theorem states that there is a _unique_ line in the hyperbolic plane that is perpendicular to each of a given pair of ultraparallel lines.

In Euclidean geometry, the _angle of parallelism_ is a constant; that is, if you try to make a triangle where 2 sides are asymptomatic (in Euclidean geometry, parallel) to each other (and the remaining side has to have non-zero length), you’ll have 2 angles of 90° (and it won’t really look like a triangle). In hyperbolic geometry, the angle of parallelism varies with the  function, where p is the length of the non-parallel. This function, described by Nikolai Ivanovich Lobachevsky, produces a unique angle of parallelism for each distance . As the distance gets shorter,  approaches 90°, whereas with increasing distance  approaches 0°. Thus, as distances get smaller, the hyperbolic plane behaves more and more like Euclidean geometry. Indeed, on small scales compared to , where  is the (constant) Gaussian curvature of the plane, an observer would have a hard time determining whether he is in the Euclidean or the hyperbolic plane.

## History

For centuries, geometers tried to prove the parallel postulate. They failed, but their efforts gave birth to hyperbolic geometry. The theorems of Alhacen, Khayyam on quadrilaterals, were the first theorems on hyperbolic geometry. Their works on hyperbolic geometry had an influence on its development among later European geometers, including Witelo, Alfonso and John Wallis.

In the nineteenth century, hyperbolic geometry was explored by János Bolyai and Nikolai Ivanovich Lobachevsky, after whom it sometimes is named. Lobachevsky published in 1830, while Bolyai independently discovered it and published in 1832. Karl Friedrich Gauss also studied hyperbolic geometry, describing in a 1824 letter to Taurinus that he had constructed it, but did not publish his work. In 1868, Eugenio Beltrami provided models of it, and used this to prove that hyperbolic geometry was consistent if Euclidean geometry was.

The term "hyperbolic geometry" was introduced by Felix Klein in 1871. For more history, see article on non-Euclidean geometry.

## Models of the hyperbolic plane

There are three models commonly used for hyperbolic geometry: the Klein model, the Poincaré disc model, and the Lorentz model, or hyperboloid model. These models define a real hyperbolic space which satisfies the axioms of a hyperbolic geometry. Despite the naming, the two disc models and the half-plane model were introduced as models of hyperbolic space by Beltrami, not by Poincaré or Klein.

1. The **Klein model**, also known as the **projective disc model** and **Beltrami-Klein model**, uses the interior of a circle for the hyperbolic plane, and chords of the circle as lines. However, angles are not preserved.
1. The **Poincaré disk model** takes the inside of a circle in the Euclidean plane, and uses arcs intersecting at 90° and lines through the center as lines. Angles are preserved.
1. The **Poincaré half-plane model** takes one-half of the Euclidean plane, as determined by a Euclidean line _B_, to be the hyperbolic plane (_B_ itself is not included).
    - Hyperbolic lines are then either half-circles orthogonal to _B_ or rays perpendicular to _B_.
    - Both Poincaré models preserve hyperbolic angles, and are thereby conformal. All isometries within these models are therefore Möbius transformations.
    - The half-plane model is identical (at the limit) to the Poincaré disc model at the edge of the disc.
1. The **hyperboloid model** uses a hyperboloid in Minkowski geometry, where the pythagorean theorem has ons axis subtracted instead of added. This axis is the one the hyperboloid is symmetric around.
    - This model has direct application to special relativity, as Minkowski 3-space is a model for spacetime, suppressing one spatial dimension. One can take the hyperboloid to represent the events that various moving observers, radiating outward in a spatial plane from a single point, will reach in a fixed proper time. The hyperbolic distance between two points on the hyperboloid can then be identified with the relative rapidity between the two corresponding observers.

## Visualizing hyperbolic geometry

M. C. Escher's famous prints _Circle Limit III_  and _Circle Limit IV_ illustrate the conformal disc model quite well. In both one can see the geodesics. (In _III_ the white lines are not geodesics, but hypercycles, which run alongside them.) It is also possible to see quite plainly the negative curvature of the hyperbolic plane, through its effect on the sum of angles in triangles and squares.

In the Euclidean plane, their angles would sum to 450°; i.e., a circle and a quarter. From this we see that the sum of angles of a triangle in the hyperbolic plane must be smaller than 180°. Another visible property is exponential growth. In _Circle Limit IV_, for example, one can see that the number of angels and demons  within a distance of _n_ from the center rises exponentially. The demons have equal hyperbolic area, so the area of a ball of radius _n_ must rise exponentially in _n_.

There are several ways to physically realize a hyperbolic plane (or approximation thereof). A particularly well-known paper model based on the pseudosphere is due to William Thurston. The art of crochet has been used to demonstrate hyperbolic planes with the first being made by Daina Taimina. In 2000, Keith Henderson demonstrated a quick-to-make paper model dubbed the "hyperbolic soccerball".

Using a computer, it is possible to raymarch lines in hyperbolic space to get an “internal” view, where the camera is actually inside of the space. This works well for 3d, as it is very hard to visualize the 4d space required to curve a 3d space into hyperbolic geometry.

## Other websites

- Visions of Infinity: Tiling a hyperbolic floor inspires both mathematics and art  Science News: Dec. 23, 2000; Vol. 158, No. 26/27, p. 408
- Java freeware for creating sketches in both the Poincaré Disk and the Upper Half-Plane Models of Hyperbolic Geometry University of New Mexico
- "The Hyperbolic Geometry Song" A short music video about the basics of Hyperbolic Geometry available at YouTube.
- More on hyperbolic geometry, including movies and equations for conversion between the different models  University of Illinois at Urbana-Champaign
- , interactive instructional website.
- Raymarched hyperbolic space

## Literature

- Coxeter, H. S. M. (1942) _Non-Euclidean geometry_, University of Toronto Press, Toronto
- Nikolai I. Lobachevsky, Pangeometry, Translator and Editor: A. Papadopoulos,  Heritage of European Mathematics Series, Vol. 4, European Mathematical Society, 2010.
- Milnor, John W. (1982) _Hyperbolic geometry: The first 150 years_, Bull. Amer. Math. Soc. (N.S.) Volume 6, Number 1, pp. 9–24.
- Reynolds, William F. (1993) _Hyperbolic Geometry on a Hyperboloid_, American Mathematical Monthly 100:442-455.
- Stillwell, John. (1996) _Sources in Hyperbolic Geometry_, volume 10 in AMS/LMS series _History of Mathematics_.
- Samuels, David. (March 2006) _Knit Theory_ Discover Magazine, volume 27, Number 3.
- James W. Anderson, _Hyperbolic Geometry_, Springer 2005,