|
Distance is a numerical description of how far apart things lie. In physics or everyday discussion, distance may refer to a physical length, a period of time, or an estimation based on other criteria (e.g. “two counties over”). In mathematics, distance must meet more rigorous criteria.
Physics According to special relativity, distances through physical time and space can only be measured as a spacetime interval. See the introduction to special relativity. When excluding relativity and other such theories (general usage): Distances through space are equal to the geometric formulas given below. It can, generally, be described as “how far apart things lie”. Distance relates to speed and time as: Geometry In neutral geometry, the minimum distance between two points is the length of the line segment between them. In algebraic geometry, one can find the distance between two points of the xy-plane using the distance formula. The distance between (x1,y1) and (x2,y2) is given by This formula could also be used as follows: Similarly, given points (x1,y1,z1) and (x2,y2,z2) in three-space, the distance between them is Which is easily proven by constructing a right triangle with a leg on the hypotenuse of another (with the other leg orthogonal to the plane that contains the 1st triangle) and applying the Pythagorean theorem. In the study of complicated geometries, we call this (most common) type of distance Euclidean distance, as it is derived from the Pythagorean theorem, which does not hold in Non-Euclidean geometries. This distance formula can also be expanded into the arc-length formula. Distance in Euclidean space In the Euclidean space Rn, the distance between two points is usually given by the Euclidean distance (2-norm distance). Other distances, based on other norms, are sometimes used instead. For a point (x1, x2, ...,xn) and a point (y1, y2, ...,yn), the Minkowski distance of order p (p-norm distance) is defined as: p need not be an integer, but it cannot be less than 1, because otherwise the triangle inequality does not hold. The 2-norm distance is the Euclidean distance, a generalization of the Pythagorean theorem to more than two coordinates. It is what would be obtained if the distance between two points were measured with a ruler: the "intuitive" idea of distance. The 1-norm distance is more colourfully called the taxicab norm or Manhattan distance, because it is the distance a car would drive in a city laid out in square blocks (if there are no one-way streets). The infinity norm distance is also called Chebyshev distance. In 2D it represents the distance kings must travel between two squares on a chessboard. The p-norm is rarely used for values of p other than 1, 2, and infinity, but see super ellipse. In physical space the Euclidean distance is in a way the most natural one, because in this case the length of a rigid body does not change with rotation. General case In mathematics, in particular geometry, a distance function on a given set M is a function d: M × M → R, where R denotes the set of real numbers, that satisfies the following conditions: For example, the usual definition of distance between two real numbers x and y is: d(x,y) = |x - y|. This definition satisfies the three conditions above, and corresponds to the standard topology of the real line. But distance on a given set is a definitional choice. Another possible choice is to define: d(x,y) = 0 if x = y, and 1 otherwise. This also defines a metric, but gives a completely different topology, the "discrete topology": with this definition numbers cannot be arbitrarily close. Distances between non-empty sets One might attempt to define the distance between two non-empty subsets of a given set as the infimum of the distances between any two of their respective points, which would agree with the every-day use of the word. However, this does not define a metric, since with this definition the distance between two different but overlapping sets is zero. A definition that does work defines the distance as the larger of two values, one being the supremum, for a point ranging over one set, of the infimum, for a second point ranging over the other set, of the distance between the points, and the other value being likewise defined but with the roles of the two sets swapped. This is a metric, called the Hausdorff metric. Distinguish
Other "distances" See also | ||||||||||
|
| |||||||||||
 |
|
| |