# Minkowski space

A four-dimensional pseudo-Euclidean space of signature $(1,3)$, suggested by H. Minkowski (1908) as a geometric model of space-time in the special theory of relativity (see ). Corresponding to each event there is a point of Minkowski space, three coordinates of which represent its coordinates in the three-dimensional space; the fourth coordinate is $ct$, where $c$ is the velocity of light and $t$ is the time of the event. The space-time relationship between two events is characterized by the so-called square interval:
$$s^2=c^2(\Delta t)^2-(\Delta x)^2-(\Delta y)^2-(\Delta z)^2.$$
The interval in Minkowski space plays a role similar to that of distance in Euclidean geometry. A vector with positive square interval is called a time-like vector, one with negative square interval, a space-like vector, one with square interval zero, a null or isotropic vector. A curve with a time-like tangent vector at each point is called a time-like curve. Space-like and isotropic curves are similarly defined. An event at a given moment of time is called a world point; a set of world points describing the development of some process or phenomenon through time is called a world line. If a vector joining neighbouring world points is time-like, then there is a frame of reference in which the events project to one and the same point of three-dimensional space. The time separating the events in this frame of reference is equal to $\Delta t=\tau=s/c$, where $\tau$ is the so-called proper time. There is no frame of reference in which these events can be simultaneous (that is, have the same time coordinate $t$). If the vector joining the world points of two events is space-like, then there is a frame of reference in which these two events occur simultaneously; they are not connected by a causal relation; the modulus of the square interval defines the spatial distance between the points (events) in this frame of reference. A tangent vector to a world line is a time-like vector. The tangent vector to a light ray is an isotropic vector.