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A four-vector, used in special relativity, and maybe also general relativity, is a 4 dimentional vector, where the scalar product between two four-vectors a and b is defined as follows: (Strictly speaking, four-vectors aren't proper vectors, since the scalar product can be negative.)

${\displaystyle ab={\begin{vmatrix}a_{0}\\a_{1}\\a_{2}\\a_{3}\end{vmatrix}}.{\begin{vmatrix}b_{0}\\b_{1}\\b_{2}\\b_{3}\end{vmatrix}}=a_{0}b_{0}-a_{1}b_{1}-a_{2}b_{2}-a_{3}b_{3}}$

In the above, a₀ would represent time, and a₁, a₂ and a₃ would represent space, same for b. The proportion between space coordinates and time coordinates should be c, for example, if the time coordinate is given in seconds, the space coordinates should be given in light seconds.

The scalar product of two four-vectors is invariant under any lorentz transformation. The laws of physics are also supposed to be invariant under a lorentz transformation.

An object will percieve the universe as if the universe was lorentz transformed so that the percieving object was stationary. Stationary is only defined with respect to a chosen inertial reference frame.

See also: four-velocity, four-acceleration, four-momentum, four-force, four-lots of stuff.