The

Special Theory of Relativity encourages one to think of space and time as being different labels for

coordinates in a single entity called

spacetime. As such, instead of using

**x** (three dimensional) and t (one dimensional) in equations, treat the two together as a single object x

^{μ}=(t,

**x**)

^{T} (four dimensional): this is called a

four-

vector.

Four vectors extend to covering other things besides space and time: they can represent any quantity which transforms in the correct way between different frames of referrence (ie. by

Lorentz Transformations: x'

^{μ}=Λ

^{μ}_{ν}x

^{ν}). Such possible quantities arise from pairs of

(three) vectorial and

scalar quantities, eg:

Four-vectors and their associated higher order

tensors are conventionally written down using the

abstract index notation.