 In the Minkowski metric the space-like interval formula_9 is defined as

We use the space-like Minkowski metric in the rest of this article.The velocity in spacetime is defined as

where

The 4-acceleration is always perpendicular to the 4-velocity

Therefore, we can regard acceleration in spacetime as simply a rotation of the 4-velocity.The inner product of the acceleration and the velocity is a Lorentz scalar and is zero.