inner product tensor
Invariant means that it takes the same value in all inertial frames, because it is a scalar (0 rank tensor), and so no Λ appears in its trivial transformation. The magnitude of the 4-vector T is the positive square root of the inner product with itself:





|


T


|

=



T

α



T

α






{\displaystyle |\mathbf {T} |={\sqrt {T^{\alpha }T_{\alpha }}}}


