spacetime vector = (cΔt, Δx, Δy, Δz)

STI = \sqrt{(cΔt)^{2} − (Δx)^{2}}

\frac{STI}{c} = \sqrt{(Δt)^{2} − (\frac{Δx}{c})^{2}}

momentum − energy vector = (E, p_{x}, p_{y}, p_{z})

ME = \sqrt{E^{2} − p^{2}c^{2}}

\frac{ME}{c} = \sqrt{(\frac{E^{2}}{c^{2}} ) − p^{2}}

\frac{ME}{c^{2}} = \sqrt{(\frac{E^{2}}{c^{4}} ) − (\frac{p^{2}}{c^{2}} )}