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The four-wavevector combines the angular frequency and spatial wavevector of a plane wave. Phase invariance gives $K^{\prime \mu }={\Lambda }^{\mu }{}_{\nu }{K}^{\nu }$. Under a boost $\mathbf{v}$, ${\omega }^{\prime }=\gamma (\omega -\mathbf{v}\cdot \mathbf{k})$, ${\mathbf{k}}_{\parallel }^{\prime }=\gamma ({\mathbf{k}}_{\parallel }-\omega \mathbf{v}/c^2)$ and ${\mathbf{k}}_{\perp }^{\prime }={\mathbf{k}}_{\perp }$. For metric signature $(+,-,-,-)$, $R_{\mu }{K}^{\mu }=\omega t-\mathbf{k}\cdot \mathbf{r}$ is a Lorentz scalar. A vacuum light wave has $K_{\mu }{K}^{\mu }=0$.

The frequency and longitudinal wavevector formulas are exactly the boost of a contravariant Lorentz four-vector:where the matrix is understood on the time component and the signed scalar component along the boost direction. Hence the four-wavevector obeysFor the metric signature $(+,-,-,-)$, lowering the position index gives $R_{\alpha }=(ct,-\mathbf{r})$, soIt is a dimensionless Lorentz scalar, equal to minus the wave phase used in the stated complex wave. Invariance follows by contracting the mutually inverse position and wavevector transformations. Reversing metric signature reverses this displayed scalar's sign, but not its invariance or any observable phase difference. In vacuum, $K_{\alpha }{K}^{\alpha }=0$ is an additional null-vector check; the wave remains lightlike in every inertial frame.