 Lorentz invariance of optical cavity resonance

A Lorentz transformation of a resonant optical cavity transforms both the electromagnetic field and the mirror worldlines. Boundary conditions and event-by-event phase matching remain satisfied. For a longitudinal boost, a cavity of rest length $L$ has length $L/\gamma$ and unequal forward/backward frequencies $\gamma {\omega }_0(1\mp \beta )$. The coordinate round-trip time is $2\gamma L/c$, while the phase rate on a moving mirror is ${\omega }_0/\gamma$, so the round-trip phase is still $2{\omega }_{0}L/c$. A stationary-mirror resonance formula with only length contraction inserted describes the wrong boundary-value problem.