  We posit a second massless photon, uncoupled to known forms of matter but
undergoing Lorentz non-invariant velocity mixing with ordinary photons. Our
speculation within a speculation suffers from the sin of implausibility but
enjoys the virtue of verifiability. To avoid unacceptable distortion of the
well-measured microwave background spectrum, the velocity difference of the
photons cannot exceed $\sim10^{-32} c$. Stronger constraints (or observable
effects!) can arise from optical measurements of distant sources.
