For twenty years, the Crab Pulsar's radio signal has shown a striking zebra pattern — evenly spaced bright and dark stripes that no single mechanism could explain. Plasma diffraction alone produces the wrong spacing. Gravitational lensing alone produces the wrong contrast. Medvedev's solution: both lenses operate simultaneously, and the pattern is their interference.
The plasma in the pulsar's magnetosphere acts as a defocusing lens, spreading radio waves apart. Gravity acts as a focusing lens, bending them back together. Where the two effects are in phase, you get a bright stripe. Where they're out of phase, dark. The zebra pattern isn't produced by either mechanism — it's produced by the mismatch between them.
This is a general principle hiding in a specific observation: when two optical effects with opposite signs compete, the residual isn't noise — it's structure. The spacing of the stripes encodes the ratio of plasma density to gravitational curvature. Erase either lens and you don't get half the pattern — you get no pattern at all.
The twenty-year delay wasn't because the individual pieces were unknown. Plasma lensing was understood. Gravitational lensing was understood. What was missing was the recognition that neither piece was the explanation — the explanation lived in the space between them. The pattern that looked like it needed a new mechanism actually needed the interference of two old ones.