https://sci-hub.se/https://doi.org/10.1038/scientificamerican1285-54I wouldn't say its backward in time, as light is timeless, but one can get them to time-reversed, a backwards version of the lightwave
In one version the mirror is vibrating naturally due to its temperature, creating microscopic fluctuations in density, and its refractive index (concern oneself with charge density, longitudinal light waves). These scatter the light beam slightly differently. In the optimal case, these scattered beams interfere with the incoming beam creating more pressure-density fluctuations, triggering a pressure-density resonance cascade that creates the conjugated beam.
