Wolf predicted a new mechanism that produces redshift and blueshift, that is not due to moving sources (Doppler effect), that has subsequently been confirmed experimentally (called the Wolf effect). Technically, he found that two non-Lambertian sources that emit beamed energy, can interact in a way that causes a shift in the spectral lines. It is analogous to a pair of tuning forks with similar frequencies (pitches), connected together mechanically with a sounding board; there is a strong coupling that results in the resonant frequencies getting "dragged down" in pitch. The Wolf effect can produce either redshifts or blueshifts, depending on the observer's point of view, but is redshifted when the observer is head-on. A subsequent 1999 article by Sisir Roy "et al." have suggested that the Wolf effect may explain discordant redshift in certain quasars.