The Wolf Effect (sometimes Wolf shift) is a frequency shift in the electromagnetic spectrum. The phenomenon occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light. It was first predicted by Emil Wolf in 1987  and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox, and a year later in optic sources by Dean Faklis and George Morris in 1988.
In optics, 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 requires that the waves from the sources are partially coherent - the wavefronts being partially in phase. Laser light is coherent while candlelight is incoherent, each photon having random phase. It can produce either redshifts or blueshifts, depending on the observer's point of view, but is redshifted when the observer is head-on.
For two sources interacting while separated by a vacuum, the Wolf effect cannot produce shifts greater than the linewidth of the source spectral line, since it is a position-dependent change in the distribution of the source spectrum, not a method by which new frequencies may be generated. However, when interacting with a medium, in combination with effects such as Brillouin scattering it may produce distorted shifts greater than the linewidth of the source.
Application to cosmology
In his 1987 Nature paper, Wolf argues that the mechanism outlined "...may be responsible for some of the so far unexplained features of quasar spectra, including line asymmetries and small differences in the observed redshifts of different lines". Wolf goes on to relate this to a 1966 paper in the same journal on quasar coherence emission by astrophysicists Hoyle, Burbidge and Sargent.
- Emil Wolf, "Selected Works of Emil Wolf: With Commentary" (2001) p.638, ISBN 981-02-4204-2. See also: Marco Marnane Capria, Physics Before and After Einstein (2005) edited by M. Mamone Capria, p.303 ISBN 1-58603-462-6. See also: S. Roy, S. Data, in Gravitation and Cosmology: From the Hubble Radius to the Planck Scale (2002) by Colin Ray Wilks, Richard L Amoroso, Geoffrey Hunter, Menas Kafatos; page 104, ISBN 1-4020-0885-6
- James, Daniel, "The Wolf effect and the redshift of quasars" (1998) Pure Appl. Opt. 7: 959-970. (Full text, PDF)
- Wolf, Emil "Noncosmological redshifts of spectral lines" (1987) Nature 326: 363—365.
- Wolf, Emil, "Redshifts and blueshifts of spectral lines caused by source correlations" (1987) Optics Communications 62: 12—16.
- Mark F. Bocko, David H. Douglass, and Robert S. Knox, "Observation of frequency shifts of spectral lines due to source correlations" (1987) Physical Review Letters 58: 2649—2651.
- Faklis, Dean, and Morris, George Michael, "Spectral shifts produced by source correlations" (1988) Optics Letters 13 (1): 4—6.