Wolf effect

The Wolf Effect (sometimes Wolf shift) is a frequency shift in the electromagnetic spectrum.^[1] The phenomenon occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light.^[2] It was first predicted by Emil Wolf in 1987 ^{[3] [4]} and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox,^[5] and a year later in optic souces by Dean Faklis and George Morris in 1988 ^[6]. Wolf and James noted:

"Under certain conditions the changes in the spectrum of light scattered on random media may imitate the Doppler effect, even though the source, the medium and the observer are all at rest with respect to one another.^[7]

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Wolf, Daniel James and Sisir Roy et al. suggested that the Wolf Effect may explain discordant redshift in certain quasars ^[8][3][2], in reference to the historical controversies surrounding the nature of quasars.

Theoretical description

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 candle light 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.^[3]

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. Under suitably controlled scenarios, it may even be possible to roughly mimic Doppler redshifts.

An example of such a medium which could produce Doppler-like shifts was found in 1990 by Daniel James, Malcolm Savedoff, Malcolm and Emil Wolf,^[9] and involved a highly statistically anisotropic scattering medium. A "no blueshift" condition has also been found by Datta, S. et al., ^{[10] [11]}.

Wolf and James noted ^[7] that:

"Although we make no claim that correlation-induced spectral shifts account for all, or even for a majority, of the observed shifts of lines in the spectra of extra-galactic objects, we note the possibility that correlation-induced spectral shifts may contribute to the shifts observed in the spectra of some astronomical objects such as quasars."

James also noted that ^[2] that:

"One can easily spot potential problems with the theory presented here. For example, we have been deliberately vague about the underlying physical nature of the scattering medium, other than specifying its anisotropic coherence properties. The scatterer, which is assumed to have a ‘white noise’ power spectrum (implying that its fluctuations are very energetic), is situated further away from the central engine than the line emitting clouds (which cannot be too hot, otherwise they would have completely ionized, making line radiation impossible). Although our results apply for a broad spectral range, we have not considered the shifts of absorption lines or of the 21cm radio line. We have ignored the unscattered radiation and the efficiency of the scattering process (although it is possible a stimulated version of the spontaneous scattering process considered here is applicable). Also the issue of spectral linewidths has not been addressed."

Notes

See also

• Scattering