The Wolf Effect and Gravitational Redshifts

Spectral lines can be redshifted toward longer wavelengths or blueshifted toward shorter ones. The Doppler effect explains how these shifts occur because of relative motions of the source and the observer along the line of sight. Approach causes a blueward shift and recessional a redward one.

Scientists have long believed that only the Doppler effect or Gravity as described by Einstein could account for wavelength shifts in the spectrum of light as it travels through space. Where neither factor applies, scientists have always assumed spectral invariance—the spectrum remains the same no matter how far the light travels. This is the case with ordinary sources—called "Lambertian" after Johann Heinrich Lambert—such as the blackbody radiation from stellar surfaces.

In the past few years, however, experiments have shown that there is a third way to shift spectral lines. This mechanism involves non-Lambertian sources that emit beamed energy, such as lasers and devices producing synchrotron light. The discoverer of this new effect is physicist Emil Wolf, who, along with Max Born, wrote the definitive textbook Principles of Optics.

A mechanic analog to Wolf's discovery is a pair of tuning forks with nearly identical resonant frequencies (pitches). If these forks are connected together mechanically by, say, a sounding board, the coupling is strong and the resonant frequencies tend to get "dragged down" to lower ones. In other words, the wavelength is lengthened, or redshifted. This phenomenon has been verified experimentally with light waves and for sound waves from coupled speakers.

The actual frequency shift due to the Wolf effect depends on the geometry. As the illustration above shows, whether an observer sees a redshift or a blue shift depends on his or her locations with respect to the source.

The mechanism can be extended from the case of two radiating point sources to that of a whole collection of such objects, for example a plasma cloud. Wolf and his colleagues have shown that such a cloud can produce shifts that closely mimic the Doppler effect. The figure shows an example.

Thus the assumption that quasars—beamed electromagnetic radiators with large redshifts—are part of the "Hubble flow" of an expanding universe could be wrong. This effect should also apply to normal galaxies, most of whose matter is in the plasma state.

Left: Due to the Wolf effect beamed emission from two separate sources can interact and shift the wavelengths of the lines in their spectra. The change can be redward or blueward by different amounts depending on the observer's point of view, but from "head-on" the shift is to the red. This is a distinctly different process than the Doppler effect. Right: Here the two mechanisms are compared for the case of oxygen lines with a redshift of 0.07. However, the light sources involved in the Wolf effect are stationary, not fleeing the observer at 84 kilometers per second.