A new way of making spectral redshifts.

A new way of making spectral redshifts

In science and technology, people haveproceeded on the assumption that once a beam of light has left its source, its properties--particularly the shape of its spectrum--do not change. It has caused some surprise, therefore, that Emil Wolf, a professor of physics at the University of Rochester, N.Y., proposes that in general this is not so: The spectrum of a given source should vary, he says, as the light moves through space (SN: 9/13/86, p. 166). It is becoming even more surprising that recently reported experiments support his contention.

The light from a source such as a lamp,a star or anything sized in between is the sum of the outputs of millions or billions of individual microscopic radiators. Traditionally physicists have believed that the spectrum of the light depends only on the physical characteristics of those individual radiators, and that the sum of the individual outputs, which provides the spectrum of the total source, is the same wherever one looks at it.

Wolf, who has been studying the coherenceproperties of light sources, contends that there is another factor to be taken into account: the coherence relations among the individual radiators. The output of each one of them fluctuates, and whether these fluctuations are more or less coherent or incoherent with one another produces the extra factor. This factor happens to be variable, and it will make the spectrum vary as the light proceeds through space, so that observers at different distances will see a different spectrum. In an experiment reported in OPTICAL COMMUNICATIONS earlier this year, Wolf's colleagues G. Michael Morris and Dean Faklis of the University of Rochester showed that in some cases spectra are in fact not constant.

For hundreds of years no one workingwith light had noticed such an effect. The reason is that most of the natural and artificial light sources that people deal with are thermal sources, in which the light arises from thermal agitation of the emitting material. These all tend to have a particular degree of incoherence that nullifies the effect, and so their spectra do not change, Wolf told SCIENCE NEWS. Stars, even galaxies, are thermal sources, Wolf points out, but there is one important class of astrophysical sources, quasars, that is generally believed to be nonthermal, and to which his theory might apply.

Believing that the redshifts (a changein frequency and wavelength toward the red portion of the spectrum) of the lines in the quasar spectra are due to velocity alone, astrophysicists use them as estimates of distance, and from such estimates they draw many important cosmological conclusions. Lines are reasonant emissions of particular chemical substances that stand out brightly against the rainbow. In the latest experiment, reported in the June 22 PHYSICAL REVIEW LETTERS, Mark F. Bocko, David H. Douglass and Robert S. Knox, also of the University of Rochester, show that lines from a partially coherent source can be shifted to the red or the blue according to Wolf's prediction.

After determining that Wolf's theoryapplies to acoustics as well as to optics, Bocko, Douglass and Knox set up an acoustical experiment with a partially coherent source because it was easier than an optical one. As radiators they used a pair of loudspeakers. They used electronic circuitry to produce two line-like shapes centered on 1,180 hertz and 1,020 hertz. One of these was split and fed to both speakers equally as a correlated component. The other was split, and half of it was inverted to form an anticorrelated component. Each half was fed to one of the speakers.

With two speakers operating together,the researchers heard a tone that was "redshifted' by 160 hertz from what they heard with each speaker operating alone. By changing the central frequency of the anticorrelated component, they could also produce a blueshift. This, they say, confirms Wolf's prediction.

The significance goes beyond astrophysics,as these developments seem to open a new way of modulating signals for communications technology. In communications, signals have been modulated by amplitude or by phase. Now it may be possible to modulate them by coherence relations. Wolf says he does not know whether such a procedure would have practical use in communications, but he and his colleagues are trying to find out.