By exploiting a feature of light they discovered only a few years ago, University of Rochester physicists believe they've found a way to reduce the size and cost of many astronomical experiments. Scientists in India recently reported in the journal Optical Engineering that they have verified the theory using small light sources in the laboratory.
Emil Wolf, Wilson Professor of Optical Physics at the University of Rochester, and former student Daniel James, now at Los Alamos National Laboratory, will discuss their most recent work in this general area at the annual meeting of the Optical Society of America Friday, Oct. 7 in Dallas.
The new technique, which Wolf and James call space-coherence spectroscopy, would replace with just two quick measurements a tedious set of measurements astronomers now must obtain to learn about some celestial objects.
"The technology would give comparable results to what astronomers are doing now, but for a much lower cost," says James. Scientists would need less time and far fewer telescopes to do the same experiments, freeing up expensive equipment for other tasks, he says.
Wolf and James believe the technique could be used to observe thermal sources, such as stars and galaxies, but perhaps not non-thermal sources, such as quasars and pulsars. They say the technique would work for sources with broad-band radiation, which most astronomical sources have. Although the radiation reaching the earth is usually not broad enough because the atmosphere blocks out many colors of light, orbiting telescopes routinely detect broad-band radiation and could be equipped to take advantage of the finding, says Wolf.
While the correctness of the theory has been demonstrated in the laboratory, James says, "it's a big leap from the laboratory to astronomy. Even though our predictions have been verified in a nice clean laboratory, the real world is a different matter. There can always be experimental difficulties that you can't foresee."
The work is a direct result of a property of light discovered by Wolf in 1986 and now widely known as the Wolf Effect. Then Wolf found that the way in which atoms in a source are ordered affects the way they emit light and the way the light travels through space. For example, in a laser the photons are in step with one another and are said to be "coherent." In contrast, the photons from a light bulb or candle flame are emitted randomly and are incoherent. Light in between -- where some of the photons are in step and others aren't -- is partially coherent light, a subject about which Wolf is widely known in the optics community as the world's expert. It is this type of light that can cause spectral shifts.
Wolf and James say that two measurements of the spectrum of a source that emits partially coherent light tells how the photons emitted by that source are in step, giving a measurement known as the light's "correlation," from which astronomers can calculate the size and other features of the source. Detecting correlation of sources currently requires a long set of experiments.
The technique would be used in stellar interferometry, which scientists use to detect very small or faint stellar objects. Stellar interferometers typically consist of anywhere from two to a few dozen telescopes or mirrors scattered miles apart that effectively form one giant telescope by collecting light from a wide area and combining the signals to piece together the stellar source. Such facilities include the National Radio Astronomy Observatory's VLA, a group of 27 telescopes scattered across the Plains of St. Augustine in New Mexico, and the Australia Telescope, an array of 12 telescopes concentrated in New South Wales.
"We obtain information from all the frequencies at once," Wolf explains. "With current stellar interferometry astronomers must cut out most of the radiation because they work with a very narrow band of it. We look at all the radiation."
Wolf and James believe that by using the new information, the number of telescopes astronomers need to make certain measurements falls dramatically.
Some astronomers, however, are skeptical about some aspects of the theory because one feature of the proposed technique was discovered 20 years ago and is embodied in a formula known as the space-frequency equivalence theorem.
But James and Wolf say the objections are based on an incomplete understanding of their work. The Rochester team says that the idea of measuring spectra to determine the correlation between multiple broad-band signals of an interferometer is completely new. Using the old formulation, astronomers took measurements at hundreds of frequencies, one at a time; with the new method, just two readings of the spectrum yields the same information.
"It's much more rapid and efficient," says James.
Since Wolf first announced the result known as the Wolf Effect, other scientists have raised objections which he has shown to be irrelevant. Wolf and James have demonstrated in several papers (most recently in a May issue of Physics Letters A) that conditions commonly found in atmospheres can shift light spectra, and he believes the theory may explain some inconsistent quasar data. But Wolf remains skeptical of scientists who have adopted his work as evidence that the universe is not expanding (expansion is generally accepted among physicists).
The work by Wolf and James is finding other uses. Japanese scientists are using it to explore basic properties of light, and some astronomers are using it to study gravitational lensing. And Wolf believes the ability to modulate coherence properties of a beam may make it possible to develop a new technique for signal coding.
"Instead of FM (frequency modulation), AM (amplitude modulation), or PM (phase modulation), why not CM (coherence modulation)?" Wolf asks. "People are used to thinking of the two extremes, either completely incoherent light such as generated by a light bulb, or coherent light such as produced by a laser. But there is a whole world in between," says Wolf, whose work is funded by the Department of Energy, the U.S. Army, the National Science Foundation, and the New York State Science and Technology Foundation. tr