A University of Rochester professor has extended his theory challenging a traditional tenet of modern physics -- the cause of spectral shifts. The work by Emil Wolf, Wilson Professor of Optical Physics in the College of Arts and Science, could have repercussions in many areas of science and suggests our view of the universe may need amending. Already, other scientists across the globe are applying his theory to longstanding problems of physics.
Wolf's latest paper, "Correlation-Induced Doppler-Like Frequency Shifts of Spectral Lines," will appear in the Nov. 13 issue of Physical Review Letters. It represents an expansion of a principle he described in earlier work. In the new paper, Wolf shows how something other than gravity or the Doppler Effect can produce dramatic changes in the behavior of light or sound.
Scientists have known for decades that light from different sources has different spectral patterns, and that these patterns can help identify the nature of a source. By studying the spectral patterns emanating from a star, for example, astronomers can determine its chemical composition.
Scientists have also known that light spectra can be shifted: the spectral pattern that is characteristic of a certain chemical at a certain wavelength, for example, can be shifted to shorter (bluer) or longer (redder) wavelengths. Scientists have long assumed that only two mechanisms could account for such spectral shifts: motion (as seen in the Doppler Effect) and gravity.
Three years ago Wolf theorized a new cause for spectral shifts. What has come to be known as the "Wolf Effect" or "Wolf Shift" has since been confirmed several times experimentally. In his latest paper, Wolf shows that the Wolf Effect can shift spectral lines as completely as the Doppler Shift. In addition, the shift is consistent -- each line within a spectrum is shifted in the same proportion.
"This mechanism can exactly imitate the Doppler Shift, using known principles of physics and optics," Wolf says. "The redshift can be as large or as small as you want -- in principle, it can have any magnitude."
Wolf's theory could have consequences in a wide range of areas, including astronomy, standards specification, coding of signals, and satellite tracking. The latest paper is one of more than a dozen Wolf has published on the subject in such journals as Physical Review Letters, Physical Review, Nature, Optics Communications, and Journal of the Optical Society of America.
Previously, Wolf showed that the manner in which atoms in a light source are ordered (source correlation) affects the way these atoms emit light and the way this light travels through space. When the fluctuations, or light emissions, from these atoms are neither fully ordered, or coherent (as in a laser), nor fully incoherent (as in a light bulb or candle flame), shifts can occur. Spectral shifts occur in the realm of partial coherence.
To explain partial coherence, Wolf uses this analogy: "Picture a contingent of 1000 soldiers walking across a bridge," Wolf says. "If all the soldiers are walking in step, that's an example of coherence. If they are all drunk and wander randomly across the bridge, that's an example of incoherence. But if some walked in step, and some wandered across, that's an example of partial coherence."
Instead of studying the light emitted by an initial source, in his latest paper Wolf looks at the way atoms in a secondary medium emit light after they are hit by light. The coherence of such "secondary" light can vary much more than "initial" light and creates the possibility of larger shifts.
Despite confirmations in the laboratory, it is not known whether and under what conditions the Wolf Effect occurs in nature. Wolf and several associates are continuing their investigations. A graduate student, Daniel James, has developed a model of a medium giving rise to such shifts. Whether such media exist in nature is now being studied by Wolf, James, and University astronomer Malcolm Savedoff.
Applications of this technology include a new method of coding information transmitted by light, sound, or radio waves. Also, several scientists from India recently suggested that the Wolf Effect may account for some puzzling differences which national laboratories of different countries have found in connection with the establishment of certain standards based on optical measurements.
Wolf's work is supported by the National Science Foundation and the U.S. Army.
Spectral lines can either be "redshifted" toward the lower frequencies at the red end of the spectrum, or "blueshifted" toward the higher frequencies at the blue end of the spectrum. The Doppler Effect explains how shifts can occur because of motion: as an object moves away from an observer, the spectrum of light emitted from the object is "redshifted"; similarly, as the object approaches the observer, the spectrum is "blueshifted." An example of the Doppler Effect (in sound instead of light waves) is the change in pitch of a train whistle as the train approaches and then moves past an observer: the pitch is higher until the train passes, then has a lower pitch as it moves away.
Scientists have long believed that only the Doppler Effect or gravity as described by Einstein could account for shifts in the spectrum of light as it travels through space. In all cases not affected by gravity or motion, scientists have assumed "spectral invariance" -- that is, the spectrum is thought to remain exactly the same irrespective of how far the light travels. Such is the case with light from ordinary sources; this may explain why no one has seriously studied the question before.
Three years ago Emil Wolf first theorized a new cause of spectral shifts and said some light sources could produce redshifts without moving away from the observer and without the effect of gravitation. When the effect was demonstrated in several laboratories, some scientists called it "trivial" because it could only lead to very small shifts. Wolf's most recent paper shows that this mechanism can completely imitate the Doppler shift to any magnitude.
While the work has many practical applications, probably the area where it has received the most attention concerns its implications for the study of quasars, high-energy objects which dot the heavens. Many quasars display a high degree of redshift, leading astronomers employing traditional assumptions about the causes of redshift to presume the quasars must be moving away from the Earth at tremendous speeds -- some nearly as fast as the speed of light. If Wolf's theory is correct, it's possible that scientists have miscalculated the age of the quasars and their distance from Earth -- long considered an important yardstick for measuring the size of the universe.
But Wolf himself thinks that current judgments about most quasars and about the age and size of the universe that are based on traditional theories of redshift are probably reasonably accurate. However, he notes, his theory could account for the discrepancies found by some astronomers, such as Halton Arp, that some quasars appear to be linked to galaxies known to be closer to Earth than the quasars are thought to be. Despite an apparent link, the amount of "redshift" of the two objects is very different. For this reason most astronomers conclude that the quasars and the galaxies are really very far apart, leaving the apparent links unexplained.
Emil Wolf, the world's leading expert in the theory of partial coherence, is utilizing more than four decades of experience studying light as he examines the canons of physics in a way no one has done before. While his results surprise many physicists and leave many astronomers skeptical, Wolf is confident of his findings.
Wolf's theory has begun to spark interest in the astronomical community because of the implications stated above. He has spoken to physicists and astronomers around the country and abroad; within the astronomy community, he has found radio astronomers most open to his theories because they are often well-acquainted with coherence theory, upon which his theories are based.
Wolf is past president and one of only nine honorary members of the Optical Society of America. The winner of several international optics prizes, Wolf co-authored with Nobel laureate Max Born the book, Principles of Optics, which is now in its sixth edition and has been translated into several languages, including Russian, Chinese and Japanese. Wolf is also the editor of Progress in Optics, an ongoing series of 27 volumes keeping pace with the field's developments. He joined the University of Rochester faculty in 1959.
tr