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A Doppler twist. When light scatters off a rotating object, its orbital angular-momentum components split, shifting in frequency, or color.

Martin Lavery

A Doppler Effect for Distant Planets

An ambulance whizzes by, and suddenly its siren drops in pitch. We are all familiar with the Doppler effect, even if we don’t know it by name. Now, scientists have found an alternative version of the phenomenon, for when sound, or light, scatters off a rotating object. The discovery could enable astronomers to measure a distant planet’s rotation, or even improve the performance of wind turbines.

Here’s how the Doppler effect works: When a noisy object is moving toward you, its sound waves bunch up, producing a higher frequency, or pitch. Conversely, as soon as the object is moving away from you, the sound waves stretch out, and the pitch lowers. The faster the object, the greater the pitch change.

The Doppler effect occurs for light as well as sound. For instance, astronomers routinely determine how fast stars and galaxies are moving away from us by measuring the extent to which their light is “stretched” into the lower frequency, red part of the spectrum. Redshifts like this were famously used in the 1920s to infer that most stars and galaxies are moving away from us and that the universe must be expanding.

Redshifts in light, and the pitch-drop of passing sirens, are both examples of a linear Doppler effect. In recent decades, however, scientists have discovered that the Doppler effect also exists for objects that are rotating. In 1981, chemist Bruce Garetz at the Polytechnic Institute of New York University showed that if a rotating object emits circularly polarized light, that light’s frequency is shifted. Then in 1997, physicists Iwo Bialynicki–Birula at the Center for Theoretical Physics in Warsaw and Zofia Bialynicka–Birula at the Institute of Physics, also in Warsaw, came up with a variant of the rotational Doppler effect, exploiting the so-called orbital angular momentum (OAM) of light.

When light has OAM, its electric and magnetic fields rotate so as to give its phase a corkscrewlike twist as the light propagates through space. Iwo and Zofia Bialynicki–Birula showed that a rotating object emitting light would imbue that light with OAM; as a result, a measurement of OAM would enable someone to infer the object’s rotation. In a paper published online today in Science, however, physicist Miles Padgett and Martin Lavery at the University of Glasgow in the United Kingdom and colleagues from the University of Strathclyde, also in Glasgow, have gone one step further: demonstrating that light gets a Doppler shift in OAM simply by scattering off a rotating object.

The researchers made their discovery by shining a laser onto a fast-rotating plastic rotor. They collected the scattered light at a detector, which filtered it into two signals of positive (clockwise-rotating) and negative (counterclockwise-rotating) OAM. Although the rotor did not change the light’s total OAM, it did shift its positive OAM higher in frequency and its negative OAM lower in frequency. The researchers used this frequency separation to determine the speed of the rotor’s rotation.

Most objects don’t emit light; they scatter it toward our eyes from other sources, such as light bulbs or the sun. For this reason, Padgett suggests that the Doppler-shift of scattered light—which should also be generated in sound waves—could find useful applications. For example, astronomers could use it to determine the rotation of distant planets, he says. Back on Earth, he adds, lasers could be sent through wind farms to determine the rotation of air currents. “If you’re a windmill, you’d like to know in advance of blustery wind, so you can tether your blades.”

Engineer Alan Willner at the University of Southern California in Los Angeles applauds the fact that “fundamental” discoveries about waves are still being made—particularly with light’s OAM, which itself was discovered only some 20 years ago. “[Padgett] has kindled this excitement throughout the scientific community about this newly understood property of light,” ​he says.