Doppler effect takes a spin

Light's twistiness allows researchers to measure rotating object's speed

A twist on the physics that cops use to clock speeding drivers can determine how fast an object is spinning. The approach could be used to protect wind turbines from damaging winds, learn about distant astronomical objects or detect tornadoes.

The Doppler effect is familiar to anyone who has heard the pitch of a siren rise and fall as an ambulance whizzes past. It describes how the frequency of light waves (or the pitch of sound waves) emitted by a moving object becomes higher as the object approaches an observer and lower as it moves away. Today people exploit the Doppler effect to track the motion of raindrops, cars and galaxies.

But Doppler’s kryptonite has always been rotating objects that are neither moving toward nor away from the observer, like the second hand on a clock.

Physicist Martin Lavery at the University of Glasgow in Scotland and colleagues thought that they could measure rotational speed by determining how a spinning object changes light waves’ orbital angular momentum. In focused beams such as lasers, successive waves of light move like ocean waves on a beach: straight on, with periodic crests and troughs. But most light is messier: The waves wind around like a corkscrew.

Lavery’s team plated a plastic disk with aluminum foil and hooked it up to a spinning motor. Then the researchers bounced a light beam off the disk that had a special property: Its waves twisted either clockwise or counterclockwise.

When Lavery’s team measured the light after it bounced off the disk, they found that the frequency of waves twisting in the same direction as the disk was spinning had become higher, while the light twisting in the opposite direction had a lower frequency. The change allowed the researchers to calculate the disk’s speed of rotation, they report in the Aug. 2 Science.

“This is the first experimental evidence that this is possible,” says Bo Thidé, a physicist at the Swedish Institute of Space Physics in Uppsala.

Lavery and his team then did a similar experiment with ordinary light to prove that, even with waves twisting in all directions, they could take advantage of the Doppler change in angular momentum to measure the speed of the disk’s rotation.

Engineers could exploit the technique to protect wind turbines, which can get damaged by violent, swirling air currents. Lavery envisions a sensor on the nose of a turbine that would detect dangerous vortices and switch the turbine off.

Thidé says that weather radar could exploit the rotational Doppler effect to detect tornadoes before they reach the ground. Two years ago, he proposed a similar method to detect the rotation rate of supermassive black holes like the one lurking at the center of our galaxy (SN: 3/12/11, p. 14). “This experiment tells the rest of the world that there’s a lot to be gained by using this technique,” he says. “It will push people to go ahead and start to build applications.”