Scientists prove long-standing theory of wave amplification

Physicists at the University of Southampton have tested and proven a 50-year-old theory for the first time using electromagnetic waves. They have shown that the energy of the waves can be increased by bouncing “twisted waves” (those with angular momentum) off an object that is rotating in a certain way.

This is called the “Zel’dovich effect”, named after the Soviet physicist Yakov Zel’dovich who developed a theory based on this idea in the 1970s. Until now, it was thought that this effect was unobservable with electromagnetic fields.

“The Zel’dovich effect is based on the principle that waves with angular momentum, which would normally be absorbed by an object, are actually amplified by that object if it is rotating at a fast enough angular velocity. In this case, the object is an aluminium cylinder and it must be rotating faster than the frequency of the incoming radiation,” explains Dr Marion Cromb, a researcher at the University of Southampton.

“My colleagues and I successfully tested this theory on sound waves a few years ago, but until this recent experiment it had not been proven with electromagnetic waves. By using relatively simple equipment – ​​a resonant circuit interacting with a rotating metal cylinder – and creating the specific conditions required, we have now been able to achieve this.”

The scientists’ results are published in the journal Nature Communications .

The Zel’dovich effect is difficult to observe, but it is related to a well-known phenomenon called the Doppler effect that we all experience around us every day.

Imagine you are on a busy road and a police car is speeding towards you with its siren blaring. From your perspective, as it approaches, the siren sounds louder than when it passed.

This is because the sound waves in front of the car approaching you are compressed to a high frequency, thus to a higher pitch. Behind the car, as it moves away, they are more dispersed to a lower frequency, thus to a lower pitch. This is the Doppler effect.

This method can also be applied to light waves. Indeed, astronomers use it to understand whether a planetary body is moving closer to or further away from Earth, based on the frequency of light waves observed from their observation point.

A similar “rotational Doppler” frequency shift occurs for twisted waves and relative rotation.

In the Zel’dovich effect, the metal cylinder must spin fast enough that, from its perspective, it “sees” a “twisted wave” shifting in angular frequency, so much so that it actually shifts to a negative frequency. This changes the way the wave interacts with the cylinder. Normally, the metal absorbs the wave, but when the wave’s frequency “goes negative,” the wave is actually amplified, reflecting off the cylinder with more energy than when it approached it.

“The amplification condition is the rotational perspective of the object,” explains Marion Cromb. “The torsional electromagnetic fields that hit it have undergone a rotational Doppler shift, so much so (or so low) that they have passed through zero and entered a ‘negative’ angular frequency. A negative frequency then means negative absorption, and that means amplification.”

Scientists say the demonstration of the Zel’dovich effect in different physical systems, both acoustic and now electromagnetic, suggests that it is quite fundamental in nature. The electromagnetic tests also open the way to observing the effect at the quantum level, where the waves could be generated by the cylinder amplifying the quantum vacuum.

Professor Hendrik Ulbricht of the University of Southampton, who led the project, said: “I am very pleased that we now have experimental evidence for the electromagnetic Zel’dovich effect. In the electromagnetic setting, it will be easier to tackle the next big challenge, which is the quantum version of the effect.”

“Our setup is relatively simple and I was happy to set up this experiment and collect the first data during the COVID pandemic. Seeing the results now is very rewarding and I am grateful to the fantastic team involved.”

The researchers also say their findings could be useful to electrical engineers exploring improvements to induction generators, such as those used in wind turbines.