Scientific Theory Confirmed After Decades

In a study published in Nature Communications, physicists at the University of Southampton tested and proved a 50-year-old theory for the first time using electro-magnetic waves.

The researchers demonstrated that the energy of waves can be amplified by bouncing "twisted waves" — waves with angular momentum — off a rotating object.

This phenomenon, known as the "Zel'dovich effect," is named after scientist Yakov Zel'dovich, who proposed the theory in the 1970s. Until now, it was believed to be unobservable with electromagnetic fields.

The Zel’dovich effect works on the principle that waves with angular momentum, that would usually be absorbed by an object, actually become amplified by that object instead, if it is rotating at a fast enough angular velocity. In this case, the object is an aluminum cylinder and it must rotate faster than the frequency of the incoming radiation.

Dr. Marion Cromb, Research Fellow, University of Southampton

Cromb continued, “Colleagues and I successfully tested this theory in sound waves a few years ago, but until this most recent experiment, it hadn’t been proven with electromagnetic waves. Using relatively simple equipment–a resonant circuit interacting with a spinning metal cylinder–and by creating the specific conditions required, we have now been able to do this.”

The Zel'dovich effect, though challenging to observe, is related to the familiar Doppler effect, which we encounter regularly.

For example, when a police car with its siren on speeds toward a person, the pitch of the siren sounds higher as it approaches and lower as it moves away. This occurs because the sound waves in front of the car are compressed, increasing their frequency and pitch. As the car moves away, the sound waves become more spread out, lowering the frequency and pitch. This phenomenon is the Doppler effect.

The same principle applies to light waves, which astronomers use to determine whether a celestial body is moving toward or away from Earth by analyzing the frequency shift of light waves. Similarly, "twisted waves" and rotating objects experience a "rotational Doppler" frequency shift.

In the Zel'dovich effect, a rotating metal cylinder must spin fast enough to "see" the twisted wave's angular frequency shift into the negative range. This change in frequency alters how the wave interacts with the cylinder. Normally, the metal would absorb the wave, but when the wave's frequency turns negative, the wave is amplified, reflecting off the cylinder with more energy than it had when it first approached.

Crumb added, “The condition for amplification is from the rotating perspective of the object. Twisting electromagnetic fields hitting it have become rotationally Doppler shifted, so much (or so low) that they’ve gone through zero and into a ‘negative’ angular frequency. Negative frequency then means negative absorption, and this means amplification.”

The scientists suggest that demonstrating the Zel'dovich effect across various physical systems, including acoustics and now electromagnetic circuits, shows that the phenomenon is fundamental in nature. Electromagnetic experiments also enable the observation of the effect at the quantum level, where waves generated by the rotating cylinder amplify the quantum vacuum.

I am very pleased that we have now experimental proof of the electromagnetic Zel’dovich effect. In electromagnetic settings it will be more straight forward to go for the next big challenge, which is the quantum version of the effect.

Hendrik Ulbricht, Professor University of Southampton

Ulbricht added, “Our setup is comparably simple, and it was my joy at work during COVID to set up this experiment and take the first data. To see the results out now is very rewarding, and I am grateful to the fantastic team involved.”

The researchers further propose that their findings could assist electrical engineers in improving induction generators used in wind turbines.

Journal Reference:

Braidotti, C, M., et al. (2024) Amplification of electromagnetic fields by a rotating body. Nature Communications. doi.org/10.1038/s41467-024-49689-w