A 50-year-old idea about electromagnetic waves known as the Zel’dovich effect has been tested by physicists in the lab, and proven to be correct. 

The idea behind the Zel’dovich effect came from a strange place. In 1969, British physicist and mathematician Roger Penrose suggested that energy could be extracted from black holes by lowering an object into the ergosphere (the region just outside of the event horizon) and allowing it to accelerate the object, stealing some of the black hole’s energy. The idea, known as the Penrose process, requires negative energy to be acquired by the object in order for it to be recovered from the black hole – otherwise, all you’d be doing is feeding the black hole.

We don’t have any conveniently close black holes to play with (probably thankfully), but a few years later, Belarusian physicist Yakov Zel’dovich came up with a far more practical way to test the concept of stealing extra energy from a rotating system. 

The idea has links to the doppler effect, which can make light appear red or blue shifted depending on how the emitting object is moving relative to us, as well as the rotational doppler effect.

“The linear version of the doppler effect is familiar to most people as the phenomenon that occurs as the pitch of an ambulance siren appears to rise as it approaches the listener but drops as it heads away. It appears to rise because the sound waves are reaching the listener more frequently as the ambulance nears, then less frequently as it passes,” study author Dr Marion Cromb, now Research Fellow at the University of Southampton, explained in a statement concerning a previous study.

“The rotational doppler effect is similar, but the effect is confined to a circular space. The twisted sound waves change their pitch when measured from the point of view of the rotating surface. If the surface rotates fast enough then the sound frequency can do something very strange – it can go from a positive frequency to a negative one, and in doing so steal some energy from the rotation of the surface.”

To test the idea, the team previously bounced sound waves off a spinning disc, and listened for a shift in frequency indicating that energy had been gained from the disc’s rotation. Now, the team has conducted the experiment using electromagnetic waves.

“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 aluminium cylinder and it must rotate faster than the frequency of the incoming radiation,” Cromb said in a statement on the new study.

“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.”

To conduct the experiment, the team needed to rotate the aluminum cylinder so fast that, from its perspective, it sees a twisted wave shifted in angular rotation with a “negative frequency”. 

“The condition for amplification is from the rotating perspective of the object,” Cromb explained. “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 team hopes to investigate the phenomenon at the quantum level.

“These findings open the way to the merging of ideas from two previously disconnected fields,” the team concludes in their paper. “In particular, a suggestive prospect is the realisation of Zel’dovich electromagnetic amplification from a rotating body in the quantum regime, i.e., the generation of photons out of the quantum vacuum stimulated by a mechanical rotation.”

The study is published in the journal Nature Communications.