Amplification of electromagnetic fields by a rotating body

Nature Communications volume  15, Article number: 5453 (2024 ) Cite this article

In 1971, Zel’dovich predicted the amplification of electromagnetic (EM) waves scattered by a rotating metallic cylinder, gaining mechanical rotational energy from the body. This phenomenon was believed to be unobservable with electromagnetic fields due to technological difficulties in meeting the condition of amplification that is, the cylinder must rotate faster than the frequency of the incoming radiation. Here, we measure the amplification of an electromagnetic field, generated by a toroid LC-circuit, scattered by an aluminium cylinder spinning in the toroid gap. We show that when the Zel’dovich condition is met, the resistance induced by the cylinder becomes negative implying amplification of the incoming EM fields. These results reveal the connection between the concept of induction generators and the physics of this fundamental physics effect and open new prospects towards testing the Zel’dovich mechanism in the quantum regime, as well as related quantum friction effects.

Electromagnetic (EM) wave amplification from a rotating cylinder was predicted by Yakov Zel’dovich in 19711. His proposal is illustrated in Fig. 1a - a wave with angular momentum reflecting off a rotating and absorbing (e.g., metallic) cylinder will be amplified if the rotational Doppler shifted frequency of the incoming wave becomes negative (in the frame of the rotating cylinder)1,2,3. Negative frequencies or energies in a rotating system had already been pointed to lead to amplification by Penrose in the context of rotating black holes: particles falling into a black hole will acquire a negative energy as they pass through the ergosphere (point at which the spacetime drag velocity becomes larger than the speed of light)4. Penrose’s reasoning points out that if the particle or mass splits so that part of the mass escapes or does not fall in, then this must gain energy in order to compensate for the negative energy of the part that falls into the black hole. In Zel’dovich’s proposal, the black hole is replaced by a rotating cylinder but this does need not rotate faster than the speed of light. Rather, through purely classical calculations based on Maxwell’s equations, Zel’dovich predicted the amplification of EM waves with frequency ω and angular momentum ℓ when

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