In 1971, Zel’dovich predicted that quantum fluctuations and classical waves reflected from a rotating absorbing cylinder will gain energy and be amplified. This concept, which is a key step towards the understanding that black holes may amplify quantum fluctuations, has not been verified experimentally owing to the challenging experimental requirement that the cylinder rotation rate must be larger than the incoming wave frequency. Here, we demonstrate experimentally that these conditions can be satisfied with acoustic waves. We show that low-frequency acoustic modes with orbital angular momentum are transmitted through an absorbing rotating disk and amplified by up to 30% or more when the disk rotation rate satisfies the Zel’dovich condition. These experiments address an outstanding problem in fundamental physics and have implications for future research into the extraction of energy from rotating systems.
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This work was supported by the UK EPSRC (grant no. EP/P006078/2) and the Horizon 2020 research and innovation programme of the European Union (grant agreement no. 820392).
The authors declare no competing interests.
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Photograph of the set-up showing the detail of the interaction region where the acoustic waveguides conduct the sound directly on to the absorber, supported by a plastic disk.
Microphone calibration: measurements of response when both microphones have no absorber placed in front of them, showing that the microphones are both calibrated and measure the same signal, as desired.
Microphone calibration: measurements of response when both microphones have absorbers placed in front of them, showing that the microphones are both calibrated and measure the same signal, as desired.
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Cromb, M., Gibson, G.M., Toninelli, E. et al. Amplification of waves from a rotating body. Nat. Phys. 16, 1069–1073 (2020). https://doi.org/10.1038/s41567-020-0944-3
Nature Physics (2020)