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Inadequacy of the Casimir force for explaining a strong attractive force in a micrometre-sized narrow-gap re-entrant cavity

Nature Physics volume 21, pages 1714–1716 (2025)Cite this article

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Matters Arising to this article was published on 21 October 2025

The Original Article was published on 03 August 2020

arising from J. M. Pate et al. Nature Physics https://doi.org/10.1038/s41567-020-0975-9 (2020)

Pate et al.1 investigated a macroscopic optomechanical system with a narrow-gap re-entrant cavity coupled to a silicon nitride (SiN) membrane resonator coated with gold (Au) or niobium (Nb). They observed a large increase in the membrane’s effective spring constant keff for sub-2-μm gaps x. This increase scales roughly with x−4, suggesting an attractive force pulling the membrane towards the re-entrant aluminium (Al) post, with an x−3 dependence. A calculation based on the proximity force approximation (PFA) reveals that the Casimir force, at the investigated gap sizes, is orders of magnitude weaker than the observed force. This large discrepancy necessitates an alternative explanation for the observed attraction.

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Fig. 1: Casimir spring constant as a function of gap size.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

I acknowledge discussions with A. Cassinese.

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  1. Dipartimento di Fisica E. Pancini, Universitá di Napoli Federico II, Naples, Italy

    Giuseppe Bimonte

  2. INFN Sezione di Napoli, Naples, Italy

    Giuseppe Bimonte

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  1. Giuseppe Bimonte
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Correspondence to Giuseppe Bimonte.

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Bimonte, G. Inadequacy of the Casimir force for explaining a strong attractive force in a micrometre-sized narrow-gap re-entrant cavity. Nat. Phys. 21, 1714–1716 (2025). https://doi.org/10.1038/s41567-025-03062-w

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