Re-examining magnetic tuning of Casimir forces

Abstract

arising from Y. Zhang et al. Nature Physics https://doi.org/10.1038/s41567-024-02521-0 (2024)

Casimir–Lifshitz forces have attracted much attention due to their relevance in controlling various phenomena^1,2 and as a manifestation of quantum fluctuations³. Lifshitz and colleagues proposed a theory connecting the response function of materials to an electromagnetic field with the magnitude of these forces⁴. According to this theory, precise control over the permittivities of interacting materials allows both the magnitude and the sign of Casimir forces to be tuned, a phenomenon that has been experimentally demonstrated^5,6. In principle, a similar tuning can be achieved by manipulating the permeabilities (μ) of the interacting materials at optical frequencies. However, this task becomes challenging when the contrast in permeability diminishes at much lower frequencies (approximately in the terahertz range)⁷. Zhang et al. reported the ability to tune Casimir forces using a magnetic field⁸. They demonstrated that the measured force between a gold sphere and a silica (SiO₂) plate interacting through aqueous suspensions containing magnetite (Fe₃O₄) nanoparticles underwent a reversal—shifting from attraction to repulsion at a specific separation distance—when a magnetic field was applied. However, we argue that both their theoretical framework and experimental data fail to substantiate this claim.