Why thermal Casimir data favor the dissipationless plasma model
In plain words
The Casimir force between metals at room temperature depends on how electrons in the metal lose energy. Calculations using the standard description of a real, resistive metal disagree with most precision measurements, which instead match a description that ignores resistance.
Precise statement
Lifshitz theory for Au or Ni sphere-plate geometries at $T = 300\,\mathrm{K}$ and separations $2e-5\text{ to }1e-4\,\mathrm{cm}$: the Drude permittivity $\epsilon(\omega) = 1 - \omega_p^2/(\omega(\omega + i \gamma))$ removes the zero-frequency transverse-electric Matsubara term, while the plasma model ($\gamma = 0$) keeps it. Micromechanical, dynamic AFM and isoelectronic differential measurements agree with the plasma model and exclude Drude, whereas a torsion-pendulum measurement at larger gaps (Sushkov et al., 2011) favored Drude after subtracting electrostatic patch forces. An answer is a physical mechanism (spatial dispersion, surface states, low-frequency response) that reproduces the data while staying consistent with measured optical and dc conductivity, confirmed at separations above $3e-4\,\mathrm{cm}$ where the thermal term is large.
What would settle it
A measurement of the thermal Casimir force at gaps above 3e-4 cm with patch potentials characterized independently, plus a dielectric model that fits it and the metal's measured transport and optics.
Status in the literature
Unverified note
Nonlocal and surface-response resolutions were proposed in 2025 (J.-S. Wang, Physical Review B 111, 245404) and magnetic-metal data were reanalyzed in 2026 (Klimchitskaya, Korikov and Mostepanenko, Physics 8, 33, https://doi.org/10.3390/physics8020033); none is accepted.