Work from fluctuations
Can asymmetry and ratchet forces be used to extract energy from fluctuations?
This is not possible in thermal equilibrium due to time reversal symmetry, but allowed out of equilibrium (e.g. energy flux):
Random motions of bacteria: (R. Di Leonardo, et al PNAS 2010)
Proposal for a Contactless Heat Engine:
Use thermal pressure to stably levitate one cylinder inside another.
Use Non-reciprocal electromagnetic media that intrinsically break reflection (or time reversal symmetry):
[2022KITP program on "Emerging regimes and implications of Quantum and Thermal fluctuational electrodynamics"]
This symmetry breaking is similar to that by external magnetic fields (magneto-optical coupling), where
Asymmetry is quantified by the ratio of cyclotron frequency to a material (e.g. plasma) frequency
.
Propulsive force in the near-field limit:
Extract work from cylinder rotating at angular speed
against frictional force of vacuum
Maximum extracted power, at optimal rotation speed,
, limited by required thermodynamic (Carnot) efficiency.
"Near Field Propulsion Forces from Nonreciprocal Media,"
D. Gelbwaser, N. Graham, M. Kardar, M. Krüger, Phys. Rev. Lett. 126, 170401 (2021).
At least in this case, Casimir energy does not come free.
From the extracted power, the engine efficiency is given by
The velocity
cannot be increased indefinitely, due to
Frictional forces, which reduce the force
This leads to a final limit for efficiency at maximum power of