I have just read the paper of Wilson et al. Nature 479, 376–379 (17 November 2011), which seems to be a remarkable step beyond the status of the research concerned with dynamic Casimir effect as outlined in Dodonov, Phys. Scripta 82, 038105 (2010).
Given the fact that each photon carries momentum, I wonder what the momentum balance of a system undergoing dynamic Casimir effect is like.
This seems to be a non-trivial issue, as Dalvit er al. http://arXiv.org/abs/1006.4790v2 (2010) show that such effect may lead to effective friction (out of 'nowhere', i.e. out of vacuum fluctuations). Indeed, this makes sense as it is only total (electromagnetic plus mechanical) momentum which is conserved.
But this raises also the following natural question: is dynamic Casimir effect a net source of mechanical momentum? To put it in more paradoxical words: does Nature allow us to build a propeller without propellant? Recent press reports (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20140006052.pdf) seems to hint at that tantalizing direction. Just a fringe-science delusion?
@ Martila. On behalf of my employer, I used to be a debunker of hoaxes for many years (something like James Randi, on a much smaller scale). There's not much to respect in those who scam people for a living: this is why I am in no danger of doing 'the damned magic' :-). Even now, when I read press reports like the NASA report quoted in my Question, I cannot help thinking about possible debunking. EmDrive in its original version seems to be crap (possibly in good faith). All the same, I started collecting papers about Casimir effect, and such collection triggered further thoughts.
Here is a possible thought experiment.
If I perturb the position of just one out of two parallel walls I perturb the Hamiltonian of the electromagnetic field, both between the walls and outside them. If the perturbation has zero frequency, the net outcome is a static force (static Casimir effect). If the perturbation has finite frequency, then the theory of perturbations in quantum mechanics ensures that both the vacuum between the wall and the vacuum outside the wall are excited (at the expense of the energy source which drives the wall motion - say, a small motor). Excited states decay through emission of photons. As for the region outside the walls, photons escape, i.e. energy is radiated (dynamic Casimir effect) at the expense of the motor. These photons have been detected experimentally. Of course, photons carry momentum. Net momentum loss per unti time (i.e., net thrust) is non-zero unless radiation is isotropic. No matter is expelled.
Am I wrong?
@ Martila. The net loss of the entire system is zero, as the total momentum (electromagnetic + kinetic) is conserved. No expulsion of matter occurs.
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