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Stingray comments on Open Thread, Jul. 27 - Aug 02, 2015 - Less Wrong
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I would give at least .00001 probability to the following: momentum per se is not conserved, but instead some related quantity, call it zomentum, is conserved, and momentum is almost exactly equal to zomentum under the vast majority of normal conditions.
In general, since we can only do experiments in the vicinity of Earth, we should always be wondering if our laws of physics are just good linearized approximations, highly accurate in our zone of spacetime, of real physics.
This is not a very meaningful claim since in modern physics momentum is not "mv" or any such simple formula. Momentum is the Noether charge associated with spatial translation symmetry which for field theory typically means the integral over space of some expression involving the fields and their derivatives. In general relativity things are even more complicated. Strictly speaking momentum conservation only holds for spacetime asymptotics which have spatial translation symmetry. There is no good analogue of momentum conservation for e.g. compact space.
Nonetheless, the EmDrive drive still shouldn't work (and probably doesn't work).