The theory is simply wrong, it neglects the electromagnetic pressure between the electromagnetic field and the tapered wall of the waveguide, it should produce the missing force component for a net force of 0.
It's impossible to arrive at a non-zero net Force with classical electrodynamics. It's theoretically proven that it conserves momentum.
Very possible reasons for measuring non-zero force:
> Systematic measurement errors.
> Leaking EM field carrying momentum.
> Energy channel from outside carrying momentum (failing measurements including battery mentioned in comments)
Unlikely explanation: All currently used theories of electrodynamics are wrong.
But explaining non-zero force from classical ED is simply impossible.
Also note that breaking conservation of momentum + relativity means that conservation of energy is broken as well.
edit: typo
edit 2:
I mean the theoretical approach presented in the paper makes obvious mistakes. I'm not arguing that breaking conservation of momentum is impossible (although I think it's really unlikely), I argue that even if it would happen it couldn't be described by classical electrodynamics (or even QED). We would have to come up with better laws for electrodynamics.
It's impossible to arrive at a non-zero net Force with classical electrodynamics. It's theoretically proven that it conserves momentum.
Very possible reasons for measuring non-zero force:
> Systematic measurement errors.
> Leaking EM field carrying momentum.
> Energy channel from outside carrying momentum (failing measurements including battery mentioned in comments)
Unlikely explanation: All currently used theories of electrodynamics are wrong.
But explaining non-zero force from classical ED is simply impossible.
Also note that breaking conservation of momentum + relativity means that conservation of energy is broken as well.
edit: typo
edit 2:
I mean the theoretical approach presented in the paper makes obvious mistakes. I'm not arguing that breaking conservation of momentum is impossible (although I think it's really unlikely), I argue that even if it would happen it couldn't be described by classical electrodynamics (or even QED). We would have to come up with better laws for electrodynamics.