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Any apparently reactionless drive is treated with skepticism by the physics community because a truly reactionless drive would violate the law of conservation of momentum.
Shawyer claims that his drive does not violate conservation of momentum and is not reactionless.[9] Shawyer has posted an updated theory paper (version 9.4) for the EmDrive.[17] Shawyer's paper includes the fundamental assertion underlying the theory: "[t]his force difference is supported by inspection of the classical Lorentz force equation F = q(E + νB). (1) If ν is replaced with the group velocity νg of the electromagnetic wave, then equation 1 illustrates that if vg1 is greater than vg2, then Fg1 should be expected to be greater than Fg2."
This statement makes two assumptions which Shawyer does not substantiate and which may explain the discrepancy between Shawyer's predictions and those of conventional physics. First, Shawyer assumes that radiation pressure is the result of the Lorentz force acting on charged particles in the reflecting material. This is analyzed by Rothman and Boughn[18] who point out that the standard theory of radiation pressure is somewhat more complicated than the simplified analysis suggests.
Second, Shawyer asserts that quantum energy is transferred at the group velocity, and thus momentum of the photon and the consequent radiation pressure must vary with group velocity. Photon momentum varies with phase velocity. Group velocity measures the rate of propagation of information. The phase velocity is constant throughout the frustum resonator, consequently radiation pressure would not be expected to produce unbalanced forces.

http://en.wikipedia.org/wiki/EmDrive

Various hypotheses and theories have been proposed explaining the underlying physics for how the EmDrive and related designs might be producing thrust. Shawyer claims that thrust is caused by a radiation pressure imbalance between the two faces of the cavity caused by the action of group velocity in different frames of reference within the framework of special relativity.[19] Yang from NWPU calculated the net force/thrust using classical electromagnetism.[12]
Harold G. "Sonny" White, who investigates field propulsion at Eagleworks, NASA's Advanced Propulsion Physics Laboratory, speculated that such resonant cavities may operate by creating a virtual plasma toroid that could realize net thrust using magnetohydrodynamic forces acting upon quantum vacuum fluctuations.[20]
Likewise, the paper describing the Eagleworks test of the Cannae drive referred to a possible interaction with a so-called "quantum vacuum virtual plasma".[14] This reference has been criticized by mathematical physicists John Baez and Sean M. Carroll because in the standard description of vacuum fluctuations, virtual particles do not behave as a plasma.

http://en.wikipedia.org/wiki/EmDrive

Sounds great, but any trips to stars might be tricky. Hitting ANY object - even something much smaller than a grain of sand - at those fractional light speeds might release spectacular amounts of energy that no physical shielding can protect the ship from. We THINK that interstellar space is “empty” but it certainly isn’t COMPLETELY empty. So, now we need “shields”.