The EmDrive is actually <i>less</i> physically plausible than the E-Cat if you get into it, since reactionless propulsion is identical to perpetual motion and&#x2F;or infinite energy. Someone had a nice explanation of this over on Reddit but I don&#x27;t have a link handy. Basically if momentum is not conserved then you can build kinetic energy without bound on finite power or something like that. The only way the EmDrive could work is if either (a) we are <i>really</i> wrong about some fundamental stuff, or (b) it&#x27;s not <i>actually</i> reactionless but is pushing against something &quot;exotic&quot; like dark matter or dark energy.<p>E-Cat or anything like it (LENR) would mean new nuclear physics -- some way of bypassing the Coulomb barrier at low energies via some novel probably quantum mechanism. But fundamentally it&#x27;s not violating anything as foundational as a conservation law. We know the energy is there via E=mc^2 and all. We just don&#x27;t <i>currently</i> know of any way to get at that energy at sub-plasma temperatures and pressures (and for the proposed fusion reaction in the E-Cat, by no known physical means). LENR would require new physics but would not break existing physics.<p>I do recall reading some interesting stuff about how some of the gas giant planets seem a bit warmer than they should be, and that if LENR were a thing it would provide a possible explanation for some anomalous heating observed in astronomical observations.