I first heard about the "EM Drive" from a (not good) television series a few years back - Salvation. The EM Drive was - there - one way around that jerk's equation. I didn't think too hard about it then.
As you know I've acquired an interest in propulsion this year, non-Tsiolkovsky (aerofoils, ramjets, solar-sails...) being a sideline. So I looked into this EM Drive again. Maybe it's some sort of Pluto Ramjet type thingy, using propellant from the local ambience. I next figured said ambient propulsion be an iron planet's magnetic field - somehow. For the sake of the story I was prepared to handwave it.
But nah. It's MAD Magazine type thingology. It gets its propulsion from magnetising... itself. Momentum is not conserved. Instinctually, I like physics to work. This thread has some good comments by Carl Pham and hairless_joe on why momentum should be conserved. Most forcefully David Gillies, bringing up Emily Amalie Emmy Noether.
Meanwhile I've also heard about chemical structures that fluctuate through time. They are, thereby, time crystals. NicoNicoNekomancer moots to me that these crystals, also, do not conserve momentum.
I point out here that I do not take the "EM Drive" seriously, myself. Salvation is inferior to When Worlds Collide inasmuch as the Orion is feasible. I literally take Alcubierre more seriously than EM, if we're doing science fantasy. Hence why this is my first post about it.
I'll let the smarter physicists figure all this out before I get into it. Currently I agree with Elon Musk: we must be rocket engineers. Rocket "science" needs to give us something that we can build.
RECORDING 2/10/21: Time crystals again. Room temperature.
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