EM drive actually works???? EAT THAT NEWTONIANS!

[Anomalocaris]

Because it seems to violate conservation of momentum.

In a solar sail, net momentum is supplied by incoming photons. The momentum is transferred to the solar sail when the solar sail either absorb or reflect the photons. So momentum is conserved in a solar sail.

In other forms of propulsion, reaction mass is used to creat thrust. The spaceship gains momentum as the thrust accelerates the ship, but the gained momentum is precisely cancelled by the movement of reaction mass in the opposite direction, thus net momentum is conserved.

EM drive suggests radiation bouncing inside a completely enclosed and sealed vessel can produce a net thrust on the vessel. Nothing is bringing momentum into the system. The but alleged thrust on the vessel would accelerate the vessel, giving it momentum. but since no reaction mass leaves the vessel to cancel the momentum gained by the vessel, momentum is not conserved.

Saying EM radiation can create momentum were none existed before does not only go against overwhelming empirical evidence that momentum is conserved in all reactions hitherto observed, it also goes against maxwell's equation of how EM radiation works. So far there is no other hint that things are amiss with accuracy and precision of maxwell's equation. So it would be surprising if implicit momentum conservation of maxwell equation turn out to be in error.

Gravity moves things "for free," and we don't accuse it of breaking the laws of momentum. So does magnetism in a way.

That's because actions of gravity and magnetism are not for free, nor do they change the total net momentum of the entire system of objects upon which they operate.

Gravity and magnetism are not free because their actions merely exchange potential energy and kenetic energy in a system, and do not change total energy. They don't give you net energy out of nothing. If the system has no available energy to start with, then neither gravity nor magnetism, no matter how strong, can cause any action to happen. Thus gravity and magnetism observe conservation of energy.

Even when they cause things to happen they do not violate conservation of momentum because for every force and action they cause in one object, they excert an equal and opposite force in another, and also cause an action in the other object that exactly zero out total changes to momentum of the system. So sum of all momentum in a system before and after any action of magnetism and gravity always remains the same. Thus momentum is also conserved.