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Comment by lordnacho | Hacker News Reader

If you draw the boundary enclosing the power source, you still have a closed system for which conservation laws ought to work, so long as nothing travels through the boundary. It's not having no source that makes it closed, it's having a boundary that leads to all the sums having to add up.

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So traditional thrusters, which also convert energy into momentum, shouldn't work either?

Conservation of momentum only applies to mechanical processes, such as force and collisions. Converting energy breaks out of those bounds, and so it no longer applies - and the emdrive breaks no physics.

At least, that's how I've always understood it.

They eject something from the system.

Rather the closed system includes both the craft and the ejected mass.

If you include the ejected mass, then momentum is conserved in that system. The ejected mass goes one way, the rocket goes the other. The momentum of the rocket is equal and opposite to the momentum of the ejected fuel.

Not true for the EmDrive.

No, momentum is simply outright conserved in the same way that energy is conserved. There are the two simplest to derive conservation laws, stemming from spatial symmetry (momentum) and time symmetry (energy).

These relationships are derivable via Noether's theorem, one of the most foundational results in physics.

https://en.wikipedia.org/wiki/Noether%27s_theorem

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