How does the "space drive" conserve momentum?

The readership of science fiction gets smarter over time, so making your SF "smart" (even if it's not hard SF) is an ever-moving target. This earlier question makes me think about Conservation of Momentum.

Just as every modern reader understands that there is no air on the moon and we need that to survive, certain fundamental principles are becoming more widely known, even when specifics of physics are not. In particular, the Second Law of Thermodynamics is the subject of a famous quote on the reasonableness of any proposed idea, and there are the basic conservation laws.

Many SF stories feature some kind of "space drive" that is propulsion in some manner that goes beyond present day physics or makes extensions/changes to the physics in the story's universe.

I submit that it would be "dumb" to handwave something that, without further clarification, violates the conservation of momentum. And, it would be "smart" to address this point in any description of the drive or the new physics, even if only to hang a bell on it.

Any new physics, as well as extensions to existing ideas such as gravity, will conserve momentum. It is quite fundamental: It is true because the laws of physics don't vary with your position in space, and laws can be expressed via a principle of least action. For purposes of this discussion, consider the latter to be equivalent to there being laws of physics as we understand the notion.

So, if an author were making up some kind of "space drive", how could it be made to conserve momentum? Related, what common ideas (like the gravity drive) have traps that need further work to produce a smarter idea in this respect?

I'm looking for various things that the author might use to handwave the problem in general or for certain classes of drive, as well as details for a sci-fi drive that is smarter and well thought out. Finally, details that lend themselves to being used as plot elements (rather than just paving over disbelief) are especially interesting.

posted almost 8 years ago

JDługosz‭

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