Must it be local conservation? In other words can we rule out systems that push against something nonlocal or against everything in the universe at once? Edit:

Must it be local conservation? In other words can we rule out systems that push against something nonlocal or against everything in the universe at once? Edit: elsewhere in the thread you explain that yes, it must be local. So if this isn't experimental error or some other minor effect, it seems safe to assume that however it works it is not violating conservation of momentum. Either that or the universe is much weirder than we thought.