About interference, momentum and thrust....

[Dale]

Can you explain what would make the forces asymmetric in the sources if the fields are interfering on another region far from them?

The whole point of finding conservation laws is that the details are totally irrelevant. From the basic laws governing the system we know that momentum is conserved, regardless of the specific details.

If you don't have momentum leaving the system then both the magnitude and the direction of the momentum of the system is constant. That implies that, on average, any net force on one part of the system is balanced by an equal and opposite net force on another part of the system.

Do you understand that?

 

[sophiecentaur]

The whole point of finding conservation laws is that the details are totally irrelevant

I think that cala is just not recognising this.

If the sources are not inside the interference region,

For two coherent sources there is no "interference region". The interference is there over all space. If what you claim were true then directional antennae would not work over large distances (inter-galactic, even). By concentrating on the near field situation around your two sources, you are not grasping the whole problem and your conclusions are not valid. Actually, the same thing would apply if you consider tennis balls being fired from a serving machine. If you operate it in a box with just one hole in it, that hole will provide drive from the balls that happen to get out (possibly after bouncing around inside for some time).