TTex600:
You can't have an equal and opposite reaction without something to push against.
Absolutely NOT true, and the obvious example is a rocket engine in a vacuum. The thrust is the result of the mass flow of gas and its exit velocity, and does not require "something to push against".
So the example of the piston in a cylinder has no relevance to the propulsive utility of a jet or rocket. The piston
does not propel the aircraft; its mechanical linkage (connecting rod, crankshaft, perhaps a gearset) is to a propeller, which
does.