“But in a vacuum (e.g. space), there is nothing to push against. You have to carry everything with you, pusher and pushes.”
Can you measure a change in momentum in a vacuum? How about velocity?
Even if the space between celestial objects is a vacuum, there are objects to push against and to provide a frame of reference to measure velocity (which is relative to the frame of reference).
A laser can project a beam from a spacecraft and will change the craft’s momentum when the beam hits some other object. The momentum of both the craft and the other object will be changed slightly even though the laser beam has no mass.
For a craft to be propelled by an energy field without “pushing” against some other object, it would require being able to dynamically alter the curvature of space the way gravity and acceleration do (statically). This would probably rely on a relationship between electromagnetism and gravity that is not currently understood by modern science.
Could a switchable gravity “circuit” operate without violating the laws of conservation of momentum and mass?
Perhaps. If space is curved, time will also be dilated. I would expect this curvature and dilation to be symmetrically mirrored when the circuit is turned off. I think this is essentially what happens with radio waves. The change in the electrical field through a wire induces a magnetic field at a right angle to it but in the path of least resistance.
In a static case (i.e. DC), the path of the magnetic field follows the right-hand rule and forms a field around the electrical field. But in alternating current, the direction of the magnetic field is constantly reversing which causes the magnetic lines of flux to be momentarily perpendicular to the electrical field (with the magnetic field alternating polarity just like the electrical field).
Whether the invention claimed here is real, I think the idea is feasible without breaking the laws of physics. Sometimes inventions and technologies are discovered without fully understanding the laws of nature being exploited.
But these are extraordinary claims requiring extraordinary proof.
> ... I think the idea is feasible without breaking the laws of physics...
That would be the way I could accept it as possible. It may be occasionally necessary to, shall we say "extend", the laws to encompass a verified but heretofore unobserved phenomenon, but "break" or "defy" the existing laws? Well, as you stated:
> "... these are extraordinary claims requiring extraordinary proof..."