[avian]

Way back in another life I did some calculations similar to that.

I don't remember details anymore, but there were some observations that some (deep space) astronomical object somewhere was accelerating and there were no models at the time that would explain that. I tried to explain it by a magnetic dipole in an non-homogeneous interstellar magnetic field.

If you know the gradients and the dipole moment you can calculate the force on the dipole.

I think it required a magnetar-level dipole to get some non-trivial forces out of the interstellar field. Anyway, I don't know if that theory ever got anywhere. Probably not. I never got any calls back from the people I presented that work to.

A spaceship would have a much lower mass than a star or whatever I was calculating that for, so I guess the requirements would be lower. Still, my gut feeling would be a magnet much beyond what you can do with today's superconductors. It would also only allow a one-way travel along the local field gradient. No round trips.

[api]

Hmm... so bottle a magnetar and you get apparently (but not really) "massless" propulsion. All we have to do is bottle a magnetar... umm... without the magnetic field ripping the spacecraft and anything in it to shreds... and do it with quantities of energy small enough to control (and dissipate waste heat) without melting the spacecraft into a blob of liquid metal. Got it. :)

So slightly more imaginable than the Alcubierre drive in that it does not break known laws of physics or require vast quantities of unobtainium (negative mass, lots of antimatter, etc.), but completely impractical with current or foreseeable technology. Might also be fundamentally impractical since the energy/mass requirements to generate a field powerful enough to overcome gravity and other forces acting on the spacecraft might be too large... the thrust/weight ratio math might not work even for micro-gravitational fields.

[whatshisface]

>It would also only allow a one-way travel along the local field gradient. No round trips.

Turn the magnet around?

[avz]

As with the compass, the orientation of your magnetic dipole against the field is unstable, so now you need a second propulsion system for maintaining the unstable orientation of the dipole.

[whatshisface]

You don't need a second propulsion system, you could use reaction wheels[0].

[0] https://en.wikipedia.org/wiki/Reaction_wheel

[avz]

Reaction wheels saturate and are therefore more suited for precise attitude control than as a source of sustained torque. Once they saturate you need to unload the accumulated momentum via some other mechanism. Thus, you do need a second propulsion system such as thrusters, light sail etc.

[whatshisface]

You can take the momentum back out of the wheels by rolling the magnet to the other side of the unstable equilibrium so that the wheels have to accelerate in the opposite direction to keep it steady.