NASA Funded Scientists Claim Breakthrough in Propellantless Propulsion

It's worth noting that if a propellentless drive works, it will probably generate free energy. This may adjust your expectations on how likely this idea is. How likely is a free energy device (ie, one that reliably gives you more energy out than you put in)? Well, this thruster can't be any more likely, because it would produce free energy.

https://arxiv.org/abs/1506.00494

> the kinetic energy of a mass with a constant force applied will increase with the square of time, while the electrical energy input only increases linearly with time. A mass with such a drive attached would eventually have a kinetic energy greater than the energy input.

...

> For example, an evacuated track 10km long and supplied with 1 GW of power that is fed via induction along the track to a 100 kg mass equipped with an EM drive would result in the mass experiencing an acceleration of 4000 m/s2(i.e., 407 g’s) and having a velocity of 8.94 km/s at the end of the track, at which point the mass would have a kinetic energy of 4 GJ. The time to accelerate the mass is only 2.23 seconds,however, so the energy input required is only 2.23 GJ. At the end of the track, the mass could be decelerated via regenerative braking, generating more energy out (4 GJ) than was input (2.23 GJ), for a net gain of 1.76 GJ free energy.

See

https://en.wikipedia.org/wiki/Dean_drive

particularly this article

http://www.rexresearch.com/dean/stine.htm

The EMdrive also had a "why." Note, if you investigate these sorts of devices, they typically have a why. It's just that the why tends to ignore a bunch of other things and it's a disappointment in the end.

Really, if you're curious, you should dig into the history of all kinds of devices like these, anything in the Impossible Class. The inventor spends decades grinding away at it, getting inconsistent, odd little results which are just tantalizing enough to keep them stumbling forward.

I have often considered a taxonomy of these sorts of enterprises, with three orthogonal systems of reckoning. The first, a type really, is what principle is being violated (Second Law of Thermodynamics, Newton's Third ...). The second is psychological: misunderstanding, self-deception, or duplicity (typically to bilk inventors), with the understanding that the inventor typically starts with misunderstanding and proceeds onward. The third would, finally, be the discovered actual source of the odd results, which could be bad experimental design, not accounting for oddly-radiating services, vibration, friction, and so forth.

Not precisely free; it moves energy around(1) by interacting with the inertial mass of other objects in the galaxy and applying force to them at a distance (the details of how, which objects, and how much force, at least in the Wikipedia summary, are really sparse).

In theory, if we could run an unbreakable string from Earth to Mars and twist it around a spindle, the differing orbits of Earth and Mars would pull that string taut and that string yanking the spindle around would generate energy that wasn't "free," but (like solar) would be free for all practical purposes. Do too much of it, and Mars crashes into Earth of course, but there's a LOT of energy that could be siphoned off from the planets' different velocities before something like that happened.

It's the same kind of "free" spacecraft get making a gravity-assist maneuver; they slow (or speed up) the orbit of the body they're assisting off of imperceptibly.

(1) ... of course hypothetically. The details of the mechanism are not well-understood and appear mostly theoretical.

Isn't it vanishingly unlikely to ever encounter one particle in interstellar space?