[roywiggins]

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.

[shadowgovt]

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.

[PaulAJ]

> [..] while the electrical energy input only increases linearly with time.

This is the fallacy. In an electric motor the power required to exert a force is proportional to the speed, so if you want to keep the acceleration constant you need to increase the electrical power (i.e. rate of energy use) linearly over time. Hence the total energy input increases with the square of the time, just like the kinetic energy imparted. If the system is 100% efficient then the total electrical energy used will be equal to the kinetic energy of the mass.

[roywiggins]

> proportional to the speed

In what reference frame? It's not obvious to me how this works in a space drive. How does it "know" how much speed it gets per unit energy?

I guess if it's a sort of Dean drive that swims through the gravitational field of the visible universe, then that's the reference frame. It's still weird!

[cryptonector]

The schemes I'm familiar with claim to extract energy from the vacuum, essentially. It sounds a lot like hocus pocus to explain away free energy. Great if physically possible, but exceedingly doubtful.

[firebaze]

Did you read Mach's Principle? It would not create free energy. And yes, it's quite probably people more specialized in this field would've asked this question before.