Ok so i was searching google on EM drive's (crazy i know but i wanted to see what i would find) so this is what i found: Reactionless Propulsion (Not) Here's a common fallacy that sometimes leads people to think it's possible to create a reactionless propulsion system. First, consider two positively charged particles, held a fixed distance D apart by some framework. The particles exert equal and opposite forces on each other, and they transmit these forces to the framework, so there is a net zero force on the framework. Now, people sometimes imagine that is should be possible to somehow "turn off" the charge of one of the particles, so that it is no longer subject to any electro- magnetic force. In addition, they suppose that the other particle will continue to experience a net repulsive force for a period of time equal to D/c, reasoning that it will take this long for the effects of turning off one charge to reach the other (at the speed of EM wave propagation). ::::::::::::::THis is the part I Don't fully understand::::::::::::::::::::: It's easy to dispose of this idea, simply by noting that charge is conserved, and we cannot simply "turn off" the charge of a particle. When this is pointed out, the proponent of reactionless propulsion will sometimes change the scenario, so that instead of considering two charged particles, we have two electro-magnets, repelling each other. It's certainly possible to "turn off" an electro-magnet, so it might seem that this provides a means of achieving reactionless propulsion. This would be true if the force on the de-powered coil instantly becomes zero when the circuit is opened, and if the full force continues to be exerted on the powered coil until the effect of turning off the first coil has time to propagate across the distance between the two coils. :::::::::::What does this mean by Transient effects?::::::::::::::::::::: However, the field surrounding an electromagnet doesn't just vanish when we open the circuit, because a changing electric field induces a magnetic field, and vice versa. As a result, there will be significant transient effects when we "turn off" one of the electro- magnets. These transients will also affect the other coil and its field, because the two coils are inductively coupled. As Faraday would have said, the "lines of force" linking the two coils will collapse. It isn't correct to view this as two superimposed static fields, one of which can simply be instantaneously deleted at will. In a sense, the fallacy with this idea is the same as with the idea of just "un-charging" a particle, i.e., it is the failure to take account of the conservation laws of electro-dynamics, which automatically ensure that momentum is conserved. Of course, another consequence of the abrupt change in the combined field of the two coils is that some energy would be radiated away in the form of an EM wave. (You've probably heard a "click" on a nearby AM radio when you de-power any kind of inductive coil.) Since our setup is non-symmetrical, the radiated wave would be non-symmetrical too, so it could carry away a net momentum in some particular direction. In this sense, we certainly can achieve a propulsive effect - but it isn't reactionless. It is reacting against the momentum of electro-magnetic radiation. So in the end why exactly would the EM not feel a force after the other was turned off?