Can a powerful capacitor be used to create a flying machine?

[DrStrange]

A couple of days ago while i was thinking about several stuff an idea came to me. Here goes. Imagine you have a really powerfull capacitor that has enough power to accelerate an electron to the speed of light, when the electron is put to the negative plate. The electron would never reach the speed of light because while it is
accelerating its mass will increase, right? So since it is still being accelarated the electron's mass would reach the capacitor's mass at some time. According to the conservation of the momentum (mv) the capacitor will then instead of moving the electron, move itself on the opposing direction. So what i came up with is whether it is possible (theoritically speaking) to make a really powerfull capacitor float by shooting electrons from the negative to the positive plate[?] I've been thinking this over and over but i have this feeling I'm missing something important, because I'm sure I'm not the first man who ever thought this. Anyway, i know the technology forum is not really made for posting ideas on imaginary machines, but i found it the most appropriate among the others. Sorry for the big message

 

[FZ+]

It depends on what happens on the postive plate. If the electron carries on going downwards, then what you have is a powerful variant of ion propulsion, which is already used by NASA in certain space probes. If the electron stops at the positive plate, then the device wouldn't work as the electron is deaccelerated to a halt with an impulse in the opposite direction, pushing the capacitor down again.