So, to give a nerdy answer, you can actually already power a spacecraft by, well, firing a laser out of the back of it. There is a problem though, the thrust is tiny for the amount of power you put in. The equation is F (thrust) = P (power) / c (speed of light). So a one gigawatt laser will apply less force than a pint of beer does resting on your hand. Okay, I hold my pints, I don't balance them on my palm, but you get the drift. You could fire a laser from Earth at a spaceship and use the power to ablate part of it in such a way that it gets fired out of a nozzle, but that is also not going to be hugely propelling. Or, at least, not propelling enough to solve the problem...
Pretty convincing, sadly😏
It is only a matter of time before someone invents a rocket that travels on a laser beam.
Stay positive
So, to give a nerdy answer, you can actually already power a spacecraft by, well, firing a laser out of the back of it. There is a problem though, the thrust is tiny for the amount of power you put in. The equation is F (thrust) = P (power) / c (speed of light). So a one gigawatt laser will apply less force than a pint of beer does resting on your hand. Okay, I hold my pints, I don't balance them on my palm, but you get the drift. You could fire a laser from Earth at a spaceship and use the power to ablate part of it in such a way that it gets fired out of a nozzle, but that is also not going to be hugely propelling. Or, at least, not propelling enough to solve the problem...
But what would happen if a mirror is placed behind the exhaust?
Perpetual motion?