Well if you can propel something forward you can propel it backwards as well.

I'm assuming some sort of fixed laser type propulsion mechanism would leverage a type of solar sail technology. Maybe you could send a phased laser signal that "vibrates" a solar sail towards the source of energy instead of away.

> Well if you can propel something forward you can propel it backwards as well

Not necessarily - at least with currently known science. Light sails work ok transferring momentum from photons, allowing positive acceleration from a giant laser Earth. Return trip requires a giant laser on the other side.

As well as a way around Newton's Third Law.

I meant to say the "simpler" (but still very complicated) solar sail approach was for a one-way trip. On paper, our civilization can muster the energy required to accelerate tiny masses to relativistic speeds. A return trip at those speeds would require a nee type of science to concentrate that amount of energy in a small mass and use it for controlled propulsion.