I can see a 1G being achieved with a propellent, but sustaining & actually increasing in speed seems unlikely without a propellent.
To me it sounds like science fiction, but since it is not my field I may be missing something. In other words, I am not saying it is impossible, but rather if possible, how is it possible. It doesn't seem that space objects increase their acceleration rates.
We have been taught that space is a vacuum, meaning there is no air resistance or friction to slow down or speed up objects. Which we see with comets that seem to move at a constant speed and movement that they can be predicted as to when they will once again pass by us, which indicates that there is something that makes their movement controlled by some source, wouldn't it?
Transfer of momentum in a standard resonant cavity. By this method, hitting light speed is not possible... Since the elasticity of the transfer is never perfect... the imbalance is what causes the motor to move...
That is a highly simplified explanation and leaves a LOT out...