[Greysard]

But, if the aliens have a magical energy source, then they probably have a magical FTL stardrive, too, and then, no, we won’t see them coming.

Most likely the aliens would have to have that magical energy source and a matching stardrive. Otherwise travel between stars is not possible:

1. Travel at near-c speeds in normal space requires enormous energy for acceleration
2. Waste energy cannot be dumped in vacuum, other than by radiating it away - which is extremely inefficient and cannot handle large amounts of power
3. Therefore, the engine that accelerates the ship to near-c speeds must be very efficient, such as to generate very little waste energy - otherwise the engine will melt down.

[Little Ray]

That waste energy is why you see in the IR spike. Right now, our understanding of physics doesn’t allow a power source without the waste heat. That is why it would be “magic” energy source.

[MHGinTN]

Inertia arises because of the zero point filed. Find a way to shield from or divert the zp field and speed is no longer hampered by the inertial restraints.