http://en.wikipedia.org/wiki/Alcubierre_drive
The link above is the evolution of the wormhole hipotisys in Eistein's theory. Read it
Its impossible because the amount of energy required to make it viable would require you to tame a black hole...
Even a tiny black at the edge of our solar system would have catatrophic results to the solar system... For exemple, we would notice huge earthquakes here on Earth... Guess what? None of those Huge earthquakes have been traked, also taming a black hole is... Dude, even using a magnetic field its isane due to the gravitical forces that ship would experience.
I really didn't wanted to be the alien inside that ship... Just the amount of radiation the black hole would realese would be insane.
They would have technology beyond our compreension, true... But Technology requires energy... And that we can track... And until now we haven't found anything beyond logic.
I 100% agree, the point is, why would they want to come here? We have nothing they need, and even our planet is trash, its a mid-age planet with just another 500 milion year of habitable life... Such a civilization could just move to a newborn system.
The only thing that makes me doubt my previous statment is knowledge, if they are here, the reason its us, as they can get whatever they want from much better sources, and cheaper, then Earth.
And what is the point of taking control of Earth? We have NOTHING those beings could need. There was a NatGeo show in 2012 whose theorised reason for Aliens coming to Earth was carbon farming, which is a nonsense, there are planets made of carbon, and if we can farm that carbon (and we can) that means they would be able to do it aswell, and that would be a lot cheaper for them... We have NOTHING they need... Not even our planet.