"...even if a Galactic Empire spanning half the galaxy does exist it might not actually be emitting radio signals at all"
It indeed seems likely that a civilization more advanced than ours would at some point progress beyond the use of radio. It is only logical that a space faring civilization would transmit in the x-ray or gamma band, since that would allow much tighter beams and sending much more data for the same transmission power over far greater distances. Perhaps there are FM modulations all around us at wavelengths far shorter than what we call gamma. We don't even know what the theoretically shortest wavelength is.
Since NASA is already experimenting with x-ray comms, it would seem logical to assume that a civilization's radio comms period would last only around 100 years. Also, nearly all of those radio transmissions would be, like ours, unfocused. Far away signals would be extraordinarily weak. Tight, very short-wavelength beams should stand out more, once we have the tech to look for them. I believe SETI has the right idea, but perhaps were ahead of their time.
Biting the hand that feeds IT
