[flangola7]

>I can't see it worth the effort for any advance civilization to mount an invasion of a backwater solar system many light years away.

Because that backwater civilization goes from turning on the BBC transmitter for the first time to interstellar terminators in a cosmic eyeblink. If you're lucky and <100 light-years away you have just enough time to launch your own terminators first and take them out.

Our galaxy is 100,000 light-years across. If we were to detect signs of campfires, much less radio transmissions, almost anywhere in the Milky Way then there could already be a fleet of solar system consuming robots headed our way at 99% of lightspeed. They might have already completed most of the journey..

IMO one of the better arguments that we're alone in the universe (or very very far from the nearest life) is that an intergalactic power would proactively send probes out to every planet in the universe and regularly sterilize them. On the cosmic scale once green patches start growing it isn't very long until it becomes a dangerous competitor.

[arcastroe]

"green patches" came about around 500 million years ago. I consider that a pretty long time, even on the cosmic scale! Especially If we believe the universe is only 14 billion years old.