[bluGill]

CHON are also very common in the universe. Most proposed alternatives (not all) depend on things less common. This doesn't rule them out but it makes the odds worse.

Radio communications between tween solar systems require more energy than we have. We couldn't detect earth level civilizations in the nearest solar system (which probably doesn't have earth like life) even if both of us by chance aimed at each other at the correct time.

[Animats]

> Radio communications between tween solar systems require more energy than we have.

Supposedly, the Arecibo radio telescope, before it collapsed, could in theory communicate with a similar setup across most of the galaxy. The dishes would have to be aimed at each other.

[bluGill]

I will admit to not being a RF engineer, but I've heard the claim from people who seem to know enough about astro physics that I tend to believe it. I know enough about physics (the inverse square law) to not doubt the claim. Now if you said this could have reached the nearest stars I'd be more inclined to believe you, but the galaxy is a large place.

> before it collapsed

This is a real problem - communication implies two-way, or at least intentional one-way. Across light-years of the galaxy anything that doesn't last for 200,000 years (or at least close to that) can't communicate. Arbitrary advanced technology has only detected life advance enough to detect their signals out a few light-years (assuming they care to look), and then they need to get a return signal back to us (again assuming they care, and we are looking)