@moonbus
One article I read said if the planet the aliens lived on had much higher gravity than Earth spaceflight like we developed would be near impossible since hydrogen powered rockets like we have today would require way more massive rocket fuel than we have in our rockets which means we are lucky as hell to live on a planet with just the right amount of gravity, enough to hold in an atmosphere but not so high a gravity well that makes spaceflight near impossible. They talk about rockets needing to weigh 50,000 tons to get into low orbit and may never be developed by even an advanced civilization.

@moonbus said
@bunnyknight

There is no reason to think that intelligent life on other planets must have opposable thumbs and use electricity, just because we do.

@deepthought said
To qualify as intelligent on galactic scales the species has to have build radio-telescopes. If they haven't then from everyone else's point of view they may as well not be intelligent, or even exist. All we'll see is oxygen lines from their atmosphere.

@metal-brain said
He brings up a good point though. Maybe Humpback Whales are more intelligent than we think, but they cannot make technology. The Octopus is very clever, but the same applies despite having eight limbs that work well at problem solving. A water dwelling species would have a difficult time leaving the water for long. How could they build a furnace to melt metal?

We are a ...[text shortened]... shortcomings of SETI. The only real way to know is to travel there and it is probably too far away.

@DeepThought
But any real distance and RT would be just one way, still good enough to say we are not alone in a vast universe, which I doubt very seriously but still not proven as of yet. Wouldn't it be a gas if RT aliens were on the triple Alpha Centauri system?

@deepthought said
Yes, but we'd still be able to observe oxygen lines - which is pretty much a smoking gun as far as life is concerned. Anaerobic respiration just doesn't provide enough energy for multi-cellular life, never mind intelligent. So if we see an exoplanet with an oxidizing atmosphere then it almost certainly harbours life and may harbour intelligent life. But non-radiotelesc ...[text shortened]... nd receive the civilizational equivalent of a selfie, which we and they need radio-telescopes to do.

@metal-brain said
"we'd still be able to observe oxygen lines"

I doubt that is possible at the distances that are likely to have a planet with life on it. Too far away and too close to stars to see.