Thank you for the response. I tend to disagree with you. I will respond to your points in the order you have written them.
Your first objection was that it is unlikely that space junk would come our way. I implied it by talking about exponential growth, but didn't directly say that a likely assumption was that aliens may be sending probes to every solar system. I quote:
Initially, a von Neumann probe, (...) would be launched from the home star toward a neighboring stellar system. Upon arrival it would seek out raw materials, from local sources such as asteroids, and use these to make several copies of itself. The copies would then be launched at the next set of neighboring stars. This process would be repeated, over and over again, so that increasing numbers of identical probes would be involved in penetrating ever more remote regions of the Galaxy. (...)
He proposed a conservative value of 300 million years, or less than 5% of the present age of the Galaxy, for complete galactic colonization. [Link]
Your second point. We don't send anything anywhere interesting.
Humanity has been submitting everything from talk radio to commercials into space for the last 100-or-so years. At the release of The Day the Earth Stood Still the movie was transmitted to Alpha Centauri.
You are right that my example, a planet needing a gas giant in the solar system to sustain life, not always being the case. Though it does not change my overall premise - that there are hundreds of factors with each a very small probability that all have to be the case to sustain life. Another solar system might not have an asteroid belt, but gamma radiation.
We tend to not be able to grasp how tiny those probabilities are, while a ig number is something we can grasp. When you see 1/10 you have a concept of how big that number is. But you might have trouble understanding 0.0000000000000000001/10000000000000000000000000000 even if it's the same number. That's the "trick" behind the "there are sooo many planets" argument.
The Drake equation is a more general framework (which is what makes it so worthless), but we have no accurate concept of any of it's variables. We can't even make an educated guesses about them, as that would just be summing up our prejudges.
Life might develop and extinct so quickly that no two civilizations are around to notice each others. But they should be noticing each others space junk or probes flying around.
Plus the whole point with the exponential growth segment was that aliens are either not there (extinct or never existed) or are everywhere.
Because the phase between those two is so short that it is unlikely that we witness it. As demonstrated with the bacteria in a bottle. Or rather they came to beat the crap out of us.
Though I must grant that just because we don't see any probes flying around doesn't mean there aren't any. You would expect that a species mastering interstellar propulsion would be able to invent some with some sort of cloak.
---------- Post added 09-20-2009 at 05:23 AM ----------
Yes. SETI is a religion. I point this out when atheists claim to reject religion for the reason that logic dictates it is unlikely. They don't seem to apply this strict a logic to their other beliefs. Which shows that they really just rebel against Christianity (oddly the only religion atheists have a problem with) because it represents "the man".
