The reason people keep bringing up ridiculous notions about aliens is their failure to grasp history on an astronomical scale. This is especially the problem of the SETI-type chanting yoyos who keep bringing up Drake's equation to say that other civilizations must exist but be "improbable" so that we haven't "contacted" them. In their ossified brains they've got some small notion of the many billions of stars which are in our galaxy, but not of the billions of years which lie in its past.
We will start with a very simple assumption. The galaxy as we see it is the most probable way for us to see it. And from this assumption we will recover that aliens (other advanced technological species) do not currently exist in our galaxy and will never exist.
To start off with, how long does it take for an intelligent species to rise and colonize the entire galaxy? It takes roughly 100,000 years. Because the galaxy is 100,000 light-years in diameter and civilization arose on Earth a mere 10,000 years ago. Even under the most pessimistic assumptions, within 100,000 years of its rise, an intelligent technological species will have colonized half of the galaxy.
How many opportunities have there been in our galaxy for an intelligent species to rise and colonize the galaxy? In the last 65 million years alone, since the extinction of the dinosaurs, there have been 650 chunks of 100,000 years during which a civilization could have arisen and colonized the galaxy. In the last 13 billion years, since the first generation of stars seeded the universe with heavy metals from which planets and living beings could form there have been 130,000 such chunks of 100,000 years.
What is the probability that an intelligent species will rise and colonize the galaxy? We will not derive this theoretically because we don't have the necessary astronomical data to do so. We will rather derive this statistically using the observations that 1) we exist, and the assumption that 2) the galaxy as we see it is the most probable way for us to see it.
The fact that we exist means that a previous civilization has not colonized the galaxy. It simply isn't to a civilization's benefit to "preserve" a planet for hundreds of millions of years on the off-chance that a species might evolve to intelligence and technology. Far better to colonize the planet and use it productively. Since aliens have not done so, because we exist, it follows that aliens haven't colonized our galaxy in the last 13 billion years.
So the probability of us seeing the galaxy as we do is ((1-X)^650)*X if we unreasonably assume that civilizations could only have arisen in the last 65 million years. And ((1-X)^130,000)*X) if we reasonably assume that such a civilization could have arisen at any time in the last 13 billion years. In both cases, X is the probability of a civilization arising and colonizing the galaxy within a specific chunk of 100,000 years. These two probability functions have maxima which correspond to the value of X that is most compatible with us seeing the galaxy as we actually see it.
So in the 65 million year case, the probability of a civilization arising in any given 100,000 year chunk is about 1.5 in 1000. And in the 13 billion year case, the same probability is about 7.7 in 100,000. This is the chance that we will meet aliens as we colonize our galaxy. So yes, SETI is a gigantic waste of time, effort and resources because the aliens really aren't waiting for us. With greater than 99.8% certainty. If you are very, very optimistic.
See also the continuation What Galactic Colonization Really Looks Like. And the meta-argument SETI types are Creationists.
Reorienting our discussion of city growth
2 weeks ago