ET, standing, of course, for "extraterrestrial advanced technological civilization" from the movie, is all the talk today. I add the words "advanced" and so on to be sure we are talking about something important, not just a few microbes or even bunches of whales cavorting on an ocean planet. Not that these things would not be important, but pale in comparison to a technological society.
Of course I don't suppose whales couldn't evolve technology. That they did not for quite a long time swimming around here on the earth is evidence, and thinking about it makes the difficulties clear enough, but anything is still possible -- well, maybe not.
I am pretty much persuaded, if not convinced, that ET must be excruciatingly rare, if not completely absent, at least from our galaxy and maybe from the entire universe we have direct knowledge of.
That is at best a heterodox opinion, although not an uncommon one among religious types, but among non-religious people like me. The religious types have their unique creation to deal with and it is easier to deny, deny, deny (although to be sure many of them jump through various hoops to get around it).
We now know enough to project that planetary systems, and even solar-system type planetary systems with at least one earth-like planet, are numerous -- by numerous here I mean really mind-graspingly large numbers.
I think, though it's likely that a Jupiter-Saturn sort of pair like the one we have is needed to bring stability to the orbits (early on, it seems they did a dance that kept both of them out of the inner system where in most systems a giant migrates inward and destroys any budding earths).
It also seems that an earth-moon "almost" double planet ("almost" because the center of gravity remains under the surface of the earth) is rare, but needed for the inclination stability over billions of years that would be needed for evolution. Theories as to the origin of the moon have settled on some sort of early collision with another planet sized body, and not just any collision, but one involving relative masses and orbits in a limited and therefore unlikely range.
Then there is the problem of the system, after this violent beginning, being left alone for several billion years for life to evolve. There are any number of things that, while individually unlikely, can serve to sterilize or almost sterilize a planet, and over several billion years each of them should happen several times.
The assumption is often made that given the right conditions the rise of life is almost a certainty, based on the statistical sample of one we have that it happened quickly after the earth settled down. Not so fast. We don't know the likelihood of there being the right conditions for the origin of life. Just the right temperature and mass may well be nowhere near enough.
One would think that with huge oceans in a reducing atmosphere for several million years, with energy sources and no big disasters, molecules that take from the environment to make at least rough copies of themselves would happen, and once started natural selection would step in to make the process better and better until you had living things. But you need several million or so years, a reducing atmosphere, lots and lots of water and of course a variety of other elements. While the presence of some of these may be taken as given, the combination may not. And of course, there is the unresolved problem of protecting these increasingly complex molecules in your soup from all sorts of solar radiation. An example is that the planet would need a strong magnetic field from day one, something that is by no means a given.
Then there are several major events in the evolution of life that may be one-off affairs with almost no chance of happening, including but not limited to the appearance of multi-cellular ("complex") living organisms. That this event seems to have been long delayed tells us either that it is unlikely or that necessary precursors are unlikely. That once it happened, it appears to have happened many times almost at once speaks more to the latter, but it's hard to say, since only one of those many occurrences of the appearance of complex life actually persisted. It may be that it is not so easy to get it right.
Millions of years, then were spent, first with pre-mammalian-reptilian forms, then with reptiles and mammals, and then with just mammals, all evolving around in circles, punctuated by occasional mass extinctions, until the appearance out of the blue of apes and hominids and mankind. Again long delays while nothing really takes place except evolution of new flavors of the same things indicates that the appearance of intelligence is not so automatic nor so predictable.
Nor, if we look at the history of our species, does the appearance of technology seem so predictable. I think slavery and the domestication of animals prevent technology for awhile, and there seems no real reason to rid a society of slaves except accidents of history. That slavery had disappeared in Europe before elsewhere and that Europe is where technology got started tells a tale.
Finally, there is the "where are they" problem, not easily pushed aside. It may be that the technologies self-destruct, or maybe (a more optimistic thought) they find ways to exit physical existence into computers or other dimensions or whatever and find things much better there. Or maybe some equivalent of bird flu eventually appears and spreads so fast and is so deadly that that is that.
There exists another set of issues not often addressed in this topic, the nature of our intelligence and how likely that might be. AI machines have been promised now for quite a while, but haven't appeared. The fact is not just our intelligence, but even our sentience (our experience of existence and of senses and emotions and so on) is not understood. Not only is it not understood, no one has any idea how to approach it. How can we make predictions as to whether it is likely to appear elsewhere when we don't even know what it is?
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