We know there are billions of stars in our galaxy alone. We know a large proportion of these stars have planets. Unless our Earth is a very unique planet, it seems not unreasonable that our galaxy would contain many Earth-like planets.
Having speculated this far, it doesn't seem much of a conceptual leap to the idea that there are ET's -- extraterrestrial humans.
Enrico Fermi, during lunch at Los Alamos, asked the simple question "So? Where is everybody?" He thought we should have had ET visitors by now. Surely at least one of the extraterrestrial humans would have passed our technical level of space travel and made a voyage to us from another star. Fermi pointed out that, even at under lightspeed, the time required for ships to cross and colonize our galaxy is shorter than the age of our galaxy.
The evident fact that we have yet to receive an ET visitor has become known as the "Fermi Paradox."
The simplest resolution is that we are unique, or at least very rare. There's a book abuout this: "Rare Earth: Why Complex Life is Uncommon in the Universe" by Ward & Brownlee. Here's the ebook version. This rare Earth idea neatly dodges the paradox -- where is everybody? They aren't there. We are unique and alone in the universe; there are no ETs.
Well, there's no way of being sure of that.
Our own spacefaring facilities have gotten as far as robotic investigation of the outer planets of our solar system, but has yet to produce a launch toward even so close a destination as putative planets of Alpha Centauri. Perhaps no ET spacefarer has progressed much beyond our level of local spacefaring, or if they have, these neighbor ETs are located too far away to get to us.
Can we hear them? The SETI project has been hopefully listening for radio messages from an ET civilization, but so-far, no frequencies have turned out to carry aliens talking. Of course, we might not be listening in the right way. And of course the transmissions might be coming from so far away that we can't detect them. I wrote an amusing speculative science fiction story about what might happen if we did figure out how to hear the ETs. It is available to be read online.
Here's a really big-time paradox dodge. The ETs are here, and have been for some time, but we have failed to recognize their presence. A variant of this dodge is the claim that ET visitors came here before humans evolved; all they found was trilobites or pterodactyls, so they went elsewhere and did not return. Yet another variant is the notion that the ETs are so uterly different from us that there's no basis of communication.
My guess is that Earth and Earthlings are sufficiently rare and thinly distributed that we are unlikely to encounter ET, either as a physical visitor or a radio transmission. The galaxy is just too big and the technical issues too daunting.
All we can hope for is a spectacular discovery which either allows us to detect ET radio or execute rapid interstellar jumps. I'd even be pleased with the development of some means of transporting small quanties of matter (or just patterns) fast enough to act as an interstellar telephone.
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