One of my first observations at this blog was that "We aren't even going to have a substantial percentage of the human population outside of Earth orbit 10,000 years from now."
This does not mean that I think that life on Earth is likely to be unique. As turbo-1 over the Physics Forums aptly observed DNA based life can live in very extreme environments and there is no reason to think that some other form of life, perhaps silicon based, couldn't evolve to survive in even more adverse environments. You can't just take one look at a place and say "No life could exist there." You have to look for the "sweet spots". If you are looking a Jupiter, you look at its volcanic and ice covered moons, or floating in its atmosphere. If you are looking at Venus, you look at the clouds, not the incredibly hot surface. On Mars, you look at buried ice that may periodically take liquid form when the sun shines on it, not on the peak of Mons Olympus. Astronomy so far has provided abundant evidence that gas giants planets are common, and there are early indications that rocky planets, which are much harder to detect because they are smaller and don't emit light, may be common as well.
I don't even think that humans are alone in our galaxy as a relatively advanced tool using species. Evolution is a process which can take place just as easily on some other planet 50,000 light years away as it can on Earth. The laws of nature are out there to discover and manipulate.
Will we ever encounter life elsewhere? Who knows? There are "formulas" for determining the likelihood of that happening, but those forumulas are so devoid of good data that they better resemble ways of thinking about the problem than any meaningful tool for determining a real probability.
On the other hand, I think that one can make some reasonable assumptions about what a first contact is likely to resemble if we ever have one. You see, any other technologically advanced species is going to face the same constraints, more or less, that prevent humans from doing much interstellar space exploration in the next 10,000 years or so. Space is big. There is a speed of light speed limit. Living things don't live forever. The amount of resources necessary to carry out space travel is immense. Even if one is engaged in space travel, that doesn't mean one will go very far or make a very comprehensive examination of all of the stellar systems that are out there.
The fastest way to spread your impact across the galaxy if with photons, like radio waves. They go as fast as it is possible to go, and the energy costs are far less demanding than sending something with mass at anything approaching that speed.
It could be that some alien species has already detected our radio waves. Even if they replied in short order, unless they were very close to us, we might not have received a response yet. But, if we ever do encounter an alien species it may very well be in the form of messages picked up by SETI or some other radiotelescope array.
Even if that alien species is quite close, say 100 lightyears away in a galaxy that is 100,000 light years across, it would take many centuries before we could ever exchange even the slightest artifacts, let alone encounter a member of that other species. And, when even a simple message and response loop takes 200 years, that communication wouldn't be anything like talking on the phone. Most likely, if we do encounter an alien species (or many of them), the relationship will be, more than anything else, like having a pen pal in a foreign country who doesn't speak your language. It would be an odd, mysterious thing indeed.
An encounter like this would profoundly change how we view the universe. But, it seems unlikely to lead to clashing interstellar empires or a War of the Worlds.
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