We may remain alone forever

1. Life is probably incredibly rare, given that it arose on Earth only once.

2. Where life exists, intelligence is probably incredibly rare, given that it arose on Earth only once.

3. Where intelligent life exists, it is probably impossible for it to travel beyond its star system, given that there is no evidence for the existence of worm holes, and that our fastest space shuttles would take over 150,000 years just to reach Alpha Centauri, our closest neighbor. Artificially intelligent shuttles could be sent in the place of those operated by organic lifeforms, but, in any case, we have no evidence for the existence of anything of that sort.

There are at least 10,000,000,000,000,000,000,000 stars currently in the universe -- a number larger than the number of grains of sand on all of the Earth's beaches combined, and not even representing the total number of stars to have ever existed (we can still detect leftovers from long-extinct civilizations, remember) -- and it would only take one with advanced life in almost fourteen billion years for us to have evidence for the existence of such life anywhere. If only a quarter of those stars has orbiting planets, with only one hundredth of those stars having Earth-like planets orbiting them, with only a fifth of those stars having Earth-like planets containing life, with only a tenth of those stars having Earth-like planets containing intelligent life, with only one hundredth of those stars having Earth-like planets containing intelligent life capable of interstellar travel, that means that there are currently 5,000,000,000,000,000 (5 quadrillion) advanced civilizations in the universe capable of interstellar travel.

That would come out to be about one intelligent civilization capable of interstellar travel per two million stars. If, say, only a trillion of them have had this capability for at least a billion years, even with our technology, they'd each be capable of visiting close to 6,700 stars in that time (assuming that the average closest star is only a few light-years away). That means that, even with such conservative estimates, 6,700,000,000,000,000 (almost 7 quadrillion) stars have been visited by space-faring civilizations in the last billion years. Admittedly, that's still only about 1/1,500,000th of all of the stars in the universe, but premises 1 and 2 concern me enough to prevent me from being optimistic about our ever encountering extraterrestrials. If life arose here only once out of who knows how many trillions upon trillions of opportunities, how likely does that really make its emergence?

Of course, arbitrarily stopping at a billion years ago doesn't make too much sense, anyway, given that stars have existed for almost as long as the universe has, and a trillion is only 1/5,000th of our original number of interstellar civilizations; raise it to 100 trillion existing for five billion years instead of one billion, and you get 33,300 stars visited per civilization instead of 6,700, or 3,330,000,000,000,000,000 (over 3 quintillion) stars visited in the last five billion years -- over 1/3,000th of all stars in the entire universe -- by civilizations currently existing (in other words, not counting all the ones who've gone extinct). You can play with these numbers all day, because they're highly variable and far from certain, but I suspect that even the most conservative of estimates will yield similarly gargantuan numbers of civilizations.

The question that you should be asking yourself: Where is all of their trash? If we take the scientific community's assumption that where there's water, there's life, then in our original scenario, there'd currently be (again, disregarding even the ones who've gone extinct, which, if counted, would only increase the odds that we'd find "space trash") 25,000,000,000,000,000 (25 quadrillion) extraterrestrial civilizations capable of interstellar travel. The number of stars in the universe is fairly certain at this point; the number of stars with planets orbiting them is starting to become clear, as is the number of stars with planets similar in size and composition to the Earth; we're already fairly certain that it takes extraordinary circumstances for language and syntax to emerge from the process of natural selection; as demonstrated, given human lifespans, interstellar travel is currently looking next to impossible. Are the odds of encountering extraterrestrials really as good as scientists claim?

4. Even if it were possible for intelligent extraterrestrials to contact us, it is highly unlikely that they would have any groundbreaking, philosophy-altering information to give us. They would possibly be able to help us, but what they knew would likely not change our assumptions regarding anything meaningful. In other words, understanding the functionality of a particular process is all that is required in order to ascertain whether that process is worth perpetuating; you don't need to know every minute detail at every scale in an infinitude of abstraction.