The age of the Earth is approximately five billion years. That’s an awfully long time. I see no evidence that the evolution of sentient creatures on this planet took place faster here than it possibly could anywhere else. We get more cosmic radiation than some other places, and much less than others. The course of biological evolution was drastically altered several times by catastrophic impacts with extraterrestrial bodies; I don’t see any reason to believe that these impacts were timed to minimize the time required to evolve sentient life.
Thus, if this galaxy were destined to develop one other sentient race in its entire history, I’d give at least even odds of it having occured already. If it were to develop many other sentient races over the course of its entire history, odds approach certainty that at least one of them evolved a long time ago.
“Long time” as in hundreds of millions or billions of years ago.
A race that evolved to spacefaring stage as recently as 100 million years ago anywhere in our galaxy would have to be spreading outward at less than 1/943 lightspeed; any faster, and they’d have settled our solar system by now. One hundred million years of technological advancement seems unlikely to end in drives that can only do a small fraction of lightspeed. Even at 1% of lightspeed, that other spacefaring race would have had to evolve and acquire spaceflight less than 10 million years ago, the blink of an eye in the grand scheme of things.
So where is everybody? Why was this planet empty when we evolved? The most plausible (and most pessimistic) explanation I can think of is that the problem of maintaining a good government and a free society long enough to develop civilian spaceflight doesn’t have a workable solution.
In total, I can think of only a few good reasons. One, intelligent creatures don’t exist anywhere else in this galaxy and never did. Two, they don’t want to go anywhere near star systems; perhaps they have a method of converting “dark matter” to regular matter and tend to look for concentrations of dark matter rather than stars and planets. Or three, intelligent aliens exist, but they never become starfaring for some reason.
The first possibility seems ludicrous, at least to me. There’s just too many stars in this galaxy, and too much time has gone by. I find the second more plausible But the third possibility, that intelligent creatures never become starfaring, is more than a little frightening and chillingly plausible to my mind.
If intelligent creatures are common, and none of them ever become starfaring, then whatever is stopping them should also affect us. Carl Sagan, among others, speculated that the development of nuclear weapons was the dead end; if so, we might be in the clear. Alternatively, if the development of cheap and abundant antimatter production happens before the development of good space habitats that can be widely separated, then small groups or even individuals can acquire the means to destroy their civilizations, and at least among humans there’s a small fraction (but frighteningly large in absolute numerical terms) that would eagerly do so if they could. But we’d have space habitats already if we hadn’t made some disastrous political decisions over the past century, so I don’t think it’s anywhere close to universal that antimatter production will come first.
To find another factor, let’s take another look at humanity. Humanity is not, of course, a single civilization. Human beings have formed thousands of civilizations since our race came into existence, most of them either falling to the sword or decaying and falling on its own. Humanity apparently went tens of thousands of years (or maybe > 100,000 years, depending on just when real, honest-to-God humans as opposed to more-or-less human-appearing animals first walked the Earth) without developing any civilizations past the tribal stage. Since we have so many examples of civilizations to look at, and since nearly all of them failed long before reaching the spacefaring stage, perhaps that is what happens to aliens – some never develop civilizations, others develop them but fail to sustain them long enough to reach the stars.
To get to the stars, your civilization has to endure long enough to develop the requisite technology. Not only must it endure, it must continue to be governed by the correct policies to facilitate technological development and economic growth generation after generation for centuries. Let the government go wrong during this time, and the drive to the stars stagnates.
The government is going to have to stand by as large numbers of respectable and even high and mighty people find themselves made completely redundant and unemployed by new technology – not once, but several times. The government is going to have to stand by as upstart nobodies, foreigners, and other riffraff become wealthy beyond anybody’s wildest dreams with the new stuff and take away lots of other people’s livelihoods – repeatedly. The government is going to have to resist the overwhelming temptation to raid the society’s capital for loot, either for the ruler and his family or for the “welfare” of the voters, and the perhaps greater temptation to take over and micromanage the productive enterprises in order to steal the credit for all the technological advancement going on. The government is going to have to resist demands by respectable people to prevent their jobs from being “destroyed” or “exported”, to prevent their tasks from being changed and their lives from being turned upside down – and resist those demands again and again and again for hundreds of years straight. It will have to resist the demands of busybodies to stop young people from being corrupted, made soft, and otherwise led astray by all the changes being wrought. It will have to refrain from asserting the authority to demand that the people working on new technologies ask permission before they can sell it, demand that people ask permission before they can buy it, or otherwise insert appeasements of the authorities as a necessary step before every incremental step of development. It will have to stand back as people do all sorts of nutty things, sometimes hurting themselves in the process, as they work out the right way to do things that no one’s ever done before.
The record of human societies shows that sustaining such an unswerving, centuries-long dedication to the necessary laissez-faire policies on the part of the governing classes, regardless of who those governing classes are, is extremely problematic, at least among human beings. Assuming aliens aren’t that much different from us, I could easily see a galaxy populated by hundreds or thousands of sentient species, all of which find their civilizations stagnating and falling before they can escape their home planets. Which in turn would, if true, strongly suggest that there isn’t a good way to straighten out our own government and prevent the eventual collapse of our own civilization before we build any spacecraft.
I really hope there’s a lot of aliens out in the dark matter…