In the eyes of all other alien civilizations, we are aliens to them. Perhaps there is an alien somewhere in the galaxy who is a bit curious about what we might do over the next millennium or so. We spend a lot of thought about how to find them, and perhaps they return the favor.
Suppose we are the good guys, and figure out our social problems, and keep our technology progressing, and eventually get to the stage where we can build a star ship. Yes, distant alien, we decide to do it, and we look long and hard at all the solar systems around us to pick one we want to go to. We first build a giant telescope, with every attachment possible, a wide range of wavelengths to observe in, and we look at all the exo-planets within 300 light years. With this big a machine, we can actually image the exo-planet itself, in poor resolution of course, but we see it going around its star. We can pick one the same size as Earth, around a G star, in the habitable zone, with oxygen in the atmosphere, and no radio emissions to indicate advanced life was already there. We finally seize on a choice, 270 light years out, and begin to design the ship. Experiments alone take a century. Design takes thirty more years, and then construction takes another forty. The ship is a probe, designed to stay in intermittent communication with Earth, and to go into orbit around Sperantia, which is the name we gave the planet.
Earth’s first space ship can accelerate to 0.02 c at peak, just before it slows down, and with all the maneuvering, that means a voyage of over two thousand years. The design of such a long-lived, autonomous system was full of challenges, but Earth succeeded and the ship was built and launched. Then Earth waited. The probe relayed back every ten years that it was doing well, was on course, hadn’t slammed into any space rock, and would keep on chugging along.
Then the long wait was over, the probe had reached Spernatia and gone into orbit. Gobs of data was collected, and then, about 270 years later, reached Earth. Earthlings finally knew what was on the planet they had selected. A climate ranging from ice at the poles to tropical heat near the equator, sea life and plant life and even a variety of animal life, lakes and rivers and glaciers and funny mountains. Instead of mountain ranges, there were plateaus and gigantic pits. If you want to leave a monument for ten thousand years, you can build a colossal pyramid. If you want to leave a monument for ten million years, you can flatten large mountains and spread the spoil around. There was no doubt that this planet had been inhabited, and mined for resources everywhere. Many lakes had the shape of huge quarries, filled in with rain and runoff. Even the coastal seashore had the unmistakable signs of being devoted to resource extraction.
The only conclusion was that this planet had been used up. No New Earth. A faubt ripple of laughter echoes through the galaxy as observing aliens, if there were any, watch the new guys fail.
It doesn’t take that many solo planets, with alien civilizations on them, to travel throughout the galaxy to any sweet spot world and set up a colony there. When they were all gone, and even the penumbra worlds that were almost good enough gone, the civilizations expired. A new world, out on the spiral arms, giving birth to intelligent creatures, would find no place to go when they got ready to travel between stars.
Knowing the exact number of solo worlds doesn’t make much difference. If there are ten million sweet spot worlds in the Milky Way, and an alien civilization needs a new one every fifty thousand years, that means that a single alien civilization will need to go through a hundred thousand planets to last five billion years. A hundred solo worlds leading to star travel will use them all up. Two hundred would use them up in half that time, only two and a half billion years. The Milky Way is over ten billion years old. If only two hundred alien civilizations get started, out of ten million suitable planets, this means the probability of successful civilizations climbing the ladder to technology to the top is very low. More likely, there is simply nothing left of the really good worlds in the Milky Way. Double the number of sweet spot worlds, triple or quadruple them and it doesn’t make a speck of difference.
A previous post noted that if other things didn’t get in the way of an alien civilization climbing the technology ladder up to its highest rungs, resource exhaustion would do it. One possible reprieve might be thought to be colonization. Colonization works if there are almost no planets with alien civilizations in thte galaxy, but otherwise the available planets get used up.
We have been talking about what might be called linear colonization. This is what one category of alien civilizations might do, just transfer their civilization to another planet when the resource exhaustion problem became too much for them, or for that matter, if some peril threatened them. But another category of alien civilization might be called an exponential one, where a single planet founds not one, but two or three or four or more colonies, and that tradition continues. Then the planets get used up much, much faster than with linear colonization. A few of this category and everything is all used up in under a billion years.
New worlds are being born all the time in the Milky Way, and some percentage of them turn into sweet spot worlds. If the alien civilizations formed near the beginning of the galaxy have not already died out, they would be taken over quickly. Otherwise, each generation would consume those of the next generation. Either way, the scenario for Earth getting to the closest sweet spot world would likely play out the same way. Nothing left here. And there is likely nobody left to talk to, no matter which plan for colonization they adopted.
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