Friday, September 25, 2015

The Effect of Short Duration Alien Civilizations

In a previous post, there was some detailed thinking about how hard it would be to maintain an alien civilization on its home planet for a million years. The problem that arises is resource exhaustion. Even supposing that the alien civilization is good at recycling the materials of the planet, the inevitable losses will run the alien civilization out of resources in shorter times than that. Perhaps there are solutions, such as using the resources of moons within their home solar system. These will do little more than double or triple the duration of the civilization, at best. There is no option for interstellar resupply, as the costs are too great. Some concepts about turning part of the civilization into a regenerative system will help with certain materials, but again, this is a factor of a few. This post considers what would be the effect of alien civilizations all having short durations on their home planets, for example, a hundred thousand years.

This concept promotes resource exhaustion as a reason for interstellar migration. Instead of waiting until there was stellar death in their solar system, which is of the order of billions of years, or some galactic peril, or some planetary peril, which might come earlier, of the order of tens of millions of years, they have to move on to a different solar system in a short time, relatively speaking. Suppose the alien civilization recognizes what the limitation on its survival is, and has a meme that induces the civilization to maintain itself. If it was capable of multiple colonization attempts at one time, this would be sort of suicidal. With only a certain number of planets able to host them, for reasons of habitability, having multiple worlds going at one time would run them out of planets even faster. So, the best they can do to survive in the galaxy, is to have one colony at a time, and when, despite their best efforts at recycling and minimization of consumption, they start to hit the exhaustion limit of that colony planet, they need to move on.

The galaxy is of the order of ten billion years old. If this alien civilization was a early starter, and life evolved on the planet after only a billion years, they would have the whole galaxy to colonize before being forced to either leave the galaxy or die out in defeat. Suppose they have, on the average, a hundred thousand years on a planet before resource exhaustion sets in. That means, to survive a billion years, they must do ten thousand migrations. Now comes the question of how many habitable planets are there, at a particular time in the galaxy’s history. There may be only of the order of thousands. This would mean that no alien civilization can endure for the life of the galaxy.

Note also that finding successive colony worlds will become harder and harder. At first they can explore near their home world, perhaps traveling up to a hundred light years away. But this sphere will become empty of future worlds for them, and they are forced to travel further each time. We have not considered what might be a maximum travel distance for a colony ship, but this effect could exacerbate the difficulty they face. To travel further would require more expense, in terms of resources, which also makes the problems more difficult.

What would the galaxy look like after a billion years of this one early starter moving from world to world. There would be thousands of hulks left behind, where there had been mining and extraction of resources, and then abandonment. A second alien civilization that managed to get started somewhere, in another solo world, would either find itself on a planet that had been mined already, or in an galactic environment where they had few worlds left to migrate to. They might be in a solar system that formed later in the life of the galaxy. Their planet could have formed long after the original alien civilization had burnt out, in other words, after they had exhausted all the solar systems they could reach. Now comes a new civilization circling a newer star, and after life evolves on that planet and reaches intelligence, moves up in technological capability, starts star flight development, only to find there is almost nowhere for them to go.

They would find that all the solar systems around older stars in which there were planets meeting their habitability conditions had only hulks awaiting them. On the other hand, if Earth is a good example, there would be geological processes on these old hulks that submerged the original crust and recycled newer crust material to the surface, giving them a new start. On such a planet, there are subduction zones in which older crust materials sinks back into the mantle, and the opposite as well, where new mantle material seeps out onto the surface. This brings new materials to the surface. The time scale for this continental recycling is of the order of billions of years for a complete recycling. Thus, it might not be hulks that the new civilization finds, but pristine planets, or ones that have been scavenged on the older crustal material but not on the new part.

The results of these two competing timescales leads to some interesting possibilities. Can the original alien civilization we initially used as an example, which originated early in the history of the galaxy, find enough worlds to move to so they can survive long enough to eventually return to their home world after a billion years to find it rejuvenated, crust-wise, and able to support another short duration of resource extraction by the same occupants who originally stripped it dry?
The answer would likely be yes, if the alien civilization in the example decided to only maintain a minimum population on their home world and especially on any colony worlds. Recall that there is a tradeoff between population and resource exhaustion time. If the alien world decides to reduce their population down to something like a minimum, perhaps ten million citizens, they would greatly extend their potential duration on each world they inhabit, and in the galaxy in general.

There would certainly be many interesting aspects of this situation. One is the living standard of the aliens. If the average alien has fifty robots and fifty intellos engaged in providing him/her/it with all the needs and wants that they might have, the average consumption rate per alien citizen would be much higher. If a much smaller number of robots and intellos were used, their duration is equivalently longer. Another aspect is that performing a migration might be more difficult with a smaller population. There might be a minimum number of citizens needed to pull off this stunt.

The density of hulk planets is controlled by the duration of an average civilization on a colony planet. Thus, the future of a new civilization is controlled by how many previous civilizations there were before them, exhausting the supply of new planets. This may explain why no aliens have showed up here. There was nowhere for them to colonize and they all went extinct.

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