Tuesday, July 14, 2015

How Old Are Alien Civilizations?

Our sun is about 4.6 billion years old. The oldest stars in the galaxy are about 13.6 billion years old. This does not mean there are alien civilizations 9 billion years older than we are.

According to current astronomy knowledge, the universe is about 13.7 billion years old. Before the first generation of stars formed, there was almost no heavier elements, just hydrogen and helium, the two lightest elements. Astronomers use the term metals not in the way chemists do, but to refer to any elements higher than helium. Early stars formed out of the two lightest elements, and were typically large stars, as the lack of metals makes the fusion burning harder to ignite. More gas has to be collected. Large stars burn very brightly and they burn themselves out in a short time. After burning, they emit the metals resulting from their fusion processes into the surrounding gas. This surrounding gas eventually becomes another star which might be cooler, like the sun, or much cooler, like the majority of stars in the galaxy, the red dwarf population.

If we assume that life needs rocky planets or moons, there would be no starting the formation of life around the earliest generation of stars. No metals, no rock, no life. So only in the second generation of stars might there be planets which could start along the pathway to life. Thus, an old civilization's planet might be something like 8 billion years older than ours. If we assume that the time needed to get to intelligent life is about the age of our sun, 4.6 billion years, and it is roughly the same on this earlier generation planet as ours, then they have a head start on us of 8 billion years.

It’s hard to imagine a civilization lasting billions of years. We have the Chinese Empire lasting, albeit in several incarnations, for a couple of thousand. If we consider all of mankind one civilization, it has been around five thousand years. It is hard to imagine our civilization a hundred years from now, to say nothing of a thousand. We simply have no basis in our existence to project time beyond a thousand years. Only thinking about basic principles will provide us any clue. For that matter, once a civilization gets beyond some tens of thousands of years, what is the difference? How is a million year civilization different from a billion year civilization? Does anything else happen other than endless repetition?

Yes, it does. First of all, a billion years is a long time in a mid-sized star’s life. A large star can expire in just millions of years, and the lifetime of stars extends as the mass of the star diminishes. The sun's lifetime is around 10 billion years, and at the far end of the scale are red dwarfs, which are thought to last a trillion years. So, depending on where on the distribution of stars the civilization’s sun is, they could be given an eviction notice long before they reach a billion. If they were out in the suburbs of the stellar kingdom, they might just get it after they reach a billion, and living in the sticks, where there is not much power and probably not many resources, they can keep living there ad infinitum.

Their planet might not be a pleasant place to live that long on, however. Stars do not stay quiescent and unchanging, and then all of a sudden they decide to die. They age, and the stellar output power changes also. So does the radius. So, the alien civilization’s planet may have been in the habitable zone when life formed on their planet and then evolved into really bright creatures, but it might not stay there long afterward. Again, out in the red dwarf region, nothing much will happen, but for stars further up the main sequence, they could be forced out of their home planet before they even reach a billion.

Their planet might be even less stable than that. The time scale for geologic processes can be in the tens or hundreds of millions of years, and surface changes can be much less than that. Seas form, glaciers form and cover much of the globe, then they disappear and sea levels rise, rivers form or go dry, and so on. There is likely enough time during any of these processes to move the entire civilization from one place to another on the planet, hopscotching around to the best available areas when the old ones get unpleasant. Orbits of planets change also, and what was a splendid orbit might drift into a wretched orbit. Eccentricity in the orbit means excesses in the seasons. Could this advanced a civilization maintain their planet’s orbit where they want it? Perhaps.

As noted in another blog, there are galactic perils to be avoided. Here on Earth, there were several supernovas close enough to be visible to the naked eye within the last couple of millennium, perhaps many more. That means, if the ratio is about the same, several million might have been visible on the alien planet we are discussing in their first few billion years of civilization. The ones we saw were too far away to injure our planet, but if you think of the chance that one of these several million was not that far away from the alien civilization’s planet, it doesn’t seem unlikely that they would have to take measures to protect themselves from it. Perhaps many went off close to them.

Stars move in the galaxy, both in their general orbits around the galactic center, but also with a statistical random motion, in the plane of the disk and perpendicular to it. Since they are random, there is a chance that two will come close enough to alter the orbits of the planets of one or both. This would be an event that might be catastrophic, requiring migration to another star’s solar system. Rogue planets are also present in the galaxy, and the numbers of them are uncertain. They have to virtually penetrate the solar system to disturb the system’s planetary orbits, but if the numbers of them are large enough, one may buzz by during the alien civilization’s first few billion years. We think about protecting our planet from asteroids, which might be a mile or so in diameter, and of doing it by modifying the orbit. Modifying the trajectory of a Jupiter-sized planet barreling toward your planet’s orbit is a different matter.

Other perils exist, and probably many we are unclear about, so that during a billion years, besides worrying about their sun’s output freezing them out or roasting them, and geological changes screwing up the continents and oceans of the planet itself, they have to worry about things the galaxy throws at them. But out of a billion years, only a few million might have been devoted to these survival problems. Moving to another world, if they can find one that meets their specifications, would be included in this few million. This assumes they are at least a category 2 civilization, one that cares about its own survival. Category 1 is probably not going to last a billion years.

Colonization would have taken up some time, if they were a category 3 civilization. Depending on how long it takes, and the grand strategy they follow, and the resources they can devote to it, it should be done in some millions of years. This figure is for covering the galaxy, and the home world is probably not going to be doing all the galaxy, only the local neighborhood. Finding suitable worlds for quick colonization, even a thousand or two light years distant, will let the home world sink back into relaxation while the clone worlds take the next step to the next almost-perfect planet to be cloned. If the civilization is following the leap-frog strategy, finding clone worlds and then slipping back into slow motion colonization on the rest, the not-so-great-for-life planets, the colonization process could take up the whole billion years and more, depending on how not-so-great the planets were.

Slow motion colonization of some nearby planet is not going to occupy the civilization during the whole billion years. It is an activity which might use up a block of time, and then the civilization would be waiting for the planet to respond to their actions. Perhaps if there were a great many planets to do, they could keep a bit busier doing it. That is, between evacuating their home planet for supernova explosions or trying to move a rogue planet that was bulls-eyed on them. Some number of millions of years out of a few billion is not many. Seems like they could have a lot of time on their hands. So one question to ask is, what do they do to keep from being bored?

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