Tuesday, February 2, 2016

Did Aliens Visit the Dinosaurs?

We ask the questions: “Where are all the aliens?” and “Why haven’t aliens visited us?” These are excellent questions and have motivated this blog for a while. But maybe aliens did visit Earth, but they just got here a little early. Maybe they arrived when the dinosaurs were here.

When we talk about the timing of alien civilizations, there are certain basic estimates that need to be made. One is the length of time between the first light-off of a sun, and the first light-off of a stellar vessel propulsor taking the vessel away from an alien civilization on a planet of that same sun. Let’s just say it is between 2 billion and 5 billion years, with the average about 3.5 billion years. Another estimate relates to the production of stars in a galaxy like the Milky Way. There is an initial burst of them when the galaxy first forms, and then the rate levels out. In our dear Milky Way, maybe 300 billion stars got formed in the first few hundred million years, which was the initial formation time. It was sudden, wasn’t it?

After that, stars get formed, mostly in the disk, and let’s say, every revolution, a galactic year which takes 200 million years, another half billion stars get formed. The galaxy is said to be about 12 billion years old, which is about 60 galactic years. That puts the total number up to 30 billion formed late plus 300 billion formed early for a total of 330 billion total, which is close enough.

Another estimate, wholly unknown, is how many alien civilizations form per star. Let’s just take a number, and say 0.01. One percent of stars have all the right conditions to form an alien civilization. Maybe this number is 10 or 100 or 1000 times too large, so it might be thought of as an upper bound. That means every galactic year, five million appropriate stars with solar systems get formed which lead to, about 3.5 billion years later, to starship launchers.

So, during the 200 million year reign of the dinosaurs on our planet, there were five million civilizations that toured the galaxy, and maybe some of them dropped down to look at Tyrannosaurus Rex.

The last estimate is how long a civilization can last. This is governed by the usage rate of the resources on the planet and any other planets they can take control of. The number might have an upper bound of 10,000 years, assuming a store of resources that will last them 1000 years at full population and full technology, and 10,000 with 90% recycling. This means all the alien civilizations that visited the dinosaurs have all died out by now.

Since visiting a star takes a terribly long time, and a terribly lot of resources, let’s suppose on the average an alien civilization visits 10 other planets before becoming extinct. We are assuming that colonizing is too expensive to happen, otherwise some multiplier needs to be added.

By simple arithmetic, there is on the average only .025 alien civilizations starting per year, and only 250 that exist at any time. More arithmetic says that in a galactic year, 50 million visits by aliens to exo-planets happen, or about 0.25 per year. Then, during our last 3000 years, which is about as long as we might have records of an alien visit, 750 planets were visited. If aliens consider visiting 10% of the stars in the disc, maybe 30 billion, and each has one planet that aliens might want to visit, that is 3 billion worthwhile planets, out of which 750 were visited during the time we would have noticed them. So, while it is quite possible that some dinosaur looked up and saw an alien ship, or was actually hunted down by an alien team of collectors, or whatever, it is very, very improbable that any aliens have visited our planet when we would have noticed. The ratio of 750 to 3 billion is quite small, and even if you throw a factor of ten or a hundred or a thousand into the mix somewhere, such as by assuming 99% recycling or colonization or anywhere else, it still does not change from very, very improbable.

This means that there is no reason to look further than resource exhaustion to explain why aliens have not visited Earth, and that digging deeper into the question of resource usage might provide some important insights. One tradeoff is population reduction. If an alien civilization reduces its population to increase the longevity of its civilization, it can extend the exhaustion time inversely proportionally to the population reduction up to a point. There are certain fixed costs which must be met, out of resources, and maintaining a population of something as low as 1 million might not use much less resources than a population of 10 million.

The reason there is this saturation effect is that some things cannot be done small. For example, perhaps the most important example, is the power source of the population. If it is DD fusion, there may well be a minimum size that even the asymptotically clever scientists and engineers of an alien civilization cannot beat. That power station might be able to serve 30 million aliens, and keeping it going costs a certain amount of resources every year. Reducing the population down to 1 million doesn’t affect the power plant costs very much.

Recycling is another mandatory activity of an alien civilization that hopes to last a long time. It may well be that a complex recycling plant, able to recycle all resources that are consumed by an advanced civilization with a high living standard, has to be some minimum size in order to be efficient. The amount of cleverness that can be used to make this number small could be large, we simple don’t know as we do not do much recycling ourselves, but it might set a minimum population size as well, and that number might be comparable with the power station minimum. We don’t know enough about either of these things to know if either, or something else more abstruse, poses a lower bound on the population, but the concept of a lower bound is clear. Maybe it is not possible to have a small biological factory for some things. Maybe there are high fixed asset costs for maintaining a robot population or an intello population. Perhaps knowledge preservation imposes high costs on the civilization. Whatever does, means that there is a limit to the extension of the time between when a civilization starts to seriously consume resources, perhaps at the industrial grand transition, to when it hits the scarcity and then exhaustion barriers.

No substantive conclusions can be drawn here, but it appears that die-off is a possible cause of the absence of aliens. Nobody came by during the window of opportunity our species provided because there are too few of them surviving long enough to get here.

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